JIA (juvenile idiopathic arthritis) in the Nordic countries. Aspects of classification, epidemiology and pain assessment

Abstract: Different classification criteria for chronic arthritis in childhood have been used in different parts of the world, which has made comparison between studies difficult. In 1995, a new set of classification criteria was proposed by the ILAR (International League of Associations for Rheumatism) to overcome such difficulties and to create more homogenous subgroups. A new name was proposed: juvenile idiopathic arthritis (JIA). This thesis took advantage of the homogeneity of the Nordic health care systems to study the epidemiology of chronic childhood arthritis in the Nordic countries and to evaluate the construct validity of the new ILAR criteria compared with the EULAR (European League Against Rheumatism) criteria, which were used in Europe previously. This prospective, population-based study had its origin in a group of Nordic pediatric rheumatologists and pediatricians and it was performed over an 18-month period, commencing on July 1st 1997. Twenty doctors collected data from the incidence cases in their area, collaborating continuously within the study group. A classification was made according to the EULAR and ILAR classification criteria and an evaluation was made of the differences in how the two sets of criteria work. Incidence figures according to both sets of criteria in the different catchment areas were studied as well as incidence of the subgroups of the disease. Heredity for psoriasis and its influence on the ILAR classification were also explored. In a separate study on how to measure and evaluate pain, 26 children and their parents were involved. The VAS (visual analogue scale) is the most commonly used pain scale as it is considered to be easy to use. But is also criticized for being difficult to interpret and an evaluation was made of VAS in comparison with the results obtained using a category scale. Use was also made of a statistical method designed for this type of study. The ILAR and EULAR criteria sets classify children differently and cannot replace each other. In the ILAR classification, 21% of the patients could not be classified into any specific subgroup, only into the "other arthritis" subgroup. The average annual incidence rate of JIA (ILAR criteria) in the Nordic countries was 15 (13-17) per 100,000 children under 16 years of age. It varied from 7 (95% confidence interval 1-13) to 23 (10-36) in different areas. The distribution of age of onset and of ANA-positivity was different for boys and girls, girls showing an early peak in incidence. Heredity for psoriasis has a strong influence on the current ILAR classification and 21% of the patients had heredity for psoriasis among first-degree relatives or second-degree relatives or both. In an assessment of pain, parents and children used the VAS and the VDS-4 differently and the results for VAS revealed considerable variation.Conclusions: The ILAR classification criteria are a promising instrument in the search for more biologically homogenous groups in chronic arthritis in childhood. Incidence figures in the Nordic countries were in accordance with earlier incidence figures and higher compared with other European countries. In the patient follow-up the results for VAS were difficult to interpret.