Type 2 diabetes is growing fast in the U.S. more than 23 million Americans have the disease and another 57 million are hovering dangerously close to developing it and the diagnosis automatically puts patients at increased risk of other health problems, including heart disease, stroke, kidney problems and eye abnormalities.
But exactly how great that added risk is appears to be in question. The results of a large, multicenter trial raise the possibility that the danger of some diabetes complications may not be as great as earlier data has indicated and that doctors may be screening diabetes patients to no benefit. Reporting from a group of institutions in the U.S. and Canada, researchers involved in the Detection of Ischemia in Asymptomatic Diabetics (DIAD) study found that screening diabetes patients for heart risk fails to predict which patients are most likely to have a heart attack. DIAD also found that the risk of heart disease among diabetes patients may be exaggerated overall, according to the data published April 14 in the Journal of the American Medical Association (JAMA). (See the top 10 medical breakthroughs of 2008.)
Based on the number of type 2 diabetes patients who typically go on to develop heart problems, DIAD researchers began with the assumption that as many as 60% of the study's 1,123 volunteers with diabetes, who showed no outward signs of heart disease, might be harboring silent heart problems. Researchers expected that screening these patients using the common treadmill stress test and then imaging their hearts would help root out any heart abnormalities, such as early blockages or irregular heart rhythms, quickly enough to be treated before leading to a potentially deadly cardiac event. (Read "The Year in Medicine 2008: From A to Z.")
"We proved our expectations three times wrong," says Dr. Frans Wackers, professor of diagnostic radiology and medicine at Yale University School of Medicine and an author of the DIAD study. "We found to our surprise that there was not an increase in heart abnormalities among diabetic patients, but actually fewer abnormalities. And the next surprising thing was that this was true in both the group that received screening and the group that received no screening at all."
Among the patients who were screened, 5.5% needed procedures, such as bypass or angioplasty, to restore blood flow to the heart during the course of the trial. A similar proportion, 7.8%, of unscreened patients required similar procedure. The difference was not statistically significant, meaning that the screening did little to predict or prevent heart problems in diabetes patients.
One reason that screening didn't appear to provide any health advantage, Wackers theorizes, may be that patients with diabetes (particularly the ones being monitored carefully in the study) are already benefiting from well controlled blood sugar in patients, both with diabetes and without, high blood sugar is associated with increased heart risk. So, if diabetes patients are already being treated for potential heart risk factors before they become hazardous, screening becomes redundant.
Wackers stresses, however, that these findings do nothing to diminish the very real risk of heart disease in diabetics. A 1998 Finnish study documented that diabetes patients who had not suffered a heart attack had the same poor health profile as those who had findings that prompted the American Diabetes Association to recommend heart-disease screening for all diabetes patients with two or more additional risk factors for heart disease, such as high cholesterol or hypertension, even in the absence of symptoms. "That study really changed the field," says Wackers, "and told us we cannot miss the risk of heart disease and should start testing all of our patients."
With his latest findings, however, Wackers thinks the ADA guidelines are ready for a revision. Heart screenings may not be as important as basic primary prevention strategies, such as ensuring that diabetes patients control their weight, cholesterol and blood pressure, and stop smoking. He argues that if the rate of heart problems is indeed declining in diabetes patients because they are being adequately treated for the risk factors for heart disease, then the stress test recommendation becomes redundant and expensive.
"Our results show that with standard of care, diabetes patients actually do quite well," he says. "I believe now that it's far more important to do primary prevention, such as keeping cholesterol levels on target, and blood pressure controlled, and not smoking."
Dr. John Buse, a DIAD investigator and immediate past president of the American Diabetes Association, agrees that the screening should be limited. "We probably should not be doing stress tests in people without heart symptoms," he says. "But doctors need to make sure to ask questions of their patients about any possible symptoms they may be having of heart trouble."
Some experts note that while the rate of heart disease in the people without diabetes may be improving due in part to increasing efforts to lower patients' cholesterol and blood pressure, among other risk factors diabetes patients who have already had heart attacks appear not to be benefiting as much from the same preventive measures, and continue to suffer and die from higher-than-average rates of heart problems.
"How do we decrease the bad outcomes in people who get heart disease within the setting of diabetes?" says Dr. David Nathan, director of the diabetes center at Massachusetts General Hospital. "There is just no clear answer to that."
In fact, the answers are sometimes conflicting. In March, scientists from Australia and New Zealand reported in the New England Journal of Medicine that aggressively lowering blood sugar in diabetes patients who have had a heart attack does not reduce their future risk of heart disease, but in fact puts these patients at higher risk of hypoglycemia (low blood sugar) and death. Meanwhile, in the current issue of JAMA, another study found that intensive blood-glucose therapy in diabetes patients was not linked with greater mortality.
Such results make it difficult to know for sure whether the risks of abnormally high or abnormally low levels of glucose are more dangerous for diabetes patients who land in the hospital with a heart attack and high blood sugar levels. "It's a moving target," says Nathan. "But we are winning the battle. It's been an incredibly exciting several decades in diabetes research, but a lot more work needs to be done. We know what we need to do, we just need to apply what we learn better to the right patients at the right time."