Natural history and indications for treatment

Abdominal aortic aneurysm (AAA) is a localized bulge in the abdominal aorta. Left untreated, aneurysms dilate slowly, usually producing no symptoms until they burst. Death from aneurysm rupture is often rapid and at least 62% of patients die before reaching the hospital [1]. Overall mortality for ruptured AAA, including hospital deaths, may approach 90% [2]. The greatest potential for a reduction in aneurysm-related mortality therefore lies in the identification, and prophylactic repair, of aneurysms that have not yet ruptured.

National Center for Health Statistics (NCHS) data indicate that there are approximately 15 000 deaths resulting from aneurysm rupture each year in the USA [3]. Risk factors include advanced age, male gender, smoking, and a family history of aneurysms. They may affect 1-2% of men older than 50 years [4].

A diagnosis of AAA presents three options: observation, surgery, or endovascular repair. This choice is influenced by the diameter and shape of the aneurysm and by the age, gender, and health of the patient. Although aneurysm diameter is a strong predictor of rupture risk, patient size is also a factor, so that for any given diameter, the risk of rupture is higher in a women than in a man. Gender also influences the decision to intervene through its effects on the risk of treatment: women fare less well than men following aneurysm repair, by open or endovascular means [5]. Life expectancy is important in assessing the value of elective AAA repair, because the patient has to live long enough for the cumulative risk of aneurysm rupture to exceed the immediate risks of intervention [6].

Data on rupture risk in patients who are followed with conservative (i.e. nonsurgical) management should be interpreted cautiously, especially because the factors on which patients are selected for nonsurgical management may strongly affect the prognosis. For example, severe comorbidity will increase the likelihood of nonsurgical management (while increasing the risk of rupture), whereas an aneurysm that appears to be expanding rapidly (which also increases the risk of rupture) will be more likely to be treated surgically. That being said, a review of the literature of the natural history of AAAs revealed that 14 studies involving 393 patients with aneurysms < 5 cm in diameter were associated with a rupture rate of 1.0% at 1 year, whereas four studies involving 131 patients with aneurysms > 5 cm in diameter were associated with a 1 year rupture rate of 8.5% [7]. The same review also demonstrated that large aneurysms expand at a greater rate than small aneurysms (0.2-0.4 cm/year for aneurysms < 54 cm, 0.3-0.7 cm/year for aneurysms > 5 cm).

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