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Carbamazepine-induced hyponatraemia — SCE Neurology MCQ

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ModerateClinical PharmacologyCarbamazepine-induced hyponatraemiaSCE Neurology

A 69-year-old woman starts carbamazepine for trigeminal neuralgia. Three weeks later she is confused and nauseated. Sodium is 121 mmol/L, serum osmolality is low and urine osmolality is inappropriately high. She is clinically euvolaemic. What is the most likely underlying mechanism?

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Correct answer: B — Syndrome of inappropriate antidiuresis

Syndrome of inappropriate antidiuresis is the best answer. Carbamazepine can cause SIADH, producing euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine. Adrenal failure has cortisol features; cerebral salt wasting is hypovolaemic; osmotic diuresis gives high solute losses; polydipsia usually has dilute urine. Clinical pearl: Older patients on sodium-channel antiseizure medicines need sodium monitoring when symptomatic.

Reference: BNF; NICE CKS hyponatraemia