Aneurysmal SAH on non-contrast CT — grade by SAH thickness and presence of intraventricular hemorrhage.
CT appearance
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This is the MODIFIED Fisher scale — not the original Fisher scale
The grade numbers are not interchangeable. The original Fisher scale (Fisher CM, Kistler JP, Davis JM, 1980) grades: 1 no SAH detected; 2 diffuse or vertical layer of subarachnoid blood <1 mm thick; 3 localized clot and/or vertical layer ≥1 mm thick; 4 intracerebral or intraventricular hemorrhage with diffuse or no SAH.
In Frontera's own cohort the original scale's risk was non-monotonic — 21%, 25%, 37%, 31% for grades 1–4 — so original Fisher 3 carries the peak risk while grade 4 drops. That is precisely why the modified scale exists. If a grade was handed to you without the scale named, confirm which scale was used before acting on it.
Rates above are symptomatic vasospasm (clinical deterioration on days 5–12 after SAH) in 1355 placebo-arm patients with angiographically proven saccular aneurysms; the cohort-wide rate was 451/1355 (33%). The scale does not predict angiographic or TCD vasospasm. "Thick" = SAH completely filling one or more cisterns or fissures is the conventional gloss (Claassen 2001); Frontera applied no explicit thick/thin or focal/diffuse criteria — scans were rated by local investigators — and interrater agreement on this call is modest. Decision support only — not a substitute for clinical judgment.
Frontera JA, et al. Prediction of symptomatic vasospasm after SAH: the modified Fisher scale. Neurosurgery. 2006;59(1):21–27. · Original scale: Fisher CM, Kistler JP, Davis JM. Neurosurgery. 1980;6(1):1–9.