Modified Fisher Scale Interpreter
Modified Fisher Scale Interpreter
Grade the subarachnoid blood load 0 to 4 from the presenting CT, and read the symptomatic vasospasm incidence reported for each grade. Grades 2 and 3 carry the same reported risk, and rater agreement is only moderate.
Modified Fisher scale
CT blood pattern to grade 0 to 4Thick diffuse subarachnoid blood filling the basal cisterns, with blood layering in both lateral ventricles, on the presenting CT
The grid
Thin is under 1 mm in depth, thick is over 1 mm
- what the modification changed
- the original Fisher scale (Fisher, Kistler and Davis, 1980) handled intraventricular and intracerebral clot in a grade 4 that could contain little subarachnoid blood at all. The 2006 modification makes intraventricular haemorrhage an independent axis, which is why the grid is 3 × 2 rather than a single ordered list
- grades 2 and 3 are equal
- both carried a reported symptomatic vasospasm incidence of 33 per cent. The ordering follows the pattern of blood, not a strictly monotonic risk, and a reader who assumes grade 3 is worse than grade 2 is reading something the data does not say
- rater agreement
- a four-observer study of 165 patients found a Fleiss kappa of 0.42 for the modified Fisher scale, against 0.53 for the original Fisher scale, 0.38 for Claassen, 0.20 for the Barrow Neurological Institute scale and 0.66 for the quantitative Hijdra sum score. Scales that ask an observer to count agree better than scales that ask them to categorise
- a source disagreement
- three independent references give the vasospasm incidence as 0, 24, 33, 33 and 40 per cent for grades 0 to 4. One widely used calculator gives 6, 15, 35 and 34 per cent for grades 1 to 4 instead. This page uses the first set, because three sources support it, and records the disagreement rather than burying it
- what it predicts, and what it does not
- symptomatic vasospasm — a clinical endpoint — in a single cohort. Not angiographic vasospasm, not transcranial Doppler velocities, not delayed cerebral ischaemia as currently defined, and not death. The 2023 AHA/ASA guideline carries no recommendation for radiographic grading on its own, noting that radiographic and clinical grades have been combined into composite scores such as VASOGRADE, HAIR and SAHIT
Worked example
Thick diffuse subarachnoid blood filling the basal cisterns, with blood layering in both lateral ventricles, on the presenting CT
Thick subarachnoid blood, over 1 mm in depth, with intraventricular haemorrhage present is modified Fisher grade 4, the highest grade
The reported symptomatic vasospasm incidence at grade 4 is 40 per cent, against 24 per cent at grade 1 — a 16-point absolute spread across the whole scale
Clear the ventricular blood and the same patient is grade 3, reported incidence 33 per cent. Thin the subarachnoid blood instead and they are grade 2 — also 33 per cent
Two different grades with the same published risk is a property of the scale, not an error on this page: the grades are ordered by pattern rather than by risk
The grid, and the reported vasospasm incidence
| Grade | Subarachnoid blood | Intraventricular blood | Reported symptomatic vasospasm |
|---|---|---|---|
| 0 | None | None | 0% |
| 1 | Thin, focal or diffuse | Absent | 24% |
| 2 | Thin, focal or diffuse | Present | 33% |
| 3 | Thick | Absent | 33% |
| 4 | Thick | Present | 40% |
| — | None | Present | Not a cell the published scale defines |
How the CT grading scales compare on reproducibility
| Scale | Fleiss kappa, four observers, 165 patients |
|---|---|
| Hijdra sum score (quantitative) | 0.66 |
| Fisher (original, 1980) | 0.53 |
| Modified Fisher (2006) | 0.42 |
| Claassen | 0.38 |
| Barrow Neurological Institute | 0.20 |
Two axes, five cells, and two of them carrying the same risk
The original Fisher scale of 1980 graded the CT appearance of subarachnoid blood as a predictor of vasospasm, and it had a structural problem: its grade 4 was defined by intracerebral or intraventricular clot and could be assigned to a patient with very little subarachnoid blood, so the scale was not monotonic in the thing it was supposed to measure. The 2006 modification pulls intraventricular haemorrhage out as an independent axis. What is left is a grid: no blood, thin blood or thick blood, crossed with intraventricular haemorrhage absent or present, giving five defined grades.
