A 55-year-old woman started on carbamazepine for trigeminal neuralgia returns after 4 weeks with fatigue, headache, and confusion. Serum sodium is 118 mEq/L and urine osmolality is inappropriately concentrated. The mechanism of carbamazepine-induced hyponatremia is:

• A Inhibition of vasopressin V2 receptors causing nephrogenic diabetes insipidus
• B Enhanced ADH secretion and potentiation of ADH action on the collecting duct ✓
• C Primary polydipsia induced by hypothalamic stimulation
• D Salt-wasting nephropathy from tubulointerstitial injury

Explanation

Carbamazepine increases both the secretion of ADH from the posterior pituitary and the sensitivity of the renal collecting duct to ADH, producing a syndrome of inappropriate ADH secretion pattern with low serum osmolality and concentrated urine. Oxcarbazepine causes hyponatremia even more frequently. V2 antagonism would cause dilute urine and hypernatremia, the opposite picture, which eliminates option A.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.