An aneurysm is defined as a pathologic dilation of a segment of a blood vessel. A true aneurysm involves all three layers of the vessel wall and is distinguished from a pseudoaneurysm, in which the intimal and medial layers are disrupted and the dilated segment of the aorta is lined by adventitia only and, at times, by perivascular clot. Aneurysms also may be classified according to their gross appearance. A fusiform aneurysm affects the entire circumference of a segment of the vessel, resulting in a diffusely dilated artery. In contrast, a saccular aneurysm involves only a portion of the circumference, resulting in an outpouching of the vessel wall. Aortic aneurysms also are classified according to location, i.e., abdominal versus thoracic. Aneurysms of the descending thoracic aorta are usually contiguous with infradiaphragmatic aneurysms and are referred to as thoracoabdominal aortic aneurysms.
Aortic aneurysms result from conditions that cause degradation or abnormal production of the structural components of the aortic wall: elastin and collagen. The causes of aortic aneurysms may be broadly categorized as degenerative disorders, genetic or developmental diseases, vasculitis, infections, and trauma (Table 274-1). Inflammation, oxidative stress, proteolysis, and biomechanical wall stress contribute to the degenerative processes that characterize most aneurysms of the abdominal and descending thoracic aorta. These are mediated by B cell and T cell lymphocytes, macrophages, inflammatory cytokines, and matrix metalloproteinases that degrade elastin and collagen and alter the tensile strength and ability of the aorta to accommodate pulsatile stretch. The associated histopathology demonstrates destruction of elastin and collagen, decreased vascular smooth muscle, in-growth of new blood vessels, and inflammation. Factors associated with degenerative aortic aneurysms include aging, cigarette smoking, hypercholesterolemia, hypertension, and male sex.
TABLE 274-1Diseases of the Aorta: Etiology and Associated Factors ||Download (.pdf) TABLE 274-1 Diseases of the Aorta: Etiology and Associated Factors
|Aortic aneurysm |
| Degenerative |
| Aging |
| Cigarette smoking |
| Hypercholesterolemia |
| Hypertension |
| Atherosclerosis |
| Genetic or developmental |
| Marfan’s syndrome |
| Loeys-Dietz syndrome |
| Ehlers-Danlos syndrome type IV |
| Turner’s syndrome |
| Familial |
| Bicuspid aortic valve |
| Chronic aortic dissection |
| Aortitis (see below) |
| Infective (see below) |
| Trauma |
|Acute aortic syndromes (aortic dissection, acute intramural hematoma, penetrating atherosclerotic ulcer) |
| Degenerative disorders (see above) |
| Genetic/developmental disorders (see above) |
| Hypertension |
| Aortitis (see below) |
| Pregnancy |
| Trauma |
|Aortic occlusion |
| Atherosclerosis |
| Thromboembolism |
| Vasculitis |
| Takayasu’s arteritis |
| Giant cell arteritis |
| Rheumatic |
| Rheumatoid aortitis |
| HLA-B27–associated spondyloarthropathies |
| Behçet’s syndrome |
| Cogan’s syndrome |
| IgG4-related systemic disease |
| Idiopathic aortitis |
| Infective |
| Syphilis |
| Tuberculosis |
| Mycotic (Salmonella, staphylococcal, streptococcal, fungal) |
The most common pathologic condition associated with degenerative aortic aneurysms is atherosclerosis. Many patients with aortic aneurysms have coexisting risk factors for atherosclerosis, as well as atherosclerosis in other blood vessels.
Medial degeneration, previously designated cystic medial necrosis, is the histopathologic term used to describe the degeneration of collagen and elastic fibers in the tunica media of the aorta as well as the loss of medial cells that are replaced by multiple clefts of mucoid material, such as proteoglycans. Medial degeneration characteristically affects the proximal aorta, results in circumferential weakness and dilation, and leads to the development of fusiform aneurysms involving the ascending aorta and the sinuses of Valsalva. This pathologic condition occurs in patients with Marfan’s syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome type IV (Chap. 406), hypertension, congenital bicuspid aortic valves, Turner’s syndrome, and familial thoracic aortic aneurysm syndromes; sometimes it appears as an isolated condition in patients without any other apparent disease.
