OVERVIEW

What is juvenile idiopathic arthritis?

Juvenile idiopathic arthritis (JIA) is the most common rheumatic disease in children and one of the most prevalent chronic illnesses in pediatric populations.

It refers to a group of chronic arthritis conditions with onset before the age of 16, which can cause symptoms such as joint stiffness, swelling, and pain in children.

Before the late 1990s, JIA was also referred to by older terms like juvenile rheumatoid arthritis (JRA, commonly used in the U.S.) and juvenile chronic arthritis (JCR, commonly used in Europe).

It must be noted that the naming and classification of JIA are provisional. As our understanding of chronic arthritis in children deepens, significant changes to its classification and terminology may occur.

Is juvenile idiopathic arthritis common?

Current research suggests the annual incidence of JIA ranges from 0.8 to 23 per 100,000, while its prevalence ranges from 3 to 401 per 100,000.

Asia has a relatively lower incidence, with data from Japan showing an annual incidence of 0.8 per 100,000, whereas Europe and the U.S. typically report rates above 3 per 100,000. Unfortunately, reliable data for China is currently lacking.

What are the types of juvenile idiopathic arthritis?

According to the International League of Associations for Rheumatology (ILAR) criteria, JIA can be classified into the following major categories:

• Systemic juvenile idiopathic arthritis (sJIA): Previously known as Still's disease, systemic-onset juvenile rheumatoid arthritis, or subacute septicemia, these outdated terms should no longer be used.
• Polyarticular juvenile idiopathic arthritis (pJIA): Defined as JIA involving more than five joints within the first six months of onset. pJIA can be further subdivided based on the presence or absence of rheumatoid factor.
• Oligoarticular juvenile idiopathic arthritis (oJIA): Defined as JIA involving fewer than five joints. oJIA can be categorized as persistent or extended based on disease progression within six months.
• Enthesitis-related arthritis (ERA): Although ERA is associated with juvenile spondyloarthritis, it excludes reactive arthritis and inflammatory bowel disease-related arthritis. Psoriatic arthritis is also classified separately, making ERA the most controversial category. For details, refer to the section on juvenile spondyloarthritis.
• Psoriatic arthritis (PsA): For specifics, refer to the section on pediatric psoriatic arthritis.
• Undifferentiated arthritis: Refers to JIA cases that cannot be classified or fit into two or more of the above categories.

Since different JIA subtypes have distinct clinical features, disease progression, and treatment approaches, this section provides only a brief overview. Detailed discussions are covered in other dedicated entries.

SYMPTOMS

What are the manifestations of systemic juvenile idiopathic arthritis?

International data indicate that systemic juvenile idiopathic arthritis, abbreviated as sJIA, primarily occurs before the age of 5, with a peak around 2 years old. There is no gender difference in the incidence of this disease. However, research on hospitalized patients in China shows that the peak onset occurs between 5 and 7 years old, with males slightly outnumbering females.

sJIA mainly manifests as fever, rash, joint pain, hepatosplenomegaly, lymphadenopathy, and serous cavity effusion. Approximately 5%–10% of sJIA patients develop life-threatening macrophage activation syndrome (MAS), while another 30% experience milder subclinical MAS.

What are the manifestations of polyarticular juvenile idiopathic arthritis?

Polyarticular juvenile idiopathic arthritis, abbreviated as pJIA, has a bimodal age distribution, with the first peak between 2–5 years old and the second between 10–14 years old. Girls are more affected than boys.

In children under 10 years old, pJIA resembles oligoarticular juvenile idiopathic arthritis, initially involving only 1–2 joints, then progressing insidiously to affect at least 5 joints within 6 months of onset. The affected joints are usually symmetrical.

In children and adolescents over 10 years old, multiple joint inflammation typically develops rapidly, involving small joints of the hands and feet within 2–3 months of onset.

Additionally, pJIA may present with uveitis, though its incidence is lower than in oligoarticular juvenile idiopathic arthritis.

Blood tests for pJIA patients often reveal: positive antinuclear antibody (ANA), elevated erythrocyte sedimentation rate (ESR) (≥ 40 mm/h), hemoglobin concentration ≤ 110 g/L (anemia), and elevated gamma globulin.

What are the subtypes of oligoarticular juvenile idiopathic arthritis?

Oligoarticular juvenile idiopathic arthritis, abbreviated as oJIA, affects fewer than 5 joints and can be further classified into:

• Persistent type: No additional joint involvement after 6 months of onset.
• Extended type: Additional joint involvement after 6 months, ultimately affecting ≥ 5 joints. About 50% of oJIA cases progress to the extended type.

