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Modified Fisher Grading Scale for SAH

Grades CT findings in subarachnoid hemorrhage to predict risk of cerebral vasospasm and delayed ischemic neurological deficit

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Why use it

Cerebral vasospasm and delayed ischemic neurological deficit (DIND) are the most preventable causes of death and disability in SAH survivors. Identifying patients at high risk (Modified Fisher Grade 3–4) enables targeted monitoring, prophylactic nimodipine therapy, and aggressive vasospasm detection protocols (transcranial Doppler, CT perfusion, angiography). The mFS provides objective, reproducible CT-based risk stratification at admission that guides neurocritical care management.

About

The Modified Fisher Scale (mFS) is a CT-based grading system for aneurysmal subarachnoid hemorrhage (SAH) developed by Frontera and colleagues in 2006. It was created to address limitations of the original Fisher Scale (1980), which was poorly calibrated for predicting symptomatic cerebral vasospasm and did not adequately account for the presence of intraventricular hemorrhage (IVH). The Modified Fisher Scale grades SAH from 0 to 4 based on two CT features: the thickness/distribution of the subarachnoid blood and the presence of intraventricular blood (IVH). Higher grades predict higher rates of symptomatic cerebral vasospasm and delayed ischemic neurological deficit (DIND), which occurs in 30–40% of SAH patients at 4–14 days and is a major determinant of outcome. Grades 3 and 4 (thick SAH and/or IVH) are associated with vasospasm rates exceeding 35–40%. The Modified Fisher Scale has superior predictive validity for vasospasm compared to the original Fisher Scale and is now the preferred CT grading system for SAH management.

When to use

  • Initial CT scan in any patient with confirmed or suspected subarachnoid hemorrhage
  • Predicting risk of symptomatic cerebral vasospasm (days 4–14 post-ictus)
  • Guiding intensity of vasospasm monitoring: TCD frequency, timing of follow-up CT angiography
  • Communicating prognosis to family regarding risk of delayed neurological deterioration
  • Stratification in clinical trials of SAH management and vasospasm prevention

Formula

Modified Fisher Scale — CT-Based Grading: Grade 0: No subarachnoid hemorrhage or intraventricular hemorrhage Vasospasm risk: ~0% Grade 1: Thin (≤1 mm) diffuse or focal SAH; NO IVH Vasospasm risk: ~24% Grade 2: Thin (≤1 mm) SAH WITH bilateral IVH Vasospasm risk: ~33% Grade 3: Thick (>1 mm) SAH in any cistern; NO IVH Vasospasm risk: ~33% Grade 4: Thick (>1 mm) SAH WITH bilateral IVH Vasospasm risk: ~40% Thick SAH = clot or blood layer >1 mm in the interhemispheric fissure, sylvian fissure, or basal cisterns Thin SAH = diffuse or focal subarachnoid blood ≤1 mm in any cistern

Pearls & pitfalls

  • Nimodipine 60 mg q4h for 21 days is indicated for ALL SAH patients regardless of Modified Fisher grade — it reduces DIND by 34% but does NOT reduce angiographic vasospasm.
  • Symptomatic vasospasm typically occurs 4–14 days post-SAH — peak incidence is at days 7–10. Clinical monitoring must be intensive during this window.
  • TCD (transcranial Doppler) monitoring: MCA velocity >120 cm/s = mild-moderate vasospasm; >200 cm/s = severe vasospasm. Lindegaard ratio (MCA velocity/ICA velocity) >6 indicates severe vasospasm.
  • Avoid hypovolemia and hyponatremia in SAH — both worsen DIND risk. Maintain euvolemia with isotonic fluids; SIADH and cerebral salt wasting are common causes of hyponatremia post-SAH.
  • IVH at admission (grade 2 and 4) predicts not only vasospasm but also communicating hydrocephalus — early ventriculostomy (EVD) may be required for grade 3–4 IVH with hydrocephalus.
  • The original Fisher scale graded "thick" SAH as >1 mm thick clot — the Modified Fisher uses the simpler dichotomy of thin vs. thick, with IVH separately weighted.
  • Early (<72h) aneurysm securing (coiling or clipping) is preferred to prevent rebleeding — rebleeding carries >50% mortality.
  • Rebleeding before treatment: the most feared early complication. Initial antifibrinolytic therapy (tranexamic acid) for <24h may reduce rebleeding in patients awaiting delayed treatment but is not standard practice.
  • Hunt-Hess grade correlates with clinical neurological status (Grade 1–5) and complements the Modified Fisher CT grade for overall prognosis.

Next steps

Modified Fisher Grade 0–2: Lower vasospasm risk. Routine SAH monitoring. Nimodipine 60 mg q4h (or 30 mg q2h) for 21 days regardless of grade. TCD monitoring every 1–2 days. Grade 3–4: High vasospasm risk. Intensive monitoring protocol: daily TCD, close clinical neurological assessment q2–4h, low threshold for CT perfusion or conventional angiography at TCD velocity changes or clinical change. Hypertension-hypervolemia-hemodilution (HHH) therapy for symptomatic vasospasm. Endovascular treatment (intra-arterial verapamil, balloon angioplasty) for refractory vasospasm.

Evidence

Frontera JA et al. Predicting symptomatic vasospasm after subarachnoid hemorrhage: the Modified Fisher Scale. Neurosurgery. 2006;59(1):21-27. PMID:16823296. Derivation: 196 SAH patients. Modified Fisher Grade 3 vasospasm rate: 33%; Grade 4: 40% vs Grade 1: 24%, Grade 2: 33%. Internal validation showed superior calibration over original Fisher scale. Validated in multiple subsequent cohorts.

References

  1. Frontera JA et al. Predicting symptomatic vasospasm after subarachnoid hemorrhage: the Modified Fisher Scale. Neurosurgery. 2006;59(1):21-27. — link
  2. Fisher CM et al. Relation of cerebral vasospasm to subarachnoid hemorrhage visualized by computerized tomographic scanning. Neurosurgery. 1980;6(1):1-9. — link
  3. Connolly ES Jr et al. Guidelines for the management of aneurysmal subarachnoid hemorrhage. Stroke. 2012;43(6):1711-1737. — link