Hyponatraemia and unsteadiness in a 45-year-old man on carbamazepine

Mr A, a 45-year-old man, presented to the emergency department with a two-day history of nausea, generalised weakness, and unsteadiness when walking. He had been well until three weeks earlier, when he was started on carbamazepine 400 mg twice daily for trigeminal neuralgia. He had also been taking hydrochlorothiazide 25 mg once daily for hypertension for several years. He reported no vomiting, headaches, or seizures. On examination, he was apyrexial, blood pressure 128/78 mmHg, pulse 72 bpm, and oxygen saturation 96% on air. Neurological examination showed mild ataxia and subtle confusion, but no focal deficits. Laboratory results revealed a serum sodium of 124 mmol/L (reference range 135 to 145), potassium 4.1 mmol/L, creatinine 78 µmol/L, blood urea 5.2 mmol/L, serum osmolality 262 mOsm/kg, urine osmolality 520 mOsm/kg, and urine sodium 68 mmol/L. TSH and cortisol were within normal ranges. His medication history was otherwise unchanged. Carbamazepine was stopped, and he received careful sodium correction. Over the following 48 hours, his sodium rose to 131 mmol/L and his symptoms resolved completely. He remained on hydrochlorothiazide throughout. A rechallenge was not performed. The presentation was consistent with syndrome of inappropriate antidiuretic hormone secretion (SIADH).