Long-term studies have shown that juvenile idiopathic arthritis (JIA) is not as benign as previously thought, with approximately 50% of adults suffering from persistent inflammation and disability.¹ Patients with erosions and disability tended to have received treatment later.² Trials in rheumatoid arthritis suggest a 1–2 year therapeutic window to limit joint damage, with optimal control of disease achieved with the use of disease modifying drugs during the first three months of onset.³ Within paediatrics, there has been a shift towards early aggressive treatment of JIA to limit inflammation and achieve a normal lifestyle, particularly in those with poor prognostic indicators (table 1⇓).^4–,⁶

DIAGNOSIS AND CLASSIFICATION OF JUVENILE IDIOPATHIC ARTHRITIS

JIA is defined as arthritis of unknown aetiology beginning before the 16th birthday and persisting for at least six weeks where other known conditions are excluded. The International League of Associations for Rheumatology (ILAR) classification of JIA was revised in 2001 and recently published.⁷ The aim of the ILAR classification is to define for research purposes mutually exclusive categories of idiopathic arthritis based on predominant clinical and laboratory features. Each category has a list of possible exclusions (table 2⇓).

EPIDEMIOLOGY AND AETIOLOGY

JIA has a UK incidence of 1 in 10 000 children and a prevalence of 1 in 1000, with a female predominance.⁸ Although described in all races and geographic areas, large regional variations in epidemiology exist with different distributions of patients among subsets of JIA, probably due to differences in human leucocyte antigen (HLA) alleles and environment.⁹

DIFFERENTIAL DIAGNOSIS