More than 100 conditions can be classified as arthritis. Most of the forms that affect adults also affect children. The most common types of juvenile arthritis (JA) include:

• Juvenile rheumatoid arthritis (JRA). This is the most common form of arthritis that affects children. JRA may affect a few joints, many joints or the entire system. Children with JRA may have complications that involve inflammation in the eyes, heart, lungs and spleen. There are three categories of this disease:
  
  
• • Systemic JRA. Involves joint swelling and pain in many areas of the body, as well as fevers, swollen lymph nodes and rash. Systemic JRA is also known as Still's disease. It can develop later in life as adult-onset Still's disease.
    
    
  • Polyarticular JRA. Involves five or more painful and swollen joints, and may develop into rheumatoid arthritis. This condition often affects the same joint on both sides of a child's body. This condition is more common in girls than in boys. It normally lasts for several years until symptoms gradually diminish.
    
    
  • Pauciarticular JRA. Affects four or fewer joints. This condition accounts for roughly half of the cases of JRA.
    
    
• Juvenile spondyloarthropathy. Arthritis that generally affects people over the age of 10. The condition is more common in boys than in girls. The joint inflammation common with this condition usually persists into adulthood and tends to affect the spine and legs. There are several types of spondyloarthropathy, including:
  
  
• • Juvenile ankylosing spondylitis. Arthritis that inflames the joints between the vertebrae in the back, joints in the pelvis and the areas where the bones connect to tendons and ligaments. Systemic complications may involve the heart valves, lungs and eyes.
    

• • Juvenile reactive arthritis. Arthritis that develops in reaction to an infection in the body. Many cases are reactions to bacterial infections in the gastrointestinal system. However, only a small percentage of children with these infections go on to develop reactive arthritis. A common type of juvenile reactive arthritis is juvenile Reiter's syndrome. Children with this condition experience inflammation in the joints (arthritis), urinary system (urethritis) and the lining of the eye (conjunctivitis).
    
    
  • Juvenile psoriatic arthritis. May include a rash of scaly patches on the skin and pitted nails and affects the large joints in the same way as ankylosing spondylitis. Some children may develop the arthritis symptoms before the psoriasis. Some may also have family members with psoriasis or with other forms of arthritis.
    
    
  • Juvenile enteropathic arthritis. Condition that involves arthritis in the spine and large joints (e.g., knees, hips) and inflammatory bowel diseases, such as Crohn's disease or ulcerative colitis. In some children, the arthritis symptoms may begin years before the bowel problems.
    
    
  • SEA syndrome (seronegativity, enthesopathy, arthropathy). A condition that is often diagnosed when a child has one of the juvenile spondyloarthropathies but the condition cannot be classified into a more clear-cut category.
    
    
• Juvenile dermatomyositis. Autoimmune condition that causes skin rashes, weak muscles near the trunk of the body and fever. This type of myositis may also cause swollen joints in some children.
  
  
• Juvenile scleroderma. A rare disease in children that involves hard skin and inflammation in the muscles, joints and tendons. A systemic form of the disease may involve the kidney, lungs or heart.

Other conditions can also cause arthritic pain in children, including lupus and Lyme disease.