The Fisher scale is the initial and best known system of classifying the amount of subarachnoid hemorrhage on CT scans, and is useful in predicting the occurrence and severity of cerebral vasospasm, highest in grade 3 .
Numerous other scales have been proposed, incorporating various parameters, and aimed at predicting mortality, morbidity and/or vasospasm.
Nonetheless, the Fisher scale and more recently the Modified Fisher scale are still widely used, the latter preferred by many over the original grading system, as the risk of symptomatic vasospasm in the original description was highest in grade 3 and lower in grade 4.
- grade 1
- no subarachnoid (SAH) or intraventricular hemorrhage (IVH) detected
- incidence of symptomatic vasospasm: 21%
- grade 2
- diffuse thin (<1 mm) SAH
- no clots
- incidence of symptomatic vasospasm: 25%
- grade 3
- localized clots and/or layers of blood >1 mm in thickness
- no IVH
- incidence of symptomatic vasospasm: 37%
- grade 4
- diffuse or no SAH
- ICH or IVH present
- incidence of symptomatic vasospasm: 31%
Differences between the Fisher scale and modified Fisher scale
- in Fisher scale no SAH is grade 1 (grade 0 in modified Fisher scale)
- in Fisher scale thin SAH, no IVH is grade 2 (grade 1 in modified Fisher scale)
- in Fisher scale thick SAH with no IVH is grade 3 (grade 3 in modified Fisher scale)
- in Fisher scale any IVH is grade 4, irrespective of presence of SAH (in modified Fisher scale it is either grade 2 if thin or no SAH, or grade 4 if thick SAH)