4.07 Polymyositis and Dermatomyositis MSK/Rheum review for the USMLE Step 1 Exam Polymyositis and dermatomyositis are autoimmune inflammatory myopathies. They are caused by abnormal activation of T cells that attack skeletal muscle and both cause proximal muscle weakness, especially of the shoulders and pelvic girdle muscles. Polymyositis develops when there is abnormal activation of CD8 T cells, while dermatomyositis is primarily attacked by CD4 T cells. Both are diagnosed through a muscle biopsy and the presence of elevated CK levels and several different autoantibodies. Dermatomyositis includes dermatologic manifestations, such as gottron papules, heliotrope rash, and shawl rash. Both are associated with MI, interstitial lung disease, and various types of cancer (dermatomyositis more so). Both diseases require prompt treatment with steroids and immunosuppressive agents.