An aneurysm is a dilation ("ballooning out") of an artery, due to yielding of the wall of the vessel and gradual stretching by the pressure of the blood. True aneurysms occur when the vessel wall is merely thinned and stretched but is still intact; false aneurysms occur when the wall has been thinned away from the inside until the blood is contained only by the greatly thickened fibrous tissues surrounding the artery. Two main factors combine to cause an aneurysm: 1) strain and 2) weakening of the wall of the artery. The artery wall may have been weakened by disease or the weakness may be congenital.
Aneurysms less than a centimetre in diameter rarely rupture and can be monitored with ultrasound. Without control, the aneurysms will continue to grow about a half centimetre a year. Eventually, all aneurysms will grow to a size that justifies repairing them. Because of their slow growth, they're usually found in people 65 and older.
The most common type of aneurysm is of the abdominal aortic artery, located in the belly just above its branch into each leg. Most people die when the artery ruptures.
The incidence of abdominal aortic aneurysms has increased three-fold over the past 40 years. It is the 13th-leading cause of death in the USA, accounting for 15,000 deaths a year. These aneurysms occur in 5 to 7 percent of people over age 60, but often carry no symptoms. Additional risk factors are high blood pressure, hardening of the arteries, smoking and diabetes. A person with a primary relative, such as a parent or sibling, with this condition has an 11-fold increased risk for developing aneurysms.