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(<4 cm in diameter) to 11% per year for aneurysms larger than 5 cm in diameter. 1 Thoracic aneurysms, which are less common than abdominal aortic aneurysms, may involve one or more aortic segments. Most thoracic aneurysms are due to atherosclerosis. Disorders of connective tissue, such as Marfan syndrome (Chapter 6), are rare causes but of important clinical significance. The majority of thoracic aneurysms are asymptomatic. When symptoms occur, they depend largely on the size and position of the aneu- rysm. Substernal, back, or neck pain may occur. There may be dyspnea, stridor, or a brassy cough caused by pressure on the trachea. Hoarseness may result from pressure on the recurrent laryngeal nerve, and there may be difficulty swallowing because of pressure on the esophagus. 23 The aneurysm also may compress the superior vena cava, causing distention of neck veins and edema of the face and neck. Abdominal aortic aneurysms, which are the most common type of aneurysm, usually develop after age 50 and are associated with severe atherosclerosis. They occur more frequently in men than women, and over half of affected persons are hypertensive. Although abdominal aortic aneurysms usually occur in the con- text of atherosclerosis, it is thought that other factors such as smoking and a genetic predisposition may play a role. 2,24 Abdominal aortic aneurysms are most commonly located below the level of the renal artery (>90%) and involve the bifurcation of the aorta and proximal end of the common iliac arteries. 1,2 They can involve any part of the vessel circumference (saccular) or extend to involve the entire circumference (fusiform). Most abdominal aneurysms are asymptomatic. Because an aneurysm is of arterial origin, a pulsating mass may provide the first evidence of the disorder. Typically, aneurysms larger than 4 cm are palpable. The mass may be discovered dur- ing a routine physical examination or the affected per- son may complain of its presence. Calcification, which frequently exists on the wall of the aneurysm, may be detected during abdominal radiologic examination. Pain may be present and varies from mild midabdomi- nal or lumbar discomfort to severe abdominal and back pain. As the aneurysm expands, it may compress the lumbar nerve roots, causing lower back pain that radi- ates to the posterior aspects of the legs. The aneurysm may extend to and impinge on the renal, iliac, or mes- enteric arteries, or to the vertebral arteries that supply the spinal cord. An abdominal aneurysm also may cause erosion of vertebrae. Stasis of blood favors thrombus formation along the wall of the vessel (Fig. 18-11), and peripheral emboli may develop, causing symptomatic arterial insufficiency. Diagnostic methods include ultrasonography, echocardiography, CT scans, and MRI. Unruptured aneurysms are generally asymptomatic and are often diagnosed incidentally during clinical examina- tion. Measures to slow aneurysm growth and lower the risk of rupture include risk factor modification. Hypercholesterolemia and high blood pressure should be controlled and smoking discontinued. Surgical repair,

in which the involved section of the aorta is replaced with a synthetic graft of woven Dacron, frequently is the treatment of choice. 23,24 Aortic Dissection Aortic dissection (dissecting aneurysm) is an acute, life- threatening condition. 1,2,28 It involves hemorrhage into the vessel wall with longitudinal tearing of the vessel wall to form a blood-filled channel (see Fig. 18-10B). Unlike atherosclerotic aneurysms, aortic dissection often occurs without evidence of previous vessel dila- tion. The dissection can originate anywhere along the length of the aorta but most often involves the ascend- ing aorta. Aortic dissection is caused by conditions that weaken or cause degenerative changes in the elastic and smooth muscle layers of the aorta. There are two risk factors that predispose to aortic dissection: hypertension and degeneration of the medial layer of the vessel wall. It is most common in 40- to 60-year-old men with an ante- cedent history of hypertension. 1 Aortic dissection also is associated with connective tissue diseases, such as FIGURE 18-11. Atherosclerotic aneurysm of the abdominal aorta.The aneurysm has been opened longitudinally to reveal a large thrombus in the lumen.The aorta and common iliac arteries display complicated lesions of atherosclerosis. (From Gotlieb AI, Lui A. Blood vessels. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine, 6th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams &Wilkins; 2012:471.)