Chapter 29: Thoracoabdominal Aortic Aneurysms

GENERAL CONSIDERATIONS AND HISTORY

Aortic aneurysm disease overall leads to 10,000 deaths annually, much of which is likely secondary to the failure to identify and diagnose disease in these patients. Aortic aneurysms are the ninth overall cause of death in the United States.¹ These are defined as localized, full-thickness arterial dilations measuring ≥50% of the normal diameter. Descending thoracic aortic aneurysms (TAAs) and thoracoabdominal aortic aneurysms (TAAAs) account only for 2% to 5% of these, as the vast majority are abdominal.

The descending thoracic aorta is defined as beginning distal to the aortic arch, at the origin of the left subclavian artery, and continuing down until just proximal to the takeoff of the celiac axis. Thoracoabdominal aneurysms account for ~10% of thoracic aneurysms and include any portion of the descending thoracic aorta as well as the visceral and/or infrarenal segments.²

The pathogenesis of most TAAs is degenerative in nature, characterized by a loss of elastic fibers from the medial layer, as well as loss of smooth muscle and proteoglycan deposition. This is thought to be secondary to increased activity of matrix metalloproteinases (MMP-2, MMP-9), leading to lysis of elastic fibers and medial remodeling. TGF-b is also involved in regulation of these matrix metalloproteinases, and mutations or dysregulation of this cytokine is often seen in associated diseases.³

A small fraction of TAAs is secondary to genetic and connective tissue disorders, inflammatory vasculitides, or infection (mycotic). Genetic disorders include Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome, Turner syndrome, bicuspid aortic valve, and familial TAA and dissection syndrome. Inflammatory conditions include Takayasu arteritis, giant cell arteritis, and Behcet’s disease.

Associated risk factors include hypertension, smoking, cocaine use, COPD, large initial diameter, female gender, history of cardiac/renal transplant, and certain patterns of wall stress. Rupture risk is most closely correlated with the maximal cross-sectional diameter of the aneurysm but is also associated with the development of a dissection within the aneurysm. The annual rupture risk when the aneurysm size is <5 cm is ~1.5%, but this increases sharply when aneurysm size is >6 cm, with risk of dissection, rupture, or death occurring in ~15%.^4,5Thoracic aneurysms, on average, tend to grow ~0.1 cm/year.²

CLINICAL FINDINGS

Most TAA are discovered incidentally during workup for an unrelated diagnosis. These patients are usually asymptomatic. For those who are symptomatic, this is usually secondary to local compression on surrounding structures, including the esophagus, recurrent laryngeal nerve, trachea, or vertebral bodies, or from mediastinal pleural inflammation. Back pain is the most commonly experienced symptom. Symptomatic TAA are associated with an increased risk of rupture.

Descending thoracic aortic aneurysms (DTAA) are classified by location within the aorta (Figure 29-1).⁶ Type A DTAAs extend from distal to the left subclavian to the sixth intercostal space. Type B extend from the sixth intercostal space ...