Abdominal aortic aneurysm

Abdominal aortic aneurysms account for about three-quarters of aortic aneurysms, affecting 0.5-3.2% of the population. The prevalence in men is 3 times higher than in women.

Abdominal aortic aneurysms usually begin below the origin of the renal arteries but may involve the orifices of the renal arteries; approximately 50% involve the iliac arteries. In general, an aortic diameter > 3 cm suggests an abdominal aortic aneurysm. Most abdominal aortic aneurysms are fusiform, and some are saccular. Many may contain laminar thrombi. Abdominal aortic aneurysms involve all layers of the aorta and do not result in dissection, but thoracic aortic dissection may extend into the distal abdominal aorta.

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Causes of abdominal aortic aneurysm

The most common cause of arterial wall weakening is usually due to atherosclerosis. Other causes include trauma, vasculitis, cystic necrosis of the tunica media, and postoperative anastomotic failure. Occasionally, syphilis and local bacterial or fungal infection (usually due to sepsis or infective endocarditis ) lead to weakening of the arterial wall and formation of infected (mycotic) aneurysms.

Smoking is the most significant risk factor. Other factors include hypertension, advanced age (peak incidence is recorded at 70-80 years), family history (in 15-25% of cases), Caucasian descent, and male gender.

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Symptoms of an abdominal aortic aneurysm

Most abdominal aortic aneurysms are asymptomatic. When symptoms occur, they may be nonspecific. As abdominal aortic aneurysms enlarge, they can cause pain that is persistent, deep, aching, visceral, and most noticeable in the lumbosacral region. Patients may notice a visible abdominal pulsation. Rapidly enlarging aneurysms that are prone to rupture often cause symptoms, but most aneurysms grow slowly and are asymptomatic.

In some cases, the aneurysm may be palpable as a pulsatile mass, depending on its size and the patient's constitution. The probability that a patient with a pulsatile palpable mass has an aneurysm >3 cm in size is approximately 40% (positive predictive value). A systolic murmur may be heard over the aneurysm. Unless death has occurred immediately from ruptured abdominal aortic aneurysm, patients in this acute situation usually experience abdominal or lumbar pain, hypotension, and tachycardia. There may be a history of recent upper abdominal trauma.

In silent AAAs, symptoms of complications (eg, limb pain due to embolism or thrombosis of organ vessels) or underlying disease (eg, fever, malaise, weight loss due to infection or vasculitis) may occasionally be present. Occasionally, large AAAs lead to disseminated intravascular coagulation, possibly because large areas of abnormal endothelium initiate rapid thrombosis and consumption of coagulation factors.

Diagnosis of abdominal aortic aneurysm

Most abdominal aortic aneurysms are diagnosed incidentally during physical examination or abdominal ultrasound, CT, or MRI. Abdominal aortic aneurysms should be suspected in elderly patients who present with acute abdominal or low back pain, regardless of the presence or absence of a palpable pulsatile mass.

If symptoms and physical examination findings suggest an abdominal aortic aneurysm, abdominal ultrasound or CT (usually the imaging modality of choice) is performed. In hemodynamically unstable patients with suspected ruptured aneurysm, ultrasound provides rapid bedside diagnosis, but intestinal gas and abdominal distension may reduce its accuracy. Laboratory studies, including complete blood count, electrolytes, blood urea nitrogen, creatinine, coagulation profile, blood typing, and cross-matching, are performed in preparation for possible surgery.

If rupture is not suspected, CT angiography (CTA) or magnetic resonance angiography (MRA) can more accurately characterize the aneurysm size and anatomy. If thrombi line the aneurysm wall, CTA may underestimate its true size. In this case, non-contrast CT may provide a more accurate assessment. Aortography is essential if renal or iliac artery involvement is suspected or if endovascular stenting (endograft) is contemplated.

Plain abdominal radiography is neither sensitive nor specific, but if performed for other purposes, calcification of the aorta and aneurysm wall may be seen. If a mycotic aneurysm is suspected, bacteriologic examination to obtain bacterial and fungal blood cultures is indicated.

