Effectiveness and Safety of IVIG for the Treatment of HMGCR Immune‐Mediated Necrotizing Myopathy

ABSTRACT Introduction/Aims Immune‐mediated necrotizing myopathy (IMNM) is an autoimmune myopathy. We aimed to compare clinical outcomes in patients with antibodies against 3‐hydroxy‐3‐methylglutaryl‐CoA reductase (HMGCR) treated on immunotherapy regimens with and without maintenance intravenous immunoglobulin (IVIG). The secondary aim was to assess outcomes in a subset that received IVIG monotherapy. Methods This was a retrospective longitudinal cohort study. Multivariate logistic regression was used to analyze the association between IVIG and the probability of reaching normal creatine kinase (CK) and normal/near‐normal proximal muscle strength at 3‐ and 6‐month follow‐ups. In patients treated with IVIG monotherapy, changes in CK and strength were analyzed using Wilcoxon signed rank tests. Results Patients treated with IVIG ( n = 40) had higher odds of CK normalization at 6 months (OR 9.44, 95% CI 1.19–74.89, p = 0.03) although not at 3 months ( p = 0.08) compared to patients not treated with IVIG ( n = 13). Increased odds of normal/near‐normal proximal muscle strength was not observed at 3 months ( p = 0.14) or 6 months ( p = 0.35). Subgroup analysis of patients on IVIG monotherapy ( n = 15) showed a median CK reduction of 84.5% at 3 months ( p < 0.001) and 95.7% at 6 months ( p < 0.001). CK normalized in 40% by 6 months. Proximal muscle strength improved at 3 months ( p = 0.003) and 6 months ( p = 0.008). 76.9% had normal/near‐normal proximal strength by 6 months. Discussion Patients treated with immunotherapy regimens that included IVIG were more likely to reach normal CK at 6 months. Maintenance IVIG monotherapy also induced marked improvements in CK and strength. IVIG monotherapy can be an effective first‐line treatment for HMGCR IMNM.