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.he 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 (that normally fight disease) attack blood vessels, normal muscle fibers, and connective tissue in organs, bones, and joints.