SAT0331 CHIPPING AWAY AT POLYMYOSITIS: A RETROSPECTIVE REVIEW AT A TERTIARY MYOSITIS CENTRE
Authors: J. Loarce-Martos, J. B. Lilleker, M. Parker, A. Herrick, N. Mchugh, Z. E. Betteridge, H. Chinoy
Journal: Annals of the Rheumatic Diseases (2020)

Background: Since first described, our understanding of polymyositis (PM) has evolved substantially. Patients previously diagnosed with PM may now be better described as having an alternative diagnosis, including immune-mediated necrotizing myopathy (IMNM), connective tissue disease overlap myositis (CTD-OM), anti-synthetase syndrome (ASS) or inclusion body myositis (IBM). Objectives: To apply the current understanding of IIM subtypes to retrospectively review the diagnosis of patients classified as PM at our centre. Methods: We reviewed a cohort of 255 patients from a UK tertiary myositis clinic, with 37 patients classified as PM according to the EULAR/ACR IIM criteria and expert opinion (1). Consensus decisions made as to whether reclassification as an alternative IIM subtype would be appropriate in each case. Results: Thirty-seven patients with PM diagnosis according to EULAR/ACR IIM criteria and expert opinion were included in the analysis. Twenty-six patients (26/37, 68.4%) were female. The mean age at diagnosis was 57 years (SD 16.25) and mean follow up was 5.2 years (SD 4.8). After consensus discussions, ten patients (27%) retained classification as PM, representing 3.9% (10/255) of the original cohort of 255 patients. The remaining 27 were reclassified as follows: 7 CTD-OM (18.9%), 6 IMNM (16.2%), 6 unspecified myopathy (16.2%), 2 DM (5.4%), 1 IBM (2.7%), and 1 non-inflammatory myopathy (2.7%, myofibrillar myopathy). Four patients (10%) had insufficient data available to allow for a defined diagnosis. Of the 10 patients retaining classification as PM, 7 (70%) were female and mean age at diagnosis was 58.9 years. Clinical characteristics are represented in table 1. Mean CK at diagnosis was 2321 IU/L. Four patients (40%) were ANA positive, 8 patients (80%) were tested for the complete myositis antibody panel and 8 (80%) were specifically tested for the presence of HMGCR antibodies (figure 1). EMG revealed a myopathic pattern in 6/7 patients tested, while MRI demonstrated muscle oedema compatible with myositis in 3/5 of the patients that were tested. A complete muscle biopsy report was available in 8/10 PM-classified patients (80%), diffuse HLA-1 upregulation was present in 5 patients (50%), endomysial inflammatory infiltrates in 5 (50%) and perimysial infiltrates (in conjunction with endomysial infiltrates) in 3 (30%). No other specific features were found. Table 1. Clinical features. PM (n=10 ) CTD-OM (n=7 ) IMNM (n=6 ) All (n=37 ) Pyrexia 0 0 0 1 (2.7 %) Weight loss 2 (20%) 2 (28.6%) 2 (33.3%) 6 (16.2 %) Cutaneous rash 0 1 (14.3%) 0 2 (5.4 %) Mechanic’s hands 1 (10%) 2 (28.6%) 0 4 (10.8 %) Sclerodactyly 0 1 (14.3%) 0 1 (2.7 %) Periungual erythema 0 1 (14.3%) 0 2 (5.3 %) Raynaud’s 0 5 (71.4%) 0 8 (21.6 %) Arthralgia 0 5 (71.4%) 0 8 (21.6 %) Arthritis 0 2 (28.6%) 0 5 (13.5 %) Muscle weakness 10 (100%) 7 (100%) 6 (100%) 36 (97.2%) Myalgia 3 (30%) 5 (71.4%) 3 (50%) 20 (54 %) Dysphagia 3 (30%) 5 (71.4%) 1 (16.7%) 13 (35.1 %) Interstitial lung disease (ILD ) 0 3 (42.9%) 0 4 (10.5 %) Malignancy 1 (9.1%) 0 2 (33.3%) 3 (8.1%) Conclusion: This study confirms that PM can now be considered a rare IIM subtype. A thorough examination, complete myositis antibody panel including HMGCR testing and careful interpretation of the biopsy results is recommended to accurately classify these patients. References: [1]Parker MJS, Oldroyd A, Roberts ME, Lilleker JB, Betteridge ZE, McHugh NJ, et al. The performance of the European League Against Rheumatism/American College of Rheumatology idiopathic inflammatory myopathies classification criteria in an expert-defined 10 year incident cohort. Rheumatol (United Kingdom). 2019 Mar 1;58(3):468–75 Disclosure of Interests: None declared