Differences in Organ Damage Based on Age at Onset in Idiopathic Inflammatory Myopathies: A Retrospective Multicenter MYKO Study

Objective To analyze the influence of age of the onset on myositis organ damage and to identify the factors influencing myositis organ damage, as clinical manifestations of myopathies differ by the age of onset and the background of patients. Methods Factors influencing organ damage [the Systemic Lupus International Collaborating Clinics/American College of Rheumatology (SLICC/ACR) Damage Index (SDI)] were identified using the Japanese multicenter myositis registry (MYKO, n=220). Factors influencing organ damage were identified using a multivariate analysis. SDI was compared among juvenile-onset (<20 years old), adolescent-onset (20-64 years), and elderly-onset (>64 years) groups. Results There was a correlation between the age at onset and the SDI score (Spearman's rank correlation coefficient ρ=0.28). Elderly patients exhibited more widespread organ damage, including neuropsychiatric, renal, pulmonary, cardiovascular, peripheral vascular, gastrointestinal, skin, and diabetes, whereas juvenile-onset patients exhibited musculoskeletal damage. Adolescent-onset patients had the lowest incidence of ocular and malignant damage. A regression analysis revealed that an older onset age (coefficient, β=0.03), longer disease duration (β=0.05), and total dose of glucocorticoid (β=3.35×10^-5) influenced SDI. After adjusting for disease duration, the influences of anti-melanoma differentiation-associated gene 5 (MDA5) [hazard ratio (95% confidence interval), 4.47 (2.17-9.21)] on pulmonary fibrosis and a history of steroid pulse [2.16 (1.16-4.05)] on muscle atrophy or weakness were shown. Conclusion There were associations between the age of onset and autoantibodies with myositis organ damage. Musculoskeletal damage was greater in patients with a juvenile onset. An older age of onset is associated with severe organ damage. These findings highlight the importance of considering the age of onset and autoantibodies for assessing the prognosis and developing treatment plans for myopathies.