Synopsis: Annual, or less frequent, surveillance intervals are effective for aneurysms < 45 mm in diameter.

Source: Brady AR, et al. Abdominal Aortic Aneurysm Expansion: Risk Factors and Time Intervals for Surveillance. Circulation. 2004;110:16-21.

Studies have shown that abdominal aortic aneurysms (AAA) can be safely followed until a diameter of 55 mm is reached before considering surgery. However, the size surveillance frequency is poorly understood. Thus, Brady and colleagues, from the United Kingdom (UK) Small Aneurysm Trial, analyzed repeated AAA diameter measurements by ultrasonography from a large national cohort to characterize AAA expansion and its determinants. In 93 UK hospitals, patients referred to vascular surgeons with aneurysms 40 to 50 mm in diameter, who were fit for surgery, were asked to participate in a trial comparing immediate surgery to surveillance and surgery, if the aneurysm became > 55 mm, grew by > 10 mm/year, or they had symptoms due to the aneurysm. The surveillance frequency was every 3 months for aneurysms > 50 and < 55, and 6 months for those < 50 mm. Sophisticated, statistical methods were used to eliminate bias.

Among 2366 patients recruited, 1743 had more than 1 (9125 total) AAA diameter measurement over a mean of 2 years follow-up (maximum 8 years). The mean, initial AAA diameter was 43 mm, and the growth rate was 2.6 mm/year (95% range, 1.0-6.1 mm/year). The strongest predictor of growth rate was initial size. Growth rates were lower in diabetics and those with peripheral vascular disease, but higher in current smokers. Age, hypertension, and other cardiovascular disease risk factors were not related to growth. The surveillance intervals to keep < 1% at 55 mm were 36, 24, 12, and 3 months for aneurysms of 35, 40, 45, and 50 mm, respectively. Brady and colleagues concluded that annual, or less frequent, surveillance intervals are effective for aneurysms < 45 mm in diameter.

Comment by Michael H. Crawford, MD

This study presents highly practical information that is of value to patients, physicians, and health care systems. Brady et al set the goal of keeping the discovery of patients with AAA diameter > 55 mm to 1% as their surveillance interval standard. This should be acceptable to all. They found that the growth rate of AAAs was slower than appreciated in previous studies because the earlier studies used linear regression modeling, which is biased toward larger aneurysms with higher growth rates. AAA size is the major factor in predicting aneurysm growth rates. Thus, the surveillance frequency proposed varies with initial AAA size.

Since the upper limit of the abdominal aortic diameter is 30 mm, why not operate on everyone above that limit and save the cost of all this screening? Two studies, 1 in the United States, and this study from the United Kingdom, did not show a mortality benefit of such a strategy. However, some studies have shown a low rupture rate up to a diameter of 60 mm. Thus some have recommended advising surgery based upon AAA size and the risk of surgery. For example, a healthy 60-year-old with an aneurysm of 55 mm would get surgery, but in a more risky patient, one might wait until 60 or 65 mm. This approach makes some sense, but has not been studied prospectively.

Interestingly, traditional atherosclerosis risk factors do not seem to be a factor in aneurysm growth, with the exception of smoking. However, the effect of smoking on AAAs may not be related to its effect on atherosclerosis. Pathologic studies have shown that AAAs are not typically atherosclerotic, and the main findings are inflammation and proteolysis. Thus, we don’t fully understand the pathogenesis of AAAs, but smoking cessation would make sense since it increases the growth rate of AAAs by up to 20%. Because we don’t understand the pathogenesis of AAAs, some have suggested that every 70-year-old man should have 1 abdominal ultrasound screening or other imaging study. The effectiveness of this recommendation is not proven, but it makes some sense. We are not told how the patients in this study were identified. Some may have had symptoms, a positive abdominal physical exam, or routine screening.

Again, because of our ignorance about the pathogenesis of AAA, Brady et al have suggested that surveillance be applied to post-stent graft and post-operative patients, under the theory that the untreated segments of aorta may expand over time and cause leaks.

Dr. Crawford is Professor of Medicine, Chief of Clinical Cardiology, University of California ,San Francisco.