Snapshot A 53-year-old woman with a past medical history of thyroid disease presents to her physician’s office for weakness. She reports that she has been feeling weak, has difficulty climbing the stairs, and combing her hair every morning. Today, she also has difficulty arising out of her chair. Physical exam reveals decreased strength, especially in the shoulders. She also has an impressive lilac periorbital rash and a sunburn on her cheeks. She is sent for further laboratory workup and counseled on the likely initiation of high-dose steroids. Introduction Clinical definition dermatomyositis is an autoimmune myopathy characterized by symmetric proximal muscle weakness and rash login to view 1 more bullet polymyositis an autoimmune myopathy characterized by symmetric proximal muscle weakness and no rash Epidemiology incidence rare demographics female > male typically in adults 30-50 years of age if in children, considered juvenile dermatomyositis risk factors drugs physical exertion Pathogenesis dermatomyositis an antibody-driven autoimmune disease characterized by perimysial inflammation and atrophy, resulting in muscle tissue damage login to view 1 more bullet associated with CD4 and complement activation polymyositis T-cell driven autoimmune disease characterized by endomysial inflammation login to view 1 more bullet associated with CD8 Associated conditions interstitial lung disease malignancy which includes breast or ovarian cancer prostate cancer lung cancer other connective tissue diseases Prognosis dermatomyositis ↑ risk of malignancy (typically develop within 5 years of diagnosis) Presentation Symptoms progressive weakness in hips and shoulders dysphagia dysphonia laryngeal muscle weakness Physical exam dermatomyositis characteristic cutaneous findings login to view 4 more bullets polymyositis and dermatomyositis proximal muscle weakness in the shoulders and hips login to view 3 more bullets Studies Labs ↑ creatinine phosphokinase (CPK or CK) ↑ creatine kinase-MB (CK-MB) ↑ aldolase ↑ lactate dehydrogenase (LDH) autoantibodies + antinuclear antibody (ANA) + anti-histidyl tRNA synthetase antibodies (anti-Jo-1) (~20% of patients) + anti-signal recognition particle (SRP) (~5% of patients) login to view 1 more bullet + anti-Mi-2 (<10% of adults) login to view 1 more bullet Muscle biopsy indication diagnostic gold standard to differentiate between inflammatory and non-inflammatory myopathies findings degenerating muscle fibers and inflammatory infiltrates dermatomyositis login to view 1 more bullet polymyositis login to view 1 more bullet Making the diagnosis based on clinical presentation and laboratory studies Differential Systemic lupus erythematosus Localized scleroderma Infectious myopathy Myasthenia gravis Treatment Management approach both diseases are often treated initially with a course of steroids followed by long-term immunosuppression Conservative sun protection indication login to view 1 more bullet Medical systemic steroids (high-dose) indications login to view 2 more bullets antimalarial indications login to view 3 more bullets non-steroidal immunosuppression indications login to view 5 more bullets Complications Cardiomyopathy Macrophage activation syndrome