Pathology of skeletal muscle in mixed connective tissue disease.

To characterize the pathology of muscle involvement in mixed connective tissue disease (MCTD), skeletal muscle biopsies from 13 patients with MCTD were examined by routine light microscopy, histochemistry, and direct immunofluorescence. The histologic and histochemical changes observed corresponded closely to changes seen in idiopathic polymyositis and the myopathy associated with systemic lupus erythematosus. Eight of 13 cases examined by direct immunofluorescence demonstrated immunoglobulin deposition either within normal appearing vessels, within normal fibers, around or on the sarcoplasmic membrane, or within the perimysial connective tissue. The histologic findings support Sharp's observation of the high incidence of focal inflammatory lesions in skeletal muscle biopsies of patients with MCTD. Immunoglobulin deposition in these muscle biopsies suggests an immunologic basis for the muscular symptomatology in MCTD.