Drug-Induced Myositis

Introduction to Myositis-Inducing Medications

The American College of Cardiology and other guideline societies recommend being aware that statin therapy carries a definite risk of myopathy, with severe myopathy reported in approximately 0.08% of patients taking lovastatin and simvastatin 1
Elevations of muscle enzyme levels greater than 10 times the upper limit of normal have been reported in 0.09% of patients treated with pravastatin 1
The American Heart Association notes that statin-associated autoimmune myopathy (with anti-HMGCR antibodies) is a rare but serious condition that requires statin cessation and immunosuppressive therapy 1

Advanced age (especially >80 years), female sex, small body frame, chronic renal insufficiency, and polypharmacy are risk factors for statin-induced myositis 1
Drug interactions that affect statin metabolism, particularly those involving the cytochrome P-450 3A4 isozyme, increase the risk of myositis 1

The National Lipid Association recommends avoiding the combination of statins with fibrates, particularly gemfibrozil (contraindicated with simvastatin), as it significantly increases the risk of myositis 2
Niacin at lipid-modifying doses (≥1 gram/day) also increases the risk of myositis when combined with statins 2, 3

The American Society of Clinical Oncology notes that myositis is a rare but potentially severe and fatal complication of ICI therapy, more common with PD-1/PD-L1 inhibitors (like pembrolizumab) than with CTLA-4 inhibitors 4, 5, 6
Myositis can present as reactivation of pre-existing paraneoplastic polymyositis/dermatomyositis or as de novo myositis, and may have a fulminant necrotizing course with muscle damage and can involve vital skeletal muscle, including the heart 5, 6

Fibrates when used alone can cause muscle damage, according to the American Association of Clinical Endocrinologists 2, 3
Long-term use of corticosteroids can cause steroid myopathy, as reported by the Mayo Clinic 5, 7

The European League Against Rheumatism notes that main symptoms of myositis include proximal muscle weakness, difficulties in standing up, lifting arms, and moving around 5, 7
Laboratory findings include markedly elevated muscle enzyme levels and inflammatory markers, and diagnostic tests may include EMG, MRI, and muscle biopsy 5, 6

The American College of Rheumatology recommends that for statin-induced myopathy, discontinuation of the statin is the first step, and for immune-mediated myositis, immunosuppressive therapy is required 1, 5, 6
High-dose corticosteroids are the cornerstone of initial treatment for severe cases, and for refractory cases, additional immunosuppressants, IVIG, or plasmapheresis may be necessary 5, 6