Surveillance Intervals for Small Abdominal Aortic Aneurysms: A Meta-analysis

It is accepted that most small AAAs grow slowly. However, there is substantial variation in growth rates between different individuals (Brady AR et al, Circulation 2004;110:16-21). Various groups have recommended different intervals between ultrasound screenings of patients with AAAs, with these intervals largely dependent on aneurysm diameter. There is, however, no consensus regarding the optimal time between ultrasound examinations in patients with small AAAs. In this study, the authors sought to determine the rates at which small AAAs progress to reach a threshold diameter of 5.5 cm and also to determine the risk of AAA rupture over time. Using these data, they hope to develop more scientifically based recommendations to guide AAA surveillance protocols. The authors identified, through a systematic literature review, studies with individual patient data recording AAA growth and rupture. Original study authors were contacted. The authors identified 18 data sets that provided repeat ultrasound measurements of AAA diameter over time in 15,471 patients. AAA diameters were analyzed using a random-effects model accounting for between-patient variability in size and growth rates of AAA. Rupture rates were analyzed by proportional hazard regression using the model AAA diameter as a time-varying covariant. Predictions of the risk of growing to a diameter >5.5 cm and of rupture within given time intervals were estimated and pooled across studies by random effects meta-analysis. Variation in AAA growth and rupture rates varied considerably among studies. For each 0.5-cm increase in AAA diameter, growth rates increased on average 0.59 mm/y (95% confidence interval [CI], 0.51-0.66 mm/y). Rupture rates increased by a factor of 1.91 (95% CI, 1.61-2.25). As an example, to monitor the AAA growth risk in men to >5.5 cm to <10%, on average, a 7.4-year surveillance interval (95% CI, 6.7-8.1 years) is sufficient for a 3-cm AAA. However, an 8-month interval (95% CI, 7-10 months) is necessary for a 5-cm AAA. To control the risk of rupture in men to <1%, the corresponding estimated surveillance intervals are 8.5 years (95% CI, 7.0-10.5 years) and 17 months (95% CI, 14-22 months).