An aneurysm (or aneurism) is a blood-filled bulge of a blood vessel. It is usually caused by disease or by the walls of the blood vessel becoming weak. Aneurysms usually happen in arteries at the base of the brain and in the aorta (the main artery coming out of the heart) - this is an aortic aneurysm. This bulge in a blood vessel can burst or break open and cause the person to die at any time. The larger an aneurysm becomes, the more likely it is to burst. Aneurysms can be treated.