JUVENILE IDIOPATHIC ARTHRITIS
Juvenile idiopathic arthritis (JIA; juvenile rheumatoid arthritis) encompasses a group of disorders that are clinically characterized by a chronic synovitis of unknown origin that begins when the patient is less than 16 years of age, and persists for more than 6 weeks. Subgroups of JIA include pauciarticular (4 or fewer joints involved), polyarticular (5 or more joints), and systemic-onset varieties. Synovial inflammation in these patients stimulates synovial proliferation and joint effusion. Pannus formation in the synovium can lead to erosion of cartilage and bone.1
Pauciarticular Juvenile Idiopathic Arthritis
Pauciarticular arthritis is the most common form of JIA. These patients have involvement of 4 or fewer joints during the first 6 months of clinical disease; additional joints may become involved subsequently. The major areas of involvement include the knee, ankle, elbow, and wrist. The distribution is often asymmetric. There are 2 subtypes of pauciarticular disease.
Pauciarticular JIA type I accounts for approximately 35% to 40% of patients with JIA. Approximately 80% of these patients are girls. The clinical onset is usually prior to the age of 4 or 5 years. Many patients with this subtype are seronegative for rheumatoid factor, but 90% have positive tests for antinuclear antibodies. The hips and sacroiliac joints are not affected in these patients. About one-quarter of children with pauciarticular disease have iridocyclitis or anterior uveitis.
Pauciarticular disease type II involves approximately 10% to 15% of children with JIA. This is predominantly a disease of boys, with a clinical onset that is usually after the age of 8 years. Serological tests for rheumatoid factor and antinuclear antibodies are usually negative; approximately 75% of these patients have the histocompatibility antigen HLA-B27. This disorder predominantly involves large joints; the hips and sacroiliac joints are usually affected. Some patients develop manifestations of ankylosing spondylitis.
Polyarticular Juvenile Idiopathic Arthritis
Polyarticular JIA is characterized by involvement of multiple joints (i.e., 5 or more). There are 2 major subtypes of polyarticular disease: rheumatoid factor–negative polyarthritis, which accounts for 20% to 25% of cases of JIA, and rheumatoid factor–positive polyarthritis, which accounts for 5% to 20% of cases. Girls are affected more frequently than boys with both subtypes of polyarticular JIA.
Rheumatoid factor–negative polyarticular arthritis may present at any time during childhood. The clinical course is relatively mild, although up to 15% of these patients have severe clinical manifestations. Rheumatoid nodules are rare with this form of polyarthritis. Both large and small joints are usually involved, often with a symmetric distribution. The onset of the rheumatoid factor–positive polyarticular disease is most often late during childhood. There is a frequent association with the HLA DR4 histocompatibility antigen. About half of these patients have a clinically severe form of arthritis. Subcutaneous rheumatoid nodules are common.
Children with polyarticular JIA may experience an insidious onset, with joint stiffness, swelling, ...