Ab1252 Vasculitis Is a Rare Finding on Muscle Biopsy and Defines a Unique Clinical Sub-Group

Background: Muscle biopsy, performed for the workup of patients with muscle weakness, myalgia, raised serum creatine kinase (CK) levels or myositis autoantibodies, helps delineate inflammatory, genetic and metabolic contributions to myopathy. A rare finding on muscle biopsy is vasculitis, which may occur in the context of systemic vasculitis (SV) or may be limited to the muscles, termed isolated muscle vasculitis (IMV). The diagnostic significance and clinical associations of muscle vasculitis are not well understood. Objectives: To assess the spectrum of diagnoses, clinical presentations, laboratory investigations and histopathological features associated with muscle vasculitis. Methods: The electronic database of the Anatomical Pathology Laboratory at SA Pathology, wherein all muscle biopsies in South Australia are assessed, was searched from 2007 to 2023, to identify muscle biopsy reports containing the term, "vasculitis". Detailed demographic, clinical, serological and histological data were gathered for these patients. Muscle symptoms and the presence of CK elevation were compared between the muscle vasculitis group and a control group of South Australian patients with idiopathic inflammatory myopathy (IIM), using Fisher's Exact Test. Additionally, the characteristics of patients with muscle vasculitis in the context of SV were compared with those with IMV, defined as patients without clinical features or autoantibodies to support another diagnosis, and without symptoms or histology of vasculitis in organs other than the muscles. Results: From 2,303 muscle biopsies, 30 showed vasculitis. These patients (46.7% female, median age 69.3 years (IQR 59.2-75.6)) had a median of 4.4 years follow-up (IQR 2.6-8.0). Six patients were deceased: four from infection and two from unknown causes. Neuromuscular symptoms were the most common indication for muscle biopsy (weakness – 14/30, myalgia – 10/30, mononeuritis multiplex – 2/30). Among patients with muscle vasculitis, 16 had SV, 8 had IMV and 6 had other diagnoses (Table 1). Twelve patients had biopsies of other tissue types additionally performed but only two (both peripheral nerve biopsies) showed vasculitis. There was evidence of a systemic inflammatory response with median CRP 78mg/L and ESR 44mm/hr. At the time of biopsy, 13/30 patients had a CRP greater than 100mg/L. Fibrinoid necrosis was present in 15/30 biopsies and diffuse MHC-I upregulation was only observed in 10/30 biopsies. Compared with IIM controls, patients with muscle vasculitis less frequently had muscle weakness (19/30 vs 201/244, p=0.03) or CK elevation (8/30 vs 192/237, p<0.05), although there was no difference in rates of myalgia (12/30 vs 88/213, p=1). No patients with IMV had positive ANCA whereas 9/15 patients with SV had a positive MPO and 2/15 had a positive PR3. Patients with IMV were less likely to have received cyclophosphamide and/or rituximab than those with SV (2/8 vs 11/14, p=0.03). Conclusion: Muscle vasculitis most commonly occurs in the context of SV, although in a substantial proportion, is limited to the muscles. A normal CK is frequently observed in muscle vasculitis, unlike IIM, and should not deter from pursuing a muscle biopsy, particularly in the presence of systemic inflammation and muscle symptoms. IMV poses a unique management dilemma; prospective cohorts are needed to assess if this entity warrants the use of cyclophosphamide and/or rituximab as is the case with other organ involvement.REFERENCES: NIL. Acknowledgements: NIL. Disclosure of Interests: None declared.