Severe arteriosclerosis a major factor. Mutation of gene encoding type III procollagen implicated. Syphilis and other bacterial infections. Cystic medial necrosis.
Arteriosclerosis causes gradual destruction of the media with focal weakening of the wall.
An incremental increase in pressure causes an increased pressure on the inner surface of the evolving aneurysm with an incremental increase in the radius, establishing a vicious cycle. Familial cases indicate the importance of genetic defects in connective tissues making the aorta susceptible to the formation of aneurysms due to arteriosclerosis. More common in men. Typically involves the abdominal aorta from just below the ostia of the renal arteries to the bifurcation of the aorta. Aneurysmal thrombus does not organize due to the paucity of functioning vasa vasorum in the fibrotic thinned out aneurysmal wall.