Immune-mediated necrotizing myopathy (IMNM) is a subtype of myositis that is associated with a refractory phenotype and poorer prognosis. The aim of the study was to provide single large center experience of outcomes of intravenous immunoglobulin (IVIg) for patients with IMNM using longitudinally collected data.

This case series longitudinally evaluated 4 of the 6 myositis core set measures at baseline and at 3 and 6 months after IVIg on 20 adult IMNM patients from 2014 to 2019 at the University of Pittsburgh. We assessed patients for improvement in core set measures, prednisone dose, adverse effects, and by the "limited" ACR/EULAR 2016 myositis response criteria. The mean differences in CK and manual muscle testing (MMT-8) were compared using a paired t test. A clinically significant response was defined as a >10% absolute improvement in the MMT-8 and a >50% absolute reduction in serum CK at 6 months of IVIg.

Intravenous immunoglobulin treatment was associated with marked improvement in IMNM patients, with 85% of patient meeting clinically significant response. The median (interquartile range) relative percent improvement in CK level was 96% (85%-98%) and in MMT was 29% (14%-36%) at 6 months.There was a significant reduction in the mean (SD) dose of prednisone at 6 months and had minimal adverse effects. In addition, with IVIg, most (13/14) patients had at least minimal improvement as per ACR/EULAR 2016 myositis response criteria.

Based on objective, meaningful improvement in MMT-8 and CK as well as marked reduction in prednisone doses with acceptable tolerability, early implementation of IVIg should be considered in adult IMNM.