Five subgroups of myositis or inflammatory myopathy were suggested by Bohan and Peter in the 1970s. These included:

1. Dermatomyositis
2. Polymyositis
3. Myositis with cancer
4. Childhood dermatomyositis or polymyositis
5. Myositis overlapping with another collagen vascular disorder

The researchers suggested that the myopathies may be preceded by a disease of the skin but more recently, cases of amyopathic dermatomyositis have been observed, where only the skin and not the muscles are affected.

In addition, a seventh form of the condition called inclusion body myositis was recognized in 1979. Some research has also suggested an eight group that includes skin disease caused by drugs.

In 2002, Richard Sontheimer suggested that in addition to classic dermatomyositis and amyopathic dermatomyositis, two further subsets of cutaneous disease exist. He proposed that the conditions featuring cutaneous disease could be divided into the following:

• Classic dermatomyositis
• Amyopathic dermatomyositis
• Hypomyopathic dermatomyositis, a form of the condition where skin disease is present but there is only mild involvement of the muscle.
• Post-myopathic dermatomyositis, where a previous case of classic dermatomyositis is followed by active skin disease, but no muscle disease.

Two common symptoms of dermatomyositis include the heliotrope rash and Gottron’s papules. The heliotrope rash is a violet or dusky rash with or without edema that erupts around the eyes. Gottron’s papules are scaley, violet patches that usually appear across bony prominences such as the knuckles, elbows and knees.