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Juvenile idiopathic arthritis (JIA) is an umbrella term encompassing a heterogeneous group of chronic arthritis in children. Many of these have significant differences from adult arthritis, while others possibly represent similar diseases in children and adults. Classification aims to enhance the understanding of the disease’s pathogenesis, patterns of phenotypes, disease trajectories, and treatment responses. 

Patient-reported outcome measures (PROMs) are invaluable for assessing disease burden and play an important role in co-producing optimal health care for the child with JIA.

This thesis has its origin in the Nordic JIA study, a population-based, prospective study of 510 children with newly diagnosed JIA who were included between 1997 and 1999. In the 8-year follow-up, we found that features associated with psoriasis were linked to more severe disease progression over time. A significant proportion of children with both psoriasis and arthritis were not classified as having juvenile psoriatic arthritis (JPsA). To ensure that these children receive early, personalized treatment, future classifications should include psoriasis and psoriasis-related characteristics as criteria. 

In the 18-year follow-up, we studied temporomandibular joint (TMJ) arthritis. Orofacial symptoms and dysfunctions were common, and two-thirds of participants showed condylar deformities or erosions on cone-beam computed tomography (CBCT). An interdisciplinary approach is recommended to optimize management throughout the course of the disease.

The presence of HLA-B27 was associated with an increased risk of not being in remission off medication after 18 years of disease duration in males but not in females. Uveitis in HLA-B27 positive individuals is not always symptomatic, which clinicians need to be aware of. 

The Juvenile Arthritis Multidimensional Assessment Report, JAMAR, was translated into Swedish and validated in a clinical setting. The Swedish JAMAR proved to be a reliable tool that can be used in both routine clinical practice and research. The responses from the questionnaire can also serve as a basis for discussions between patients and caregivers.

In conclusion, JIA is a complex disease requiring attention to multiple aspects. Our results highlight the need for better classification of psoriasis in JIA, the importance of careful and multidisciplinary follow-up for TMJ arthritis, and an association between HLA-B27 positivity and more severe disease. The Swedish version of JAMAR serves as a valuable complement to existing PROMs and has the potential to enhance health care for children with JIA.