39: Ascending and Arch Aneurysms of the Aorta

Key Concepts

• Epidemiology

  • The leading cause of aneurysms of the ascending aorta is medial degeneration, which may be idiopathic or accelerated by heritable disorders of connective tissue; conversely, arch aneurysms are due to chronic dissection or long-standing hypertension and atherosclerosis.

• Pathophysiology

  • Histopathologic investigations of aneurysmal tissue have shown that a majority of ascending aortic and arch aneurysms are associated with medial degeneration in the layers of the aortic wall. This is characterized by a triad of (1) loss of noninflammatory smooth muscle cells, (2) fragmentation of elastic fibers, and (3) accumulation of basophilic ground substance in cell-depleted areas of the medial layer of the vessel wall. This combination of factors weakens the aortic wall, thereby increasing wall tension; this can induce aortic dilatation and aneurysm formation, leading to higher wall stress and eventually resulting in intramural hemorrhage, aortic dissection, or rupture. Medial degeneration can be accelerated by inborn errors of metabolism that affect any component of the aortic connective tissue. Known heritable disorders of connective tissue that commonly manifest as ascending and arch aneurysms of the aorta are Marfan syndrome, Ehlers–Danlos syndrome, Loeys–Dietz syndrome, familial forms of thoracic aortic aneurysm and dissection, and bicuspid aortic valve disease; Turner syndrome and Noonan syndrome occur far less frequently.

• Clinical features

  • Most patients with thoracic aortic aneurysms are asymptomatic at the time of presentation, and aneurysms of the ascending aorta usually are detected as incidental findings during testing for other disorders. In contrast, because of their anatomic location and the space-occupying nature of the aneurysm, aneurysms of the aortic arch may impinge on important mediastinal structures and are therefore symptomatic more often than aneurysms of the ascending or descending aorta. Symptoms attributable to the aneurysm are cause for concern and consideration for early surgical intervention. Patients with rupture of an ascending or arch aneurysm often present in extremis.

• Diagnostics

  • The most common finding on chest x-ray for a thoracic aortic aneurysm is widening of the superior mediastinum; other findings include displacement and compression of the mediastinal contents and curvilinear calcification in the aneurysmal wall. Echocardiography is widely available, can be noninvasive [transthoracic echocardiography (TTE)] or minimally invasive [transesophageal echocardiography (TEE)], and is relatively inexpensive. TTE can visualize the aortic root and ascending aorta accurately and is suitable for evaluating the size of moderate aneurysms and assessing cardiac complications such as aortic regurgitation (AR), impaired left ventricular function, and hemopericardium. Intraoperative application of TEE allows continuous monitoring of the surgical procedure without disturbing the sterile field. Because of its speed and wide availability, computed tomography/computed tomography angiography (CT/CTA) is currently the most common diagnostic imaging method for studying the aorta. CTA has the advantage of demonstrating aortic wall thickening, calcification, and luminal thrombus, thus displaying the true axial extent of an aneurysm. Magnetic resonance angiography is emerging as a diagnostic tool but is less readily available and has limited utilization in ...