Subarachnoid haemorrhage (SAH) is bleeding into the subarachnoid space — the area between the brain and the arachnoid membrane — most commonly caused by rupture of a brain aneurysm (80% of cases). The hallmark is the sudden 'thunderclap' headache — the worst headache of the patient's life — that comes on in seconds. SAH is a life-threatening emergency with 10–15% of patients dying before reaching hospital and significant morbidity from delayed vasospasm (cerebral artery narrowing occurring 4–14 days after bleed). Urgent CT brain, lumbar puncture if CT is negative, and neurovascular treatment are mandatory.
