Venlafaxine Hyponatraemia: Incidence, Mechanism and Management

Abstract

Objective: This prospective study was performed on patients aged >65 years commencing therapy with venlafaxine, in order to determine the incidence of hyponatraemia induced by the drug, to investigate the underlying pathophysiological mechanisms, and to evaluate a simple approach to management of this condition. Method: All patients aged >65 years seen by one author (MR) from all referral sources were entered into the study. Baseline biochemical tests were ordered, and if hyponatraemia developed (plasma Na <130 mmol L−1) additional tests were performed to ascertain the mechanism, while the patient continued on venlafaxine and fluid restriction was instituted. Results: A total of 58 patients were seen, of whom 10 developed hyponatraemia, giving an incidence of 17.2%. Of these 10 patients, five were excluded from prolonged observation because of either severe medical illness, side-effects from the antidepressant or being lost to follow up. When hyponatraemia developed, it invariably did so within a few days of starting venlafaxine, and was associated with non-suppression of antidiuretic hormone in the face of a low serum osmolality. Fluid restriction (800 mL day−1) was effective in raising the plasma sodium to the normal range within 2 weeks, after which the fluid restriction could be relaxed without relapse occurring. These patients remained well for the follow-up period of up to 6 months. Conclusions: Patients >65 years of age should have their electrolytes measured 3–5 days after starting venlafaxine therapy. If hyponatraemia develops, it can be managed with modest fluid restriction without discontinuing drug treatment, subject to close continued clinical observation and biochemical monitoring.