[Impaired vasopressin secretion in patients with myotonic dystrophy]

Hypernatremia has occasionally been observed in patients with myotonic muscular dystrophy (MyD). To elucidate the possibility of osmoregulatory dysfunction, we investigated hypothalamo-posterior pituitary function as well as serum electrolytes in eight patients with MyD. Blood samples were obtained early in the morning after overnight dehydration. Renal function was estimated by blood urea nitrogen, serum creatinine and creatinine clearance. Posterior pituitary function was evaluated by direct measurement of plasma vasopressin (AVP) during a 5% hypertonic saline infusion. Plasma AVP concentrations were determined by sensitive radioimmunoassay. In five patients, circulating blood volume (CBV), plasma renin activity (PRA) and serum aldosterone (S-Aldo.) were also measured. The mean serum sodium level (143.9 +/- 1.7mEq/1: Mean +/- SD) was significantly higher than in the controls (139.4 +/- 2.2mEq/1). A 5% hypertonic saline infusion showed a subnormal increase in AVP and diminished thirst, despite sufficient elevation of plasma osmolality, in all patients as compared with healthy adults. Renal function was intact. Biochemical evidence of dehydration, estimated by PRA, S-Aldo and CBV, was unremarkable in four of the five patients. These findings suggest that patients with MyD have neurogenic disorders of osmoregulation in addition to previously reported endocrine abnormalities. Impaired AVP secretion in response to osmotic stimuli and reduced thirst might be responsible for such failure.