Brain haemorrhage. Coloured 3D angiogram of the skull and right common carotid artery (red) of a 28-year-old patient as part of an assessment into a subarachnoid haemorrhage (SAH) caused by a ruptured aneurysm. An aneurysm occurs when an artery wall weakens and swells like a balloon. If it bursts (as here), an aneurysm can cause severe internal haemorrhage. SAH is bleeding into the subarachnoid space - the area between the arachnoid membrane and the pia mater surrounding the brain.