VP.45 Clinical features of anti-mitochondrial M2 antibody-positive myositis: Case series of 17 patients

Abstract

To clarify the prevalence, clinical features, treatment outcome, and severity determinants of anti-mitochondrial M2 antibody-positive myositis (AMA-M2-positive myositis). Clinical data for AMA-M2-positive myositis patients who were treated at Hokkaido University Hospital and other participating facilities from 2012 to 2021 were retrospectively evaluated. Of a total 182 patients with inflammatory myositis, 17 patients (13 females and 4 males) were AMA-M2-positive (mean age, 57.4 years; SD, 9.4 years). The median duration before diagnosis was 24 months (interquartile range [IQR]: 12-85), indicating a trend of delayed diagnosis of this disease. Weakness mainly involving paravertebral muscles, weight loss, respiratory failure, and cardiac complications were typical symptoms, but the course and severity of each symptom varied from case to case. Twelve patients (71%) had vital capacity percentage (%VC) of less than 80% and 4 patients needed ventilator support. The average body mass index (BMI) at diagnosis was 19.6 (SD, 4.1). Thirteen patients (76%) had cardiac complications and 6 patients showed lower left ventricular ejection fraction (LVEF) less than 50%. A strong correlation was found between %VC and mRS scores (R=0.68, p=0.002), indicating a trend for the degree of respiratory dysfunction to be linked to a decline in ADL. The correlation between %VC and BMI was also strong (R=0.68, p=0.002), indicating that weight loss can be an indicator of potential progression of respiratory failure. Fourteen patients received immunotherapy and CK level improved within a few months. %VC and mRS scores also improved significantly. We found significant correlations of %VC with BMI and mRS scores in AMA-M2-positive myositis patients. Immunotherapy improved the CK level, respiratory dysfunction, and ADL. We propose that CK level, %VC and BMI should be monitored as disease indicators in treatment of AMA-M2-positive myositis.