A localized, abnormal arterial dilation usually caused by a weakening of the vessel wall. Aneurysms are commonly found in the abdominal aorta, intracranial arteries, and thoracic aorta; however, they may also involve the femoral, popliteal, splenic, carotid, or renal arteries. Aneurysms vary in size from less than 1 in. (2–3 cm) to more than 4 in. (10 cm). They are usually classified as true or false: true aneurysms involve all layers of the artery (inner endothelium or intima, middle muscular layer or media, and outer connective tissue or adventitia); false aneurysms do not involve all layers. With the exception of intracranial aneurysms, atherosclerotic vessel disease is generally the cause of aneurysms; other causes include syphilis, trauma, cystic medial necrosis, bacterial infections, and arteritis.