Familial clusterings of aortic aneurysms occur in 20% of patients, suggesting a hereditary basis for the disease. Mutations of the gene that encodes fibrillin-1 are present in patients with Marfan’s syndrome. Fibrillin-1 is an important component of extracellular microfibrils, which support the architecture of elastic fibers and other connective tissue. Deficiency of fibrillin-1 in the extracellular matrix leads to excessive signaling by transforming growth factor β (TGF-β). Loeys-Dietz syndrome is caused by mutations in the genes that encode TGF-β receptors 1 (TGFBR1) and 2 (TGFBR2). Increased signaling by TGF-β and mutations of TGFBR1, TGFBR2, TGFBR3, as well as TGFB2 and TGFB3, may cause thoracic aortic aneurysms. Mutations of SMAD3, which encodes a downstream signaling protein involved with TGF binding to its receptors, have been described in a syndrome of thoracic aortic aneurysm; craniofacial, skeletal, and cutaneous anomalies; and osteoarthritis. Mutations of the genes encoding the smooth muscle–specific alpha-actin (ACTA2), smooth muscle cell–specific myosin heavy chain 11 (MHC11), and myosin light chain kinase (MYLK) and mutations of TGFBR2 and SMAD3 have been reported in some patients with nonsyndromic familial thoracic aortic aneurysms. Mutations of type III procollagen have been implicated in Ehlers-Danlos type IV syndrome.
The infectious causes of aortic aneurysms include syphilis, tuberculosis, and other bacterial infections. Syphilis (Chap. 177) is a relatively uncommon cause of aortic aneurysm. Syphilitic periaortitis and mesoaortitis damage elastic fibers, resulting in thickening and weakening of the aortic wall. Approximately 90% of syphilitic aneurysms are located in the ascending aorta or aortic arch. Tuberculous aneurysms (Chap. 173) typically affect the thoracic aorta and result from direct extension of infection from hilar lymph nodes or contiguous abscesses as well as from bacterial seeding. Loss of aortic wall elasticity results from granulomatous destruction of the medial layer. A mycotic aneurysm is a rare condition that develops as a result of staphylococcal, streptococcal, Salmonella, or other bacterial or fungal infections of the aorta, usually at an atherosclerotic plaque. These aneurysms are usually saccular. Blood cultures are often positive and reveal the nature of the infective agent.
Vasculitides associated with aortic aneurysm include Takayasu’s arteritis and giant cell arteritis, which may cause aneurysms of the aortic arch and descending thoracic aorta. Spondyloarthropathies such as ankylosing spondylitis, rheumatoid arthritis, psoriatic arthritis, relapsing polychondritis, and reactive arthritis (formerly known as Reiter’s syndrome) are associated with dilation of the ascending aorta. Aortic aneurysms occur in patients with Behçet’s syndrome (Chap. 357), Cogan’s syndrome, and IgG4-related systemic disease. Aortic aneurysms also result from idiopathic aortitis. Traumatic aneurysms may occur after penetrating or nonpenetrating chest trauma and most commonly affect the descending thoracic aorta just beyond the site of insertion of the ligamentum arteriosum. Chronic aortic dissections are associated with weakening of the aortic wall that may lead to the development of aneurysmal dilatation.
