Abdominal Aortic Aneurysm

Abdominal aortic aneurysm (AAA) is a true aneurysm, meaning a localized dilation of the aorta caused by weakening of its wall, and involving all three layers (intima, media, and adventitia) of the arterial wall (Fig. 86-1). A false aneurysm, or pseudoaneurysm, is a collection of flowing blood that communicates with the arterial lumen but is not enclosed by the normal vessel wall; it is contained only by the adventitia or surrounding soft tissue. Pseudoaneurysms can arise from a defect in the arterial wall or a leaking anastomosis after AAA repair. AAA is distinct from aortic dissection, which sometimes is incorrectly called a “dissecting aortic aneurysm.” In aortic dissection, blood enters the media of the aorta and splits (dissects) the aortic wall (see Chapter 85). Aortic aneurysm and aortic dissection are very different diseases, with different clinical presentations, complications, diagnostic methods, and treatments. An aneurysm can develop in any segment of the aorta, but most involve the aorta below the renal arteries. The diameter of the normal adult infrarenal aorta is approximately 2 cm, and a diameter of 3 cm or more defines an AAA.