Treating aneurysms that arise from the aortic segments distal to the left subclavian artery poses challenges that are distinct from those presented by aneurysms of the more proximal ascending and arch segments. The distal aortic segments comprise the descending thoracic aorta, which extends from the left subclavian artery to the diaphragm within the chest, and the abdominal segment that extends from the diaphragm to the iliac bifurcation. The diaphragm divides the thoracic aorta from the abdominal aorta.
An aortic aneurysm that is limited to the chest is classified as a descending thoracic aortic aneurysm (DTAA). An aortic aneurysm that traverses the diaphragm and extends into both the chest and the abdomen to any degree is considered a thoracoabdominal aortic aneurysm (TAAA) (Fig. 53-1). Aneurysms in these locations can be extensive and can involve many or all of the aortic branch vessels. Modern critical care and surgical adjuncts for organ protection have improved the outcomes of surgical repair; however, operative treatment of DTAA and TAAA continues to represent a significant clinical challenge to the cardiovascular surgeon.
Drawing depicting a thoracoabdominal aortic aneurysm.
The etiology of DTAA and TAAA has changed over time. Whereas tertiary syphilis was the most common cause of thoracic aneurysms in the early 1900s, other causes are more prevalent today. Well-established causes of DTAA and TAAA include medial degeneration, atherosclerosis, aortic dissection, connective tissue disorders, aortitis (eg, Takayasu's arteritis), aortic coarctation, infection, and trauma. As our understanding of genetics increases and as more advanced genetic testing becomes available, classification systems are likely to evolve to include more molecular factors. Perhaps partly because of improved screening for aneurysmal disease and the increasing age of the population, it is certain that the incidence and prevalence of thoracic aortic aneurysm are increasing over time.1
The most common types of aneurysms of the descending and thoracoabdominal aorta today are grouped into the category of atherosclerotic aneurysms. Unfortunately, although this term may be descriptive, it may not accurately describe the mechanism of aneurysmal changes. Although atherosclerosis and aortic aneurysms share common risk factors and frequently coexist, thoracic aortic aneurysms are primarily the result of age-related medial degeneration, which is characterized by changes in elastin and collagen that reduce aortic integrity and strength. Subsequent aortic enlargement and aneurysm formation provide fertile ground for superimposed intimal atherosclerosis and further degeneration of the aortic wall. The usual histologic changes in the aging aorta include elastin fragmentation, fibrosis with increased collagen, and medial degeneration.2 As with most aneurysmogenic processes, medial degeneration usually causes diffuse, fusiform aortic dilatation. In some cases, however, medial degeneration produces saccular aneurysms along the descending thoracic aorta. These saccular aneurysms may be superimposed on or coexist with more generalized, fusiform aneurysmal disease of the thoracoabdominal aorta.
Risk factors for aortic dissection and aortic aneurysm overlap to a ...