An aneurysm can result from atherosclerosis (hardening and narrowing of the inside of arteries). As atherosclerosis develops, the artery walls become thick and damaged and lose their normal inner lining. This damaged area of artery can stretch or "balloon" from the pressure of blood flow inside the artery, resulting in an aneurysm.
An aneurysm also can develop from constant high blood pressure inside an artery.
A thoracic aortic aneurysm can result from an injury to the chest (for example, an injury that occurs from an auto crash). Certain medical conditions, such as Marfan syndrome, that weaken the body's connective tissues, also can cause aneurysms.
In rare cases, infections such as untreated syphilis (a sexually transmitted infection) can cause aortic aneurysms. Aortic aneurysms also can occur as a result of diseases that cause inflammation of blood vessels, such as vasculitis.
This information has been provided by National Heart Lung and Blood Institute