Klinefelter Syndrome and X Polysomies

Literature and regulatory information updated: 4 October 2026.

Spectrum of sex chromosome polysomies in males (47,XXY; 48,XXXY; 48,XXYY; 49,XXXXY)

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0. Initial Reference Table

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1. Introduction

The original description of the condition now known as Klinefelter syndrome was published in 1942 by Harry F. Klinefelter Jr., Edward C. Reifenstein Jr. and Fuller Albright in the Journal of Clinical Endocrinology and Metabolism, under the title "Syndrome characterized by gynecomastia, aspermatogenesis without a-Leydigism and increased excretion of follicle-stimulating hormone" [21]. The authors described nine men who shared a characteristic set of clinical findings: gynaecomastia, small firm testes, absent spermatogenesis without evident impairment of Leydig cell function, and increased urinary excretion of follicle-stimulating hormone (FSH). At the time, the aetiological mechanism remained unknown; the prevailing hypothesis located the defect in a primarily testicular endocrine axis, without any suspicion of a chromosomal cause.

The causal link would only be established seventeen years later. In 1959, Patricia A. Jacobs and John A. Strong published in Nature a case report of human intersexuality associated with a probable XXY sex-determining mechanism, demonstrating for the first time that men with the phenotype described by Klinefelter had an additional X chromosome, resulting in the 47,XXY karyotype [22]. This finding places Klinefelter syndrome among the first human conditions in which chromosomal aneuploidy was unequivocally correlated with a defined clinical phenotype, a foundational milestone in modern medical cytogenetics, contemporary with the characterisation of chromosome 21 trisomy in Down syndrome.

Over the following decades, further milestones expanded the clinical and genetic spectrum of the condition. In 1960, Fraccaro, Kaijser and Lindsten described in Lancet a child with 49 chromosomes — the first report of what is now called 49,XXXXY syndrome, sometimes referred to as Fraccaro syndrome [23]. In 1966, Zaleski and colleagues assembled three new cases and reviewed 30 published cases of 49,XXXXY, consolidating the clinical delineation of this pentasomy [11]. Subsequent studies by Caldwell and Smith (1972) on childhood detection and treatment, and by Jacobs and colleagues (1988), who used molecular probes to determine the parental origin of the additional X chromosome, consolidated the contemporary understanding of its aetiopathogenesis [11]. More recently, national population follow-up studies — notably the Danish and British cohorts conducted by Bojesen, Gravholt and colleagues — provided the epidemiological, morbidity and mortality data that now underpin clinical guidelines [9, 24, 25]. In 2011, Tartaglia and colleagues systematically described the differences between the 47,XXY form and the tetrasomies and pentasomies 48,XXYY, 48,XXXY and 49,XXXXY, proposing that these should not be treated merely as "variants" of Klinefelter syndrome [12], and in 2021 the European Academy of Andrology published the first international guideline dedicated to the syndrome, developed using the GRADE system (Grading of Recommendations, Assessment, Development and Evaluation) [15].

Klinefelter syndrome is now recognised as the most common sex chromosome aneuploidy in men and the most frequent single genetic cause of male hypogonadism and infertility [17, 26]. It accounts for approximately 3% of infertile men and 5–10% of cases of oligospermia [1]. Its clinical relevance extends beyond the reproductive axis: it is a multisystem condition with neurocognitive, skeletal, metabolic, cardiovascular and oncological consequences, whose adequate understanding requires a coordinated multidisciplinary approach from diagnosis — increasingly prenatal, owing to the spread of non-invasive prenatal testing (NIPT) — through follow-up in adulthood.

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2. Molecular Genetics and Pathophysiology

2.1 Chromosomal mechanism

The classical form of Klinefelter syndrome results from a chromosomal nondisjunction event during parental gametogenesis, producing a gamete with two X chromosomes (maternal) or an X gamete accompanied by an additional paternally derived X chromosome, which, on combining with the normal complementary gamete, generates a zygote with a 47,XXY constitution. Approximately 50 to 60% of cases originate from errors in paternal meiosis I (nondisjunction during spermatogenesis), whereas the remaining 40 to 50% arise from errors in maternal meiosis I or meiosis II, and a small fraction results from a postzygotic mitotic error [1, 11]. This pattern contrasts with chromosome 21 trisomy (Down syndrome), which is predominantly maternal in origin and strongly associated with advanced maternal age. In Klinefelter syndrome, early studies did not demonstrate an association with advanced paternal age in cases of paternal origin; however, more recent evidence suggests that older men produce a greater proportion of XY spermatozoa, potentially placing them at a slightly increased risk of fathering children with the syndrome. For cases of maternal origin, there is a consistent association with advanced maternal age, restricted to meiosis I errors [11].

Paternal nondisjunction arises from failure of normal recombination between Xp and Yp in the pseudoautosomal region, which is required for correct segregation of the XY pair in meiosis I; maternal age above 35 years is associated with maternal meiosis I errors [1]. The maternal age effect is quantifiable: the prevalence of 47,XXY rises from 1/2,500 live births at a maternal age of 33 years to 1/300 at 43 years (Hook, 1992) [2], and, in the Danish cytogenetic register, the crude prenatal prevalence of 213 per 100,000 male fetuses fell to 153 per 100,000 after standardisation for maternal age [9]. The EAA guideline summarises the origin of the supernumerary X as paternal (nondisjunction in meiosis I) in 50% of cases and maternal (meiosis I or II) or postzygotic in the remaining 50%, attributes the association with maternal age to maternal meiosis I errors and notes that an association between paternally derived 47,XXY and paternal age has also been described [15]. Among male blastocysts investigated by preimplantation genetic testing, the prevalence of 47,XXY was 0.9% (17/1,794), correlated with maternal age; the discrepancy from birth prevalence may reflect reduced implantation or increased early loss of 47,XXY embryos, or greater production of 47,XXY blastocysts by infertile women of advanced maternal age referred for IVF/PGT — a population that does not represent the general population [15]. The distribution of chromosomal constitutions and their respective mechanisms is summarised in the following table.

Source: adapted from Table 1.5.1 in Ribeiro (coordinator), Síndromes Genéticas: Reconhecer para Tratar [1], whose frequencies total more than 100%. The review by Tartaglia and colleagues cites postzygotic mitotic error in 8% of 47,XXY individuals (Hassold et al., 2007) and a distribution of approximately 50% maternal and 50% paternal origin [12].

Mosaicism — coexistence of a 46,XY cell line with a 47,XXY (or variant) cell line in the same individual — occurs in approximately 15% of cases and tends to be associated with milder phenotypes, particularly regarding residual testicular function, since the distribution of the two cell lines between tissues is random and cannot be accurately inferred from the proportion observed in peripheral blood leucocytes. For this reason, an isolated "normal" peripheral blood cytogenetic result does not exclude tissue mosaicism, and persistent clinical suspicion warrants investigation with a more sensitive karyotype (50-cell count) or analysis of another tissue, such as buccal mucosa. The natural fertility exceptionally reported in men with Klinefelter syndrome is probably explained by cryptic 47,XXY/46,XY mosaicism [2].

2.2 Gene dosage effect and X inactivation

The central pathophysiological mechanism of Klinefelter syndrome is the gene dosage effect arising from the presence of supernumerary X chromosomal material. Under physiological conditions, one of the two X chromosomes in female cells is predominantly silenced by an epigenetic process mediated by the non-coding RNA XIST (X-inactive specific transcript), which coats the inactive X chromosome in cis and promotes its heterochromatinisation. This dosage compensation mechanism also operates in men with a 47,XXY karyotype: the additional X chromosome undergoes preferential inactivation. However, inactivation is incomplete — an estimated 12% to 25% of genes located on the X escape this silencing, wholly or partly, and are expressed from both X chromosomes [27]. More precisely, approximately 12% of genes constitutively escape inactivation and a further 15% have an inactivation status that varies between individuals, tissues or cells [27]. These "escape genes" are concentrated in pseudoautosomal regions 1 and 2 (PAR1 and PAR2), which share homology between the X and Y chromosomes and, by definition, are not subject to inactivation, as well as at other loci dispersed along the short arm of the X.

Overexpression of these escape genes — many of which act in immune regulation, metabolism and neurodevelopmental pathways — constitutes the proposed molecular substrate for much of the syndrome's clinical phenotype, including the predisposition to autoimmune disease, metabolic abnormalities and the characteristic neurocognitive profile [27, 28]. A study combining registry data for the entire Danish population (6.8 million individuals) with systems biology identified 78 overrepresented comorbidities in Klinefelter syndrome — including pituitary hypofunction and dental caries, in addition to infertility and osteoporosis — and implicated disruption of the Jak-STAT pathway and dysregulation of genes involved in the immune system (IL4), energy balance (POMC, LEP) and erythropoietin signalling [28]. An instructive and classically cited example is SHOX (short stature homeobox-containing gene), located in the pseudoautosomal region, whose overexpression in 47,XXY individuals is proposed as the molecular determinant of the tall stature and body proportions with disproportionately long limbs observed in these patients — in contrast with haploinsufficiency of the same gene, which causes short stature in Turner syndrome [11]. Studies using induced pluripotent stem cell (iPSC) models derived from patients with 47,XXY and higher-grade aneuploidies (48,XXXY; 49,XXXXY) demonstrated that PAR1 region genes are particularly susceptible to dosage-dependent transcriptional dysregulation, helping explain why phenotypic severity gradually increases as the number of supernumerary X chromosomes rises — a phenomenon known as the "graded chromosomal dosage effect". For pseudoautosomal genes expressed from all sex chromosomes, expression is expected to increase 1.5-fold in 47,XXY, 2.0-fold in 48,XXYY and 48,XXXY, and 2.5-fold in 49,XXXXY relative to 46,XY men [12]. Height, however, does not follow this gradient linearly: increasing sex chromosome number affects height non-linearly (Ottesen et al., 2010, a study of 305 patients with sex chromosome aneuploidies), with tall stature in 47,XXY, 48,XXYY and 48,XXXY and below-average height in 49,XXXXY [11, 12]. Among phenotypic modifiers, the EAA guideline highlights the androgen receptor gene (AR, Xq11.2–q12), whose (CAG)n repeat polymorphism in exon 1 is inversely related to androgen sensitivity and may be associated with androgen-dependent KS traits, gynaecomastia, body composition and response to testosterone; FSH-related polymorphisms may influence serum FSH, and skewed X inactivation is another proposed factor [15]. These genotype–phenotype correlations derive from observational studies, and the guideline itself calls for their validation [15].

2.3 Gonadal pathophysiology

In the testis, excess X chromosome material progressively impairs Sertoli cell function and spermatogonial survival, a process that accelerates at the onset of puberty, when activation of the hypothalamic–pituitary–gonadal axis imposes functional demands on germ cells. The characteristic histological result is progressive hyalinisation and fibrosis of the seminiferous tubules, with almost complete depletion of the germ cell compartment in adulthood, culminating in azoospermia in the vast majority of cases. In parallel, Leydig cell function also becomes insufficient throughout puberty and adulthood, producing variable degrees of testosterone deficiency, although typically less pronounced than the impairment of spermatogenesis. The hypothalamic–pituitary–gonadal axis responds to the deficit of gonadal steroids and reduction in inhibin B (produced by Sertoli cells) with a compensatory but ineffective elevation of pituitary gonadotrophins — LH and, particularly, FSH — producing the laboratory picture of hypergonadotrophic hypogonadism, the defining biochemical hallmark of the syndrome [15, 26].

The chronology is characteristic: 47,XXY boys enter puberty normally and, from mid-puberty onwards, the testes begin to involute and hypergonadotrophic hypogonadism develops with reduced testosterone production [2]; testosterone levels decrease in late adolescence and early adulthood [11]. In polysomies 48,XXYY, 48,XXXY and 49,XXXXY, adult testicular histology shows the same seminiferous tubular hyalinisation and fibrosis, germ cell loss and Leydig cell hyperplasia; during childhood, FSH, LH and total testosterone are within the prepubertal range, FSH and LH rise at the onset of puberty and testosterone increases spontaneously in most patients, subsequently stabilising in the low-normal range or declining — a pattern similar to that in 47,XXY, although without dedicated longitudinal hormonal studies [12].

2.4 Polysomy variants (48,XXXY; 48,XXYY; 49,XXXXY)

In addition to the classical 47,XXY form, rarer chromosomal variants exist with further supernumerary X (and, in 48,XXYY, also Y) chromosomes. The 49,XXXXY syndrome, originally described by Fraccaro in 1960, results from successive maternal nondisjunction events in meioses I and II, generating an oocyte with four X chromosomes which, when fertilised by a Y-bearing spermatozoon, produces the 49,XXXXY karyotype; this maternal origin was consistent in the 20 cases studied [12, 23]. Its estimated incidence is approximately 1 in 85,000 to 1 in 100,000 male births, with just over one hundred cases published in the worldwide literature. The 48,XXXY syndrome shares an analogous mechanism, with only three X chromosomes; of the five cases in which parental origin was studied, two resulted from successive nondisjunction events during spermatogenesis (XᵖXᵖYᵖ fertilising an Xᵐ oocyte) and three from double nondisjunction during oogenesis (XᵐXᵐXᵐYᵖ) [12]. The 48,XXYY syndrome most often results from fertilisation of a normal oocyte by an aneuploid XᵖYᵖYᵖ spermatozoon produced by nondisjunction in meioses I and II during spermatogenesis — in the 11 cases with studied parental origin, the additional X and Y chromosomes were always paternal — and may also result from postzygotic mitotic nondisjunction [12]. The 48,XXYY constitution is the most frequent of the three (1:18,000–1:40,000), followed by 48,XXXY (approximately 1:50,000) [12]. As the number of X chromosomes increases from XXY to XXXXY, there is a progressive and predictable worsening of growth deficiency, intellectual impairment, the degree of hypogenitalism and the frequency of associated malformations — a phenotypic gradient directly correlated with the magnitude of the gene dosage effect described above [11]. Cognitive abilities are estimated to decrease by 10–15 intelligence quotient (IQ) points per additional X chromosome [12]. Although historically classified as "variants" of Klinefelter syndrome, more recent reviews — such as that by Tartaglia and colleagues (2011) — argue that these polysomies constitute distinct clinical entities with their own care needs and should not simply be treated as more severe forms of the same diagnosis [12]. Nevertheless, the same authors consider that complete separation would have negative implications — reduced inclusion in research projects, fragmentation of patient advocacy efforts and a risk of undertreating hypogonadism, whose replacement protocol currently does not differ between the four constitutions — and recommend coordinating education and patient advocacy across all sex chromosome aneuploidies [12].

