Invited Review: Granulomatous myopathy: Sarcoidosis and beyond-Article

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Granulomatous myopathy is a rare finding in skeletal muscle biopsy characterized by the presence of non-necrotizing granulomas. It is associated with a limited number of diseases, with sarcoidosis being the most common cause. When granulomatous myopathy is identified, it is important to evaluate for evidence of extramuscular granulomatosis and signs of sarcoidosis. About half of the patients with granulomatous myopathy have sarcoid myopathy, while the remaining patients may have inclusion body myositis (IBM) or other autoimmune disorders. It is important to distinguish between sarcoid myopathy and IBM, as the treatment approaches differ. IBM should be considered in patients with granulomatous myopathy who present with finger flexor weakness and other clinical and pathological clues of IBM. Further investigations should aim to identify myasthenia gravis and other autoimmune disorders, especially those known to cause granulomatous inflammation in other organs. Laboratory investigations may include the evaluation of specific autoantibodies. In cases of unexplained granulomatous myopathy, immune checkpoint inhibitors, chronic graft-vs-host disease, and extranodal natural killer/T-cell lymphoma should be considered as possible causes. Proper identification of the underlying etiology of granulomatous myopathy in each patient can guide appropriate treatment. Additionally, muscle imaging, such as MRI and PET-CT scans, can provide valuable information for diagnosis and guiding biopsy sites.

Keywords

granulomatous myopathy

skeletal muscle biopsy

sarcoidosis

extramuscular granulomatosis

sarcoid myopathy

inclusion body myositis

autoimmune disorders

myasthenia gravis

muscle imaging

biopsy sites