Modified Fisher Scale Calculator

Aneurysmal subarachnoid hemorrhage (SAH) is a devastating stroke subtype caused by rupture of an intracranial aneurysm. Blood spreads into the subarachnoid space, and a serious delayed complication is cerebral vasospasm, which occurs 4-14 days after rupture and causes ischemic stroke in up to 30% of SAH patients.

The Modified Fisher Scale grades the appearance of blood on the initial CT (computed tomography) scan and predicts vasospasm risk. Frontera and colleagues published it in 2006 to repair the original Fisher scale (1980), which had two problems. Observers disagreed with each other about grades 2 and 3, and the grades were not ordered by risk: original grade 3 carried a higher vasospasm risk than grade 4, so a higher number did not mean a worse outlook. The modified version turns on two questions only, which is why it can be scored quickly and reproducibly at three in the morning.

The two questions are: is the subarachnoid blood thin or thick, and is there IVH (intraventricular hemorrhage, blood inside the ventricles)?

Grade 0: No SAH and no IVH. Symptomatic vasospasm risk approximately 0%. Grade 1: Thin SAH, no IVH. Risk approximately 12-24%. Grade 2: Thin SAH with IVH. Risk approximately 21-33%. Grade 3: Thick SAH, no IVH. Risk approximately 19-33%. Grade 4: Thick SAH with IVH. Risk approximately 40%.

Thick blood means >= 1 mm in any basal cistern or Sylvian fissure. IVH independently raises vasospasm risk and marks a heavier bleed.

Look again at those two middle rows. Grade 3 starts at 19% and grade 2 starts at 21%, so on the low end grade 3 reads slightly better than grade 2, and the two ranges sit almost on top of each other. That is not a typo. Reported figures vary between series, and the modified scale never claimed a clean step from 2 to 3, so treating a move from grade 2 to grade 3 as an escalation reads more into the number than it holds. The separation this scale is actually good at is grade 4 against everything below it.

The scale describes subarachnoid blood. It has no grade for blood confined to the ventricles with a clean subarachnoid space, because that picture usually is not an aneurysmal SAH: primary intraventricular hemorrhage, extension from a deep intracerebral bleed, or an aneurysmal bleed imaged late enough that the subarachnoid blood has cleared. This calculator says so rather than inventing a grade.

After aneurysmal SAH, management includes:

• Securing the aneurysm (surgical clipping or endovascular coiling) to prevent rebleeding
• Nimodipine (a calcium channel blocker) for all SAH patients, regardless of grade, reduces delayed ischemia
• Transcranial Doppler ultrasonography to monitor for vasospasm
• Prophylactic hypervolemia is no longer routine; euvolemia is the current standard
• If symptomatic vasospasm occurs, induced hypertension and endovascular intervention are options

This calculator is a reference tool. Radiology interpretation and clinical context determine the actual grade.