Neuroradiology · Subarachnoid haemorrhage · Modified Fisher scale
Modified Fisher Scale for Aneurysmal Subarachnoid Haemorrhage
CT-based grading of subarachnoid and intraventricular blood after aneurysmal subarachnoid haemorrhage, primarily used to stratify the risk of symptomatic cerebral vasospasm.
Purpose
To grade the amount of subarachnoid blood and the presence of intraventricular haemorrhage on the initial non-contrast CT after aneurysmal subarachnoid haemorrhage, thereby stratifying the risk of symptomatic cerebral vasospasm.
| Grade | Subarachnoid blood | Intraventricular haemorrhage |
|---|---|---|
| 0 | Absent | Absent |
| 1 | Focal or diffuse thin SAH | Absent |
| 2 | Focal or diffuse thin SAH | Present |
| 3 | Focal or diffuse thick SAH | Absent |
| 4 | Focal or diffuse thick SAH | Present |
How to use it
- The scale separates two CT features: the thickness of subarachnoid blood and the presence or absence of intraventricular haemorrhage.
- Thin SAH is conventionally defined as less than 1 mm in maximal thickness, whereas thick SAH is approximately 1 mm or greater.
- The grade generally increases from thin SAH without intraventricular haemorrhage to thick SAH with intraventricular haemorrhage.
- Grade 4 represents the highest-risk imaging pattern in the original validation because both thick cisternal blood and intraventricular haemorrhage are present.
- The scale supports risk stratification and monitoring but does not independently diagnose angiographic vasospasm or delayed cerebral ischaemia.
Common mistake
Do not confuse the modified Fisher scale with the original Fisher scale: in the modified scale, Grade 4 specifically combines thick SAH with intraventricular haemorrhage, and an intracerebral haematoma does not independently define the grade.
Exam pearl
Modified Fisher is a CT blood-burden scale, not a clinical severity scale: remember the sequence as none, thin without IVH, thin with IVH, thick without IVH, and thick with IVH.
Viva questions
- What is the modified Fisher scale?
- It is a five-grade CT scale that classifies aneurysmal subarachnoid haemorrhage according to the thickness of subarachnoid blood and the presence of intraventricular haemorrhage.
- What is modified Fisher Grade 0?
- Grade 0 means that neither subarachnoid blood nor intraventricular haemorrhage is visible on CT.
- How are modified Fisher Grades 1 and 2 differentiated?
- Both have thin subarachnoid haemorrhage. Grade 1 has no intraventricular haemorrhage, whereas Grade 2 has intraventricular haemorrhage.
- How are modified Fisher Grades 3 and 4 differentiated?
- Both have thick subarachnoid haemorrhage. Grade 3 has no intraventricular haemorrhage, whereas Grade 4 has intraventricular haemorrhage.
- Which modified Fisher grade represents thick SAH with intraventricular haemorrhage?
- Modified Fisher Grade 4.
- What is the principal clinical implication of the modified Fisher grade?
- It stratifies the risk of symptomatic cerebral vasospasm after aneurysmal subarachnoid haemorrhage and helps guide the intensity of neurological monitoring.
- Does intracerebral haemorrhage independently determine the modified Fisher grade?
- No. The modified Fisher grade is determined by subarachnoid blood thickness and intraventricular haemorrhage; intracerebral haemorrhage does not independently define the grade.
- What is the important limitation when interpreting the modified Fisher scale?
- It estimates risk from the initial CT blood pattern but does not itself diagnose vasospasm or delayed cerebral ischaemia and should not replace clinical and physiological monitoring.