Carbamazepine-induced hyponatremia in a patient with partial central diabetes insipidus

A 51-year-old Japanese man was referred for the evaluation of persistent hyponatremia. The serum sodium level remained around 120 mmol/l despite mild water restriction. His past history included chronic alcoholism, myocardial infarction and lumbar disc herniation. Carbamazepine (200 mg, b.i.d.) has been used for more than 8 years for low back pain. Serum sodium returned to normal after carbamazepine was stopped, and rechallenge produced acute symptomatic hyponatremia (117 mmol/l) on day 2 after a total dose of 600 mg. Hepatic, renal and endocrine function were within normal limits, and the response to a water load (20 ml/kg) was also normal. Partial central diabetes insipidus was diagnosed by his response to water restriction and nasal desmopressin administration. Polyuria and hypernatremia were not evident in this case, probably due to a combination of low solute intake and low, but not deficient, levels of plasma ADH. This case demonstrates that carbamazepine may cause acute hyponatremia even in central diabetes insipidus, probably by sensitizing the distal renal tubules.