Juvenile Myositis: Insights on Classification, Pathogenesis and Therapies over the Past 50 Years

The juvenile idiopathic inflammatory myopathies (JIIM) are rare serious, multisystem autoimmune diseases affecting children and young people. Fifty years ago the prognosis for these conditions was poor, with high morbidity and mortality, and very few treatments were available. Since then, there has been major growth in our understanding of the pathogenesis and evidence-basis for treatment. Early descriptions considered juvenile dermatomyositis (JDM) to be essentially one disease. Our modern understanding has shown that that there are different types of JIIM, and that while JDM remains the most prevalent, even JDM itself is highly heterogenous. Much progress has been made possible through concomitant growth in the research community with a focus on JIIM, and the fact that the myositis research and clinical communities are highly collaborative internationally. Here, we review the growing understanding of the different types of childhood myositis, the developments in classification, driven in part by recognition of associations of particular phenotypes with specific myositis autoantibodies, the novel insights into pathogenesis of the JIIM, the therapeutic targets that these have revealed, and the evidence for, and development of, modern treatment care pathways, which have changed radically in recent decades. This has resulted in improved outcomes for children and young people with JIIM, and hope for further groundbreaking developments.