NEJM: CIMT of the internal carotid artery best classifies CV disease
While both maximum internal and mean common carotid intima-media thickness (CIMT) can help predict cardiovascular (CV) outcomes, maximum intima-media thickness of the internal carotid artery can improve the classification of risk of CV disease, according to a study published July 21 in the New England Journal of Medicine.

“Carotid-wall intima–media thickness is a surrogate measure of atherosclerosis associated with cardiovascular risk factors and with cardiovascular outcomes,” Joseph F. Polak, MD, MPH, of the Tufts Medical Center in Boston, and colleagues wrote. “Intima-media thickness of the walls of the common carotid artery and internal carotid artery may add to the Framingham risk score for predicting cardiovascular events.”

During the study, Polak and colleagues measured the mean IMT of the common carotid artery and maximum IMT of the internal carotid artery in 2,965 patients in the Framingham Offspring Study. The authors analyzed CV disease outcomes for an average of 7.2 years. The reclassification of CV disease was evaluated on the basis of the eight-year Framingham risk score categories—low, intermediate or high—after adding IMT values.

Patients had a mean age of 58 years and no history of CV disease; 55.3 percent were women.

The study authors reported that 296 patients experienced a CV event. While Framingham risk factors were all significant predictors of CV disease, when the researchers added mean IMT for the common carotid artery it was significantly associated with the risk of CV disease.

“Carotid-artery intima–media thickness, measured noninvasively with the use of carotid-artery ultrasonography, is an independent predictor of new cardiovascular events in persons without a history of cardiovascular disease,” wrote Polak and colleagues.

Maximum IMT of the internal carotid artery was associated with the risk of CV disease in those models that included the Framingham risk factors. When Polak et al added in internal carotid artery IMT thickness, the C statistic increased significantly, from 0.748 to 0.758.

The researchers found that internal carotid artery IMT significantly increased the net reclassification index for men and women, 6.7 percent and 9.2 percent, respectively. Internal carotid artery IMT also increased the net reclassification for patients 60 years of age or younger and those 60 years and older, 9.1 percent and 7.6 percent, respectively.

Reclassification is a practical approach to gauging the effects of adding new risk factors for traditional Framingham Risk Score factors when differences in the C statistics are marginal, according to the authors.

Additionally, Polak and colleagues found that the presence of plaque (a maximum IMT of greater than 1.5 mm in the internal carotid artery), was a significant predictor of CV events.

“Even within the three Framingham risk categories we used, the presence of plaque, defined as an intima-media thickness of more than 1.5 mm, was a significant predictor of cardiovascular events, suggesting that further analyses are needed to evaluate the effect of plaque on risk stratification,” the authors wrote.

“It is not clear whether the intima-media thickness incrementally adds value to the Framingham risk factors for cardiovascular-risk prediction,” Polak and colleagues offered. “The addition of intima-media thickness measurements slightly increased the predictive power with respect to cardiovascular risk assessment in one study and with respect to stroke in another study.”

These results may affect how IMT is assessed for the primary prevention of CV disease, the authors wrote. Currently, American College of Cardiology Foundation and American Heart Association guidelines issue carotid IMT a level IIa recommendation for CV risk evaluation; however, emphasize an indication of high risk if the common carotid artery IMT is above the 75th percentile.

“We believe the intima-media thickness of the internal carotid artery should be measured in addition to the thickness of the common carotid artery for purposes of cardiovascular risk assessment,” the authors wrote. However, a limitation to this could be the method used to measure the IMT of the internal carotid artery.

“The maximum intima-media thickness of the internal carotid artery, as either a continuous measurement or a surrogate for the presence of plaque (above a threshold of 1.5 mm), contributed significantly but modestly to the predictive power of the risk factors used in calculating the Framingham Risk Score and improved risk classification on the basis of the Framingham Risk Score,” the authors concluded.