Muscle-dominant ANCA-associated vasculitis: a single-center cohort study.

To investigate the clinical characteristics of muscle-dominant ANCA-associated vasculitis (muscle-dominant AAV) in a consecutive single-center cohort. We retrospectively analyzed consecutive patients newly diagnosed with microscopic polyangiitis (MPA) and granulomatosis with polyangiitis (GPA) at a single center. Cases with muscle-dominant AAV were defined as those with muscle involvement due to AAV in the absence of other organ manifestations, including glomerulonephritis, cutaneous vasculitis, vasculitic neuropathy, or alveolar hemorrhage. We compared the muscle-dominant AAV group with all remaining AAV patients, who served as the comparison group. Among 72 consecutive patients newly diagnosed with MPA or GPA, 8 (11%) were classified as having muscle-dominant AAV. All patients with muscle-dominant AAV (100%) were MPO-ANCA-positive. In all patients with muscle-dominant AAV, serum creatine kinase levels remained within the normal range despite MRI-confirmed muscle inflammation. Compared with the comparison group, the muscle-dominant AAV group had significantly higher rheumatoid factor (RF) levels. ROC analysis identified an exploratory RF cutoff value of 47.55 U/mL (AUC = 0.79). The prevalence of interstitial lung disease (ILD) tended to be more frequent in the muscle-dominant AAV group than in the comparison group (75 vs. 37.5%, p = 0.060). Muscle-dominant AAV was characterized by MPO-ANCA positivity, elevated RF levels, normal CK levels, and MRI-confirmed muscle involvement. ILD was often observed but did not reach statistical significance. These findings suggest a possible clinical pattern within AAV, which we tentatively refer to as the hypothesized IMARM pattern (ILD, MPO-ANCA, RF, myopathy).