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Arthritis is inflammation of synovial tissue within a diarthrodial joint*. Infection-related arthritides are discussed in Chapter 152, but idiopathic arthritides constitute another group. Because the pathogenesis of the idiopathic forms is not completely understood, a variety of schemes have been devised to classify childhood arthritides according to the pattern of involvement, associated symptoms, and genetic factors. The classification criteria for juvenile idiopathic arthritis (JIA) proposed by the International League of Associations for Rheumatology is the most current,1 and is followed in this chapter. Based on the number of inflamed joints, laboratory features, family history, and extra-articular features, seven subtypes are recognized: oligoarticular arthritis, rheumatoid factor (RF)-positive polyarthritis, RF-negative polyarthritis, systemic onset JIA, enthesitis-related arthritis (ERA), psoriatic arthritis, or undifferentiated arthritis (Table 150-1).

TABLE 150-1Clinical Features of Juvenile Idiopathic Arthritis

The diagnosis of JIA is clinical, defined as joint swelling or limitation of joint range of motion with joint pain or tenderness. These signs must be present consistently for at least 6 weeks in a patient younger than age 16 years. The annual incidence of JIA is estimated at 10 per 100,000 children, and the prevalence of JIA in the United States is about 1 in 1000 children younger than 16 years.2 Estimates of the actual number of American children with JIA vary between 70,000 and 100,000, including both active and inactive cases.

*Supported by The National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01-AR060893), The Arthritis Foundation, and the Marcus Foundation Inc., Atlanta, GA.

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