Introduction:
Subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space, the area between the arachnoid and pia mater, which contains cerebrospinal fluid (CSF) and major cerebral vessels. It is a life-threatening neurological emergency and accounts for ~5% of all strokes.
The two main types of SAH are:
- Traumatic SAH – most common (e.g., in road traffic accidents).
- Non-traumatic (spontaneous) SAH – usually due to ruptured aneurysms, particularly berry aneurysms at the Circle of Willis.
Imaging Modality of Choice:
- Non-contrast CT (NCCT) head is the first-line investigation for suspected SAH.
- It is fast, widely available, and highly sensitive within the first 72 hours of symptom onset.
- CT can detect even small amounts of blood in the CSF spaces due to its inherent hyperdensity compared to CSF.
CT Technique:
- Axial thin-section imaging (≤5 mm) from skull base to vertex.
- Images viewed in brain and subarachnoid window settings.
- Supine position; motionless acquisition is critical to avoid artifacts.
CT Features of Acute SAH:
1. Hyperdense Blood in the Subarachnoid Space:
- Fresh extravasated blood appears hyperdense (bright) relative to CSF (measured as 60–70 Hounsfield units).
- Common sites of blood accumulation:
- Basal and perimesencephalic cisterns
- Sylvian fissures
- Interhemispheric fissure
- Convexity sulci (in diffuse SAH)
2. Intraventricular Hemorrhage (IVH):
- Blood may reflux into the ventricular system, particularly:
- Occipital horns of the lateral ventricles
- Third ventricle
- Fourth ventricle
- IVH is associated with worse prognosis.
3. Hydrocephalus:
- Blood can obstruct CSF flow or resorption → communicating hydrocephalus.
- Seen as:
- Dilatation of lateral and third ventricles
- Effacement of cortical sulci
- “Ballooning” of temporal horns (early sign)
4. Parenchymal Extension:
- Associated intraparenchymal hemorrhage can occur with large ruptured aneurysms.
- Most often in the temporal lobe (rupture of MCA aneurysm) or frontal lobe (ACoA aneurysm).
5. Vasospasm & Infarction (Delayed):
- May show low-attenuation areas due to secondary ischemia.
- Occurs typically 4–14 days post-bleed, especially in aneurysmal SAH.
Grading Systems Based on CT:
1. Fisher Grade (for aneurysmal SAH):
| Grade | Description | Risk of Vasospasm |
|---|---|---|
| I | No blood seen | Low |
| II | Thin SAH (<1 mm), no clots | Moderate |
| III | Localized clots (>1 mm) | High |
| IV | SAH with IVH or intraparenchymal extension | Very High |
2. Modified Fisher Grade:
Takes into account both subarachnoid and intraventricular blood and is better at predicting vasospasm.
Sensitivity of CT for SAH Over Time:
- <6 hours post ictus: 95–99% sensitive
- 6–24 hours: 85–90%
- >72 hours: Sensitivity decreases further due to:
- Resorption of blood
- Redistribution of blood into dependent areas
- Isoattenuation relative to brain parenchyma
