Impact of glucocorticoids on muscle biopsy findings in antineutrophil cytoplasmic antibody-associated vasculitis

Several studies have examined the utility of quadriceps femoris muscle biopsies for diagnosing antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). We aimed to investigate the effects of glucocorticoid therapy on the muscle biopsy findings of patients with AAV. Data from patients with AAV who underwent a biopsy of the bilateral vastus lateralis of the quadriceps femoris at Tokyo Metropolitan Tama Medical Center between January 2015 and December 2023 were retrospectively analyzed. In total, 126 patients with suspected small vessel vasculitis (SVV) or medium vessel vasculitis (MVV) underwent a muscle biopsy at the study center. Of these, the present study included 71 patients with a final diagnosis of AAV who received a muscle biopsy of the bilateral vastus lateralis at disease onset and began glucocorticoid therapy at the study center. Of the 71 patients, 41 (57.7%) had a positive muscle biopsy finding as defined by the presence of inflammatory infiltrates with fibrinoid necrosis in the vessels. Fifty-six and 15 patients underwent a biopsy before and after starting glucocorticoid therapy, respectively. Of the former, 33 (58.9%) had a positive finding compared to eight (53.3%) of the latter (Pearson's chi-square test: p = 0.697). Among the patients with a positive biopsy result, 22 (53.7%) and 19 (46.3%) had a unilaterally and bilaterally positive finding, respectively. Glucocorticoid therapy did not significantly affect the findings of quadriceps femoris muscle biopsies. Unilateral biopsies may have a lower diagnostic sensitivity for AAV than bilateral biopsies.