The Fisher scale is the initial and best-known system of classifying the amount of subarachnoid hemorrhage on CT scans, described in 1980 4. Its primary use was in predicting the occurrence and severity of cerebral vasospasm, highest in grade 3 2,4.
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Usage
Although it is known as a scale, from grade 1 to grade 4, it should be noted that the risk of vasospasm only increases from grade 1 to 3, and is lower again in grade 4 2.
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. It is important to note that the latter is more than a mere refinement of the original description and it is, therefore, essential to be explicit as to which scale is being used 6.
The modified Fisher scale is 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.
Scale
-
grade 1
no subarachnoid (SAH) or intraventricular hemorrhage (IVH) detected
incidence of symptomatic vasospasm: 21% 3
-
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) 4,5
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