Systemic juvenile idiopathic arthritis

Systemic juvenile idiopathic arthritis (sJIA) is a distinct subtype of JIA characterised by arthritis accompanied by quotidian (daily spiking) fever lasting at least 2 weeks, along with characteristic systemic features including evanescent salmon-coloured rash, lymphadenopathy, hepatosplenomegaly, and serositis. sJIA accounts for approximately 10–15% of all JIA cases and is now recognised as an autoinflammatory disease rather than a classic autoimmune condition, driven by dysregulation of the innate immune system with central roles for IL-1β and IL-6. sJIA is distinguished from other JIA subtypes by the absence of ANA and RF, the potential for severe life-threatening complications including macrophage activation syndrome (MAS), and its distinct response to IL-1 and IL-6 inhibitors rather than conventional DMARDs.

sJIA is now classified alongside adult-onset Still disease (AOSD) as an autoinflammatory condition, sharing the same pathogenic mechanism of innate immune dysregulation. This distinguishes sJIA fundamentally from other JIA subtypes, which are T cell-mediated autoimmune diseases.

Innate Immune Dysregulation

• IL-1β overproduction — central cytokine driving the systemic features of sJIA; IL-1β is produced by activated macrophages and neutrophils through NLRP3 inflammasome activation; IL-1 inhibition (anakinra, canakinumab) is dramatically effective
• IL-6 dysregulation — drives the acute phase response (elevated ferritin, CRP, fibrinogen), fever, and anaemia of chronic disease; tocilizumab (anti-IL-6R) is highly effective for sJIA
• S100 proteins (S100A8/A9/A12) — alarmin proteins released by activated neutrophils and macrophages; highly elevated in sJIA; S100A12 is a more sensitive disease activity marker than ESR in sJIA
• Macrophage hyperactivation — pathological macrophage activation causes the life-threatening cytokine storm of MAS; IL-18 is markedly elevated and drives MAS pathophysiology; inhibiting IL-18 is an emerging therapeutic strategy

Macrophage Activation Syndrome (MAS) Pathophysiology

• MAS is a form of haemophagocytic lymphohistiocytosis (HLH) — uncontrolled macrophage and T cell activation causes cytokine storm with multi-organ failure
• Triggers — often precipitated by intercurrent infection (particularly EBV, CMV, parvovirus), disease flare, or medication change; can occur de novo at sJIA diagnosis
• Rapidly fatal if unrecognised — mortality 10–20% even with treatment; early recognition and aggressive immunosuppression are critical

sJIA presents with the combination of arthritis and systemic features. Arthritis may be minimal or absent at onset, with systemic features dominating the early clinical picture. The quotidian fever pattern and evanescent salmon rash are pathognomonic when present.

Systemic Features (ILAR Diagnostic Criteria)

• Quotidian fever — daily temperature spikes ≥39°C (once or twice per day), returning to normal or subnormal between spikes; persists ≥2 weeks; NOT a low-grade persistent fever
• Evanescent rash — salmon-coloured, macular or maculopapular; non-pruritic; typically on trunk and proximal extremities; appears during fever spikes and disappears with defervescence; Koebner phenomenon positive
• Lymphadenopathy — generalised, non-tender; prominent in cervical chain; may be striking on examination
• Hepatomegaly and/or splenomegaly — common; hepatosplenomegaly significant enough to cause abdominal discomfort; LFT elevation in some cases
• Serositis — pericarditis most common; pleuritis less common; pericardial effusion may cause chest pain and dyspnoea; rarely causes tamponade

Articular Features

• Arthritis may lag behind systemic features by weeks to months at onset; polyarticular in the majority when established
• Wrists, knees, and ankles most commonly involved; hip involvement common and carries poor functional prognosis
• Cervical spine involvement — common; atlanto-axial instability risk; relevant for anaesthesia management

sJIA is a clinical diagnosis based on ILAR criteria. Investigations confirm systemic inflammation, exclude alternative diagnoses (infection, malignancy, other autoinflammatory syndromes), and monitor for MAS.

