FGFR3 Spectrum (Achondroplasia, Hypochondroplasia and Thanatophoric Dysplasia)

Literature and regulatory information updated: 6 October 2026.

Diseases of the FGFR3 Spectrum/Complex: from Achondroplasia to Thanatophoric Dysplasia and the Oncogenomic Spectrum — Medical Genetics and Cancer Genomics Lecture (Allelic Spectrum of the FGFR3 Gene)

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0. Initial Reference Table

Reference HGVS nomenclature: MANE Select transcript NM_000142.5 (protein NP_000133.1; MANE version 1.5; see Section 5.7). All the above entities — seven to eight entities, of which six to seven constitute the classic spectrum of “FGFR3 complex disorders” — belong, in the International Nosology of Skeletal Dysplasias (2023 revision), to the “FGFR3 chondrodysplasias” group [25] — a continuum of severity determined by the degree of constitutive receptor activation rather than different genes [22, 26]. Added to this germline core is a somatic/oncogenic spectrum (urothelial carcinoma, epidermal naevi, multiple myeloma with t(4;14)) sharing the same gene but entirely different mechanisms, inheritance and clinical implications, discussed separately in Section 9.3.

Note on frequency estimates in reference manuals: the Oxford Desk Reference gives a birth prevalence of ~1/27,000 for achondroplasia and ~1/20,000 for thanatophoric dysplasia [1, 27]; Smith's Recognizable Patterns gives ~1/15,000 for achondroplasia and hypochondroplasia incidence around one-twelfth that of achondroplasia [28, 29]; the Brazilian manual Síndromes Genéticas: Reconhecer para Tratar gives 1:10,000–30,000 for achondroplasia, without racial predominance, and notes that there are still no epidemiological studies in Brazil [30]. These values, from older series or syntheses, have been retained as a reference and discussed in Section 9.1.

Organisation of care: in the European Union, ERN BOND (skeletal dysplasias) and ERN CRANIO (Muenke syndrome and craniosynostoses) [31, 32]; in France, achondroplasia has a PNDS (French National Diagnosis and Care Protocol; Protocole National de Diagnostic et de Soins) issued by the Haute Autorité de Santé (April 2025), drawn up by the reference centre for constitutional bone diseases (CRMR MOC; coordinated at Hôpital Necker-Enfants malades, with constituent sites at Cochin, Lariboisière, Lyon and Montpellier, and some twenty competence centres) and integrated into the OSCAR health network (filière), with the participation of the patient association APPT (see Sections 9.2 and 11.1) [33].

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1. Introduction

Achondroplasia (MIM #100800; Orphanet ORPHA:15) is the most frequent skeletal dysplasia causing disproportionate dwarfism, resulting from activating variants in FGFR3 (locus 4p16.3), encoding fibroblast growth factor receptor 3. Recognition of achondroplasia as a distinct clinical entity dates to the nineteenth century, but its current designation and systematic description are chiefly due to Parisian work in the late nineteenth and early twentieth centuries. Jules Parrot, a French surgeon, consolidated in 1878 the current designation and recognition of the pattern of rhizomelic dwarfism with relative macrocephaly, distinguishing it from other forms of skeletal disproportion then grouped under the generic term "fetal chondrodystrophy". The history of the FGFR3 complex disorders illustrates, perhaps better than any other topic in skeletal genetics, the transition from descriptive to molecular nosology. For almost a century, achondroplasia, hypochondroplasia and thanatophoric dysplasia were classified exclusively by clinical–radiological criteria, grouped under the broad designation "chondrodystrophies". The most influential clinical description of achondroplasia as a distinct entity dates to the work of Pierre Maroteaux and Maurice Lamy in Paris, who also coined in 1967 the term "thanatophoric dwarfism" to describe a lethal skeletal dysplasia with very short extremities and a narrow chest, distinct from classic achondroplasia [28, 34]. Its systematic clinical–radiological characterisation is chiefly due to Langer, Baumann and Gorlin (1967) [35], whose work still underpins the descriptive criteria used in skeletal radiology.

Hypochondroplasia, a milder allelic form, was only clinically delineated by Ravenna in 1913, with its designation and autosomal dominant transmission pattern formally established by Léri and Linossier in 1924 [36, 37]; for decades, most cases remained incorrectly classified as attenuated achondroplasia, given the substantial phenotypic overlap between the two entities [29]. Thanatophoric dysplasia — the lethal form of the spectrum — was described as a distinct nosological entity by Pierre Maroteaux, Maurice Lamy and J. M. Robert in 1967 in Paris, under the designation “le nanisme thanatophore” (“thanatophoric dwarfism”), derived from the Greek thanatos (death) and phoros (bearer), reflecting the almost universal perinatal mortality observed in initial series [34, 38]; Langer and colleagues subsequently separated the condition into two types [39].

Conceptual unification of these conditions within a single allelic spectrum became possible only with the advent of molecular genetics in the 1990s. The turning point came in 1994, when two independent groups identified almost simultaneously mutations in the fibroblast growth factor receptor 3 gene (FGFR3) in patients with achondroplasia as its cause: Rita Shiang and colleagues at the University of California, Irvine (published in Cell) and Frédérique Rousseau and colleagues in Paris (published in Nature) [7, 8]. Shiang and colleagues demonstrated that more than 97% of cases resulted from a single point substitution at codon 380 of the transmembrane domain, replacing glycine with arginine (p.Gly380Arg) [7], revealing unprecedented mutational homogeneity in human genetics: virtually all cases result from the same single nucleotide substitution — cDNA position 1138, a particularly mutable CpG dinucleotide — responsible for the overwhelming majority of cases worldwide. This discovery was remarkable not only for its mechanistic elegance but also for its almost unparalleled homogeneity in human genetics: a single nucleotide (cDNA position 1138) proved to be one of the most mutable sites in the entire human genome, subject to a well-documented advanced paternal age effect. This finding catalysed a rapid succession of related discoveries, and the following year, 1995, consolidated the concept of an “allelic family” of diseases: Peter Tavormina and colleagues demonstrated that thanatophoric dysplasia types 1 and 2 — the lethal form of the spectrum — also result from distinct mutations in the same gene, FGFR3, but at different positions (including introduction of supernumerary cysteine residues in the extracellular domain in type 1, and p.Lys650Glu substitution in the tyrosine kinase domain in type 2) [5]. Almost simultaneously, Gary Bellus and colleagues identified the recurrent p.Asn540Lys (N540K) mutation as the cause of approximately half of hypochondroplasia cases [15], and later, in 1999, the same team described p.Lys650Met as the cause of a new, particularly severe entity — SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans) [6]. In 1996, Michael Naski and colleagues provided the unifying biochemical basis of the spectrum, demonstrating experimentally in a controlled system that phenotypic severity correlates with the degree of constitutive activation of receptor kinase activity — a “molecular dose effect” subsequently systematised by Zoltan Vajo, Clair Francomano and David Wilkin in 2000 in their reference review of “the achondroplasia family of skeletal dysplasias” [22]. In parallel, in 1997, Maximilian Muenke and colleagues at the National Institutes of Health described a new coronal craniosynostosis syndrome associated with p.Pro250Arg (P250R) substitution in the extracellular receptor domain — in the linker region between immunoglobulin-like domains II and III of the ectodomain — a common and phenotypically highly variable coronal craniosynostosis form, now known as Muenke syndrome, definitively expanding the gene's clinical spectrum beyond pure skeletal dysplasia [18]. In 1999, two groups — Bellus and Tavormina — independently characterised SADDAN syndrome, caused by the p.Lys650Met (K650M) mutation at the same codon as the mutation causing thanatophoric dysplasia type 2, paradigmatically illustrating the concept of a continuous allelic severity spectrum [6, 40].

This nosological unification under a single gene — bringing together lethal and non-lethal, skeletal and cranial forms — made FGFR3 one of the most instructive paradigms of human molecular genetics and would inspire, decades later, development of the first targeted molecular therapies for a skeletal dysplasia, with the first approval of the C-type natriuretic peptide analogue vosoritide in 2021 [41, 42]. This trajectory — from Maroteaux and Lamy's clinical description to twenty-first-century precision therapy, including oncological repurposing of the same gene through tyrosine kinase inhibitors for urothelial carcinoma — structures this lecture.

Estimated achondroplasia prevalence lies between 1/25,000 and 1/30,000 live births, with recent meta-analyses suggesting around 4.6–4.7 per 100,000 births (approx. 1/21,000) [12], higher in North Africa and the Middle East (up to 34.3/100,000) and sub-Saharan Africa (12.6/100,000), suggesting a possible influence of population factors not yet fully understood (see Section 9.1). Around 80% of cases result from a de novo mutation, markedly associated with advanced paternal age — attributed to positive clonal selection of spermatogonia carrying the FGFR3 variant; the remaining 20% have autosomal dominant transmission from an affected parent, with complete penetrance and relatively uniform expressivity. The clinical impact of achondroplasia is substantial: beyond disproportionate short stature (mean adult height 131 ± 5.6 cm in men and 124 ± 5.9 cm in women), the disease involves increased lifelong risk of neurological, respiratory, otological and orthopaedic complications, associated with an estimated overall life expectancy reduction of around 10 years relative to the general population, with early infant mortality predominantly of central origin (brainstem compression) and mortality in middle age predominantly cardiovascular [26, 43].

This lecture addresses in an integrated manner the entities constituting the clinical and molecular spectrum associated with pathogenic gain-of-function FGFR3 variants: thanatophoric dysplasia (types 1 and 2), achondroplasia (including the lethal homozygous form), hypochondroplasia, SADDAN syndrome, Muenke syndrome and Crouzon syndrome with acanthosis nigricans. CATSHL syndrome (camptodactyly, tall stature and hearing loss) is included as a pathophysiological contrast, caused by loss-of-function variants in the same gene, whose skeletal overgrowth phenotype confirms FGFR3's role as a dose-dependent negative regulator of endochondral bone growth. The somatic oncogenic role of FGFR3 in urothelial neoplasms and multiple myeloma is also addressed — including targeted therapy with erdafitinib — establishing the bridge between constitutional genetics and cancer genomics that characterises this area of integrated genomic medicine.

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2. Molecular Genetics and Pathophysiology

2.1 The gene and protein

FGFR3 is located on the short arm of chromosome 4 at band 4p16.3 and encodes a transmembrane tyrosine kinase receptor belonging to the fibroblast growth factor receptor family (FGFR1–4; FGF, fibroblast growth factors). The mature protein comprises three extracellular immunoglobulin-like domains (D1, D2, D3), a single transmembrane domain and an intracellular domain with tyrosine kinase activity, divided into two lobes (TK1 and TK2) by an activation loop [22, 26]. Physiologically, FGF binding to the ectodomain — particularly FGF9 and FGF18 in the presence of heparin or heparan sulphate as cofactor — induces receptor dimerisation and trans-autophosphorylation of tyrosine residues in the intracellular domain, activating downstream signalling cascades. Unlike most cell surface receptors, whose activation promotes proliferation (and most growth receptors, whose loss of function reduces cell proliferation), FGFR3 physiologically acts as a negative regulator of endochondral bone growth and chondrogenesis in the epiphyseal growth plate: its activation inhibits proliferation and promotes premature differentiation/senescence of chondrocytes in the growth plate's proliferative zone through predominant activation of the RAS-RAF-MEK-ERK pathway (MAPK; RAS/rapidly accelerated fibrosarcoma – mitogen-activated protein kinase), with secondary contributions from STAT1/STAT5 (signal transducer and activator of transcription), phospholipase C-gamma and PI3K-AKT (phosphoinositide 3-kinase – protein kinase B) pathways [44, 45].

2.2 Common pathogenic mechanism: constitutive gain of function; the mutational spectrum and genotype–phenotype correlation

Pathogenic variants responsible for the classic FGFR3 spectrum (achondroplasia, hypochondroplasia, thanatophoric dysplasia and SADDAN) invariably cause gain of function, that is, ligand-independent constitutive receptor activation, through spontaneous dimerisation (mutations introducing extra cysteine residues in the extracellular domain, promoting aberrant disulphide bridges between monomers), direct alteration of transmembrane domain conformation (p.Gly380Arg), or stabilisation of the active kinase domain conformation or intrinsic activation of the catalytic domain itself (codon 650 mutations) [5, 6, 9, 15, 26]. The common final result is exaggerated inhibition of chondrocyte proliferation in the growth plate's proliferative zone and acceleration of premature differentiation/senescence, with consequent disorganisation of growth plate architecture and deficient endochondral ossification — the process responsible for around 90% of axial and appendicular skeletal bone formation [16]. In achondroplasia, altered FGFR3 is constitutively activated, with unregulated signal transduction through the receptor, resulting in inappropriate differentiation of the cartilaginous growth plate and ultimately deficient endochondral growth [26]; the receptor in growth plate chondrocytes is in a state of constant activation, altering long bone growth [30]. There is a consistent genotype–phenotype correlation between variant location along the protein, degree of constitutive activation measured in vitro and observed clinical severity:

• Extracellular domain (Ig-like III) and stop codon loss mutations — associated with thanatophoric dysplasia type 1, the most severe and always lethal phenotype in the classic spectrum. The most frequent variants are p.Arg248Cys (R248C, ~55%), p.Tyr373Cys (Y373C, ~24%) and p.Ser249Cys (S249C, ~6%), with around 10% being stop codon mutations [1, 4]; they introduce a new cysteine residue at various locations in the receptor's extramembranous segment [34].
• Transmembrane domain — the p.Gly380Arg substitution (corresponding to c.1138G>A, present in more than 98–99% of cases, or more rarely c.1138G>C) constitutes the molecular basis of virtually all classic achondroplasia cases [9, 46]; the Oxford Desk Reference states that the two common variants (G1138A and G1138C; historical numbering, = NM_000142.5:c.1138G>A and c.1138G>C, both NP_000133.1:p.(Gly380Arg)) account for 98% of mutations [27]. Rare variants described in achondroplasia include c.1043C>G (p.Ser348Cys) and p.Gly375Cys [26]. The French achondroplasia PNDS (French National Diagnosis and Care Protocol; 2025) attributes 97% of cases to p.Gly380Arg and lists, based on Ornitz and Legeai-Mallet (2017), four other substitutions associated with the achondroplasia phenotype, classified in ClinVar on the MANE transcript: NM_000142.5:c.649A>T, p.(Ser217Cys), likely pathogenic (single submission); NM_000142.5:c.835A>T, p.(Ser279Cys), and NM_000142.5:c.1031C>G, p.(Ser344Cys), pathogenic or likely pathogenic (several concordant submissions); and NM_000142.5:c.1123G>T, p.(Gly375Cys), pathogenic; p.(Ser344Cys) is a variant distinct from the p.(Ser348Cys) mentioned above [33, 44, 47]. This domain also contains the p.Ala391Glu mutation, causing Crouzon syndrome with acanthosis nigricans, which combines severe craniosynostosis with skin hyperpigmentation.
• Proximal tyrosine kinase domain — p.Asn540Lys substitution (N540K; C1620A transversion, or more rarely C>G at the same codon) is responsible for around 50 to 70% of hypochondroplasia cases (around 60% according to Bone Dysplasias and the Oxford Desk Reference), a qualitatively similar but quantitatively much milder phenotype than achondroplasia [16, 27, 48]; other variants have been described (for example, p.Ile538Val), and complete gene screening is recommended in cases with typical radiographic signs without the common variants [48].
• Codon 650 of the tyrosine kinase domain (activation loop) — exemplarily illustrates the spectrum's dose–response relationship: the p.Lys650Met substitution (K650M) causes SADDAN syndrome; the p.Lys650Glu substitution (K650E) causes thanatophoric dysplasia type 2, a more severe phenotype than SADDAN despite paradoxically producing a lower degree of kinase activation in in vitro assays (approximately 10 times basal activity versus 18 times for K650M) — a finding already emphasised in Vajo and colleagues' original studies, demonstrating that isolated enzymatic activity does not fully explain clinical severity, with probable contributions from alternative signalling pathways and effects on protein stability [22, 40]. Different substitutions at the same codon (K650N and K650Q; historical designation, = NP_000133.1:p.(Lys650Asn), from NM_000142.5:c.1950G>T or c.1950G>C, and p.(Lys650Gln), from c.1948A>C) instead cause forms of hypochondroplasia, sometimes associated with acanthosis nigricans.
• Extracellular domain, linker region between IgII-IgIII domains — p.Pro250Arg substitution (P250R) causes Muenke syndrome [18, 49]. Muenke syndrome occupies a distinct molecular position: p.Pro250Arg lies in the linker region between ectodomain Ig II and Ig III domains, a region critical for ligand-binding specificity, rather than the transmembrane or intracellular domain. This variant thus occupies a different molecular position from the others, and its predominant effect (the result) appears to be increased affinity and/or signalling mediated by certain FGFs in cranial tissues, rather than ligand-independent constitutive activation — a mechanism helping to explain clinically the incomplete penetrance and markedly variable expressivity of this syndrome, from asymptomatic carriers to severe pansynostosis forms, whose predominant phenotype is coronal craniosynostosis, with typically minor and variable appendicular skeletal involvement [18, 19].
• Loss of function (CATSHL syndrome) and LADD syndrome — at the opposite end of the spectrum, heterozygous p.Arg621His in the tyrosine kinase domain causes partial FGFR3 loss of function and CATSHL syndrome (camptodactyly, tall stature, scoliosis and hearing loss), a phenotype reproducing that of the Fgfr3 knockout mouse and demonstrating that abnormal FGFR3 signalling can both inhibit and promote endochondral bone growth [23]. Homozygous p.Thr546Lys (c.1637C>A) was described in two brothers born to first cousins with tall stature, severe lateral tibial deviation with inability to walk, scoliosis, hearing loss, camptodactyly and arachnodactyly — the first report of a homozygous loss-of-function variant in human FGFR3, with autosomal recessive inheritance, located near the hypochondroplasia hotspot [24]. Heterozygous variants in the tyrosine kinase domains of FGFR3 (and FGFR2), as well as in FGF10, were identified in lacrimo-auriculo-dento-digital (LADD) syndrome, with lacrimal duct aplasia, malformed ears and deafness, small teeth and digital abnormalities — expanding the spectrum of abnormalities associated with abnormal FGF signalling [50].

