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Case of immune‐mediated necrotizing myopathy associated with anti‐signal recognition particle antibodies: Dramatic improvement after rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone therapy for intravascular large B‐cell lymphoma

Immune‐mediated necrotizing myopathy associated with anti‐signal recognition particle antibodies (anti‐SRP myopathy) is representative of several subtypes of necrotizing immune‐mediated myopathy, and is characterized by rapidly progressive proximal muscle weakness, dysphagia and poor responsiveness to conventional immunosuppressive therapies. We report a 71‐year‐old man who developed anti‐SRP myopathy followed by malignant lymphoma, and whose condition dramatically improved with rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone therapy. He showed progressive proximal muscle weakness with severe dysphagia, and was diagnosed with anti‐SRP myopathy by antibody tests and pathological examination. He was treated with conventional immunosuppressive therapies, specifically corticosteroids, intravenous immunoglobulin and tacrolimus; however, none of these therapies were effective. Two months after starting tacrolimus, the patient developed intravascular large B‐cell lymphoma, which was suspected to be an iatrogenic immunodeficiency‐associated lymphoproliferative disorder. Rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone therapy resulted in not only complete remission of lymphoma, but also dramatic improvement of anti‐SRP myopathy. The present case suggests that rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone therapy is a potential treatment for anti‐SRP myopathy.