Benefit May Not Outweigh Harm in AAA Screening

Stephanie Edwards

June 27, 2018

A study on the benefits and harms of screening for abdominal aortic aneurysm (AAA) is raising the question of whether the practice significantly improves outcomes. 

The study, including data on more than 130,000 men in Sweden, did show a small decrease in mortality with screening, although the difference did not reach statistical significance. However, reductions in mortality were similar in counties that offered screening and those that didn't, suggesting other factors may be at play.

"Our new findings suggest that this screening program may be outdated because the number of deaths from abdominal aortic aneurysm has been greatly reduced, likely due to lower smoking rates," Minna Johansson, MD, Department of Public Health and Community Medicine, Institute of Medicine, University of Gothenburg, Sweden, said in a statement.

The article was published online June 16 in The Lancet.

Other Factors to Consider

Following the results of four randomized trials done in the 1980s and 1990s, AAA screening has been available in the United Kingdom, United States, and Sweden for men aged 65 years and older, the researchers write. 

Screening includes one ultrasound examination of the abdomen that measures the size of the aorta. Those with an aorta 30 mm wide or more are diagnosed with AAA and monitored regularly by ultrasound; those with an aorta wider than 55 mm are offered preventive surgery. 

The researchers looked at individual data on the incidence of, mortality from, and surgery for AAA in 25,265 men aged 65 years or older who were invited to join the AAA screening program between 2006 and 2009. They simultaneously followed a contemporaneous cohort of 106,087 age-matched men who were not invited to screening. 

To account for background trends, they also analyzed national trends from Swedish men 40 to 99 years during 1987 to 2015. They used the Swedish cause-of-death registry, which has 98% complete mortality data for the country, and data on surgical procedures for AAA (acute and elective) were retrieved from the Swedish national registry for vascular surgery (Swedvasc), which includes 99% of all surgeries for AAA.   

Adjustment was made by weighting the analysis with a propensity score obtained from a logistic regression model on cohort year, marital status, educational level, income, and whether the patient had an AAA diagnosis at baseline, they note.

"Adjustment for differential attrition was also done by weighting the analysis with the inverse probability of still being in the cohort 6 years after screening," they write. "Generalised estimating equations were used to adjust the variance for repeated measurement, and in response to the weighting."

AAA mortality in Swedish men decreased from 36 to 10 deaths per 100,000 men aged 65 to 74 years of age between the early 2000s and 2015, the authors note. However, mortality decreased at similar rates in all Swedish counties whether or not screening was offered, they point out.

After 6 years of screening, they found a nonsignificant 24% reduction in AAA mortality associated with screening (adjusted odds ratio [aOR], 0.76; 95% confidence interval [CI], 0.38 - 1.51; P = .001), "corresponding to a 0.02 percentage point absolute reduction in disease-specific mortality (95% CI –0.03 to 0.07) or that two men (95% CI –3 to 7) avoid death from AAA for every 10 000 men offered screening."

Screening was associated with increased odds of AAA diagnosis (aOR, 1.52; 95% CI, 1.16 - 1.99; P = .002) and an increased risk for elective surgery (aOR, 1.59; 95% CI, 1.20 - 2.10; P = .001). They write, "For every 10 000 men offered screening, 49 men (95% CI 25–73) were likely to be overdiagnosed, 19 of whom (95% CI 1–37) had avoidable surgery that increased their risk of mortality and morbidity."

The authors note that a limitation of the study is that it was not a randomized trial and many other factors contributing to AAA mortality and incidence  predate organized AAA screening.  Because socioeconomic status is strongly related to mortality from AAA, the authors adjusted for such status, but stated, "we cannot exclude residual confounding, most notably related to smoking status."

Johansson and colleagues say that since meaningful follow-up was limited to 6 years, the full effect of screening might not have been captured, which could lead to potentially underestimating the benefits and overestimating the overdiagnosis and overtreatment.

"The small benefit and substantially less favourable benefit-to- harm balance of AAA screening at present means that the continued justification of the intervention should be revisited," the researchers conclude.

Smoking Reduction a Priority

In an accompanying Comment, Stefan Acosta, MD, PhD, Department of Clinical Sciences, Lund University, Malmö, Sweden, acknowledges that the data suggest a decreasing benefit to harm ratio but do indicate that "in terms of a potential avoidable surgery, both overdiagnosis and overtreatment might not be as harmful as one thinks." 

This could provide the opportunity for "pharmacological secondary intervention (eg, with statins, anti-platelet therapy, and blood pressure reduction) for individuals with increased burden of cardiovascular disease," he says.

Following up on the authors' own acknowledged limitation that a 6-year follow-up may be too narrow a time frame, Acosta points to a report by Wanhainen and the Swedish Aneurysm Screening Study Group, "who showed that at least 10 years of implementation of AAA screening might be needed to have solid data on changes in AAA mortality."    

Primary prevention programs to reduce the prevalence of tobacco smoking is a top priority, whereas screening for AAA is not. Dr Stefan Acosta

 

Acosta further suggests there may be an underestimation of the benefit of screening due to a substantial decrease in the number of autopsies in men age 65 to 74 years in Sweden, leading to missing information on deaths from AAA ruptures outside of the hospital.

Acosta concludes that perhaps the focus should be on the relationship between smoking and AAA. He points out that "smoking is eight times more common in individuals with AAA than in healthy controls and is implicated in 75% of AAA cases," and perhaps the lower rates of smoking may be the main factor to consider when speculating on the decrease in incidence and mortality of AAA.

"Every percentage drop in the prevalence of smoking will have a huge effect on smoking-related diseases such as cancer and AAA," Acosta writes. "Primary prevention programs to reduce the prevalence of tobacco smoking is a top priority, whereas screening for AAA is not."  

The study was funded by the Research Unit and Section for General Practice, FoUU-centrum Fyrbodal, Sweden, and the region of Vastra Gotaland, Sweden.  The authors and Acosta have disclosed no relevant financial relationships.

Lancet. Published online June 16, 2018. Abstract, Comment

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