A 68-year-old woman on carbamazepine for trigeminal neuralgia for 4 months presents with lethargy and confusion. Serum sodium is 118 mEq/L, urine osmolality is high, urine sodium is elevated, and euvolemia is confirmed. The most likely mechanism is:
- A Stimulation of ADH release and enhanced renal response to ADH (SIADH) ✓
- B Carbamazepine-induced nephrogenic diabetes insipidus
- C Osmotic diuresis from glucosuria secondary to hyperglycemia
- D Direct tubular toxicity causing salt wasting
Correct answer: A. Stimulation of ADH release and enhanced renal response to ADH (SIADH)
Explanation
Carbamazepine causes euvolemic hyponatremia through a syndrome of inappropriate ADH secretion, by enhancing ADH release from the posterior pituitary and potentiating its effect on collecting ducts. It is more common in elderly patients and those on higher doses. Nephrogenic diabetes insipidus causes hypernatremia with dilute urine, and salt-wasting produces volume depletion rather than euvolemia, eliminating options B and D.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.