Clinical Spectrum and Outcome of Idiopathic Inflammatory Myopathy at Neurological Institute of Thailand (NIT)

Abstract

Background: Immune-mediated necrotizing myopathy (IMNM) is a severe inflammatory myopathy with a high relapse rate. Objective: We aimed to compare clinical characteristics, lab data, and treatment outcomes in Thai patients with immune-mediated necrotizing myopathy (IMNM) to other idiopathic inflammatory myopathies (IIMs). Methods: This retrospective study included IIMs patients aged 18 or older, treated at the Neurological Institute of Thailand (NIT) between 2015 and 2021. Patients had relevant clinical and lab data, including muscle enzymes, from a minimum of 3 follow-up visits within a year. Baseline clinical and lab data, including serum myositis-specific and associated autoantibodies (MSAs and MAAs), were documented. Results: Out of 25 IIMs patients, 18 had IMNM (15 anti-SRP, 2 anti-HMGCR, and 1 sero-negative). Five (20%) had dermatomyositis (2 anti-Mi2, 2 anti-NXP2, and 1 anti-1TIF1γ positive), and 2 (8%) had overlap myositis (OM) with anti-KU positive. The mean age of onset was 49±14.3 years, with 72% being female. Median symptom duration was 90 days (IQR, 45-150). The mean initial MRC sumscore for IMNM was 42.68±10.1, and for non-IMNM (DM and OM) it was 44.5±6.6 and 56±2.8, respectively. However, median CK level was significantly higher in IMNM patients compared to non-IMNM (DM and OM) (4952.5 vs. 1,816 U/L, p = .03). Eighty percent received 2 immunosuppressive drugs. Only 3 IMNM and 1 DM needed IVIg or rituximab as rescue therapy. Around 40-50% of patients had full motor strength at 1 year, but 38.9% of IMNM patients experienced relapses. Conclusions: Most patients with IIMs responded well to two immunosuppressive drugs. However, approximately 40% of IMNM experienced relapses and persistent mild hip flexor weakness.