The smallest AAAs often do not appear to change significantly over many years, according to a meta-analysis of previous studies reported in the February 27 issue of
"The survival rate following rupture of an abdominal aortic aneurysm is only 20 percent, making AAAs an important cause of mortality," according to background information in the article. "In patients with small AAA (diameter <5.5 cm), the risk of rupture is lower than the risk of surgery and surveillance is indicated. The majority of small AAAs grow slowly, but there is substantial variation in growth rates between different individuals. 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."
To better guide AAA surveillance efforts, Simon G. Thompson, D.Sc., of the University of Cambridge, England, and colleagues conducted a study to determine the rates at which small AAAs progress to reach the surgery threshold diameter of 5.5 cm and the risk of AAA rupture over time. Via a meta-analysis, the authors assessed individual patient data from studies of small AAA growth and rupture. A total of 18 studies containing records from 15,471 individual patients (13,728 men and 1,743 women) under surveillance for small AAAs were included in the analyses. Most studies used 5.5 cm as the threshold for surgical intervention, used only ultrasound scans, and recorded external aortic diameters.
The researchers found that AAA growth and rupture rates varied considerably across studies. Each 0.5-cm increase in baseline AAA diameter resulted in a 0.59-mm per year increase in average aortic growth rate. Rupture rates in men increased by a factor of 1.9 for every 0.5-cm increase in baseline AAA diameter. For men with a 3.0-cm AAA, the estimated average time taken to have a 10 percent chance of reaching the surgery threshold diameter 5.5 cm was 7.4 years. The corresponding average times for 4.0 cm- and 5.0 cm-AAAs were 3.2 years and 8 months, respectively.
To control the risk of rupture in men to below 1 percent, the corresponding estimated surveillance intervals are 8.5 years for a 3.0 cm and 17 months for a 5.0-cm AAA.
While absolute growth rates were similar for women and men (particularly for larger baseline AAA diameters), there were marked differences in the absolute risks of rupture. Women had a 4-fold greater rupture risk for all AAA sizes and reached a rupture risk of greater than 1 percent in a much shorter time than men.
"Current recommendations for surveillance intervals vary widely although the intervals usually decrease with increasing AAA diameter (for example, 1 year for AAAs measuring 3.0-4.4 cm and 3 months for those measuring 4.5-5.4 cm in the current screening program in England)," the authors write. The findings of this study indicate that for men "these surveillance intervals could be extended to 3 years for AAAs measuring 3.0 to 3.9 cm, 2 years for 4.0 to 4.4 cm, and yearly for 4.5 to 5.4 cm; the risk of rupture would be maintained at less than 1 percent. 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."
"There is a need for more research regarding women with aneurysms in the diameter range of 4.5 to 5.4 cm. Since national rates of AAA rupture are declining, recommended surveillance intervals may need to be reassessed. There is also a need to establish the cost effectiveness of different surveillance policies. Decreasing surveillance frequency would reduce surveillance costs. However, it may also slightly increase rupture rates and increase patient anxiety. This would decrease overall life expectancy and quality of life in AAA patients under surveillance and increase costs attributable to emergency surgery," the researchers conclude.