While the Hunt and Hess scale evaluates the clinical severity of a subarachnoid hemorrhage (SAH), the Fisher Grade is a radiological tool used specifically to predict the most feared delayed complication of SAH: cerebral vasospasm.
The Threat of Vasospasm
Days after an aneurysm ruptures, the breakdown products of the blood irritating the brain's arteries can cause those arteries to violently constrict. This vasospasm can lead to delayed cerebral ischemia (DCI), causing strokes and death in patients who initially survived the hemorrhage.
Grading Criteria on Non-Contrast CT
The Fisher Grade is determined by the distribution and thickness of the blood:
- Grade 1: No blood detected. (Low vasospasm risk ~21%)
- Grade 2: Diffuse or vertical layers of blood less than 1 mm thick. (Low risk ~25%)
- Grade 3: Localized clot and/or vertical layer of blood 1 mm or greater in thickness. (High risk ~37%)
- Grade 4: Intracerebral or intraventricular clot with diffuse or no SAH. (Intermediate risk ~31%)
Grade classification based purely on the amount and distribution of blood visualized on a non-contrast CT head.
The Grade 3 Paradox
It is often confusing that Grade 3 carries a higher risk of vasospasm than Grade 4. This is because vasospasm is a reaction to blood bathing the arteries in the subarachnoid space. In Grade 3, there are thick, dense clots sitting directly on these vessels. In Grade 4, the bleeding has ruptured outward into the brain parenchyma or inward into the ventricles, meaning there is paradoxically less blood sitting in the subarachnoid space to irritate the major arteries.