Etiology

The causes of thoracic aortic aneurysms are summarized in Table 11-1. Atherosclerotic degeneration is the most common cause of aneurysms involving the descending thoracic and thoracoabdominal aorta. Risk factors include increased age, smoking, and hypertension.

Table 11-1 Causes of Thoracic Aortic Aneurysms

Cystic medial necrosis is a degenerative condition characterized by fragmentation of the elastic fibers and loss of smooth muscle within the medial layer of the aortic wall, resulting in a loss of tensile strength. Cystic medial necrosis typically affects the ascending aorta and can lead to aneurysm and dissection. The condition is more common in males than females and typically develops in people over 40 years of age. Some degree of medial degeneration of the aorta is part of the normal aging process and accounts for the age-related increase in aortic dimensions. Marfan syndrome is an autosomal-dominant connective tissue disorder characterized by defective synthesis of the glycoprotein fibrillin, predisposing to cystic medial necrosis. Patients with Marfan syndrome are at high risk for developing early-onset aneurysms and dissections of the thoracic aorta.

Aneurysms of the proximal ascending aorta may result from chronic flow turbulence due to aortic valve stenosis (poststenotic dilatation). In addition, patients with bicuspid or unicuspid aortic valves are at increased risk for aortic aneurysm formation due to primary structural abnormalities of the aortic wall.¹ Thoracic trauma, usually a sudden deceleration injury, is a rare cause of chronic aneurysm formation. This is usually caused by intimal disruption in the area of the aortic isthmus. A pseudoaneurysm (false aneurysm) is a collection of thrombus and connective tissue outside the aortic wall that occurs as a result of a contained rupture. Pseudoaneurysms typically occur secondary to infection, trauma, or previous aortic surgery.

Classification

Aneurysms of the thoracic aorta can be classified on the basis of their anatomic position. Ascending aortic aneurysms arise anywhere from the aortic valve to the innominate artery. Aortic arch aneurysms involve aorta bounded by the brachiocephalic vessels. Descending thoracic and thoracoabdominal aneurysms arise distal to the left subclavian artery. Thoracoabdominal aneurysms are further divided according to the Crawford classification (see Fig. 11-1).

Figure 11.1 The Crawford classification of thoracoabdominal aneurysms. Type I aneurysms arise distal to the left subclavian artery and extend to the level of the superior mesenteric artery but above the renal arteries. Type II aneurysms arise distal to the left subclavian and extend to below the renal arteries. Type III aneurysms arise from below the level of the sixth intercostal space and extend to below the renal arteries. Type IV aneurysms arise from the level of the twelfth intercostal space (supraceliac) and extend to below the renal arteries.

Natural History

An aortic aneurysm is a slowly progressive but intrinsically lethal condition that eventually undergoes rupture or dissection. The primary determinant of adverse outcome is aneurysm size: a patient with a maximal aortic dimension of 6 cm has an annual risk for rupture, dissection, or death of 14%.²

The growth rate of aneurysms is not linear, tending to accelerate as the aneurysm increases in size (see Wall Stress and Aneurysm Growth in Chapter 1). Aneurysms grow faster in patients who smoke, in patients who are hypertensive, in aortas with dissections, and in the thoracic aorta rather than the abdominal aorta.

Surgical intervention is recommended when the maximal aortic diameter exceeds 5.5 cm in the ascending aorta and 6 to 6.5 cm in the descending aorta.^2,3 These recommendations are based on the relative risks for rupture or dissection compared with the risks involved in surgery.

