Polymyositis & Dermatomyositis Support Group
Polymyositis (PM) is a type of chronic inflammation of the muscles (inflammatory myopathy) related to dermatomyositis and inclusion body myositis. The inflammation is predominantly of the endomysium in polymyositis, whereas dermatomyositis is characterized by primarily perimysial inflammation.
I googled it quick. this may help you figure it out, or lead you to an answer. (You will have to cut and paste, I can never get my links to work right.)