Granulomatous myositis. Clinicopathologic study of 12 cases.

Granulomatous inflammation is infrequently encountered in skeletal muscle biopsy material. Of 2,985 muscle biopsy specimens reviewed over 12 years, 12 (0.4%) with granulomatous inflammation were identified. The patients included 9 women who ranged in age from 24 to 76 years (mean 50.3 years). The most common clinical findings included decreased strength or weakness in the extremities (n = 8), muscle pain (n = 5), and weight loss (n = 3). All muscles exhibited nonnecrotizing granulomas; an associated vasculitic process was identified in 2. Endomysial chronic inflammation consisting primarily of lymphocytes and plasma cells was present in 10 muscles, and perivascular chronic inflammation in 8. Degenerating muscle fibers were noted in 10 cases, and regenerating fibers in 11. Evidence of neurogenic atrophy was seen in 8 muscles. Increased endomysial fibrosis was observed in 5 muscles, and type II muscle fiber atrophy in 5 muscles. Stains for acid-fast bacilli and Gomori methenamine silver stain were performed in all but 2 cases and failed to demonstrate organisms. In 3 cases, concomitant sural nerve biopsies were performed, and granulomas were identified in 2 of those cases. Clinicopathologic diagnoses included sarcoidosis (n = 6), vasculitis (n = 2), and granulomatous myositis not otherwise specified (n = 2). In 2 cases, there was insufficient clinical information or follow-up data to determine a cause. In conclusion, granulomatous myositis is infrequently found in muscle biopsy specimens (0.5% of all biopsies in this series); most muscles demonstrate evidence of chronic endomysial or perivascular inflammation accompanied by muscle fiber degeneration and regeneration; and the most common cause for granulomatous myositis was sarcoidosis in this series.