• Essential
  FBC, film, ferritin, LFTs, ESR, CRP, fibrinogen, D-dimer

  FBC: anaemia of chronic disease; thrombocytosis (leukocytosis common). Ferritin: markedly elevated in active sJIA (>500 μg/L); extremely high ferritin (>10,000 μg/L) suggests MAS. ESR: elevated; paradoxically FALLING ESR in a sick sJIA patient suggests MAS (fibrinogen consumption). D-dimer and fibrinogen: coagulopathy in MAS. Film: leukaemoid reaction possible; blast cells indicate malignancy.
• Essential
  ANA, RF, anti-CCP

  Negative in sJIA by definition. Positive ANA or RF suggests alternative diagnosis. HLA-B27 may be checked to exclude ERA. Anti-dsDNA and complement if SLE is a differential.
• Essential
  Blood cultures, viral serology (EBV, CMV, parvovirus B19)

  Exclude sepsis and viral arthritis. EBV, CMV, and parvovirus can cause fever and polyarthritis mimicking sJIA. Viral triggers must be excluded before commencing immunosuppression. Positive blood cultures require antibiotic treatment before immunosuppression.
• Essential
  Bone marrow aspirate and trephine

  Mandatory when malignancy cannot be excluded (bone pain, blast cells on film, thrombocytopenia, marked hepatosplenomegaly). Also required if MAS is suspected and diagnosis uncertain. Haemophagocytosis on bone marrow biopsy confirms MAS. Must be performed urgently — do not delay treatment if clinical urgency is high.
• Essential
  Echocardiogram

  Mandatory in all patients at diagnosis to assess for pericarditis and pericardial effusion. Effusion may be subclinical. Repeat if chest symptoms develop. Tamponade requires urgent drainage.
• Recommended
  IL-18 level (specialist centres)

  IL-18 is markedly elevated in active sJIA and extremely elevated in MAS; emerging role as early MAS biomarker. Available in specialist centres; not routine in all Australian hospitals. Very high IL-18 in a deteriorating sJIA patient is an early MAS signal.

sJIA has two broad disease phenotypes: a self-limited monocyclic course (single episode, remission within months) and a polycyclic or persistent course with ongoing arthritis and systemic flares. Risk stratification guides treatment intensity.

sJIA treatment differs fundamentally from other JIA subtypes. IL-1 and IL-6 pathway inhibitors are the cornerstone of therapy for steroid-refractory or moderate-severe disease. Methotrexate has limited efficacy. Early biologic use is now favoured to achieve rapid disease control and avoid corticosteroid toxicity.

Macrophage activation syndrome (MAS) is the most dangerous complication of sJIA and requires immediate specialist-level management. MAS is a form of secondary HLH driven by uncontrolled macrophage and T cell activation causing a cytokine storm.

Diagnostic Criteria for MAS in sJIA (2016 Classification)

• Febrile patient with known or suspected sJIA with ferritin >684 μg/L AND any 2 of: platelets ≤181×10³/L; AST >48 U/L; triglycerides >156 mg/dL; fibrinogen ≤360 mg/dL
• Clinical vigilance is more important than strict criteria — treat on suspicion while investigations are pending

MAS Treatment

• High-dose IV methylprednisolone — 30 mg/kg/day (max 1 g/day) ×3–5 days; first-line treatment; rapid anti-cytokine effect
• Cyclosporine A — 2–4 mg/kg/day IV then oral; added when methylprednisolone alone insufficient; impairs T cell and macrophage activation; calcineurin inhibitor
• Anakinra (high dose) — 4–10 mg/kg/day SC or IV; evidence accumulating for efficacy in MAS; rapidly titratable due to short half-life; preferred in refractory MAS
• Etoposide — reserved for severe MAS refractory to steroids and cyclosporine; cytotoxic; used per HLH-94 or HLH-2004 protocol; bone marrow toxicity
• Infection workup and treatment — EBV, CMV, parvovirus B19, and bacterial cultures; treat identified infections concurrently; infection is both a trigger and a mimicker of MAS
• ICU admission — for severe MAS with multi-organ failure, coagulopathy, or haemodynamic instability

Pericarditis Management

• NSAIDs + colchicine — for mild pericarditis; echocardiogram monitoring
• Corticosteroids — for moderate-severe or NSAID-refractory pericarditis in sJIA
• Pericardiocentesis — for tamponade; cardiology involvement required

Non-pharmacological management in sJIA supports joint function, mitigates treatment toxicity, and provides psychosocial support for children with a chronic and potentially severe illness.

