Polymyositis and dermatomyositis
Polymyositis and dermatomyositis sit within the idiopathic inflammatory myopathies, a group of multisystem autoimmune diseases causing objective muscle weakness and sometimes characteristic skin, lung, swallowing and cardiac disease. In adults, dermatomyositis and selected antibody phenotypes also signal increased malignancy risk.
Definition
Polymyositis and dermatomyositis are historical clinical labels within the idiopathic inflammatory myopathies, a group of autoimmune diseases causing inflammatory or immune-mediated muscle disease with possible skin, lung, swallowing and cardiac involvement. Dermatomyositis has characteristic cutaneous features; the label polymyositis requires specialist exclusion of other inflammatory myopathy phenotypes.
First principles
Use the idiopathic inflammatory myopathy framework
The historical labels polymyositis and dermatomyositis do not capture the full range of modern phenotypes. Specialist assessment distinguishes dermatomyositis, clinically amyopathic dermatomyositis, antisynthetase syndrome, immune-mediated necrotising myopathy, overlap myositis and other entities, while true polymyositis is a diagnosis of exclusion. This matters because the associated lung, cancer, skin, cardiac and treatment risks differ.
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