Juvenile rheumatoid arthritis (JRA) is divided into three categories, which are characterized by the number of joints and other body systems affected. These categories are:

• Pauciarticular JRA. This form involves four or fewer joints. It is the most common form of JRA and affects half of the children with the disease, according to the National Institutes of Health (NIH). Pauciarticular JRA usually involves the larger joints, such as the knee, ankle, wrist or shoulder. It usually occurs asymmetrically, affecting a joint on only one side of the body (e.g., one knee). Children with pauciarticular JRA frequently have eye inflammations.
  
  
• Polyarticular JRA. This form affects five or more joints and is more common in girls than boys. It involves the small joints in the fingers and hands, and usually occurs symmetrically, in the same joints on both sides of the body. The knees and hips are also commonly affected. Some children may have jaw pain, which can cause discomfort when chewing and may affect eating habits. Polyarticular JRA is more severe and may also include eye inflammations. It affects about 30 percent of children with JRA, according to the NIH. Some children with polyarticular JRA actually have adult rheumatoid arthritis that develops at an early age.
  
  
• Systemic JRA (Still's disease). Systemic JRA affects about 20 percent of the children with JRA, according to the NIH. It can affect many body systems and have other symptoms, such as fever and rash, that may appear several months before any joint pain. Systemic JRA can cause inflammation of the heart (pericarditis) and lungs (pleuritis) and can enlarge the liver, spleen or lymph nodes. Flare-ups may occur after a child has a viral infection, such as chicken pox. Systemic symptoms may disappear in adulthood, but the arthritis may persist. The development of systemic JRA later in life is known as adult-onset Still's disease.