Clinical Characteristics of Anti‐Neutrophil Cytoplasmic Antibody‐Associated Vasculitis Patients Proven by Muscle Biopsy and Its Relevance to Kidney Involvement

Muscle symptoms including myalgia are common in patients with anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV). Muscle biopsy (MB) is sometimes used in the diagnosis of AAV, but which AAV patients benefit from MB remains unclear. We retrospectively assessed the clinical characteristics of Japanese AAV patients who underwent MB and examined the relationship between MB showing positive results for muscular vasculitis and kidney involvement. We enrolled all 38 of the 118 patients with AAV who also underwent MB. Clinical characteristics and histopathological findings on kidney biopsies were assessed in patients with MB showing positive or negative results for muscular vasculitis. Among the 38 patients, 34 (89.5%) were diagnosed with microscopic polyangiitis, and 17 patients (44.7%) showed positive MB. Myalgia and edema of the lower extremities were observed in 12 MB-positive patients, higher proportions than seen in MB-negative patients (70.6% vs. 38.1%, p = 0.046 and 70.6% vs. 33.3%, p = 0.022, respectively). Kidney involvement was more frequent in MB-positive patients (70.6%) than in MB-negative patients (28.6%, p = 0.010). Twelve of the 38 patients with MB underwent kidney biopsy. Proportions of crescents or vascular fibrinoid necrosis were higher in MB-positive patients than in MB-negative patients (83% vs. 33% and 67% vs. 20%, respectively). Myalgia and edema of the lower extremities may offer a guide to the utility of MB. Glomerulonephritis should be considered when muscular vasculitis is confirmed by MB.