Definition. Dermatomyositis (dur-muh-toe-my-uh-SY-tis) is an uncommon inflammatory disease marked by muscle weakness and a distinctive skin rash. Dermatomyositis affects adults and children alike. In adults, dermatomyositis usually occurs from the late 40s to early 60s.efinition. Dermatomyositis (dur-muh-toe-my-uh-SY-tis) is an uncommon inflammatory disease marked by muscle weakness and a distinctive skin rash. Dermatomyositis affects adults and children alike. In adults, dermatomyositis usually occurs from the late 40s to early 60s.