Juvenile idiopathic arthritis (JIA) is a heterogeneous condition with very few clinical and laboratory signs that can help predict the course and severity of the disease in the individual patient. The cell-surface antigen HLA-B27 is well known to be associated with spondyloarthropathies, reactive arthritis, and enthesitis. HLA-B27 plays an important role in the classification of JIA, since evidence of sacroiliitis most often evolves after years of arthritis in other joints. We investigated the associations of HLA-B27 and the clinical manifestations of JIA using a method as close to a population-based study as possible.
We studied an incidence-based cohort of 305 patients collected prospectively in 3 Nordic countries (Sweden, Norway, Denmark). Clinical and serological data of the first 3 years of the disease were collected.
HLA-B27 was found to be positive in 25.5% of the patients, and we found a higher proportion of HLA-B27-positive boys with older age at disease onset (p=0.034). Regression analysis showed a correlation of 0.7 in the HLA-B27-positive boys, pointing to a higher risk of more joint involvement with older age at disease onset. By Fisher's exact test, involvement of small joints in the lower extremities was associated with HLA-B27 in boys (p=0.011), but not in girls (p=0.687). HLA-B27 was associated with inflammatory back pain in both sexes (p=0.041 in boys, p=0.042 in girls), but with enthesitis only in boys (p<0.001 in boys, p=0.708 in girls).
HLA-B27 is of increasing importance with older age at disease onset in boys with JIA, predicting more active joints within the first 3 years of disease, and also involving small joints in the lower extremity to a greater degree than in HLA-B27-negative boys. During the first 3 years of disease the occurrence of HLA-B27 is associated with inflammatory back pain in both sexes, but with enthesitis only in boys. Our data present new challenges for the ILAR classification of JIA.