Physiotherapy and Exercise

• Range of motion exercises — maintain joint mobility during and after active disease episodes; hip and knee flexion contractures are a particular concern in sJIA
• Aerobic conditioning — reduced during active systemic disease; gentle reintroduction as systemic features resolve; hydrotherapy preferred during active joint disease
• Intra-articular corticosteroid injections — for highly active joints with minimal systemic disease activity; under general anaesthesia if multiple joints involved

Bone Health and Metabolic Monitoring

• Calcium and vitamin D — all patients on corticosteroids; prophylactic supplementation
• Bisphosphonate — for significant corticosteroid-induced osteoporosis; DEXA bone density at baseline and 12 months
• Blood glucose monitoring — monthly during high-dose corticosteroid therapy; steroid-induced hyperglycaemia

Growth Monitoring

• Height, weight, and growth velocity — every 3–6 months; both chronic inflammation and corticosteroid use suppress growth
• Endocrinology referral — for significant growth failure; growth hormone therapy may be considered in selected cases

Monitoring in sJIA requires vigilance for MAS, disease activity assessment, treatment toxicity, and growth. Ferritin is the primary biomarker for both sJIA activity and MAS detection.

When to Refer Urgently

• Suspected MAS: Sustained fever, rapidly rising or dramatically falling ferritin, cytopenias, hepatomegaly, coagulopathy — emergency paediatric rheumatology and haematology; do not wait for bone marrow results before commencing treatment
• Pericarditis with haemodynamic compromise: Urgent cardiology and PICU — pericardiocentesis for tamponade
• ILD in sJIA on biologic: Dyspnoea, hypoxia, or cough on IL-4/IL-13-pathway biologics (dupilumab) or after biologic exposure — urgent respiratory review; sJIA-ILD is a recently recognised complication

Specific clinical considerations apply to particular sJIA patient groups.

sJIA with Macrophage Activation Syndrome at Diagnosis

• MAS can be the presenting feature of sJIA — the diagnosis of sJIA may be established simultaneously with MAS; arthritis may not be apparent at this stage
• Treat MAS immediately — high-dose IV methylprednisolone and cyclosporine; commence anakinra as systemic inflammation resolves
• Haematology and infectious diseases co-management — mandatory; infection triggers must be identified and treated

sJIA-Associated Interstitial Lung Disease (sJIA-ILD)

• Emerging complication — progressive lung disease with clubbing, hypoxia, and HRCT ground-glass opacification; more common in patients with prior MAS and on IL-4/IL-13-pathway therapies
• Early HRCT screening — in patients with respiratory symptoms or unexplained hypoxia on treatment
• Management: immunosuppression review, respiratory specialist, lung transplantation in severe cases

Long-Term Outcomes

• ~30–40% of sJIA patients have a monocyclic course with long-term remission; ~30% have a polycyclic course; ~30% have persistent active disease with ongoing joint damage
• Hip involvement and persistent polyarthritis predict poor functional outcome; early aggressive biologic therapy reduces long-term joint damage
• Transition to adult rheumatology required for persistent disease — adult-onset Still disease (AOSD) management mirrors sJIA; same biologic therapies apply

Stewardship in sJIA focuses on early appropriate biologic use to minimise corticosteroid exposure, vigilance for MAS, infection screening before immunosuppression, and appropriate use of IL-1 versus IL-6 inhibitors.

GP Role in sJIA Management

• Febrile child with sJIA — fever in a child with sJIA is a MAS trigger until proven otherwise; FBC, ferritin, LFTs, blood cultures, and urgent contact with paediatric rheumatology team
• Monitoring — FBC, LFTs, and ferritin monthly; biologic injection site reactions; growth monitoring 3–6 monthly
• Immunisation — all vaccinations (including varicella and MMR) must be completed before biologic initiation; annual influenza vaccine; no live vaccines on biologics
• Infection management — low threshold for investigation of febrile illness; report to rheumatology team any admission or serious infection; hold biologic during serious infection

sJIA requires close specialist follow-up, with frequency determined by disease activity, biologic therapy, and MAS risk. The goal is inactive disease with minimal corticosteroid exposure.

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