Aneurysms caused by Marfan syndrome display accelerated unpredictable growth patterns and have a tendency to rupture or dissect at a smaller size than do aneurysms in patients without Marfan syndrome. This is particularly the case if there is a family history of early complications. Surgery is recommended for patients with Marfan syndrome when the maximal diameter exceeds 5 cm in the ascending aorta and 6 cm in the descending aorta.²

Clinical Features and Investigations

Thoracic aortic aneurysms are commonly asymptomatic and are usually discovered incidentally when a chest radiograph or echocardiogram is performed for some other reason. A proportion of patients experience chest pain, which is caused by rapid expansion of the aneurysm itself or, rarely, by bony erosion. Occasionally, a large aneurysm causes symptoms that result from pressure on adjacent structures. Hoarseness may develop due to stretching of the recurrent laryngeal nerve. Pressure on the trachea and left main bronchus can cause stridor, dyspnea, atelectasis, or pneumonia. Pressure on the esophagus may cause dysphagia.

Aneurysms involving the aortic root commonly cause aortic regurgitation secondary to annular dilatation. Thus, an aortic aneurysm may be identified in a patient who presents with aortic regurgitation or congestive cardiac failure.

The aneurysm may be apparent on a chest radiograph. With an ascending aneurysm, there may be a prominent right aortic border on the frontal projection and filling in of the upper substernal space on the lateral projection. With an arch aneurysm there may be widening of the mediastinum on the frontal projection. An enlarged descending thoracic aorta may be seen behind the heart on the frontal and lateral projections (Fig. 11-2).

Figure 11.2 Chest radiograph showing a descending thoracic aortic aneurysm and the abnormal lateral border of the aneurysm (arrows).

Imaging of thoracic aortic aneurysms may be undertaken by echocardiography, computed tomography (CT), or magnetic resonance (MR) imaging. Echocardiography allows accurate assessment of aortic regurgitation, aortic leaflet morphology, and ventricular function, but it may not fully delineate the anatomic extent and relationships of the aneurysm. Echocardiography is particularly poor at imaging the distal ascending aorta and aortic arch. To image the aneurysm itself accurately, either a contrast CT scan (Fig. 11-3A) or MR imaging scan is required. For patients undergoing aneurysm surveillance, conventional axial CT scanning is sufficient. For operative planning, CT or MR imaging with three-dimensional reconstruction is extremely helpful (Fig. 11-3B).

Figure 11.3 CT scan of a descending thoracic aortic aneurysm. A, The aneurysm in transverse section. B, The aneurysm in coronal reconstruction. Eccentric mural thrombus is present (arrows). The aorta is very tortuous.

Treatment of Thoracic Aneurysms

In patients with small aneurysms, regular surveillance by CT scanning is indicated; the frequency of scanning is determined by the size and growth pattern of the aneurysm. Medical treatment primarily involves control of hypertension. β Blockers are a good choice for blood pressure control because they reduce tension in the aneurysm wall and, unlike arteriolar vasodilators, do not increase shear forces in the aorta.⁴ Drug treatment of hyperlipidemia and diabetes may also be indicated. Smoking cessation, diet, and exercise are all important preventive strategies for atherosclerosis.

Aneurysms involving the ascending aorta, aortic arch, and the thoracoabdominal aorta typically require open surgical repair. Aneurysms limited to the descending thoracic aorta may be suitable for endovascular stent grafting. Concomitant aortic valve and coronary artery bypass graft surgery may also be indicated.

Preoperative Assessment

Aortic aneurysms and dissections typically occur in older patients with histories of smoking, hypertension, and chronic obstructive pulmonary disease. There is frequently coexisting coronary artery, carotid artery, and peripheral vascular disease. Furthermore, open repair of thoracic aortic aneurysms imposes a huge physiologic insult on patients. Thus, it is essential to fully investigate and optimize patients prior to surgery.

An echocardiogram should be performed on all patients; with particular emphasis on aortic valve and ventricular function. Patients over 40 years of age and those with risk factors for coronary artery disease should undergo coronary angiography.⁵

More than one third of patients undergoing surgery for thoracic aortic aneurysms have either a diagnosis of chronic obstructive pulmonary disease or a history of heavy smoking.⁶ These patients should have spirometry and arterial blood gases performed preoperatively. They are also likely to benefit from a period of smoking cessation, chest physiotherapy, and appropriate treatment of the infective and bronchospastic components of their respiratory diseases. The management of patients with obesity, renal failure, carotid disease, and diabetes is similar to that outlined in Chapter 9.

