When Should an Abdominal Aortic Aneurysm Be Treated?
Case
A generally healthy, 74-year-old man presents with sudden-onset abdominal pain due to acute pancreatitis. Computed tomography (CT) of his abdomen shows pancreatic inflammation and an incidental finding of a 4.5-cm abdominal aortic aneurysm. He had never had any imaging of his abdomen prior to this study and described no prior episodes of abdominal pain.
When should his abdominal aortic aneurysm be treated?
Overview
An abdominal aortic aneurysm (AAA) is an abnormal dilation of the abdominal aorta between the diaphragm and the aortic bifurcation of the iliac arteries. An AAA is usually defined as a dilatation with a diameter of >3 cm or 50% greater than the typical diameter. Most AAAs are located in the infrarenal aorta, proximal to the iliac bifurcation.
Population screening programs show a prevalence of AAA of 4% to 8% in men aged 65 to 80 years.1 AAA prevalence is approximately six times greater in men than women, though the prevalence in women might be increasing.1 AAA is most common in white men, with black men and those of Asian heritage having lower risk. A combination of genetic predisposition and environmental and physiologic factors lead to initiation and progression of AAAs; family history, male sex, advanced age, and history of smoking are major risk factors.
Mortality after AAA rupture is high. Approximately 62% of patients die prior to hospital arrival.2 Of those who undergo emergent AAA surgery, 50% will die.1
Aortic repair with a prosthetic vascular graft reduces morbidity and mortality from rupture, but the risks of repair are not trivial.2
Review of the Data
Risk of rupture. An AAA should be repaired when the risk of rupture outweighs the risks of surgical repair. Symptomatic aneurysms—such as those causing back or abdominal pain—have a higher risk of rupture than asymptomatic aneurysms. Most AAAs are asymptomatic and, in the absence of imaging, not identified until the time of rupture. Given the significant mortality associated with rupture, there is benefit to intervening on asymptomatic aneurysms before rupture.
The risk of AAA rupture has been studied in patients who either have been unfit for surgical repair or uninterested in intervention. Risk of rupture increases substantially with aneurysm size. Lederle et al estimated a two-year aneurysm rupture risk of 22.1% for AAA with a diameter of 5.0 to 5.9 cm, 18.9% for 6.0 to 6.9 cm, and 43.4% for a diameter ≥7.0 cm.3 In another study of 476 patients, the average risks of rupture in male and female patients with an AAA of 5.0 to 5.9 cm were 1.0% and 3.9% per year, respectively. For male and female patients with ≥6.0 cm AAAs, risks of rupture were 14.1% and 22.3% per year.4 Women with AAA have been found to have a higher risk of rupture in all studies in which female patients were included.
Because rupture risk increases with size, predicting the rate of growth is clinically important. Powell et al conducted a systematic review of growth rates of small AAAs.5 In 15 studies that examined 7,630 patients, the growth rate for a 3.5-cm aneurysm was estimated at 1.9 mm/year and for a 4.5-cm aneurysm was 3.5 mm/year. Given an exponentially increasing aneurysm diameter, this suggests an elapsed time of 6.2 years for a 3.5-cm aneurysm to grow to 5.5 cm, and 2.3 years for a 4.5-cm AAA to grow to 5.5 cm. This prediction does not account for individual variability in growth rate. Some AAAs grow quickly, others erratically, and others not at all. This growth variability is influenced by individual characteristics including cigarette smoking, sex, age, and other factors.