The 2022 French PNDS on sex chromosome tetrasomies and pentasomies in boys extends the group to seven karyotypes — 48,XYYY, 48,XXYY, 48,XXXY, 49,XYYYY, 49,XXYYY, 49,XXXYY and 49,XXXXY — and leaves out 47,XXY and 47,XYY, a distinction it considers important for these patients [13]. For 48,XXYY, the proposed mechanism is fertilisation of a normal oocyte by a 25,XYY spermatozoon formed through two successive nondisjunctions (meiosis I and II) in a euploid father, or a meiotic nondisjunction followed by early mitotic missegregation in the zygote; the karyotype is homogeneous when the postzygotic error is very early and mosaic when it is later [13]. For 48,XXXY two routes are accepted (a 23,Y spermatozoon with a 25,XXX oocyte, or a 25,XXY spermatozoon with a 23,X oocyte), and for 49,XXXXY a double nondisjunction in oogenesis, producing a 26,XXXX oocyte [13]. The Y polysomies (48,XYYY and 49,XYYYY) result from Y nondisjunction during spermatogonial mitosis associated with meiotic nondisjunction, or from a father with a supernumerary Y (constitutional 47,XYY or limited to the primordial germ cells); when the error is postzygotic, mosaicism with a variable number of Y chromosomes arises [13]. To explain the clinical variability within each karyotype, the PNDS lists, in addition to mosaicism, overexpression of X genes that escape inactivation and of Y pseudoautosomal genes, polymorphisms of X and Y genes, DNA methylation changes at various loci across the genome induced by the supernumerary sex chromosomes and, on the endocrine side, the number of CAG repeats in the AR gene, with lower androgen sensitivity the greater the number of repeats [13]. It attributes the tall stature of 48,XXYY to overexpression of SHOX, also present on the supernumerary Y, and the shorter stature of 49,XXXXY to marked overexpression of the escape genes located on the additional inactive X chromosomes [13].

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3. Clinical Spectrum and Manifestations by Organ System

The clinical presentation of Klinefelter syndrome is notably heterogeneous, ranging from almost asymptomatic forms — often identified only during investigation of infertility in adulthood — to pronounced phenotypes recognisable in childhood. There is no single pathognomonic finding; diagnosis depends on recognising a combined pattern of findings. Features vary with the timing of onset and magnitude of androgen deficiency [11].

Reproductive system. In descending order of frequency in adulthood, the most evident findings are infertility (virtually universal in the non-mosaic form), small testes of firm or hard consistency (usually < 2.5 cm long in adults, or volume < 4 mL measured with a Prader orchidometer), reduced facial hair, gynaecomastia (present in approximately one-third of adolescents), reduced pubic hair and reduced penile size. Testosterone production is, with rare exceptions, inadequate, averaging less than half the normal adult value. Virilisation is partial and insufficient [11]. In childhood, the penis and testes are relatively small [11]; newborns have a normal appearance, but cryptorchidism is more frequent (one-third required orchidopexy in the study by Ratcliffe, 1999), and gynaecomastia, often transient, rarely requires surgical reduction (1/19 in the same study) [2]. Apart from the small testes, adult 47,XXY genitalia and sexual life are normal; sexual and gender orientation do not differ from those of 46,XY men [2]. The EAA guideline qualifies this statement: most men with KS identify as male, but gender incongruence has been described in some studies, and psychosexual development is often delayed, with possibly more frequent sexual problems [15]. According to the same guideline, testosterone is low in approximately 50% of adults, but LH is elevated in almost all, so overt or subclinical hypogonadism is almost a hallmark of the syndrome; in adults, testicular volume ranges from 1 to 5 mL, gynaecomastia is present in approximately one-third, and prominent or persistent pubertal gynaecomastia remains a red flag [15]. In the largest series, 27–37% of patients had a history of cryptorchidism, and micropenis (< −2.5 SD) is rare, although one study found 17% [15]. Among the genital abnormalities described, the Brazilian textbook includes complete sex reversal, true hermaphroditism, testicular feminisation and ambiguous genitalia at birth, as well as reduced secondary sexual characteristics and seminiferous tubular atrophy [1].

Growth and skeletal system. From childhood, there is a tendency towards long limbs, a reduced upper/lower segment ratio and relatively tall, slender stature; height typically lies between the 25th and 99th percentiles, averaging near the 75th percentile, while weight and head circumference remain at the 50th percentile [11]. Mean final adult height in 47,XXY is 186 cm, compared with 177 cm in 46,XY men [2]; other series report means of 179 to 188 cm [12]. The body habitus may acquire a feminised appearance, with relatively wide hips, a narrow chest, sparse hair and a female distribution of adipose tissue (eunuchoid habitus) [1]. Additional skeletal findings include mild elbow dysplasia, fifth-finger clinodactyly and taurodontism (enlargement of the dental pulp chamber with thinning of the enamel) [11]; flat feet, kyphosis and reduced muscle tone and strength are also described [1], and scoliosis may develop during adolescence [11]. Osteoporosis is relatively common even in young or middle-aged men, reflecting the cumulative effect of hypoandrogenism on bone mineral density. The EAA guideline reports low bone mass, osteopenia or osteoporosis in up to 40% of patients, without a clear relationship between serum testosterone and bone mineral density (BMD), which may be reduced despite normal testosterone; the deficit results from decreased bone formation and increased resorption, probably from insufficient peak bone mass at the end of puberty, and fracture risk also depends on hypovitaminosis D, the fat/lean mass ratio, comorbidities, low INSL3, X inactivation and androgen receptor sensitivity [15]. In childhood and early puberty, BMD is usually normal (whole-body DXA in boys and adolescents aged 4.3 to 18.6 years); in a small study, 7 of 40 boys (11.96 ± 3.97 years) had a Z-score ≤ −2.0 on phalangeal quantitative ultrasound, with higher PTH and reduced 25-hydroxyvitamin D, a finding requiring confirmation; the guideline estimates that probably fewer than 20% of children with KS will have osteopenia or osteoporosis [15].

Neurocognitive and behavioural system. Although the mean overall intelligence quotient (IQ) typically ranges from 85 to 90 — thus within or near the normal range — there is wide individual variability. Where a discrepancy exists between verbal IQ and performance (non-verbal) IQ, verbal IQ is lower than performance IQ — a relative verbal weakness, more evident in childhood — although several cohorts (including series unbiased by postnatal diagnosis) find no significant difference [12, 29]; associated specific difficulties in auditory processing, auditory memory and expressive language result in impaired reading and spelling. Between 20% and 50% of patients have mild to moderate intention tremor. There is a tendency towards behavioural problems, notably immaturity, insecurity, shyness, impaired social judgement and disproportionate assertive behaviour, with difficulty forming peer relationships; the risk of autistic spectrum and schizotypal traits is increased relative to the general population [11]. Specific cognitive deficits in language comprehension and executive function are consistently described as part of the syndrome's neuropsychological phenotype. Smith's Recognizable Patterns describes the opposite pattern ("verbal IQ is usually higher than performance") [11], whereas the review by Tartaglia and colleagues (Table 2: "Verbal IQ < Performance IQ" in 47,XXY, 48,XXYY, 48,XXXY and 49,XXXXY) and the review of the cognitive phenotype by Boada and colleagues (Denver, Toronto and Edinburgh cohorts) describe verbal IQ as lower than or equal to performance IQ, consistent with the expressive language, auditory processing and reading difficulties described in this paragraph [12, 29].

Compared with siblings, there is a modest decrease of 10–15 IQ points, with a wide range (67–133 in the study by Linden and Bender, 2002); approximately two-thirds of 47,XXY boys have greater reading and spelling difficulties than 46,XY peers, almost half require speech and language therapy, and 70% received additional educational support (usually partial support in mainstream school) in the study by Linden and Bender, a proportion of ~60% in the cohort of 19 boys studied by Bishop and colleagues (2011) [2]. Autism rates of ~10% (Bishop et al., 2011) and attention deficit hyperactivity disorder (ADHD) rates of 36% (Tartaglia et al., 2012) have been described in 47,XXY [2]; the 2011 review places ADHD in 47,XXY at 35–45% [12]. Other neurological and psychiatric findings described include dyslexia, deficits in fine motor skills, coordination and praxis, reduced short-term memory, impaired visual information encoding and verbal fluency, epilepsy, schizophrenia and other psychiatric disorders [1]; mild to moderate ataxia occurs occasionally [11]. The syndrome has been proposed as a genetic model of psychotic disorders (DeLisi et al., 2005) [11]. The EAA guideline reports increased risks of schizophrenia, bipolar disorder, depression, anxiety, autism and ADHD in adults, deficits in executive functions (attention, flexibility, planning, response inhibition) and language (verbal processing speed, expressive grammar, word retrieval), difficulty identifying and verbalising emotions and increased distractibility; however, it emphasises that many studies suffer from selection bias because they are based on diagnosed patients, and that undiagnosed cases may have a more neutral phenotype [15].

Source: Table 5.3 of the Oxford Desk Reference, reproduced from Bishop, Jacobs et al., Arch Dis Child 2011 [2]. OR = odds ratio relative to the comparison group; * diagnoses coded as mutually exclusive. National data: Department for Children, Schools and Families, England, 2008.

Endocrine–metabolic system. Hypergonadotrophic hypogonadism is the central biochemical hallmark. There are associated increased risks of type 2 diabetes mellitus (approximately 8% of adults according to classical series, with more recent estimates indicating an even higher prevalence of metabolic syndrome), central obesity and dyslipidaemia [11]. The lipid profile described includes elevated total and LDL cholesterol and reduced HDL, accompanied by insulin resistance, osteopenia and osteoporosis [1]. Central precocious puberty is an occasional abnormality [11]. The EAA guideline describes increased fat mass and reduced lean mass through the combined effect of the genetic background and hypogonadism, with insulin resistance and a predisposition to type 2 diabetes; body fat is already increased in childhood and early adolescence despite normal body mass index and lean mass (DXA, skinfolds), suggesting causes beyond low testosterone [15]. A meta-analysis of 21 observational and 22 intervention studies, cited by the guideline, showed worse fasting glucose, HOMA index, HDL, LDL, body mass index and waist circumference, and lower BMD, in untreated patients than in controls; testosterone replacement improved body composition and lumbar BMD, with less evident effects on lipid and glucose profiles, and does not fully attenuate cardiometabolic risk [15]. Thyroid function does not appear to be affected [15].

Cardiovascular and haemostatic system. There are increased risks of venous thromboembolism, congenital cardiovascular disease, lower-limb vascular insufficiency with pretibial skin ulceration, and varicose veins [11]. Mitral valve prolapse and recurrent venous ulcers are also reported [1]. Cohort studies demonstrate hypofibrinolytic states, with reduced fibrin clot lysis, elevated fibrinogen and factor XIII, and increased plasminogen activator inhibitor type 1 (PAI-1), consistent with a hypercoagulable state attributable, at least in part, to androgen deficiency. The EAA guideline estimates a risk of deep vein thrombosis and pulmonary embolism three to six times that in the general population, partly attributable to elevated PAI-1, linked to both the genetic background and low testosterone; testosterone may reduce this risk, but the evidence is low [15]. The same guideline reports evidence, based on small numbers of patients, of altered cardiac rhythm integrity with shortening of the QTc interval, attributed to excess X gene dosage, potentially predisposing to sudden cardiac arrest [15].

Respiratory system, skin and eyes. Chronic bronchitis is more frequent in adults; severe acne and glaucoma are among the occasional abnormalities [11]. Retinal dysfunction and altered daytime and night-time vision, taurodontism and increased dental caries risk have been described in small series that the EAA guideline considers require validation [15].

