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What is it? Juvenile idiopathic arthritis (JIA) is a chronic disease characterized by persistent joint inflammation; the typical signs of joint inflammation are pain, swelling and limitation of movement. “Idiopathic” means that we don’t know the cause of the disease and “juvenile”, in this case, means that symptoms appear before 16 years of age.
What does chronic disease mean? A disease is said to be chronic when the appropriate treatment does not lead to an immediate recovery, but only to an improvement of symptoms and laboratory test results. This also means that when the diagnosis is made, it is impossible to say for how long the child is going to be sick.
How frequent is it? JIA is a rare disease that affects about 80-90 per 100,000 children.
What are the causes of the disease? Our immune system protects us from infections (virus and bacteria). In doing so, it is able to distinguish what is harmless and part of our body and what is foreign and potentially dangerous, which it destroys. It is believed that chronic arthritis is a consequence of an abnormal response of our immune system, which, due to unknown causes, loses part of its capacity to distinguish between dangerous and normal cells and attacks its own joint components. For this reason, diseases such as JIA are called autoimmune, meaning that the immune system reacts against the organs of its own body. However, the precise mechanisms that cause JIA, as with as most human chronic inflammatory diseases, are unknown.
Is it a hereditary disease? JIA is not a hereditary disease, since it cannot be transmitted directly from parents to their children. Nevertheless, there are some genetic factors, largely still to be discovered, that increase the chance of developing the disease. The agreement in the scientific community is that this disease is multifactorial, which means it is the result of a combination of genetic factors and exposure to environmental factors (probably infections). Even when there may be a genetic predisposition, it is very rare to have two children affected in the same family.
How is it diagnosed? Doctors diagnose someone as having JIA when the onset of the disease is before the age of 16, arthritis lasts for more than six weeks and the causes are unknown (which means that all other diseases responsible for arthritis have been ruled out). The arthritis must be present for more than six weeks in order to exclude forms of temporary arthritis that may follow viral infections. The diagnosis of JIA is, therefore, based on the presence and persistence of arthritis and the careful exclusion of any other disease by medical history, physical examination and laboratory tests.
What happens to the joints? The synovial membrane is the cellular lining surrounding the joint and is usually very thin. In JIA it becomes much thicker and filled with inflammatory cells, while the amount of the synovial fluid inside it increases. This causes swelling, pain and limitation of movement. A characteristic feature of joint inflammation is joint stiffness, which occurs after prolonged rest. It is, therefore, particularly pronounced in the morning (and referred to as morning stiffness). Often the child attempts to reduce pain by keeping the joint in a position half-way between flexion (fully bent) and extension (straight), this position is called antalgic indicating the fact that it is maintained to reduce pain. If not properly treated, joint inflammation may produce damage by two main mechanisms: a) The synovial membrane may get very thick and form what is called the synovial pannus, which, through the release of various substances, provokes the erosion of articular cartilage and bone. b) Keeping the joint in the antalgic position for a long time causes muscle atrophy, which is the wasting away of muscles and soft tissues, leading to flexion deformity.
Are there different types of the disease? There are several different forms of JIA. They are mainly distinguished on the presence or absence of systemic symptoms. Systemic symptoms are symptoms that affect many organs, such as fever or rash and on the number of joints involved. By convention, the different forms of JIA are defined according to the symptoms presented during the first six months of the disease. For this reason, they are often referred as onset forms.
Systemic JIA. This form is diagnosed because of the presence of systemic features, besides arthritis. The main systemic symptom is represented by high spiking fever, often accompanied by a salmon coloured rash that appears during fever spikes. Other symptoms may include muscle pain, enlargement of the liver, spleen or lymph nodes (groups of cells that filter out bacteria etc., as a critical part of the immune system), and inflammation of membranes around the heart (pericarditis) and lungs (pleuritis). Arthritis may be present at disease onset, or appear later on. The disease may affect children at any age. About half of all patients are diagnosed with systemic features. These patients tend to have the best long-term prognosis (predicted outcome). In the other half of patients, systemic symptoms often tend to subside with time and joint involvement becomes more important. In a minority of these patients, systemic symptoms persist together with joint involvement. Systemic JIA accounts for less then 10% of all JIA cases, but is seldom observed in adults.
Polyarticular JIA. This is diagnosed because of the involvement of five or more joints during the first six months of disease and in the absence of the above mentioned systemic symptoms. The presence or absence of an autoantibody in the blood called rheumatoid factor (RF) allows for polyarticular JIA to be distinguished into two subforms: RF negative and RF positive. 1) RF positive polyarticular JIA. This is rare in children (<5% of all JIA patients). It is considered the equivalent of adult RF positive rheumatoid arthritis (the major type of chronic arthritis in adults). It often causes symmetric arthritis affecting mainly the small joints of hands and feet, initially, extending to the other joints as disease progresses. It is much more common in females than in males and usually has its onset after 10 years of age. It is often a severe form of arthritis. 2) RF negative polyarticular JIA. This accounts for 15-20% of all JIA cases and can occur at any age. It is a complex form, which probably includes different diseases. The variable course and eventual outcome of the disease in different patients reflect this complexity.