Aneurysms

Abdominal Aortic Aneurysms

Abdominal aortic aneurysms (AAA) are defined as an aortic diameter of at least 1.5 times the diameter measured at the level of the renal arteries. In most people, this diameter works out to be 3 centimeters. Abdominal aortic aneurysms are the most common type of aneurysms and occur between the renal and inferior mesenteric arteries. (Anything located more superiorly is a thoracic aortic aneurysm.) They are true aneurysms and therefore involve all layers of the aorta. Atherosclerosis is considered the most common cause of AAA, although other etiologies include cystic medial necrosis and infections. Aneurysms are also associated with diseases such as Marfan and Ehlers-Danlos syndromes. In the United States, AAA kills 15,000 people per year and is the 10th most common cause of death for people older than 55 years.

Why and how abdominal aortic aneurysms develop is uncertain, although there are theories. The fact that AAA develops in many members of some families suggests a genetic basis, and chromosome 19 has been implicated. Another possibility involves enzymes called proteases that destroy important components of the arterial wall such as elastin and collagen. Finally, it is thought that inflammation in the arterial wall may play a role in the development of AAA as well.

Risk factors for the development of AAA include age greater than 60 (5 to 10 percent prevalence), male sex, Caucasian race, atherosclerosis, hypertension, and a family history of the disease. The strongest risk factor for developing AAA, however, is cigarette smoking. Cigarette smoking carries an odds ratio of 5 (i.e., a smoker is five times more likely to develop AAA than a nonsmoker) and a longer history of smoking confers a greater risk. It is estimated that smoking alone causes 75 percent of all AAA greater than 4 centimeters in diameter. Smoking also increases the rate at which an aneurysm grows.

Patients with AAA are frequently asymptomatic and the disease is discovered only incidentally via imaging studies obtained for other reasons. The two most important symptoms are abdominal or back pain as well as abdominal tenderness to palpation. Other findings on physical examination may include a pulsatile mass at or above the navel, which is approximately where the aorta divides and a common location for AAA, as well as an audible bruit (a harsh, shooting sound of blood flowing) when the abdomen is auscultated with a stethoscope. Patients with a ruptured aneurysm will be hypotensive due to exsanguination of blood into the abdominal cavity; the survival rate of patients with ruptured AAA is less than 50 percent. The diagnosis of AAA can be accomplished with physical examination and imaging studies. As mentioned, physical examination may reveal a pulsatile abdominal mass. Imaging studies include ultrasonography (preferred), computed tomography (CT), and magnetic resonance imaging (MRI).A small percentage of patients with AAA have an entity called inflammatory aneurysm. Additional findings of inflammatory aneurysm include weight loss and an elevated erythrocyte sedimentation rate (a type of blood measurement indicative of systemic inflammation).

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