Insulin secretion in patients with myotonic dystrophy and their relatives

Oral glucose, i.v. tolbutamide and i.v. arginine tolerance tests were performed in 11 patients with myotonic dystrophy and 9 of their clinically unaffected relatives. Five of the myotonic patients had glucose intolerance; 7 had exaggerated immunoreactive insulin (IRI) response to glucose. One of the 9 relatives demonstrated glucose intolerance and none had exaggerated insulin response to glucose. Three relatives, all obese, hyperresponded to arginine and one of them also responded excessively to tolbutamide. The results indicate that an exaggerated IRI response to glucose is common in myotonic dystrophy. Enhanced responses to other stimuli are less frequent. Although glucose intolerance occurred in half of the patients, the fact that the highest IRI levels were seen in non-diabetic patients suggests that this excessive response may protect against glucose intolerance. Our studies in relatives do not support the potential usefulness of testing for hyperinsulinemia in the early detection of myotonic dystrophy.