Dermatomyositis is seen more frequently in children than polymyositis, and it's course is often severe and rapidly progressive. In cases of acute dermatomyositis, the muscles may be tender and swollen so that movement is painful. Polymyositis is less likely to involve pain. While an association with malignancy has been found in both adult dermatomyositis and polymyositis, it occurs much more frequently in dermatomyositis. This is not the case in juvenile dermatomyositis, where no association between the disorder and malignancy has been identified.
Polymyositis is much more diverse clinically than dermatomyositis and more difficult to define.
According to many authorities, polymyositis is more likely than dermatomyositis to be slowly progressive. Dermatomyositis is a recognizable condition. The primary criteria for diagnosis are clinical and recognizable during a physical exam and detailed history. These criteria include a combination of the distinctive erythematous skin rash with a typical onset and distribution onset of muscle weakness. However, because of the rarity of the disorder, especially in children, it may be difficult to diagnose for a physician who has had no prior experience with the disorder.