A 68-year-old woman on carbamazepine for trigeminal neuralgia presents with two weeks of lethargy and gait instability. Serum sodium is 121 mEq/L, urine osmolality is elevated, and she is clinically euvolemic. The most likely mechanism is:
- A Carbamazepine-induced nephrogenic diabetes insipidus
- B Osmotic diuresis from glycosuria
- C Carbamazepine-enhanced ADH action producing a syndrome of inappropriate antidiuresis ✓
- D Primary polydipsia
Correct answer: C. Carbamazepine-enhanced ADH action producing a syndrome of inappropriate antidiuresis
Explanation
Carbamazepine potentiates the effect of ADH on renal collecting ducts, partly by upregulating aquaporin-2 expression, producing dilutional hyponatremia indistinguishable from SIADH. It occurs more often in the elderly and at higher doses. Nephrogenic diabetes insipidus causes water diuresis with hypernatremia, the opposite picture, and the euvolemic state with concentrated urine excludes polydipsia and osmotic diuresis.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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