A haemorrhage into the subarachnoid space in the BRAIN. It is usually the result of rupture of an ANEURYSM on the CIRCLE OF WILLIS. It may occur in someone with no previous suspicion of any problem, sometimes following head injury or intense physical exercise. It causes a severe headache, often referred to by doctors as ‘a thunderclap’ and is a major emergency. The diagnosis is confirmed by CT scan or by identifying blood in the CEREBROSPINAL FLUID at LUMBAR PUNCTURE. Cerebral ANGIOGRAPHY will usually pinpoint the site of bleeding. Immediate treatment includes life-support measures and procedures to reduce blood pressure with referral to a neurosurgical team for consideration of surgery.