Oncological system. Breast cancer risk is 20 to 50 times higher than in men without the syndrome, although, in absolute terms, this corresponds to an incidence of approximately 1 in 5,000 affected men over their lifetime — a figure sufficiently low not to justify systematic mammographic screening, with guided monthly breast self-examination preferred [11]. The Oxford Desk Reference reports a relative risk of 20, mean age at diagnosis of 72 years and an actual risk of ~3%, lower than in women [2]; in the British cohort, the standardised mortality ratio (SMR) for breast cancer was 57.8 (95% CI 18.8–135.0), particularly elevated in 47,XXY mosaicism (SMR 222.8) [30]. There is also an increased risk of extragonadal germ cell tumours (mainly mediastinal), with a relative risk of 67 in the Danish cohort of 696 men (four mediastinal tumours observed, all malignant germ cell tumours) and risk concentrated between early adolescence and 30 years [11, 31]; in absolute terms, the risk of primary mediastinal germ cell tumour is below 1%, potentially presenting with precocious puberty or respiratory symptoms between 10 and 30 years [2]; however, there is no consistent increase in testicular cancer risk. Leukaemia, non-Hodgkin lymphoma and possibly lung and gallbladder cancer have also been associated in cohort series [11]. In the British cohort of 3,518 men, the SMR for all cancers was 1.2 (95% CI 1.0–1.4), with increased mortality from lung cancer (SMR 1.5) and non-Hodgkin lymphoma (SMR 3.5; 36.7 in 48,XXYY men) and zero prostate cancer mortality (SMR 0; 95% CI 0–0.7) [30]; the Brazilian textbook mentions central nervous system cancer and the protective nature of the syndrome against prostate cancer [1]. A Danish study of 696 men with Klinefelter syndrome (Hasle et al., 1995) found no increase in overall cancer incidence and concluded that routine cancer screening was not justified [2, 31]. The EAA guideline summarises the literature as follows: increased cancer mortality, particularly from breast cancer, lung cancer and non-Hodgkin lymphoma (standardised incidence ratio, SIR, of 3.02 in a large cohort, and 3.62 for various forms of leukaemia), lower prostate cancer mortality, and breast cancer incidence 4 to 30 times that of 46,XY men, with a mean age at diagnosis of 58 years (versus 67 years in 46,XY men) and a calculated cumulative risk by age 75 of 1% — low in absolute terms given the rarity of male breast cancer [15]. Extragonadal germ cell tumours arise mainly between 15 and 30 years, are located predominantly in the mediastinum and are usually non-seminomas (mature teratomas or mixed tumours); younger boys may present with precocious puberty due to hCG production and older boys with thoracic symptoms [15]. The relationship with testicular germ cell tumours is undocumented, no cases of germ cell neoplasia in situ (GCNIS) have been described, and the frequency of Leydig cell tumours, which may merely reflect hyperplasia of these cells, does not appear higher than in infertile men [15]. The three sources differ in their breast cancer figures (relative risk 20–50 and approximately 1 in 5,000 in Smith's; relative risk 20, mean age 72 years and risk ~3% in the Oxford Desk Reference; 4–30-fold, 58 years and 1% by 75 years in the EAA), reflecting different cohorts, periods and denominators; the EAA cautions that these epidemiological studies should be interpreted carefully, given the low KS diagnosis rate and small number of longitudinal cohorts [2, 11, 15].

Autoimmune disease. National registry linkage studies demonstrate a significant association between Klinefelter syndrome and autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis and Sjögren syndrome, a finding attributed partly to overexpression of immune regulatory genes escaping X inactivation (for example, TLR7, although this mechanism is more classically described in women with lupus; in 47,XXY men, an analogous gene dosage hypothesis has been proposed) [11, 32]. The EAA guideline considers a higher prevalence of autoimmune diseases likely, while noting that the data derive from small numbers of patients [15].

Comparative table: phenotypic gradient between 47,XXY and higher-grade polysomies (48,XXYY, 48,XXXY and 49,XXXXY)

Methodological note: data on higher-grade polysomies derive from historical case series and reviews (rather than controlled trials), reflecting the extreme rarity of these conditions; interindividual phenotypic variability remains considerable even within each karyotypic category. In the review by Tartaglia and colleagues, the mean age at diagnosis of 48,XXYY appears as 6.8 years in the comparative table and 7.7 years in the text and in the original study of 95 patients [12, 33].

3.1 48,XXYY syndrome

The multicentre cross-sectional study of 95 males with 48,XXYY aged 1 to 55 years (mean age 14.9 years) constitutes the principal data source on this tetrasomy [33]. The mean age at diagnosis was 7.7 years, and the most frequent indication for genetic testing was developmental delay or behavioural problems (68.4%). Tall stature develops during adolescence and adulthood (adult mean 192.4 cm; SD 7.5; n = 22). Hypertelorism, clinodactyly, flat feet and dental problems are common at all ages. The most frequent medical problems were allergies and asthma (> 50%), congenital heart defects (19.4%), radioulnar synostosis (17.2%), inguinal hernia and/or cryptorchidism (16.1%) and seizures (15%); in adulthood, hypogonadism (100%), deep vein thrombosis (18.2%), intention tremor (71%) and type 2 diabetes (18.2%). Brain magnetic resonance imaging (n = 35) showed white matter abnormalities in 45.7% and ventricular dilatation in 22.8%. Developmental delays and learning difficulties were universal, although variable in severity; 26% had full-scale IQ within the intellectual disability range and 68% had adaptive functioning composite scores < 70. Rates of ADHD (72.2%), autism spectrum disorders (28.3%), mood disorders (46.8%) and tics (18.9%) were high, with 55.9% receiving psychopharmacological medication [33]. Increased mortality from non-Hodgkin lymphoma has also been described [12, 30]. Smith's Recognizable Patterns summarises that 48,XXYY individuals have a greater likelihood of intellectual disability and behavioural problems than 47,XXY individuals [11].

The French PNDS (2022) adds further frequency data for 48,XXYY: insulin resistance in 18%, asthma-type bronchial hyperreactivity and allergies in more than half, generalised tremor in 71%, epilepsy in 15%, deep vein thrombosis in 18%, varicose veins and leg ulcers, and inguinal hernia in 16%; it also mentions discussion of an increased risk of autoimmune thyroid disease and dyslipidaemia, and a publication suggesting an excess of non-Hodgkin lymphoma [13]. Motor delay is present in 75% [13]. In the psychiatric domain, anxiety and mood disorders affect 47% (generalised anxiety 21%, obsessive–compulsive disorder 8%, depression 18%, bipolar disorder 8%, cyclothymia 1%), schizophrenia is rare (3%), oppositional defiant disorder affects 10%, tics 19% and autism spectrum disorder 28–52%; one third of patients are admitted to psychiatric care during their lifetime [13]. In adolescence and early adulthood, episodes of violence may worsen and disorders of sexual behaviour may arise, and difficulties managing money expose patients to precariousness [13]. As for the treatment observed, 84% take at least one psychotropic drug and prescribing directed at psychiatric disorders is estimated at 56%, with psychostimulants in 31%, selective serotonin reuptake inhibitors in 18%, antipsychotics in 23%, mood stabilisers (lithium or valproate) in 11% and other drugs (atomoxetine, clonidine, bupropion, buspirone, clonazepam, amitriptyline) in 9%; 40% receive more than one psychotropic drug, and the response to symptom-directed treatments is generally good [13]. Treatment for tremor is given to 4%, for respiratory problems to 38% and for gastrointestinal problems to 13%; testosterone is used by 35% of boys aged 11–14 years and by 65% of those aged 15–19 years, and by only 64% of adults — owing to loss to follow-up, to levels considered normal or to behavioural worsening at the start of treatment, although other authors describe improved behaviour with testosterone [13]. These figures come from series of already diagnosed patients and not from population samples.

3.2 48,XXXY and 49,XXXXY syndromes

The greater the aneuploidy, from XXY to XXXXY, the more severe the growth deficiency, intellectual disability, hypogenitalism and other features; the findings in 48,XXXY extend from the milder XXY traits towards the severe extreme represented by 49,XXXXY [11]. In 49,XXXXY, in addition to the findings described in the table, hypotonia, joint laxity or both occur in approximately one-third of patients, together with a short neck, limited elbow pronation, flat feet, usually mild epiphyseal dysplasia, early degeneration of articular cartilage, hypoplastic seminiferous tubules and reduced Leydig cells [11]. Occasional abnormalities include obesity, a flat occiput, microcephaly, arhinencephaly, corpus callosum hypoplasia, delayed myelination, cortical dysgenesis, cavum septum pellucidum, seizures, downslanting palpebral fissures, Brushfield spots, myopia, cleft palate, cleft lip, small conical teeth, delayed tooth eruption, taurodontism and enamel defects with early loss of anterior deciduous teeth, a webbed neck, preauricular pit, pectus excavatum, cervical rib, gynaecomastia, congenital heart disease, umbilical and/or inguinal hernia, scoliosis, single palmar crease, talipes equinovarus, toe abnormalities with a gap between the first and second toes, intravesical ureterocele, hypospadias, bifid scrotum and growth hormone deficiency [11]. Expressive language is more affected than receptive language, and language-based learning deficits impair social interactions, causing frustration and behavioural problems; declining intellectual performance with age is described, and relatively preserved non-verbal skills can be used to reduce behavioural problems and improve learning [11]. In 7 children with 49,XXXXY screened using the Gilliam scale, no autism concerns were identified; autistic symptoms have not been studied in 48,XXXY [12].

In all three polysomies, facial dysmorphism is variable and often subtle, more apparent in 49,XXXXY; common findings include hypertelorism, epicanthal folds, upslanting palpebral fissures, hooded eyelids, fifth-finger clinodactyly, short nail beds (often with onychophagia), flat feet, joint hyperextensibility and prominent elbows with cubitus varus; dental problems (frequent caries, thin enamel, malocclusion, taurodontism) are very common, and only approximately 30% have gynaecomastia, usually mild [12]. Seizures (10–15%), strabismus, constipation and recurrent otitis are common in childhood; scoliosis, tremor, deep vein thrombosis, pulmonary embolism and type 2 diabetes develop during adolescence and early adulthood [12]. Developmental dyspraxia contributes to early language and motor deficits, and emotional immaturity, anxiety, obsessive–compulsive behaviours, impulsivity, behavioural dysregulation and tics are more frequent than in 47,XXY [12].

3.3 Other sex chromosome tetrasomies and pentasomies in boys (HAS PNDS 2022)

The French PNDS brings seven karyotypes together in a common protocol on account of the features they share: advanced height and weight (except in 49,XXXXY), learning disorders or intellectual disability, behavioural and psychiatric disorders, hypergonadotrophic hypogonadism with genital anomalies and infertility, skeletal and morphological signs, and an excess of comorbidities [13]. The table summarises the karyotypes not described in the previous sections and the PNDS data for 48,XXXY and 49,XXXXY.

In 48,XXYY, the incidence of 1:18,000–1:40,000 male births corresponds to 1:36,000–1:80,000 in the general population, and in the rarer karyotypes the incidence in the general population is half that calculated on male births (1:100,000 to < 1:2,000,000) [13]. The data on 48,XYYY, 49,XYYYY and the mixed pentasomies come from isolated reports, almost always mosaic, and do not allow the frequency of each sign to be estimated; the coexistence of other sex chromosome cell lines modifies the clinical picture and follow-up [13]. In the X polysomies, lack of follow-up in adult services is associated with regression of acquired skills (language, motor skills) and with the absence of prevention of endocrine, psychiatric and ageing-related complications [13]. 47,XYY, excluded from the PNDS, is not developed here.

Reference figures. Photographs of 47,XXY (an untreated 16-year-old adolescent with gynaecomastia; a 9-year-old child with a small penis and long limbs) and 48,XXXY (an adolescent and preadolescent with short stature, facial dysmorphism, elbow abnormalities and hypogonadism) appear in Figures 1 and 2 of the chapter in Smith's Recognizable Patterns of Human Malformation (pp. 66–67) [11]. Open-access photographs of facial features and physical findings (clinodactyly, prominent elbows, flat feet) in 48,XXYY, 48,XXXY and 49,XXXXY are available: Figure 1 of Tartaglia et al., 2011 and Figure 2 of Tartaglia et al., 2011 [12].

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4. Clinical Recognition and Red Flags

Clinical recognition of Klinefelter syndrome should be stratified by age group, since the phenotype evolves throughout development and the presentation "window" differs markedly between childhood, puberty and adulthood. In the Danish register, fewer than 10% of the expected number of cases were diagnosed before puberty, and only approximately one-quarter of affected adult men had been diagnosed [9]. The EAA guideline estimates that 50–75% of men with KS never receive a diagnosis and, among diagnosed individuals, reports 21% prenatally, 10–12% in childhood, 16% at puberty and 51% in adulthood; it recalls that the classical phenotype described in older literature is present only in a minority, that, except for small testes, there are no consistent signs at all ages, and that small firm testes are fairly specific, as is the combination of signs in adults [15]. Screening all male newborns has been proposed (for example, by detecting increased copy number in DNA from dried blood spots on filter paper), but the same guideline acknowledges the lack of empirical evidence that early diagnosis followed by intervention improves adult outcomes [15].

Prenatal period. With the spread of non-invasive prenatal testing (NIPT) based on cell-free fetal DNA circulating in maternal plasma, a progressive increase in the frequency of prenatal diagnosis is expected. Non-specific ultrasound findings, such as increased nuchal translucency or ambiguous genitalia, may also prompt investigation. Parents should be told that an NIPT result positive for 47,XXY constitutes screening, not a definitive diagnosis, and must be confirmed by karyotyping on a sample obtained by amniocentesis or chorionic villus sampling. Prenatal diagnosis of 47,XXY is usually an unexpected finding during amniocentesis or chorionic villus sampling performed for another indication [2, 34].

Childhood. In the prepubertal period, findings are usually subtle or absent, making clinical suspicion difficult. Where present, they may include micropenis, hypospadias, cryptorchidism, slightly small testes, mild global delay in gross and fine motor development, speech and language delay and learning difficulties that become more evident on starting school [11]. Cryptorchidism is the most studied route to recognition: in a series of 600 newborns with isolated cryptorchidism cited by the EAA, KS prevalence was 1.3%, rising to 2.6% in boys still cryptorchid at 12–24 months and reaching 4.2% in persistent bilateral forms versus 1.6% in unilateral forms; chromosomal abnormalities overall affect 3–4% of cryptorchid boys, with KS the most frequent [15]. In childhood, growth may accelerate from 5–6 years, with height above that expected from parental target height and longer legs even before puberty [15]. Minipuberty data are contradictory: some studies showed low testosterone, another normal testosterone but below controls, and a recent study found higher FSH, LH, inhibin B and testosterone than in controls, without differences in height, weight, testicular volume or penile length; before puberty, testosterone, oestradiol, FSH and LH are generally normal, and the low sensitivity of hormonal assays at this age limits interpretation [15]. The syndrome was identified in 0.4% of boys with special educational needs without a previously known diagnosis and in 1.2% of boys with intellectual disability of unknown aetiology, a population in which these patients were initially referred for fragile X syndrome investigation — reinforcing the recommendation that karyotyping should be considered in the diagnostic algorithm for unexplained intellectual disability in boys, alongside (rather than replacing) molecular analysis of FMR1. In polysomies, facial dysmorphism, hypotonia and congenital malformations lead to earlier diagnosis (mean 4 months in 49,XXXXY and 7.7 years in 48,XXYY) [12].

