The inflammatory myopathies are a group of diseases, with no known cause, that involve chronic muscle inflammation accompanied by muscle weakness. The four main types of chronic, or long-term, inflammatory myopathies are polymyositis, dermatomyositis, inclusion body myositis, and necrotizing autoimmune myopathy. Although the cause of many inflammatory myopathies is unknown, the majority are considered to be autoimmune disorders, in which the body’s immune response system that normally defends against infection and disease attacks its own muscle fibers, blood vessels, connective tissue, organs, or joints.
These rare disorders may affect both adults and children, although dermatomyositis is more common in children. Polymyositis and dermatomyositis are more common in women than in men. Inclusion body myositis is most common after age 50. General symptoms of chronic inflammatory myopathy include slow but progressive muscle weakness that starts in the proximal muscles—those muscles closest to the trunk of the body.
Other symptoms include fatigue after walking or standing, tripping or falling, and difficulty swallowing or breathing. Some individuals may have slight muscle pain or muscles that are tender to the touch. Polymyositis affects skeletal muscles (involved with making movement) on both sides of the body. Dermatomyositis is characterized by a skin rash that precedes or accompanies progressive muscle weakness.
IBM is characterized by progressive muscle weakness and wasting. Juvenile myositis has some similarities to adult dermatomyositis and polymyositis. Symptoms of necrotizing autoimmune myopathy include weakness in both the upper and lower body, difficulty rising from low chairs, climbing stairs, or lifting objects, fatigue, weight loss, and muscle pain.
Most cases of dermatomyositis respond to therapy. Approximately one-third of individuals with juvenile-onset dermatomyositis recover from their illness, one-third have a relapsing-remitting course of disease, and the other third have a more chronic course of illness. The prognosis for polymyositis varies. Most individuals respond fairly well to therapy, but some people have a more severe disease that does not respond adequately to therapies and may have significant disability.
IBM is generally resistant to all therapies and its rate of progression appears to be unaffected by currently available treatments. Necrotizing autoimmune myopathy generally responds well to long-term combination immunosuppressive therapies.
The chronic inflammatory myopathies can’t be cured in most adults but many of the symptoms can be treated. Options include medication, physical therapy, and rest. Polymyositis, dermatomyositis, and necrotizing autoimmune myopathy are first treated with high doses of corticosteroid drugs such as prednisone. Immunosuppressant drug may reduce inflammation in individuals who do not respond well to prednisone.
Injections of adrenocorticotropic hormone gel may be another option for people who do not respond to or cannot tolerate other drug treatment options. Physical therapy is usually recommended to prevent muscle atrophy as well as to maintain muscle strength and range of motion. Some individuals may use a topical cream to treat skin problems associated with the disorder. IBM has no standard course of treatment.