THORACIC AORTIC ANEURYSMS
The clinical manifestations and natural history of thoracic aortic aneurysms depend on their location. Medial degeneration is the most common pathology associated with ascending aortic aneurysms, whereas atherosclerosis is the condition most frequently associated with aneurysms of the descending thoracic aorta. The average growth rate of thoracic aneurysms is 0.1–0.2 cm per year. Thoracic aortic aneurysms associated with Marfan’s syndrome or aortic dissection may expand at a greater rate. The risk of rupture is related to the size of the aneurysm and the presence of symptoms, ranging approximately from 2–3% per year for thoracic aortic aneurysms <4.0 cm in diameter to 7% per year for those >6 cm in diameter. Most thoracic aortic aneurysms are asymptomatic; however, compression or erosion of adjacent tissue by aneurysms may cause symptoms such as chest pain, shortness of breath, cough, hoarseness, and dysphagia. Aneurysmal dilation of the ascending aorta may cause congestive heart failure as a consequence of aortic regurgitation, and compression of the superior vena cava may produce congestion of the head, neck, and upper extremities.
A chest x-ray may be the first test that suggests the diagnosis of a thoracic aortic aneurysm (Fig. 274-1). Findings include widening of the mediastinal shadow and displacement or compression of the trachea or left main stem bronchus. Echocardiography, particularly transesophageal echocardiography, can be used to assess the proximal ascending aorta and descending thoracic aorta. Contrast-enhanced CT, magnetic resonance imaging (MRI), and conventional invasive aortography are sensitive and specific tests for assessment of aneurysms of the thoracic aorta and involvement of branch vessels (Fig. 274-2). In asymptomatic patients whose aneurysms are too small to justify surgery, noninvasive testing with either contrast-enhanced CT or MRI should be performed at least every 6–12 months to monitor expansion.
A chest x-ray of a patient with a thoracic aortic aneurysm.
A magnetic resonance angiogram demonstrating a fusiform aneurysm of the ascending thoracic aorta. (Courtesy of Dr. Michael Steigner, Brigham and Women’s Hospital, Boston, MA, with permission.)
TREATMENT Thoracic Aortic Aneurysms
β-Adrenergic blockers currently are recommended for patients with thoracic aortic aneurysms, particularly those with Marfan’s syndrome, who have evidence of aortic root dilatation to reduce the rate of further expansion. Additional medical therapy should be given as necessary to control hypertension. Angiotensin receptor antagonists may reduce the rate of aortic dilation in patients with Marfan’s syndrome by blocking TGF-β signaling. Clinical outcome trials have found that the rate of aortic root enlargement in patients with Marfan’s syndrome was similar with atenolol and losartan. Operative repair with placement of a prosthetic graft is indicated in patients with symptomatic ascending thoracic aortic aneurysms and for most asymptomatic aneurysms, including those associated with bicuspid aortic valves, when the aortic root or ascending aortic diameter is ≥5.5 cm, or when the growth rate is >0.5 cm per year. Replacement of the ascending aorta >4.5 cm is reasonable in patients with bicuspid aortic valves undergoing aortic valve replacement because of severe aortic stenosis or aortic regurgitation. In patients with Marfan’s syndrome, ascending thoracic aortic aneurysms of 4–5 cm should be considered for surgery. Operative repair is indicated for patients with degenerative descending thoracic aortic aneurysms when the diameter is >6 cm, and endovascular repair should be considered if feasible when the diameter is >5.5 cm. Repair is also recommended when the diameter of a descending thoracic aortic aneurysm has increased >1 cm per year.
ABDOMINAL AORTIC ANEURYSMS
Abdominal aortic aneurysms occur more frequently in males than in females, and the incidence increases with age. Cigarette smoking is a potent modifiable risk factor. Abdominal aortic aneurysms ≥4.0 cm may affect 1–2% men aged >50 years. At least 90% of all abdominal aortic aneurysms >4.0 cm are related to atherosclerotic disease, and most of these aneurysms are below the level of the renal arteries. Prognosis is related to both the size of the aneurysm and the severity of coexisting coronary artery and cerebrovascular disease. The risk of rupture increases with the size of the aneurysm: the 5-year risk for aneurysms <5 cm is 1–2%, whereas it is 20–40% for aneurysms >5 cm in diameter. The formation of mural thrombi within aneurysms may predispose to peripheral embolization.