Puberty and adolescence. This is the period of greatest diagnostic value for recognition of the syndrome. Delayed, incomplete or dissociated puberty (for example, pubic hair development without corresponding testicular volume) should prompt in-depth assessment. The most specific finding is small testicular volume associated with absent or incomplete secondary sexual characteristics — testes that remain small and progressively firmer despite pubertal advancement, measured objectively by Prader orchidometer or ultrasound. Pubertal-onset gynaecomastia, sparse facial hair relative to age and family background, and a growth spurt with disproportionately long legs are additional red flags. According to the EAA guideline, puberty begins at a normal age (the onset window does not differ from that of 46,XY boys); the testes grow little and then shrink, gonadotrophins rise to hypergonadotrophic levels from mid-puberty, testosterone and INSL3 cease to rise, inhibin B falls in the year following pubertal onset and becomes undetectable in most by the end of puberty, and the pubertal decline in AMH is later [15]. More than 60% of adolescents have compensated hypergonadotrophic hypogonadism, allowing spontaneous completion of puberty, and approximately 25% have below-normal testosterone; in some cases diagnosis follows prolonged pubertal progression, which should not be confused with delayed pubertal onset [15].

Adulthood. Most cases are still diagnosed at this stage, typically during investigation of couple infertility (azoospermia or severe oligospermia) or persistent/progressive gynaecomastia. Non-specific symptoms of hypogonadism — fatigue, reduced libido, decreased muscle mass and strength, mood changes — may be underestimated or attributed to other causes, contributing to the average diagnostic delay, with diagnosis usually in the third or fourth decade of life. The EAA guideline quantifies the probability of KS by semen category: 3–4% of infertile men, 6% of men with a total count < 10 million per ejaculate and 10–15% of men with non-obstructive azoospermia; most men with KS are azoospermic, approximately 10% have severe oligozoospermia or cryptozoospermia and the prevalence in normozoospermia is practically zero [15]. Infertility assessment leads to diagnosis in approximately 15–20% of identified 47,XXY cases, but no non-mosaic cases of 48,XXYY, 48,XXXY or 49,XXXXY identified through this route have been reported [12].

In the tetrasomies and pentasomies, the French PNDS places diagnosis of 48,XXYY at around 7–8 years of age, prompted in 68% of cases by learning difficulties or behavioural problems; without intellectual disability, diagnosis may come only in adulthood, because learning disorders do not always lead to genetic testing, a delay that impairs access to care and to entitlements and that first-line use of chromosomal array in these disorders should reduce [13]. The signs that prompt testing, at any age, are advanced height and weight, learning disorders or intellectual disability, psychiatric and behavioural disorders, morphological or skeletal features and hypergonadotrophic hypogonadism; in an adult with a mosaic karyotype and mild neurodevelopmental disorders, infertility may be the point of entry, and prenatally the finding arises from malformative ultrasound signs that lead to karyotyping or array [13].

The following table summarises the predominant clinical features by life stage and the proportion of diagnoses at each stage according to the Brazilian textbook [1].

The distribution cited by the EAA, compiled from different studies, gives greater weight to prenatal diagnosis (21% prenatally, 10–12% in childhood, 16% at puberty and 51% in adulthood) [15]; both distributions concern diagnosed cases only and depend on the period and prenatal diagnosis coverage in each population.

The key points in the postnatal history are developmental milestones, speech and language development and schooling; on physical examination, height and pubertal development (according to age) [2].

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5. Diagnosis: Criteria, Tests and Algorithms

Definitive diagnosis of Klinefelter syndrome is cytogenetic. The test of choice is the peripheral blood karyotype, with an explicit recommendation to count at least 50 metaphase cells, a practice that increases sensitivity for detecting low-level mosaicism (for example, 46,XY/47,XXY), which could be missed with standard 20-cell counts. When clinical suspicion of mosaicism is not confirmed in peripheral blood — a plausible situation given the random tissue distribution of cell lines — investigation can be supplemented with karyotyping of another tissue, such as buccal mucosa (buccal smear). Chromosomal microarray also identifies the additional X and/or Y chromosomes in all four constitutions [12]. Sex chromatin testing (Barr bodies in buccal epithelial smears) is no longer used because of false-negative results [1].

Hormonal profile. The expected biochemical pattern in boys of middle school age and adults with Klinefelter syndrome is hypergonadotrophic hypogonadism: reduced or low-normal total (and free) testosterone, accompanied by elevated FSH and LH. In addition, reduced inhibin B and insulin-like factor 3 (INSL3, a marker of Leydig cell function) and, in some cases, slightly elevated oestradiol are observed, with sex hormone-binding globulin (SHBG) comparable to controls [15]. It is important to recognise that this characteristic hormonal profile is usually absent in prepubertal childhood, since the hypothalamic–pituitary–gonadal axis remains relatively quiescent until pubertal onset — explaining why diagnostic suspicion in this age group should rely predominantly on phenotypic findings and karyotype, rather than baseline hormone values. Free and total testosterone samples should be collected in the morning to avoid circadian fluctuations [1].

Prenatal screening (NIPT). NIPT, based on sequencing cell-free fetal DNA circulating in maternal plasma, has a reported sensitivity of approximately 91.0% and specificity of 99.6% for sex chromosome aneuploidies overall, but the positive predictive value (PPV) specifically for 47,XXY varies considerably between cohorts — from approximately 57% to over 90% in different series, reflecting differences in baseline prevalence and laboratory methodology [35]. As NIPT is a screening rather than diagnostic test, every positive result should be followed by confirmatory karyotyping on a sample obtained invasively (amniocentesis or chorionic villus sampling), particularly because discordance between the NIPT result and the true fetal karyotype may occur through several biological mechanisms, including confined placental mosaicism and unsuspected maternal chromosomal abnormalities. Depending on the technology, maternal sex chromosome aneuploidy (including maternal mosaicism) should be considered when NIPT suggests fetal sex chromosome aneuploidy [34]. In a Chinese series of 58,720 pregnancies screened between 2020 and 2024, 252 (0.43%) had a high-risk result for sex chromosome aneuploidy, of which 28 were maternal in origin; among fetal cases, 55 corresponded to 47,XXY, and the overall PPV confirmed by invasive diagnosis was 52.0% [36]. Sex chromosome aneuploidies occur in 1/400 live births and are even more frequent in prenatal diagnosis (1/250–1/300) [34].

Additional assessment. A full multidisciplinary neurodevelopmental assessment is recommended as soon as the diagnosis is established, particularly during childhood and early childhood, including assessment of speech and language, motor function and school performance. Bone densitometry should be considered from late adolescence, given the risk of osteoporosis. Baseline cardiovascular assessment and screening for metabolic risk factors (glucose, lipid profile) are recommended during transition to adulthood, in view of the comorbidity profile described in Section 3.

Source: Table 1.5.3 in Ribeiro (coordinator) [1].

Summary diagnostic algorithm

1. Clinical suspicion (compatible phenotype at any age, infertility, prenatal finding or incidental detection).
2. Request peripheral blood karyotype with a 50-cell count.
3. If karyotype normal and suspicion of mosaicism persists → karyotype in a second tissue (buccal mucosa) or complementary molecular techniques (FISH, array-CGH, sequencing).
4. Hormonal profile (total and free testosterone, LH, FSH, inhibin B, oestradiol, SHBG) — more informative from pubertal onset.
5. Multidisciplinary assessment: paediatric or adult endocrinology, urology/andrology, clinical genetics, neuropsychology, speech and language therapy (where applicable), and reproductive medicine in adulthood.

Diagnostic recommendations of the EAA/ESE guideline (2021)

The EAA guideline uses GRADE terminology: “we recommend” corresponds to a strong recommendation (1) and “we suggest” to a weak recommendation (2); evidence quality is very low (⊕◯◯◯), low (⊕⊕◯◯), moderate (⊕⊕⊕◯) or high (⊕⊕⊕⊕) [15]. The diagnostic recommendations are as follows.

Source: sections 3.1 and 3.2.1 and recommendation list (section 6) of Zitzmann et al. [15].

The guideline adds that karyotyping should be considered with testicular volume < 5 mL even without overt primary hypogonadism, but that other causes of non-obstructive azoospermia or severe oligozoospermia should be considered — particularly central hypogonadism, which may also involve small testes and does not always justify karyotyping [15]. With strong clinical suspicion and a normal karyotype, it suggests investigating mosaicism in a larger number of metaphases, possibly in fibroblasts or by array-CGH, but does not recommend more expensive analyses such as array-CGH for routine use [15]. Regarding NIPT, it notes that sensitivity and specificity data for sex chromosome aneuploidies are sparse, with a tendency towards more false positives, and that NIPT is not diagnostic and requires cytogenetic confirmation; prenatal diagnosis uses conventional karyotyping or array-CGH in chorionic villi or amniotic fluid [15]. Determining the length of CAG repeats in AR may help decide on androgen replacement in patients with signs of hypogonadism and low-normal testosterone, and tests based on hypermethylation and differential gene expression remain experimental, without direct clinical consequences [15]. The guideline does not stipulate a minimum number of metaphases; the 50-cell count and mosaicism exclusion thresholds given in this article do not originate from this guideline (see Hook's statistical thresholds) [37], and the chromosomal array, which Tartaglia's review describes as capable of identifying all four constitutions [12], is considered by the EAA to be a supplementary rather than first-line test [15].

Limitations of cytogenetic tests and report content

Coverage of each method. The number of metaphases determines the mosaicism that karyotyping can exclude: with 95% confidence, 20 cells exclude mosaicism at or above approximately 14%, 30 cells approximately 10% and 50 cells approximately 6%; a 46,XY or 47,XXY cell line below that threshold may be missed [37]. Karyotyping examines only dividing lymphocytes and does not indicate the constitution of the testes, so a non-mosaic blood result does not exclude gonadal mosaicism, and the exceptional natural fertility described in section 2.1 is attributed to cryptic mosaicism [2]. Chromosomal array detects the supernumerary X and Y chromosomes in all four constitutions and, on SNP platforms, may provide information on parental origin and uniparental disomy, but does not detect balanced rearrangements (for example, X–autosome translocations), does not characterise isochromosomes or markers without karyotyping and has limited sensitivity for minor cell lines, so it is confirmed by karyotyping [12]. Interphase FISH with X and Y probes in several hundred nuclei increases sensitivity for mosaicism, and SRY FISH distinguishes the 46,XX male (section 6) [38]. NIPT is a screening test rather than a diagnostic one, with variable positive predictive value and the possibility of maternal aneuploidy or confined placental mosaicism [34–36]. With a convincing phenotype but a normal blood karyotype, a second tissue is studied (buccal mucosa, fibroblasts).

Minimum report content. The report should use current ISCN nomenclature (ISCN 2024), state the number of metaphases counted and analysed and the banding resolution, describe mosaicism with the number of cells of each line in square brackets — for example mos 47,XXY[12]/46,XY[38] —, and identify the supplementary technique and limitations (tissue, mosaicism exclusion threshold) [39]. When diagnosis is made by array, the report provides ISCN array notation with coordinates on the reference genome (GRCh38), and any additional copy number variation is classified according to ACMG/ClinGen standards [40]. HGVS sequence variant nomenclature and ACMG/AMP classification do not apply to the aneuploidy itself; they are only relevant if a second cause is investigated by exome or panel for a phenotype unexplained by the karyotype (for example, severe intellectual disability in a 47,XXY man), in which case the report should state the gene, versioned MANE transcript, HGVS, zygosity and classification, and a variant of uncertain significance is not reclassified by response to treatment or partial segregation [41].

Confirmation and initial assessment in the tetrasomies and pentasomies (HAS PNDS 2022)

Diagnosis is confirmed by identifying two or three supernumerary sex chromosomes on karyotyping of blood (postnatal) or of chorionic villi, amniotic fluid or fetal blood (prenatal) [13]. When the finding results from chromosomal array or from exome or genome sequencing, increasingly used as first-line tests in neurodevelopmental disorders, the PNDS requires blood karyotyping or FISH to confirm the diagnosis and quantify mosaicism [13]. With these first-line tests, it considers discussion of the historical differential diagnoses unnecessary [13]. The initial work-up, adjusted for age, is summarised in the following table [13].

Before prescribing hormones, psychotropic drugs or antiepileptic drugs, the contraindications of each are checked, and bronchial hyperreactivity or asthma must be excluded before a beta-blocker, with a respiratory medicine opinion if necessary [13].

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6. Differential Diagnosis

The most robust discriminator between Klinefelter syndrome and most of its reproductive differential diagnoses is the hormonal pattern of hypergonadotrophic hypogonadism (elevated gonadotrophins), which distinguishes it from central hypogonadotrophic forms, together with definitive karyotypic confirmation.

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7. Treatment and Risk Management Strategies

Treatment of Klinefelter syndrome is multimodal and depends on age, hormonal phenotype and patient priorities (particularly fertility). There is no curative therapy for the underlying chromosomal defect; treatment aims to correct androgen deficiency, mitigate associated comorbidities and — where desired — enable biological fatherhood through assisted reproduction techniques.

