[Juvenile idiopathic arthritis: Definition and classification].

Juvenile idiopathic arthritis (JIA) is a group of diseases defined by the presence of arthritis of more than 6 weeks duration in patients aged less than 16 years and with unknown etiology. The international classification based on clinical and biological criteria define each type of JIA: systemic, oligoarticular, polyarticular with and without rheumatoid factor, enthesitis-related arthritis, and psoriatic arthritis. However, some discussions persist concerning systemic-onset juvenile idiopathic arthritis, whose clinical symptoms and pathogenic mechanisms are quite similar to those observed in autoinflammatory diseases, arthritis with antinuclear factors (poly- and oligoarticular) that could be considered as a homogenous group, and a family history of psoriasis that frequently led to unclassified arthritis. Better knowledge of the pathogenic mechanisms should improve the initial clinical classification with more homogeneous groups of patients and reduce the number of unclassified cases of arthritis.