Incidence of juvenile idiopathic arthritis in the Nordic countries.: A population based study with special reference to the validity of the ILAR and EULAR criteria

Objective. To find the incidence of juvenile arthritis according to the ILAR and EULAR criteria within defined areas in the Nordic countries, and to study the validity of the ILAR and EULAR criteria from this perspective. Method. A longitudinal, prospective, population based study with patients enrolled according to the ILAR and EULAR criteria. Twenty doctors in Iceland, Norway, Sweden, Denmark, and Finland collected data from the incidence cases within their catchment areas over a period of 1.5 years, beginning July 1, 1997. Clinical and serological data from the first year of the disease were collected. Results. In the whole group of 315 patients, the incidence rate was 15 per 100,000 children/year (95% CI 13-17) according to the ILAR criteria, varying from 7 (1-13) in Iceland, 19 (7-31) and 23 (10-36) from 2 different regions in Norway, and 9 (5-12) and 16 (9-23) from 2 different areas in Denmark, to 15 (12-18) in Sweden and 21/100,000/year (15-26) in the Helsinki region in Finland. An early peak in distribution for age of onset was found in girls but not in boys. The number of antinuclear antibody (ANA) positive children in the whole group, made up of children who had undergone at least one analyzed ANA test, was 123/315 (39%). Girls were ANA positive in 83/197 (42%) and boys in 40/118 (34%). Uveitis developed in 27/315 (8.6%) children during the first 6 months of the disease. Conclusion. Incidence rates of juvenile arthritis for areas within the Nordic countries were in accord with previous data. The ILAR criteria present slightly higher incidence rates, with a shorter disease duration for inclusion, compared to the EULAR criteria. Patients in one subgroup in either of the criteria sets do not necessarily belong to the expected subgroup in the other set of criteria; e.g., for juvenile ankylosing spondylitis (EULAR) and enthesitis related arthritis (ILAR). Our epidemiological findings are a reminder to be aware of possible new subgroups in children with juvenile arthritis.