Aneurysm formation in experimental atherosclerosis: relationship to plaque evolution.

To determine whether aneurysms form in experimental diet-induced atherosclerosis, we reviewed our experience with cynomolgus monkeys (n = 268) and rhesus monkeys (n = 175) fed an atherogenic diet for various lengths of time. Many animals in long-term experiments were fed "regression" diets and cholestyramine to lower cholesterol levels after lesions were established. No aneurysms were found in animals on normal diet. There were no aneurysms in 252 animals fed an atherogenic diet with or without regression for 12 months or less. However, aneurysms formed in 13% of cynomolgus monkeys (4 of 31) and 1% (1 of 107) rhesus monkeys on an atherogenic regimen for 16 to 24 months. Four of the five animals with aneurysms were on a regression diet and cholestyramine for 4 to 12 months. The fifth was fed the atherogenic diet for 20 months without subsequent regression. Aneurysms were prominent and involved the thoracic and abdominal aorta, innominate artery, carotid arteries, iliac and femoral arteries, and formed in areas most involved with plaque formation in both species. Histologic evidence was found of thinning of the media and atrophy with loss of normal architecture. The higher incidence of aneurysms in cynomolgus monkeys was associated with greater media destruction than was noted in the rhesus. These data support the thesis that aneurysm formation is a manifestation of atherosclerosis. In primate atherosclerosis, aneurysms form only after prolonged exposure to the atherogenic regimen, even in the presence of declining serum cholesterol levels. Matrix fibers in plaques may provide structural support to the aortic wall where there is underlying atrophy of the media. With time or declining serum cholesterol levels or both, plaques may atrophy leaving an aortic wall too thin to support increasing mural tension, leading to aneurysmal enlargement.