There are two major classifications of aneurysm: true aneurysms and false aneurysms or pseudoaneurysms. True aneurysms occur as a result of atrophy of the medial layer of the artery. The wall dilates but at least one wall of the artery remains intact. True aneurysms are further subdivided based on shape into a saccular aneurysm, a bulge with a narrow neck or spherical in shape connecting it to one side of the artery, and a fusiform aneurysm, circumferential and relatively uniformly shaped. Saccular aneurysms are associated with syphilis or congenital malformations. A false aneurysm involves the dissection of all the layers of the arterial wall and forms as a result of trauma, infection, or disruption of a suture line. This causes an extravascular hematoma or tamponade. Aortic dissection, sometimes referred to as dissecting aneurysm, occurs when a tear in the inner layer of an arterial wall allows blood to collect between layers of the wall. It is an acute, life-threatening condition requiring emergency surgery and has a high mortality rate.