MRI-guided muscle biopsy improves diagnostic yield in small- to medium-vessel vasculitis: a retrospective single-center study.

To evaluate the utility of magnetic resonance imaging (MRI)-guided muscle biopsy (MB) in diagnosing small- to medium-vessel vasculitis. We retrospectively included patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV) or polyarteritis nodosa (PAN) between April 2020 and March 2025 who underwent MRI and MB. The primary outcome was the diagnostic sensitivity of MRI-guided MB for AAV and PAN. The secondary outcome was the diagnostic sensitivity for PAN alone. MB was considered positive when it demonstrated either necrotizing vasculitis or non-necrotizing vasculitis. Eighteen patients who underwent MRI and MB were included: 11 patients had AAV and 7 had PAN. The median interval between MRI and MB was 3.5 days. The mean Birmingham Vasculitis Activity Score was 13.6. Muscle pain was observed in 11 patients; however, none of the patients exhibited elevated creatine kinase levels. The sensitivity of MB for diagnosing AAV and PAN was 83.3% (15/18; 95% confidence intervals [CI] 58.6–96.4%), whereas that for PAN alone was 85.7% (6/7; 95% CI 42.1–96.3%). No biopsy-related complications were observed. There were no apparent differences in clinical characteristics between the MB-positive and MB-negative groups. MRI-guided MB may represent a diagnostic option for small- to medium-vessel vasculitis, even in patients without muscle pain or when other suitable biopsy sites are unavailable. Although this was a small exploratory retrospective single-center study, these findings should be validated in larger multicenter prospective studies.