Snapshot A 49-year-old woman with polymyositis presents to her physician’s office for weakness. She reports that she feels unusually tired and weak. She works as a makeup artist and in the past few weeks has experienced difficulty applying makeup on her clients. She finds it difficult to work while she has her arms raised. She denies any fevers, weight gain or loss, or any other motor deficits. Physical exam reveals decreased strength in her shoulders. No rash is appreciated. Laboratory evaluation reveals increased creatinine phosphokinase, positive antinuclear antibody, and positive anti-signal recognition particle. She is started on high-dose corticosteroids. (Polymyositis) Introduction Clinical definition dermatomyositis is an autoimmune myopathy characterized by symmetric proximal muscle weakness and rash login to view 1 more bullet polymyositis is an autoimmune myopathy characterized by symmetric proximal muscle weakness and no rash Epidemiology Demographics female > male typically in adults 30-50 years of age if in children, considered juvenile dermatomyositis Risk factors drugs physical exertion ETIOLOGY 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+ lymphocytes and complement activation polymyositis T-cell driven autoimmune disease characterized by endomysial inflammation login to view 1 more bullet associated with CD8+ lymphocytes Associated conditions interstitial lung disease malignancy including breast or ovarian cancer prostate cancer lung cancer other connective tissue diseases Presentation Symptoms progressive weakness in hips and shoulders dysphagia dysphonia laryngeal muscle weakness Physical exam dermatomyositis characteristic cutaneous findings login to view 6 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-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 Inclusion body myositis presents with weakness and elevated CRP/CK biopsy demonstrates endomysial inflammation with basophilic-rimmed vacuoles 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 Prognosis Dermatomyositis ↑ risk of malignancy (typically develops within 5 years of diagnosis)