A 55-year-old man started on carbamazepine for trigeminal neuralgia returns after 4 weeks with lethargy, nausea, and confusion. Serum sodium is 118 mEq/L, urine osmolality is high, and euvolemia is present. The most likely explanation is:
- A Carbamazepine-induced nephrogenic diabetes insipidus
- B Carbamazepine-induced syndrome of inappropriate ADH secretion ✓
- C Carbamazepine-induced primary polydipsia
- D Carbamazepine-induced cerebral salt wasting
Correct answer: B. Carbamazepine-induced syndrome of inappropriate ADH secretion
Explanation
Carbamazepine potentiates the effect of ADH on renal collecting ducts and can cause true SIADH with euvolemic hypo-osmolar hyponatremia and concentrated urine, particularly in elderly patients. Nephrogenic diabetes insipidus produces dilute urine and hypernatremia, the opposite picture. Primary polydipsia gives low urine osmolality. Cerebral salt wasting features volume depletion. Oxcarbazepine carries an even higher risk of hyponatremia than carbamazepine, a frequent exam point.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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