Excerpt from Aneurysm, Abdominal

Synonyms, Key Words, and Related Terms: abdominal aneurysm, abdominal aortic aneurysm, AAA, mycotic aneurysm, abscess formation, atherosclerosis, smoking, chronic obstructive pulmonary disease, COPD, hypertension, syncope, shock, cyanosis, sudden cardiovascular collapse, peripheral atherosclerotic vascular disease, Marfan syndrome, Ehlers-Danlos syndrome, collagen vascular diseases, mycotic aneurysm, claudication, pulsatile abdominal mass, abdominal bruit, aortic rupture

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Background: Abdominal aortic aneurysm (AAA) is a relatively common, potentially life-threatening condition. It has a wide spectrum of presentations and should be considered in the differential diagnosis for a number of symptoms. AAA is usually the result of degeneration in the media of the arterial wall, resulting in a slow and continuous dilatation of the lumen of the vessel. In fewer than 5% of cases, AAA is caused by mycotic aneurysm of hematogenous origin. In these cases, local invasion of the intima and media gives rise to abscess formation and aneurysmal dilation of the vessel. Gram-positive organisms most commonly cause mycotic aneurysm. As with aneurysm of the thoracic aorta, AAA may be described as fusiform, which is circumferential, or saccular, which is more localized.

Pathophysiology: The 3 layers comprising the normal aorta are the intima, media, and adventitia. Structural and elastic properties of major arteries are mostly imparted by the media, which is composed of smooth muscle cells surrounded by elastin, collagen, and proteoglycans. AAA develops following degeneration of the media due to atherosclerotic changes. The degeneration ultimately may lead to widening of the vessel lumen and loss of structural integrity.

Most AAAs occur in association with advanced atherosclerosis. Atherosclerosis may induce AAA formation by causing mechanical weakening of the aortic wall with loss of elastic recoil, along with degenerative ischemic changes, through obstruction of the vasa vasorum. Many patients with advanced atherosclerosis do not develop AAA, while some patients having no evidence of atherosclerosis do. The observed association between atherosclerosis and AAA is probably not causative; however, atherosclerosis may represent a nonspecific secondary response to vessel wall injury that is induced by multiple factors.

Frequency:

In the US: Ruptured AAA is the 13th-leading cause of death in the US, causing an estimated 15,000 deaths per year. The incidence of AAA is 2-4% in the adult population, and 11% of cases in that subset occur in males older than 65 years. Despite increased survival following diagnosis, incidence and crude mortality seem to be increasing.

Mortality/Morbidity: In 1988, 40,000 surgical reconstructions for AAA were performed in the US, with substantial mortality differences between elective versus emergency operations. As elective aneurysm repair has a mortality rate drastically lower than that associated with rupture, the emphasis must be on early detection and repair free from complications.

Race: White males have the highest incidence of AAA.

Sex: Males are affected 7 times more often than females.

Age: More than three fourths of patients with AAA are older than 60 years.