What causes it?

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Arthritis is triggered by an abnormal response of the immune system which leads to swelling, inflammation and pain in the lining of one or several joints. The immune system normally makes antibodies to attack bacteria, viruses, and other germs. In people with autoimmune diseases, the immune system makes antibodies against tissues of the body. It is not clear why this happens, but some people have a tendency to develop autoimmune diseases.

In some cases of arthritis there may be a family connection. Children should be able to move freely with mild arthritis but with severe forms mobility will be affected. Bacterial infections and viruses such as mumps, rubella, herpes and Lyme disease, particularly in under two's age group, can be a trigger.

Polyarthritis can be caused by a variety of things, including Crohn's disease (inflammatory bowel disease), inflammatory blood vessel disease, blood disorders such as sickle cell disease or haemophilia, cystic fibrosis and connective tissue disorders often because of immune system problems (e.g. ankylosing spondylitis, erythematosus and systemic lupus).

With rheumatoid arthritis, antibodies are formed against the synovium (the tissue that surrounds each joint). This causes inflammation in and around affected joints. Over time, the inflammation can cause damage to the joint, the cartilage, and parts of the bone near to the joint.

How is arthritis diagnosed?

If your child complains of pain in their joints, or if you're concerned that they have other symptoms of juvenile arthritis, see your GP as soon as possible.

If your GP suspects that your child may have joint problems your child will probably be referred for X-rays or bone scans of the affected joints and blood tests that can check for antibodies associated with this condition. Fluid may be taken from the joint to check for infection.