The battery of stains should include congo red for diagnosis of myopathy, especially inclusion body myositis, Dr. Lacomis said. Immunostains may indicate inflammatory conditions or dystrophies, he said. The lab will try to be very specific, but clinical correlation is advised. Various inflammatory myopathies present unique images on biopsy results. For example, in dermatomyositis, basophilic stippling may be seen in the cytoplasm of the perifascicular muscle fibers. One might also see dense, continuous staining of capillaries for membrane attack complex, Dr. Lacomis showed. In polymyositis and inclusion body myositis, invasion of nonnecrotic muscle fibers by inflammatory cells is often seen, although not always in polymyositis, making diagnosis tricky.
Rheumatologists should note that some myopathies may be triggered by drugs commonly used to treat rheumatic diseases, Dr. Lacomis said. Hydroxychloroquine may cause vacuolar myopathy, and glucocorticoids may also lead to a toxic myopathy, he noted.
Susan Bernstein is a freelance medical journalist based in Atlanta.
