Confusion and Severe Hyponatraemia in an 82-Year-Old Woman on Multiple Medications

An 82-year-old woman is admitted to the acute medical unit with a two-day history of confusion and a fall at home. Her family reports that she has been progressively drowsy and disoriented. On examination, she is drowsy but rousable, with no focal neurological deficit. She appears clinically euvolemic, with no oedema, and no signs of dehydration or fluid overload. Her blood pressure is 138/76 mmHg, pulse 82 bpm, temperature 37.1°C. Her medical history includes type 2 diabetes, hypertension, and low mood. Her regular medications are metformin 500 mg twice daily, hydrochlorothiazide 25 mg once daily, and citalopram 10 mg once daily. The citalopram was started ten days before admission. Her other medications have been unchanged for over a year. Two days ago, she was found confused by her daughter. On admission, her serum sodium is 121 mmol/L (reference 135-145), potassium 4.1 mmol/L, urea 5.0 mmol/L, creatinine 65 µmol/L, glucose 5.2 mmol/L, TSH 1.2 mIU/L, and cortisol 450 nmol/L. Urine osmolality is 450 mOsm/kg, and urine sodium is 40 mmol/L. Chest radiograph and CT head are unremarkable. On admission, citalopram was withheld. Over the next 48 hours, with cautious fluid restriction, her serum sodium rose to 131 mmol/L and her confusion resolved. She was not rechallenged with citalopram. The possibility of thiazide-induced hyponatraemia was considered, but hydrochlorothiazide had been taken at the same dose for many years, with normal sodium levels documented on a blood test one month before admission. There was no evidence of hypothyroidism, adrenal insufficiency, or other causes. The clinical presentation and temporal relationship are consistent with drug-induced syndrome of inappropriate antidiuretic hormone secretion (SIADH), most likely related to the recently started medication.