Juvenile rheumatoid arthritis (JRA) is a form of arthritis that affects children, usually before the age of 16. JRA involves inflammation and stiffness in a joint that lasts for more than six weeks. The joint may be red, swollen and sore, although the child may not complain of joint pain.

JRA is the most common form of juvenile arthritis. It may affect one joint or many joints. Like the rheumatoid arthritis that affects adults, JRA is a systemic disease and may also affect other systems in the body. Other than the joints, the eyes are the most common area affected. Some forms of JRA may also affect the skin and other organs such as the heart, lungs, liver and spleen.

An estimated 11,700 to 69,000 American children have JRA, according to the Centers for Disease Control and Prevention (CDC). JRA usually does not develop in infancy but can develop any other time in childhood. Certain types more commonly develop among 6- to 8-year-olds, whereas others occur in teenagers.

Juvenile rheumatoid arthritis affects the joints, where two or more bones meet. There are several types of joints. Arthritis occurs mostly in synovial joints, which can move, such as those in the hands, wrists, ankles, knees and feet. For each synovial joint, a space enclosed by the ligaments and adjoining bones forms a cavity called the joint capsule. The outer layer of the capsule is formed by a fibrous membrane.

The inside of the capsule is lined with a membrane called the synovium. This membrane secretes synovial fluid, which fills the joint capsule and provides lubrication. The ends of the bones encased in the capsule are cushioned in soft cartilage. The cartilage and synovial fluid permit the bones to move without rubbing against each other.

JRA is an autoimmune condition, which means patients have an abnormal immune response. Normally, the immune system protects the body from outside invaders, such as germs. Immune cells, such as white blood cells, attack these invaders and flush them out or make them inactive. Part of this process normally produces some inflammation in tissue.

For someone with autoimmune response, the immune system misidentifies normal body tissue as an outside invader. It attacks the tissue and tries to destroy it. In JRA, certain types of white blood cells attack parts of the synovium, causing the inflammation that characterizes JRA. The synovium then thickens, which causes the joint to swell. The synovium can form a body called a pannus, which has granular tissue that covers the bone and cartilage. The pannus tissue reacts with proteins called enzymes and erodes the bone surface.

The specific cause of JRA is not well understood. Although the immune system attacks the tissue in the joints, no one knows what triggers such an attack. Some researchers think that a genetic component is involved that makes a child more susceptible to developing JRA, but that some outside factor (e.g., viral infection) or a combination of factors trigger the initial inflammation.

JRA develops in several forms, which may affect a few joints in the body or many joints. Joints commonly affected by JRA include:

• Hips, knees and ankles
• Shoulders, elbows, wrists and hands
• Neck
• Lower spine
• Jaw

All forms of arthritis may cause joint pain and swelling. Such joint changes in JRA may affect the growth and proper development of bones in children.

Many children with JRA also experience inflammations in the eye. Some of the inflammations involve the uveal tract, the middle layer of the eye that includes the iris, ciliary body and choroid. Some of these inflammations may not have symptoms but can cause vision problems if left untreated. Some children also develop recurrent rashes. The most severe cases of JRA can affect the heart, lungs, spleen, liver and lymph nodes.

The course of JRA may vary. Some children may have mild joint pain for a few months or a year. For others, the symptoms are recurrent over years and may affect their activity levels and result in frequent absences from school. Children may be reluctant to exercise or participate in activities because of pain.

Most children eventually outgrow JRA and are not affected by it as adults. Some patients may experience joint problems as adults, especially if their bones were affected during a growth period. If the inflammation of joints is not stopped, it can destroy the inflamed joints. About 75 percent of children with JRA enter remission eventually with a minimal loss of function and deformity, according to the National Institutes of Health. Other cases of JRA may actually be the adult form of rheumatoid arthritis starting at an early age.