Rhabdomyolysis in a patient with suspected drug interaction

A 68-year-old man presented to the emergency department with a two-day history of progressive muscle pain, weakness, and dark urine. He had no fever, rash, or other symptoms. His medical history included type 2 diabetes mellitus, hypertension, and hypercholesterolaemia. His medications were atorvastatin 40 mg once daily for three years, metformin 850 mg twice daily, amlodipine 5 mg once daily, and clarithromycin 500 mg twice daily for seven days, started for a chest infection. The patient reported that the muscle symptoms began two days after starting clarithromycin. On examination, he was alert, temperature 37.1°C, pulse 88 beats/minute, blood pressure 152/88 mmHg. There was diffuse muscle tenderness in the proximal limbs, but no rash, no jaundice, and no neurological deficit. Blood tests showed creatine kinase (CK) 12,500 U/L (reference range 50 to 200), creatinine 160 µmol/L (baseline 90 µmol/L), potassium 6.0 mmol/L, and normal liver function tests. Urinalysis was positive for blood on dipstick, but microscopy showed no red cells, consistent with myoglobinuria. Thyroid function and autoimmune screen were negative. The patient was admitted. Atorvastatin and clarithromycin were both stopped. Over the next five days, the muscle pain resolved, CK fell to 1,200 U/L, and creatinine returned to 110 µmol/L. The patient was treated with intravenous fluids and a fixed-dose insulin and glucose infusion for hyperkalaemia. No rechallenge was performed. Drug history included no other recent changes.