About JIA

Juvenile Idiopathic Arthitis
Juvenile Idiopathic Arthritis (JIA) is the most common childhood rheumatic disease in the Western world, affecting approximately 1 per 1000 children overall. It is characterized by persistent inflammation of the joints for at least six weeks, with an onset prior to the age of 16 years. As JIA is a diagnosis of exclusion, other causes of joint inflammation (e.g. infections etc) should be excluded.

Multiple names have been associated with this illness, including Juvenile Chronic Arthritis (JCA) and Juvenile Rheumatoid Arthritis (JRA). Currently the disease is classified according to the International League of Associations for Rheumatology (ILAR) revised classification, referring to the disease as JIA. The primary aim for the reclassification of JIA was to define relatively homogeneous, exclusive subsets of arthritis for both prognostic and research purposes. This classification consists of 7 heterogeneous subgroups (table 1) based on predominantly clinical manifestations and laboratory features within the first 6 months of disease. The new classification includes systemic-onset, oligoarticular persitent and extended, polyarticular rheumatoid factor negative and positve subsets, psoriatic arthritis, enthesitis related arthritis and undifferentiated arthritis. All subsets have their own inclusion- and exclusion criteria (see table 1).

The present criteria rely on clinical and laboratory features but as the immunological basis of JIA becomes better understood classification may be defined by genetic predispositions for gene expression and cytokine profiles. Mononuclear cells in peripheral blood display different gene expression profiles among the different JIA subtypes, and further understanding of pathophysiology of disease may lead to more specific disease patterns and subsequent tailored treatment.

Table 1: ILAR criteria

JIA subtypes
Systemic Arthritis (sJIA)
This subtype affects girls and boys equally and can occur at any age but with a peak onset at age 2. Often the characteristical systemic features precede the arthritis in sJIA. Daily fever has to be present for at least 2 weeks, with at least three days of documented quotidian spikes (>39°C). Besides the fever, at least one or more criteria need to be present (see Table 1). The distribution of arthritis seen in sJIA includes both the small and large joints. Monoarthritis is uncommon and the disease course is usually polyarticular. Inflammatory markers such as ESR and CRP are often markedly elevated. Both uveitis and autoantibodies are not typically part of the features of sJIA, and, in contrast to the other JIA subtypes, cells and cytokines of the innate immune response are involved, suggesting that sJIA may be considered an autoinflammatory disease.
 
Oligoarticular JIA
Oligoarthritis affects girls more than boys. Age at onset distribution is characterized by a peak incidence between 1 and 2 years of age. Oligoarthritis accounts for 50-70% of all children with chronic arthritis. Oligoarthritis is defined by the involvement of four or less joints. If no more than four joints are involved during the total disease course it is classified as oligoarticular persistent JIA. If the total count of affected joints after the first six months exceeds four, the disease is defined as oligoarticular extended. Oligoarthritis tends to affect mainly the joints in the lower extremities, with approximately 50% of monoarthritis, usually of the knee. Uveitis may occur in up to 20% of these children as an extraarticular disease manifestation and is usually asymptomatic. Antinuclear antibody (ANA) testing is positive in 65-85% of children with oligoarthritis, particularly in girls with associated uveitis.  

Polyarticular JIA
Polyarthritis is defined as more than four joints affected within the first six months of onset. Two subtypes are recognized: polyarticular rheumatoid factor negative JIA and polyarticular rheumatoid factor positive disease. Polyarticular JIA accounts for approximately 20% of JIA, and approximately 85% of these children are rheumatoid factor negative. Polyarticular  RF negative JIA; boys are more frequently affected than girls; two phases in the age distribution are seen, with one peak at 1-3 years of age and another later in childhood and adolescence; knees, ankles and wrists are the most commonly involved joints early in the disease course although small joints can be involved at disease onset. The distribution of affected joints is more commonly asymmetric, compared to RF positive polyarthritis.  In RF positive polyarthritis girls outnumber the boys and the mean age of onset is 9-11 years.

Enthesitis Related Arthritis (ERA)
This is the only subset in which boys tend to be more frequently affected than girls. ERA involves mostly the lower extremities and eventually the axial skeleton and accounts for approximately 1-7% of all JIA cases. It is typically associated with HLA-B27 positivity. The onset can be of an insidious character with musculoskeletal pain and stiffness. Lower extremities are most commonly affected together with enthesitis at one or more sites around the knee or foot.

Juvenile Psoriatic Arthritis (jPsA)
This diagnosis is complicated by the fact that the diagnosis of psoriasis in a young child might be subtle, atypical and transient. Therefore in the new classification system patients do not always have to have classic psoriasis with arthritis. The reason for the link between psoriasis and arthritis is still unknown. JPsA has a biphasic age distribution with one peak during preschool years and a second one in late childhood. It is slightly more frequent in girls than boys. JPsA is clinically heterogeneous. The arthritis involves the wrists and small joints of the hands and feet, and progresses to polyarticular disease in the absence of treatment. The older children resemble adult Psoriatic Arthritis with a gender ratio closer to 1:1 and a tendency to enthesitis and axial skeleton involvement.