Risk factors for aneurysms include connective tissue disorders, familial history of aneurysm, and atherosclerotic risk factors (age, smoking, hypertension, hyperlipidemia, and diabetes). These risk factors combine to increase the expansile force on the aortic wall or to impair the patient's ability to withstand these forces. The Laplace law [wall tension = (pressure * radius)/tensile force] dictates that as the aorta dilates, the force on the aortic wall increases, therefore causing further aortic dilation.
The destruction of the media of the aorta is a prominent feature in aneurysm pathogenesis. Elastin and collagen are markedly reduced in the aneurysmal aorta and fibrolamellar units are dramatically decreased. In addition, the normal abdominal aorta has a decreasing number of elastic lamellae as the aorta becomes more distal. This may help account for the prominent infrarenal location of aneurysm in many patients.
Histologic examination of an aneurysmal wall will show an intima that is infiltrated by atherosclerosis and a thinned media. There may be intraluminal thrombus, and the adventitia has often been infiltrated by inflammatory cells. Patients with saccular outpouchings (blisters) may have an increased risk of rupture. Such blisters have been shown to increase the incidence of rupture of small (less than 5-cm) aneurysms.4
Rate of aneurysmal dilation is variable. As a consequence of the LaPlace law, larger aneurysms will expand more quickly than smaller ones. An average rate may be 0.25 to 0.5 cm per year.4 Patients with known aneurysms must be followed closely for unpredictably fast expansion.
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