SAH is bleeding beneath the arachnoid layer of the dura. All bleeding into cerebrospinal fluid (e.g. basal cisterns, cerebral ventricles) is subarachnoid haemorrhage.

Rupture of an intracranial aneurysm accounts for 85% of cases. Identification and obliteration of such aneurysms is needed to reduce the risk of rebleeding. Of the remainder, 10% have no identifiable cause, and are labelled non-aneurysmal SAH.

Rarer causes account for the last 5% of cases, including arterial dissection, rupture of an arteriovenous malformation, and posterior reversible encephalopathy syndrome (PRES). Subarachnoid haemorrhage is often seen in trauma, but is a completely different clinical entity and is not considered here.

Accounts for approximately 3% of all strokes, and occurs with an incidence of 10.5 per 100,000 person-years. It occurs more commonly in women, and is strikingly more common in Finland and Japan. Family history is an important risk factor, and having a first-degree relative with SAH increases the relative risk three to seven-fold. Three main modifiable risk factors have been identified: smoking, hypertension and heavy alcohol consumption.

SAH is a potentially devastating condition. Approximately half of all patients with SAH will die. Of the remainder, one-third will not recover functional independence.