JDM can be a very variable illness. It can range from being mild, where children may have few symptoms, to be a more severe disease which can affect different organs of the body (such as the heart, lungs, brain) as well as the muscles and skin. Organ involvement in JDM can vary from child to child. When JDM affects children's organs such as heart, lungs, abdomen (gut) or brain, treatment needs to be given quickly. Even if a child has very mild disease it is usual to treat them with medicines to prevent any complications in the future (such as calcinosis or reduced movement of a joint) and to switch off the disease activity.
There are some cases of JDM where the child doesn’t show any muscle weakness at all and will just have skin changes (we call this amyopathic dermatomyositis). Others will just have muscle disease without any skin disease (so called polymyositis). Sometimes the rash can appear later than muscle weakness in JDM.