7.1 Testosterone replacement in adults

The European Academy of Andrology (EAA) guideline, endorsed by the European Society of Endocrinology (2021), recommends that testosterone replacement in patients with Klinefelter syndrome follow established guidelines for male hypogonadism in general, with haematocrit and prostate-specific antigen (PSA) monitoring and dose adjustment according to clinical and laboratory response [15]. Specifically, the EAA recommends starting testosterone in men with KS and hypogonadism diagnosed according to hypogonadism guidelines, if possible after fertility has been addressed (1, ⊕⊕⊕◯), following usual intervals for clinical assessment, safety parameters (haematocrit, PSA and others) and dose titration (1, ⊕⊕⊕◯), and endocrine assessment every 12 months in adults not receiving testosterone (1, ⊕⊕◯◯) [15]. It considers that treatment of hypogonadism should begin as soon as the diagnosis is made, that the replacement effect may be modulated by the AR CAG polymorphism, and suggests not starting testosterone when TESE is planned, because of possible suppression of gonadotrophins and residual spermatogenesis (2, ⊕⊕◯◯); owing to insufficient data, it neither recommends nor advises against stopping ongoing testosterone before TESE [15]. Human chorionic gonadotrophin (hCG) and antioestrogens (aromatase inhibitors or selective oestrogen receptor modulators), used compassionately in hypogonadal men not treated with testosterone, lack controlled trials and, according to the EAA, cannot be recommended generally [15]. The guideline does not specify testosterone doses or formulations, referring to general hypogonadism guidelines; doses in the following table derive from prescribing information and the Endocrine Society guideline [15, 16, 42]. The Endocrine Society guideline on testosterone therapy in men with hypogonadism (2018) is the most widely adopted complementary reference for formulation selection, monitoring and management of adverse effects [16].

Androgen replacement produces a more masculine physique, increased facial and pubic hair, more goal-directed thinking, improved self-esteem, less fatigue and irritability, and increased libido, strength and bone mineral density [11]; it has a protective effect against osteoporosis risk [2].

Adverse effects and risk management: erythrocytosis/polycythaemia (monitor haematocrit; reduce dose or extend administration intervals if haematocrit > 54%), acne and oily skin, obstructive sleep apnoea (worsening), fluid retention, gynaecomastia (paradoxical, through excess peripheral aromatisation), further suppression of already impaired residual spermatogenesis (relevant to reproductive planning — exogenous testosterone should not be started before sperm or testicular tissue cryopreservation if fertility is a priority), and theoretically stimulation of prostate neoplasm growth — hence the recommendation to monitor PSA in men over 40 years, following the same parameters applicable to any man receiving androgen replacement [16].

7.2 Androgen therapy in children and adolescents

The EAA guideline recommends against testosterone administration to infants and prepubertal boys with KS (1, ⊕⊕⊕◯) and, in early childhood, recommends it only for micropenis (1, ⊕⊕⊕◯), where hormonal assessment may justify low-dose systemic testosterone for a short period or topical dihydrotestosterone (no doses specified in the guideline); it suggests LH and testosterone measurement in the first 2–3 months of life in prenatally diagnosed boys only where this has therapeutic consequences, i.e. in micropenis (2, ⊕◯◯◯) [15]. High-dose testosterone to arrest excessive growth is rarely used because of physical and psychological adverse effects and unknown effects on testicular function [15]. The guideline acknowledges that early testosterone has been proposed mainly in the United States, with improved neurodevelopmental outcomes described in non-randomised, unblinded studies, and concludes that current knowledge does not support this treatment [15]. Trials in prepubertal boys are therefore interpreted differently by their authors and the EAA. A randomised controlled clinical trial (Ross JL et al., J Pediatr, 2017 [51]; Davis SM et al., J Clin Endocrinol Metab, 2017 [52]) assessed low-dose oxandrolone in boys with 47,XXY syndrome, demonstrating improved visuomotor function, psychosocial benefits (reduced anxiety, depression and social problems) and improved cardiometabolic health parameters; there was no effect on cognitive, hyperactive or aggressive behaviour [11]. The double-blind trial randomised 84 boys aged 4 to 12 years to oxandrolone 0.06 mg/kg/day (n = 43) or placebo (n = 41) for 24 months; benefit occurred in 1 of 5 primary outcomes (Bruininks visuomotor scale, p = 0.005) and, in secondary analyses, in the anxiety/depression (p = 0.03) and social problems (p = 0.01) scales of the Child Behavior Checklist [51]. The EAA adds that boys treated with oxandrolone had greater risks of early gonadarche and pubarche and advanced bone age, without differences in testosterone or gonadotrophins, and that the same trial described reductions in body fat, triglycerides and HDL [15]. Oxandrolone is not approved for this indication and its use remains investigational. Furthermore, the FDA withdrew marketing authorisation for oxandrolone in 2023; it is no longer available in the United States and has no centralised authorisation in the European Union [53, 54]. In infants, the hypothesis that testosterone during minipuberty improves the phenotype was tested in TESTO (71 prenatally diagnosed 47,XXY infants, testosterone cypionate 25 mg IM every 4 weeks, three doses, versus placebo): adiposity decreased through lean mass gain, but gonadotrophins and inhibin B were suppressed and there was no short-term motor, cognitive or language effect, so the authors do not support routine infant treatment and long-term follow-up is awaited [55]; an earlier open-label pilot trial with 20 infants had shown a favourable effect on body composition [56]. The decision to start androgen therapy during secondary school (around 12 to 14 years) should be based on direct laboratory evidence of testosterone deficiency or hypergonadotrophism suggestive of such deficiency, rather than age alone. In adolescence, the EAA recommends assessing Tanner stages, pubertal development, testosterone and gonadotrophins, signs and symptoms of hypogonadism, height, weight, waist circumference and body proportions from before expected pubertal onset and subsequently at individualised intervals (1, ⊕⊕⊕◯); it recommends testosterone for delayed puberty and/or signs and symptoms of hypogonadism with low-normal testosterone and supranormal LH (> 2 SD for age), after fertility has been addressed (1, ⊕⊕⊕◯); and suggests not treating adolescents with compensated hypergonadotrophic hypogonadism (2; quality ⊕⊕⊕◯ in the text and ⊕⊕◯◯ in the guideline's own final list), since there are no published data on benefits or harms in this group [15]. There are no controlled testosterone trials during puberty in KS; extrapolation from other hypogonadal adolescents suggests normal completion of puberty and better physical and psychological development, and fertility assessment should ideally precede replacement because exogenous testosterone may suppress residual spermatogenesis [15]. The Oxford Desk Reference recommends referral to paediatric endocrinology around age 10 for growth monitoring and testosterone, FSH and LH measurement, offering testosterone supplementation if hypergonadotrophic hypogonadism is present [2]; the Brazilian textbook advises starting testosterone at the beginning of puberty when increased LH and FSH are observed, to ensure development of secondary sexual characteristics, increased bone mineral density and muscle mass, and prevention of osteoporosis [1]. These two approaches differ from the EAA, which does not consider isolated gonadotrophin elevation a sufficient indication: with compensated hypergonadotrophic hypogonadism and no signs or symptoms of hypogonadism, the EAA suggests no treatment and requires delayed puberty or symptoms with low-normal testosterone and LH > 2 SD, following discussion of fertility, before recommending testosterone [1, 2, 15].

Specific considerations in polysomies. Replacement protocols follow those for 47,XXY, but recommended starting points vary between 11–12 years, mid-adolescence, or elevated LH and/or low testosterone [12]; in 49,XXXXY, Smith's recommends considering replacement at 11–12 years according to the overall situation [11]. Formulation selection should consider the psychological profile: topical preparations for adolescents with needle anxiety, and injections where cognitive deficits, ADHD or poor self-care are present, to ensure adherence [12]. In a study of 22 adults with 48,XXYY, all had laboratory evidence of hypogonadism, but only 64% (14/22) received testosterone because of fears of behavioural worsening; six of the eight untreated individuals had consequences such as osteoporosis. Clinical experience suggests behavioural improvement or no change after testosterone initiation; starting in early puberty with progressively increasing low doses is recommended, and testosterone doses should not be changed simultaneously with psychopharmacological medication, to distinguish the effects of each change [12]. Psychopharmacological medication combined with behavioural therapies is effective in tetrasomies and pentasomies with moderate to severe behavioural or emotional symptoms [12].

7.3 Management of gynaecomastia

When gynaecomastia is significant and causes physical or psychological discomfort, or for cosmetic reasons, surgery (subcutaneous mastectomy or associated liposuction) is the definitive option, since established (fibrotic) gynaecomastia responds poorly to pharmacological interventions alone. Surgical reduction is rarely necessary (1 in 19 in Ratcliffe's study) [2]. The EAA recommends breast examination (with breast ultrasound if needed) to detect gynaecomastia at the first visit and subsequently on an individualised basis, with treatment according to guidelines (1, ⊕⊕⊕◯), refers to its own gynaecomastia guideline and considers that permanent gynaecomastia before or during pubertal transition should raise suspicion of KS and, if necessary, be treated surgically; it justifies the importance of diagnosis and treatment by preservation of self-esteem and body image [15].

7.4 Management of infertility

See Section 8 for a detailed description of assisted reproduction techniques. For infertility management, the Oxford Desk Reference recommends referral to reproductive medicine to consider artificial insemination with donor semen (AID) or medically assisted reproduction with in vitro fertilisation (IVF) and ICSI, using sperm obtained from the ejaculate or testicular biopsy [2].

7.5 Neurodevelopmental and psychosocial support

Physiotherapy, early stimulation programmes, speech and language therapy and individualised educational support (specialised educational plans) should be offered according to needs identified during the initial multidisciplinary assessment and reassessed periodically throughout development. Parents should be advised to have a low threshold for seeking professional help with developmental, behavioural or learning difficulties, since early intervention improves prognosis [2]. Disclosure of the karyotype is generally recommended on a "need-to-know" basis, so that the child is not treated or perceived differently; disclosure to the child should be a gradual process over several years, with healthcare professionals supporting parents (Linden et al., 2002) [2, 34]. In polysomies, early intervention therapies are particularly indicated because early speech and motor delays are almost universal [12]. In prepubertal children and adolescents, the EAA recommends speech and language monitoring and therapy, monitoring of learning and school difficulties, social training and psychological support where needed (1, ⊕⊕⊕◯), and, in adults, consideration of psychosexual and psychiatric issues with specialist referral where needed (1, ⊕⊕◯◯) [15]. It also suggests attention to possible gender incongruence, with specialist follow-up in a multidisciplinary setting (2, ⊕◯◯◯): these patients may not want testosterone and may seek feminisation, and feminising hormone treatment may increase thrombosis risk, already elevated in KS [15]. Organisationally, it recommends multidisciplinary centres or structures (geneticists, paediatricians and paediatric endocrinologists, psychologists, speech and language therapists, adult endocrinologists and andrologists, urologists, reproductive gynaecologists, sexologists and psychiatrists), improved transition from paediatric to adult endocrinology and structured KS teaching in undergraduate and postgraduate medical training (all three 1, ⊕⊕⊕◯) [15].

Source: adapted from Table 1.5.4 in Ribeiro (coordinator) [1].

7.6 Risk management strategies by system

7.7 Pivotal trials and studies and limitations of comparison

Limitations of comparison. These studies do not answer the same question and should not be ranked against one another: paediatric trials use intermediate outcomes (fat mass, motor and behavioural scales) in small samples with short follow-up, and the only hard clinical outcome (mortality) comes from an observational cohort, subject to confounding by indication (diagnosed and treated men may differ in access to care, smoking and physical activity), so the relative reduction of 44% cannot be converted into a causal effect or an individual absolute benefit [58]. The oxandrolone trial was positive in only 1 of 5 primary outcomes, requiring caution in interpreting secondary outcomes [51]. In TESTO, the body composition effect accompanied gonadal axis suppression without neurodevelopmental benefit, and the open-label 2019 pilot without placebo is not comparable to TESTO [55, 56]. The 44% sperm retrieval rate is an average per cycle, with substantial heterogeneity between centres, rather than each man's individual probability [57]. The TRAVERSE trial, which underpinned the 2025 US prescribing information changes, was conducted in middle-aged and older men with hypogonadism, rather than men with KS; extrapolation to this population is indirect [59]. Ongoing trials without published results include a phase 4 testosterone gel versus placebo trial during puberty in boys with KS (NCT06294990; 32 planned participants) and a phase 3 trial of anastrozole, semaglutide and human chorionic gonadotrophin on sex steroid balance in men with KS (NCT05586802; 150 planned) [60].

7.8 Comparative regulatory status: United States and European Union

No medicine is authorised specifically for Klinefelter syndrome; testosterone is authorised for primary male hypogonadism, and several US prescribing information documents name Klinefelter syndrome among the causes of congenital testicular failure. In the European Union, no testosterone for male use has centralised authorisation (the EMA data file lists only withdrawn female products), so brands depend on national, mutual recognition or decentralised authorisations, with national SmPCs [54]. Authorisation does not guarantee marketing, reimbursement or access: in Portugal these depend on marketing and the funding assessment by INFARMED; in Brazil, on ANVISA registration and incorporation into the SUS.

7.9 Evidence extrapolation and off-label use

Off-label use is defined by prescribing information, age and jurisdiction. In adult men with KS and confirmed hypogonadism, testosterone is within the authorised indication in the United States and European Union. In adolescence, most prescribing information (gel, IM and oral undecanoate, Xyosted®) does not establish safety below 18 years, so pubertal induction or maintenance with these products constitutes use outside the authorised age indication, even where recommended by guidelines; US generic enanthate, which covers pubertal delay, and cypionate, whose safety is unestablished only below 12 years, are partial exceptions [15, 59]. In infants, testosterone has been used only in trials, and TESTO does not support routine use [55]. Oxandrolone in prepubertal boys is investigational and no longer has an authorised product in the United States [51, 53]. In polysomies (48,XXYY, 48,XXXY, 49,XXXXY), there are no dedicated trials: replacement follows 47,XXY protocols by extrapolation, and fertility, doses and monitoring should be presented to families as extrapolation rather than condition-specific evidence [12]. Aromatase inhibitors, chorionic gonadotrophin and semaglutide used before micro-TESE or for the metabolic profile are off-label for these purposes and under evaluation in a trial [57, 60].

7.10 EAA surveillance schedule by life stage

The EAA guideline summarises parameters to assess from childhood to adulthood in two tables; the following table combines them, retaining the intervals specified by the guideline itself [15].

Source: Tables 1 and 2 of Zitzmann et al. [15]. In the growth and vitamin D rows, guideline table 1 repeats the same text for childhood and adolescence.

