05PConsistent clinical features of anti-Mi-2 myopathy: a Korean retrospective cohort study

Anti-Mi-2 is a myositis-specific autoantibody associated with a distinct dermatomyositis phenotype. However, its clinical characteristics in Korean patients are not well documented. This study aimed to characterize the clinical features of anti-Mi-2 positive myopathy in a Korean cohort. We conducted a retrospective review of patients diagnosed with inflammatory myopathy at Gangnam Severance Hospital between October 2003 and March 2025. Among 247 patients tested for myositis-specific antibodies using a 16-antibody line blot assay, eleven (4.5%) were positive for anti-Mi-2 antibody (signal intensity ≥2+). Clinical, serological, histopathological, and radiological data were analyzed. The median age at symptom onset was 62.0 years [Interquartile range (IQR): 52.0-69.5]. Among the eleven patients, five were male and six were female. All patients exhibited classic dermatomyositis-type cutaneous features, including heliotrope rash, Gottron's sign, V-sign, and shawl sign. Symmetric proximal limb weakness was observed in 10 patients, though the overall severity was mild. The median MRC sum score was 48.0 [IQR: 47.5–50.5], with 5 patients scoring ≥50, indicating relatively preserved muscle strength in a substantial proportion of this subgroup. Myalgia was reported in five patients, and oropharyngeal involvement was noted in one. No patients showed facial weakness, interstitial lung disease, or coexisting connective tissue disease. Cardiomyopathy and malignancy were identified in two patients each. The median creatine kinase level at diagnosis was 4,183 U/L [IQR: 3,205-5,678]. Muscle biopsies were available for ten patients, revealing necrotic fibers in seven and perifascicular atrophy in four. EMG was performed in eight patients, showing myopathic patterns in five. MRI findings, available in five patients, were consistent with inflammatory myopathy in all cases. In this Korean cohort, anti-Mi-2 positive myopathy showed a consistent clinical phenotype, with classic dermatomyositis-type cutaneous features, relatively preserved muscle strength, and minimal systemic involvement. These findings contribute real-world evidence that may support early recognition and classification of this myositis subtype in clinical settings.