A 45-year-old woman on carbamazepine for trigeminal neuralgia develops nausea and confusion over two weeks. Serum sodium is 118 mEq/L, urine osmolality is high, and euvolaemia is present. The most likely mechanism is:
- A Stimulation of ADH release and potentiation of its renal action (SIADH-like effect) ✓
- B Carbamazepine-induced nephrogenic diabetes insipidus
- C Primary polydipsia induced by carbamazepine
- D Salt-losing tubulopathy from chronic interstitial nephritis
Correct answer: A. Stimulation of ADH release and potentiation of its renal action (SIADH-like effect)
Explanation
Hyponatraemia is a recognised dose-related effect of carbamazepine and oxcarbazepine, mediated by enhanced ADH secretion and increased renal responsiveness to ADH, producing a syndrome indistinguishable from SIADH. Euvolaemia with concentrated urine confirms this. Nephrogenic diabetes insipidus causes dilute urine and hypernatraemia, so option B describes the opposite abnormality and is excluded by the biochemistry.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.