Muscle Vasculitis: A Novel Delineation of Distinct Subsets of Disease

The objective of this study was to assess the clinical and histopathologic features of muscle vasculitis.

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.

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.

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.