A case of myotonic dystrophy (MD) associated with glucose-induced hyperinsulinemia followed by reactive hypoglycemia and increased number of cytosine-thymine-guanine (CTG) trinucleotide repeats in MD gene.

A 39-year-old man with myotonic dystrophy consulted our hospital for nausea, vomiting and dizziness that occurred after 75 g oral glucose tolerance test (OGTT). Reexamination of OGTT revealed remarkable hyperinsulinemia (622 microU/ml) followed by reactive hypoglycemia (50 mg/dl) and such hypoglycemic symptoms as nausea, vomiting, dizziness and palpitation. DNA analysis of the circulating lymphocytes revealed increased (1,500 times) number of cytosine-thymine-guanine (CTG) trinucleotide repeats in myotonic dystrophy protein kinase (DM kinase) gene. Gel chromatographic analysis of the plasma in combination with sensitive enzyme immunoassay of insulin revealed that the ratio of proinsulin to total immunoreactive insulin was elevated at fasting (12.9%), and was decreased to 8.9% at 60 min after glucose administration. These findings may indicate that biologically active authentic insulin was predominantly secreted after glucose administration in the present case. This is the first case report of myotonic dystrophy with hyperinsulinemia associated with reactive hypoglycemia induced by oral glucose administration.