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Drug-Induced Hyponatraemia — RACP Adult Medicine MCQ

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ModerateNeurologyDrug-Induced HyponatraemiaRACP Adult Medicine

A 60-year-old woman with epilepsy on carbamazepine presents with hyponatraemia (Na 122 mmol/L). She is euvolaemic and asymptomatic. Her urine osmolality is 580 mOsm/kg. What is the mechanism of carbamazepine-induced hyponatraemia?

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Correct answer: A — Enhanced renal response to ADH (SIADH-like)

Carbamazepine causes hyponatraemia through an SIADH-like mechanism – it enhances the renal tubular response to ADH (vasopressin) by increasing aquaporin-2 expression in the collecting duct, leading to water retention. It does NOT increase ADH secretion. Incidence is 5–40% and is dose-dependent. Management includes fluid restriction or switching to an alternative AED. Oxcarbazepine has an even higher risk.

Reference: AMH – 2025 – Antiepileptics; eTG – 2025 – Neurology