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JIA classification

Juvenile idiopathic arthritis (JIA) is the commonest cause of chronic arthritis in children in many parts of the world.  In other countries (e.g., North America and North Europe) Lyme disease is common.

The incidence is 1 in 10,000 per year and prevalence is 1 in 1,000 - this is similar to epilepsy in children. 

The term JIA covers a group of diseases. It is a diagnosis of exclusion, with arthritis for six weeks or more, and the cause is unknown (likely a combination of genetic and environmental factors). JIA rarely is familial. JIA, by definition, requires onset of arthritis prior to a child's 16th birthday.

The classification of JIA is essentially based on onset and progression (especially the number of joints affected in the first 6 months), and the presence or absence of rheumatoid factor.  

There are several JIA subtypes, these are summarised in the Table below. This knowledge is useful as the JIA subtype broadly determines the management approach. 

The classification of Juvenile Idiopathic Arthritis, with clinical criteria
Characteristic
Clinical features
Age at onset
<16 years
Minimum duration
6 weeks
Subtypes
Systemic
Arthritis.
Fever, rash and can also have adenopathy, serositis, organomegaly.
Oligoarthritis
• 1–4 joints affected during the first 6 months.
Persistent – affects no more than four joints throughout course.
Extended – affects more than four joints after first 6 months.
 
Polyarthritis
Rheumatoid factor positive – affects five or more joints in first 6 months. Rheumatoid factor is positive on at least two occasions with tests taken at least 3 months apart.
Rheumatoid factor negative – affects five or more joints in first 6 months. Rheumatoid factor is negative.
 
Enthesitis-Related arthritis (sometimes called ERA)
Arthritis and enthesitis, or arthritis or enthesitis with at least two of the following:
•Sacroiliac joint tenderness.
•Inflammatory back pain.
•HLA-B27+.
•Family history of HLA-B27+ related disease.
Psoriatic arthritis
Arthritis and psoriasis or arthritis and at least two of:
•Dactylitis.
•Nail changes.
•Family history of psoriasis.
Undifferentiated
Arthritis not fulfilling the above categories or fulfilling more than one category.

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