Differences between sources. The EAA (2021) and other sources in this article differ on points with practical consequences. (i) Testosterone in prepubertal boys: the EAA recommends no treatment except for micropenis, whereas US trials of oxandrolone and infant testosterone tested this treatment — TESTO, subsequent to the guideline, also does not support routine use [15, 51, 55]. (ii) Testosterone initiation in adolescence: the Oxford Desk Reference and Brazilian textbook associate initiation with elevated gonadotrophins, whereas the EAA suggests not treating compensated hypogonadism [1, 2, 15]. (iii) Breast surveillance: Smith's proposes monthly self-examination and rejects screening mammography, the EAA proposes biennial clinical examination with possible mammography or ultrasound for family history or suspicion, and neither proposes systematic mammography [11, 15]. (iv) Fertility preservation in minors: the EAA recommends against testicular tissue or spermatogonial cryopreservation in prepubertal children and restricts adolescent TESE to selected cases, whereas some literature considers immature tissue cryopreservation a justifiable experimental option (section 8) [15, 61, 62]. (v) Gender identity: the Oxford Desk Reference states that sexual and gender orientation do not differ from those of 46,XY men, and the EAA highlights gender incongruence described in some studies [2, 15]. (vi) Testosterone monitoring: the EAA defines no specific intervals or doses and refers to hypogonadism guidelines such as the Endocrine Society guideline [15, 16]. The EAA also acknowledges the scarcity of randomised trials in almost all domains and some internal grading inconsistencies between the text and final list (QTc, adolescent compensated hypogonadism), noted above.

7.11 Management of the tetrasomies and pentasomies according to the French PNDS (2022)

Testosterone. The PNDS sets out both sides of the debate on minipuberty: testosterone in this period contributes to brain maturation, including the development of areas such as the left inferior parietal lobule, and some studies suggest better neurodevelopment, language and behaviour with treatment at around 6–12 months, whereas others point to a deleterious medium-term effect on testicular volume and penile length, through reduced Sertoli cell activity [13]. Some endocrinologists propose testosterone in early childhood to increase penile length, an option to be weighed against these data [13]. At puberty, testosterone improves hormonal balance (body hair, voice, muscle mass, prevention of breast development), bone mineralisation, strength, fatigue and exercise tolerance [13]. In the polysomies, FSH is generally normal until 8 years of age but rises earlier than in 47,XXY (sometimes as early as 8 years), and AMH and inhibin B, markers of Sertoli function, may fall very early; genital hypoplasia and cryptorchidism may reflect insufficient testosterone in fetal life [13]. Some endocrinologists start testosterone enanthate or undecanoate systematically at the onset of puberty, others are guided by testosterone, LH and FSH; treatment may continue into adulthood, with monthly, three-monthly or other injection intervals adjusted to each patient, and prescribing is individualised, if necessary in a multidisciplinary meeting [13]. In France, at the time of the PNDS, three-monthly undecanoate was not reimbursed, testosterone patches and capsules had ceased to be available in 2016, and the remaining gels, considered less effective and not reimbursed for this indication, carry a risk of transfer through contact [13]. In patients with bipolar or psychotic disorder, testosterone may destabilise the condition and worsen disorders of sexual behaviour: it is started slowly, with close psychiatric monitoring and without changing other psychotropic drugs or the environment at the same time [13]. The doses of the formulations are given in the table in section 7.1.

Psychotropic drugs. No specific psychotropic drug is recommended; the response to the usual treatments for inattention, impulsivity, anxiety and mood instability is generally good, with a favourable effect on schooling, well-being and social integration [13]. Psychostimulants, selective serotonin reuptake inhibitors, atypical antipsychotics and mood stabilisers are used, directed at the comorbidities and in accordance with the recommendations and marketing authorisations of each drug; an initial psychiatric opinion is recommended, given the scarce information on efficacy and tolerability in these patients, together with coordination between prescribers, with attention to endocrine and metabolic effects [13]. The PNDS accepts off-label use of aripiprazole for severe behavioural disorders associated with neurodevelopmental disorders, with metabolic monitoring, and of clozapine for very severe behavioural disorders, with full blood counts because of the risk of leuconeutropenia [13]. Before a psychotropic drug for behavioural purposes, or in the event of lack of efficacy, a somatic or environmental cause is sought (pain, a dental or ear, nose and throat problem, constipation, an unexpected change, lack of means of communication), especially in patients with limited communication [13].

Other drugs. For tremor, the PNDS proposes propranolol, amantadine or primidone; for asthma, gastro-oesophageal reflux, constipation, diabetes, hypothyroidism and deep vein thrombosis there are no specific recommendations, and the general guidelines are followed [13]. Antiepileptic drugs require prior checking of contraindications and precautions, which are frequent in patients with digestive and psychiatric problems [13]. In very severe disorders of sexual behaviour that do not respond to environmental, educational and psychological interventions, an antiandrogen — mainly cyproterone acetate or a GnRH analogue — may be considered after multidisciplinary discussion, taking into account the negative effect on bone mineralisation and, with cyproterone, the risk of meningioma, which led to prescribing being restricted [13]. Since 2020, the European Medicines Agency has restricted medicines containing 10 mg or more of cyproterone per day, in men, to the reduction of sexual drive in paraphilias when other options are not appropriate, with monitoring for signs of meningioma and permanent discontinuation if one is diagnosed; the risk, between 1 and 10 per 10,000 patients, increases with cumulative dose [63]. As the PNDS gives no doses for these drugs, the following table presents the label doses, with the corresponding extrapolation.

Rehabilitation and support. Psychomotor therapy strengthens muscle tone, facilitates motor acquisitions and treats coordination disorders and atypical sensory profiles; occupational therapy teaches activities of daily living broken down into subtasks and, when fine motor skills and intention tremor make handwriting slow, introduces the keyboard early and later the tablet [13]. Speech and language therapy begins at the first signs of delay and, in the absence of verbal language, uses pictograms and signs (PECS, Makaton) or partial sign language [13]. Psychological support covers the patient, parents and siblings; cognitive remediation programmes for attention and social cognition are under development, and the behavioural and cognitive behavioural therapies used in autism may improve behaviour; for anxiety, the PNDS mentions animal-assisted therapy and therapeutic horse riding [13]. The plan is completed by adapted physical activity (prevention of osteoporosis and excess weight), orthoptics (exophoria, esophoria), podiatry (insoles), physiotherapy (orthopaedic problems, hypotonia) and oral health with orthodontics [13].

Surgery. Testicular prostheses are an option when testicular size is a source of distress, after careful assessment of expectations in view of the irreversibility of the procedure; mastectomy is indicated mainly when adolescent gynaecomastia causes psychological distress; inguinal hernias, clubfoot and other malformations are also operated on [13].

Emergency cards. The PNDS includes emergency cards for three karyotypes, with fields for contacts, allergies, treatments and means of communication [13].

Divergences between the PNDS and the other sources. (i) Incidence of 48,XXXY: 1:50,000 male births in the PNDS, versus 1:17,000–1:50,000 in the review by Tartaglia and colleagues [12, 13]. (ii) Behaviour and IQ in 48,XXXY: the PNDS describes passive patients without aggression, with an IQ of 40–60, whereas the comparative table in section 3, based on the 2011 review, records irritability, aggression and a mean IQ of 40–75; the series are small and the two descriptions coexist in the literature [12, 13]. (iii) Gonadal timeline: according to the PNDS, FSH may rise from 8 years of age and AMH and inhibin B fall very early, whereas the 2011 review describes gonadotrophins in the prepubertal range until the onset of puberty (section 2.3) [12, 13]. (iv) Start of testosterone: the PNDS accepts systematic initiation at the onset of puberty, the practice of some French endocrinologists, whereas the EAA, for 47,XXY, suggests not treating compensated hypogonadism without signs or symptoms; the PNDS also mentions testosterone in early childhood for penile length, which the EAA reserves for micropenis [13, 15]. In the polysomies, in which androgen deficiency appears earlier, the PNDS position does not directly contradict that of the EAA, which does not address these karyotypes. (v) Psychotropic drugs in 48,XXYY: the PNDS cites 84% on at least one psychotropic drug and 56% on treatment for psychiatric disorders, and the series of 95 patients records 55.9% on psychopharmacological medication (section 3.1) [13, 33]. (vi) The PNDS sentence on breast surveillance attributes the karyotype "47,XYY" to Klinefelter syndrome, a typographical error (the karyotype is 47,XXY); the PNDS does not describe an excess of breast cancer in the polysomies and recommends only clinical attention [13].

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8. Reproductive Options and Family Counselling

Infertility is the most consistent and universal manifestation of non-mosaic Klinefelter syndrome, resulting from progressive hyalinisation and fibrosis of seminiferous tubules. Historically considered absolutely sterile, men with Klinefelter syndrome began, in the late 1990s and early 2000s, to gain access to biological fatherhood through the combination of testicular sperm extraction (TESE, or its microsurgical variant, micro-TESE) with intracytoplasmic sperm injection (ICSI). Occasionally some spermatozoa are found in the testes even when absent from the ejaculate [2].

Sperm retrieval rates. Published series and meta-analyses report a mean surgical sperm retrieval rate of approximately 40 to 50% of micro-TESE procedures in men with non-mosaic Klinefelter syndrome, with variation between studies of approximately 28% to 70%, reflecting differences in surgical technique, patient age, previous adjuvant hormonal therapy (for example, aromatase inhibitors to optimise the testosterone/oestradiol ratio before the procedure) and selection criteria [57]. In the meta-analysis by Corona and colleagues (37 studies; 1,248 patients with mean age 30.9 years), the retrieval rate per TESE cycle was 44% (95% CI 39–48%), without a difference between conventional TESE (43%) and micro-TESE (45%); the pregnancy rate was 43% (218 biochemical pregnancies in 410 ICSI cycles) and the live birth rate 43% (95% CI 34–53%), with no influence of age, testicular volume, LH, FSH or testosterone, or of the use of fresh or cryopreserved sperm [57]. A reference retrospective series (Schiff JD et al., J Clin Endocrinol Metab, 2005 [72]) reported adequate sperm retrieval for ICSI in 70.4% of TESE attempts in a dedicated cohort, with a clinical pregnancy rate of 58% and 14 live births from 19 cycles with retrieved sperm — data which, although from a single highly experienced centre, illustrate the potential of the technique. Greco and colleagues (2013) reported 38 azoospermic patients with non-mosaic KS, with sperm retrieval in 15 (~40%), 26 ICSI cycles (16 with cryopreserved sperm), 15 pregnancies and 16 children born [2]. Larger population studies comparing different aetiologies of non-obstructive azoospermia place the retrieval rate in men with Klinefelter syndrome at approximately 42% — lower than in obstructive causes, but comparable to or higher than other causes of idiopathic non-obstructive azoospermia. Patient age at the time of the procedure does not appear, on its own, to be a consistent predictor of sperm retrieval success according to several series [57]. Donor insemination and adoption remain relevant options [2]. The EAA guideline, citing another synthesis, reports cryopreservable motile sperm in the ejaculate in approximately 10% of men with KS, testicular retrieval in 30–60% and an assisted reproduction live birth rate of approximately 16%, without a difference between conventional TESE and micro-TESE and with apparently lower ICSI success than in the general population (without direct comparison) [15]; the difference from the 43% live birth rate per ICSI cycle in Corona's meta-analysis results, at least partly, from different denominators (per ICSI cycle with sperm versus an overall rate not defined in the guideline) [15, 57]. The EAA recommends semen analysis and sperm cryopreservation in all adults with KS who desire fatherhood (1, ⊕⊕⊕◯), and multifocal conventional TESE or micro-TESE with cryopreservation in all adults with confirmed azoospermia and a current or future desire for fatherhood (1, ⊕⊕⊕◯) [15]. Whether the probability of finding foci of spermatogenesis decreases with age remains debated; in adults, no clinical, biochemical or hormonal parameter has been consistently associated with successful retrieval [15].

Sperm aneuploidy. In spermatozoa retrieved from men with Klinefelter syndrome, the aneuploidy rate is slightly increased relative to the general population, estimated at approximately 5%, reinforcing the recommendation for preconception genetic counselling and, where available and desired by the couple, consideration of preimplantation genetic testing (PGT-A) of the embryos generated. Hennebicq and colleagues (2001) found higher frequencies of 24,XX and 24,XY spermatozoa than in controls and a much higher frequency of disomy 21 (6.2% versus 0.4%); other authors demonstrated that, where focal spermatogenesis exists in non-mosaic KS, it originates from euploid germ cells and produces normal mature gametes — and, indeed, the vast majority of children born to fathers with non-mosaic KS are chromosomally normal (Greco et al., 2013) [2]. The EAA considers that, since offspring do not appear affected by the father's genetic condition, it is unclear whether PGT or prenatal diagnosis is necessary, but declares genetic counselling mandatory [15].

