From the Guidelines
Myositis is primarily caused by autoimmune conditions, with the most common causes including idiopathic inflammatory myopathies such as polymyositis, dermatomyositis, and inclusion body myositis, as well as immune-related adverse events (irAEs) associated with immune checkpoint inhibitor (ICPi) therapy, as reported in up to 40% of patients treated with ICPi therapy 1. The clinical presentation of myositis can vary, with symptoms including proximal muscle weakness, myalgia, and fatigue, and can be associated with other systemic autoimmune disorders, such as lupus or rheumatoid arthritis 1. Myositis can also be triggered by viral infections, such as influenza, HIV, and coxsackievirus, as well as bacterial infections, particularly those causing trichinosis or toxoplasmosis 1. Certain medications, especially statins used to lower cholesterol, can cause drug-induced myositis in susceptible individuals, and genetic factors may predispose some individuals to developing myositis, particularly in cases of inherited myopathies 1. Environmental triggers, such as exposure to certain toxins or ultraviolet light (in dermatomyositis), can also contribute to the development of myositis in genetically susceptible individuals 1. The diagnosis of myositis is based on a combination of clinical presentation, laboratory tests, including muscle enzymes and autoantibody panels, and imaging studies, such as MRI and EMG 1. Treatment of myositis depends on the underlying cause and severity of the disease, and may include high-dose corticosteroids, immunosuppressive agents, such as methotrexate, azathioprine, or mycophenolate mofetil, and biologic agents, such as rituximab or IVIG 1. In cases of ICPi-associated myositis, discontinuation or interruption of ICPi therapy may be necessary, and treatment with high-dose corticosteroids and immunosuppressive agents may be required to manage the condition 1. Overall, myositis is a complex and multifactorial disease, and a comprehensive approach to diagnosis and treatment is necessary to improve outcomes and quality of life for affected patients.
From the Research
Myositis Causes
- Infections such as influenza, HIV, and SARS-CoV2 can cause myositis 2
- Certain drugs, including checkpoint inhibitors and statins, may also lead to myositis 2, 3
- Rheumatologic diseases, including overlap myositis, are important causes of myositis 2
- Malignancies can also be associated with myositis, particularly in cases of malignancy-associated myositis 2
- Autoimmune responses, including those involving cytotoxic CD8-positive T cells, can contribute to the development of myositis subtypes such as polymyositis (PM) and inclusion body myositis (IBM) 3
- Complement-mediated microangiopathy can lead to destruction of capillaries and inflammatory cell stress in dermatomyositis (DM) 3
- Viral infections, such as HTLV1 virus and toxoplasmosis, can also trigger myositis 4