A 40-year-old woman started on carbamazepine for focal epilepsy presents 4 weeks later with fatigue and mild confusion. Serum sodium is 122 mEq/L, urine osmolality is inappropriately high, and euvolaemia is confirmed clinically. The most likely explanation is:

• A Pseudohyponatraemia from hypertriglyceridaemia induced by carbamazepine
• B Carbamazepine-induced syndrome of inappropriate ADH secretion ✓
• C Primary polydipsia caused by carbamazepine-induced dry mouth
• D Salt-wasting nephropathy due to tubulointerstitial injury

Explanation

Carbamazepine is a recognised cause of drug-induced SIADH, typically appearing within the first weeks of therapy and producing euvolaemic hypo-osmolar hyponatraemia with concentrated urine. Oxcarbazepine causes hyponatraemia even more frequently. Pseudohyponatraemia gives a normal measured osmolality, salt wasting produces hypovolaemia, and primary polydipsia dilutes the urine rather than concentrating it.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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