An aneurysm is an abnormal bulge in the wall of an artery. Normally, the walls of arteries are thick and muscular, allowing them to withstand a large amount of pressure. Occasionally, however, a weak area develops in the wall of an artery. This allows the pressure within the artery to push outwards, creating a bulge or ballooned area called an "aneurysm."
Aneurysms can form in any blood vessel, but they occur most commonly in the aorta (aortic aneurysm). The aorta is the largest artery in the body. It carries blood from the heart to the rest of the body. Aortic aneurysms can occur in two main places:
Congenital abnormality. A number of conditions, such as Marfan syndrome or bicuspid aortic valves are present at birth and can cause weakness of the artery walls.
Syphilis used to be a common cause of thoracic aneurysms, but it is no longer as common.
Note: Instead of causing a bulge in a thinned artery wall, aortic aneurysms occasionally occur between layers of the artery itself. This is called a "dissecting aneurysm." Blood starts to flow in the separated artery layers cutting off blood flow in the artery. This condition can rapidly lead to rupture of the artery.