Chronic synovial inflammation of unknown etiology in at least 1 joint, for at least 6 weeks. Age of onset must be <16 years old. In 1997, a new classification system was introduced to classify JIA into 7 subtypes:

• Oligoarticular arthritis affects <5 joints during the 1st 6 months of the disease. Tends to involve large joints, especially the knee. Peak age of onset is 1–6 years; 80% are antinuclear antibody (ANA) positive.
  • Persistent oligoarticular JIA remains in <5 joints.
  • Extended oligoarticular JIA spreads to involve 5 or more joints. Has worse prognosis than persistent oligoarthritis.
• Polyarticular juvenile idiopathic arthritis affects ≥5 joints. Can occur at any age: peak ages of onset are 1–4 and 7–10 years.
  • Rheumatoid factor positive (RF+) polyarticular juvenile idiopathic arthritis is like adult-onset idiopathic arthritis that occurs in a child. It is often quite aggressive.
  • Rheumatoid factor negative (RF-) polyarticular juvenile idiopathic arthritis is usually less aggressive and easier to control.
• Systemic-onset idiopathic juvenile arthritis:
  • Characterized by high, spiking quotidian or diquotidian fevers and an evanescent pink/salmon-colored macular rash
  • Affected children may also have lymphadenopathy, hepatosplenomegaly, pericarditis, or pleuritis.
  • Arthritis may not appear until weeks to months after the onset of the systemic symptoms.
  • Can occur at any age
• Enthesitis-related arthritis (ERA) generally affects boys, many of whom are human leukocyte antigen (HLA)-B27 positive, in late childhood or adolescence
• Psoriatic arthritis is associated with psoriasis. It often begins in a few joints and then becomes polyarticular. Often involves small joints of hands and feet, as well as knees. Dactylitis seen in nearly 50% patients.