Incidence of juvenile idiopathic arthritis in children in Estonia: a prospective population-based study

Objective: To study the incidence rate of juvenile idiopathic arthritis (JIA) and its clinical subtypes in Estonia, to follow the course of the disease in newly diagnosed patients for 2 years, and to find the frequency of human leucocyte antigens (HLA) B27, DR1 and DR4 in JIA patients. Method: A population-based study involving prospective registration of new cases of JIA in 1998–2000 and their clinical follow-up for 2 years. Results: In 1998–2000, 162 new cases of JIA were diagnosed. The mean annual incidence rate of JIA was 21.7 per 100 000 children aged 0–15 years (22.9 in girls and 19.3 in boys). During the investigation period, the incidence rate rose 3.5-fold. Oligoarthritis was the most frequent subtype (mean annual incidence rate of 11.7 per 100 000), followed by seronegative polyarthritis (4.4 per 100 000). HLA-DR1, B27 and DR4 were found respectively in 44.4, 28.6 and 11.1% of cases in which the analysis was performed. In HLA-B27-positive patients, inflammation markers of blood remained at a high level for a longer period compared with HLA-B27-negative patients. Conclusions: This is the first population-based study on the epidemiology of juvenile arthritis in Estonia in which the new classification criteria defined by the International League of Associations for Rheumatology (ILAR) have been used. In addition to environmental factors, an increase in awareness among family doctors is a probable reason for the rise in incidence during the study period. HLA-B27 might have predictive value as a marker of chronicity of inflammation.