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Treatment of abdominal aortic aneurysm

Some abdominal aortic aneurysms grow gradually at a constant rate (2-3 mm/year), others grow in leaps and bounds, and for unknown reasons approximately 20% of aneurysms remain at a constant size indefinitely. The need for treatment is related to size, which correlates with the risk of rupture.

Abdominal Aortic Aneurysm Size and Rupture Risk*

ABA diameter, cm	Risk of rupture, %/year
<4	0
4-4.9	1
5-5.9*	5-10
6-6.9	10-20
7-7.9	20-40
>8	30-50

* Surgical treatment is considered the method of choice for aneurysms measuring > 5.0-5.5 cm.

Rupture of an abdominal aortic aneurysm is an indication for immediate surgical intervention. Without treatment, the mortality rate approaches 100%. With treatment, the mortality rate is approximately 50%. The figures are so high because many patients have concomitant coronary thrombosis, cerebrovascular and peripheral atherosclerosis. Patients who develop hemorrhagic shock require restoration of circulating fluid volume and blood transfusion, but the mean arterial pressure should not be raised > 70-80 mm Hg, because bleeding may increase. Preoperative control of hypertension is important.

Surgical treatment is indicated for aneurysms > 5-5.5 cm (when the risk of rupture exceeds 5-10% per year), unless contraindicated by concomitant pathological conditions. Additional indications for surgical treatment include aneurysm size increase > 0.5 cm for 6 months regardless of size, chronic abdominal pain, thromboembolic complications, or an iliac or femoral aneurysm that causes lower limb ischemia. Before treatment, it is necessary to evaluate the condition of the coronary arteries (to exclude ischemic heart disease), because many patients with abdominal aortic aneurysm have generalized atherosclerosis, and surgical intervention creates a high risk of cardiovascular complications. Appropriate medical therapy for ischemic heart disease or revascularization is very important to reduce morbidity and mortality in the treatment of abdominal aortic aneurysm.

Surgical treatment consists of replacing the aneurysmal portion of the abdominal aorta with a synthetic graft. If the iliac arteries are involved, the graft must be large enough to cover them. If the aneurysm extends above the renal arteries, these arteries must be reimplanted into a graft or a bypass graft must be created.

Placement of an endoprosthesis within the aneurysm lumen via the femoral artery is a less invasive alternative treatment used when the surgical risk of complications is high. This procedure eliminates the aneurysm from the systemic circulation and reduces the risk of rupture. The aneurysm eventually closes with thrombotic masses, and 50% of aneurysms decrease in diameter. Short-term results are good, but long-term results are unknown. Complications include kinking, thrombosis, migration of the endoprosthesis, and the formation of a persistent blood flow into the aneurysmal space after endoprosthesis placement. Thus, the patient should be followed more closely (with examinations performed more frequently) after endograft placement than after traditional grafting. If there are no complications, imaging studies are recommended at 1 month, 6 months, 12 months, and every year thereafter. Complex anatomical features (for example, a short aneurysm neck below the renal arteries, severe arterial tortuosity) make it impossible to implant an endoprosthesis in 30-50% of patients.

Repair of aneurysms < 5 cm does not appear to improve survival. Such aneurysms should be followed with ultrasound or CT after 6 to 12 months until they enlarge to a degree that would warrant repair. The duration of follow-up for incidentally discovered asymptomatic aneurysms has not been established. Control of atherosclerotic risk factors, especially smoking cessation and use of antihypertensive agents, is important. If a small or moderate-sized aneurysm becomes larger than 5.5 cm and the preoperative risk of complications is lower than the estimated risk of rupture, surgical repair is indicated. The risk of rupture versus the preoperative risk of complications should be discussed in detail with the patient.

Treatment of mycotic aneurysms consists of active antibacterial therapy directed at the microorganism and subsequent removal of the aneurysm. Early diagnosis and treatment improve the outcome.