What is the Fisher grade?

The Fisher scale (1980) classifies subarachnoid hemorrhage on CT into four grades based on the amount and distribution of blood: Grade 1 (no blood), Grade 2 (diffuse thin layer <1 mm), Grade 3 (thick clot or vertical layer ≥1 mm), and Grade 4 (intraventricular or intracerebral extension). Higher grades correlate with greater vasospasm risk.

Which grade carries the highest vasospasm risk?

Grade 3 (thick cisternal clot ≥1 mm or vertical layer ≥1 mm) carries the highest risk of symptomatic vasospasm, typically cited at 20–40% in the original series. Grade 4, despite appearing severe, paradoxically carries a somewhat lower vasospasm risk than Grade 3.

How does the Fisher scale differ from the Modified Fisher scale?

The Modified Fisher scale (Claassen 2001) adds a score for bilateral intraventricular blood and re-grades the severity to better stratify vasospasm risk. This calculator implements the original Fisher (1980) four-grade classification.

Should I use Fisher grade alone to manage SAH?

No. Fisher grade is one of several tools. Combine it with the Hunt-Hess or WFNS clinical grade, GCS, aneurysm location/size, and clinical course to guide management. Always apply clinical judgment.

What counts as a thick clot for Grade 3?

The original Fisher criteria define a thick layer as ≥1 mm in the vertical plane (e.g., interhemispheric fissure) or a clearly identified dense clot in any cistern (ambient, sylvian, basal) without necessarily measuring 1 mm exactly. Thin diffuse subarachnoid blood without a focal clot is Grade 2.

Is this tool validated for clinical use?

This tool implements the published Fisher (1980) criteria faithfully, but it is intended for educational reference only. Clinical decisions must be made by a qualified physician with access to the full patient history and imaging.

Fisher Grade Calculator

Select CT findings to assign a Fisher Grade (1–4) and estimate the risk of symptomatic cerebral vasospasm after subarachnoid hemorrhage.

CT Scan Findings

Select the single description that best matches the CT appearance.

Grade 1 — No subarachnoid blood detected

CT scan is normal; no blood visible in the subarachnoid space.

Grade 2 — Diffuse thin layer (<1 mm)

Diffuse or vertical subarachnoid blood present but no clot and no layer ≥1 mm thick.

Grade 3 — Thick clot or vertical layer (≥1 mm)

Localized clot or thick layer ≥1 mm in the interhemispheric fissure, insular cistern, or ambient cistern.

Grade 4 — Intracerebral or intraventricular blood

Blood extending into the brain parenchyma or ventricular system, with or without diffuse SAH.

Select a CT finding on the left to see the Fisher Grade and vasospasm risk.

Fisher Scale Reference Table

Grade	CT Finding	Vasospasm Risk
1	No subarachnoid blood detected on CT	Low (~0–5%)
2	Diffuse thin SAH, no clot, vertical layer <1 mm	Moderate (~10–15%)
3	Localized clot or vertical layer ≥1 mm	High (~20–40%)
4	Intraventricular or intracerebral blood ± SAH	Moderate–High (~10–35%)

Summary

The Fisher grading scale classifies subarachnoid hemorrhage (SAH) on non-contrast CT into four grades based on the distribution and thickness of cisternal blood. Originally described by Fisher et al. in 1980, the scale predicts the likelihood of symptomatic cerebral vasospasm — a major cause of delayed neurological deterioration after SAH. Grade 3 (thick cisternal clot) carries the highest vasospasm risk, while Grade 1 (no visible blood) carries the lowest.

How it works

Review the CT scan findings for blood in the subarachnoid space.
Select the single CT pattern that best matches your patient.
The tool instantly assigns a Fisher Grade (1–4).
Read the associated symptomatic vasospasm risk estimate.
Use the grade to guide monitoring intensity and treatment planning.

Use cases

Stratify vasospasm risk in patients admitted with SAH.
Communicate severity concisely in handoffs and referrals.
Guide frequency of transcranial Doppler or angiographic monitoring.
Document CT grade for research or quality improvement registries.
Teach neurology and neurosurgery trainees the Fisher classification.
Compare to Hunt-Hess clinical grade for combined risk assessment.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu