A patient receiving carbamazepine for trigeminal neuralgia develops hyponatraemia with serum sodium of 128 mEq/L. The most likely mechanism is:

• A Inhibition of renal Na+/K+-ATPase reducing tubular sodium reabsorption
• B Adrenal suppression decreasing aldosterone secretion
• C Osmotic diuresis from glucose-lowering effect of carbamazepine
• D SIADH-like effect due to enhanced renal tubular sensitivity to ADH ✓

Explanation

Carbamazepine causes hyponatraemia by an ADH-potentiating (SIADH-like) mechanism — it enhances the sensitivity of renal collecting duct cells to antidiuretic hormone, promoting excessive free-water retention. This is a well-recognised adverse effect, particularly in elderly patients, and is not related to adrenal or glucose effects. Oxcarbazepine causes this effect even more frequently than carbamazepine.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.