An abdominal aortic aneurysm commonly produces no symptoms. It usually is detected on routine examination as a palpable, pulsatile, expansile, and nontender mass, or it is an incidental finding observed on an abdominal imaging study performed for other reasons. As abdominal aortic aneurysms expand, however, they may become painful. Some patients complain of strong pulsations in the abdomen; others experience pain in the chest, lower back, or scrotum. Aneurysmal pain is usually a harbinger of rupture and represents a medical emergency. More often, acute rupture occurs without any prior warning, and this complication is always life-threatening. Rarely, there is leakage of the aneurysm with severe pain and tenderness. Acute pain and hypotension occur with rupture of the aneurysm, which requires an emergency operation.
Abdominal radiography may demonstrate the calcified outline of the aneurysm; however, about 25% of aneurysms are not calcified and cannot be visualized by x-ray imaging. An abdominal ultrasound can delineate the transverse and longitudinal dimensions of an abdominal aortic aneurysm and may detect mural thrombus. Abdominal ultrasound is useful for serial documentation of aneurysm size and can be used to screen patients at risk for developing an aortic aneurysm. In one large study, ultrasound screening of men aged 65–74 years was associated with a risk reduction in aneurysm-related death of 42%. For this reason, screening by ultrasonography is recommended for men aged 65–75 years who have ever smoked. In addition, siblings or offspring of persons with abdominal aortic aneurysms, as well as individuals with thoracic aortic or peripheral arterial aneurysms, should be considered for screening for abdominal aortic aneurysms. CT with contrast and MRI are accurate noninvasive tests to determine the location and size of abdominal aortic aneurysms and to plan endovascular or open surgical repair (Fig. 274-3). Contrast aortography may be used for the evaluation of patients with aneurysms, but the procedure carries a small risk of complications such as bleeding, allergic reactions, and atheroembolism. Since the presence of mural thrombi may reduce the luminal size, aortography may underestimate the diameter of an aneurysm.
A computed tomographic angiogram depicting a fusiform abdominal aortic aneurysm before (left) and after (right) treatment with a bifurcated stent graft. (Courtesy of Drs. Elizabeth George and Frank Rybicki, Brigham and Women’s Hospital, Boston, MA, with permission.)
TREATMENT Abdominal Aortic Aneurysms
Statins are indicated to reduce the risk of cardiovascular events related to atherosclerosis. Medical therapies, such as β-adrenergic blockers and renin-angiotensin inhibitors, have not proven effective in reducing the rate of aneurysm growth. Operative repair of the aneurysm with insertion of a prosthetic graft or endovascular placement of an aortic stent graft (Fig. 274-3) is indicated for abdominal aortic aneurysms of any size that are expanding rapidly or are associated with symptoms. For asymptomatic aneurysms, abdominal aortic aneurysm repair is indicated if the diameter is ≥5.5 cm. In randomized trials of patients with abdominal aortic aneurysms <5.5 cm, there was no difference in the long-term (5- to 8-year) mortality rate between those followed with ultrasound surveillance and those undergoing elective surgical repair. Thus, serial noninvasive follow-up of smaller aneurysms (<5.5 cm) is an alternative to immediate repair. The decision to perform an open surgical operation or endovascular repair is based in part on the vascular anatomy and comorbid conditions. Endovascular repair of abdominal aortic aneurysms has a lower short-term morbidity rate, but a comparable long-term mortality rate with open surgical reconstruction. Long-term surveillance with CT or MR aortography is indicated after endovascular repair to detect leaks and possible aneurysm expansion.
In surgical candidates, careful preoperative cardiac and general medical evaluations (followed by appropriate therapy for complicating conditions) are essential. Preexisting coronary artery disease, congestive heart failure, pulmonary disease, diabetes mellitus, and advanced age add to the risk of surgery. With careful preoperative cardiac evaluation and postoperative care, the operative mortality rate approximates 1–2%. After acute rupture, the mortality rate of emergent operation is 45–50%. Endovascular repair with stent placement is an alternative approach to treat ruptured aneurysms and may be associated with a lower mortality rate.