Abdominal Aortic Aneurysms

Abdominal aortic aneurysms, which are common among the elderly, increase in frequency and size with age. These aneurysms expand much more rapidly than thoracic aortic aneurysms. The distal aorta is the site of the most common and most dangerous atherosclerotic aneurysms. About 98% of abdominal aortic aneurysms are infrarenal in origin, often involving the proximal common iliac arteries. Less than 2% of abdominal aortic aneurysms extend above the level of the renal arteries, affecting the thoracoabdominal portion of the aorta, the celiac axis, the superior mesenteric artery, or the renal arteries. Abdominal aortic aneurysms must be distinguished from aortic dissection.

Symptoms, Signs, and Diagnosis

Aortic aneurysms are almost always silent until they reach or are close to the point of rupture. A rupture usually starts as a small perforation, blocked from leaking for hours or even days by pressure from a retroperitoneal blood clot. If it is diagnosed rapidly, lifesaving surgical repair may be possible. The pain pattern can be misleading. The pain is usually referred to the back and is indistinguishable from general backache. Because the elderly frequently have backache due to other conditions, a symptomatic abdominal aortic aneurysm can be easily overlooked. No examination for backache in the elderly is complete without palpation of the abdomen for an aneurysm. Unexplained abdominal or lower back pain with a prominent pulsation should suggest a ruptured aneurysm until proved otherwise. In elderly obese patients, sudden pain suggests the diagnosis, even if a pulsation is undetectable.

Most large abdominal aneurysms can be detected by gentle, thorough palpation of the abdominal aorta, which is an essential component of the physical examination of the elderly. Detecting these aneurysms in asymptomatic patients is imperative because the operative risk is so high after a rupture. Typically, an aneurysm appears as a painless expansile mass that has both lateral and anterior pulsations. However, often only a strong pulse is felt, making it difficult to distinguish a normal aorta from an aneurysm or from generalized ectasia and tortuosity. In patients of normal girth, the normal aortic pulse is generally palpable in the epigastrium, and a strong pulse is normal in thin patients, whereas any pulse may signal an aneurysm in obese patients. About 50% of aneurysms are associated with a bruit.

An aneurysm may be suspected only after an abdominal x-ray is taken for another reason. An anteroposterior view may indicate curvilinear aortic calcification near the midline, but calcification may be better seen in a lateral view, which may outline the aneurysm's calcified anterior and posterior walls.

Ultrasonography is the method of choice for confirming the diagnosis. It is virtually 100% accurate, providing precise information about the aneurysm's size, shape, and location. The likelihood of rupture is directly related to the aneurysm's transverse and anteroposterior diameters and inversely related to its length. Rupture is not likely when the diameter is < 5 cm; when the diameter is larger, the rupture rate rises quickly.

If patients have symptoms and signs of a rupturing aneurysm, immediate exploratory laparotomy is indicated. However, if the index of suspicion is low and the onset of pain is recent, contrast CT of the abdomen can be performed. If the aneurysm has already ruptured, retroperitoneal swelling can usually be seen. Diagnosis of rupture is the only advantage CT has over ultrasonography.

Prognosis

The mortality rate associated with unrepaired abdominal aneurysms is high. The 5-year survival rate varies from 14 to 37%. The likelihood that an aneurysm will lead to death is directly related to its size. In one study, the mortality rate was about 25% for patients with an aneurysm < 7 cm in diameter and about 61% for patients with an aneurysm >= 10 cm. Data from the 1960s suggest that 1 of 250 persons > 50 years died of a ruptured abdominal aortic aneurysm. This figure may have decreased in recent years because of better diagnostic techniques.

Complications other than rupture occur infrequently. Mural thrombi may embolize to the legs. Rarely, consumption coagulopathy occurs, resulting in thrombocytopenia, elevated thrombin time, fibrin split products in the blood, and a bleeding diathesis. An infection in the aneurysm is even rarer; if it occurs, Salmonella sp is most often implicated. Patients with recurrent Salmonella septicemia of unknown origin should be evaluated for an aneurysm.

Surgical repair prolongs life. With an experienced surgical team, elective repair has an operative mortality rate of < 3%, even though most patients have other manifestations of atherosclerotic disease. Contraindications to surgery include recent transient ischemic attacks and unstable angina.

Treatment

The operative risk associated with elective aneurysm repair is dramatically lower than the operative risk after rupture. Age should not determine whether elective repair of a large abdominal aortic aneurysm is performed in otherwise healthy elderly patients. Abdominal aortic aneurysms > 5 cm in diameter usually should be repaired. Repair of slightly smaller lesions may be considered, particularly if serial ultrasonograms show progressive enlargement and if patients are otherwise healthy. Patients with small aneurysms can be followed clinically and with ultrasonography every 6 months to 1 year. When the aneurysm is > 4 cm, ultrasonography should be performed every 3 to 4 months.

Angiography is not necessary before repair. During surgery, the aneurysm is opened, a graft is inserted, and then the walls of the aneurysm are closed around the graft. A promising experimental technique, the percutaneous insertion of a collapsed graft with a metallic ring at each end, avoids surgery. The graft is threaded through a femoral artery into the area of the aneurysm and then opened. The rings anchor the graft at each end. If perfected, this technique may allow repair of aneurysms even in very elderly patients with severe heart disease.

Treatment of patients who have both coronary artery disease and an abdominal aneurysm is controversial. Some authorities advocate coronary angiography and bypass surgery as the first intervention, but most reserve this approach for patients with severe heart disease. Most surgeons forgo coronary angiography for patients with little or no angina and a good ejection fraction (as determined by radionuclide left ventricular cineangiography).

The management of patients with significant stable angina is open to question. Thallium scanning of the heart before and after IV injections of dipyridamole is useful. Evidence of blood flow redistribution after dipyridamole administration is well correlated with postoperative myocardial infarction. Conventional submaximal stress tests and 48-hour ambulatory ECG monitoring also can help assess the need for coronary bypass before aneurysm repair.