Hypodipsic hypernatraemia in a miniature schnauzer

Normovolaemic hypernatraemia as a result of a suspected congenital primary hypodipsia was diagnosed in a young male Miniature Schnauzer. Despite an elevated serum sodium concentration, the dog did not appear dehydrated on physical examination and the urine osmolality: plasma osmolality ratio was greater than 4; antidiuretic hormone deficiency was therefore not suspected. Basal serum cortisol and thyroxine concentrations were normal. Plasma aldosterone concentration and plasma renin activity (37 pmol l-1 and 1.55 ng dl-1 h-1 respectively) were within normal range. A defective central thirst regulation mechanism was suspected as the dog was totally disinterested in drinking water despite the chronically elevated serum sodium concentration. Excessive ingestion of water mixed with food, and milk resulted in hyponatraemia and associated cerebral oedema. On stabilisation of the dog's condition, a calculated fluid intake based on daily maintenance fluid requirements was prescribed to prevent recurrence of hypernatraemia and hyponatraemia, and associated signs of central nervous system disease. The dog was in apparent good health with controlled fluid intake when examined 230 d later.