Juvenile Idiopathic Arthritis

Introduction

Juvenile idiopathic arthritis (JIA) is the commonest chronic inflammatory arthritis of childhood and one of the commonest chronic diseases of childhood overall. It is defined as arthritis of unknown cause with onset before the age of 16, persisting for at least 6 weeks, once infection, malignancy and other identifiable causes have been excluded.¹

JIA is not a single disease but a family of subtypes, classified by the International League of Associations for Rheumatology (ILAR), each with a distinct pattern of joint involvement, associated features, and - critically for exam purposes - a different risk profile for uveitis and other complications. Recognising which subtype fits a given presentation is the central skill this topic tests.

Classification and clinical features

Oligoarticular JIA

The commonest subtype, accounting for roughly half to two-thirds of cases. Defined by involvement of 4 or fewer joints in the first 6 months of disease, typically large joints (knee, ankle) and often asymmetrical. It predominantly affects young girls, frequently under the age of 6, and is often ANA-positive.

Polyarticular JIA (RF-negative and RF-positive)

Defined by involvement of 5 or more joints in the first 6 months.

Systemic JIA (Still's disease)

A distinct, systemic illness rather than primarily a joint disease at onset. The classic picture is quotidian (daily) spiking high fevers, often twice daily, accompanied by a salmon-pink, evanescent, macular rash that comes and goes with the fever, plus lymphadenopathy, hepatosplenomegaly and serositis. Arthritis may be absent at presentation and appear only later, which can make early diagnosis difficult.

Enthesitis-related arthritis (ERA)

The paediatric counterpart of the adult seronegative spondyloarthropathies, associated with HLA-B27, more common in older boys, and characterised by lower limb arthritis and enthesitis (Achilles tendon, plantar fascia). It can progress to axial involvement resembling juvenile ankylosing spondylitis.

Psoriatic JIA

Arthritis in a child with psoriasis, or arthritis plus at least two of dactylitis, nail pitting/onycholysis, or a first-degree relative with psoriasis - mirroring the adult condition covered in the psoriatic arthritis article.

Growth and skeletal complications

Chronic synovitis in a growing child causes problems not seen in adult arthritis: localised overgrowth from increased blood flow to an inflamed joint (for example, leg length discrepancy from unilateral knee disease), micrognathia from temporomandibular joint involvement, and generalised growth retardation from chronic inflammation and, where used, long-term corticosteroids.

Differential diagnosis

• Septic arthritis and osteomyelitis - acutely unwell child, single joint, needs urgent exclusion before assuming a chronic diagnosis
• Reactive arthritis - post-infective, usually self-limiting
• Leukaemia and other malignancy - can present with joint pain, especially at night, sometimes with disproportionate pain relative to examination findings, cytopenias, or bone pain
• Henoch-Schonlein purpura (IgA vasculitis) - arthralgia plus the characteristic purpuric rash and abdominal symptoms
• Rheumatic fever - migratory polyarthritis following streptococcal infection, now rare in the UK but still tested
• Non-accidental injury - always consider in an unexplained pattern of joint or limb symptoms in a young child
• Hypermobility - can cause joint pain without true synovitis or inflammatory markers

Investigations

JIA is a clinical diagnosis of exclusion - there is no single confirmatory test.

• FBC, CRP/ESR - may be normal, particularly in oligoarticular disease; markedly abnormal in systemic JIA
• ANA - does not diagnose JIA, but a positive result identifies the subgroup at highest risk of uveitis and guides screening frequency
• Rheumatoid factor and anti-CCP - used to subclassify polyarticular disease and predict a more aggressive course
• HLA-B27 - supports a diagnosis of enthesitis-related arthritis in the right clinical context
• Ferritin - markedly elevated in systemic JIA, and dramatically so in evolving macrophage activation syndrome
• Blood film +/- bone marrow examination - if there is any concern about leukaemia, particularly before starting corticosteroids
• Joint aspiration - if septic arthritis cannot be confidently excluded
• Plain radiographs, ultrasound or MRI - assess joint damage, synovitis and, in the growing skeleton, growth plate involvement

Management

Management follows a similar treat-to-target philosophy to adult inflammatory arthritis, adapted for the growing child and delivered through a paediatric rheumatology multidisciplinary team.²

• NSAIDs - first-line for symptom control in milder disease
• Intra-articular corticosteroid injection - highly effective, particularly in oligoarticular disease, and can be used as definitive treatment for a limited number of affected joints
• Methotrexate - first-line DMARD for polyarticular disease, or oligoarticular disease that fails to respond adequately to joint injections
• Biologic therapy - anti-TNF agents (etanercept, adalimumab) for polyarticular and enthesitis-related disease; anti-IL-1 (anakinra, canakinumab) and anti-IL-6 (tocilizumab) agents are particularly effective in systemic JIA, reflecting its distinct innate-immune-driven pathophysiology
• Systemic corticosteroids - for systemic JIA flares and, at high dose, for macrophage activation syndrome
• Physiotherapy and occupational therapy - maintain joint function and range of movement, and address growth-related complications
• Regular ophthalmology screening - slit-lamp examination on a schedule determined by subtype, ANA status, and age and duration of disease, since uveitis is so often silent

Complications

Uveitis, if missed, can cause synechiae, cataract, glaucoma and permanent visual loss. Uncontrolled arthritis causes joint damage, contractures and growth disturbance, including leg length discrepancy and micrognathia. Macrophage activation syndrome carries significant mortality if not recognised and treated promptly. As with adult inflammatory arthritis, cumulative corticosteroid exposure adds osteoporosis, growth suppression and metabolic risk on top of the disease itself.

Red flags

Prognosis

Outcomes vary considerably by subtype: oligoarticular disease often remits, particularly with prompt treatment, though a subset extends to involve more joints over time. RF-positive polyarticular disease and systemic JIA tend to run a more persistent, aggressive course and are more likely to continue into adulthood, sometimes as adult rheumatoid arthritis. Early diagnosis, treat-to-target DMARD and biologic therapy, and systematic uveitis screening have together transformed long-term outcomes for most children with JIA over the past two decades.