Fertility preservation in adolescents. Since spermatogonial depletion begins even before the end of puberty, early cryopreservation has been proposed — of ejaculated sperm (where present in late puberty), or immature testicular tissue containing spermatogonial stem cells (SSC), in protocols that remain experimental. Testicular biopsy studies in adolescents with Klinefelter syndrome demonstrate extensive hyalinisation and fibrosis already at this stage in practically all patients assessed, although residual spermatogonia may be identified in a minority, particularly younger individuals with preserved Sertoli cell function (normal FSH and inhibin B) [61]. The scientific community remains divided over routinely recommending this procedure in asymptomatic children and adolescents: some groups consider immature testicular tissue cryopreservation ethically justifiable as an experimental fertility preservation option, while others argue that the absence of a demonstrated increase in success rates, combined with the invasive and still experimental nature of the technique (without proven generation of functional mature sperm from cryopreserved immature human tissue), does not justify systematically offering it to minors unable to provide full consent [61, 62]. This is an area of active controversy and developing research; decisions should be individualised, shared with the family and, wherever possible, involve the adolescent. Work by Rives and colleagues (2013) suggests a critical point shortly after pubertal onset when sperm may be detected in the ejaculate or at least testicular tissue, allowing extraction, cryopreservation and subsequent ICSI use (Kliesch et al., 2011); the testis may also harbour spermatogonia and incompletely differentiated germ cells [2]. The systematic review by Ly and colleagues (six studies, patients aged 13 to 24 years) concluded that no current recommendation exists on the minimum age for proposing fertility preservation [61]. The EAA guideline takes a position: it recommends against cryopreservation of testicular tissue or spermatogonial stem cells in prepubertal children with KS (1, ⊕⊕⊕◯), because no in vitro spermatogenesis method exists; recommends informing adolescents (and parents, where appropriate) about fertility, as ungraded good clinical practice; suggests testicular ultrasound at puberty and subsequent visits (2, ⊕◯◯◯), semen collection with cryopreservation where motile sperm are present, following careful information and assessment of the adolescent's own wishes (2, ⊕⊕◯◯), and TESE or micro-TESE with cryopreservation in selected cases, with specific counselling, if physical and mental maturity permit (2, ⊕⊕◯◯) [15]. The decision should belong to the adolescent rather than the parents; grading is weak because not all adolescents are psychologically prepared and success rates are similar when procedures are performed later [15]. In data cited by the EAA, TESE found sperm in 10% of attempts at 13–14 years and 45% at 15–19 years, comparable to men aged 20–30 years; adolescents with LH ≤ 17.5 IU/L and testosterone ≥ 7.5 nmol/L had the highest success rate (54%), falling to 44% with higher LH, and there was no retrieval with testosterone < 7.5 nmol/L regardless of LH [15]. The caption to guideline Figure 2 (histological sections from an adult with Sertoli-cell-only tubules, nodular Leydig hyperplasia and hyalinised tubules, and a non-mosaic 47,XXY 18-year-old with elongated spermatids in some tubules) describes this focal pattern: Figure 2 of Zitzmann et al., 2021 [15]. In tetrasomies and pentasomies, lower cognitive abilities, social deficits and limited expressive language make consent issues even more delicate, requiring individual assessment of each adolescent [12].

Genetic counselling and recurrence risk. Since the vast majority of Klinefelter syndrome cases result from sporadic nondisjunction events rather than a structured hereditary predisposition, recurrence risk in future pregnancies of the same couple is slightly increased relative to the general population but remains low in absolute terms — analogous to the small increase in recurrence risk observed in other sporadic aneuploidies. Recurrence risk is estimated at < 1% [2] or 1% [1], probably no higher than the maternal age-specific risk, and parents of 47,XXY boys are not routinely karyotyped [2, 34]. Genetic counselling should therefore provide reassurance proportionate to this low risk while offering non-invasive prenatal screening or invasive diagnosis in subsequent pregnancies, according to the parents' informed preference. The indication for invasive prenatal diagnosis is weak, with a delicate balance between the very low recurrence risk and procedure-associated pregnancy loss risk; NIPT may be offered as an alternative where available [2, 34]. For patients with Klinefelter syndrome themselves who achieve biological fatherhood through TESE/ICSI, preconception genetic counselling should address the slightly increased risk of autosomal and sex chromosome aneuploidies in offspring arising from the higher sperm aneuploidy rate already mentioned, with PGT-A a tool to discuss rather than an absolute requirement. Prenatal diagnosis is indicated when a man with KS fathers a pregnancy [1, 2].

Counselling in the tetrasomies and pentasomies. According to the French PNDS, these karyotypes, homogeneous or mosaic (with 46,XY cell lines or with other sex chromosome abnormalities), almost always arise sporadically and are not inherited; parental karyotyping is unnecessary, although it may be performed on request, and healthy relatives are given the general population risk [13]. The parents of an affected boy are given a recurrence risk below 1%, corresponding to the theoretical risk of germline mosaicism, and prenatal diagnosis by karyotyping of chorionic villi (from 11 weeks of gestation) or amniotic fluid (from 15 weeks) may be discussed for reassurance [13]. Only one paternal transmission has been described — a 48,XXYY boy whose father was 47,XYY, with supernumerary X and Y of maternal and paternal origin — an isolated case that does not demonstrate an increased risk in the offspring of 47,XYY men, in whom greater production of 24,XY and 24,YY spermatozoa is possible [13]. For the patient himself, the risk of transmitting the sex chromosome aneuploidy is probably very low and has not been demonstrated; data from 47,XXY suggest an excess of autosomal aneuploidies, such as trisomy 21, owing to an abnormal spermatogenic environment, a controversial finding similar to that observed in secretory azoospermia with a normal karyotype [13]. Regarding fertility, the PNDS recommends, in patients without severe cognitive impairment, semen analysis in a reproductive biology laboratory as soon as collection is possible and gamete self-preservation if spermatozoa are present; in the context of a parental project, after semen analyses and testicular biopsy or aspiration, IVF with ICSI is proposed if spermatozoa are found, sperm donation if they are not, or adoption, while also assessing the couple's capacity to raise the child, with specific parenting support [13]. The diagnosis is disclosed in a dedicated consultation, preferably with a psychologist, followed by another consultation a few weeks later; at prenatal disclosure, all options are presented to the couple, including termination of pregnancy for medical reasons, with referral to a multidisciplinary prenatal diagnosis centre [13].

Counselling after prenatal diagnosis. Prenatal diagnosis of a sex chromosome aneuploidy is almost always incidental. The recommended approach includes obtaining the pregnancy history and indication for the invasive procedure, emphasising where appropriate that the information obtained was not what was being sought; explaining chromosomes and the unexpected finding; describing the identified aneuploidy and what the couple can expect if they continue the pregnancy (a normally appearing baby, mainstream schooling in most cases) and the follow-up plan (for example, referral of a 47,XXY boy to paediatric endocrinology); exploring the couple's hopes and fears and the validity of information obtained elsewhere; emphasising the rarity of diagnosis compared with population frequency, reflecting how infrequently these individuals reach specialist care; and exploring pregnancy options while discouraging hasty decisions [34]. As Biesecker wrote (BMJ, 2001), the loss experienced by parents after a prenatal diagnosis is often not that of the diagnosed fetus, but of the fetus they expected, and this grief does not preclude welcoming the affected child [34].

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9. Special Considerations: Population Diversity and Epidemiology

Klinefelter syndrome has a relevant classificatory peculiarity: Orphanet explicitly does not include it in its rare disease nomenclature, because it does not meet the European Union rarity criterion (prevalence below 1 in 2,000) — the 47,XXY form, with estimated prevalence between 1 in 500 and 1 in 660, therefore falls within the category of a common genetic condition, albeit widely underdiagnosed [3, 4]. Higher-grade polysomy variants — 48,XXXY (ORPHA:96263) and 49,XXXXY (ORPHA:96264) — remain classified as rare diseases, with estimated birth prevalence for 49,XXXXY of 0.55 per 100,000 (≈ 5.5 per million; class 1–9 per 1,000,000) in Europe [5, 6]. The 48,XXYY constitution (ORPHA:10) has an estimated prevalence of 1:18,000–1:40,000 male births [7, 33].

Regional data gaps. The available literature on Klinefelter syndrome is heavily concentrated in cohorts of European (particularly Danish and British) and North American origin. No robust, specific population-based epidemiological data on the prevalence of Klinefelter syndrome in Latin America, sub-Saharan Africa, the Middle East or Southeast Asia were located in the sources consulted; most of the information available for these regions is limited to isolated case reports or small clinical series (for example, case reports from Iran, Sri Lanka and China identified in this search), without population-based studies allowing regional prevalence to be estimated with statistical confidence. Similarly, the literature consulted did not identify a specific population founder effect or a systematically documented ethnic/racial variation in the frequency of the nondisjunction that gives rise to Klinefelter syndrome — unlike many monogenic diseases, sporadic chromosomal aneuploidy does not, as far as current evidence allows us to state, follow a particular pattern of ethnic or geographical aggregation; the main risk variable consistently identified in the literature is advanced maternal age (for cases of maternal origin), rather than ancestry. This regional data gap should be explicitly acknowledged rather than filled by extrapolation. Estimates from the Global Burden of Disease (GBD) 2021 study for individuals under 20 years illustrate the weight of diagnostic capacity: between 1990 and 2021, the global number of recorded paediatric cases rose from 589,674 to 690,885 (a 17.2% increase), with stable prevalence (26.1 and 26.2 per 100,000), but with rates of 17.1 per 100,000 in low sociodemographic index (SDI) countries and 32.5 per 100,000 in high-SDI countries — a difference attributed to diagnostic capacity and access to genetics services rather than to biological variation [73]. The Brazilian textbook Síndromes Genéticas: Reconhecer para Tratar reports a prevalence of 1/448 male births, without indicating whether this is Brazilian population data [1].

Underdiagnosis as a global phenomenon. A finding across practically all cohorts analysed, regardless of region, is pronounced underdiagnosis: only an estimated 25% of men with Klinefelter syndrome receive a lifetime diagnosis and, among those diagnosed, only half receive adequate testosterone replacement, according to the latest Danish study [9, 58]. In the Danish register for 1970–2000, 696 men among 2,480,858 live births were diagnosed postnatally (prevalence of ~40 per 100,000 diagnosed adult men, versus 153 per 100,000 expected) [9]. This underdiagnosis and undertreatment pattern appears to be global, resulting from the frequently subtle or non-specific phenotype rather than access barriers restricted to particular healthcare systems — although it is reasonable to presume that karyotyping, prenatal screening and specialised multidisciplinary care are significantly less available in lower-income regions, a hypothesis not systematically quantified in the reviewed literature. For polysomies, a higher proportion is presumed to be diagnosed during life, given the greater frequency of dysmorphism, malformations and developmental delays, but equivalent studies do not exist [12].

Special populations. Within the syndrome spectrum, two populations warrant specific mention: (i) prenatally diagnosed individuals, whose numbers tend to grow with the spread of NIPT and who, according to observational data, appear to have less severe phenotypes and better neurodevelopmental prognosis — attributed both to possible ascertainment bias (fetuses incidentally diagnosed during autosomal aneuploidy screening tend to have milder phenotypes than those diagnosed on clinical grounds) and the benefit of early intervention; and (ii) individuals with 46,XY/47,XXY mosaicism, representing approximately 15% of cases and tending to have residual testicular function and a milder overall phenotype, although — as discussed — the proportion of mosaic cells in peripheral blood does not accurately predict the individual phenotype [11].

Patient and family support organisations. AXYS – Association for X and Y Chromosome Variations (United States, founded in 1989, formerly Klinefelter Syndrome and Associates) is the largest international advocacy, education and support organisation for individuals with X and Y chromosome variations and their families, offering individual telephone support, biennial community conferences and a clinical and research consortium (AXYS Clinic and Research Consortium) [18]. In Europe, notable organisations include Germany's 47xxy Klinefelter Syndrom e.V. and the United Kingdom's Klinefelter Syndrome Association (http://www.ksa-uk.net), alongside Unique – The Rare Chromosome Disorder Support Group (http://www.rarechromo.org), which produces a detailed information leaflet [2], among other national organisations with similar scope. In the European Union, the relevant European reference network is Endo-ERN (European Reference Network on Rare Endocrine Conditions), through the “Sex Development & Maturation” thematic group, which covers sex chromosome aneuploidies; for polysomies with intellectual disability or malformations, ERN ITHACA (the European network for rare malformation syndromes and rare intellectual developmental and neurodevelopmental disorders) is the complementary network [19, 20]. In the United States, besides AXYS, Living with XXY focuses specifically on 47,XXY [55]. These organisations play a relevant role in reducing stigma, disseminating updated information and providing continuing psychosocial support to patients and families.

Organisation in France. The PNDS on sex chromosome tetrasomies and pentasomies in boys (Haute Autorité de Santé, 2022) was coordinated by the Dijon reference centres ("Déficiences intellectuelles de causes rares" and "Anomalies du développement et syndromes malformatifs" of the Eastern interregion) and by the "Maladies rares à expression psychiatrique" competence centre [13]. Follow-up is coordinated by a geneticist, (neuro)paediatrician or (child) psychiatrist at a reference or competence centre of the AnDDI-Rares or DéfiScience networks, with the FIRENDO network for the endocrine aspects; the family doctor refers patients to these centres, ensures multidisciplinary follow-up and monitors for complications [13]. The coordination and guidance platforms for neurodevelopmental disorders, with reimbursed assessments extended in 2021 up to 12 years of age, and level 3 centres (language and learning disorders; autism resource centres) support the assessment [13]. The care protocol allows recognition as a long-term condition (ALD 31 or 32), sometimes from birth, and the departmental centre for people with disabilities (MDPH) grants benefits and educational and vocational guidance; schooling takes place in mainstream settings with human support and a personalised plan or in specialised settings, and in adulthood vocational guidance, place of residence and legal protection are discussed [13]. Therapeutic education may be offered at any stage, including the transition programme "En route vers l'autonomie" of the AnDDI-Rares and DéfiScience networks for patients with mild intellectual disability, and the Valentin APAC association supports families and advocates assessment of neurodevelopmental disorders rather than their automatic attribution to the syndrome [13]. The relevant European networks are ERN ITHACA, for intellectual disability and malformation syndromes, and Endo-ERN, for the endocrine aspects [19, 20].

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10. Illustrative Clinical Case

10.1 Composite clinical vignette: adolescent with dissociated puberty

The following case is a composite clinical vignette, illustrative and didactic in nature, developed for educational purposes; it does not correspond to a specific real patient.

Identification: M.S., male, 16 years old, referred to the paediatric endocrinology clinic by his attending paediatrician.

Chief complaint: "My son is not developing like his peers."

