Mirtazapine-induced hyponatremia in an elderly hospice patient

Mirtazapine, which enhances central noradrenergic and serotonergic activity, is a commonly prescribed drug for mood disorders in elderly patients due to the low incidence of adverse effects. A heterocyclic antidepressant, mirtazapine has pharmacodynamics similar to selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants. Nevertheless, geriatric patients in general are more prone to experience adverse drug effects.

To report a case of hyponatremia in an elderly hospice patient associated with mirtazapine use.

A 72-year-old Latino male with stage IV colon cancer entered into hospice care and was treated for major depressive disorder with mirtazapine. On day 6 of treatment, he was somnolent and confused. He was found to have severe hyponatremia (serum sodium of 116 mmol/L) without any immediately identifiable physiologic cause. His previous sodium levels were within normal limits over a 6-month period including when mirtazapine was started. Upon discontinuation of mirtazapine, the patient's mental status improved, and his sodium level returned to normal.

Hyponatremia resulting from antidepressant use, particularly SSRIs, is rare in the general population, but in the elderly population the incidence increases because of multiple factors. There are few reports in the literature regarding hyponatremia induced by mirtazapine.

The benefit of treating mood disorders at the end of life outweighs the risks of untreated depression. Hyponatremia, although an uncommon adverse effect of mirtazapine therapy, should be considered in the elderly patient with altered mental status or delirium who has recently initiated mirtazapine therapy.