To determine whether QT interval is influenced by genetic factors and whether QT-interval prolongation occurs in type 1 diabetes or is related to diabetic autonomic neuropathy, QT intervals were measured, and autonomic function was assessed in 44 pairs of identical twins who were discordant for type 1 diabetes. Twins were compared with 44 normal control subjects of similar age and sex. QT intervals were corrected for heart rate (QTc). QTc in diabetic twins correlated with that in their nondiabetic co-twins (r = 0.41; p = 0.006). Diabetic twins had significantly longer QTc than did their nondiabetic co-twins and control subjects (416 +/- 18 vs 407 +/- 16 and 403 +/- 19 ms, respectively; p0.005). A greater number of abnormal autonomic function tests were detected in diabetic twins than in their nondiabetic co-twins and control subjects (8 vs 2 and 0%, respectively; p0.01). Diabetic twins with disease duration14 years (n = 22) had longer QTc than did their nondiabetic co-twins (420 +/- 17 vs 402 +/- 14 ms; p0.0005). Twins with diabetes for14 years had a greater frequency of abnormal autonomic function tests than did those with diabetes14 years (15 vs 2%; p0.001). QTc did not correlate with autonomic function in diabetic twins. It is concluded that QT interval is influenced by genetic factors, and in type 1 diabetes, QTc can be prolonged independently of autonomic neuropathy.
