A Case Report of Blonanserin-Induced Hyponatremia

A 54-year-old female patient who developed a single episode of major depressive disorder, with psychotic symptoms (DSM-IV-TR) of 3-month duration, was put on venlafaxine 150 mg/day and risperidone 4 mg/day. Over a period of 2 months, her depressive symptoms decreased considerably, and the psychotic symptoms subsided. Because she complained of sedation, which distressed the patient, risperidone was stopped, and blonanserin was started at a dose of 4 mg/day, which was increased to 8 mg/day after 1 week. She reported further improvement and no longer complained of sedation. Four days after the dose of blonanserin was changed to 8 mg/day, she developed light headedness, difficulty in concentration, tremulousness, fatigue, and severe generalized weakness; she needed support to walk. She was hospitalized and extensively evaluated for organic conditions. All her investigations were normal, but hyponatremia was detected (sodium level, 128 mmol/L). Blonanserin and venlafaxine were both discontinued. Her sodium levels returned to normal in 4 days. Her clinical condition improved, and the side effects disappeared within a week. She was discharged from the hospital after gradually reintroducing venlafaxine 150 mg/day, under close monitoring of sodium levels, which remained normal. One month later, she was in complete remission and had normal electrolyte levels. She is being regularly followed up.

A number of psychotropic drugs are known to cause hyponatremia and syndrome of inappropriate secretion of antidiuretic hormone.¹ Among antidepressants, selective serotonin reuptake inhibitors have been most reported to cause clinically significant hyponatremia. Although venlafaxine has potent serotonergic activity, it is much less likely to cause hyponatremia, and the reported cases of hyponatremia are generally clinically insignificant.² Hyponatremia seen with venlafaxine usually manifests in a few days after onset.³ In this case, hyponatremia appeared more than 2 months after venlafaxine was started; reintroduction of venlafaxine did not cause hyponatremia. Therefore, it is reasonable to conclude that blonanserin caused the hyponatremia.