This molecular dose–effect relationship explains the observed clinical spectrum: mutations in the extracellular or transmembrane domain (p.Gly380Arg in achondroplasia) produce relatively moderate kinase activation; tyrosine kinase domain mutations (p.Asn540Lys in hypochondroplasia) produce still subtler activation, corresponding to the spectrum's mildest phenotype; codon 650 mutations (p.Lys650Glu in thanatophoric dysplasia type 2, p.Lys650Met in SADDAN) produce the highest degrees of constitutive activation measured in vitro, although the correlation between magnitude of biochemical activation and clinical severity is not strictly linear — documented since Vajo and colleagues' original studies, emphasising the discrepancy between kinase activation in cellular assays and observed clinical severity — indicating participation of additional modifying mechanisms not yet fully elucidated [6, 22]. This severity gradient — thanatophoric dysplasia type 1 and type 2 (lethal) > SADDAN (severe, prolonged survival possible) > achondroplasia (moderate) > hypochondroplasia (mild) > Muenke syndrome (predominantly craniofacial, incomplete penetrance) — approximately corresponds to a decreasing degree of constitutive receptor activation and is one of the most instructive genotype–phenotype correlation examples in all human genetics. In counterpoint, Síndromes Genéticas: Reconhecer para Tratar emphasises that within thanatophoric dysplasia there is no consistent genotype–phenotype correlation between different type 1 variants, with phenotypic variability independent of mutation except for mutation-dependent differences in the severity of the endochondral disorder in long bones [4]. In hypochondroplasia, patients with N540K have a more severe phenotype (disproportionate short stature, macrocephaly, unchanged or narrowed interpedicular distance and a fibula longer than the tibia), whereas patients with hypochondroplasia not linked to 4p16.3 have milder radiological abnormalities, with normal hands and long bones and no metaphyseal widening [29]. Illustrating the continuum is the occurrence of thanatophoric dysplasia type 1 (p.Arg248Cys) and hypochondroplasia (p.Asn540Lys) variants in patients with an achondroplasia phenotype, and demonstration that San Diego-type lethal platyspondylic dysplasia is a particular manifestation of FGFR3 variants [4, 34].

2.3 Inheritance pattern and parental origin of de novo mutations

All entities of the spectrum have autosomal dominant transmission with complete penetrance — 100% penetrance in achondroplasia, with relatively uniform expressivity [27, 30] — with the partial exception of Muenke syndrome, whose penetrance is incomplete and expressivity markedly variable, including asymptomatic carriers, without a penetrance difference between sexes in an international cohort of 106 carriers [51]. However, the overwhelming majority of cases result from de novo germline mutations: approximately 80% in achondroplasia, more than 85% (virtually all) in thanatophoric dysplasia and SADDAN — conditions which, moreover, are practically incompatible with reproduction — and around 50-60% in hypochondroplasia [6, 10, 11]. De novo mutations originate almost exclusively on the paternally derived chromosome (allele): parental origin studies demonstrated that c.1138G>A occurs almost exclusively on the paternal allele, suggesting that FGFR3 nucleotide 1138 is one of the most mutable sites in the human genome during spermatogenesis, attributed to a selective replicative advantage ("selfish spermatogonial selection", spermatogonial clonal selection) of spermatogonia carrying the activating variant, explaining the well-documented advanced paternal age effect in all lethal and severe entities of the spectrum [7, 9]; the same paternal origin pattern was confirmed for the Muenke syndrome p.Pro250Arg mutation [18], and advanced paternal age was also documented in presumably de novo cases of hypochondroplasia [29] and thanatophoric dysplasia [34]. Reference manuals differ regarding the proportion of de novo achondroplasia cases: around three-quarters according to Bone Dysplasias, 80% according to the Oxford Desk Reference and around 90% according to Smith's Recognizable Patterns [26–28]. The French PNDS states that 85% of people with achondroplasia are born to parents of average stature and stresses complete penetrance: there are no carriers of the pathogenic variant without clinical signs [33].

Recurrence risk in siblings of an affected child born to average-stature parents is low but not zero, attributable to parental gonadal (germline) mosaicism — usually in testicular tissue, demonstrated in sperm DNA of the father of three affected siblings (Natacci and colleagues, 2008) [26] — less frequent in this spectrum than in other chondrodysplasias such as osteogenesis imperfecta. Published estimates vary by series: values around 0.1–0.2% are cited for achondroplasia, whereas other series place empirical recurrence risk between 0.2 and 1%, above baseline population risk but substantially below the 50% expected in classic dominant transmission [26]; reference manuals give divergent values — around 0.02% in the Oxford Desk Reference (based on Mettler and Fraser, 2000) and Bone Dysplasias, 0.2% in Smith's Recognizable Patterns and “<1%” in the Brazilian manual [26–28, 30]. Sensitive parental screening (ideally capable of detecting low-level mosaicism) allows a more precise estimate of familial recurrence risk and should be systematically offered to parents of an affected child even without parental phenotypic signs.

2.4 Homozygosity and compound heterozygous forms

When two parents carrying pathogenic FGFR3 variants have offspring together — the most common example being two individuals with achondroplasia, but also combinations such as achondroplasia and hypochondroplasia — there is a 25% risk of the fetus inheriting two mutated copies (homozygosity or compound heterozygosity). Homozygous achondroplasia — resulting from a reproductive union between two individuals with achondroplasia, each transmitting the mutated allele — produces a radiographically intermediate phenotype between heterozygous achondroplasia and thanatophoric dysplasia, with severe thoracic stenosis (thoracic hypoplasia) and fatal respiratory failure, typically lethal in the neonatal (perinatal) period due to respiratory failure secondary to severe thoracic hypoplasia [26, 34]; radiological abnormalities are more extreme and diagnosis rests on parental history [26, 27]. Compound heterozygosity for variants producing p.Asn540Lys (hypochondroplasia) and p.Gly380Arg (achondroplasia) on different alleles instead produces a more severe skeletal phenotype than typical achondroplasia but is not uniformly lethal, unlike homozygous achondroplasia, with the potential for severe disability and possibly reduced life expectancy; when both partners have hypochondroplasia, counselling is complicated by genetic heterogeneity and scarce data on the prognosis of offspring inheriting variants from both parents [16]. This directly affects reproductive counselling of concordant couples in whom both partners have achondroplasia or another condition of the spectrum (see Section 8).

2.5 Somatic oncogenic relevance of FGFR3

Although this lecture's central focus is constitutional pathology, it is relevant from an integrated genomics perspective that somatic activating variants of the same gene — mechanistically analogous to the germline variants described above, including the same p.Ser249Cys (S249C) substitution and others in the extracellular domain — are implicated in urothelial neoplasms and multiple myeloma [52–55]. These observations underpinned clinical development of small-molecule pan-FGFR inhibitors such as erdafitinib, currently FDA-approved and included in NCCN guidelines for locally advanced or metastatic urothelial carcinoma with susceptible FGFR3 or FGFR2 alterations [56]. This oncological relevance, including targeted erdafitinib therapy and repositioning of infigratinib from oncology to achondroplasia, is developed in detail in Sections 7.3 and 9.3.

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3. Clinical Spectrum and Manifestations by System

3.1 Shared finding: skull and axial skeleton

The most consistent and earliest finding throughout the FGFR3 disease family is relative macrocephaly, with frontal bossing, midface hypoplasia, a depressed nasal bridge and, in more severe forms, true skull base stenosis with reduction of the foramen magnum [16, 26]. In achondroplasia, this cranial configuration results from deficient endochondral growth at the skull base, contrasting with preserved membranous growth of the calvarium — an asymmetry explaining characteristic craniofacial disproportion; the calvarium is large relative to facial bones and the skull base short, with early spheno-occipital closure [27, 28]. Macrocephaly, predominantly caused by a large brain, is a usual finding [28]. In thanatophoric dysplasia type 2 and less frequently type 1, a cloverleaf skull deformity may be observed, resulting from severe multisuture craniosynostosis associated with hydrocephalus [34, 39]; the equivalent German term is Kleeblattschädel anomaly.

At vertebral level, the spectrum is characterised by a progressive reduction in interpedicular distance from the upper to lower lumbar region (the reverse of physiological widening), associated with short pedicles on lateral views — the anatomical basis of lumbar spinal canal stenosis, a major adult complication in achondroplasia — and, at its extreme, severe platyspondyly (marked vertebral body flattening with an “H” configuration on lateral radiographs) and insufficient ossification of vertebral centres in thanatophoric dysplasia — a pathognomonic sign of severity and lethality [26, 34].

3.2 Appendicular skeleton

The most evident clinical sign is rhizomelia — disproportionate shortening of the proximal limb segment (humerus and femur) relative to middle and distal segments — more pronounced in achondroplasia than hypochondroplasia and extreme in thanatophoric dysplasia. Hands show the classic “trident hand” configuration, with separation between the 3rd and 4th fingers and short broad fingers (brachydactyly), more evident in achondroplasia than hypochondroplasia and associated with limited elbow extension due to radiohumeral joint abnormalities [26]. In SADDAN and thanatophoric dysplasia, a characteristic lower-limb posture with extended knees, termed “puppet posture”, is also described [34]. Tibial bowing is a common late achondroplasia complication, occurring in up to 40% of older children, generally with progressive varus [26]; relative fibular overgrowth may accentuate bowing and require early epiphysiodesis (stapling) [28].

3.3 Thanatophoric dysplasia (types 1 and 2)

Presentation is always prenatal or neonatal. It is characterised by disproportionate dwarfism with very short extremities and a relatively preserved trunk (birth length 36 to 46 cm, mean 40 cm), redundant skin folds along the limbs, small “sausage-like” fingers, a narrow chest with short ribs (the principal determinant of mortality through secondary pulmonary hypoplasia), a protruding abdomen and relative macrocephaly (head circumference 36 to 47 cm, mean 37 cm) with a large fontanelle, depressed nasal bridge, prominent forehead, small face and, in some cases, ocular proptosis [1, 34, 39]. In thanatophoric dysplasia type 1, the commonest form, femora have characteristic “telephone receiver” bowing and vertebral bodies are very flattened (≤35% of the adjacent disc space in the lumbar region); in type 2, long bones are typically straight and vertebral bodies taller, but almost all cases have a cloverleaf skull deformity due to severe multisuture craniosynostosis associated with hydrocephalus — only occasional in type 1 [39]. In both thanatophoric dysplasia and SADDAN, a characteristic lower-limb posture with extended knees, termed “puppet posture”, is described. Encephaloceles occur in some cases, particularly thanatophoric dysplasia type 2 [34]. Central nervous system abnormalities — temporal lobe dysplasia (the most frequent), megalencephaly, polymicrogyria, neuronal heterotopia, temporal hyperplasia, gyral disorganisation through premature development of aberrant sulci, hippocampal dysplasia, nuclear dysplasia, abnormal axonal bundles, cerebellar and brainstem hypoplasia, hydrocephalus, encephalocele and, rarely, semilobar holoprosencephaly — are described in both subtypes and contribute to neurological impairment in rare long-term survivors and, in rare survivors with intensive support, to profound cognitive impairment [34, 39, 57, 58]; neuropathological abnormalities are speculated to begin between the 11.5th week of gestation (formation of the medial temporal fissure and hippocampus) and the 13.5th week (a phase of intense cell migration and proliferation) [4]. Occasional abnormalities include patent ductus arteriosus, atrial septal defect, horseshoe kidney, hydronephrosis, imperforate anus, chylous ascites, radioulnar synostosis, cutaneous syndactyly, flat haemangioma on the nose and eyelids, hypertelorism, bilateral hearing loss and acanthosis nigricans in long-term survivors [4, 39]. Reduced fetal activity and polyhydramnios are frequent [39].

3.4 Achondroplasia

The cardinal phenotype combines rhizomelic dwarfism (predominant shortening of proximal limb segments — upper arm and thigh more affected than forearm and lower leg), relative macrocephaly with a prominent forehead, midface hypoplasia with a depressed nasal bridge and narrow nasal cavities, a relatively large mandible, and the characteristic “trident hand” due to inability to fully approximate the third and fourth fingers, with short broad phalanges, associated with limited elbow extension due to radiohumeral joint abnormalities [26, 27]. The trunk shows thoracolumbar kyphosis in infancy — present in up to 87% of children aged 1–2 years, resolving spontaneously in most cases by 5–10 years [11] — followed in the walking child by compensatory lumbar hyperlordosis, a protruding abdomen and prominent buttocks due to anterior pelvic tilt. Axial muscle hypotonia is constant in the first year, delaying motor milestones (unsupported sitting between 9 and 20 months and walking between 14 and 27 months), although it progressively normalises [27]. Cognitive development is typically normal, except for language delay secondary to recurrent otitis and untreated hearing loss. Around half of newborns have birth length within normal limits (around 47 cm), with progressive growth deceleration from infancy [28, 30]. Mean adult stature is 131 ± 5.6 cm in men and 124 ± 5.9 cm in women [26, 28], with ranges of 120–145 cm in men and 115–137 cm in women according to the Oxford Desk Reference [27] (the Brazilian manual gives means of 125 cm and 120 cm and weights of 55 kg and 46 kg, respectively [30]); GeneReviews gives, based on more recent data, mean adult heights of 129.9 ± 6.25 cm in men and 122.4 ± 5.9 cm in women and refers to updated growth charts for length, weight, head circumference and height–weight relationship [46]. Multisystem involvement includes:

• Central nervous system — foramen magnum stenosis with a risk of cervicomedullary compression; communicating hydrocephalus of variable degree (usually stable), predominantly related mechanistically to jugular venous stenosis at the skull base (increased venous pressure due to narrowing of the jugular foramen), manifested by excessive head circumference increase in the first three years, usually stabilising spontaneously without shunting; temporal lobe abnormalities on magnetic resonance imaging [28, 30];
• Respiratory system — obstructive sleep apnoea due to midface hypoplasia and relative adenotonsillar hypertrophy (potentially affecting most children when systematically assessed by polysomnography), central apnoea due to brainstem compression, airway malacia and ventilatory restriction due to a small rib cage, potentially causing tachypnoea and sweating, particularly during meals, with particular risk during intercurrent infections such as bronchiolitis [27, 28];
• Axial skeletal system — childhood thoracolumbar kyphosis (generally resolves spontaneously with maturation of axial muscle tone), compensatory lumbar hyperlordosis and progressive lumbar spinal canal stenosis, symptomatic in adults; low back pain is the commonest complaint in older children and adults [26];
• Appendicular skeleton — tibial bowing, a common late complication occurring in up to 40% of older children, generally with progressive varus; joint hypermobility common in all joints except the elbow [26, 27];
• Auditory system — dysfunction of the (short) auditory tube with recurrent otitis media with effusion, present in most patients, and conductive hearing loss [27, 28];
• Stomatognathic system — dental malocclusion due to maxillary hypoplasia, present in around 50% of children after 10 years, and crowding of mandibular teeth [26, 28];
• Metabolism — relative glucose intolerance (oral glucose tolerance test) and a tendency towards obesity beginning in later childhood [28];
• Female reproductive system — higher incidence of menorrhagia, uterine fibroids, large breasts and need for caesarean delivery due to pelvic disproportion [28]; pregnancy in women with achondroplasia also entails specific anaesthetic considerations related to a potentially more difficult airway (see Section 9.2).

Occasional abnormalities include spinal cord or nerve root compression, pulmonary hypertension and synostosis of multiple cranial sutures [28]; the Brazilian manual also mentions umbilical hernia, congenital radial head subluxation, ligamentous laxity, varus or valgus feet, bronchial asthma and recurrent pneumonia [30]. Some developmental delay, particularly motor, is common, as is a reduction in intelligence quotient relative to siblings; an achondroplasia-specific developmental milestone screening test exists up to 2 years (Todorov and colleagues) [28].

The French PNDS (French National Diagnosis and Care Protocol; 2025) adds quantitative natural history data, mainly from recent retrospective cohorts (Hoover-Fong and colleagues, 2021; Stender and colleagues, 2022; Maghnie and colleagues, 2023; Pimenta and colleagues, 2023) [33]. Birth length is 47.28 ± 2.9 cm in girls (versus 49.1 ± 1.9 cm in the reference population) and 47.9 ± 3.2 cm in boys (versus 49.9 ± 1.9 cm). At least five specific growth charts have been published (Horton 2007, Del Pino 2011, Tofts 2017, Merker 2018, Hoover-Fong 2021), differing slightly from one another; a downward deviation from the specific chart should prompt a search for another cause of growth failure, and the stature of average-statured parents appears to correlate with that of the child. In early childhood there may be relative failure to gain weight, owing to feeding difficulties, gastro-oesophageal reflux, oro-oesophageal dysmotility, tachypnoea with increased work of breathing, excessive sweating or tachycardia; later, the dominant risk is excess weight, with early abdominal adiposity that worsens in adolescence. Adult stature is around 127–135 cm in men and 120–126 cm in women (Ozono and colleagues, 2024), with mean weights of 55 and 46 kg; growth restriction is around 50% in the lower limbs and 25% in the upper limbs, with trunk length at the lower limit of normal. The mean age of walking is 18–19 months; generalised joint hypermobility, limited elbow extension, macrocephaly and hypotonia explain distinctive motor strategies, and brachydactyly with hypermobility of the small joints explains slowness and fatigue in writing. Cognitive function is normal; when a neurodevelopmental disorder is present, hearing, sleep (obstructive apnoea) and the environmental context are checked first, before looking for another associated cause. Otitis media with effusion affects 50–70% of children from the first year, with hearing impact in 38–60%; 56% of 3-year-olds have had tympanostomy tubes, 53% of patients older than 15 years have undergone adenotonsillectomy, around 25% have language or articulation delay and around 40% of people with achondroplasia have conductive hearing loss. Genu varum, with internal tibial torsion and tibiotarsal varus, is associated with a relatively long fibula and knee instability (recurvatum and mediolateral instability), becomes apparent with walking, progresses at around 3–4 years and affects 10% of children at 5 years and 93% of adults; between one-third and one-half of children have lower-limb deformity and around a quarter of these require surgery; the metaphyses show pseudo-enchondromatous trabeculation (Nahm and colleagues, 2023), and varus greater than 15° predisposes to knee osteoarthritis. Dislocation or subluxation of the radial head is rare and should be sought when pronation–supination is limited; the wrists are hypermobile in almost all children and adults. From a cardiometabolic standpoint, diabetes does not appear more frequent than in the general population (Legare and colleagues, 2021), but arterial hypertension is frequent and underdiagnosed because of the use of inappropriate cuffs [33]. The figure of 42% that the PNDS attributes to achondroplasia corresponds, in the source study, to the whole group of 403 adults with various short-stature skeletal dysplasias, assessed at patient association meetings and specialist clinics in the USA, with hypertension defined as systolic pressure >140 mmHg, diastolic >90 mmHg or use of antihypertensives; in the 234 adults with achondroplasia (106 men and 128 women), the study describes an 8–10% increase in the probability of hypertension per unit of body mass index, without individual clinical diagnosis [59]; the use of achondroplasia-specific body mass index charts is recommended (Tofts and colleagues, 2017). Isolated seizures have been described in up to 2% of individuals in some cohorts, with no established aetiology. Craniofacial dysmorphism (frontal bossing, maxillozygomatic retrusion, depressed nasal bridge, relative prognathism) develops in the first two years and becomes more marked with age, and is graded in three degrees (mild, moderate, severe; Morice and colleagues, 2023); malocclusion results from maxillary hypoplasia with relatively excessive mandibular growth [33].

