Myositis

Injury, medicines, infection, or an autoimmune disorder can lead to myositis. It can also be idiopathic.

• Injury - A mild form of myositis can occur with hard exercise.[3] A more severe form of muscle injury, called rhabdomyolysis, is also associated with myositis.[3] This is a condition where injury to the patient's muscles causes them to quickly break down.[3]
• Medicines - A variety of different medicines can cause myositis. One of the most common drug types that can cause myositis is statins. Statins are drugs that are used to help lower high cholesterol. One of the most common side effects of statin therapy is muscle pain.[4] Rarely, statin therapy can lead to myositis.[4]
• Infection - The most common infectious cause of myositis is viral infections, such as the common cold.[3] Viruses, such as COVID-19, are also shown to be a rare cause of myositis.[5] Benign acute childhood myositis has been described in children after prodromal viral infections with different viral agents.
• Autoimmune - Autoimmune disease is an abnormal immune response to a functioning body part, in this case the muscles. The three main types of autoimmune myositis are dermatomyositis, polymyositis, and inclusion body myositis.[3] Other autoimmune diseases, such as systemic lupus erythematosus, can also cause myositis-like symptoms.[3]

There are various tools that can be used to help diagnose myositis. The most common ones are physical exam, electromyography, magnetic resonance imaging, muscle biopsy, and blood tests. The first thing a doctor will likely do is perform a physical exam.[1] The doctor looks for various things such as muscle weakness and rashes.

Another possible test is an electromyography (EMG). This is a test that inserts tiny needles into the patient's muscles.[3] This allows a physician to look at the muscles’ response to various electrical nerve signals and evaluate which muscles potentially have myositis.[3] Magnetic resonance imaging (MRI) is also useful.[6] This test uses a big magnet to create images of muscles on a computer.[3] It is painless.

Muscle biopsies are the most reliable tests for diagnosing myositis.[3]

There are also a variety of blood tests available that help in the diagnoses of myositis. The doctor may look for elevation of creatine kinase in blood, which is indicative of muscle inflammation.[3] Certain autoantibodies (antibodies that target muscle cells) can also be found in the blood, which can indicate that myositis is caused by an autoimmune disease.[2] Some specific examples of autoantibodies are Anti-Jo-1, Anti-HMGCR, Anti-TIF1, etc.[2]

Treatment for myositis depends on the underlying cause.[3] For myositis that is caused by a viral infection, no treatment is typically needed.[3] For myositis caused by a bacterial infection, antibiotics can be used.[3] For myositis caused by a medication, it is important to stop use of that medication.[3]

There are a variety of treatment options available if myositis is caused by an autoimmune disease. Glucocorticoids are often the first choice for treatment.[7] This drug works to weaken the immune system, so that the immune system is not able to attack the muscles. It is a type of steroid and can cause a wide array of side effects such as mood changes, increased hunger, trouble sleeping, etc. Another treatment option is a steroid-sparing immunosuppressive agent.[7] This also works to weaken the immune system, but does not cause the side effects that the steroids do. Another treatment option is a class of drugs called biologics.[7] Also, intravenous immunoglobulins (IVIg) have also been shown to be effective in the treatment of myositis caused by an autoimmune disease.[8]

• Myopathy (muscle disease)
• Myalgia (muscle pain)
• Masticatory muscle myositis (a disease in dogs)
• Perimyositis

• Myositis: NIH

Myositis Association https://www.myositis.org