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, and diminished range of motion. Fulminant onset of disease is less common. The affected joints are swollen and warm, but are usually not erythematous. Joint tenderness and pain are usually mild. The initial manifestations are most often in the large joints, such as the knees, ankles, wrists, and elbows. The proximal interphalangeal joints are the most common sites of involvement in the hands, sometimes leading to a fusiform deformity. Metacarpophalangeal joint involvement is also common. About half of patients with polyarticular disease have involvement of the cervical spine, which is clinically characterized by neck stiffness. Temporomandibular joint involvement is common. Hip joint involvement occurs in more than half of children with polyarticular JIA. The sacroiliac joints are usually spared.

Systemic-Onset Juvenile Idiopathic Arthritis

Systemic-onset JIA (Still disease) accounts for 10% to 20% of cases. Males and females are affected approximately equally. Although this type is identified in children of all ages, onset prior to the age of 5 years is most common. These patients have prominent extra-articular manifestations of disease, including daily spiking fever, rheumatoid rash, serositis, lymphadenopathy, and hepatosplenomegaly. The rheumatoid rash has an evanescent, recurrent character. There are small, pale, erythematous macules that often have central pallor.

Approximately one-third of children with systemic-onset JIA develop pleuritis, pericarditis, or peritonitis. Symptomatic joint disease occurs at presentation or within a few months of the onset of systemic symptoms. Only a few joints may be involved at the onset, but the arthritis tends to be progressive. Up to half of these children suffer a chronic destructive arthritis. Common sites of involvement include the knees, wrists, ankles, cervical spine, and hips.

Imaging

The major site of pathology in JIA is the synovium. Inflammatory cells infiltrate the synovium. Synovial proliferation occurs. There is reactive secretion of increased quantities of synovial fluid. Lymphoid follicles and granulation tissue may form, leading to focal synovial thickening and pannus formation. There is secondary destruction of cartilage. Pannus may erode into adjacent bone. The synovial coverings of tendons and bursae may also be inflamed.

The major radiographic features of JIA consist of osteopenia, joint effusion, periarticular soft tissue swelling, joint space narrowing, and erosions (Table 60-1; Figures 60-1, 60-2, and 60-3). Other potential findings include periostitis in the periarticular regions of the phalanges, metacarpals, and metatarsals, epiphyseal overgrowth, premature physeal closure, leg length discrepancy, short broad phalanges and metacarpals, temporomandibular joint hypoplasia, synovial cysts, and joint malalignment. Subchondral synovial cysts are common. Popliteal synovial cysts can occur.

The earliest radiographic finding of JIA is articular and periarticular soft tissue prominence, due to synovial thickening, a joint effusion, or both (Figure 60-4). Periarticular osteoporosis is a prominent and early radiographic manifestation of JIA, and is helpful in the differentiation from many other forms of arthritis (Figure 60-5). The periarticular demineralization is apparently related to hyperemia in response to the synovial inflammation. Generalized osteopenia can also occur, due to disuse or steroid therapy. Radiolucent metaphyseal bands are occasional findings, as in patients with other systemic diseases. Growth arrest lines may develop, and sometimes coexist with metaphyseal radiolucent bands. Manifestations of periostitis along the diaphyses of small tubular bones adjacent to affected joints are frequent early radiographic findings. Chronic inflammation and hyperemia may lead to overgrowth of the epiphyses and acceleration of skeletal maturation (Figure 60-6). In the wrist, carpal bone undergrowth is common in children with JIA.