Fluvoxamine and SIADH

Dear Sir, I read Dr McHardy's report (BJCP March/April 1993) with interest.1 Hyponatraemia has been classified into three types:2 the first is due to excessive salt loss, as in polycystic disease, or most commonly secondary to diuretic therapy, which this woman was undergoing. The second is due to an expansion of the extracellular volume, which may be due to excessive water intake by the patient. (I note that fluid restriction was difficult in this case, and it is reported that polydipsia may be a symptom of certain psychiatric disorders.3) The third is due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which may be caused by a chest infection.4 The course of recovery was related not only to the fluvoxamine but to the resolution of the chest infection, the cessation of the diuretic, and improved monitoring of the fluid intake. The case reported by Dr McHardy represents one of the major difficulties in assigning a particular side-effect to a particular drug in old age psychiatry and medicine, especially when the effect is a rare one. The presence of multiple pathologies and the need to do several things at once makes any causal relationship hard to establish except by rechallenge, which is unethical. Reporting of the side-effect depends much on individual awareness, and such a rare side-effect may become lost in the general incidence of metabolic and endocrine abnormalities found in this age group. Most reports of hyponatraemia associated with selective serotonin reuptake inhibitors (SSRIs) have to date been with fluoxetine;5-8 there has been one report with paroxetine9 but none of sertraline. The SSRIs are structurally disparate, and it is most likely that the hyponatraemia represents a class effect. However, in any patient recently started on an SSRI whose mental state deteriorates, a full physical and biochemical work-up would be advisable.