Incidence of juvenile idiopathic arthritis in Finland, 2000-2020

Abstract

Objective: Previous epidemiological data of JIA in Finland are from the turn of the millennium. We aimed to determine the recent annual incidence of JIA in several consecutive years in Finland and to explore the differences in incidence between sexes, age groups and regions.

Methods: We analysed all children <16 years of age who met the ILAR classification criteria for JIA. Cases from 2000-2020 were identified from two national registers: the Care Register for Health Care of the Finnish Institute for Health and Welfare and the Reimbursement Register containing medication data from the Social Insurance Institution of Finland; cases from 2016-2020 were identified from the Finnish Rheumatology Quality Register.

Results: The incidence of JIA was 31.7 per 100 000 (95% CI 30.2, 33.1), according to the Care Register in 2000-2020 and peaked in 2010-2014. No considerable differences in incidence rates were observed among registers. In all age groups, incidence in girls was predominant compared with boys. The incidence in girls peaked at the ages of 2 years and 14-15 years. Decreasing incidence was observed among boys 0-3 years old during the entire study period, whereas increasing incidence was observed among teenage girls and boys 4-7 years old in 2000-2013.

Conclusion: The incidence of JIA is not only very high with respect to that in other parts of the world but also higher than previously reported in Finland. The incidence varied by region and year but was not higher at the end than the beginning of the study period.

Keywords: children; epidemiology; incidence; juvenile idiopathic arthritis; register study.

Figure 1.

Annual age-adjusted incidence of JIA among children in Finland. Annual age-adjusted incidence rates of JIA per 100 000 children <16 years of age among girls (A) and boys (B). Joinpoint analyses were performed to detect the best fit model for annual trends in incidence. The figure indicates changes in incidence rates during 2000–2020 in the Care Register of National Institute for Health and Welfare (Care Register) and the Reimbursement Register of the Social Insurance Institution (SII register). Segments (dotted lines) with a statistically significant annual percentage change (APC) are marked a–g

Figure 2.

Incidence of JIA in the indicated age groups by sex. Incidence rates in girls (A) and boys (B) per 100 000 children <16 years for different age groups in 2000–2020 in Finland. Joinpoint analyses were performed to detect the best fit model for age groups. The incidence was stable in boys (B) in all groups, whereas in girls (A), the incidence peaked at 2 years of age and also increased at 6–15 years of age. Segments (dotted lines) with statistically significant annual percentage change (APC) are marked a–d

Figure 3.

Incidence of JIA in the three study registers in eight hospital districts. Annual incidence rates of JIA from the Care Register of National Institute for Health and Welfare (Care register), the Reimbursement Register of the Social Insurance Institution of Finland (SII register), and the Finnish Rheumatology Quality Register (FinRheuma) in 2016–2020 in eight hospital districts in Finland

Figure 4.

Changes in the incidence of JIA in the indicated age groups. Incidence rates per 100 000 children <16 years of age among boys and girls in different age groups during the study period in the Care Register of National Institute for Health and Welfare. Age groups 0–3 years old (A), 4–7 years old (B), 8–11 years old (C) and 12–15 years old (D). Joinpoint analyses were performed to detect the best fit model for annual trends in incidence. The figure shows statistically significant changes among boys 0–3 years old (A) and 4–7 years old (B), and among girls 12–15 years old (D). Segments (green dotted lines for boys and orange for girls) with statistically significant annual percentage change (APC) are marked with a and b

Figure 5.

Regional differences in JIA incidence in Finland. Geographical distribution of the mean annual incidence rates of JIA per 100,000 children in all 21 Finnish hospital districts during the years 2000–2020. Data are from the Care Register of National Institute for Health and Welfare