3.5 Hypochondroplasia

A phenotype qualitatively but not quantitatively overlapping with achondroplasia, with typically minor or absent craniofacial involvement (a normal or slightly macrocephalic skull) and milder hand and spinal features — a distinguishing trait in clinical differentiation [29, 48]. Mean birth length is 47.7 cm and mean weight 2.9 kg [29]. Limbs are relatively short, without rhizomelic, mesomelic or acromelic predominance, with a relatively long trunk, brachydactyly, short broad hands and feet, generalised laxity, mildly limited elbow extension and forearm supination, a long distal fibula (potentially causing foot inversion), short distal ulna and long ulnar styloid, childhood genu varum (tending to correct spontaneously although potentially requiring surgical correction), slightly increased lumbar lordosis, a protruding abdomen and sometimes scoliosis [29, 48]. Final stature is reported somewhat variably between series: a classic range of 118–152 cm, with means of 138–165 cm in men and 128–151 cm in women, whereas other series place adult stature between 132 and 147 cm — variation probably reflecting sampling differences between historical cohorts [29, 48, 60]. Onset is usually postnatal, becoming evident around 2-3 years or even only at school age (the usual age of manifestation), contributing to late or missed diagnoses; prenatal ultrasound diagnosis has been achieved in some cases, particularly in children of affected parents [48]. Relative macrocephaly is a useful discriminator from idiopathic short stature. Mild intellectual disability associated with medial temporal lobe dysgenesis on imaging has been described in a significant proportion of cases in historical series (up to 9% in some cohorts — 9% in Hall and Spranger's series; 4 of 13 cases in another series, with intelligence quotient between 50 and 80), more prevalent in N540K carriers [29, 60]. Epilepsy may arise at various ages, typically with a temporal lobe focus; infants may show only subtle signs (apnoea, cyanosis, eye deviation, a vacant gaze), and electroencephalographic abnormalities (3 Hz spike–wave complexes or temporal slowing) may be difficult to capture, so active enquiry about temporal lobe epilepsy symptoms and conventional seizure treatment are recommended [16]. Occasional abnormalities include brachycephaly with a short skull base, mild frontal bossing, esotropia, cataract, ptosis, cleft palate, postaxial foot polydactyly, tall or flat vertebrae and acanthosis nigricans [29]. Exercise may cause mild knee, ankle or elbow pain in childhood, more intense in adults and sometimes with low back pain; caesarean delivery is frequently necessary in pregnant women with hypochondroplasia [29, 48].

3.6 SADDAN

A phenotype intermediate between severe achondroplasia and thanatophoric dysplasia, distinguished by the combination of moderate to severe neurodevelopmental impairment (with seizures, frequently refractory, and hydrocephalus), acanthosis nigricans (typically absent at birth, manifesting in early childhood in survivors) and thoracic hypoplasia with respiratory difficulty. Unlike thanatophoric dysplasia, prolonged survival, although rare, is well documented [6, 40].

3.7 Muenke syndrome

Unlike the skeletal spectrum, Muenke syndrome is primarily characterised by unilateral or bilateral coronal craniosynostosis — bilateral causing brachycephaly or turribrachycephaly and unilateral causing anterior plagiocephaly — potentially associated with macrocephaly without synostosis, temporal bossing, hypertelorism, eyelid ptosis, sensorineural or conductive hearing loss (deafness), variable developmental delay and behavioural abnormalities, and minor appendicular skeletal abnormalities (short widened middle phalanges, carpal or tarsal fusions, clinodactyly). Expressivity is markedly heterogeneous — from completely asymptomatic carriers to complex multisystem phenotypes — and penetrance incomplete, without a difference between sexes (bicoronal craniosynostosis in 50% of women and 44.4% of men; p = 0.84) [51]; a significant proportion of p.Pro250Arg carriers have no detectable clinical manifestation, illustrating this entity's characteristic incomplete penetrance [18, 19, 49]. In the largest natural history study (106 carriers from 71 families), 85% had craniosynostosis (bilateral in 47.5% and unilateral in 28.2%), with hearing loss recorded in 70.8%, developmental delay in 66.3%, intellectual disability in 35.6%, attention deficit hyperactivity disorder in 23.7% and seizures in 20.2%; 75% of those with a complete skeletal survey had at least one radiographic abnormality beyond the skull [51]. Hearing loss is predominantly bilateral sensorineural, may develop after normal neonatal hearing screening, and adaptive and executive difficulties appear independent of craniosynostosis or hearing loss [19]. Smith's Recognizable Patterns adds strabismus in 45% of patients, with amblyopia, astigmatism and nystagmus, describes the characteristic hearing loss as mild to moderate sensorineural loss in the low to mid frequencies, sometimes progressive, and states that around two-thirds of cases are inherited from a parent, with or without a phenotype; it recommends testing for the variant in all patients with coronal synostosis without another confirmed diagnosis and offering testing to the parents of positive patients, whether or not they have clinical signs [61]. Bone Dysplasias describes a generally benign course, with possible feeding and swallowing difficulties in early infancy and medial temporal lobe dysgenesis, as in hypochondroplasia, which may give rise to temporal lobe seizures; it also reports greater severity in females, which diverges from the absence of a sex difference in the frequency of bicoronal craniosynostosis in the natural history study [51, 62].

3.8 Crouzon syndrome with acanthosis nigricans

Combines the craniofacial features of classic Crouzon syndrome (multisuture craniosynostosis, exorbitism, midface hypoplasia) — usually more severe than in the FGFR2-associated form — with progressive acanthosis nigricans, frequent hydrocephalus and, in some described cases, Chiari type I malformation [22].

3.9 Central and peripheral nervous system (cross-spectrum overview)

Neurological complications are the principal source of morbidity and mortality in achondroplasia, largely resulting from foramen magnum and upper cervical canal stenosis. In infants, cervicomedullary compression may manifest as marked hypotonia, obstructive and central apnoea and, in the most severe cases, sudden death due to cardiorespiratory failure — making systematic neurological monitoring in the first year mandatory [16, 63, 64]. The severity of these complications correlates with the degree of foramen magnum and jugular and hypoglossal canal stenosis and is reflected in the degree of muscle hypotonia; cervicomedullary compression may also cause neurological impairment of the extremities, with hyperreflexia, clonus and ultimately paresis, and the frequency of neurological abnormalities correlates inversely with foramen magnum diameter on computed tomography [26]. Communicating hydrocephalus (internal and external), predominantly mechanistically related to jugular venous stenosis at the skull base, develops in a significant proportion of infants, manifested by excessive head circumference increase in the first three years, stabilising spontaneously in virtually all cases without shunting and without correlation between hydrocephalus degree and intellectual performance [26]. Hydrocephalus requiring treatment occurs in fewer than 5% of individuals; it is thought that foramen magnum stenosis contributes to hydrocephalus in some patients, which may then be treated by posterior fossa decompression or endoscopic third ventriculostomy, with achondroplasia-specific skull and brain anatomical differences to be considered before the latter [46, 65]. In middle or later adulthood, lumbar canal stenosis, exacerbated by anterior vertebral body wedging, disc protrusions and osteophytes, causes neurogenic claudication, necessitating decompressive laminectomy in around 30% of adults with achondroplasia during their lifetime [26]; numbness, pain, weakness, hyperreflexia and bladder dysfunction are early symptoms and signs, and by 10 years a significant number of affected children have already developed neurological symptoms with claudication and increased lower-limb reflexes [26, 28]. In adults, leg and back pain is reported in 50% of patients [30]. According to the French PNDS (French National Diagnosis and Care Protocol), ventriculomegaly and excess extra-axial fluid in the newborn are frequent without active hydrocephalus and stabilise during the second year; true hydrocephalus, rare and progressive, due to raised jugular venous pressure, is treated by ventriculocisternostomy or, more rarely, by ventriculoperitoneal shunting, the indication for which must be weighed against its high complication rate [33]. Craniocervical junction instability is a very rare complication, to be suspected in the presence of positional neurological signs, and craniosynostosis, characteristic of Muenke syndrome, Crouzon syndrome with acanthosis nigricans and thanatophoric dysplasia, occurs rarely in achondroplasia [33]. Lumbar and lower-limb pain increases with age — around 20% at 20 years and 80% at 60 years — and may be associated with bladder–sphincter and anorectal disorders (dysuria, voiding disorders, terminal constipation of neurological origin); lumbar stenosis in achondroplasia appears early, often before 30 years, predominates in the upper lumbar segments (from L1 to the sacrum, especially L2–L3) and at the thoracolumbar junction, may extend to T10–T12 and, more rarely, the cervical spine, and correlates with thoracolumbar kyphosis and disc degeneration, not with age, sex, stature, weight or body mass index (Huet and colleagues, 2020); pyramidal signs should raise suspicion of spinal cord compression at a higher level [33].

In the lethal spectrum (thanatophoric dysplasia), severe structural cortical malformations — polymicrogyria, neuronal heterotopia, temporal hyperplasia and gyral disorganisation — are described, associated with profound cognitive impairment in rare survivors with intensive support [34].

3.10 Respiratory system (cross-spectrum overview)

In achondroplasia, the combination of narrow upper airways, relative adenotonsillar hypertrophy, muscle hypotonia, recurrent upper respiratory infections and a small thorax results in a high prevalence of obstructive sleep apnoea syndrome, potentially affecting most children when systematically assessed by polysomnography; severe obstructive apnoea may result in cardiorespiratory failure and death [26]. Most individuals with achondroplasia snore [27]. The French PNDS describes, in infants, a small thorax with increased, sometimes marked, compliance, resulting in paradoxical inspiratory movement in most young infants (inferolateral and often substernal retraction), lateral indentations or pectus excavatum, and frequent tachypnoea in the first months, without impact on feeding when uncomplicated; restrictive respiratory insufficiency may be problematic in the first year, especially during infections or asthma. Central apnoeas in the first months relate to compression of the brainstem and spinal cord, and obstructive apnoeas may arise at any age, owing to midface hypoplasia aggravated by adenotonsillar hypertrophy [33]. In thanatophoric dysplasia, severe thoracic hypoplasia due to short ribs directly causes fatal neonatal respiratory failure, constituting this condition's central prognostic determinant [34].

3.11 Skin and connective tissue: cross-spectrum note

Acanthosis nigricans — velvety skin thickening and hyperpigmentation in flexural areas (axillae, neck, groin) — is a distinguishing SADDAN finding, typically absent at birth and emerging in early childhood in long-term survivors, sometimes associated with specific codon 650 variants, including hypochondroplasia forms with acanthosis nigricans [6]. Beyond acanthosis nigricans associated with SADDAN and certain codon 650 variants, it should be noted that the same gain-of-function FGFR3 variants, when occurring postzygotically in mosaicism (that is, outside the context of a constitutive germline mutation), may produce circumscribed skin phenotypes — keratinocytic epidermal naevi and localised acanthosis nigricans — without any systemic skeletal manifestation (see Section 9.3). In achondroplasia itself, acanthosis nigricans (neck and, less frequently, axillae and groin) appears in late childhood or adolescence in around 10% of patients, in a consecutive series of 477 individuals from a North American bone dysplasia clinic, more frequently in the non-white population and in obese patients [66]. The French PNDS recommends screening for insulin resistance and metabolic abnormalities when this finding is present [33], but the study it cites found no association with hyperinsulinaemic states or neoplasia and concludes that the acanthosis nigricans of achondroplasia, mild and not requiring treatment, does not warrant targeted investigation [66]; metabolic assessment is therefore indicated by the patient's risk factors (obesity, hypertension, family history) and not by the isolated skin lesion.

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4. Clinical Recognition and Warning Signs

Early recognition of FGFR3 complex disorders rests on three axes: temporal context of presentation, pattern of body disproportion and craniofacial involvement, and on distinct clinical scenarios, each with specific warning signs that should trigger referral to medical genetics.

Prenatal period. Routine second-/third-trimester obstetric ultrasound may reveal femoral shortening below the 3rd–5th centile for gestational age — a finding whose isolated specificity for an FGFR3 dysplasia (achondroplasia) is nevertheless low (below 25%, or 0.25), so isolated femoral shortening without other suggestive findings should not generate disproportionate alarm or be communicated to parents as a presumptive skeletal dysplasia diagnosis, always requiring molecular confirmation [16, 26]. Femoral length is routinely measured at the 18–20-week anomaly scan; in achondroplasia, limb lengths are usually at or above the 5th centile until 24 weeks, so many cases present only in the third trimester, with head circumference around the 95th centile; observing trident hands (detectable from 25 weeks) and frontal bossing is useful [1]. In a fetus with short femora identified after 24 weeks of gestation, a narrow chest and hydramnios, thanatophoric dysplasia should be the first diagnosis excluded given its lethality; absence of severe skull deformity and presence of femoral bowing point towards type 1, whereas a cloverleaf skull — a virtually pathognomonic finding — points towards type 2 [34]. Prenatal warning signs actually increasing pretest probability of an FGFR3 dysplasia include the combination of progressive rhizomelia, relative macrocephaly, a narrow chest, polyhydramnios (more marked in lethal forms) and, when present, cloverleaf skull deformity. Conversely, a fetus or newborn with rhizomelic disproportion, relative macrocephaly and a characteristic face but without the preceding signs or severe respiratory impairment suggests achondroplasia — nowadays frequently anticipated prenatally in the third trimester. In thanatophoric dysplasia, the first trimester may show long bone shortening and increased nuchal translucency (due to vascular flow compression by the narrow chest) [4]. Ultrasound predictors of lethal skeletal dysplasia include early severe long bone shortening, femoral length/abdominal circumference ratio below 0.16, a small chest with short ribs and a high cardiothoracic index, and marked bowing or fractures [1].

Neonatal period and early childhood. Major warning signs of achondroplasia include the combination of macrocephaly with a large anterior fontanelle, depressed nasal bridge, evident rhizomelia, trident hands and marked axial hypotonia; marked neonatal hypotonia associated with disproportionate macrocephaly should prompt immediate genetic assessment. The most critical infant warning sign is foramen magnum stenosis, with a risk of cervicomedullary compression manifested by severe hypotonia, central/obstructive apnoea and, in the worst scenario, sudden death — the principal cause of mortality in the first 4 years. Disproportionate hypotonia, clonus, hyperreflexia, marked motor delay, stridor or apnoea documented by polysomnography should alert the clinician. It is particularly important for paediatricians and neonatologists to recognise neurological warning signs early — recurrent central apnoea, lower-limb hyperreflexia, clonus, stridor or other cervicomedullary compression signs — justifying urgent neurosurgical assessment with craniocervical junction imaging, given the sudden death risk in this age group [63, 64]. In children with achondroplasia, mortality is twice normal, with excess sudden death below 4 years, 50% due to brainstem compression; Hecht and colleagues (1987) found a 7.5% death risk in the first year [27, 67]; this risk, attributed to central apnoea through injury to respiratory control centres, may fall to only 0.3% with systematic assessment of all infants and selective neurosurgical intervention [46]. Hydrocephalus (communicating, due to jugular foramen stenosis) may cause excessive head circumference increase in the first three years, usually stabilising spontaneously; transfontanelle ultrasound should be performed if the fontanelle is particularly large, occipitofrontal circumference increases too rapidly or exceeds the 95th centile on specific charts, or hydrocephalus symptoms develop [28]. Respiratory signs (obstructive sleep apnoea, recurrent respiratory infections due to nasopharyngeal hypoplasia) and otological signs (recurrent otitis media, conductive hearing loss) require systematic surveillance given the cumulative language delay risk. Marked thoracolumbar kyphosis persisting beyond two years, particularly with neurological impairment, is another warning sign requiring exclusion of associated spinal cord compression. Every child with severe hypotonia, life-threatening apnoea, progressive stridor or focal neurological signs should be urgently referred for neurosurgical assessment and monitoring in a specialist skeletal dysplasia unit.

Later childhood, adolescence and adulthood. A frequently undervalued warning sign is low back pain with neurogenic claudication, hyperaesthesia or bladder dysfunction, which should prompt assessment for lumbar canal stenosis even in patients with an established achondroplasia diagnosis, given its relevance as a major progressive complication [26]. In adulthood, attention shifts to lumbar spinal canal stenosis causing neurogenic claudication, paraesthesiae, pain and bladder dysfunction — a surgical indication in around 30% of patients. Walking capacity above 100 m is a useful history parameter in older children and adults [27]. Adults are also at risk of nerve root or spinal cord compression unrelated to exertion, potentially requiring urgent neurological/neurosurgical assessment [27].

Hypochondroplasia. The main clinical recognition challenge is phenotypic overlap with idiopathic/familial (“constitutional”) short stature: some patients with presumed normal-variant short stature may have hypochondroplasia and, conversely, normal short children may be incorrectly diagnosed with hypochondroplasia [48]. Postnatal-onset short stature with relative macrocephaly and no classic facial dysmorphism should suggest hypochondroplasia — frequently underdiagnosed as a normal familial variant; relative macrocephaly is, according to dedicated studies (head circumference has high specificity for recognising N540K), an important discriminator favouring hypochondroplasia, but molecular confirmation is essential before definitively labelling a child as having a “normal familial variant” of short stature, particularly with a family history of disproportionate stature [16, 48]. Slow growth, if not evident at birth, is usually obvious by 3 years [29].

Muenke syndrome. The central warning sign is unilateral or bilateral coronal craniosynostosis, isolated or syndromic, or associated with hearing loss: even without other evident syndromic signs or dysmorphic features, every patient with this finding should be referred for targeted FGFR3 genetic testing (P250R), given this entity's high relative prevalence within syndromic craniosynostoses and incomplete penetrance [18, 19].

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5. Diagnosis: Criteria, Tests and Algorithms

5.1 General approach

There are no formal consensus clinical diagnostic criteria for any FGFR3 spectrum entity; diagnosis rests on clinical and radiographic findings combined with molecular confirmation [68]. Diagnosis of FGFR3 dysplasias classically rests on a combination of highly characteristic clinical and radiological criteria, complemented and confirmed by targeted molecular testing. In contemporary practice, molecular confirmation is recommended in virtually all cases, for both genetic counselling and prognosis and because of increasing availability of targeted therapies whose access is conditional on genotypic confirmation in some healthcare systems [13, 46, 69]. There is an apparent clinical continuum between thanatophoric dysplasia, achondroplasia, hypochondroplasia and normal short individuals; many rarer skeletal dysplasias are incorrectly diagnosed as “achondroplasia” prenatally or neonatally until the skeletal survey is interpreted by an expert or molecular testing is normal [27].

