Dermatomyositis is a rare inflammatory disease that causes muscle weakness and a violet-colored or dusky red skin rash. The signs and symptoms of dermatomyositis usually develop gradually, over weeks or months. It is unclear what causes dermatomyositis, but doctors suspect it is an inherited condition. Dermatomyositis may occur at any age, but it mostly affects adults in their late 40s to early 60s, or children between 5 and 15 years of age. Follow the links below to find WebMD's comprehensive coverage about how dermatomyositis is contracted, what it looks like, how to treat it, and much more.