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

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.