5.2 Clinical–radiological criteria and radiographic assessment by entity

Complete skeletal radiographic assessment (anteroposterior and lateral skull, chest, anteroposterior and lateral spine, pelvis with hips, and a segment of each limb including hand and foot) remains the cornerstone of initial diagnostic suspicion; good-quality radiographs with specialist interpretation are essential for all but the commonest dysplasias [70]. In achondroplasia, diagnosis primarily rests on clinical–radiographic assessment: the gold-standard investigation is the postnatal skeletal survey, revealing the characteristic combination — caudally decreasing lumbar interpedicular distance, a square pelvis with a narrow sciatic notch, short widened tubular bones and proximal femoral oval radiolucency in the newborn [26]. The most discriminating findings by entity include:

• Achondroplasia — major clinical criteria include disproportionate rhizomelic dwarfism; macrocrania with a prominent forehead; a hypoplastic midface; infant thoracolumbar kyphosis with later compensatory lumbar lordosis; trident hands [26]. Radiologically: a large calvarium with prominent frontal, parietal and occipital regions and reduction of the skull base and foramen magnum; progressive craniocaudal reduction in lumbar interpedicular distance (rather than physiological widening) between L1 and L5, with short pedicles and posterior vertebral body border scalloping; an "elephant ear" pelvic configuration (square short ilia, small and rounded without widening, horizontal acetabular roofs, a small/narrow sciatic/ischial notch and a spur on the medial border of the triradiate cartilage); a short widened femoral neck; shortened long bones with widened metaphyses and essentially normal epiphyses (except the knee, where ossification is delayed and irregular); in infants, oval or square radiolucent areas in the proximal femur and humerus (proximal femoral oval radiolucency in the newborn); short ribs; and relative disproportion between humeral length (disproportionately short) and fibular length (relatively long compared with the tibia) [26–28]. The French PNDS (French National Diagnosis and Care Protocol) does not recommend routine radiographs outside orthopaedic and neurosurgical follow-up and describes how the signs evolve with age: in infants, a tapered and curved proximal femoral end ("ice-cream scoop"), horizontal acetabular roofs with a medial spur, small vertebral bodies, sometimes platyspondylic, with increased intervertebral space and a small anterior spur, brachymetacarpia and stubby middle and proximal phalanges; in older children, chevron metaphyses with epiphyseometaphyseal impaction, short femoral necks with a small prominent trochanter, tibial bowing, varus obliquity of the ankles, excessive visualisation of the Y cartilages and the ischiopubic synchondrosis, and horizontalisation of the sacrum; in adults, preserved vertebral body height, interpedicular narrowing of the lower lumbar vertebrae, prominent muscle insertion ridges (especially the deltoid V), bulky trochanters and frequent hip and spinal osteoarthritis [33].
• Hypochondroplasia — findings resembling achondroplasia but considerably subtler: moderate narrowing of interpedicular distances (or absent caudal widening), anteroposterior pedicle shortening and increased posterior concavity of lumbar vertebral bodies; short relatively square tubular bones with mild metaphyseal widening; slightly small ilia with narrow sciatic notches; a short broad femoral neck; a distal fibula elongated relative to the tibia; occasionally a moderately enlarged neurocranium [29, 48]. The most reliable radiographic signs are short ilia, a short greater sciatic notch, horizontal acetabula and short broad femora [48]. These findings usually require comparison with specific reference charts and frequently become evident only at school age — diagnostic abnormalities may not be visible on radiographs before 2 years [27]; proposed neonatal criteria have limited sensitivity, contributing to this entity's characteristic mean diagnostic delay [48, 71].
• Thanatophoric dysplasia — diagnosis rests on almost pathognomonic radiological findings: very short broad long bones with cupped metaphyses; a narrow chest due to short ribs on anteroposterior and lateral views; severe platyspondyly with marked vertebral body flattening (“H” appearance on lateral radiographs), less severe in type 2 than type 1, with notched ossification defects in central endplate portions; reduced vertical diameter and horizontal inferior iliac borders, a narrow sacrosciatic notch and short broad pubes and ischia; accessory ischial and iliac ossification centres before 24 weeks; acorn-shaped proximal femoral radiolucency; small facial bones and a relatively large calvarium with craniosynostosis signs; and, in type 2, a cloverleaf skull [34, 39, 72]. Differentiation between types is essentially based on femoral morphology (bowed into a “telephone receiver” in type 1, straight in type 2) and craniosynostosis, more constant in type 2 [5, 34].

5.3 Molecular tests

Genetic confirmation is usually performed by targeted molecular testing, Sanger sequencing or a focused next-generation sequencing panel of critical FGFR3 exons (notably exons 8, 10, 13, 15 and 19 encoding extracellular, transmembrane and tyrosine kinase domains — particularly exon 10 for achondroplasia codon 380, exon 13 for hypochondroplasia codon 540 and exon 15 for thanatophoric dysplasia type 2 and SADDAN codon 650), confirming diagnosis in virtually 100% of classic achondroplasia cases (more than 99% result from only two recurrent variants at the same nucleotide, c.1138G>A or c.1138G>C) and around 70% of clinically compatible hypochondroplasia cases [15, 46, 71]. With a typical clinical–radiological presentation, molecular confirmation by direct FGFR3 sequencing is recommended, identifying in more than 99% of cases the recurrent c.1138G>A (p.Gly380Arg) variant, classified as pathogenic (class 5, ACMG-AMP criteria). Atypical forms require complete gene screening given minor variants (c.1043C>G, c.1138G>C) [26]. In N540K-negative cases with a suggestive radiographic phenotype, complete FGFR3 sequencing (expanded screening) is recommended; with a strongly suggestive phenotype but no identified FGFR3 variant, a multigene skeletal dysplasia panel should be considered to exclude phenocopies, because a relevant proportion of molecularly negative hypochondroplasia results from variants in other genes (for example, NPR2, ACAN) or remains unexplained — genetic heterogeneity contrasting with achondroplasia's almost absolute homogeneity [46, 71, 73]. As mild achondroplasia (p.Gly380Arg) and classic hypochondroplasia (p.Asn540Lys) may have very similar presentations, GeneReviews recommends testing the two variants producing p.Asn540Lys (c.1620C>A and c.1620C>G) and the two producing p.Gly380Arg (c.1138G>A and c.1138G>C) whenever molecular testing for hypochondroplasia is requested [16]. Given the point gain-of-function mechanism, intragenic deletion or duplication testing by MLPA (multiplex ligation-dependent probe amplification) or array-CGH (microarray comparative genomic hybridisation) is not indicated first-line, as these rearrangements are not a recognised pathogenic mechanism in this disease family [46]. For thanatophoric dysplasia, the Brazilian manual proposes initial polymerase chain reaction (PCR) testing targeting the commonest variants and, if negative with the diagnosis still suspected, sequencing; targeted analysis and sequencing of the entire coding region detect more than 99% of variants in both types, and p.Lys650Glu has been identified in all individuals with thanatophoric dysplasia type 2 [4]. When features suggest a chromosomal or syndromic diagnosis, genomic analysis (genomic array, gene panel, exome or genome sequencing) is indicated [1, 70].

All identified variants should be classified according to the five-category ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology) system: pathogenic, likely pathogenic, uncertain significance (VUS), likely benign and benign. The recurrent variants mentioned (p.Gly380Arg, p.Asn540Lys, p.Lys650Glu, p.Lys650Met, p.Pro250Arg) are classified as pathogenic with high confidence, given PS1/PS2 criteria (confirmed de novo mutations, independent recurrent occurrence and robust genotype–phenotype concordance in thousands of published cases) [74]. In ClinGen's clinical actionability programme, the paediatric hypochondroplasia report (AC1080) concluded insufficient evidence after expert review, and the achondroplasia report (AC1005) remains incomplete with a preliminary conclusion of insufficient evidence in paediatric and adult contexts — conclusions reflecting that programme's actionability criteria rather than availability of surveillance and targeted therapy described in this lecture [75].

5.4 Prenatal diagnosis

Serial obstetric ultrasound, focusing on long bone biometry, head circumference, thoracic morphology and amniotic fluid volume, permits suspicion from 22-24 weeks of gestation for severe forms (thanatophoric dysplasia, sometimes evident at 20 weeks) and later (usually after 26-28 weeks, sometimes only in the third trimester) for heterozygous achondroplasia, whose long-limb shortening becomes progressively more evident as pregnancy advances [16, 26, 27]. In achondroplasia, ultrasound after 26 weeks may show increased biparietal diameter, reduced limb size with broad metaphyses and a low nasal bridge, with three-dimensional computed tomography possible after 30 weeks [30]. In thanatophoric dysplasia, 2nd–3rd trimester findings include polyhydramnios, limb length below the 5th centile, a cloverleaf skull (type 2) and/or relative macrocephaly (type 1), ventriculomegaly, encephalocele, a narrow thoracic cavity, platyspondyly and bowed femora (type 1) [4]. In a series of 31 prenatally detected skeletal dysplasias, a final diagnosis was possible in 80%, including eight thanatophoric dysplasias and three achondroplasias [1]. Prenatal ultrasound has limited value for achondroplasia: femoral shortening after 24 weeks has specificity below 0.25 and is only a sign of suspicion [26]. Genetic testing is particularly useful in prenatal diagnosis (chorionic villus sampling/amniocentesis) in at-risk couples or with suggestive but inconclusive ultrasound findings. Following ultrasound suspicion, molecular diagnosis may be confirmed by targeted FGFR3 analysis in chorionic villi (11–13 weeks), amniotic fluid obtained through amniocentesis (from 15 weeks), cord blood or, more recently, analysis of circulating cell-free fetal DNA in maternal plasma (cffDNA; non-invasive prenatal diagnosis) — particularly useful when one parent already carries a known pathogenic variant (for testing the commonest recurrent variants), allowing a non-invasive result with high analytical sensitivity for detecting the paternally derived variant; it is not applicable when achondroplasia is maternally inherited [1, 27, 30]. In an ongoing pregnancy with suspected thanatophoric dysplasia, pre-delivery confirmation is indicated to plan neonatal management [1]. Preimplantation genetic diagnosis (PGD), associated with in vitro fertilisation, is an additional option for couples with a previous family history (see Section 8). With a fetal or neonatal skeletal dysplasia, DNA storage (cord blood, fetal blood, placental tissue, skin biopsy), a complete radiographic survey, photographs and clinical measurements are recommended; in the event of death, autopsy should include bone histology [1].

The French PNDS describes a prenatal pathway in which most diagnoses of achondroplasia are now made during pregnancy [33]. In transmitted forms with a known parental variant, early diagnosis is made by chorionic villus sampling at 11–12 weeks of gestation or, if transmission is paternal, by cell-free fetal DNA in maternal blood; amniocentesis, later and with a risk of miscarriage or preterm delivery, is generally discouraged in achondroplasia and reserved for when villus sampling was not possible or not wanted. When the mother has achondroplasia, the uterus may become abdominal from the first trimester and the cervix is long, which makes chorionic villus sampling (to be performed as early as possible) and any termination of pregnancy more difficult; for termination, mifepristone under epidural analgesia is preferred to aspiration, and cervical length is measured by transvaginal ultrasound before any intervention. A preconception consultation with a genetic counsellor and a psychologist is offered to the couple. In de novo forms, suspicion usually arises at the third-trimester screening ultrasound: limbs below the 3rd centile with a biparietal diameter above the 95th centile, sometimes polyhydramnios and a marked nasal arch; the signs (short long bones with stubby metaphyses, midface retrusion, frontal bossing, tapered proximal femurs, small flattened vertebrae, horizontal acetabular roof) are rarely visible at the 22-week ultrasound. On a targeted ultrasound (transmitted form or another indication), widening of the angle between the femoral diaphysis and the proximal metaphysis, with a small metaphyseal "beak", may be visible from 22 weeks (Khalil and colleagues, 2016). Confirmation is by low-dose fetal computed tomography from 28 weeks — which also shows increased intervertebral space, anterior median spurs of the vertebral bodies, round or square iliac wings and the tapered oval zone of the proximal femur — and/or by testing for p.Gly380Arg in amniotic fluid or in cell-free fetal DNA in maternal plasma, the latter being useful when combined with imaging; if the recurrent variant is not found, the rare FGFR3 variants are sought. The couple is referred to a multidisciplinary prenatal diagnosis centre, and the diagnosis should be disclosed by professionals familiar with achondroplasia, with a psychology consultation systematically offered, whatever the decision about the pregnancy [33].

5.5 Craniocervical junction assessment and imaging surveillance

Brain and craniocervical junction magnetic resonance imaging is recommended in all infants with confirmed achondroplasia to calculate the Achondroplasia Foramen Magnum Score (AFMS), a standardised instrument for stratifying cervicomedullary compression risk and deciding whether neurosurgical decompression is needed [45, 64]. According to GeneReviews, AFMS 4 requires intervention and AFMS 3 may require it; assessment as early as possible after birth includes history and neurological examination (lower-limb hyperreflexia and clonus), craniocervical junction neuroimaging and polysomnography for central and obstructive apnoea; MRI is considered standard care (rapid brain MRI can be performed without sedation), and, if CT is used, sagittal and transverse foramen magnum dimensions more than 1 standard deviation below the achondroplasia mean, associated with clinical signs, identify infants most likely to need surgery [46]. Signs indicating suboccipital decompression are lower-limb hyperreflexia or clonus, increased central apnoea for age on polysomnography, and evidence of cord compression and/or T2 signal change; neuroimaging should be repeated with any concern, because neonatal findings may change as the skull base grows into the foramen magnum [46]. MRI assesses hydrocephalus (usually communicating), cervicomedullary compression (foramen magnum dimensions, brainstem displacement) and cord compression due to stenosis or listhesis; the Oxford Desk Reference records expert debate about offering routine MRI or only with symptoms, and recommends sleep studies in all children to assess central apnoea and cranial ultrasound in newborns and infants to assess ventricular size [27]. Hecht and colleagues established computed tomography reference values for foramen magnum dimensions in children with achondroplasia, and assessment of children with cervical compression symptoms should be undertaken by professionals experienced in the disease's natural history [28].

The French PNDS recommends MRI of the brain, the craniocervical junction and the cervicothoracic spinal cord in the initial assessment of all infants, performed by the network of centres between 3 and 4 months and repeated every 6–12 months until 3 years, depending on the clinical picture and the AFMS, and no longer routine after that age; the schedule is adjusted to any late-pregnancy fetal MRI, to detect severe stenoses early [33]. The foramen magnum is small and often "keyhole"-shaped. The best clinical indicators of compression are hyperreflexia or asymmetry of lower-limb reflexes, altered axial and peripheral tone, worsening of severe hypotonia and central apnoeas on polysomnography; clinical repercussions appear late, and there are asymptomatic or paucisymptomatic infants with significant spinal cord injury. Somatosensory evoked potentials may help when the signs are insufficient for a decision, and the surgical indication is decided at a multidisciplinary meeting with experienced neurosurgeons and paediatric radiologists; in the presence of stenosis without spinal cord injury, MRI is repeated 6 months later and thereafter according to progress. Dynamic MRI in flexion and extension is not a screening test and is reserved, with caution and possibly under monitoring, for children with clinical events suggestive of instability; dynamic radiographs are difficult to interpret in the immature skeleton, and dynamic CT involves radiation without showing the spinal cord. The PNDS attributes to Hashmi and colleagues (2018) a sudden death mortality of 4.5% in infants, explained by direct spinal cord compression or by apnoea related to compression of the vertebral arteries at the craniocervical junction [33]. This figure does not appear in the cited study, either as a proportion or as a rate. Hashmi and colleagues included 855 patients seen at least once between 1986 and 2015 at four North American bone dysplasia clinics (Houston, Johns Hopkins, Nemours and Wisconsin), with 12,117 person-years and vital status verified in the US National Death Index up to January 2015; there were 12 deaths, i.e. 0.99 per 1000 person-years. Only 489 patients (57%) were seen in the first year of life, contributing 316 person-years in that interval, and there was a single death at that age: a sudden death at 2 months, a few days before already scheduled craniocervical junction imaging, which corresponds to 3.2 per 1000 person-years. Sudden or ill-defined death accounted for 2 of the 12 deaths (17%), against 41% of deaths under 25 years in Hecht's 1987 cohort. Among infants seen in the first year, craniocervical junction imaging, polysomnography and targeted neurological examination rose from 51% (19/37), 56% (21/37) and 43% (16/37) in those seen up to 1986 to 76% (342/452), 71% (323/452) and 83% (377/452) in those seen afterwards, and 20% (99/489) underwent suboccipital or craniocervical decompression. The risk of death was lower, without statistical significance, in patients with craniocervical junction imaging (HR 0.29; 95% CI 0.07–1.11), with polysomnography (HR 0.37; 95% CI 0.07–1.93) or with decompression (HR 0.38; 95% CI 0.08–1.78). The series counts only deaths occurring after the first specialist consultation, so it underestimates early mortality in the population [76]. In a Texas population-based registry (106 children born in 1996–2003), there were 4 deaths, all in the first year (41.4 per 1000 live births, all causes) [77]. The historical reference figure of 7.5% in the first year (Hecht and colleagues, 1987) predates systematic craniocervical junction surveillance [67].

5.6 Summary diagnostic algorithm

In a child or fetus with a phenotype suggestive of disproportionate skeletal dysplasia, the recommended diagnostic pathway is: (1) detailed clinical–anthropometric characterisation, measuring body segments and calculating arm span/height and upper/lower segment ratios, recording parental heights and paternal age (if de novo) and plotting on achondroplasia-specific charts [27]; (2) a complete skeletal radiographic series (skull, spine, pelvis, limbs); (3) molecular testing targeting recurrent FGFR3 variants, with reflex to complete gene sequencing if negative but the phenotype is persuasive; (4) early multidisciplinary assessment — neurosurgery/paediatric neurology (craniocervical junction assessment), otorhinolaryngology and sleep respiratory medicine, paediatric orthopaedics and, where applicable, paediatric endocrinology to discuss targeted therapeutic options.

5.7 Limitations of genetic testing and report content

The high sensitivity of targeted testing in achondroplasia — more than 99% of patients with a classic phenotype have c.1138G>A or c.1138G>C — does not transfer to other entities. In hypochondroplasia, FGFR3 variants explain only around 70% of clinically compatible cases, so testing restricted to recurrent codons may be negative without excluding diagnosis; the next step is complete gene sequencing and, if negative, a skeletal dysplasia panel, exome or genome [16, 46, 71]. Tests restricted to selected exons or codons do not detect rare variants outside hotspots (for example, p.Ser348Cys, p.Gly375Cys or thanatophoric dysplasia type 1 stop codon loss variants), and copy number analysis adds no yield in this gain-of-function disease family [26, 46]. Low-level somatic mosaicism, such as in the woman mosaic for p.Arg248Cys described in Section 9.3, may require greater-depth techniques or testing of other tissues, and parental leucocyte DNA never excludes exclusively gonadal mosaicism [26, 57]. Cell-free fetal DNA analysis in maternal plasma tests targeted paternally derived or de novo variants, does not apply when the variant is maternally inherited, and an uninformative result requires invasive testing [1, 27].

