Polymyositis associated with monoclonal gammopathy.

Three patients presented with gradual onset of proximal muscle weakness. Electromyography and muscle biopsy were abnormal in each patient and typical of myositis. All three had an IgG kappa monoclonal paraprotein in the serum. Examination of muscle biopsy specimens by direct immunofluorescence revealed linear deposits of IgG kappa along the sarcolemmal basement membrane of individual fibres. Stains for lambda light chains, other immunoglobulins, and complement components were negative. Plasmapheresis and immunosuppressive drugs produced clinical improvement in all three patients. Muscle biopsies were taken from two of the patients after plasma exchange and drug therapy and showed no immune deposits. These observations suggest a possible cause-and-effect relation between the serum paraprotein and the inflammatory myopathy in these patients.