An aneurysm is a bulge or ballooning of the weakened wall of a blood vessel or of the heart. If the bulge stretches too far, it may leak or tear (rupture).

Aneurysms are most common in the aorta, the main blood vessel that carries blood from the heart to the rest of the body. When a rupture occurs in the aorta, it causes heavy bleeding. The sudden heavy bleeding causes shock and often rapid death.

When an aneurysm in an artery of the brain ruptures, it causes a stroke.

It is not known for sure why aneurysms occur in some people and not others. People with the greatest risk are men over age 55. Men are 5 times more likely to have abdominal aortic aneurysms than women. Brain artery aneurysms affect both men and women of all ages. Aneurysms may be caused or worsened by hardening of the arteries, birth defects, high blood pressure, infection, injuries, and smoking.