Both types of endocrine myopathy can develop anytime from childhood through adulthood. Both DM and DM affect both children and adults and are characterized by weakening of neck and limb muscles, muscle pain and swelling. DM also commonly causes a skin rash. The severity and progression of these types of myopathy vary considerably. Some people with PM and DM develop other disorders, such as abnormal heart rhythms, lung disease, gastrointestinal problems, arthritis, or cancer.
Inflammatory myopathies are autoimmune disorders.
An autoimmune disorder, or disease, is caused by the body's immune system mistakenly attacking healthy tissue. In this case, it attacks healthy muscle fibers and causes inflammation, which in turn damages the muscle. It is not known what triggers this autoimmune response. Most people with PM don't feel intense pain but do suffer muscular aches, cramping and tenderness. The muscle weakening is, however, quite intense and may fluctuate over weeks to months.