Juvenile dermatomyositis (der-MA-toe-MY-o-SY-tis) (JDMS) is a relatively rare disease that causes a skin rash and weak muscles in children. JDMS is different from dermatomyositis in adults, and from polymyositis, a disease that usually affects adults.
The skin rash and weak muscles of JDMS are caused by inflammation in the blood vessels that lie under the skin and in the muscles. This is called vasculitis. Since blood vessels run throughout the body, JDMS can also affect other areas such as the digestive tract. Usually, the worst symptoms are in the skin and muscles.
JDMS is often called an autoimmune disease. The immune system is a group of cells which normally protects the body from infections. In autoimmune diseases, these cells are signaled to “turn on” their infection-fighting process, but then can't turn it off. The process then ends up damaging the body rather than protecting it.
One way the immune system cells fight infection is by a process called inflammation. But when the cells can't “turn off” the inflammation process, tissues are damaged. In JDMS, blood vessels in the skin and muscles are inflamed. An inflamed muscle will feel weak or sore, while inflamed skin will have a rash.