38PClinical course and treatment of anti-mitochondrial M2 antibody-positive myositis: a single-center retrospective case series

Abstract

We reported that anti-mitochondrial M2 antibody (AM2A)-positive myositis may be an independent subtype of autoimmune myositis based on clinicopathological and statistical analysis. It is still unknown how effective conventional immunotherapy is among patients with AM2A-positive myositis. We chose 13 AM2A-positive patients who underwent muscle biopsy from January 2017 to March 2025. We retrospectively examined their clinical history including reactivity to immunotherapy. There were 6 men and 7 women. The mean age at muscle biopsy was 58 ± 4.1 years. The average of pre-biopsy disease duration was 27.5 ± 24.3 months. Twelve patients showed high creatine kinase (CK), and 3 had severe muscle weakness with an MMT score lower than 4 in the extremities and/or the neck muscles. Although all patients are ambulatory, 4 patients had respiratory failure and 2 of them needed mechanical ventilation support. All patients showed arrhythmia and/or cardiac dysfunction on echocardiogram. Except for 2 patients who have not been treated, 11 patients had prednisolone as the first line. Six had additional medication, including immunosuppressants and/or intravenous immunoglobulin. Five patients exhibited a favorable clinical course, whereas symptoms relapsed as the medication dosage was reduced in 4 patients. In addition, the arrhythmia persisted in 2 of them despite improvement of muscle strength. Immunotherapy including prednisolone and intravenous immunoglobulin was ineffective in one patient with ventilation support and he died with comorbidity. Another patient was weaned from the ventilator and walked by himself. However, he suddenly died of unknown causes after discharge from hospital. Although most of the patients showed good response to immunotherapy in the early state, some experienced difficulties with treatment. Cardiac problems could remain in the long term. We need to carefully follow up on the condition including cardiac function with AM2A-positive myositis.