Anti-Ku is a rare antibody which can be positive in some rheumatic diseases and it might be related to cardiac involvement. Polymyositis is an inflammatory myopathy, and its cardiac involvement seldom presents as myopericarditis and anti-Ku positive. In this case, we report a mid-aged woman with chest pain, upper limbs weakness and fever unrelated with infection. The diagnosis of this case was unquestionably myopericarditis supported by ECG, cardiac MRI and negative findings in coronary arteries. Diagnosis of polymyositis was further clarified by the evidence of persistently increased CK, degeneration of proximal muscle in MRI, muscular dystrophy with lymphocytes infiltration in muscle biopsy. In the analysis of autoantibodies, we surprisingly discovered positive anti-Ku. Glucocorticoid and mycophenolate?mofetil were then prescribed for polymyositis. Patient follow-up indicated remission of both myopericarditis and polymyositis. We finally clarified this rare case as a positive anti-Ku polymyositis with myopericarditis as cardiac involvement. This report presents a rare case with anti-Ku positive polymyositis and the cardiac involvement of polymyositis was manifested as myopericarditis. Therefore, positive anti-Ku might explain the myopericarditis as cardiac involvement in polymyositis. More cases and longer duration of follow-up is required for the comprehensive understanding of the disease.
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