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 smoking, increasing age, white race, hypertension, hyperlipidemia, connective tissue disorders, and familial history of aneurysm.¹

PATHOPHYSIOLOGY

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, 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, and larger aneurysms are more likely to rupture. Rupture is catastrophic, occurring once the stress on the vessel wall exceeds its tensile strength. Patients with low socioeconomic position are at greater risk to present with a ruptured aortic aneurysm (versus intact aneurysm) and are more likely to have poor outcomes after surgical repair.²

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.³ Small pseudoaneurysms may eventually spontaneously thrombose.

Mycotic aneurysms and infected aneurysms occur due to an infection in the vessel wall, often in an immunocompromised patient. A mycotic aneurysm occurs secondary to a septic embolization from valvular endocarditis, while an infected aneurysm develops in an existing aneurysm after bacteremia or direct extension from a neighboring infection.⁴ IV drug abuse is a significant risk factor for both types of these aneurysms. The clinician should be vigilant for this diagnosis in patients abusing IV drugs because the misdiagnosis of cellulitis or abscess is possible.⁴

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 coexist with contralateral popliteal aneurysms or abdominal aortic aneurysms.⁵ 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,⁶ and thrombosis, creating ischemia in the perfused organ.^7,8