P128 Differences in approach to immune mediated necrotizing myopathy between rheumatology and neurology: a case for developing a combined service?

Abstract Background/Aims Immune-mediated necrotising myopathy (IMNM) is a rare, severe subtype of idiopathic inflammatory myopathy, characterised by muscle weakness, elevated creatine kinase (CK), and muscle fibre necrosis. It is associated with anti-SRP or anti-HMGCR antibodies. At our centre, anti-SRP is included in all myositis panels, while anti-HMGCR requires specific requests. This study examined the presentation and management of IMNM at a UK tertiary centre, aiming to characterise patient profiles and identify management differences between specialties to inform collaborative care. Methods We analysed all anti-HMGCR and anti-SRP antibody tests from 2012-2023 at Sheffield Teaching Hospitals. Patient records were reviewed to confirm diagnosis and treatment if serology was positive. Results 114 anti-HMGCR and 12,302 anti-SRP tests were performed. Anti-HMGCR had a higher positivity rate (28%, n = 28) compared to anti-SRP (1.48%, n = 183) (p 0.01). Fifty-one antibody positive patients were assessed by rheumatology or neurology. The most common referral reason was incidental anti-SRP positivity from interstitial lung disease screening (n = 11), none of whom were diagnosed with IMNM. 47% (n = 24) were diagnosed with IMNM.44% of IMNM patients were managed by rheumatology, 52% by neurology, and one received joint care. There were no significant baseline differences between IMNM subtypes or between parent specialty in functional status (mean modified Rankin score 2.67 rheumatology vs 2.84 neurology), strength (MMT8 120 rheumatology vs 126 neurology), or CK (5939 rheumatology, 6100 neurology). All neurology patients underwent muscle biopsy versus 44% in rheumatology. All patients received corticosteroids; 74% also received a DMARD. Neurology patients were more likely than rheumatology patients to receive rituximab (58% vs 20%) or intravenous immunoglobulin (IVIG) (25% vs 0%). Patients receiving rituximab had similar baseline MMT8 (122) to those that did not (124). However, 12-month MMT8 was worse in those receiving rituximab (137 vs 143) and a higher proportion remained on corticosteroids (40% vs 15%) perhaps reflecting refractory disease. At 12 months, average MMT8 score improved by 16 points (mean MMT8 150 rheumatology vs 135 neurology patients). CK had normalised in 52%, and 65% had discontinued prednisolone. There was no apparent difference in outcomes by antibody subtype. Conclusion IMNM is rare, with only 24 cases identified here. Due to the small sample size, detailed statistical analysis of outcomes was not possible. Anti-HMGCR tests yielded a higher proportion of positives than anti-SRP, likely due to more targeted testing. Removing anti-SRP from routine myositis panels may reduce costs and unnecessary referrals. Patients treated by neurology were more likely to have a muscle biopsy (due variation in skillset of treating team). Neurology patients were more likely to receive rituximab or IVIG. We therefore propose that a collaborative approach between neurology and rheumatology for IMNM patients would be beneficial in ensuring equitable access to diagnostic tests (e.g. biopsy) and providing standardised, cost-effective treatment pathways. Disclosure M.F. Cox: None. S. Carlson: None. C. Hewamadduma: None.