In abdominal aortic aneurysm (AAA) screening programs, surveillance intervals of several years may be clinically acceptable for the majority of patients with small AAA since the smallest aneurysms often do not appear to change for many years, according to a meta-analysis published in the Feb. 27 issue of the Journal of the American Medical Association.

“The intervals between ultrasound surveillance examinations used in randomized trials of screening depend on aneurysm size. However, no consensus exists regarding the optimal time intervals between ultrasounds,” wrote Matthew J. Bown, MD, of the University of Leicester, England, and colleagues.

A rupture of an AAA has a survival rate of 20 percent, though in patients with AAA less than 5.5 cm in diameter, the risk of rupture is lower than the risk of surgery and surveillance is indicated, explained the authors.

To evaluate AAA surveillance efforts, Bown and colleagues conducted a meta-analysis of 18 studies comprised of 15,471 patients under surveillance for small AAAs. Most studies used 5.5 cm as the threshold for surgical intervention, used exclusively ultrasound for surveillance and recorded external aortic diameters.

Results showed that AAA growth and rupture rates varied considerably between studies. For each 0.5 cm increase in baseline AAA diameter, there was a 0.58 mm per year increase in average aortic growth rate, reported the authors.

Rupture rates in men increased by a factor of 1.9 for every 0.5 cm increase in baseline AAA diameter. The estimated time to have a 10 percent chance of reaching the surgery threshold was 7.4 years, 3.2 years and 8 months for men with AAA diameters of 3 cm, 4 cm and 5 cm, respectively. To control the risk of rupture in men to below 1 percent, the corresponding estimated surveillance intervals are 8.5 years for a 3 cm AAA and 17 months for a 5 cm AAA.

Absolute growth rates were similar for women and men, but there were significant differences in the absolute risk of rupture, according to Bown and colleagues. Women had a 4-fold greater rupture risk for all AAA sizes and reached a rupture risk of greater than 1 percent more quickly than men.

Reflecting the rate of AAA diameter increase, the authors suggested the following surveillance intervals that would maintain the risk of rupture at less than 1 percent:

• 3 – 3.9 cm AAA: 3 years
• 4 – 4.4 cm AAA: 2 years
• 4.4 – 5.4 cm AAA: Annual

“For a U.S. patient with a 3.0-cm AAA detected by screening, this would reduce the average number of surveillance scans from approximately 15 to 7,” wrote the authors.

Bown and colleagues noted that more research is needed on women with aneurysms with a diameter of 4.5 – 5.4 cm, and on the cost-effectiveness of various surveillance policies.