Objective To identify early factors associated with disease course in patients with juvenile idiopathic inflammatory myopathies (JIIM). Methods Univariable and multivariable multinomial logistic regression analyses were performed in a large JIIM registry (n=365), including demographics, early clinical features, serum muscle enzyme levels, myositis autoantibodies, environmental exposures, and immunogenetic polymorphisms. Results Multivariable associations with chronic or polycyclic courses compared to monocyclic included myositis-specific autoantibodies (multinomial odds ratio (M-OR) 4.2 and 2.8, respectively), myositis-associated autoantibodies (M-OR 4.8 and 3.5), and a documented infection within six months of illness onset (M-OR 2.5 and 4.7). A higher overall clinical symptom score at diagnosis was associated with chronic or monocyclic courses compared to polycyclic. Furthermore, a severe illness onset was associated with chronic compared to monocyclic or polycyclic courses (M-OR 2.1 and 2.6), while anti-p155/140 autoantibodies were associated with chronic or polycyclic courses compared to monocyclic (M-OR 3.9 and 2.3). Additional univariable associations of chronic compared to monocyclic course included photosensitivity, “V-sign” or “Shawl-sign” rashes, and cuticular overgrowth (OR 2.2–3.2). The mean and highest ultraviolet index in the month before diagnosis were associated with a chronic compared to polycyclic course in boys (OR 1.3 and 1.5), while residing in the Northwest was less frequently associated with a chronic course (OR 0.2). Conclusion Myositis autoantibodies, in particular anti-p155/140, and a number of early clinical features and environmental exposures were associated with a chronic course in patients with JIIM. These findings suggest early factors can be identified that are associated with poorer outcomes in JIIM.

In conclusion, it appears that exercise training is safe and effective in adult patients with active as well as inactive stable IIMs. However, more studies with a well-controlled design are needed. In addition, studies in children with an IIM are indicated.

QMUS can provide additional information for follow-up of JDM regarding disease severity and residual muscle damage, particularly after normalization of CMAS.