very small blood vessels (the vasovasorum) that provide nutrients to the vessel wall.

An aneurysm may occur when a blister or bulge develops in weak areas of the aorta or peripheral arteries that expand under constant pressure from flowing blood. When the bulge grows to 50 percent or greater than the normal size of the vessel, it is classified as an aneurysm (Hiratzka et al., 2010; O’Gara, 2003). Modest arterial dilation that is less than 150 percent of the normal vessel dimension is commonly defined as vascular “ectasia.” Aneurysms can occur in the aorta as well as peripheral vessels such as the popliteal artery (the vessel behind the knee) or small vessels in the brain (sometimes referred to as “berry aneurysms”). Untreated, most aneurysms may grow to sizes that pose a major risk of rupture. A ruptured aortic aneurysm is a life-threatening event, making early detection and medical management important to avoid death. Similarly, a ruptured small-vessel aneurysm in the brain can cause a fatal hemorrhagic stroke or result in long-term neurological deficits.

The histopathology of aortic aneurysm and dissection is characterized by degeneration of the muscle or medial layer of the aortic wall as well as disruption of elastic fibers and increased deposition of connective tissue components such as proteoglycans. The former use of the term cystic medial necrosis is now recognized as a misnomer that inaccurately describes the progressive loss of smooth muscle cells in the aortic aneurysm. In most cases of aneurysmal disease, the process of medial degeneration is associated with a superimposed atherosclerotic lesion.

Aortic dissection occurs with a “tear” or disruption within the degenerated medial layer of the aorta, involving bleeding within and along the aortic wall. Dissection may occur without the presence of an aneurysm (Hiratzka et al., 2010). Genetic disorders such as Marfan and Loeys-Dietz syndromes affect the composition and function of the aorta such that individuals are predisposed to aneurysm formation and dissection at early ages.

An aortic dissection should be distinguished from an “intramural hematoma,” which lacks evidence of blood flow in a false lumen channel within the medial layer or evidence of a blood channel connection to the vessel lumen through an intimal tear. Intramural hematomas typically occur in the descending aorta of elderly individuals and are postulated to arise from a rupture of the small vessels in the outer layer that feed the aortic wall (the vasovasorum).

EPIDEMIOLOGY, NATURAL HISTORY, AND TYPES OF AORTIC ANEURYSMS

Abdominal aortic aneurysms (AAAs), involving the infradiaphragmatic abdominal area, are the most common type of aneurysms. In addition,



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement