Chapter 60: Aneurysmal Disease

INTRODUCTION

An aneurysm is dilation of the arterial wall to >1.5 times its normal diameter. Aneurysms have been classically distinguished as true aneurysms, pseudoaneurysms, and mycotic aneurysms. The wall of a true aneurysm involves of all layers of the vessel. Risk factors for these include connective tissue disorders, familial history of aneurysm, and atherosclerotic risk factors (i.e., age, smoking, hypertension, and hyperlipidemia). A progressive decrease in elastin, collagen, and fibrolamellar units results in thinning of the media of the vascular wall and a decrease in its tensile strength. In aortic true aneurysm, the dilatation and increased wall force are intertwined, creating more dilatation (Laplace law: wall tension = pressure × radius). The rate of aneurysmal dilatation is variable and predictable, with larger aneurysms expanding more quickly and changing a mean 0.25 to 0.5 cm per year. However, abrupt expansion occurs and is not predictable, with larger aneurysm more likely to rupture. Rupture is catastrophic, occurring once the stress on the vessel wall exceeds its tensile strength.

The wall of a pseudoaneurysm consists partly of the vessel wall and partly of fibrous or other surrounding tissue. A pseudoaneurysm can develop at the site of previous vessel catheterization and at anastomoses from prior vascular reconstruction, trauma, or infection.¹ Small pseudoaneurysms may eventually spontaneously thrombose.

A mycotic aneurysm develops as a result of infection in the vessel wall, often in an immunocompromised patient. The source can be direct extension from a neighboring infection or embolization from valvular endocarditis.

Peripheral and visceral aneurysms are less frequent but an important subset of arterial aneurysmal disease. Popliteal artery aneurysms are the most common peripheral aneurysm; they often co-exist with contralateral popliteal aneurysms or abdominal aortic aneurysms.² Aneurysms of the femoral artery are uncommon and often accompany aneurysmal disease at other sites. Visceral artery aneurysms may occur anywhere but are most common in the renal, splenic, and hepatic arteries. Most visceral aneurysms remain silent and undetected until a complication such as rupture occurs. All but splenic artery aneurysms are more common in elderly men. Complications of aneurysms include rupture, which has an 80% mortality rate,³ and thrombosis, creating ischemia in the perfused organ.^4,5

GENERAL CLINICAL FEATURES OF ANEURYSMS

Clinical signs and symptoms can be nonspecific; often, the symptoms are driven by location, the pressure exerted upon neighboring structures, or the signs of peripheral embolization from an intramural thrombus. Visceral aneurysms are often detected after an abdominal CT scan for abdominal or flank complaints; similarly, lower extremity aneurysms are often detected during an extremity Doppler US examination in a search for deep venous thrombosis. Once rupture occurs in any truncal aneurysm, hemorrhagic shock develops and mortality is high without prompt surgical intervention.

SYMPTOMATIC ABDOMINAL AORTIC ANEURYSMS

An abdominal aortic aneurysm is defined as an aorta ≥3.0 cm in diameter; ...