ERA is defined by the International League Against Rheumatism as arthritis and enthesitis of at least 6 weeks’ duration in a child younger than 16 years, or arthritis or enthesitis plus two of the following: sacroiliac tenderness or inflammatory spinal pain, HLA-B27 positivity, onset of arthritis in a male older than 6 years, and family history of HLA-B27-associated disease.7 Common clinical manifestations of ERA include arthritis, enthesitis, and acute anterior uveitis. In the first 6 months of disease, oligoarticular (in four or fewer joints) disease is most common. The most commonly affected joints at diagnosis are the sacroiliacs, knees, ankles, and hips.6 The small joints of the feet and toes are also commonly involved.8 Midfoot joint inflammation, or tarsitis, is highly suggestive of the diagnosis.
Axial disease is common in children with established ERA. By 2, 4, and 5 years after disease onset, as many as 15%, 53%, and 92%, respectively, of children with juvenile spondyloarthropathy (SpA), a condition that encompasses most children with ERA, develop symptomatic sacroiliitis.9 Up to 35%–48% of children with ERA have clinical or radiographic evidence of sacroiliitis (Figure 1A).6,10–12 Untreated sacroiliitis may progress to spondylitis, which is a condition characterized by radiographic findings of corner lesions (Figure 1B), erosions, syndesmophytes, diskitis, and ankylosis or fusion of the axial joints. Once sacroiliitis has developed and changes are visible on radiographs, treatment can be problematic. Sacroiliitis may not respond to medications commonly used as first-line agents for JIA, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or disease-modifying antirheumatic drugs (DMARDs), and the disease may continue to progress along the axial skeleton. A subset of children with sacroiliitis will progress to spondylitis as adults, which is characterized by back pain, stiffness, and eventual fusion of the vertebra.
