Amyloid myopathy masquerading as polymyositis

Objective, It is not well appreciated that the clinical presentation of amy loid myopathy can mimic that of polymyositis. By retrospective clinicopatho logic analysis we determined distinctive features of amyloid myopathy that differentiate the 2 diseases. Methods. Two patients with clinical and histologic evidence of an inflammat ory myopathy had fatal outcomes despite appropriate treatment fbr polymyosi tis. Their clinical course and original pathologic specimens were reviewed. In addition, original tissue samples were obtained and analyzed using Cong a red staining and immunoperoxidase. Results. The initial diagnosis of polymyositis was supported in both cases by muscle biopsies showing inflammatory infiltrates and elevations of creat ine phosphokinase and by classic electromyography. Retrospective evaluation of the initial muscle biopsies disclosed subtle but incontrovertible evide nce of vascular amyloid. Further analysis of the original specimens confirm ed 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. Fa ilure to recognize amyloid myopathy deprives patients of potentially life p rolonging treatment. Congo red staining and immunohistochemical analysis of tissue could prevent misdiagnosis.