Arthritis, Juvenile Idiopathic (Rheumatoid)
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Juvenile idiopathic arthritis (JIA) is defined as chronic synovial inflammation of unknown etiology in at least one joint, for at least 6 weeks. Age of onset must be <16 years old. It is classified as one of seven subtypes:
- Oligoarticular arthritis affects <5 joints during the first 6 months of the disease. Tends to involve large joints, especially the knee. Peak age of onset is 1 to 6 years. Up to 80% are antinuclear antibody (ANA)-positive.
- Persistent oligoarticular JIA remains in <5 joints after the 6th month of disease.
- Extended oligoarticular JIA spreads to involve five or more joints (cumulatively) after the 6th month of disease; has worse prognosis than persistent oligoarthritis
- Polyarticular JIA affects ≥5 joints. Can occur at any age; peak ages of onset are 1 to 4 years and 7 to 10 years; divided into two subtypes
- Rheumatoid factor–positive (RF+) polyarticular JIA is like adult-onset rheumatoid arthritis (RA) that occurs in a child; often quite aggressive
- Rheumatoid factor–negative (RF−) polyarticular JIA 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)
- Entheses (e.g., osteotendinous junctions, osteoligamentous junctions) are sites where tendons or ligaments attach to bone.
- ERA generally affects boys in late childhood or adolescence.
- Many are human leukocyte antigen-B27–positive.
- Psoriatic arthritis is associated with psoriasis. It often begins in a few joints and then becomes polyarticular. It often involves small joints of hands and feet, as well as knees. Dactylitis (inflammation of an entire digit with joint and tendon involvement) is seen in nearly 50% of patients.
- Undifferentiated arthritis is arthritis that does not fall into any of the other categories or falls into more than one of the above categories.
Incidence ranges from 1 to 23/100,000 per year.
- Prevalence ranges from 3.8 to 400/100,000; varies but is thought to be ∼1/1,000
- Affects >100,000 children in the United States
- Girls are affected twice as often as boys, but boys are affected more frequently with ERA.
- ~50% of children with JIA have the oligoarticular type.
- 30% have the polyarticular type.
- 10% have systemic-onset JIA.
- Rare in siblings, but many studies have demonstrated increased frequencies of various human leukocyte antigen markers in JIA
- Each marker may be associated with a different subtype of JIA:
- Human leukocyte antigen-DR4: RF+ polyarticular JIA
- Human leukocyte antigen-DR1: oligoarticular disease without uveitis
- Human leukocyte antigen-DR5: oligoarticular JIA with uveitis
- Human leukocyte antigen-B27: ERA
- Human leukocyte antigen-A2: early-onset oligoarticular JIA
- Etiology is multifactorial and likely differs between onset types. Other susceptibility loci in immune response genes, both adaptive and innate, have been reported. Environmental (including infectious) and hormonal factors can also play a role in pathogenesis.
- Activated T cells and macrophages produce synovitis.