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1. I. Background

I. Background

• There is much disagreement among experts about definitions of childhood arthritis. At least three clinical classification schemes exist—juvenile rheumatoid arthritis (JRA), juvenile chronic arthritis (JCA), and juvenile idiopathic arthritis (JIA). All three schemes do not include many of the conditions considered as arthritis and other rheumatic conditions in adults. Also, a case counted in one classification system may not be a case in another system; however, all schemes define childhood arthritis as occurring in people younger than 16 years.


• Because childhood arthritis is an umbrella term covering a number of types of arthritis and because there are a number of different clinical case definitions for childhood arthritis, there is a wide range of estimates of how much childhood arthritis exists and much difficulty in describing its epidemiology.


• The most common form of juvenile arthritis is JRA (the term and classification system used most commonly in the United States). JRA involves at least 6 weeks of persistent arthritis in a child younger than 16 years with no other type of childhood arthritis. JRA has three distinct subtypes: systemic (10%), polyarticular (40%) and pauciarticular (50%). Each type has a unique presentation and clinical course and immunogenetic association. For the latter two types, girls are more commonly affected (3–5:1). In all three types about 40–45% still have active disease after 10 years. For the systemic type, the peak age of onset is 1– to 6–years-old and about 50% of cases show very short stature in adulthood as a result. For the pauciarticular form, there are two distinct subtypes- early onset and late onset. Early onset is more common in girls, late onset is more common in boys. The genetics differ as do the clinical courses. In the polyarticular form, there are also two subtypes: rheumatoid factor (RF) positive and negative. RF positive usually affect girls with onset after 8 years of age and a poorer prognosis compared with RF negative children. (1)


• For adults, more than 150 conditions are counted as arthritis and other rheumatic conditions (AORC). Many of these conditions occur in children, although much more rarely.