The report should state the gene, transcript with version — for FGFR3, MANE Select NM_000142.5 (version 1.5) — HGVS nomenclature in complementary DNA and protein, zygosity (heterozygosity; homozygosity or compound heterozygosity in children of two affected parents), ACMG/AMP classification and method limitations [78, 79]. Older literature uses abbreviated designations (G380R, N540K, K650E, P250R) and nucleotide numbering such as “G1138A” or “C1620A”, matching MANE transcript numbering. Recurrent variants are classified in ClinVar with criteria: NM_000142.5:c.1138G>A and c.1138G>C, p.(Gly380Arg), pathogenic; c.1620C>A and c.1620C>G, p.(Asn540Lys), pathogenic; c.1948A>G, p.(Lys650Glu), and c.1949A>T, p.(Lys650Met), pathogenic; c.742C>T, p.(Arg248Cys), and c.1118A>G, p.(Tyr373Cys), pathogenic; c.1172C>A, p.(Ala391Glu), pathogenic; c.749C>G, p.(Pro250Arg), pathogenic or likely pathogenic; c.1043C>G, p.(Ser348Cys), pathogenic or likely pathogenic; c.1862G>A, p.(Arg621His), pathogenic or likely pathogenic; and c.1637C>A, p.(Thr546Lys), likely pathogenic with a single submission [47]. Some older ClinVar entries describe complex alleles on an earlier transcript (NM_000142.4) without assertion criteria and should not be used as evidence [47].

In Muenke syndrome, incomplete penetrance means that absence of a phenotype in a carrier parent does not diminish p.(Pro250Arg) pathogenicity [51]. Conversely, a rare FGFR3 variant of uncertain significance in a child with short stature or craniosynostosis is not reclassified as pathogenic through response to targeted therapy or partial segregation; segregation is described as investigation until criteria are satisfied [78]. Genetic confirmation also affects access: in the European Union, the vosoritide indication requires achondroplasia diagnosis to be confirmed by appropriate genetic testing [80].

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6. Differential Diagnosis

Before proposing a fetal skeletal dysplasia diagnosis, a short femur may reflect incorrect dating, a normal small fetus, intrauterine growth restriction, a chromosomal abnormality (ultrasound marker of trisomy 21; triploidy) or a malformation syndrome (Cornelia de Lange, Russell–Silver, Seckel, Smith–Lemli–Opitz), requiring a history of drug exposure, maternal diabetes and consanguinity, maternal–fetal Doppler studies and measurement of all long bones, head circumference, thorax and abdominal circumference [1]. Infantile hypophosphatasia (ALPL/TNSALP) is distinguished by severe demineralisation with fractures, a globular hypomineralised skull and very low or undetectable serum alkaline phosphatase [1]. The French PNDS (French National Diagnosis and Care Protocol) also includes in the differential diagnosis of achondroplasia diastrophic dysplasia, Jeune asphyxiating thoracic dystrophy, osteogenesis imperfecta and the platyspondylic dysplasias, and considers the borderline forms of the same spectrum (hypochondroplasia and thanatophoric dysplasia) the main differential among causes of short long bones with preserved or increased head circumference [33].

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7. Treatments and Risk Management Strategies

7.1 Pharmacological therapy targeting the FGFR3 pathway (achondroplasia)

For decades, FGFR3 spectrum treatment was exclusively symptomatic and surgical. The past decade brought a paradigm shift with development of FGFR3-axis targeted therapies, currently centred on three complementary pharmacological approaches: C-type natriuretic peptide (CNP) pathway agonism, ligand sequestration by a soluble decoy receptor, and direct oral inhibition of kinase activity — or, in another formulation, C-type natriuretic peptide (CNP) pathway agonists, a soluble competitive receptor antagonist and direct oral kinase activity inhibition [84]. Since 2021, the therapeutic landscape has been transformed by approval of vosoritide (VOXZOGO®), a C-type natriuretic peptide analogue antagonising downstream FGFR3 signalling, promoting growth plate chondrocyte proliferation.

Vosoritide (trade name Voxzogo®; generic name vosoritide). A modified C-type natriuretic peptide (CNP) analogue, the natural physiological antagonist of FGFR3-MAPK signalling in the growth plate; by binding NPR-B (guanylate cyclase B), it activates the protein kinase G pathway, inhibiting the RAF-MEK-ERK cascade downstream of activated FGFR3, partially restoring chondrocyte proliferation and the balance between inhibitory (FGFR3) and stimulatory (CNP) signalling [41, 69]. It was the first agent (drug) approved specifically for achondroplasia, based on the pivotal phase 3 trial (study 111-301) conducted by Ravi Savarirayan and colleagues (published in the Lancet, 2020), demonstrating a mean annualised growth velocity gain of approximately 1.6 cm/year over placebo in children with open growth plates [42]. The pivotal phase 3 trial (study 111-301) demonstrated a statistically significant annualised growth velocity increase over placebo of approximately 1.57 cm/year, with long-term extension data confirming sustained effect and improved body proportionality. The FDA (Food and Drug Administration) initially approved the drug through accelerated approval in November 2021 for children from 5 years, extending the indication in October 2023 to all ages with open epiphyses/growth plates (from birth), based on additional phase 2 study data in children under 5 years; the EMA (European Medicines Agency) approved it in the same year, initially from 2 years, with subsequent extension from 4 months [69, 85]; in the European Union, European Commission authorisation (26 August 2021, orphan medicinal product) is a standard rather than conditional authorisation, and the current indication requires diagnosis confirmed by appropriate genetic testing [80]. It is formulated as lyophilised powder for subcutaneous injection (0.4 mg, 0.56 mg or 1.2 mg vials), reconstituted before administration. Dosage is defined by actual body weight bands according to the label/SmPC table (for example, 0.24 mg for 8–11 kg, 0.4 mg for 22–32 kg and 0.8 mg from 90 kg), corresponding approximately to the pivotal trial's 15 micrograms/kg, subcutaneously once daily at the same time, with daily injection site rotation and dose adjustment as weight increases; in lower weight bands used in infants, the per-kilogram dose is higher (around 30 micrograms/kg, the dose studied below 2 years) [86]. Patients should have eaten adequately and consumed around 240 to 300 mL fluid in the hour before administration to mitigate haemodynamic symptoms (dizziness, syncope) [69, 86]. The most frequent adverse effects (>10%) include injection site reactions (erythema, swelling/oedema, urticaria, pruritus, bruising — up to 85% of patients), vomiting (around 27%), nausea and symptomatic transient arterial hypotension, usually mild (dizziness, fatigue, post-injection malaise, syncope or presyncope — around 13%, present in more than 1 in 10 patients), requiring monitoring at treatment initiation and constituting the principal reason for a relative contraindication in patients with significant cardiovascular disease or concomitant antihypertensive therapy; asymptomatic serum alkaline phosphatase elevation is also described [69, 86]. The drug is not recommended in patients with estimated glomerular filtration rate below 60 mL/min/1.73m². In summary, it is administered subcutaneously daily, FDA/EMA-approved for children with open epiphyses, with the indication extended in 2023 to those under 5 years. The most frequent adverse effects are injection site reactions and vomiting; transient hypotension requires monitoring at treatment initiation (dose is fixed by weight without a formal titration schedule). International consensus recommendations on vosoritide implementation and monitoring were published in 2025 [87], and the ACMG published a therapeutic bulletin on its approval in 2023 [88]. Beyond its effect on growth velocity, vosoritide treatment showed improved sagittal spinal balance, reduced lower-limb bowing and improvement in physical aspects of health-related quality of life at three years; computational models predict a total gain of around 26 cm in boys and 22 cm in girls if treatment begins at birth, with final treated adult height still undetermined [46]. In July 2026, the FDA accepted submission of an application for traditional approval (converting the original accelerated approval), with a target decision date of 28 February 2027, based on accumulated final adult height data from more than 5,000 treated patients in more than 50 countries [45, 85]. According to the sponsor's quarterly report, confirmation of benefit required by accelerated approval rests on comparison of open-label extension studies with natural history data rather than a new randomised trial; this is sponsor information, and the regulatory decision has not yet been taken [89].

Vosoritide according to the French PNDS (2025). In France, vosoritide has been authorised and reimbursed since December 2022, after a period of early access; reimbursement was initially limited to children aged at least 2 years, and the HAS opinion of 24 March 2025 was favourable to reimbursement from 4 months to 2 years and to its maintenance from 2 to 5 years, following the European extension of the indication to 4 months (October 2023) [33]. Prescription is reserved for physicians in medical genetics, paediatrics, endocrinology or rheumatology, preferably from the network of constitutional bone disease centres, and is preceded by detailed information for the family about objectives, long-term uninterrupted treatment, supply from a community pharmacy, cold storage and transport, additional follow-up and training of at least two people in the household. The PNDS (French National Diagnosis and Care Protocol) considers ineligible families who do not understand the rules of asepsis, refuse injections or the specific follow-up, or are not covered by French social security. The usual dose is 15 µg/kg, subcutaneously once daily, at the same time, as a fixed dose by weight band from 4 kg, with a maximum of 0.8 mg/day from 90 kg (Table 7.1), rotating injection sites (thighs, abdomen, upper buttocks, back of the upper arm); eating and drinking around 30 minutes before the injection is recommended to reduce hypotension. The pre-treatment assessment proposed by the French consortium includes mandatory molecular documentation, ENT assessment with audiometry less than 1 year old, polysomnography less than 3 years old, brain and craniocervical junction MRI less than 1 year old before 5 years of age (less than 3 years old after 5), spinal MRI after 8 years and radiographs of the spine (anteroposterior and lateral) and lower limbs (anteroposterior), using EOS after 4–5 years. Initiation takes place in a day hospital or specialist clinic: assessment of motivation, quality-of-life questionnaire (PedsQL), blood pressure, pulse and temperature, blood and urine tests, bone age, measurements (height, weight, head circumference, sagittal and frontal skull diameters, arm span, sitting and supine height, tibial height, right hand length, abdominal circumference), training of caregivers by a nurse with the first injection in hospital and instructions on sharps disposal, and possible home nursing support in the first weeks. Specific follow-up includes a teleconsultation at the first month, consultations at 3 and 6 months and then six-monthly, and an annual assessment with electrolytes, urea, creatinine, transaminases, glucose, full blood count with platelets, calcium, phosphate, 25-OH-vitamin D and PTH, urinary electrolytes and calcium, bone age and EOS radiographs of the spine and lower limbs according to the clinical picture and at 5, 10 and 15 years; in treated children, densitometry at 5 and 10 years is proposed (Table 11.1). A monthly multidisciplinary meeting is dedicated to treatment problems, and adverse reactions are reported to the national pharmacovigilance portal [33].

Table 7.1 — Vosoritide: daily dose by weight band according to the European table reproduced in the French PNDS (subcutaneous, once daily) [33]

The US label table starts at 3 kg (0.096 mg/day) and has the same maximum of 0.8 mg/day from 90 kg [86]; the correspondence with 15 µg/kg is approximate, with a higher dose per kilogram in the lower weight bands.

Vosoritide in hypochondroplasia (September 2026 update). On 9 September 2026, results of the randomised placebo-controlled phase 3 CANOPY-HCH-3 trial (NCT06455059) were published in NEJM Evidence and presented at the European Society for Paediatric Endocrinology (ESPE) congress, in 81 children aged 3 to 17 years with hypochondroplasia (41 vosoritide, 40 placebo): vosoritide increased annualised growth velocity at 52 weeks, with a least-squares mean difference of 2.33 cm/year over placebo (95% CI 1.85–2.82; p<0.0001), and improved standing height, height Z-score and arm span, with a safety profile comparable to that in achondroplasia; a supplemental new drug application (sNDA) for this indication was submitted to the FDA in July 2026, with EMA submissions being prepared [89, 90]. At this review's date, vosoritide is not approved for hypochondroplasia, and use in this disease outside a trial is off-label in all jurisdictions. A placebo-controlled phase 2 trial in children with hypochondroplasia aged 0 to 36 months (NCT07126262) and a phase 2/3 infigratinib trial in hypochondroplasia (ACCEL, NCT06873035) are also ongoing [91].

Navepegritide (trade name Yuviwel®). Developed by Ascendis Pharma using the TransCon® CNP technology platform, it is a long-acting CNP prodrug designed for sustained release, administered subcutaneously once weekly — a dosing advantage over daily vosoritide, with a potential adherence advantage — providing continuous systemic exposure to active peptide throughout the weekly interval. It received FDA accelerated approval on 27 February 2026 for children from 2 years with achondroplasia and open epiphyses/growth plates [92, 93]. The pivotal ApproaCH trial (phase 2b according to publication, registered as phase 2/3, NCT05598320; 2:1 randomised, double-blind, placebo-controlled; N=84 children aged 2 to 11 years, 57 navepegritide and 27 placebo, from five countries) demonstrated increased annualised growth velocity, with an adjusted (least-squares) mean difference of 1.49 cm/year (1.5 cm/year; 95% CI 1.05–1.93) over placebo (5.89 versus 4.41 cm/year; 5.9 versus 4.4; p<0.0001) at 52 weeks, with additional documented benefits in body proportionality and limb span [94]; published secondary outcomes included improvement in tibiofemoral angle, mechanical axis deviation and fibula/tibia ratio and, below 5 years, in the physical functioning domain of an achondroplasia child experience scale, without symptomatic hypotension or fractures [94]. According to the GeneReviews synthesis, phase IIb studies showed an overall annualised growth velocity increase of 1.5 cm/year, 1.78 cm/year between 5 and 11 years and 1.02 cm/year between 2 and 4 years, with increased vertebral growth and reduced lower-limb bowing [46]. The pivotal protocol dose was 100 micrograms/kg/week; the FDA label defines weekly dosage by weight bands from 8 kg (for example, 0.88 mg for 8–9.9 kg and 2.8 mg for 23.1–30.5 kg) [93]; formulation is lyophilised powder in three strengths (1.3 mg, 2.8 mg and 5.5 mg). The most common described adverse effects include fever, nasopharyngitis, otitis media, upper respiratory infection, vomiting, nausea, headache, limb pain and injection site reactions; the label includes a hypotension risk warning, with an overall safety profile comparable to vosoritide [92, 94]. Marketing began in early second-quarter 2026, currently representing a weekly alternative to daily vosoritide. In the European Union, the marketing authorisation application is under EMA assessment, with a decision expected in the fourth quarter of 2026 [95]. The US label provides for direct switching from a daily CNP analogue, starting navepegritide the day after the last daily dose [96]. In infants (0 to <2 years), the phase 2 reACHin trial (NCT06079398) is ongoing; data from the open-label sentinel cohort (7 infants, mean age 11.7 months), released by the sponsor in September 2026 and not yet peer-reviewed, describe stable or improved foramen magnum score (AFMS) in all children, no decompression in 52 weeks and mean growth velocity of 9.9 cm/year, without a control group [97]. In the phase 2 COACH trial (published in 2026), 21 children aged 2 to 11 years received navepegritide with weekly lonapegsomatropin: in the 12 previously untreated children, growth velocity at 52 weeks was 8.69 cm/year, versus 5.95 cm/year in matched controls treated with navepegritide alone in ApproaCH — an external, non-randomised comparison; the combination is experimental and growth hormone has no approved indication in achondroplasia [98]. On 31 August 2026, BioMarin and Ascendis announced an agreement ending navepegritide patent litigation in several countries, including Brazil and Germany, through a worldwide non-exclusive licence and royalty payments; this is announcement information, relevant to continuity of access without direct clinical implications [99].

Infigratinib. A selective oral small-molecule FGFR1-3 (tyrosine kinase) inhibitor — mechanistically distinct from the preceding approaches, directly blocking constitutively activated mutant FGFR3 kinase activity rather than indirectly compensating through the CNP pathway — representing the first oral therapy in advanced development for this spectrum. It was previously developed and marketed for cholangiocarcinoma with FGFR2 fusions (alterations) (subsequently withdrawn from the market in that indication for commercial reasons), now being repositioned for achondroplasia at a much lower dose than in oncology — an example of therapeutic repurposing of an oncological agent for a constitutional indication. Phase 2 PROPEL2 results, published by Savarirayan and colleagues in the New England Journal of Medicine in November 2024, demonstrated a mean annualised growth velocity increase of 2.50 cm/year (95% CI 1.22–3.79) at 18 months of treatment in the highest-dose cohort (0.25 mg/kg/day, the highest dose tested in the escalation; no authorised maximum dose, as there is no label), without major identified safety signals; this was a dose-escalation study in 72 children without a placebo group, measuring the increase relative to each child's baseline [100]. Topline results of the pivotal phase 3 PROPEL3 (PROPEL 3) study were released in February 2026, and the complete primary results were published in the New England Journal of Medicine (online 28 June 2026; print September 2026). In the peer-reviewed publication (114 children aged 3 to 17 years, 2:1 randomised, infigratinib 0.25 mg/kg once daily or placebo for 52 weeks), the least-squares mean difference was 1.74 cm/year in annualised growth velocity (95% CI 1.31–2.17; p<0.001) and 0.32 in height z-score (96% CI 0.23–0.41), but the difference in upper/lower segment ratio in the overall population, a key secondary outcome, was not significant (−0.02; 96% CI −0.06 to 0.01); serious adverse events occurred in 5% and 3% of the infigratinib and placebo groups, none considered treatment-related [101]. Improved proportionality in the 3- to 8-year subgroup (difference −0.05; p<0.05) and arm span (+0.37 standard deviations) were released by the sponsor and should be read as results of secondary or subgroup analyses, rather than as demonstration of the key secondary outcome [84, 101, 102]. Administration is oral once daily at a weight-adjusted dose; available studies have not identified discontinuations or serious drug-related adverse events to date. The sponsor (BridgeBio Pharma) submitted the new drug application (NDA) to the FDA in the third quarter of 2026 (announcement of 10 August 2026), planning EMA submission in the fourth quarter of 2026, US launch in mid-2027 and EU approval in the second half of 2027; the drug is therefore not yet approved at this lecture's date, and development is also being accelerated for hypochondroplasia. In an announcement of 31 August 2026, the sponsor included infigratinib among medicines "under FDA review", without releasing a PDUFA date, unlike for other products [103]. In Japan, an open-label single-arm phase 3 infigratinib trial (KK8398, NCT06926491) and a phase 2 trial in children under 3 years (NCT07169279) are ongoing [91]. Infigratinib holds FDA Breakthrough Therapy designation, to date the only agent in achondroplasia development with this designation, as well as orphan drug, Fast Track and rare paediatric disease designations [102].

Recifercept — a soluble FGFR3 decoy receptor designed to sequester circulating FGF ligands before binding to the pathogenic receptor — was discontinued by Pfizer in 2024 after pharmacometric modelling of the disease's natural course demonstrated insufficient efficacy in phase 2 studies, illustrating that not all targeted pharmacological pathway approaches translate into sufficient clinical benefit [104]. Selective FGFR3 tyrosine kinase inhibitors, now beginning to enter clinical trials, and meclizine, of interest for inhibiting a RAS-MAPK pathway step, remain in early investigation; none is approved [46]. Among selective FGFR3 inhibitors, dabogratinib (formerly TYRA-300) is assessed in the open-label phase 2 BEACH301 trial (NCT06842355; children aged 3 to 10 years), with a fifth dose level opened in 2026 and first sentinel cohort results expected by the sponsor at the end of the first quarter of 2027, and ABSK061 in a phase 1/2 trial (NCT07297875); BMN 333, a long-acting CNP analogue, entered a phase 2/3 trial comparing it with vosoritide in 2026 (NCT07441876) [89, 91, 105].

