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.