When a cerebral aneurysm ruptures, blood will fill the space surrounding the brain.

What is it?  Subarachnoid hemorrhage occurs when a blood vessel just outside the brain ruptures. The area of the skull surrounding the brain (the subarachnoid space) rapidly fills with blood. A patient with  subarachnoid hemorrhage may have a sudden, intense headache, neck pain, and nausea or vomiting.  Sometimes this is described as the worst headache of one's life. The sudden buildup of pressure outside the brain may also cause rapid loss of consciousness or death.

What causes it?  Subarachnoid hemorrhage is most often caused by abnormalities of the arteries at the base of the brain, called cerebral aneurysms. These are small areas of rounded or irregular swellings in the arteries. Where the swelling is most severe, the blood vessel wall become weak and prone to rupture. View an interactive tutorial on cerebral aneurysms from the Toronto Brain Vascular Malformation Study Group. (Requires Flash.)

Surgical treatment of aneurysms involves placing clip on neck of aneurysm.

Who gets it?  The cause of cerebral aneurysms is not known. They may develop from birth or in childhood and grow very slowly. Some people have not one, but several aneuryms.   Subarachnoid hemorrhage can occur at any age, including teenagers and young adults. Subarachnoid hemorrhage is slightly more common in women than men.