The reported symptomatic vasospasm incidences are 0 per cent at grade 0, 24 at grade 1, 33 at grades 2 and 3, and 40 at grade 4. Three things in that sequence are worth saying out loud. Grades 2 and 3 are equal — thin blood with ventricular extension and thick blood without it carried the same risk, so the grades are ordered by pattern and not strictly by risk, and reading grade 3 as worse than grade 2 is reading something the data does not support. Most of the discrimination is in the step from no blood to any blood: from grade 1 to grade 4 the whole scale spans 16 absolute percentage points. And one widely used online calculator reproduces grades 1 to 4 as 6, 15, 35 and 34 per cent instead; three independent references agree on the figures used here, so those are what this page prints, with the disagreement recorded rather than buried.
The judgement the scale asks for is the depth of blood in the cisterns and fissures — under 1 mm thin, over 1 mm thick — and it is the least reproducible part. A four-observer study of 165 patients found a Fleiss kappa of 0.42 for the modified Fisher scale, below the original Fisher scale’s 0.53 and well below the Hijdra sum score’s 0.66. Intraventricular haemorrhage is also scored as simply present or absent, so a trace of layering in one occipital horn and a cast filling all four ventricles score identically. Two practical traps remain. The grade belongs to the presenting CT, because subarachnoid blood clears over days and a scan taken 48 hours in yields a grade the published risks were never attached to. And the endpoint is symptomatic vasospasm in one cohort — not angiographic narrowing, not Doppler velocities, not delayed cerebral ischaemia as currently defined. Read this grade alongside the Hunt and Hess grade or the WFNS grade rather than instead of them. A grade is not a diagnosis and a cohort risk is not this patient’s probability: a stratum in which 72 per cent died tells you about that cohort, not which 72 per cent. This page reports what a stratum predicted in a study. It recommends no action. Every threshold here comes from a named cohort, and cohorts differ in case mix, era and treatment; where your unit’s protocol differs, it takes precedence.
Frequently asked questions
What are the modified Fisher scale grades?
Grade 0 is no subarachnoid and no intraventricular haemorrhage; 1 is thin subarachnoid blood with no intraventricular blood; 2 is thin subarachnoid blood with intraventricular blood; 3 is thick subarachnoid blood with no intraventricular blood; 4 is thick subarachnoid blood with intraventricular blood. Thin is under 1 mm in depth, thick over 1 mm.
What vasospasm risk does each modified Fisher grade carry?
As reproduced from the 2006 derivation cohort: 0 per cent at grade 0, 24 at grade 1, 33 at grades 2 and 3, and 40 at grade 4, for symptomatic vasospasm. One widely used calculator gives 6, 15, 35 and 34 per cent for grades 1 to 4 instead; three independent references support the first set.
Why do grades 2 and 3 have the same vasospasm risk?
Because that is what the derivation cohort showed: thin subarachnoid blood with ventricular extension and thick subarachnoid blood without it carried the same reported incidence. The grades are ordered by the pattern of blood rather than strictly by risk.
How is the modified Fisher scale different from the original?
The original 1980 scale’s grade 4 was defined by intracerebral or intraventricular clot and could be assigned to a patient with little subarachnoid blood, so it was not monotonic in the quantity it measured. The 2006 modification makes intraventricular haemorrhage an independent axis crossed with the thickness of subarachnoid blood.
How reliable is the modified Fisher scale between observers?
Moderate. In a blinded four-observer study of 165 patients the Fleiss kappa was 0.42, against 0.53 for the original Fisher scale and 0.66 for the quantitative Hijdra sum score. The disagreement concentrates on the thin-against-thick judgement, the only measurement the scale asks for.
Related calculators
References
- Modified Fisher scale. Radiopaedia.org (accessed 7 October 2026), citing Frontera JA, Claassen J, Schmidt JM, et al., Neurosurgery 2006;59(1):21–27.
- Modified Fisher scale calculator. WikEM (accessed 7 October 2026), citing Fisher CM, Kistler JP, Davis JM, Neurosurgery 1980;6(1):1–9.
- Woo PYM, et al. Computed tomography interobserver agreement in the assessment of aneurysmal subarachnoid haemorrhage and predictors for clinical outcome. J Neurointerv Surg. 2017;9(11):1118–1124.
- Hoh BL, Ko NU, Amin-Hanjani S, et al. 2023 guideline for the management of patients with aneurysmal subarachnoid hemorrhage. Stroke. 2023;54(7):e314–e370.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/