A brain (cerebral) aneurysm is a bulging, weak area in the wall of an artery that supplies blood to the brain. In most cases, a brain aneurysm causes no symptoms and goes unnoticed. In rare cases, the brain aneurysm ruptures, releasing blood into the skull and causing a stroke. High blood pressure. 2  The risk of subarachnoid hemorrhage is greater in people who have a history of high blood pressure. 3  Smoking. 4  In addition to being a cause of high blood pressure, the use of cigarettes may greatly increase the chances of a brain aneurysm rupturing.