Thoracic Aortic Aneurysms

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CHAPTER 92 Thoracic Aortic Aneurysms

Thoracic aortic aneurysms can result from a variety of causes. The underlying cause of a thoracic aortic aneurysm can typically be predicted by its location and morphologic features and by the age of the patient.1 Whereas the overarching goal of therapy remains similar (i.e., to prevent complications, notably aortic rupture), the nature, timing, and associated operative interventions can differ significantly according to the location and cause of the aneurysm. An example is that of the ascending aortic aneurysm, which by its location is associated with the additional considerations of whether to replace the aortic valve, to reimplant the coronary arteries, and to repair the arch vessels. In addition, assessment of global cardiovascular function is paramount in directing appropriate treatment strategy.

Thoracic aortic aneurysms are less common than their abdominal counterpart.2 Imaging plays a critical role in diagnosis, treatment planning (i.e., assessment of the need for intervention, urgency of intervention, and type of intervention), and postsurgical surveillance. Moreover, some complications, such as spinal cord ischemia, are germane to surgical repair of thoracic aortic aneurysms, and imaging can provide a road map that may allow prospective modification of surgical technique to reduce the chances of such complications.

THORACIC AORTIC ANEURYSM

Definition

The traditional definition of an aneurysm is dilation of a blood vessel wall so that the resulting caliber is 50% greater.3 This size-based definition does not account for morphologic characteristics such as focal saccular dilation of the aorta due to trauma, penetrating atherosclerotic ulcer, and infection. These scenarios require an “aneurysm mentality” because saccular aortic dilations are at particular risk for rupture and are thus also classified as aneurysms.

In true aneurysms, the dilation involves all layers of the blood vessel wall. False aneurysms (also known as pseudoaneurysms or saccular aneurysms) occur from disruption of one or more layers of the aortic wall.4

Etiology and Pathophysiology

The following paragraphs classify aneurysms by underlying etiology and pathogenesis, location, and whether they are true or false. The purpose of such a classification affects the search for associated vascular lesions, the surgical approach, and the potential complications.

Etiology and Pathogenesis

Cystic Medial Necrosis

As the name implies, cystic medial necrosis affects the medial layer of the arterial wall; degeneration of the smooth muscle creates “cystic spaces,” resulting in a fusiform aneurysm.4 This is the most common cause of aneurysms of the ascending aorta. The pathophysiologic process involves the aortic root, resulting in dilation of the annulus of the aortic valve. Associated aortic regurgitation may require concomitant replacement of the aortic valve.6 Cystic medial necrosis is the hallmark of the pathologic changes in Marfan syndrome (Figs. 92-3 to 92-6). Other connective tissue disorders, such as Ehlers-Danlos and Loeys-Dietz (Fig. 92-7) syndromes, also affect the medial layer of the aorta.4 These entities are both familial and have an identifiable gene leading to the abnormal biochemistry.

Marfan syndrome is the most common of the connective tissue processes, with an incidence of 1:10,000.7 Marfan syndrome is an autosomal dominant condition that results in a mutation in the gene encoding fibrillin 1,8 an essential protein for elastic properties, causing cardiovascular and musculoskeletal abnormalities. The elastin-depleted aorta is stiffer and more prone to dilation as it incurs higher pulse pressure than the normally distensible aorta does. The dilation starts in the root and extends to the mid-ascending aorta.9 Aortic rupture and dissection are the leading causes of death in patients with Marfan syndrome.7 Repair in patients with Marfan syndrome is recommended in the asymptomatic patient when the aortic root or ascending aorta exceeds 5 cm in diameter because of the high risk of aortic rupture and aortic dissection.10 Associated cardiovascular abnormalities include aortic insufficiency and mitral valve prolapse, which frequently necessitate valve repair, and pulmonary artery aneurysms.9

Loeys-Dietz syndrome has only recently been characterized as a distinct phenotype that is caused by mutations in genes encoding type 1 or type 2 transforming growth factor β.11 Aneurysms form at an earlier age than in other connective tissue disorders and tend to rupture at a smaller size, with a greater propensity for dissection. The arteriopathy tends to be more systemic than in Marfan syndrome, and the postoperative surveillance must factor both the repaired artery and the remote arteries including the intracranial circulation.

Vascular Ehlers-Danlos syndrome is an autosomal dominant disorder caused by heterozygous mutations of the COL3A1 gene. The syndrome is characterized by fragile arterial tissue that not only is prone to aneurysm, dissection, and rupture but can also make surgical repair difficult. Noninvasive imaging, such as computed tomographic angiography (CTA) and magnetic resonance angiography (MRA), is preferred because of the risk of dissection and rupture with arterial access. Unlike Marfan and Loeys-Dietz syndromes, which have a predilection for the aortic root, Ehlers-Danlos syndrome more often affects the visceral arteries.12

Location

In general, ascending aortic aneurysms are due to cystic medial necrosis. The more common descending thoracic aortic/thoracoabdominal aortic aneurysms are caused by atherosclerosis.

Ascending aortic aneurysms may involve only the supracoronary aorta and spare the aortic root, involve the aortic root only or as well, or result in diffuse tubular dilation.

Thoracoabdominal aortic aneurysms are further divided by the Crawford classification (Fig. 92-12), which is used to determine the operative approach and to counsel the patient about postoperative complications. Crawford I and II start distal to the origin of the left subclavian artery, with Crawford II extending below the renal artery origin. Crawford III starts more distal than Crawford I in the descending thoracic aorta. Crawford IV is essentially an aneurysm of the abdominal aorta that extends to the diaphragmatic hiatus.18

image

image FIGURE 92-12 The Crawford classification of thoracoabdominal aortic aneurysms.

(Reproduced with permission from Coselli J, Lemaire S. Descending and thoraco-abdominal aortic aneurysms. In Kohn LH. Cardiac Surgery in the Adult, 3rd ed. New York, McGraw-Hill, 2007.)

Manifestations of Disease

Clinical Presentation

Aneurysms may be detected incidentally, be manifested through local mass effect or systemic symptoms, or cause symptoms from acute rupture.1,19