None of these drugs modifies established cranial, dental or neurological manifestations of achondroplasia; their effect centres on promoting linear bone growth by modulating the growth plate. Confirmation of benefit in final adult height and long-term functional outcomes (mobility, quality of life, reduced orthopaedic complications) is the subject of ongoing extension studies, a condition for maintaining accelerated approvals [42, 69, 94]. None of these agents is indicated, approved or recommended in the remaining spectrum entities (thanatophoric dysplasia, SADDAN, Muenke syndrome), which continue exclusively under symptomatic and surgical management. Growth hormone therapy has no established indication in this condition, and vosoritide remained the only approved pharmacological therapy targeting the disease's molecular mechanism until 27 February 2026, when navepegritide became the second approved therapy (FDA).

Growth hormone. Recombinant growth hormone has no recognised role in classic achondroplasia, despite historical use outside the context of robust evidence [26]; reference manuals record divergent data — prolonged treatment contributes 2.6% and 2.1% to final adult height in men and women, respectively, and vertebrae appear to grow more than limbs, slightly increasing disproportion, so it should only be considered after full discussion of uncertainties and limited gain [27, 28]. In hypochondroplasia, its role remains controversial, with studies demonstrating a small growth velocity increase in the first year without consistent impact on final stature [29, 48, 106]; growth hormone may promote rapid short-term growth but without long-term benefit in most children with skeletal dysplasias [70]. The French PNDS stresses that growth hormone secretion is normal in achondroplasia, whose height deficit corresponds to 6–7 standard deviations below the mean, that recent meta-analyses show no benefit on adult height with supraphysiological doses (Wrobel and colleagues, 2021) and that this treatment is not indicated in achondroplasia in Europe [33].

Summary table of FGFR3 pathway-targeted drug dosage in achondroplasia (regulatory status updated to 6 October 2026)

7.2 Symptomatic and surgical management; non-pharmacological risk management strategies (all entities)

Surveillance and symptomatic treatment remain the backbone of therapeutic management, particularly beyond isolated achondroplasia. The multidisciplinary approach follows the International Consensus Statement on Achondroplasia recommendations (2022; 55 specialists from 16 countries, 136 recommendations) [45] and American Academy of Pediatrics health surveillance guidelines (2020 revision; previous versions 1995 and 2005) [63, 68, 107]. First-line management remains protocol-based multidisciplinary surveillance: systematic infant polysomnography and neuroimaging (craniocervical MRI) to screen for cervicomedullary compression, with occipitocervical decompression reserved for severe hypotonia, life-threatening apnoea or documented cord compression. Baseline symptomatic treatment remains essential — serial audiometry in the first three years, active otological treatment, orthodontics for malocclusion (present in around 50% after 10 years), weight control to prevent obesity and, in adults, lumbar laminectomy for refractory neurogenic claudication. The pillars include:

• Neurological surveillance — brain and cervical MRI in all infants and systematic polysomnography in the first year, with occipitocervical neurosurgical decompression indicated for disproportionate severe hypotonia, lower-limb hyperreflexia, potentially fatal central apnoea, polysomnographic evidence or abnormal subcortical somatosensory evoked potentials, foramen magnum stenosis below the reference centile (mean) for achondroplasia, MRI findings indicating significant cord compression, cerebrospinal fluid flow blockage on dynamic flexion/hyperextension studies, or syringomyelia [26, 28, 63, 64]. Bone Dysplasias emphasises that decompression guidance remains controversial [26]; cervical decompression is performed in approximately 5-10% of achondroplasia patients [30], below the proportion documented in the CLARITY natural history study, where 20.5% of individuals with achondroplasia underwent decompression over 60 years and 9% needed a second operation, with rates of 23% to 24% in large skeletal dysplasia centres over the past 35 years [46]. Head circumference should be recorded on specific charts and monitored serially (every 1–2 months) in the first years; some patients need shunting [27]. Infant handling should include head support, avoiding bouncing seats and swings (risk of cervicomedullary compression and worsening kyphosis), and contact or impact sports should be avoided (rugby, ice hockey, skiing, trampolining, gymnastics) [27]. Discouraging unsupported sitting or other positions bending the trunk forwards until adequate trunk strength develops may prevent a gibbus or permanent kyphosis caused by anterior wedging of the first two lumbar vertebrae [27, 28].
• Respiratory surveillance — systematic polysomnography in the first year, treating obstructive sleep apnoea with adenotonsillectomy or, in selected cases, continuous positive airway pressure (CPAP) or BiPAP; obstructive apnoea associated with airway malacia may require oxygen supplementation [27, 28, 30].
• Hearing surveillance — recurrent otitis media, very frequent due to Eustachian tube dysfunction associated with skull base hypoplasia, should be treated actively, with serial audiometry in the first 3 years and tympanostomy tubes (ventilation tubes) for recurrent otitis, given the potential impact of untreated hearing loss on language development; parents should remain vigilant, with a low threshold for audiometry and otorhinolaryngology referral [26, 27].
• Orthodontic surveillance — orthodontics for malocclusion, present in around 50% of children after 10 years; mandibular dental crowding may require extraction [26, 28].
• Orthopaedic surveillance — decompressive lumbar laminectomy, needed in around 30% of adults (almost one-third) for neurogenic claudication (thoracolumbar stenosis with inability to walk more than 100 m or neurological deficits), should be performed by teams specifically experienced in achondroplasia given atypical pedicle anatomy [26, 27, 30]; corrective osteotomies for severe genu varum (tibia vara), preferably deferred until adolescence or growth completion, and correction of tibia/fibula and radius/ulna asymmetries [27, 28]; exercises to flatten the lumbosacral curve [28]; bilateral surgical limb lengthening — controversial given repeated procedures, prolonged orthopaedic apparatus use, superficial infections, non-skeletal tissue stretching and high associated transient morbidity (a lengthy procedure) — available in specialist centres as a secondary cosmetic/functional option to improve final stature (up to around 25 cm) and correct severe lower-limb angular deformities, preferably deferred until adolescence for patients requesting it with informed understanding, following thorough person-centred discussion of risks, functional benefits and psychosocial implications; the topic should be discussed with all parents [26–28].
• Metabolic surveillance — structured dietary advice to prevent obesity, using achondroplasia-specific growth charts; early dietary management with structured nutritional counselling is recommended to prevent obesity, extremely prevalent in adulthood and exacerbating multiple orthopaedic and cardiovascular complications [26, 30].
• Rehabilitation and support — six-monthly assessment by neurology, physiotherapy and occupational therapy for hypotonia and motor delay (early stimulation and adaptation of daily living activities), speech therapy (language assessment), psychology (self-esteem; psychotherapy for family and/or affected individual) and gynaecology/obstetrics (family planning, caesarean indication due to a small pelvis) [30].
• Anaesthetic care — avoid spinal anaesthesia, take special care with cervical manipulation (cervical cord impairment during intubation), attend to a potentially more difficult airway and increased risk of obstructive apnoea and post-sedation obstruction, and base dosage on body weight rather than chronological age [27].

The French PNDS (2025) details preventive and therapeutic measures by age [33]. In infants, uncontrolled head movements, especially anteroposterior ones, are avoided: supporting the head in line with the trunk during handling, prams with a rigid backrest instead of umbrella strollers, limiting baby carriers without rigid back support, abandoning upright automatic swing chairs, head-reducing supports in prams and car seats (the bulky occiput forces neck flexion when the child is strapped in) and prohibiting somersaults and jumping; in severe forms with marked hypotonia and no head control, an adapted cervical collar may be used temporarily, until craniocervical junction surgery or neurological maturation. The child should not be seated during the first year: preferably lying supine, free, on a mat, and semi-reclined in a chair with a rigid backrest for meals. Thoracolumbar kyphosis may resolve by 10 years, but 10–15% of adults retain a marked deformity with vertebral wedging; it can be limited by gentle stimulation, postural exercises mobilising the girdles and short periods prone, once the integrity of the craniocervical junction has been checked. A bivalved thoracolumbar orthosis with support (thermoformed, made from a cast) is indicated after 9 months, when the child starts to sit, if the kyphosis is marked with radiological repercussions on the vertebral bodies; it is worn during the day, mainly when sitting, never at night or during play and physical activity, and maintained until the kyphosis disappears in standing (usually around 3 years); an orthosis applied too early may worsen polypnoea. Disorders of sagittal balance begin with hip flexion contracture, with compensatory lumbosacral hyperlordosis and junctional kyphosis, which become structured without prevention; parents learn gentle hip mobilisations, without forcing, holding the thighs rather than the lower legs because of knee hyperlaxity (the OSCAR network provides a video). Physiotherapy is rarely useful in infants, and "African"-type massage and infant osteopathy are discouraged; from 5–6 years, short weekly or fortnightly sessions are proposed, without mobilisation of the cervical spine, without joint cracking and without knee stretching, focused on mobility of the pelvis, hips and lumbosacral spine, strengthening of the paraspinal muscles, girdles and abdominal wall, and breathing exercises [33].

On the therapeutic side, the PNDS provides in infants for occipital decompression, continuous positive airway pressure or non-invasive ventilation according to polysomnography, combined with upper airway clearance (adenotonsillectomy, turbinectomy, tongue base reduction, epiglottoplasty), and insertion of tympanostomy tubes; after 18 months, treatment of asthma and reflux, non-pharmacological analgesia (relaxation, massage, warm gels, adapting routes, three-wheeled scooter) and pharmacological analgesia (paracetamol), vitamin D according to French paediatric recommendations, and correction of genu varum [33]. Corrective splints for genu varum are ineffective and should be avoided; when the deformity becomes poorly tolerated, usually after 7–8 years, proximal tibial and fibular osteotomy is performed, stabilising the hyperlax knee with temporary pinning in hyperextension and adding upper fibular epiphysiodesis if the fibula is very long with tibiotarsal varus (isolated fibular epiphysiodesis can be performed after 6–7 years); complex deformities may require bifocal or femoral osteotomies, with personalised cutting guides based on three-dimensional CT. Maxillary hypoplasia is treated after 6 years by palatal expansion, sometimes with a reduction in the number of teeth, and maxillary advancement may be needed after puberty. Prevention of excess weight must be early and continuous: energy requirements appear to correspond to around two-thirds of those of a person of average stature, and the priority is to reduce sedentary behaviour and promote adapted physical activity. Sufficient calcium intake and vitamin D supplementation according to French recommendations for children and adults are recommended; two studies described reduced bone mineral density, difficult to interpret because of the smaller bone area and without a known increase in fracture risk, and vitamin D status in this population has not been described. Delayed bone maturation and its deceleration in adolescence must be taken into account before any limb-lengthening surgery. Extensive limb lengthening allowed, in Paley's series (66 patients with achondroplasia, almost all operated with external fixators), a mean gain of 27 cm (12–40 cm): 30 cm when started in childhood, 26 cm when started in adolescence and 16.8 cm in adults, with one permanent paraparesis among 75 patients; the PNDS gives 26 cm as the overall mean, which is the value for the adolescent subgroup [33, 108]. Current techniques use motorised intramedullary nails, better tolerated than external fixators, but the procedure requires successive operations and months of rehabilitation, and long-term results are poorly known; the French centres chose not to include it in the care pathway, prioritising lower-limb alignment, muscle quality and well-being. In adolescence, preparation for transition to adult care begins at 15 years [33].

In adults, the French PNDS coordinates follow-up through rheumatology or physical and rehabilitation medicine [33]. Lumbar stenosis is mainly central, rarely foraminal, so surgery does not aim to free the roots; laminectomy frequently covers five or more levels, which is destabilising in itself, and must spare the facet joints as much as possible; it improves walking distance and radicular pain, but little or not at all low back pain and sensory disturbances, and carries a risk of neurological worsening, usually transient, and, rarely, of paraplegia due to conus ischaemia, sometimes delayed. It should be performed by surgical and anaesthetic teams experienced with achondroplasia, after medical treatment by rheumatology or physical and rehabilitation medicine and before permanent lesions, with MRI of the whole spine — also before any general anaesthesia — so as not to miss cervical stenoses. Analgesics are preferred to non-steroidal anti-inflammatory drugs; periradicular injections carry a neurological risk, so non-selective epidural routes and posterior facet joint injections under radiological guidance by an experienced physician are preferred; high-approach lumbar injections are contraindicated after lumbar surgery, and spinal or epidural anaesthesia is contraindicated after spinal arthrodesis. Avoiding prolonged sitting and alternating sitting and standing positions at work is recommended. Mechanical arthropathies of the hips and knees are treated with weight control, adapted physical activity, analgesia, intra-articular corticosteroid injections or viscosupplementation and physiotherapy and, in case of failure with significant disability, total joint replacement; valgus osteotomy with derotation of the proximal tibia and fibular shortening carries risks of infection, delayed union and compartment syndrome. In the presence of chronic pain, the full neurological examination is repeated, looking for motor deficit, absent reflexes or pyramidal syndrome (hyperreflexia, sustained clonus, Babinski sign) and for bladder–sphincter or anorectal disorders [33].

In hypochondroplasia, treatment is symptomatic; limb lengthening may be considered [48]. In Muenke syndrome, treatment centres on cranio-orbital surgical correction of craniosynostosis, following general principles for syndromic craniosynostosis treatment, usually in the first year to minimise cosmetic and neurocognitive sequelae, complemented by serial audiological and ophthalmological assessment [18, 19]; depending on severity, first correction is typically performed between 3 and 6 months via a transcranial approach, with newer approaches such as endoscopic strip craniectomy and posterior distraction in use, and protocol-based surveillance includes audiograms for acquired or progressive hearing loss, developmental and behavioural assessments, neurological assessment for seizures, ophthalmological assessment for strabismus and social support [19]. In thanatophoric dysplasia and SADDAN, discussion of intervention level (comfort support versus intensive ventilatory support, including tracheostomy) should be undertaken in advance and multidisciplinarily, ideally prenatally, involving neonatology, medical genetics and paediatric palliative care, given these conditions' intrinsic prognostic severity [34]. When thanatophoric dysplasia is diagnosed prenatally, pregnancy management aims to avoid complications (prematurity, polyhydramnios, dystocia and delivery complications due to macrocephaly and/or a rigid extended neck), with cephalocentesis and caesarean delivery potentially considered to avoid maternal complications; neonatally, palliative and comfort care are instituted, with decisions agreed with the team and supported by the family through written consent [4].

7.3 The oncogenic spectrum: therapy targeting somatic FGFR3 alterations

The same gene producing this germline skeletal dysplasia spectrum also constitutes an actionable oncogene when somatically altered, particularly in urothelial carcinoma. Erdafitinib (trade name Balversa®) is an oral pan-FGFR inhibitor (FGFR1-4), approved for adults with locally advanced or metastatic urothelial carcinoma carrying susceptible FGFR3 alterations (most frequently FGFR3-TACC3 fusion or somatic activating point variants, distinct from germline variants discussed in preceding sections) after progression on at least one previous systemic therapy line [56, 109, 110]. Initial accelerated approval occurred in April 2019 and was converted to full FDA approval in January 2024 based on phase 3 THOR results, demonstrating a 36% reduction in death risk with erdafitinib versus chemotherapy (median overall survival 12.1 versus 7.8 months; hazard ratio 0.64) in patients previously treated with anti-PD-1/PD-L1 immune checkpoint inhibitors [109, 110]. The recommended starting dose is 8 mg orally once daily, increasing to 9 mg after serum phosphate assessment on treatment day 14–21 if serum phosphate is below 9.0 mg/dL and there are no ocular abnormalities or grade ≥2 adverse reactions — a pharmacodynamic titration strategy guided by hyperphosphataemia, an expected class effect of renal tubular FGFR inhibition. The most significant adverse effects include hyperphosphataemia (virtually universal, used as a pharmacodynamic biomarker), ophthalmic toxicity (serous retinal pigment epithelial detachment requiring serial monthly ophthalmological surveillance in the first four treatment months), stomatitis, hand–foot syndrome, nail abnormalities/onycholysis and xerostomia; proactive phosphate monitoring, instituting a low-phosphorus diet and, if necessary, phosphate binders, is integral to standardised risk management [109, 110]. This scenario is covered by National Comprehensive Cancer Network (NCCN) bladder cancer systemic treatment guidelines, recommending systematic FGFR2/3 alteration testing in patients with metastatic urothelial carcinoma eligible for targeted therapy [56].

It should be noted that infigratinib — discussed in Section 7.1 for achondroplasia — is the same pan-FGFR inhibitor previously approved (and subsequently withdrawn from the market for commercial reasons) for cholangiocarcinoma with FGFR2 fusions, illustrating therapeutic repurposing of an oncological agent for a constitutional indication through pharmacological repositioning at a much lower dose than in oncology.

7.4 Pivotal trials, regulatory status, evidence hierarchy and off-label use

These trials do not allow drugs to be ranked through indirect comparison. Populations differ in age (2 to 11, 3 to 17 or 5 to 17 years), which matters considerably because baseline growth velocity decreases with age; primary outcomes also differ (absolute AGV at 52 weeks in ApproaCH and 111-301; change in AGV from baseline in PROPEL 3 and CANOPY-HCH-3), and differences between group means do not describe each child's response. PROPEL2 and COACH have no concurrent placebo group, and in 111-206, the only randomised trial below 5 years, the height z-score difference did not reach statistical significance [98, 100, 111]. One-year AGV is a surrogate outcome: US accelerated approvals of vosoritide and navepegritide are based on it, and final adult height, proportionality, complications and quality of life on treatment remain to be demonstrated in controlled trials; proposed confirmation for vosoritide rests on open-label extensions compared with natural history [42, 89, 94]. The effect in hypochondroplasia cannot be transferred to achondroplasia or vice versa. Infant reACHin data come from a seven-child sentinel cohort released by the sponsor [97].

Evidence hierarchy. This article distinguishes peer-reviewed published trials (111-301, 111-206, ApproaCH, PROPEL2, PROPEL 3, CANOPY-HCH-3, COACH), results released through announcements or documents submitted to the US securities authority (PROPEL 3 subgroup results, reACHin sentinel cohort, submissions and PDUFA dates), regulatory decisions (FDA; European Commission following CHMP opinion) and national access. An accepted submission or PDUFA date is not approval, a CHMP opinion is not yet authorisation, and authorisation does not guarantee reimbursement or access in each country.

Extrapolation and off-label use. Off-label use is defined by label wording, age and jurisdiction; extrapolation refers to transferring results to unstudied populations. Vosoritide in a 2-month-old infant is an approved US indication and off-label EU use; in hypochondroplasia it is off-label in every country despite a positive phase 3 trial; navepegritide is not authorised in the EU; a CNP analogue combined with growth hormone (COACH) is experimental. None of these drugs is indicated in lethal forms, SADDAN or Muenke syndrome, and use in those entities would simultaneously be off-label and unsupported extrapolation. The hope of functional benefit (fewer decompressions, fewer spinal operations, better quality of life) is currently an extrapolation of linear growth effect rather than a demonstrated result.

