Fisher Grade Calculator

Grade subarachnoid hemorrhage on CT using the Fisher classification for vasospasm risk. Enter values for instant results with step-by-step formulas.

Share this calculator

X Facebook LinkedIn

Formula

Fisher Grade 1-4 based on CT appearance of subarachnoid blood

Original Fisher: Grade 1 = no blood, Grade 2 = diffuse thin (<1mm) blood, Grade 3 = thick (>1mm) clot, Grade 4 = ICH/IVH with diffuse or no SAH. Modified Fisher incorporates IVH presence with SAH thickness for improved vasospasm prediction.

Worked Examples

Example 1: Fisher Grade 3 with High Vasospasm Risk

Problem: A 48-year-old woman presents with thunderclap headache. CT head shows thick subarachnoid blood greater than 1 mm in the basal cisterns and left Sylvian fissure with no intraventricular hemorrhage.

Solution: CT findings: Localized thick subarachnoid blood (>1 mm layer)\nNo intraventricular hemorrhage\nOriginal Fisher Grade: 3 (thick clot/blood >1 mm)\nModified Fisher Grade: 3 (thick SAH, no IVH)\nVasospasm risk (original): 30-70%\nVasospasm risk (modified): 30-40%

Result: Fisher Grade 3 - High vasospasm risk requiring aggressive TCD monitoring and nimodipine prophylaxis

Example 2: Modified Fisher Grade 4

Problem: A 62-year-old man with sudden loss of consciousness. CT shows thick subarachnoid hemorrhage in bilateral Sylvian fissures and basal cisterns with blood in the third and fourth ventricles.

Solution: CT findings: Thick subarachnoid blood (>1 mm) in multiple cisterns\nIntraventricular hemorrhage present in third and fourth ventricles\nOriginal Fisher Grade: 4 (intraventricular clot with diffuse SAH)\nModified Fisher Grade: 4 (thick SAH with IVH)\nVasospasm risk (modified): 40-50%\nConsider EVD placement for hydrocephalus

Result: Modified Fisher Grade 4 - Very high vasospasm risk with IVH; consider EVD and aggressive monitoring

Frequently Asked Questions

What is the Fisher Grade and what does it measure?

The Fisher Grade is a radiological classification system developed by Dr. C. Miller Fisher in 1980 to categorize the amount and distribution of subarachnoid hemorrhage (SAH) visible on computed tomography (CT) scans. The primary purpose of this grading system is to predict the risk of cerebral vasospasm, a dangerous narrowing of brain blood vessels that typically occurs 4-14 days after the initial hemorrhage. The scale ranges from Grade 1 (no visible blood) to Grade 4 (intracerebral or intraventricular clot), with Grade 3 (thick subarachnoid blood) carrying the highest vasospasm risk. It is routinely assessed on the initial CT scan in emergency departments.

What is the difference between the original and modified Fisher Scale?

The original Fisher Scale, published in 1980, has four grades based on the amount of subarachnoid blood on CT. However, it has a paradoxical relationship where Grade 4 has lower vasospasm risk than Grade 3. The modified Fisher Scale, proposed by Claassen and colleagues in 2001, addresses this limitation by incorporating both the thickness of subarachnoid blood (thin vs thick) and the presence or absence of intraventricular hemorrhage (IVH). The modified version provides a more linear correlation between grade and vasospasm risk, making it more clinically useful. Many contemporary neurocritical care units now prefer the modified Fisher Scale because of its improved predictive accuracy for delayed cerebral ischemia.

How is the CT scan evaluated for Fisher grading?

Fisher grading requires careful evaluation of the initial non-contrast CT scan of the head performed after suspected subarachnoid hemorrhage. The radiologist or clinician assesses several features: the presence or absence of subarachnoid blood in the basal cisterns, Sylvian fissures, and interhemispheric fissure; the thickness of the blood layer (less than 1 mm vs 1 mm or greater for the original scale); the presence of localized blood clots; and the presence of intraventricular hemorrhage (IVH) or intracerebral hemorrhage (ICH). The CT should ideally be performed within 24 hours of symptom onset for optimal sensitivity. Modern high-resolution CT scanners have improved detection of subtle subarachnoid blood compared to older technology.

What is the role of nimodipine in Fisher Grade management?

Nimodipine is a calcium channel blocker that is the only pharmacological agent with strong evidence for improving outcomes after subarachnoid hemorrhage. It is recommended for all SAH patients regardless of Fisher Grade, typically administered as 60 mg orally every 4 hours for 21 days. While nimodipine has not been definitively shown to prevent angiographic vasospasm, it does reduce the incidence of delayed cerebral ischemia and improve neurological outcomes. The drug works through neuroprotective mechanisms beyond simple vasodilation. Patients with higher Fisher Grades (particularly Grade 3) who are at greatest vasospasm risk may benefit most from strict adherence to the nimodipine protocol, along with hemodynamic optimization and close monitoring.

What is delayed cerebral ischemia and how does Fisher Grade predict it?

Delayed cerebral ischemia (DCI) is a clinical syndrome occurring after subarachnoid hemorrhage, characterized by new neurological deficits or infarction on imaging that cannot be attributed to other causes such as rebleeding, hydrocephalus, or metabolic derangements. DCI typically occurs between days 4 and 14 after the hemorrhage and affects approximately 30% of SAH patients. The Fisher Grade, particularly the modified version, is one of the strongest predictors of DCI. Higher grades with thicker subarachnoid blood and intraventricular hemorrhage correlate with increased DCI risk. This relationship exists because larger volumes of blood in the subarachnoid space lead to greater release of blood breakdown products that trigger arterial vasospasm and inflammation.

When should the Fisher Grade be reassessed after initial CT?

The Fisher Grade is typically assigned based on the initial CT scan obtained at presentation, as this provides the most relevant prognostic information for vasospasm risk. However, repeat CT scanning is performed for several clinical indications that may require reassessment of the hemorrhage pattern. These include clinical deterioration, failure to improve as expected, suspicion of rebleeding, development of hydrocephalus, or post-procedural evaluation after aneurysm treatment. While the Fisher Grade itself is not formally reassigned on follow-up scans, the evolution of blood on serial imaging provides valuable clinical information. Clearance of subarachnoid blood over time generally correlates with decreasing vasospasm risk, while persistent or increased blood may indicate rebleeding or ongoing hemorrhage.