What are the manifestations of oligoarticular juvenile idiopathic arthritis?

The peak onset of oligoarticular juvenile idiopathic arthritis, abbreviated as oJIA, occurs between 1–3 years old, becoming rare after 5 years old and extremely uncommon after 10 years old. Girls are more frequently affected.

Key manifestations of oJIA include:

• Limping without discomfort. Families often notice the child walking "awkwardly" in the morning but returning to normal shortly after. The child rarely reports pain, and medical attention is sought primarily due to knee swelling. Families usually cannot pinpoint the exact onset time.
• oJIA mainly affects the knees and ankles, occasionally the wrists and elbows, but rarely the hips.
• oJIA typically lacks fever, rash, or other systemic symptoms.
• Long-term complications of oJIA include uveitis and leg length discrepancy, the latter being the most common and severe.
• Blood tests for oJIA patients often show positive antinuclear antibody (ANA), which correlates with uveitis.

What are the manifestations of enthesitis-related arthritis?

Enthesitis-related arthritis, abbreviated as ERA, often begins in late childhood, with girls more commonly affected. ERA most frequently involves joints in the legs and back.

What are the manifestations of psoriatic arthritis?

Psoriatic arthritis, abbreviated as PsA, rarely occurs in infants under 1 year old, with girls more affected than boys.

PsA can involve one or multiple joints. Affected children may exhibit "sausage-like" swelling of fingers or toes, accompanied by fever and rash.

CAUSES

What is the cause of systemic juvenile idiopathic arthritis?

Systemic juvenile idiopathic arthritis, abbreviated as sJIA, is actually an autoinflammatory disease related to immune dysregulation. It primarily involves abnormal hyperactivity of interleukin-1, interleukin-6, interleukin-18, neutrophils, and monocytes/macrophages.

DIAGNOSIS

What are the inclusion criteria for juvenile idiopathic arthritis?

All of the following criteria must be met simultaneously:

• Age of onset < 16 years.
• Disease duration ≥ 6 weeks.
• Joint swelling, pain; or pain/tenderness with limited joint movement.
• Exclusion of other diseases, such as infections, hereditary connective tissue disorders, inherited metabolic diseases, etc.

TREATMENT

How to treat systemic juvenile idiopathic arthritis?

For systemic juvenile idiopathic arthritis (sJIA), mild cases may be treated with nonsteroidal anti-inflammatory drugs (NSAIDs, excluding aspirin).

However, most patients require stronger treatment. When economically feasible, interleukin-1 or interleukin-6 inhibitors are preferred over glucocorticoids due to the latter's suboptimal efficacy and significant side effects.

For critically ill patients, a combination of interleukin-6 inhibitors and glucocorticoids is necessary. In some cases, methotrexate, tumor necrosis factor-alpha (TNF-α) inhibitors, or T-cell co-stimulation blockers may also be added to manage chronic arthritis.

How to treat polyarticular juvenile idiopathic arthritis?

For polyarticular juvenile idiopathic arthritis (pJIA), treatment plans should be tailored based on disease activity and adjusted according to therapeutic response. Methotrexate is typically the primary treatment, sometimes combined with TNF-α inhibitors.

Patients who test positive for rheumatoid factor or anti-cyclic citrullinated peptide (CCP) antibodies may develop rheumatoid arthritis in the future.

How to treat oligoarticular juvenile idiopathic arthritis?

For oligoarticular juvenile idiopathic arthritis (oJIA), first-line treatments include oral NSAIDs (excluding aspirin) and intra-articular glucocorticoid injections.

However, patients with ≥5 affected joints or uveitis should start methotrexate. If response is inadequate, TNF-α inhibitors may be added.

Ophthalmologic slit-lamp examinations every 6 months are essential to screen for uveitis. Anti-nuclear antibody-positive patients require more frequent screening (every 3 months). This applies to all JIA subtypes except sJIA and significantly reduces blindness or vision impairment risks.

DIET & LIFESTYLE

What should patients with juvenile idiopathic arthritis pay attention to in daily life?

Patients with different types of juvenile idiopathic arthritis (JIA) may have varying disease outcomes, but they share similar lifestyle considerations. For example:

• Get vaccinated proactively to reduce infection risks associated with treatment.
• Wash hands frequently and maintain personal hygiene.
• Exercise regularly to improve physical fitness.

PREVENTION

Can Juvenile Idiopathic Arthritis Be Prevented?

There is no effective prevention method.