7.5 Divergences between the French PNDS and other sources

The French achondroplasia PNDS (2025) agrees with the 2022 international consensus and the North American recommendations on multidisciplinary surveillance, neuroimaging and polysomnography in infants, and molecular confirmation before targeted therapy, but diverges on other points [33, 45, 68]. For doses, the guideline recommendation is the reference and the label serves for comparison.

Table 7.2 — Divergences between the French PNDS and other cited sources

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8. Reproductive Options and Family Counselling

Genetic counselling is the process through which a counsellor helps the counsellee adapt to the condition and understand related medical facts, including early diagnosis, therapy, referral to support groups and recurrence risk [30]. In FGFR3 complex disorders, it should begin by explaining accessibly and non-directively the sporadic origin of the overwhelming majority of cases. Genetic counselling in the FGFR3 spectrum should consider three distinct scenarios.

First scenario (the most frequent). An average-stature couple has a child affected by a de novo mutation — in around 80% of achondroplasia and virtually all thanatophoric dysplasia and SADDAN, parents do not carry the variant; recurrence risk in subsequent pregnancies is low, close to baseline population risk although slightly higher due to the previously mentioned possibility of parental germline (gonadal) mosaicism — documented around 0.1 to 0.2% for achondroplasia or, in other series, between 0.2 and 1% (0.02% according to the Oxford Desk Reference and Bone Dysplasias) [10, 26, 27, 114]. Targeted molecular prenatal diagnosis is offered but not mandatory in future pregnancies. In thanatophoric dysplasia, an example of a “genetic lethal” in which the altered gene is not transmitted to another generation, parents are not carriers and recurrence risk is negligible, although gonadal mosaicism should be considered [1, 4]. In sporadic cases confirmed molecularly, parental screening (ideally sensitive sequencing capable of detecting low-level mosaicism) allows more precise estimation of familial recurrence risk and should be systematically offered to parents of an affected child even without parental phenotypic signs. As achondroplasia is usually clinically apparent, testing of other relatives is rarely indicated unless there is clinical suspicion [27].

Second scenario. One parent has the condition (confirmed carrier of a pathogenic FGFR3 variant) and has a 50% risk of transmitting it to each child, regardless of sex, following classic autosomal dominant inheritance — typical of familial achondroplasia, hypochondroplasia or Muenke syndrome and the most relevant scenario for discussing assisted reproductive options [26, 46]. In these cases, preimplantation genetic diagnosis (PGD), with in vitro fertilisation, is a validated specialist laboratory option for couples wishing to select non-carrier embryos; invasive prenatal diagnosis (chorionic villus sampling, possible at 11 weeks if the variant is known, or amniocentesis) or non-invasive circulating cell-free fetal DNA analysis when the parental variant is known (except maternally inherited achondroplasia) is also available [27]. In Muenke syndrome, reproductive counselling should explicitly discuss incomplete penetrance and extremely variable expressivity — an asymptomatic p.Pro250Arg carrier parent may have offspring with clinically significant craniosynostosis and vice versa, so counselling should avoid deterministic phenotypic predictions [18, 19]. In the international natural history cohort, 64.7% of informative probands had inherited the variant [51], making parental testing mandatory before assuming a de novo variant, as a mildly affected parent is easily overlooked.

Third scenario (the most complex and of particular clinical and ethical relevance). Occurs when both parents have achondroplasia (or carry another pathogenic variant of the spectrum) — a relatively common situation within the community of people with short stature, given the historical tendency, and the greater sociability facilitated by associations and community events, towards relationships within the community itself. In this situation, offspring have a 25% probability of inheriting both mutated alleles (homozygosity or compound heterozygosity, typically lethal in the neonatal/perinatal period through respiratory failure secondary to severe thoracic hypoplasia, with features very similar to thanatophoric dysplasia), a 50% probability of the parents' usual heterozygous phenotype and a 25% probability of average stature [26, 27]; the Brazilian manual expresses this scenario as a 75% recurrence risk of affected offspring, of whom 25% are homozygous, with a more severe presentation and death in the first months of life [30]. The French PNDS (French National Diagnosis and Care Protocol) gives a transmission probability of 2/3 "in each pregnancy" when both parents have achondroplasia, and of 1/2 when only one does [33]; the 2/3 figure is the proportion of heterozygotes among surviving conceptuses (excluding lethal homozygosity), not the risk per conception, which is 50% heterozygosity and 25% homozygosity [26, 27]. Given the severity of the homozygous form, preconception counselling and targeted molecular prenatal diagnosis are particularly important in these couples, allowing informed pregnancy planning and advance decisions on the level of neonatal intervention; this 25% risk of a lethal outcome makes early prenatal diagnosis (by chorionic villus sampling or amniocentesis, with targeted molecular analysis) particularly relevant for this subgroup of couples, and the discussion should include, in a non-directive manner, the options for pregnancy monitoring and the implications of each possible result [26].

In all scenarios, preimplantation genetic diagnosis is an available reproductive option for couples with a known pathogenic variant wishing to reduce transmission probability or specifically avoid the lethal homozygous/compound state in concordant couples; it should be discussed informatively rather than prescriptively, respecting reproductive autonomy and the couple's cultural and identity values [46]. Many families and adults with achondroplasia do not view the condition as a disease to “prevent” or “treat” in its entirety, but as a human variation with which they live fully — a significant proportion of the community does not perceive their condition as something to avoid but as an integral part of their identity, so reproductive discussions should be sensitive to this sociocultural perspective, without implicit judgements about the condition's “undesirability”. Genetic counselling should therefore be systematically non-directive, centred on complete information and respect for the couple's decision-making autonomy, avoiding imposing quality-of-life values not universally shared by the affected community — a principle particularly relevant in debate about making new growth-targeted pharmacological therapies available, whose acceptance is not consensual within the achondroplasia community itself.

In France, a prenatal diagnosis is disclosed at a multidisciplinary prenatal diagnosis centre, with a psychology consultation systematically offered — also when the pregnancy continues — and a meeting with a professional from the network of constitutional bone disease centres; new therapies may be mentioned with caution in genetic counselling consultations, and preimplantation genetic testing, described as a complex option, is discussed case by case in authorised centres. Genetic counselling and information on prenatal diagnosis should be addressed in adolescence, during the transition period; fertility of men and women with achondroplasia is normal, and contraception is discussed with gynaecology from puberty [33].

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9. Special Considerations: Population Diversity and Epidemiology

9.1 Population diversity and global epidemiology

Achondroplasia is recognised as the most frequent non-lethal skeletal dysplasia (the commonest survivable dysplasia) in all studied populations, with globally concordant birth prevalence estimates around 1 in 25,000 to 1 in 30,000 live births, corresponding to an estimated worldwide population exceeding 250,000 people [1, 11, 13]; Foreman and colleagues' meta-analysis (2020) suggests around 4.6–4.7 per 100,000 births (approximately 1/21,000) [12]. Thanatophoric dysplasia is the most frequent lethal chondrodysplasia, with incidence 1:20,000–50,000 live births, without sex predominance, type 1 being commoner than type 2 [4]. Unlike many Mendelian genetic conditions, the FGFR3 spectrum has no documented relevant population founder effect for the principal pathogenic variants (p.Gly380Arg, p.Asn540Lys, p.Pro250Arg). Achondroplasia's mutational homogeneity (c.1138G>A recurring in virtually all studied populations) instead results from intrinsic hypermutability of that specific nucleotide during paternal spermatogenesis (a particularly mutation-susceptible CpG dinucleotide subject to positive spermatogonial selection), rather than descent from a common ancestor — explaining the condition's globally uniform geographical and ethnic distribution and making these variants virtually identical in relative frequency and clinical presentation across all population groups studied to date, an example of cross-cultural mutational homogeneity in medical genetics [7, 9, 18]. Surveillance studies in US birth defect programmes and recent systematic reviews suggest relatively homogeneous birth prevalence among European-, African- and Asian-ancestry populations, with global estimates around 1/25,000 to 1/30,000 births for achondroplasia, corresponding to an estimated worldwide population exceeding 250,000 people [12, 13]. Some observational studies reported higher rates in North Africa, the Middle East (34.3/100,000) and sub-Saharan Africa (12.6/100,000) [12, 115], although these differences may partly reflect methodological variation in case ascertainment, local surveillance and diagnostic access between healthcare systems rather than a true biological mutation-rate difference, without an established specific population biological basis and requiring confirmation by dedicated population studies.

Robust epidemiological data remain scarce for Latin America, sub-Saharan Africa and Southeast Asia, where rare disease registries are incipient, constituting a relevant gap in regional epidemiological data in indexed English-language literature; most available quantitative evidence derives from European (France, Denmark, United Kingdom), North American and, more recently, Japanese and Australian registries. The first national prevalence study of achondroplasia and hypochondroplasia based on a rare disease registry (Banque Nationale de Données Maladies Rares, France, data to January 2024; published 2025) estimated birth prevalence of 3.27 per 100,000 for achondroplasia and 1.31 per 100,000 for hypochondroplasia, with the authors' explicit caveat that hypochondroplasia prevalence is probably underestimated through underdiagnosis of milder forms [10]. No Portugal-specific prevalence data or exclusively Brazilian population-based studies were identified in consulted sources — the Brazilian genetic syndromes manual states that there are still no epidemiological studies in Brazil and no racial predominance [30] — although South American ECLAMC hospital data discussed below exist; absence from consulted sources does not prove studies do not exist, and global estimates should be cautiously extrapolated in these care settings. A review of 467 published achondroplasia patients systematises the clinical and mutational spectrum (table available at https://pmc.ncbi.nlm.nih.gov/articles/PMC10822181/table/Tab1/) [74], and population studies from Japan and Northern Ireland document thanatophoric dysplasia prevalence and prognosis [58, 116]; in the Northern Ireland population-based study with multiple ascertainment sources over 12 years, thanatophoric dysplasia was the most frequent perinatally lethal skeletal dysplasia (22 of 39 cases), with minimum prevalence 0.80 per 10,000 live births (around 1:12,000), higher than previous studies [116]. In South America, a study by Brazilian authors (Campinas) using ECLAMC hospital network data (Latin American Collaborative Study of Congenital Malformations; 1,544,496 births examined in 132 hospitals between 2000 and 2007) estimated birth prevalence of 0.47 per 10,000 (95% CI 0.36–0.59) for thanatophoric dysplasia and 0.44 per 10,000 (0.33–0.55) for achondroplasia; FGFR3 group dysplasias represented 29% of the best-documented osteochondrodysplasias, overall perinatal lethality of osteochondrodysplasias was 50%, and paternal age was higher in cases than controls [117]. This 1:20,000 value for thanatophoric dysplasia is cited by GeneReviews; it is birth prevalence in participating hospitals, not national population prevalence [57, 117]. For hypochondroplasia, GeneReviews emphasises that no prevalence studies have been published and that many affected individuals, with no symptoms beyond short stature, do not seek medical care; its prevalence may approach that of achondroplasia [16].

This has direct practical implications: unlike diseases with a founder effect (for example, specific BRCA1/2 mutations in Ashkenazi populations), there is no indication for population screening targeting specific ethnic subgroups in FGFR3 disorders, with clinical suspicion and molecular confirmation equally applicable, with the same expected diagnostic yield, in all populations.

9.2 Special populations

Couples in whom both parents have achondroplasia (or another FGFR3 dysplasia) constitute a subgroup with specific reproductive counselling needs already discussed in detail in Section 8 and represent a growing proportion of patients followed in specialist reference centres, reflecting greater sociability within the short-stature community through development of patient organisations and dedicated community events.

Pregnancy in women with achondroplasia entails specific obstetric considerations: women with achondroplasia can have successful term pregnancies, with principal risks being late-pregnancy respiratory impairment and neurological complications of hyperlordosis; baseline respiratory function tests early in the third trimester may be useful, and specialist obstetric follow-up is required [27]. There is a high probability of elective caesarean delivery due to cephalopelvic disproportion related to the characteristic pelvic configuration (and relative macrocephaly in an affected fetus, justifying delivery planning in a specialist unit) [1]; obstetric planning should take place in a centre experienced in anaesthesia and high-risk obstetrics in short-stature individuals, also considering potentially more difficult airway features relevant to general anaesthesia if needed and avoiding spinal anaesthesia [26, 27].

The French PNDS describes pregnancy as usually well tolerated, depending on stature and orthopaedic sequelae, and recommends a preconception consultation (neurological assessment, respiratory assessment with function tests and possibly polygraphy, and cardiological assessment), at which genetic counselling is reviewed and the care pathway of a child with achondroplasia is explained [33]. The uterus becomes abdominal from the first trimester, in anteversion, with early respiratory discomfort, and the long cervix makes transvaginal procedures difficult. Molecular prenatal diagnosis is performed using cell-free fetal DNA in maternal blood if the mother does not have achondroplasia and, otherwise, by chorionic villus sampling at 12 weeks, with amniocentesis possible later; without a first-trimester diagnosis, ultrasound surveillance is organised, accompanied by a physician–genetic counsellor pair, with intermediate scans at 18 and 26 weeks in addition to the screening scans at 22 and 32 weeks in a reference centre, and some centres offer low-dose fetal CT in case of doubt. The anaesthetic consultation should be early, with recent spinal radiographs; respiratory function tests standing and supine are requested in the second trimester if not performed before and if the pregnant woman is shorter than 1.20 m or has a respiratory problem. Epidural or spinal anaesthesia is possible if there is no osteosynthesis material at the puncture site, and regional anaesthesia is preferable to general anaesthesia because of intubation difficulties, with a case-by-case decision according to neurological and orthopaedic abnormalities. Delivery is by caesarean section, because of the narrow pelvis, preferably after 37 weeks if maternal tolerance allows; systematic psychological support is offered and home help for postpartum fatigue is planned in advance [33]. When the fetus has achondroplasia, the mode of delivery is decided case by case, according to presentation, maternal comorbidities, macrocrania and the fetopelvic relationship, with prophylactic caesarean section sometimes offered from 38 weeks; as a rule, there is no excess perinatal distress or sucking–swallowing disorder in the newborn [33].

Psychosocially, the international community of people with skeletal dysplasias — organised in associations such as Little People of America (http://www.lpaonline.org), the Restricted Growth Association (United Kingdom; https://www.restrictedgrowth.co.uk) and European and Latin American counterparts, including ANDO Portugal (Associação Nacional de Displasias Ósseas), Fundación ALPE in Spain and Instituto Nacional de Nanismo in Brazil — plays a central role in family support, accessibility advocacy and representation in the development and regulation of new therapies [27, 118]. Recent trials have begun to include outcomes beyond stature alone, such as achondroplasia-specific quality-of-life and physical functioning measures and body proportionality [94, 101]. In the European Union, the relevant European Reference Network is ERN BOND (rare bone diseases) and, for Muenke syndrome and craniosynostoses, ERN CRANIO [31, 32]. In France, the PNDS integrates social and school support: the individualised reception plan (projet d'accueil individualisé, PAI), from nursery to secondary school, specifies the adaptations (avoiding somersaults, trampoline and jumping; a ground-floor classroom or one with a lift; adapted furniture with a footrest; a lighter school bag; an extra one-third of time in examinations; a laptop with occupational therapy support in the final years of schooling); achondroplasia is not on the list of 30 long-term conditions (affections de longue durée), but may be recognised as an "off-list" condition when it requires continuous care for more than 6 months; the Maisons départementales des personnes handicapées grant the mobility and parking card, the disabled child education allowance, the school support assistant and recognition of disabled worker status, and organise the personalised schooling plan [33]. The national patient association is the Association des Personnes de Petite Taille (APPT), which took part in drafting the PNDS; the OSCAR network provides the follow-up schedule, recommendations for school and an emergency card; psychological support should be offered at any age, especially at disclosure of the diagnosis, school entry, the move to lower secondary school and the transition to adulthood [33].

9.3 The somatic and mosaic spectrum: beyond germline inheritance

An aspect frequently undervalued in classic skeletal dysplasia teaching is that the same pathogenic gain-of-function FGFR3 variants, when occurring postzygotically in mosaicism, produce entirely distinct circumscribed phenotypes — notably keratinocytic epidermal naevi and cutaneous forms of localised acanthosis nigricans without any systemic skeletal manifestation — reflecting the general principle that embryonic timing and tissue distribution of the mutation determine the final observed phenotype as much as genotype itself. A 47-year-old woman mosaic for p.Arg248Cys, characteristic of thanatophoric dysplasia type 1, had asymmetric limb lengths, bilateral congenital hip dislocation, focal bone bowing, an S-shaped humerus, extensive acanthosis nigricans and redundant limb skin folds; her only son had a lethal skeletal dysplasia compatible with thanatophoric dysplasia, documenting parental somatic and gonadal mosaicism [57].

At a purely somatic and tumour level, activating FGFR3 alterations — recurrent point mutations, including the same p.Ser249Cys variant and others in the extracellular domain, mechanistically analogous to thanatophoric dysplasia germline variants, and gene fusions involving FGFR3-TACC3 — are implicated in bladder urothelial neoplasms, occurring in approximately 65% of non-invasive papillary bladder carcinomas and around 20% of muscle-invasive urothelial carcinomas, particularly in low-grade non-muscle-invasive forms [52, 53]; in the ESMO handbook, somatic FGFR3 mutations occur in 49% of non-muscle-invasive urothelial bladder carcinomas (enriched in low-grade tumours), in 10–14% of chemotherapy-naive muscle-invasive tumours (TCGA series) and in 40% of a series of 195 upper tract urothelial carcinomas, in which they are more frequent than in bladder tumours and concentrated in low-stage, low-risk tumours; ESMO recommends testing for FGFR fusions and mutations in advanced urothelial carcinoma (ESCAT IC) [119]; they also occur, less frequently, in cervical carcinoma. In multiple myeloma, the recurrent t(4;14)(p16.3;q32) translocation, present in 15 to 20% of cases, juxtaposes FGFR3 with the immunoglobulin heavy chain enhancer (locus), resulting in receptor overexpression and constituting an adverse prognostic factor, historically associated with shorter survival before the proteasome inhibitor era [54, 55]. This molecular context underpins the current clinical use of erdafitinib and the clinical development of small-molecule pan-FGFR inhibitors (Section 7.3), and illustrates how a single gene can produce, according to the inheritance context (constitutive germline versus acquired somatic) and the affected tissue, clinical entities as distinct as a lethal newborn skeletal dysplasia and a urological cancer in an older adult [56, 109, 110].

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10. Illustrative Clinical Case

The following cases are composite clinical vignettes constructed for teaching from features typically described in achondroplasia literature (including neonatal diagnostic and orofacial manifestation case reports) [120, 121], and do not represent identifiable real patients.

10.1 Case 1 — The newborn's diagnostic pathway

Identification: Male newborn, first child of a young healthy non-consanguineous couple without a family history of skeletal dysplasia.

Pregnancy history: Second-trimester anomaly scan without relevant abnormalities. Third-trimester ultrasound (32 weeks) identified femoral shortening below the 3rd centile for gestational age, with head circumference at the 75th centile, prompting maternal–fetal medicine referral. Uncomplicated vaginal delivery at term.

