Case Report: SIADH-Induced Hyponatremia Following Sertraline and Quetiapine Use in an Elderly Patient with Dementia: A Case Report

Background: Hyponatremia — defined as serum sodium below 135 mEq/L — is the most common electrolyte disturbance in hospitalised patients and can be precipitated by psychotropic medications through the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Selective serotonin reuptake inhibitors (SSRIs) are a well-recognised cause, while second-generation antipsychotics such as quetiapine have been implicated less frequently. Cases involving the concurrent use of both drug classes in elderly patients with dementia are rarely reported. Case presentation We describe a 77-year-old man with advanced Alzheimer’s disease, type 2 diabetes, hypertension, chronic kidney disease, and multiple other comorbidities who developed severe hyponatremia (serum sodium 121 mmol/L) following concurrent treatment with sertraline 50 mg and quetiapine. He presented with worsening dyspnoea and an abrupt decline in cognition over two months, temporally associated with the reintroduction of quetiapine. Laboratory findings confirmed hypotonic hyponatremia with inappropriately concentrated urine, consistent with SIADH. Thyroid and adrenal function were normal. Cardiac evaluation revealed new-onset heart failure without an acute ischaemic event. After systematic exclusion of other aetiologies, psychotropic medications were identified as the probable cause. Management comprised discontinuation of both sertraline and quetiapine and fluid restriction to less than 1,000 mL per day, with close electrolyte monitoring. Conclusion This case underscores the importance of routine sodium monitoring in elderly patients with dementia initiated on serotonergic or antipsychotic therapy. Acute cognitive deterioration in this population should prompt evaluation for reversible metabolic causes, including drug-induced hyponatremia, before attributing decline to dementia progression. A multidisciplinary approach involving neurology, nephrology, and psychiatry is essential for accurate diagnosis and safe management.