Juvenile (childhood onset) dermatomyositis is a condition, which affects a number of systems of the body, although its main effects are seen in the muscles and skin.
It is important to stress that the childhood form of dermatomyositis differs from adult polymyositis in which inflammation of muscle, usually without any other organ involvement, is caused by infiltration of muscle by cells called lymphocytes and from the adult form of dermatomyositis which occurs in older people and may be associated with an underlying tumour. Remember to make this distinction if you read other literature.