Abdominal Aortic Aneurysm 

Aneurysms are defined as a focal dilatation in an artery, with at least a 50% increase over its normal diameter. Thus, an enlargement of at least 3 cm of the abdominal aorta fits the definition. Abdominal aortic aneurysms (AAAs) are relatively common and are potentially life threatening.

AAA usually results from degeneration in the media of the arterial wall, leading to a slow and continuous dilatation of the lumen of the vessel (see Pathophysiology). Uncommon causes include infection, cystic medial necrosis, arteritis, trauma, inherited connective-tissue disorders, and anastomotic disruption. (See Etiology.)

AAAs generally affect elderly white men. Smoking appears to be the risk factor most strongly associated with AAA. In addition to increasing age and male sex, other factors include increased height, weight, body mass index, and body surface area. A familiar clustering has been noted in 15-25% of patients undergoing surgical repair of AAA. Female sex, African American race, and presence of diabetes mellitus are negatively associated with AAA.^{[1, 2]}

Most AAAs are asymptomatic, and many are detected as an incidental finding on diagnostic imaging obtained for other reasons. There is a wide spectrum of clinical presentations and AAA should be considered in the differential diagnosis for a number of symptoms.

Ultrasonography is the standard imaging tool for AAA. Bedside emergency ultrasonography should be performed immediately if AAA is suspected. (See Workup.)

Treatment of abdominal aortic aneurysms (AAAs) is with surgical repair. When indicated, unruptured aneurysms can undergo elective repair (see Treatment). The combination of ultrasonographic screening, reduced preoperative risk, and new minimally invasive techniques extend aortic aneurysm treatment into an increasingly elderly population. Ruptured AAAs require emergent surgical repair.

See Emergent Management of Abdominal Aortic Aneurysm Rupture and Bedside Ultrasonography Evaluation of Abdominal Aortic Aneurysm for more information on these topics.

The abdominal aorta has 3 distinct tissue layers: intima, media, and adventitia. The intima is composed of the classic endothelial layer. The media comprises smooth muscle cells surrounded by elastin, collagen, and proteoglycans; to a great extent, this layer imparts the structural and elastic properties of the artery. The adventitia consists primarily of collagen, but also contains a variety of cells including fibroblasts and immunomodulatory cells, as well as adrenergic nerves.

The diameter of the aorta decreases in size from its thoracic portion to the abdominal and infrarenal portions. A normal aorta shows a reduction in medial elastin layers from the thoracic area to the abdominal portion. Elastin and collagen content are also reduced.

Most cases of AAA begin below the renal arteries and end above the iliac arteries. The size, shape, and extent of AAAs vary considerably. As with aneurysm of the thoracic aorta, AAAs aortic aneurysm may be described as fusiform, which is circumferential, or saccular, which is more localized. However, these descriptions represent a continuum.

The important surgical and endovascular anatomic considerations include associated renal and visceral artery involvement (either occlusive disease or involved in the aneurysm process) and the iliac artery (either occlusive disease or aneurysms). The length of the infrarenal aortic neck is important in helping determine the surgical approach (retroperitoneal vs transabdominal) and the location of the aortic cross clamp.

Hypogastric artery (internal iliac) outflow is important in planning surgical repair. Loss of blood flow from the hypogastric artery may result in impotence in males and sigmoid colon ischemia with necrosis.

Inflammatory aneurysms represent a subsegment of AAA and are characterized by a thick inflammatory peel. These aneurysms are associated with retroperitoneal fibrosis and adhesion of the duodenum and fibrosis (see the image below).

AAAs arise as a result of a failure of the major structural proteins of the aorta (elastin and collagen). The inciting factors are not known, but a genetic predisposition clearly exists. Although aneurysms represent a dilatation in all layers of the vessel wall, AAAs develop following degeneration of the media. The degeneration ultimately may lead to widening of the vessel lumen and loss of structural integrity.

