aortic aneurysm, a localized dilation of the wall of the aorta caused by atherosclerosis, hypertension, connective tissue disease such as Marfan’s, or less frequently, syphilis. The lesion may be a saccular distension or a fusiform or cylindrical swelling of a length of the vessel. Syphilitic aneurysms almost always occur in the thoracic aorta and usually involve the arch of the aorta. The more common atherosclerotic aneurysms are usually in the abdominal aorta below the renal arteries and above the bifurcation of the aorta. These lesions often contain atheromatous ulcers covered by thrombi that may discharge emboli, causing obstruction of smaller vessels. See also dissecting aneurysm.
