Amyloid myopathy masquerading as polymyositis.

OBJECTIVE: It is not well appreciated that the clinical presentation of amyloid myopathy can mimic that of polymyositis. By retrospective clinicopathologic analysis we determined distinctive features of amyloid myopathy that differentiate the 2 diseases. METHODS: Two patients with clinical and histologic evidence of an inflammatory myopathy had fatal outcomes despite appropriate treatment for polymyositis. Their clinical course and original pathologic specimens were reviewed. In addition, original tissue samples were obtained and analyzed using Congo red staining and immunoperoxidase. RESULTS: The initial diagnosis of polymyositis was supported in both cases by muscle biopsies showing inflammatory infiltrates and elevations of creatine phosphokinase and by classic electromyography. Retrospective evaluation of the initial muscle biopsies disclosed subtle but incontrovertible evidence of vascular amyloid. Further analysis of the original specimens confirmed the presence of immunoglobin light chain (AL) amyloid. CONCLUSION: Amyloid myopathy can mimic polymyositis. Both can have similar clinical symptoms, as well as inflammatory infiltrates on muscle biopsy. Failure to recognize amyloid myopathy deprives patients of potentially life prolonging treatment. Congo red staining and immunohistochemical analysis of tissue could prevent misdiagnosis.