A 55-year-old man started on carbamazepine for trigeminal neuralgia presents four weeks later with fatigue, mild confusion, and a serum sodium of 118 mEq/L with low plasma osmolality and inappropriately concentrated urine. The most appropriate management step is:
- A Add fludrocortisone to restore mineralocorticoid tone
- B Restrict water intake and reduce or withdraw carbamazepine ✓
- C Switch carbamazepine to oxcarbazepine, which does not cause hyponatremia
- D Give a single bolus of 3% hypertonic saline and continue the same dose
Correct answer: B. Restrict water intake and reduce or withdraw carbamazepine
Explanation
Carbamazepine commonly causes SIADH-like hyponatremia through enhanced ADH action on renal collecting ducts, particularly in elderly patients. Treatment is fluid restriction with dose reduction or withdrawal. Oxcarbazepine is not a safe swap because it causes hyponatremia even more frequently than carbamazepine, making option C actively wrong. Hypertonic saline alone without addressing the drug risks ongoing worsening.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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Written and medically reviewed by the StethoPrep medical team.