Optimal blood glucose control during 18 years preserves peripheral nerve function in patients with 30 years' duration of type 1 diabetes

OBJECTIVE - To assess the association between 18 years of mean HbA(1c) and nerve conduction parameters of the lower limb in patients with type I diabetes of 30 years' duration. RESEARCH DESIGN AND METHODS - HbA(1c) has been examined prospectively since 1982 in a group of 39 patients with type I diabetes. Mean age at baseline was 25 years (range 18-40) with 12 years' disease duration. The mean age at diagnosis of diabetes was 12.5 years. Nerve function of lower limbs was assessed at baseline, after 8 years, and after 18 years. RESULTS - A total of 23 men and 16 women were studied. Mean age was 43 years. Mean HbA(1c) was 8.2% (range 6.6-11.3) during 18-year follow-up. Nerve conduction velocity (NCV) and nerve action potential amplitude (NAPA) at the last examination were significantly associated with mean HbA(1c) (P < 0.05). From 1982 to 1999, there was a significant reduction in nerve function in patients with mean HbA(1c) ≥8.4% (highest tertile). For example, the mean NCV in the tibial nerve was reduced from 47 to 31 m/s (P < 0.01). The number of nerves with NCV (P < 0.01) and NAPA (P = 0.01) reduced to below the reference level in each patient was also significantly associated to mean HbA(1c). No significant associations were found between nerve function parameters, sex, disease duration, blood pressure, serum cholesterol, microalbuminuria, or smoking. CONCLUSIONS - The present study shows that mean HbA(1c) is a strong predictor of nerve function. Mean HbA(1c) <8.4% over 18 years was associated with near-normal nerve function.