Myopathy is a term used to describe muscle disease. The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation, accompanied by muscle weakness. Another word for chronic inflammation of muscle tissue is myositis. The three main types of chronic, or persistent, inflammatory myopathy are polymyositis, dermatomyositis and inclusion body myositis.
Muscle inflammation may be caused by an allergic reaction, exposure to a toxic substance or medicine, another disease such as cancer or rheumatic conditions, or a virus or other infectious agent. The chronic inflammatory myopathies are idiopathic, meaning they have no known cause. They are thought to be autoimmune disorders, in which the body’s white blood cells, which normally fight disease, attack blood vessels, normal muscle fibers and connective tissue in organs, bones and joints.
These rare disorders affect both adults and children, although dermatomyositis is the most common chronic form in children. Polymyositis and dermatomyositis are more common in women than in men. A rare childhood onset form of polymyositis and dermatomyositis can occur in children between the ages of 2 and 15 years. Inclusion body myositis usually affects individuals over age 50.