Muscle Vasculitis: A Novel Delineation of Distinct Subsets of Disease

OBJECTIVE: The objective of this study was to assess the clinical and histopathologic features of muscle vasculitis. METHODS: The electronic database of SA Pathology, wherein all muscle biopsy samples in South Australia are assessed, was searched (2000-2023), identifying cases of vasculitis. Cases were classified as systemic vasculitis (SV) or isolated muscle vasculitis (IMV). IMV occurred as either localized muscle disease (IMV-LMD) or with known connective tissue disease (IMV-CTD). Clinical and histologic comparisons between muscle vasculitis and biopsy-proven idiopathic inflammatory myopathy (IIM) were made. RESULTS: From 3,090 muscle biopsy samples, 59 showed vasculitis (47.5% female, median age 64.1 [interquartile range (IQR) 51.5-75.3] years). SV was present in 32 of 59, IMV-LMD was present in 15 of 59, and IMV-CTD was present in 10 of 59. Patients with SV and IMV-LMD frequently had normal creatine kinase (CK) levels (SV: median 58.5 [IQR 25-152] IU/L; IMV-LMD: median 34.5 [IQR 21.5-88.5] IU/L). Antineutrophil cytoplasmic antibodies (ANCAs) were restricted to those with SV (myeloperoxidase-ANCA [13 of 32] vs proteinase 3-ANCA [2 of 32]). Compared to patients with IIM, patients with muscle vasculitis had less frequent weakness (29 of 59 vs 305 of 369; P < 0.01) but more distal muscle symptoms (21 of 59 vs 74 of 360; P = 0.02) and elevated C-reactive protein levels (35 of 59 vs 20 of 82; P < 0.01). Salient histologic features of IIM (lymphocytic infiltration of myofibers, myofiber necrosis, major histocompatibility complex class I up-regulation) occurred less frequently in muscle vasculitis than IIM (P < 0.01), suggesting myofiber integrity despite pathology in the vasculature. CONCLUSION: Muscle vasculitis can occur with SV or be isolated to the muscles and is rarely associated with CTD. Distinct from IIM, muscle vasculitis frequently lacks myofiber necrosis and/or infiltration; consequent preservation of muscle power and normal CK levels should not deter from muscle biopsy.