Rapid-Onset SIADH Triggered by Combined Duloxetine and Levodopa Use in an Older Adult With Depression and Parkinsonism: A Previously Unreported Interaction.

ObjectiveThe syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a potentially life-threatening electrolyte disturbance commonly encountered in older adults, particularly in association with serotonergic and dopaminergic medications. Both duloxetine and levodopa have been individually implicated in the development of SIADH, although the mechanism often remains unclear.MethodsSingle case presentation and review.ResultsA 73-year-old woman with newly diagnosed depression and parkinsonism was initiated on duloxetine (30 mg/day) and levodopa/benserazide (tradename Madopar) (100/25 mg, 3 times daily). Within 24 h, she developed fatigue, nausea, confusion, and dizziness. Laboratory evaluation revealed severe hyponatremia (serum sodium: 115 mmol/L), low serum osmolality (230 mOsm/kg), high urine osmolality (310 mOsm/kg), and urinary sodium of 43 mmol/L, with a clinically euvolemic status. Thyroid and adrenal function test were normal. Both medications were discontinued. Fluid restriction and hypertonic saline infusion led to normalization of sodium levels over 72 h, with complete clinical recovery.ConclusionThis case highlights an unusually rapid onset of SIADH after combined initiation of duloxetine and levodopa/benserazide. The temporal proximity of symptom onset suggests a possible synergistic interaction. Clinicians should maintain vigilance for acute hyponatremia in older adults shortly after introducing serotonergic and dopaminergic agents.