An aneurysm occurs at a weak point in the wall of a blood vessel (artery). Because of the flaw, the artery wall bulges outward. This bulge is called an aneurysm. An aneurysm can rupture, spilling blood into the surrounding body tissue. A particularly dangerous type of aneurysm is a cerebral aneurysm. A common form of cerebral aneurysm is a berry aneurysm, so-called because of its shape. If a cerebral aneurysm ruptures, it can cause permanent brain damage, disability, or death.

A ruptured aneurysm spills blood into the brain or into the fluid-filled area that surrounds the brain tissue. Bleeding into this area, called the subarachnoid space, is referred to as subarachnoid hemorrhage (SAH). Cerebral aneurysms can be caused by brain trauma, infection, hardening of the arteries (atherosclerosis), or cancer, but most seem to arise from a congenital, or developmental, defect.

Some aneurysms may have a genetic link and...