After age 50 years, the normal diameter of the infrarenal aorta is 1.5 cm in women and 1.7 cm in men. An infrarenal aorta that is 3 cm or more in diameter is considered an AAA, even if asymptomatic. Approximately 90% of abdominal aortic aneurysms are infrarenal.

A multidisciplinary research program supported by the US National Heart, Lung, and Blood Institute identified proteolytic degradation of aortic wall connective tissue, inflammation and immune responses, biomechanical wall stress, and molecular genetics as mechanisms important in the development of AAA.^[3] Similarly, surgical specimens of AAA reveal inflammation, with infiltration by lymphocytes and macrophages; thinning of the media; and marked loss of elastin (see the image below).

Elastin

Elastin is the principal load-bearing element in the aorta. The aortic wall contains smooth muscle, elastin, and collagen arranged in concentric layers in order to withstand arterial pressure. The number of medial elastin layers from the proximal thoracic aorta to the infrarenal aorta is markedly reduced, with medial thinning and intimal thickening.

Elastin fragmentation and degeneration are observed in aneurysm walls. The decrease in content coupled with the histological changes of this matrix protein in aneurysms may explain the propensity for aneurysm formation in the infrarenal aorta.

Proteolysis, metalloproteinases, and inflammation

The aortic media appears to degrade in AAA by means of a proteolytic process. This implies an increase in the concentration of proteolytic enzymes relative to their inhibitors in the abdominal aorta as the individual ages.

Recent research has focused on the role of the metalloproteinases, a group of zinc-dependent enzymes responsible for tissue remodeling. Reports have documented increased expression and activity of matrix metalloproteinases (MMPs) in persons with AAAs. MMPs and other proteases have been shown to be secreted into the extracellular matrix of AAAs by macrophages and aortic smooth muscle cells.

MMPs and their inhibitors are present in normal aortic tissue and are responsible for vessel wall remodeling. Aneurysmal tissue tends to demonstrate increased MMP activity and decreased inhibitor activity, favoring the degradation of elastin and collagen. The mechanism that tips the balance in favor of degradation of elastin and collagen in the aortic wall of AAAs by MMPs and other proteases is presently unknown.

AAAs demonstrate a chronic adventitial and medial inflammatory infiltrate upon histological examination. Infiltration of AAAs with lymphocytes and macrophages may trigger protease activation via various cytokines (interleukin [IL]–1, IL-6, IL-8, and tumor necrosis factor-alpha).

Immunoreactive proteins are found more conspicuously in the abdominal aorta, and this may contribute to the increased frequency of aneurysms in this location. Further study has defined a matrix protein that is immunoreactive with immunoglobulin G in the aneurysm wall. This autoantigen appears to be a collagen-associated microfibril.

Certain infectious agents have been associated with the development of this protein, including Chlamydia pneumoniae and Treponema pallidum; however, a direct cause-and-effect relationship has not been demonstrated.

Insights from molecular genetics

Through gene microarray analysis, various genes involved in extracellular matrix degradation, inflammation, and other processes observed in AAA formation have been shown to be up-regulated, while others that may serve to prevent this occurrence are down-regulated. The combination of proteolytic degradation of aortic wall connective tissue, inflammation and immune responses, biomechanical wall stress, and molecular genetics represents a dynamic process that leads to aneurysmal deterioration of aortic tissue.

Atherosclerosis

Most abdominal aortic aneurysms occur in persons with advanced atherosclerosis. Atherosclerosis may induce abdominal aortic aneurysm formation by causing mechanical weakening of the aortic wall with loss of elastic recoil, along with degenerative ischemic changes, through obstruction of the vasa vasorum.

Many patients with advanced atherosclerosis do not develop AAA, while some patients having no evidence of atherosclerosis do. The observed association between atherosclerosis and AAA is probably not causative; however, atherosclerosis may represent a nonspecific secondary response to vessel wall injury that is induced by multiple factors.

