Incidence and risk factors for hyponatraemia following treatment with fluoxetine or paroxetine in elderly people

Aims To establish the incidence, time course and risk factors of hyponatraemia complicating treatment with fluoxetine or paroxetine in an elderly population. Methods Retrospective descriptive and case control study in an inpatient/outpatient assessment and rehabilitation service for people aged 65 years and over. Fourteen elderly patients with hyponatraemia complicating treatment with fluoxetine or paroxetine, matched with 56 controls drawn from 845 patients treated with fluoxetine or paroxetine over 3.5 years. No other SSRI antidepressants were used over the study period. Results The incidence of hyponatraemia was 4.7/1000 people treated/year (6.3/1000 for fluoxetine and 3.5/1000 for paroxetine). Hyponatraemia was detected at a median 13.5 (mean 18.6, range 4-64) days after commencing the drug. Mean (95% confidence intervals) body weights were lower in cases at 53.0 (95% CI 46.5-59.5) kg compared with controls at 64.5 (95% CI 60.1-68.4) kg (P<0.01). 71% of cases were women compared with 45% of controls (P=0.07) but the effect of gender was confounded by body weight. There were trends for cases to be older (odds ratio 1.10: 95% CI 0.99, 1.23) and lighter (odds ratio 0.92, 95% CI 0.86, 0.99). Conclusions Approximately 1 in 200 elderly people treated per year with fluoxetine or paroxetine developed complicating hyponatraemia. Low body weight was a particular risk factor. Most cases occurred within 3 weeks of treatment.