February 6th, 2014
Carotid IMT Testing: Misgraded and Misunderstood
The 2013 American College of Cardiology American Heart Association Guideline on the Assessment of Cardiovascular Risk has generated a lot of discussion about the accuracy and validity of the pooled risk prediction equations. Lost in the heat generated by that discussion, however, is a controversial Class III recommendation for common carotid artery intima-media thickness (CCA IMT) testing, which, essentially, recommends against its use. Yet, just three years ago, the ACC/AHA Guideline for Assessment of Cardiovascular Risk in Asymptomatic Adults gave it a class IIa recommendation — saying it was “reasonable” if published technical standards were followed, as is recommended for coronary artery calcium screening (CAC). This major downgrade was a surprise, because no new, guideline-modifying data about the predictive value of CCA IMT testing was published in the interim.
We think this downgrade is the result of two separate, but related, problems.
First, the 2010 guideline gave too high a rating to both CCA IMT and CAC — the IIa grade was undeserved, because no data at that time, or even at present, suggest major improvements in patient outcomes using either imaging test. The misrepresentation in the recent ACC/AHA guideline, however, seems to result from a misunderstanding of how CCA IMT testing is used clinically, which leads to the second problem.
The description of CCA IMT testing in the ACC/AHA Guideline focuses on its weak predictive characteristics, and references a meta-analysis by Den Ruiter et al that focused on CCA IMT from many population-based studies. However, the American Society of Echocardiography (ASE) Consensus Statement on carotid ultrasound for cardiovascular disease (CVD) risk prediction and others have specified that carotid ultrasound for CVD risk prediction should be based on a thorough scan of the carotid arteries for the presence of plaques in addition to measurement of CCA IMT. This is because the presence and extent of carotid plaque, which occurs predominantly in the carotid bifurcation and internal carotid artery, rather than the CCA, are independent predictors of future CVD events and in observational studies perform better than CCA IMT alone.
The ASE Consensus Statement, which has been widely adopted, specifically stipulated that measuring CCA IMT without considering plaque presence was insufficient; however the study that provided the recent ACC/AHA Working Group with the strongest evidence for its class III recommendation was a meta-analysis based solely on CCA IMT without regard to plaques. The ACC/AHA Working group raised important concerns about standardization and measurement issues, however, clinical use of this carotid ultrasound for CVD risk prediction has been standardized for over five years. It has published appropriate use criteria and its reproducibility is excellent, even in less experienced hands.
We believe that a Class IIb recommendation would be more appropriate, based on the test characteristics of CCA IMT plus plaque scanning. Although CAC appears to be a better predictor of CVD than CCA IMT and plaques in many (though not all) studies, neither CAC nor CCA IMT have been demonstrated to improve patient outcomes. The best either test has shown is modest improvements in risk factors, use of statins and aspirin, and patient and physician intentions.