"We've found what we believe is the earliest sign of nerve change due to diabetes," says Peter J. Dyck, M.D., Mayo Clinic neurologist and lead researcher on the study. Results were published in the September issue of Diabetes Care. "Changes begin much earlier than previously demonstrated," he says.
About 500 patients from Olmsted County, Minn., home to Mayo Clinic, participated in the longitudinal study, many for 20 years. Patients agreed to periodic measures of their diabetes and measurement of nerve, eye, kidney and blood vessel complications.
About half the people with diabetes develop some type of nerve damage (neuropathy) caused indirectly by high blood sugar levels. Symptoms can include pain, asleep-type numbness, tingling, burning and loss of feeling. Serious complications can include foot ulcers, gangrene, amputations, blindness and kidney failure.
In the study, researchers used various techniques to measure nerve changes, including patient exams, reflex and strength tests, and nerve conduction tests, which measure how quickly nerves carry electrical signals. The nerve conduction tests, over time, provided the most consistent and reliable measures of early nerve damage due to diabetes.
"Even when patients had nerve conduction values well within the normal range, our serial assessments showed steady, unequivocal and statistically significant worsening," says Dr. Dyck.
The nerve conduction measures were corrected for variations in patients' age, height and weight that could have affected results.
The study focused especially on 90 patients who at first evaluation did not have nerve damage and who had been evaluated at least six times at annual or biannual intervals. The Mayo investigators then tested which measure of neuropathy (nerve conductions, symptoms, neurologic signs, quantitative sensation tests or quantitative autonomic tests) significantly worsened, improved or remained unchanged over the study period. Of the five tests, only nerve conduction showed an unequivocal, highly significant, steady worsening over time.
Dr. Dyck says the study offers insights for diabetes care and future research on treatment.
"The aim should be to prevent neuropathy and the complications of eyes and kidneys rather than to intervene after they develop," says Dr. Dyck. Other studies have shown that rigorous control of blood sugar may prevent and possibly even reverse nerve, eye and kidney complications for people with diabetes.
About 18 million Americans have diabetes. From 60 to 70 percent of people with diabetes develop some type of neuropathy or nerve damage due to the disease, according to the American Diabetes Association. Assuming that no symptoms means good news is dangerous, according to Dr. Dyck. "The study shows that diabetes is insidious from the beginning," he says. Diabetes is like atherosclerosis and hypertension, which develop insidiously and continuously unless controlled. Later, they may result in such severe problems as strokes, heart attacks and gangrene of the feet and legs.
Mayo Clinic research results could also influence the design of future clinical trials on treatment options for diabetes. Because nerve changes begin so early, Dr. Dyck says it will be important for clinical trials to include less severely affected patients and to perform studies over longer periods.