What is myositis?

The word myositis means inflammation of muscles (“myo” means muscle and “itis” means inflammation). Myositis can be present in many different disorders. The inflammatory myopathies (muscle diseases) are a group of disorders that all have the symptoms of inflammation and weakness of the muscles, mainly those muscles closest to the trunk of the body (proximal muscles). Some of the disorders in this group in polymyositis, dermatomyositis and inclusion body myositis. In some people, myositis is associated with inflammation in other organs, such as the joints, heart, lungs, intestines and skin.

People with dermatomyositis experience a skin rash as well as muscle inflammation. People with rheumatic diseases, such as rheumatoid arthritis, lupus and scleroderma may also experience muscle inflammation.

Myositis is thought to be an autoimmune disease ̶ the body’s immune system malfunctions and attacks its own tissues. Current scientific opinion is that environmental factors trigger this kind of autoimmune disease in people that are genetically susceptible. In some cases, it is thought that being exposed to certain viruses may trigger the overactive immune response. In people with myositis, the person’s white blood cells attack muscle. The body then produces antibodies that attack healthy tissue in different parts of the body, damaging muscles and skin.