It is customary to monitor patients harboring small (<5.5 cm maximum diameter) abdominal aortic aneurysms (AAA) with ultrasound scans (or even CT) at 6-month intervals and occasionally more frequently. This may be unnecessary and wasteful. No consensus exists on the optimal time interval to perform such surveillance studies, but a new UK study (RESCAN) appearing in the February 2013 issues of the Journal of the American Medical Association goes a long way to help define this important area of vascular care. Thompson et al1 conducted the study to determine the rates at which small AAAs progress to reach the elective-repair threshold diameter of 5.5 cm and the risk of rupture over time. Using meta-analysis methodology, the authors scrutinized individual patient information from published studies of small-aneurysm growth and rupture.
In total, 18 studies containing records from 15,471 patients (13,728 men and 1,743 women) under surveillance for small AAA were analyzed. Most used 5.5 cm as the threshold for intervention and used only ultrasound scans to measure diameters. The authors found that aneurysm growth and rupture rates varied considerably across the various studies.
Each 0.5 cm increase in baseline aneurysm diameter resulted in a 0.59 mm per year increase in average growth. In men, rupture rates increased by a factor of 1.9 for every 0.5 cm increase in baseline aneurysm diameter. For those with a 3 cm aneurysm, the estimated average time to reach a 10% chance of attaining the intervention threshold diameter of 5.5 cm was 7.4 years. The corresponding average times for 4 cm and 5 cm aneurysms were 3.2 years and 8 months, respectively. They further estimated that controlling the risk of aneurysm rupture in men to under 1% would require surveillance intervals of 8.5 years for 3 cm aneurysms and 17 months for 5 cm aneurysms.
Aneurysm growth rates were found to be similar for women and men. However, there were marked differences in the absolute risks of rupture: women had a four-fold risk for all aneurysm sizes and reached the 1% rupture risk threshold in a much shorter time than men.
These findings tell us that for men, the surveillance intervals for AAA could be extended to 3 years for aneurysms measuring 3 cm to 3.9 cm, 2 years for aneurysms measuring 4 cm to 4.4 cm, and 1 year for aneurysms measuring 4.5 cm to 5.4 cm, and “the risk of rupture would be maintained at less than 1%.”1 But more research is clearly needed regarding women with 4.5 cm to 5.4 cm aneurysms, and caution should be exercised because women are exposed to a significantly greater risk of AAA rupture.
There is no doubt that decreasing surveillance frequency would lead to decreased costs and savings and fewer unnecessary scans. However, it could also increase patients’ anxiety and nervousness and potentially compromise the quality of their lives. As always, exercising judgment and individualized decision-making will be conducive to better care. But in the end, it seems almost certain that current small-aneurysm surveillance practices are off-base and unnecessarily frequent – and particularly so in the United States. It is time to re-examine.
- RESCAN Collaborators, Bown MJ, Sweeting MJ, Brown LC, Powell JT, Thompson SG. Surveillance intervals for small abdominal aortic aneurysms: A meta-analysis. JAMA. 2013;309(8):806-813.