Physical examination at birth: Head circumference 36 cm (>97th centile), length 46 cm (<3rd centile), increased upper/lower segment ratio. Relative macrocephaly with a prominent forehead and depressed nasal bridge, symmetrical four-limb rhizomelic shortening, short trident hands through separation of the third and fourth fingers, generally reduced muscle tone. Protruding abdomen. No respiratory difficulty at birth [120, 121].

Further investigation: Skeletal radiographs revealed short tubular bones with metaphyseal widening, shortened femoral necks, square ilia with a narrow sciatic notch and craniocaudal narrowing of lumbar interpedicular distance — a radiographic pattern compatible with achondroplasia. Targeted FGFR3 sequencing confirmed heterozygous pathogenic c.1138G>A (p.Gly380Arg), absent in both parents, consistent with a de novo germline mutation.

Course and management: Brain and craniocervical junction MRI at 6 weeks showed no significant cervicomedullary compression (normal AFMS score), with serial neuroimaging reassessment scheduled during the first year, together with baseline polysomnography (no relevant obstructive or central apnoea events) and audiological assessment. The family was referred to a multidisciplinary skeletal dysplasia clinic and family support organisation; currently available growth-targeted pharmacological options were discussed non-directively, with the family deciding to reassess the option after 2 years, consistent with current consensus recommendations.

Teaching comment: This case illustrates the typical contemporary diagnostic pathway — third-trimester ultrasound suspicion, clinical and radiographic confirmation at birth, molecular confirmation of recurrent c.1138G>A and early institution of internationally recommended multidisciplinary surveillance, particularly emphasising first-year neurological surveillance, the period of greatest sudden death risk related to unrecognised cervicomedullary compression.

10.2 Case 2 — Prenatal diagnosis, initiation of targeted therapy and long-term follow-up

The following case is a composite clinical vignette constructed for teaching from features typically described in achondroplasia literature and does not represent an identifiable real patient.

Routine obstetric ultrasound at 29 weeks of gestation in a 32-year-old primigravida without a relevant family history revealed femoral length below the 3rd centile for gestational age, associated with relative macrocrania (head circumference at the 95th centile) and a depressed nasal bridge in the midsagittal plane. The remaining morphological study, including chest and cardiac assessment, revealed no additional abnormalities, and amniotic fluid volume was normal. The patient was referred to maternal–fetal medicine and medical genetics, with amniocentesis proposed for targeted FGFR3 molecular analysis, identifying heterozygous pathogenic c.1138G>A, p.(Gly380Arg) — confirming prenatal sporadic achondroplasia (parents of normal stature without the variant in peripheral blood).

The male newborn was delivered by elective caesarean at 39 weeks with appropriate weight for gestational age but length below the 3rd centile. Neonatal examination confirmed four-limb rhizomelia, macrocephaly with a prominent forehead, midface hypoplasia and trident hands, without immediate respiratory difficulty. Protocol-based surveillance began according to achondroplasia-specific paediatric follow-up recommendations: polysomnography at 6 weeks (revealing mild obstructive apnoea episodes without an urgent surgical indication), craniocervical junction MRI at 4 months (foramen magnum at the condition-specific lower normal limit without cord compression or syringomyelia), and serial head circumference follow-up, showing an accelerated but spontaneously stabilising increase compatible with mild communicating hydrocephalus without shunting.

At 3 years, after thorough parental discussion of risks, expected benefits and uncertainties about long-term outcomes — including that therapy does not alter established neurological or cranial manifestations — and confirmation of no cardiovascular contraindications, daily subcutaneous vosoritide was started, with quarterly growth velocity, blood pressure and renal function monitoring. At 6 years, the child had age-appropriate cognitive and language development, mild bilateral conductive hearing loss secondary to recurrent otitis media (under otorhinolaryngology follow-up with previous transtympanic ventilation tubes) and sustained growth velocity gain since treatment began, without major reported adverse effects beyond occasional post-injection dizziness managed by increased pre-administration hydration.

Teaching comment: This case complements the preceding one by illustrating the complete trajectory to shared therapeutic decision-making and medium-term follow-up on targeted therapy, emphasising fundamental pillars of contemporary achondroplasia management: ultrasound-guided and molecularly confirmed prenatal diagnosis; systematic protocol-based neurological and respiratory surveillance in the first year; coordinated multidisciplinary care; and currently the possibility of informed discussion of FGFR3-targeted therapy, always within a shared family decision respecting diverse perspectives on the condition itself.

10.3 Real published case: long-term survivor with thanatophoric dysplasia type 1 (Carroll et al., 2020)

Thanatophoric dysplasia literature describes individually documented long-term survivors, including one individual to 28 years, all with profound developmental delay, severe growth impairment and ventilatory dependence [39]; the survivor series underpinning debate about the designation “lethal” is reviewed by Carroll and colleagues [122].

The case described by these authors in a US skeletal dysplasia programme concerns a boy whose pregnancy was complicated by polyhydramnios and suggestive ultrasound findings; amniocentesis identified FGFR3 c.742C>T, p.(Arg248Cys), confirming thanatophoric dysplasia type 1 (NM_000142.5; pathogenic in ClinVar). His parents, counselled to expect death in the delivery room, contacted two survivor families and chose to continue the pregnancy with a life-prolonging delivery plan. He was born by caesarean at 36 weeks, immediately intubated, spent five months in neonatal intensive care and underwent tracheostomy and gastrostomy at two weeks; subclinical seizures, hypoventilation and apnoea were treated. At 2.5 years, polysomnography showed periodic breathing and central apnoea, and examination revealed hypotonia, weakness, hyperreflexia and a crossed adductor reflex; MRI showed foramen magnum and cervical canal stenosis with cord signal change, and foramen magnum decompression was followed by resolution of central apnoea and developmental progress. Acanthosis nigricans appeared around 2 years and required several skin fold excisions. At 9 years, he fed himself, drove a powered wheelchair, communicated through a device, sign language and up to three-word phrases, had been off ventilation since 8 years and was preparing for decannulation; neuropsychological assessment placed him at a 1- to 3-year level [122]. The child's photograph (figure 1) and the table of clinical courses of published long-term survivors (table 1) are available in the open-access original article: https://pmc.ncbi.nlm.nih.gov/articles/PMC8241327/ .

Comment. The case illustrates prognostic uncertainty justifying the non-directive counselling described in Section 8 and debate about “lethal” (Section 11), and shows that foramen magnum stenosis, so relevant in achondroplasia, also influences the course of rare thanatophoric dysplasia survivors. This is an individual report of a family choosing sustained intensive care with prolonged technological dependence; it does not estimate survival probability or convert the described interventions into routine recommendations [122].

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11. Prognosis and Long-Term Follow-Up

Survival prognosis differs (varies) radically according to the specific spectrum entity.

In thanatophoric dysplasia and SADDAN, prognosis is substantially more guarded (see the real case in Section 10.3): in thanatophoric dysplasia without intensive ventilatory support, most newborns die within hours to days from respiratory failure due to pulmonary hypoplasia secondary to thoracic shortening, or central apnoea — early perinatal mortality exceeds half of live births in recent national series [4, 27, 58, 116]; however, with sustained intensive intervention (including early tracheostomy in some cases), long-term survivors (several years) are described, ventilator-dependent, with significant cognitive and motor impairment (language and motor function at a late first-year level), seizures, hearing loss and estimated final stature between 80 and 90 cm, not reaching reproductive age [4, 5, 34]. Prolonged survival with permanent ventilatory and technological support is documented in a growing number of cases: of the 20 long-term survivors in the Japanese national survey, 16 underwent tracheostomy and the other 4 did not only because of contraindications [57, 58]. This growing number of documented survivors has prompted active debate about the appropriateness of “thanatophoric” to describe the condition in contemporary neonatal intensive care [122]. In SADDAN, prolonged survival, although rare, is significantly more frequent than in thanatophoric dysplasia, with control of frequently refractory seizures and chronic respiratory support being the main determinants of morbidity and mortality: prolonged survival is conditioned by frequently refractory epilepsy and chronic respiratory complications requiring long-term support [6].

In achondroplasia, life expectancy is generally good but reduced by approximately 10 years in the current adult population according to the largest published mortality study (a 42-year follow-up cohort) [26, 43], with excess mortality primarily attributable to cardiovascular complications (partly related to the high prevalence of obesity and untreated sleep apnoea), respiratory complications and neurological complications related to cervicomedullary compression and progressive spinal stenosis, particularly in the first year of life and, in a second peak, in adulthood due to lumbar canal stenosis; Smith's Recognizable Patterns gives a mean reduction of 15 years [28]. Weight gain is the greatest problem associated with joint and cardiovascular dysfunction, producing early heart disease-related mortality, with contributions from lack of physical activity, persistent appetite and psychological factors [30]. The recommended surveillance, particularly monitoring and early intervention for cervicomedullary compression in the first year of life, has reduced crude mortality in infants and young children, but standardised mortality remains elevated at all paediatric ages and in young adults [76]. Mean final adult stature is 131 cm (±5.6) in men and 124 cm (±5.9) in women according to classic reference data. Intellectual development is normal in most cases, except when impaired by untreated hearing loss or, more rarely, uncontrolled symptomatic hydrocephalus, although there may be mild motor delay in the first years and impaired verbal comprehension secondary to untreated hearing loss [26]. In most cases prognosis is good, depending on complications (spinal stenosis, weight gain, hydrocephalus, cervical cord compression) [30].

The North American multicentre cohort of Hashmi and colleagues (855 patients seen at four bone dysplasia clinics between 1986 and 2015; 12,117 person-years; median age at study exit 14.8 years, maximum 58.4 years) describes mortality by age and cause during the period of systematic surveillance. Standardised to the 2010 US population, mortality ratios were 13.8 (95% CI 1.9–97.8) under 1 year, 239.8 (99.8–576) from 1 to 4 years, 29.9 (7.5–119.6) from 5 to 14 years and 7.7 (2.9–20.6) from 15 to 24 years; they could not be calculated after 24 years, the age of the latest death. Of the 12 deaths, one occurred in the first year, five from 1 to 4 years, two from 4 to 14 years and four from 15 to 24 years. From 1 to 4 years, three of the five deaths were cerebrovascular or cardiovascular: at 2.1 years, with lower-limb clonus and cor pulmonale recorded months earlier; at 3.0 years, cardiac arrest shortly after adenotonsillectomy; at 3.3 years, cardiac arrest. The other two were a death of unknown cause at 2.5 years, 1–2 months after uncomplicated foramen magnum decompression and C1 laminectomy, and a sudden unexpected death at 2.6 years. From 5 to 24 years, three of the six deaths were motor vehicle accidents (two pedestrians struck at 6.5 and 11.8 years and one passenger thrown from the open bed of a pick-up truck at 19.4 years); the remainder were a death from congenital anomalies at 19.5 years, in a patient with C1 laminectomy, foramen magnum decompression and revision in the first year and a half, epilepsy, cardiorespiratory arrest at 2 years and operated craniosynostosis, a non-traumatic intracerebral haemorrhage at 22.7 years and a malignant adrenal tumour at 24.0 years. Pedestrian deaths from 5 to 14 years were equivalent to 27.9 per 100,000 (2010 standardisation), whereas total road traffic mortality in the US population in 2014 was 10.6 per 100,000, 1.6 per 100,000 in pedestrians; the authors suggest that short stature makes pedestrians less visible to drivers and recommend counselling patients and families on road safety. Compared with Hecht's 1987 cohort, there were no deaths from pneumonia, hydrocephalus or brainstem compression (3, 2 and 1 cases in the older cohort), and cardiovascular and cerebrovascular deaths appeared in young children; in the older cohort, 19 of the 53 deaths (36%) were cardiovascular, all after 25 years [67, 76]. Crude mortality fell, but standardised ratios are higher when calculated with more recent reference populations (from 1 to 4 years, 93.6 with the 1975 population and 239.8 with the 2010 population), because mortality in the general population fell faster than that of people with achondroplasia. The limitations of the series are the small number of deaths, the wide confidence intervals, inclusion only after the first consultation at a specialist centre and two internal inconsistencies in the article: the abstract gives the 2000 population as the reference, whereas the methods and tables use the 2010 population, and the notes to Table 2 swap the units of the rates (14.0 corresponds to deaths per 1000 patients and 0.99 to deaths per 1000 person-years) [76].

In hypochondroplasia, life expectancy is considered normal, with adult stature between 132 and 147 cm in some series and within a classic range of 118–152 cm in others, a divergence reflecting sampling differences between cohorts [29, 48, 60]; an increased risk of mild learning difficulty is described more frequently than in achondroplasia, associated in some series with structural temporal lobe dysgenesis, as is exertion-related joint pain in adulthood, mechanical arthralgia being a relatively common and sometimes undervalued complaint during clinical follow-up [16]. The principal source of long-term morbidity is thus exertion-related joint pain, alongside the psychosocial implications of short stature, sometimes exacerbated by late or missed diagnosis. Neurological complications are rare [27].

In Muenke syndrome, survival prognosis is excellent, with major morbidity related to timely craniofacial surgery and hearing loss screening and correction; a subgroup has learning difficulties and behavioural abnormalities benefiting from specialist neurodevelopmental follow-up [18, 19].

Regardless of the specific entity, all surviving patients with confirmed FGFR3 dysplasia benefit from a structured lifelong follow-up programme, ideally involving a multidisciplinary skeletal dysplasia reference centre — in Portugal and the European Union integrated into the European Reference Network for Rare Bone Diseases (ERN-BOND) — with periodic growth, neurological, otorhinolaryngological/audiological, orthopaedic and mental health assessments and, in adulthood, cardiovascular and metabolic surveillance targeting excess weight and degenerative spinal stenosis complications [45, 107]. Long-term follow-up in all non-lethal forms should be provided through these multidisciplinary skeletal dysplasia clinics, with structured transition from paediatric to adult care, including orthopaedic, neurological, otorhinolaryngological/audiological, respiratory, gynaecological/obstetric, metabolic/cardiovascular and mental health assessment throughout life. Although most children and adults need prolonged medical supervision, as normal a life as possible should be emphasised, with physiotherapy and occupational therapy support for daily living activities [70]. The CLARITY natural history study documents 60 years of orthopaedic surgical experience in four skeletal dysplasia centres [123].

11.1 Achondroplasia follow-up schedule according to the French PNDS

Paediatric follow-up is coordinated by a clinical geneticist or a paediatrician from the network of reference and competence centres for constitutional bone diseases, and adult follow-up by a rheumatologist or a physical and rehabilitation medicine physician, always in liaison with the attending physician [33]. The first genetics clinic appointment confirms the clinical and molecular diagnosis; the next appointment takes place at the first month and, if necessary, at 3 months, then six-monthly until 2 years and annually until transition, with a joint consultation between the paediatric and adult teams at around 17–18 years; in adults, a consultation at a specialist centre every 12–36 months is proposed. After 2 years, the minimum complementary assessments are performed at the pivotal ages of 2, 5, 10, 15, 17–18 and 25 years and then every 5 years. Each consultation includes measurements on specific charts, a full neurological examination, respiratory and sleep assessment, quality of life (PedsQL and QoLISSY in children; SF-12 or SF-36 in adults) and dietary advice, and blood pressure is measured with an adapted cuff. The referring physician may provide an OSCAR network emergency card with contacts, treatments, allergies and recommendations for emergencies, with a QR code linking to the PNDS (French National Diagnosis and Care Protocol). Prognostic factors and correlations between the different organ involvements have not been established, and there are few data on ageing [33].

Table 11.1 — Surveillance in achondroplasia

The 2020 North American recommendations and the 2022 international consensus, described in Section 7.2, complement this schedule, and differences in intervals are given in Table 7.2 [45, 68].

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12. Summary and Final Considerations

The FGFR3 spectrum is one of the most complete genotype–phenotype correlation examples in genomic medicine: a single gene, a limited number of recurrently affected codons and a continuous clinical severity gradient extending from almost universal perinatal lethality in thanatophoric dysplasia to mild and sometimes subclinical craniosynostosis forms in Muenke syndrome, through achondroplasia — the most prevalent and best-characterised form of disproportionate dwarfism in humans — and hypochondroplasia, its subtler, clinically underdiagnosed counterpart. FGFR3 complex disorders are the paradigmatic example of an allelic family of Mendelian diseases, in which distinct but mechanistically convergent pathogenic variants, all conferring constitutive gain of function on the tyrosine kinase domain of fibroblast growth factor receptor 3, generate a continuous severity spectrum from isolated and frequently asymptomatic Muenke syndrome craniosynostosis, through mild disproportionate short stature in hypochondroplasia and classic achondroplasia, to almost invariable neonatal lethality in thanatophoric dysplasia.

Understanding the shared pathophysiological mechanism — constitutive RAS-MAPK pathway activation downstream of the receptor with inhibition of chondrocyte proliferation — has enabled, over the past decade, transition from exclusively symptomatic and surgical management to molecularly targeted therapy, with three distinct pharmacological classes (agents) — CNP pathway agonism (vosoritide, navepegritide) and direct kinase inhibition (infigratinib) — currently approved or in advanced regulatory approval stages, representing the first generation of treatments capable of modifying linear bone growth in this condition, although not yet altering established neurological, cranial or dental manifestations, tangibly transforming clinical practice over the past five years; in September 2026, phase 3 vosoritide results in hypochondroplasia extended this perspective to the spectrum's milder form. Molecular confirmation of the specific genotype remains central not only for diagnosis and genetic and reproductive counselling but increasingly for therapeutic decisions. In parallel, and instructively, the same gene acts as an actionable oncogene when somatically altered in urothelial carcinoma and multiple myeloma, a scenario already incorporated into international oncology guidelines through erdafitinib — illustrating how the molecular genetics of a rare skeletal dysplasia can directly inform precision oncology development decades later, and vice versa.

This therapeutic evolution involves ethical and social controversy within the affected community itself; the clinician should maintain rigorously non-directive counselling centred on the person's and family's informed autonomy, rather than solely maximising height outcomes. Early recognition, structured multidisciplinary surveillance according to current international guidelines and timely specialist skeletal dysplasia clinic referral remain, regardless of new therapy availability, the most robust determinants of long-term prognosis in all non-lethal spectrum entities. For the clinical geneticist, integrated mastery of this spectrum — clinical, radiological, molecular, therapeutic and ethical/reproductive — is now a central practice requirement, both for its relative frequency among skeletal dysplasias and the speed of therapeutic change over the past five years.

On 6 October 2026, vosoritide was the only targeted drug authorised in the European Union (from 4 months), navepegritide was approved only in the United States (from 2 years, accelerated approval), and infigratinib awaited a regulatory decision; positive hypochondroplasia vosoritide results had not yet translated into authorisation.

Drug doses and indications should always be confirmed in current prescribing information and adapted to the local regulatory context (INFARMED/EMA/FDA), given this therapeutic spectrum's current regulatory dynamism.

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13. Bibliographic References

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