| Summary: | Abstract Background Cutaneous and systemic plasmacytosis are skin disorders characterized by cutaneous polyclonal plasma cell infiltration accompanied by polyclonal hypergammaglobulinemia. Cutaneous plasmacytosis involvement is limited to the skin, mainly on the face and trunk, while systemic plasmacytosis also involves 2 or more organ systems. However, there have been no reports of inflammatory myositis due to plasmacytosis. Here, we report a patient with plasmacytosis who developed myalgia and easy fatigability due to inflammatory myositis. Case presentation A 54-year-old man with cutaneous plasmacytosis on the face, chest, and back complained of a history of atypical facial and lower leg pain and easy fatigability since the age of 45 years. Muscle-strength tests revealed bilateral trivial gastrocnemius weakness with myalgia. The results of routine blood analysis, including creatine kinase and thyroid function, were normal, but levels of several inflammation markers and autoantibodies were elevated. Additionally, lower leg magnetic resonance imaging and gastrocnemius muscle biopsy revealed inflammatory myositis mimicking polymyositis. His plasmacytosis, myalgia, and lower leg weakness were ameliorated by prednisolone. Conclusion The patient was diagnosed with inflammatory myositis due to plasmacytosis. Given that plasmacytosis has previously been reported to disrupt the immune status, myositis in this patient might have been associated with abnormal autoimmune inflammation. Neurologists and physicians should thus be aware that plasmacytosis might be associated with inflammatory myositis accompanied by myalgia.
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