The Fisher Scale remains the dominant radiologic tool for identifying patients requiring aggressive DCI prophylaxis in the neuro-ICU.
Ischemia Probability Probe
Input the radiological findings from the non-contrast head CT to determine the Fisher Grade and associated vasospasm risk.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Evaluation of patients with spontaneous subarachnoid haemorrhage (SAH) confirmed on non-contrast CT.
Predicting the risk of delayed cerebral ischaemia (DCI) due to cerebral vasospasm.
Initial radiologic staging within 24–48 hours of ictus.
Clinical Objective
The Fisher Scaleaims to quantify the "clot burden". Thick, localized blood is the primary trigger for the chemical cascade that leads to delayed vasospasm.
Section 2
Pearls/Pitfalls
The 1mm Rule
The key differentiator between Grade 2 and Grade 3 is the thickness of the blood. If the blood layer is ≥ 1 mm thick in any cistern, it is Grade 3. Grade 3 has the HIGHEST risk of symptomatic vasospasm (~33%).
Grade 3 vs. Grade 4
Counter-intuitively, the original Fisher Scale doesn't always mean Grade 4 is high risk. Grade 4 indicates IVH or ICH without thick SAH. Many clinicians found that Grade 4 actually had a LOWER risk of vasospasm than Grade 3. This led to the creation of the Modified Fisher Scale.
Section 3
Next Steps
Prophylaxis Protocol
Section 4
Evidence Appraisal
Primary Reference
Relation of cerebral vasospasm to subarachnoid hemorrhage visualized by computerized tomographic scanning
The scale was developed by Dr. C. Miller Fisher, a legendary Canadian neurologist at MGH, who also described the lacunar stroke syndromes and the Miller Fisher variant of GBS.
Last Comprehensive Review: 2026
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Evaluation of patients with spontaneous subarachnoid haemorrhage (SAH) confirmed on non-contrast CT.
Predicting the risk of delayed cerebral ischaemia (DCI) due to cerebral vasospasm.
Initial radiologic staging within 24–48 hours of ictus.
Clinical Objective
The Fisher Scaleaims to quantify the "clot burden". Thick, localized blood is the primary trigger for the chemical cascade that leads to delayed vasospasm.
Section 2
Pearls/Pitfalls
The 1mm Rule
The key differentiator between Grade 2 and Grade 3 is the thickness of the blood. If the blood layer is ≥ 1 mm thick in any cistern, it is Grade 3. Grade 3 has the HIGHEST risk of symptomatic vasospasm (~33%).
Grade 3 vs. Grade 4
Counter-intuitively, the original Fisher Scale doesn't always mean Grade 4 is high risk. Grade 4 indicates IVH or ICH without thick SAH. Many clinicians found that Grade 4 actually had a LOWER risk of vasospasm than Grade 3. This led to the creation of the Modified Fisher Scale.
Section 3
Next Steps
Prophylaxis Protocol
Section 4
Evidence Appraisal
Primary Reference
Relation of cerebral vasospasm to subarachnoid hemorrhage visualized by computerized tomographic scanning
The scale was developed by Dr. C. Miller Fisher, a legendary Canadian neurologist at MGH, who also described the lacunar stroke syndromes and the Miller Fisher variant of GBS.
