Recurrent severe hypernatraemia in a young patient: a disconnect between osmoreceptor function and drinking behaviour.

Chronic hypernatraemia is a rare clinical entity. In the younger population, hypernatraemia is often a consequence of failure to generate thirst in response to osmotic stimuli.We report the case of a male patient admitted with severe hypernatraemia (plasma sodium 175 mmol/L) on return from holidays. His urine was maximally concentrated at 894 mOsm/kg-suggestive of normal vasopressin reserve. MRI of the brain showed a large extra-axial cyst, with preservation of the posterior pituitary bright spot. Formal osmoregulatory studies demonstrated normal osmoregulated vasopressin secretion and normal thirst, but no appropriate drinking behaviour.This patient illustrates a unique pathophysiological disconnect between thirst appreciation and the central drive to drink, in the context of normal osmoregulatory function. It is likely that this disconnect is related to the patient's large intracranial cyst.The management challenge is to maintain appropriate fluid intake in order to prevent recurrent severe hypernatraemia.