History of the presenting illness: His mother reports that the patient has always been slightly taller than his classmates, with proportionally long legs relative to his trunk. At 13 years, he began developing pubic hair and slight penile enlargement, but without the expected accompanying increase in testicular volume. Approximately one year ago, slight bilateral retroareolar glandular enlargement was noted, causing social embarrassment reported by the adolescent himself. Teachers report reading difficulties and difficulty understanding complex verbal instructions since the early years of primary school, requiring individualised educational support despite preserved performance in non-verbal tasks and mathematics. His mother describes him as shy, struggling to make friends and occasionally impulsive when frustrated.

Physical examination: Height at the 90th percentile for age; arm span slightly greater than height; slender habitus, with relatively broad hips for a male. Bilateral gynaecomastia, Tanner B2 (palpable glandular tissue without suspicious nodularity). Tanner P3 pubic hair; sparse facial hair. Bilateral testicular volume estimated at 4 mL by Prader orchidometer (firm consistency), penis at the lower limit of normal size for pubertal stage. No facial dysmorphism. Neurological examination without evident tremor at the consultation.

Initial diagnostic hypothesis: Incomplete/dissociated puberty with primary hypogonadism requiring clarification; differential diagnosis including Klinefelter syndrome.

Investigation: Morning hormonal profile showed total testosterone 180 ng/dL (below that expected for pubertal stage), LH 12 mIU/mL and FSH 18 mIU/mL (both elevated for age), reduced inhibin B and slightly elevated oestradiol. Peripheral blood karyotype (50 cells) confirmed 47,XXY in all metaphases analysed, without evidence of mosaicism.

Management: Referral for multidisciplinary assessment (endocrinology, psychology, speech and language therapy/educational support, clinical genetics). The condition's pathophysiology, sporadic nature (non-hereditary in the usual form) and expected prognosis were discussed with the family. Discussion began regarding the appropriate timing of possible androgen replacement, considering the demonstrated hypergonadotrophism; jointly with the family, reassessment in three months was chosen before a definitive therapeutic decision, with advice on monthly breast self-examination and anticipatory counselling about future fertility preservation options (cryopreservation of ejaculated sperm if present in semen assessment to be performed at the end of puberty), to be revisited with the adolescent himself as his capacity to participate in these decisions matures.

Teaching comment: This case illustrates the most characteristic adolescent presentation of Klinefelter syndrome — dissociated puberty with small firm testes, gynaecomastia, specific language/reading difficulties with a preserved non-verbal cognitive profile, and shyness/social difficulties — reinforcing the importance of a low threshold of clinical suspicion for this combination of findings even without evident dysmorphism. Testosterone replacement should be started only after discussion of fertility preservation, given the suppressive effect of exogenous testosterone on residual spermatogenesis (see 7.1 and Section 8).

10.2 Correlation with published series

The pattern in case 10.1 corresponds to the reference description: Figure 1A of Smith's Recognizable Patterns shows an untreated 47,XXY adolescent aged 16 years with gynaecomastia, and Figure 1B a 9-year-old child with a small penis and long limbs [11]. In polysomies, the typical presentation differs: in the series of 95 patients with 48,XXYY, the most frequent reason for genetic testing was developmental delay and behavioural problems (68.4%), with diagnosis at a mean age of 7.7 years [33], and in 49,XXXXY, dysmorphism, hypotonia and malformations lead to diagnosis at a mean of 4 months [12]. A published report illustrates the consequences of late diagnosis in polysomies: a 19-year-old with facial dysmorphism, cardiac and musculoskeletal abnormalities, intellectual disability, recurrent respiratory infections while growing up and delayed puberty, until then referred only for psychological support, had a 49,XXXXY karyotype, hypergonadotrophic hypogonadism, insulin resistance and vitamin D deficiency; after 12 months of testosterone replacement, testosterone and vitamin D normalised, pubic hair, testicular volume and penile size increased, weight decreased, HOMA index fell and social interaction improved [74].

10.3 Published case: diagnosis at 76 years after a fragility fracture

Shapiro and Oprea (Brooklyn, United States, 2026) described a 76-year-old man referred to endocrinology after a non-displaced fracture of the right ninth rib during an episode of severe coughing with an upper respiratory tract infection [75]. He reported delayed puberty and small testes since childhood (noted by his paediatrician around age 2, a source of teasing in adolescence and mentioned again by his family doctor, without investigation), late-onset mild gynaecomastia, recurrent miscarriages with his first wife and a low-count semen analysis in his 30s, low testosterone known for 10 to 15 years with treatment refusal, erectile dysfunction and reduced libido. He measured 185.4 cm (approximately +1.2 standard deviations), with mild bilateral gynaecomastia, central adiposity and descended but small testes. Total testosterone was 159.2 ng/dL (5.53 nmol/L), with LH 27.63 IU/L and FSH 62.60 IU/L; karyotyping showed 47,XXY. Densitometry revealed T-scores of −2.0 at the lumbar spine and total hip and −2.1 at the femoral neck, interpreted together with the fracture as clinical osteoporosis; he was treated with intravenous zoledronic acid, with subsequent densitometric improvement, and testosterone was deferred pending urological assessment of benign prostatic hyperplasia [75]. The open-access article (PMC12983455) has no clinical figures — only a laboratory table — so there are no images to consult; a subsequent correction (luag104) merely corrected a typographical error (“30 seconds” to “30s”) [75].

The case illustrates the pattern described in 4 and 9: cardinal signs (small testes, infertility, hypergonadotrophic hypogonadism) present for decades without karyotyping. The authors state that European guidelines recommend screening mammography [75]. The EAA guideline text does not support that interpretation: it proposes clinical breast and axillary examination every two years in adults and “occasional” mammography and/or breast ultrasound, particularly with a family history of breast cancer or other grounds for suspicion, rather than systematic mammography; this article's position, following sources that do not recommend systematic mammographic screening because absolute risk is low (sections 3 and 7.6), is compatible with that of the EAA [2, 11, 15]. The case also illustrates EAA recommendations unfulfilled for decades — karyotyping for small testes with primary hypogonadism or oligozoospermia — and the recommendation for lumbar and femoral DXA with fracture risk assessment at the first adult KS visit [15]. The decision to start a bisphosphonate before testosterone was individualised in an older man with prostatic hyperplasia and does not replace timely androgen replacement recommended for young men with KS.

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11. Prognosis and Long-Term Follow-Up

The prognosis of Klinefelter syndrome, when adequately recognised and treated, is mostly favourable in terms of quality of life and functioning — most patients complete secondary education, and a substantial proportion complete higher education [11]. Most need some school support, particularly in reading and spelling, and some attend full-time special education programmes [11]. Ratcliffe (1999) observed that most XXY men held less-skilled jobs than their fathers, without increased unemployment, with wide variation and professional careers in some [2]. Prognosis is good with early diagnosis and adequate treatment, but may be limited by school difficulties, frustrations and emotional or behavioural problems [1]. Nevertheless, long-term population cohort data demonstrate excess morbidity and mortality relative to the general male population.

Mortality. The reference British cohort study (Swerdlow AJ et al., 2005 [25]), which followed 3,518 patients diagnosed from 1959 to mid-2003, identified an overall standardised mortality ratio (SMR) of 1.5 (95% CI 1.4–1.7), with excess mortality from cardiovascular disease (SMR 1.3), nervous system disease (SMR 2.8), respiratory disease (SMR 2.3), diabetes (SMR 5.8), epilepsy (SMR 7.2), pulmonary embolism (SMR 5.7) and peripheral vascular disease (SMR 7.9). In the same study (461 deaths), mortality was also elevated for intestinal vascular insufficiency (SMR 12.3), renal disease (SMR 5.0) and femoral fracture (SMR 39.4), and reduced for ischaemic heart disease (SMR 0.7; 95% CI 0.5–0.9) [25]. Smith's summarises that mortality is increased by 50%, with infections and nervous, respiratory and genitourinary system diseases among the principal causes of death [11]; the Oxford Desk Reference, by contrast, states that men with KS have normal life expectancy, acknowledging a modest increase in diabetes and cardiovascular, respiratory and digestive disease risk (Swerdlow et al., 2001) [2] — a statement unsupported by the 2005 and 2025 mortality data. Subsequent Danish cohort studies (Bojesen A et al.) estimated an average loss of approximately two years of life expectancy [24]. Of particular clinical relevance, a recent large Danish study (Chang S et al., Lancet Regional Health – Europe, 2025 [58]) demonstrated that testosterone replacement is associated with an approximately 44% reduction in all-cause mortality in men with Klinefelter syndrome (adjusted hazard ratio 0.56; 95% CI 0.37–0.85), bringing mortality in treated patients close to that of the general male population, without an associated increase in major adverse cardiovascular events (MACE). The study compared 557 testosterone-treated men with KS with untreated men with KS born in the same year (total 950) and 50,150 men from the general population between 1994 and 2022; median age at entry in the treated group was 31.1 years, median follow-up 12.9 years, and mortality in treated men did not differ significantly from the comparison population (HR 1.27; 95% CI 0.91–1.79) [58]. This finding — the first of this scale to demonstrate an association between hormonal treatment and reduced mortality in this population — reinforces the urgency of health policies aimed at timely diagnosis and treatment, given that, according to the same study, only half of diagnosed men in the Danish register received testosterone replacement. It should be noted that this is a retrospective observational study without randomisation, subject to inherent design limitations (including absent data on smoking, physical activity and diet as potential confounders, as acknowledged by the authors), so the strength of the association, although relevant, does not replace the need for confirmatory prospective trials.

Recommended follow-up. For adults receiving testosterone replacement, clinical and laboratory assessment (haematocrit, PSA) is recommended at regular intervals according to the applicable hypogonadism guideline; for untreated adults, the EAA guideline recommends annual endocrine assessment [15]. The full EAA surveillance schedule by life stage (physical examination at least annually in adults, annual metabolic and hormonal profiles, vitamin D, DXA and ECG at the first visit and subsequently individualised, thromboprophylaxis in risk situations) appears in section 7.10; the same guideline estimates life expectancy shortening at approximately 2–3 years and recommends improving transition from paediatric to adult endocrinology (1, ⊕⊕⊕◯) [15]. Lifelong follow-up should include bone density monitoring, metabolic and cardiovascular profiles, psychological assessment where indicated and periodic reinforcement of breast self-examination advice. During transition from adolescence to adulthood, particular attention should be paid to continuity of multidisciplinary care, a time historically associated with loss to follow-up in many healthcare systems.

Prognosis in polysomies. In adulthood, disability and psychological complexity are greater in tetrasomies and pentasomies: some adults with 48,XXYY and 48,XXXY live independently, but the proportion is much lower than in 47,XXY, and 36% of adults with 48,XXYY have had previous hospitalisation for behavioural or psychiatric reasons [12]. Adaptive functioning in 48,XXYY is, on average, lower than expected from IQ (mean score 68.9 in 47 patients) [12]. In 49,XXXXY, linear growth is slow, with moderately short final height; infertility and inadequate virilisation are expected, and declining intellectual performance with age is described [11]. In all three polysomies, medical problems emerging during adolescence and early adulthood (scoliosis, tremor, deep vein thrombosis, pulmonary embolism, type 2 diabetes) justify active surveillance (see 7.6) [12].

Surveillance in the tetrasomies and pentasomies. The French PNDS sets a systematic annual consultation, more frequent if there are complications, and an organised transition to adult services at the end of adolescence, to avoid interruption of follow-up; in adulthood, rehabilitation and support are maintained to avoid regression of acquired skills [13].

The PNDS does not describe an excess of breast cancer in the polysomies, but recommends clinical attention, by analogy with 47,XXY [13].

Quality of life. Qualitative studies of the lived experience of Klinefelter syndrome highlight timely diagnosis and access to adequate information as central determinants of psychosocial adaptation, beyond strictly biomedical parameters, reinforcing the value of referral to patient support organisations from the early diagnostic stages. The 2025 review in the Journal of Clinical Endocrinology & Metabolism emphasises that health-related quality of life is reduced compared with controls and that the European guideline is the only endorsed guideline, with limited attention to psychological comorbidities [14].

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12. Summary and Concluding Remarks

Klinefelter syndrome, originally described in 1942 and elucidated karyotypically in 1959, remains, more than eighty years later, widely underdiagnosed despite its high prevalence — the most common sex chromosome aneuploidy and most frequent single genetic cause of male hypogonadism and infertility. Its central pathophysiological substrate is the gene dosage effect arising from an incompletely inactivated supernumerary X chromosome, translating clinically into a multisystem spectrum — reproductive, neurocognitive, skeletal, metabolic, cardiovascular and oncological — whose severity correlates directly with the degree of chromosomal polysomy, from classical 47,XXY to the rare 48,XXXY and 49,XXXXY forms. The paternally derived 48,XXYY constitution adds a high neurobehavioural burden (ADHD in approximately 72%, autism spectrum disorders in approximately 28%) and risks of deep vein thrombosis, tremor and type 2 diabetes in adulthood to hypergonadotrophic hypogonadism. Definitive diagnosis remains cytogenetic, increasingly anticipated by non-invasive prenatal screening, although most cases are still identified in adolescence or adulthood, sometimes only during infertility investigation. Treatment combines individualised androgen replacement — associated with lower mortality in a large Danish observational cohort and recommended by the EAA guideline for confirmed hypogonadism after fertility has been addressed —, neurodevelopmental and psychosocial support, surgical treatment of gynaecomastia where indicated and, where reproduction is desired, TESE/ICSI techniques, which have definitively transformed the reproductive prognosis of a population previously considered uniformly sterile. Relevant knowledge gaps persist, particularly epidemiological ones, in populations outside Europe and North America, and the ethical debate on experimental fertility preservation in children and adolescents remains open. Early clinical recognition, supported by an appropriately calibrated threshold of suspicion for dissociated puberty, specific language difficulties with preserved non-verbal cognition and male infertility, remains the intervention with the greatest potential impact on the clinical course and quality of life of affected individuals.

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