Epidemiology

Finding a unifying definition for abdominal aortic aneurysm (AAA) that encompasses the various classification schemes related to their size, shape, and location is an arduous task. Simplistically, most AAAs occur infrarenally and are defined as an aortic diameter greater than 3 cm, mostly measured in the anteroposterior dimension.^1–3 Although normal aortic diameter differs between the sexes, it is not substantial enough to warrant changing the size limit of 3 cm that is currently used to characterize a small AAA.³ Finally, it is important to note that dilation of an aneurysm encompasses the three layers of the vascular wall; otherwise, the dilation is a pseudoaneurysm.¹ For the commonly accepted descriptions and locations of most aneurysms, please see Box 66.1.

At present, AAA is the thirteenth leading cause of death in the United States. Once an AAA ruptures, death is a certainty in more than 65% of cases,¹ with some studies reporting mortality rates as high as 90%, thus highlighting the necessity for rapid surgical repair.⁴ Low rates of postmortem examination and the high likelihood that some of these deaths have been erroneously attributed to cardiac causes severely hamper gathering true estimates of the prevalence of and mortality associated with ruptured AAAs.² Elective repair of an AAA lowers the mortality to less than 5%, so there is an obvious benefit to diagnosing and treating these aneurysms before they rupture.^4,5 Given that this disease affects 4% to 7% of adults 65 years or older, the onus increasingly falls on physicians to accomplish this task as the population ages.⁴

Pathophysiology

There is abundant research on identification of risk factors that potentiate the development and growth of AAAs (Box 66.2). Besides male gender, age, and hypertension, every study on the subject of AAAs singles out long-term tobacco smoking as the most important risk factor.² AAAs develop in tobacco smokers more than four times more frequently than in lifelong nonsmokers.¹ Lederle et al.⁶ compared the relative risk for different diseases in long-term cigarette smokers and found that risk for the development of AAAs is fivefold higher than that for cerebrovascular disease and threefold higher than that for coronary artery disease.¹ Smoking not only raises the risk for the development of AAAs but also increases the annual growth rate of existent AAAs, with some studies reporting a rate of 2.83 mm/yr in smokers versus 2.53 mm/yr in nonsmokers.^1,7 However, the exact means by which smoking enhances aneurysm formation remains a mystery.

In addition to tobacco smoking, there are other risk factors. Atherosclerosis is so highly associated with the development and expansion of AAAs that the American Heart Association (AHA) guidelines on the treatment of AAAs mandate that blood pressure and fasting serum lipid values be strictly monitored and controlled in patients with AAAs.³ The AHA also recommends that smoking cessation interventions be provided to patients with a personal or family history of aneurysms.³ Despite the importance of atherosclerosis, additional factors are probably present because not every patient with atherosclerosis has an AAA.¹

Many authorities cite a possible causal link between the development of AAAs and chronic obstructive pulmonary disease (COPD). Researchers blame tobacco smoking–induced elastin degradation for this proposed association.³ A review of the literature notes that patients with COPD undergoing long-term treatment with corticosteroids have a much higher AAA expansion rate than do COPD patients not taking steroids.³ Hence, steroid use and coexisting disease are more likely to be responsible for the high prevalence of AAAs in patients with COPD.³ Finally, from a genetic standpoint, several screening studies suggest that male first-degree relatives of people with COPD are most at risk. Female first-degree relatives appear to be at similar risk, but the data are less certain.³ Familial aneurysms differ from nonfamilial aneurysms mainly in that they may develop at an earlier age.³ Existing genetic polymorphisms probably account for the familial clustering of AAAs, but this clustering could also result from exposure to common environmental factors, such as tobacco smoke.¹

Histologically, AAAs develop as a result of a complex interplay of immunologically mediated proteases and antiproteases that cause destruction of elastin and collagen in the arterial wall. Recent research has focused on the role of matrix metalloproteinases (MMPs) in the degradation of elastin and collagen, the main proteins involved in AAA growth and rupture, respectively. In early aneurysm formation, elastic fibers are lost, fragmented, or attenuated, which contributes to development of the actual wall of the aneurysm.^1,8 Once the concentration of medial elastin is severely decreased, the collagen-rich adventitial tissue constitutes the artery’s last mode of resistance, and once completely degraded, rupture inevitably ensues.^1,8 The natural history of all arterial aneurysms encompasses gradual or sporadic growth in their diameter (or both) and accretion of mural thrombus.³ These features contribute to rupture, thromboembolic ischemic events, and compression or erosion of adjacent structures, three of the most common complications of AAAs.³

Presenting Signs and Symptoms

AAA continues to be such a difficult diagnosis because of vague and variable symptoms (Table 66.1), which carry large differential diagnoses. Vague symptoms such as back or abdominal pain often herald this diagnosis in its early (virtually asymptomatic) and potentially curative stages. The symptoms may masquerade as those of renal colic, diverticulitis, or gastrointestinal hemorrhage, thereby leading to a fatal misdiagnosis.³ Commonly, patients with nonruptured AAAs are seen only after suffering complications such as various thromboembolic events or after thorough evaluation for chronic vague abdominal and back pain.³

Table 66.1 Variations in Clinical Findings in Patients with Abdominal Aortic Aneurysms

Nonruptured aneurysms are sometimes found incidentally. An astute clinician can find them on physical examination. Palpation of AAAs is safe and has not been reported to precipitate rupture.³ Abdominal palpation is moderately sensitive for the detection of AAAs that meet the size criteria necessary for surgical intervention, but even large aneurysms may be difficult to palpate, especially in obese patients. However, in the case of AAAs that are small or have already ruptured, physical examination alone is not nearly precise enough.³

When aneurysms rupture, the extent of shock varies according to the location and size of the rupture and the amount of delay before the patient is examined.¹ AAAs that rupture anterolaterally dramatically violate the peritoneal cavity and are most often associated with sudden death. Patients with ruptured AAAs who reach a physician tend to have ruptures involving the posterolateral wall in the retroperitoneal space. Some of these patients are fortunate enough that their bodies temporarily tamponade a small tear so that they suffer relatively small initial blood loss.¹ A few of these patients with contained ruptures may even exhibit flank ecchymosis, a physical finding known as the Grey Turner sign.³ However, AAAs are not contained for long, and even these patients suffer the deleterious effects of a larger rupture if they fail to seek medical attention early or if the physician fails to recognize the signs of impending rupture.¹

Differential Diagnosis and Medical Decision Making

As a result of the variable manifestations of AAAs just mentioned, the differential diagnosis is broad. Table 66.2 lists the differential diagnostic entities for AAA, along with priority actions for each. The correct diagnosis of AAA, regardless of whether it is ruptured, hinges on maintaining a high level of suspicion and pursuing an appropriate work-up.

Table 66.2 Differential Diagnosis of Abdominal Aortic Aneurysm and Priority Actions