Coloured computed tomography (CT) scan of a coronal section through the brain showing a subarachnoid haemorrhage (SAH) with signs of bleeding. A SAH is a rare, life-threatening type of stroke caused by bleeding into the space surrounding the brain (subarachnoid space). It often occurs when a weak area in a blood vessel on the surface of the brain bursts and leaks (aneurysm). One of the first symptoms is often a severe headache, followed by loss of consciousness. A subarachnoid haemorrhage is a medical emergency and immediate treatment is needed to stop the bleeding and prevent lifelong brain damage.