Extradural, subdural, subarachnoid and intracerebral, each with a characteristic vessel and shape
Mnemonic
Four compartments, four appearances:
| Type | Shape on CT | Vessel | Typical patient |
|---|---|---|---|
| Extradural | Biconvex, does not cross sutures | Middle meningeal artery | Young, temporal trauma, lucid interval |
| Subdural | Crescent, crosses sutures | Bridging veins | Elderly, alcohol, anticoagulated, minor trauma |
| Subarachnoid | Blood in the sulci and basal cisterns | Berry aneurysm | Thunderclap headache |
| Intracerebral | Within the parenchyma | Small perforators | Hypertension, basal ganglia |
“Extradural stops at sutures, subdural stops at the midline.”
Extradural is arterial so it deteriorates quickly, classically after a lucid interval; subdural is venous so it can be chronic and present with fluctuating confusion weeks later.
Subarachnoid haemorrhage requires CT within 6 hours (over 95 per cent sensitive), and if negative and clinically suspected, a lumbar puncture at 12 hours for xanthochromia.
Expansion
| Type | Vessel | Shape on CT | Typical setting |
|---|---|---|---|
| Extradural | Middle meningeal artery | Lens (biconvex), does not cross sutures | Temporal skull fracture, young adult, lucid interval |
| Subdural | Bridging veins | Crescentic, crosses sutures | Elderly, alcohol, anticoagulants; minor trauma |
| Subarachnoid | Berry aneurysm rupture | Blood in sulci and basal cisterns | Thunderclap headache, meningism |
| Intracerebral | Charcot-Bouchard microaneurysms | Within parenchyma | Hypertension, basal ganglia |
The shape difference is anatomical: the dura is adherent at sutures, so extradural blood cannot cross them but is powerful enough to strip dura from bone. Subdural blood lies inside that attachment and spreads freely.
Subdural is common in the elderly and in alcohol dependence because cerebral atrophy stretches the bridging veins, so trivial trauma tears them, and the history of injury is often absent.
Subarachnoid haemorrhage risk factors: hypertension, smoking, polycystic kidney disease, Ehlers-Danlos and coarctation. Complications include rebleeding, vasospasm at days 3 to 14 (treated with nimodipine), hydrocephalus and hyponatraemia.