3. Juvenile Idiopathic Arthritis (JIA)

The majority of children with inflammatory rheumatologic conditions have some form of arthritis. The different categories are determined by articular and extra-articular manifestations and it may be months before one is sure of the diagnosis. Not knowing the category does not preclude treatment, but one should always be willing to change the diagnosis as the illness progresses. Juvenile idiopathic arthritis (JIA) is an umbrella term for several distinct arthritides lasting more than 6 weeks with unknown etiology (Table 3.1) [1]. The pathogenesis of these diseases involves both autoimmune and genetic factors. Dysregulation of the immune and inflammatory systems are observed and include increased immune complexes, complement activation and disordered Th1, Th2, and Th17 cell interactions with predominance of Th1 and Th17 cells in the synovium. Additionally, hormonal, infectious and other environmental agents, yet to be identified, are likely involved. The term “juvenile” refers to the onset of arthritis at 16 years of age or younger. However, it is an arbitrary distinction and, as far as we know, there are no biological reasons why nearly all these conditions cannot occur in adults. In fact, several are more common in adults, such as rheumatoid factor (RF) positive polyarthritis and psoriatic arthritis. Conversely, the onset of systemic and anti-nuclear antibody (ANA) positive oligoarticular JIA in Caucasians is most commonly below the age of 6 years and these are much less frequent in adults than in the pediatric age group.