Chronic hypernatremia from a congenital defect in osmoregulation of thirst and vasopressin.

An infant with microcephaly and delayed development was found to have chronic asymptomatic hypernatremia. Computerized brain tomography disclosed dysplasia of the midline structures, septum pellucidum and corpus collosum. Evaluation revealed defective osmoregulation, hypothalamic hypothyroidism, and hypogonadotropinism. He showed no desire to drink at plasma osmolalities over 330 mOsm/kg. His plasma vasopressin levels (less than or equal to 1.4 pg/ml) were inappropriately low relative to his high levels of plasma osmolality (greater than or equal to 310 mOsm/kg), which might be accounted for by either deficient neurohypophyseal vasopressin stores or disturbance of the hypothalamic osmoreceptors governing vasopressin. The first possibility was ruled out by demonstrating normal vasopressin response (167 pg/ml) to nonosmotic (emetic) stimulation. Under baseline conditions, his urine was concentrated up to 747 mOsm/kg and urine volume was low. With water loading, maximal water diuresis developed (urine osmolality 68 mOsm/kg), but his plasma osmolality remained in the hyperosmolar range (312 mOsm/kg). Treatment with a vasopressin analogue, desamino-D-arginine vasopressin, and forced hydration restored plasma osmolality and plasma sodium to normal. These findings indicate a severe defect in the hypothalamic osmoreceptors controlling thirst and vasopressin secretion with normal vasopressin stores and preserved vasopressin responsiveness to nonosmotic stimuli. To our knowledge, this report provides the first documentation of selective osmoreceptor defect in conjunction with congenital dysplasia of midline brain structures.