Interferon-alpha is a predisposing risk factor for carbamazepine-induced hyponatremia: A case of syndrome of inappropriate antidiuresis caused by interferon-alpha therapy

A 31-year-old man had been treated with carbamazepine (CBZ) for 6 years and warfarin with bucolome for 2 years before developing hyponatremia 7 days after an injection of interferon-alpha 2b and starting oral ribavirin for chronic hepatitis C virus infection. Despite the hyponatremia, urinary osmolality exceeded plasma osmolality, and urinary excretion volume decreased markedly after water loading. Restriction of water intake and administration of dimethylchlortetracycline improved the hyponatremia, and lithium therapy maintained the normonatremia for one year. The hyponatremia recovered 6 months after the interferon-alpha 2b therapy was completely stopped. In the present case, the syndrome of inappropriate antidiuresis may have been caused by the effect of interferon-alpha 2b on the renal distal tubules that had been sensitized by CBZ. Patients on CBZ therapy should be carefully observed for the development of hyponatremia when they are started on interferon-alpha 2b injections.