Non-necrotizing granulomatous inflammation is a rare but easily recognized histopathological finding in skeletal muscle biopsy. A limited number of diseases are known to be associated with non-necrotizing granulomatous myopathy. Once identified, a careful evaluation for evidence of extramuscular granulomatosis and other signs suggestive of sarcoidosis is warranted as about half of the patients have sarcoid myopathy. In addition, the presence of granulomatous myopathy should trigger a search for clinical and pathological clues of inclusion body myositis (IBM), which accounts for most of the remaining patients and can coexist with sarcoidosis. Recognizing the features of IBM in patients with granulomatous myopathy can potentially spare the patients from unnecessary exposure to immunosuppressive therapies. In patients whose granulomatous myopathy remain unexplained, further investigations should aim at identifying myasthenia gravis and other autoimmune disorders, especially those known to cause granulomatous inflammation in other organs. Laboratory investigations should include acetylcholine receptor, antimitochondrial, antineutrophil cytoplasmic, thyroglobulin, and thyroid peroxidase autoantibodies. In the appropriate clinical context, exposure to immune checkpoint inhibitors and chronic graft-vs-host disease can be causes of granulomatous myopathy. In cases of unexplained granulomatous myopathy, natural killer/T-cell lymphoma should be considered and careful histopathological examination for atypical cells and appropriate immunostaining is crucial. Identifying the etiology of granulomatous myopathy in each patient can guide appropriate treatment.
Keywords: antimitochondrial antibody; granulomas; immune checkpoint inhibitor myopathy; inclusion-body myositis; inflammatory myopathy; non-caseating granulomatous myopathy; non-necrotizing granulomatous myopathy; sarcoidosis.
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