What are the 5 major diagnostic features of polymyositis and dermatomyositis?
- Proximal muscle weakness
- Myopathic features on EMG testing
- Elevated muscle enzymes (Primarily the CK, though elevations of AST and LDH are also seen).
- Presence of a typical skin rash (in the case of dermatomyositis)
- Positive muscle biopsy (note that the histological features of PM and DM are different. PM is characterized by endomyseal infiltration of inflammatory cells, most of which are CD4+ cells. In DM, an inflammatory infiltrate is seen in the perivascular and perifascicular regions).