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.
Since sclero also affects skin, I don't know if a skin biopsy would given any useful information. I suggest that you ask you dermatologist if the rash is anything to worry about.
Lucky