Etiology

The causes of aortic dissection are similar to the causes of aortic aneurysm (see Table 11-1). Indeed, dissection, intramural hematoma, and atherosclerotic plaque rupture can all complicate an aortic aneurysm. An additional cause of aortic dissection is iatrogenic aortic injury due to surgical manipulations such as the placement of aortic cannulas during cardiopulmonary bypass.

Classification

Most aortic dissections begin in the ascending aorta and extend distally, although between 5% and 10% begin in the arch, and 30% to 35% begin in the first part of the descending aorta. There are two main classification systems, based on the extent of aortic involvement (Fig. 11-4). The Stanford classification is used here.⁹

Figure 11.4 The Stanford classification of aortic dissections. Type A dissections involve the ascending aorta; they may be limited to the ascending aorta (1) or extend into the descending thoracic aorta (2). Type B dissections involve the descending thoracic aorta (3). A dissection that involves only the aortic arch is not classified by the Stanford system, but if it subsequently extends proximally into the ascending aorta, it becomes type A.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Daily PO, Trueblood HW, Stinson EB: Management of acute aortic dissections. Ann Thorac Surg 10:237-247, 1970.)

Clinical Features and Investigations

Acute aortic dissection (type A and B) classically presents with the abrupt onset of excruciating chest pain that is commonly described as having a tearing quality. Anterior chest pain is more likely with type A dissection; back pain is more common with type B dissection. However, there is wide variation in presenting symptoms, and aortic dissection should be considered in all patients who present with acute severe chest pain.

Type A Dissection

Type A aortic dissections can result in the following complications:

• Pericardial tamponade and sudden death due to rupture of the aorta into the pericardium

• Aortic regurgitation due to extension of the dissection into the aortic root.

• Myocardial ischemia or infarction due to obstruction of the orifice or avulsion of the right coronary artery. (The left coronary is rarely involved.)

• Obstruction of a major branch artery, causing hypoperfusion of the brain, spinal cord, kidneys, or gastrointestinal tract.

• Free rupture into the thoracic cavity. This is usuallyrapidly fatal.

Patients commonly present with shock due to the combined effects of pericardial tamponade and aortic regurgitation. In the absence of these complications, patients may be hypertensive. Patients may develop a major neurologic deficit, such as impaired consciousness, hemiplegia, paresthesia, or paraplegia. Pulses (brachial, carotid, femoral) may be weak, absent, or asymmetric. Four-limb blood pressure measurements may show marked variability. The chest radiograph may demonstrate widened mediastinum and loss of the aortic knuckle (Fig. 11-5).

Figure 11.5 Chest radiograph showing a widened mediastinum (arrows) caused by type A dissection.

Imaging

Patients who present with symptoms and signs consistent with aortic dissection require urgent diagnostic imaging. Options include transesophageal echocardiography (TEE), CT scanning, MR imaging, and aortography, all of which have a sensitivity and specificity greater than 90%.¹⁰ Intramural hematoma may be difficult to diagnose with any of these techniques. The preferred initial investigation varies according to the institution, but CT scanning is the most widely used modality.¹¹ CT scanning has the advantage of being rapid (unlike MR imaging) and does not require sedation or general anesthesia (unlike TEE). Aortography is rarely indicated other than as part of a stenting procedure. CT scanning demonstrates the extent of the dissection (Fig. 11-6) and the presence of any pericardial blood but does not usually demonstrate the entry tears or any associated aortic regurgitation.

Figure 11.6 CT scan showing a type A dissection. A, In the transverse section, the intimal flap (arrows) is seen in the ascending and descending aorta. B, In the sagittal reconstruction, the false lumen (arrows) is seen immediately distal to the aortic arch and also within the abdomen.

TEE is the most useful investigation for determining the severity of any associated aortic regurgitation (Fig. 11-7

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