Statin-induced myopathy after a dose increase in a 66-year-old woman

A 66-year-old woman presented to the clinic with a 6-week history of progressive bilateral thigh and shoulder pain and difficulty climbing stairs. She reported no fever, rash, or change in bowel habit. She had a history of hypertension and dyslipidemia. Her medications were simvastatin 80 mg at night, amlodipine 10 mg once daily, and lisinopril 10 mg once daily. She had been taking simvastatin 40 mg for the past year; two months ago the dose was increased to 80 mg to achieve a lower LDL cholesterol target. Four weeks after the dose increase, she noticed mild aching in her thighs, which worsened over time. She attributed this to her age and did not seek medical advice initially.

On examination, she was afebrile. Blood pressure was 148/86 mmHg. There was tenderness on palpation of the quadriceps and deltoid muscles. Power was 4/5 in hip flexion and shoulder abduction bilaterally. Muscle bulk was normal; reflexes were preserved. No muscle swelling or skin rash was seen.

Investigations: creatine kinase (CK) was 2,400 U/L (normal range 30–150 U/L). Serum creatinine was 75 μmol/L (normal 50–100 μmol/L). Thyroid-stimulating hormone was 2.1 mIU/L (normal 0.4–4.0 mIU/L). Antinuclear antibodies were negative. Urine dipstick was negative for blood. Liver function tests were within normal limits. Electrocardiogram showed sinus rhythm.

The simvastatin was discontinued after the elevated CK was confirmed. Four weeks later, her muscle pain had resolved and CK was 210 U/L. She reported no recurrence of symptoms at follow-up. She was not rechallenged with simvastatin. Alternative lipid-lowering therapy with ezetimibe was initiated. The patient was admitted to the hospital for short-stay observation and monitoring of renal function during the initial presentation due to the risk of rhabdomyolysis.

This case highlights the importance of considering statin-induced myopathy in patients with proximal muscle pain and elevated CK, particularly when the statin dose has recently been increased. The presence of amlodipine, which can increase simvastatin exposure through CYP3A4 inhibition, adds to the risk.