Townsend: Sabiston Textbook of Surgery,
Copyright © 2007 Saunders, An Imprint of Elsevier
Aneurysms are classified into two main groups: true and false aneurysms. In true aneurysms, all three layers of the vessel wall are involved, whereas false aneurysms or pseudoaneurysms do not have all three layers of a vessel wall. Aneurysms are also distinguished by both morphology and etiology. The most common aneurysms are spindle-shaped fusiform aneurysms, with symmetrical enlargement involving the whole circumference of the artery. Aneurysms that affect only part of the arterial circumference are termed saccular. These eccentric aneurysms are believed to have a higher risk for rupture than fusiform aneurysms.
Based on etiology, aneurysms most commonly distinguished are degenerative aneurysms, caused by atherosclerotic changes in the vessel wall. The pathogenesis of aneurysms, as discussed later, is a multifactorial process involving genetic predisposition, aging, atherosclerosis, inflammation, and localized proteolytic enzyme activation. Congenital aneurysms and those associated with arteritides and connective tissue disorder are rare. Infected (mycotic) aneurysms are somewhat more common, and they frequently present as false aneurysms. Aneurysms may also occur with wall weakness caused by aortic or arterial dissection. Dissections lead to separation of the layers of the arterial wall due to a tear in the intima and varying thickness of the media. The term dissecting aneurysm is applied to dissections with aneurysmal dilation of the false lumen.
Aneurysmal enlargement can also result from post-stenotic dilation of an artery such as in the subclavian artery in patients with thoracic outlet syndrome or in aortic coarctation. Additional types of aneurysms include those associated with pregnancy and childhood. Etiologies of pseudoaneurysms include blunt or penetrating trauma, iatrogenic injury during arterial catheterization, and arterial graft anastomotic disruption.
The most frequent site of extracranial arterial aneurysms is the infrarenal aorta. In one large autopsy series of patients with aortoiliac aneurysms, the most frequent location was the abdominal aorta alone (65%), followed by the thoracic aorta alone (19%), the abdominal aorta and iliac arteries (13%), the thoracoabdominal aorta (2%) and iliac arteries alone (1%). Peripheral arterial aneurysms are much less common. Popliteal aneurysms account for about 70% of all peripheral aneurysms, femoral aneurysms are less frequent, and carotids constitute less than 4%. Visceral (splanchnic) and renal artery aneurysms have been considered rare, although their reported incidence due to frequent abdominal imaging has increased recently.
Aneurysm size is described by their width (anteroposterior or lateral diameter) and by their length. It is the width of the aneurysm and not the length that is the most important predicting factor of rupture. Arteriomegaly, originally described by Leriche in 1943, refers to diffuse arterial enlargement without discrete aneurysm formation, involving several segments of the arterial tree including the aorta and the iliac, femoral, and popliteal arteries. Arteries that are normally not prone to develop aneurysm, such as the external iliac and profunda femoris artery, are frequently dilated as well. In nearly 6000 patients who underwent aortofemoral arteriography at the Mayo Clinic, arteriomegaly was found in 5%. Three patterns were identified: type I, with aneurysms of the aorta to common femoral artery with more distal arteriomegaly; type II, with aneurysms of the femoropopliteal segment and more proximal arteriomegaly; and type III, with aneurysms of the aorta to popliteal artery with arteriomegaly of the intervening nonaneurysmal segments. Only males were affected, and they were about 5 years younger than the average-age patient with solitary aneurysms. This condition was distinct from multiple aneurysms that are separated by normal-diameter, nonectatic arterial segments, a condition often termed aneurysmosis. Gloviczki and associates reported on 102 patients with multiple aortic aneurysms who underwent 201 aortic reconstructions. Aneurysmosis was present in 3.4% of patients with aortic aneurysms, of whom 75% were men. Smoking history and abnormal electrocardiogram changes were present in 84%, hypertension in 75%, and obstructive lung disease in 61%. Eighteen percent with descending thoracic aortic aneurysms required surgical treatment of a second aortic aneurysm. In a population-based study by Bickerstaff and associates, 25% of patients with thoracic aneurysms had AAA. Patients with AAAs have a proclivity for lower extremity aneurysms. In a review of nearly 1500 patients with aortoiliac aneurysms, 3.5%, all men, had aneurysms in the femoral, popliteal, or visceral arteries. The likelihood of detecting an AAA in a man with a femoral artery aneurysm was 92%, and it was 64% for men with a popliteal artery aneurysm. In a recent Mayo Clinic review that included 141 (47%) unilateral and 158 (53%) bilateral popliteal artery aneurysms, AAA was present in 55% of patients, more frequently in those with bilateral than with unilateral popliteal artery aneurysms (65% versus 43%; Fig. 65-1 ).
Figure 65-1 (From Huang Y, Gloviczki P, Noel AA, et al: Early complications and long-term outcome after open surgical treatment of popliteal artery aneurysms: Is exclusion with saphenous vein by-pass still the gold standard? J Vasc Surg 45:706-715, 2007. By permission of Mayo Foundation for Medical Education and Research. All rights reserved.)