Site Specificity of Aneurysmal Disease

Aneurysms are localized pathological dilations of any vessel, and in arteries, a diameter of 1.5 times that of the normal artery is a generally accepted definition. Morphologically, aneurysms are usually considered to be either fusiform (Figures 1 and 2⇓) or saccular (Figures 3 and 4⇓). The transition from normal to aneurysmal diameter may be remarkably sudden, particularly in the case of saccular aneurysms. Different sizes and types of arteries may be affected. Although aneurysms are by definition focal, they may be multiple and associated with generalized arteriomegaly in some individuals. This highlights the importance of both systemic and focal factors in their pathogenesis. Only a minority of aneurysms have specific pathological causes, the underlying cause of most aneurysms seen in large to medium-sized arteries being unknown. Figure 1. A resected popliteal artery aneurysm demonstrating the focal nature of the pathology.Figure 2. A fusiform AAA with extension into the common (CIA) and internal (IIA) but not external (EIA) iliac arteries.Figure 3. An example of a saccular aneurysm of the posterior communicating artery (arrow).Figure 4. An example of a highly localized saccular AAA.The pathological processes involved in the formation of most degenerative aneurysms include upregulation of proteolytic pathways, inflammation, and loss of arterial wall matrix. Similar changes have been reported in cerebral, thoracic aortic, and abdominal aortic aneurysms. The natural history of most aneurysms is gradual expansion with increasing risk of rupture or, in some cases (particularly popliteal aneurysms), thrombosis, distal embolization, or both. Arterial dissection is distinct from true aneurysm formation, although in some animal models and in humans, dissection may progress to aneurysmal change and rupture. The reason that aneurysms are common in some locations and not others is equally unclear. The present review will explore possible explanations for the site specificity of aneurysmal …