Although myositis is a rare disease, people of all ages, races, and ethnic backgrounds are susceptible. Inclusion body myositis is more common in men, while dermatomyositis and polymyositis are more common in women. Myositis often begins when people are teenagers or when they are older than 60.
The cause of myositis is not known. We do know that myositis is an autoimmune disease. In myositis, this autoimmunity leads to inflammation of specific muscles. The inflammation results in muscle damage and may lead to muscle cell death. Many different muscle groups may be involved in the disease ranging from muscles of the arms and legs to those of the respiratory and gastrointestinal systems. In addition, the abnormal immune response can lead to other problems including damage to blood vessels, joints, lungs, and the heart.