Clinical phenotypes and long-term outcomes of anti-SRP versus anti-HMGCR immune-mediated necrotising myopathy: a 13-year single-centre study

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OBJECTIVES: To characterise and compare the clinical phenotypes, malignancy risk, and long-term outcomes of anti-SRP and anti-HMGCR immune-mediated necrotising myopathy (IMNM). Given the rarity of this condition, we aimed to validate established findings within our large, ethnically diverse cohort managed over a 13-year period. METHODS: We conducted a retrospective, observational single-centre study of all patients with anti-SRP or anti-HMGCR positive myopathy managed at our tertiary neuromuscular centre between 2010-2023. RESULTS: We included 57 patients (15 anti-SRP, 42 anti-HMGCR). The anti-SRP cohort was younger at disease onset (median 53 vs. 66.5 years, p<0.001) and had a higher frequency of extra-muscular features, including interstitial lung disease (26.7% vs. 2.4%) and arthritis (33.7% vs. 0%). The overall malignancy incidence was 12.3% and was low in both subgroups. In the anti-HMGCR cohort, antibody titres showed a significant positive correlation with creatine kinase (CK) (r=0.692, p<0.001) and troponin T levels (r=0.476, p=0.014). A greater proportion of anti-HMGCR patients achieved treatment-free remission compared to anti-SRP patients (21.4% vs. 13.0%). CONCLUSIONS: Our findings are in line with previous reporting, demonstrating that the majority of patients respond to immunosuppressive therapy, and the association of anti-SRP myopathy with more prominent systemic involvement. Anti-HMGCR antibody titres are a potentially valuable biomarker for disease activity and may have utility in guiding immunosuppression withdrawal.

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