Defining Clinical and Autoantibody Phenotypes in Patients with Juvenile Idiopathic Inflammatory Myopathies

Background: The juvenile idiopathic inflammatory myopathies (JIIM) are a heterogeneous group of systemic autoimmune diseases characterized by chronic muscle inflammation. The aim of this project was to study the relationship between specific clinical subgroups, autoantibodies and a variety of features in juvenile myositis patients.;Methods: Study subjects were individual participants of the Childhood Myositis Heterogeneity Study. Random Forest models and regression models were developed to determine which characteristics would be potentially informative within each clinical subgroup or the major autoantibody subgroups.;Findings: The most important differentiating features reported in our analysis included periungual capillary abnormalities and malar rash for juvenile dermatomyositis (JDM) patients; disease severity at disease onset, frequency of hospitalizations and CK levels for juvenile polymyositis (JPM) patients; and malar rash, Raynaud's phenomenon, arthralgia, interstitial lung disease, and the presence of U1RNP autoantibody for overlap myositis (JCTM) patients. We also found that myositis specific autoantibodies (MSA) can be an important method for classifying children with myositis. Univariate and multivariate analyses also showed that UV radiation intensity was associated with the relative proportion of patients with JDM compared to JPM and also of those patients who had anti-p155 autoantibody.;Interpretation: Our findings support that clinical subgroups and MSAs can be an important method for classifying children with myositis. Finally, this study was the first that looked at the distribution of myositis phenotypes and UV radiation exposure in the juvenile patients in the US and showed that UV radiation may affect the clinical and immunologic expression of autoimmune disease in patients.