How it's calculated
The Modified Fisher Scale grades the initial CT appearance of aneurysmal subarachnoid haemorrhage (SAH) by the thickness of subarachnoid blood and the presence or absence of intraventricular haemorrhage (IVH), producing a grade from 0 to 4 that correlates with the risk of symptomatic cerebral vasospasm.
Grade and approximate vasospasm risk
| Grade | CT findings | Approx. vasospasm risk |
|---|---|---|
| 0 | No SAH, no IVH | ~0% |
| 1 | Thin SAH, no IVH | ~24% |
| 2 | Thin SAH, with IVH | ~33% |
| 3 | Thick SAH, no IVH | ~33% |
| 4 | Thick SAH, with IVH | ~40% |
Clinical use
- Assessed on the initial (admission) non-contrast CT, before any intervention — it's a one-time baseline grade, not a repeated/trending score.
- Higher grades support closer vasospasm surveillance (e.g. more frequent transcranial doppler ultrasound, lower threshold for CT/catheter angiography if new deficits develop) typically over the days 4-14 window when vasospasm risk peaks.
- Used alongside clinical grading scales — Hunt & Hess or WFNS — which grade the patient's neurological status rather than the CT appearance; the two are complementary, not interchangeable.
- The original (non-modified) Fisher scale is an older, related system — the modified version is more widely used today because it better stratifies risk, particularly by separating out the added risk from IVH.
Frequently asked questions
What does the Modified Fisher Scale predict?
The risk of symptomatic cerebral vasospasm after aneurysmal subarachnoid haemorrhage, based on how much blood is seen on the initial CT and whether it has entered the ventricles (IVH).
What is the difference between the Fisher Scale and the Modified Fisher Scale?
The original Fisher scale (1980) grades subarachnoid blood thickness alone. The Modified Fisher Scale (2001) refines this by explicitly incorporating the presence of intraventricular haemorrhage, which improves its correlation with vasospasm risk.
Is the Modified Fisher Scale the same as Hunt & Hess?
No — Hunt & Hess (and WFNS) grade the patient's clinical/neurological status on presentation. The Modified Fisher Scale grades the CT imaging appearance. Both are used together in SAH management, assessing different things.
References
- Frontera JA, et al. Prediction of symptomatic vasospasm after subarachnoid hemorrhage: the modified Fisher scale. Neurosurgery. 2006;59(1):21-27.
- Fisher CM, et al. Relation of cerebral vasospasm to subarachnoid hemorrhage visualized by CT scanning. Neurosurgery. 1980;6(1):1-9.