A brain aneurysm (or cerebral aneurysm) is a weak area on the wall of a brain artery that balloons out and fills with blood. This bulging pocket or sac can put pressure on other parts of the brain, which may or may not cause symptoms. In some cases the weakened blood vessel may burst (rupture or bleed), spreading blood into the surrounding tissue (called a hemorrhage).
Not all brain aneurysms burst or cause symptoms. Approximately 6 million people in the United States (about 1 in 50) are living with an unruptured brain aneurysm. However, nearly one-third of those who have a ruptured brain aneurysm will die as a result, and more than half who survive will have some permanent disability.