An elderly woman started on carbamazepine for trigeminal neuralgia returns after one month with fatigue, nausea, and gait instability. Serum sodium is 122 mEq/L and urine osmolality is high. The most likely explanation is:
- A Carbamazepine-induced syndrome of inappropriate ADH secretion ✓
- B Carbamazepine-induced primary polydipsia
- C Carbamazepine nephrogenic diabetes insipidus
- D Carbamazepine-induced adrenal insufficiency
Correct answer: A. Carbamazepine-induced syndrome of inappropriate ADH secretion
Explanation
Carbamazepine potentiates ADH action on renal collecting ducts and can cause SIADH with euvolaemic hyponatraemia and concentrated urine, particularly in the elderly. Oxcarbazepine carries an even higher risk. Nephrogenic diabetes insipidus would produce dilute urine with hypernatraemia, so option C contradicts the biochemistry given in the stem.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.