Patients at greatest risk for AAA are men who are older than 65 years and have peripheral atherosclerotic vascular disease. A history of smoking often is elicited. The US Preventive Services Task Force recommends ultrasonography screening in men aged 65-75 years who have ever smoked.^[4] However, a recent Swedish study showed that instances of AAA in elderly men have been decreasing, which can be attributed to a nationwide decline in smoking for the past 30 years, as well as the significantly improved longevity of the elderly population. A one-time ultrasound scan is recommended for men once they reach age 65 years to detect and prevent possible occurrences of AAA.^[5]

Other risk factors include the following:

• Chronic obstructive pulmonary disease (COPD)
• Previous aneurysm repair or peripheral aneurysm (popliteal or femoral)
• Coronary artery disease
• Hypertension (1-15% of cases)

Less frequent causes of AAA include Marfan syndrome, Ehlers-Danlos syndrome, and collagen-vascular diseases. In fewer than 5% of cases, AAA is caused by mycotic aneurysm of hematogenous origin. In these cases, local invasion of the intima and media gives rise to abscess formation and aneurysmal dilation of the vessel. Gram-positive organisms most commonly cause mycotic aneurysms. Other uncommon causes include cystic medial necrosis, arteritis, trauma, and anastomotic disruption producing pseudoaneurysms.

Persons who have first-degree relatives with AAA are at increased risk for AAA. The familial prevalence rate of AAA has been estimated at 15-25%. Studies by Majumder and associates suggest that the genetic predisposition is isolated to a single dominant gene with low penetrance that increases with age.^[6]

Tilson et al described the potential for an autoimmune basis for the development of AAA involving the DRB1 major histocompatibility locus.^[7] This locus has been identified as a basis for inflammatory AAA.

AAA is thought to be a degenerative process of the aorta, the cause of which remains unclear. It is often attributed to atherosclerosis because these changes are observed in the aneurysm at the time of surgery. However, a study by Blanchard et al found that the risk factors for AAA differ from those for atherosclerosis, with no association between cholesterol and AAA.^[1] In addition, atherosclerosis fails to explain the development of occlusion, which is observed in the disease process.

United States statistics

In autopsy studies, the frequency rate of AAA ranges from 0.5-3.2%. In a large US Veterans Administration screening study, the prevalence rate was 1.4%.^[2] Ruptured AAA is the 13th-leading cause of death in the United States, causing an estimated 15,000 deaths per year. Despite increased survival following diagnosis, incidence and crude mortality seem to be increasing.

AAA is 5 times more common in men than in women and is 3.5 times more common in white men than in African-American men. The likelihood of development varies from 3-117 cases per 100,000 person-years. In men, the process appears to begin at approximately age 50 years and reaches peak incidence at approximately age 80 years. In women, the onset is delayed and appears to begin at approximately age 60 years. The reported incidence of rupture varies from 1-21 cases per 100,000 person-years.

International statistics

The frequency rate of asymptomatic AAA is 8.2% in the United Kingdom, 8.8% in Italy, 4.2% in Denmark, and 8.5% in Sweden (in males only). The frequency rate of AAA in females is much lower, 0.6-1.4%.

Racial, sexual, and age differences in incidence

White men have the highest incidence of AAA. AAAs are uncommon in African Americans, Asians, and persons of Hispanic heritage. The male-to-female incidence ratio in people younger than 80 years is 2:1. In those older than 80 years, the ratio is 1:1.

The incidence of AAA begins to increase sharply after 50 years of age and peaks at age 70-79 years (see the graph below).

The prognosis is guarded in patients who suffer rupture of an AAA prehospital. More than 50% do not survive to the emergency department; of those who do, survival rate drops by about 1% per minute. However, the survival rate is good in the subset of patients who are not in severe shock and who receive timely, expert surgical intervention.

Mortality in elective AAA repair is drastically lower than that associated with rupture. Consequently, the emphasis must be on early detection and repair free from complications.

For patient education information, see the Circulatory Problems Center and Cholesterol Center, as well as Aortic Aneurysm, High Cholesterol, and Cholesterol FAQs.