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OverviewAetiologyPathophysiologyClinical features and red flagsInvestigationsDifferential diagnosisManagementComplicationsMonitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Systemic juvenile inflammatory disease

Paediatrics and child health · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Macrophage activation syndrome

Persistent fever with falling platelets or other counts, rapidly rising ferritin, hepatitis, coagulopathy, low fibrinogen, high triglycerides, neurological change or organ failure is a rheumatological and critical-care emergency.

Action: Use ABCDE, involve paediatric rheumatology, haematology and intensive care urgently, investigate infection and malignancy in parallel and begin specialist high-dose glucocorticoid and cytokine-directed treatment without waiting for every criterion.

Overview

Recognise systemic juvenile idiopathic arthritis as a cytokine-driven fever syndrome, exclude infection and malignancy rapidly and detect macrophage activation from changing blood counts, ferritin, liver tests, triglycerides and fibrinogen.

Build a daily chronology of temperature rather than accepting recurrent fever. Ask whether the child becomes well between spikes, whether rash coincides with fever or rubbing, and about morning stiffness, sore throat, lymph nodes, abdominal pain, chest pain and weight. Examine during fever if possible for evanescent rash, arthritis, hepatosplenomegaly and serositis.

Systemic JIA is a diagnosis of exclusion. Culture suspected sources and assess travel, exposure, EBV and other infections from context. FBC and blood film, LDH, urate, imaging and marrow evaluation may be required before immune suppression when malignancy remains plausible. Positive ANA does not establish systemic JIA and may distract from the innate inflammatory phenotype.

Inflammatory trends are more important than one ferritin. Uncomplicated systemic JIA commonly produces neutrophilia, thrombocytosis, high ESR, CRP and ferritin. MAS reverses that pattern: platelets and other counts fall, AST and ferritin rise sharply, fibrinogen is consumed, triglycerides rise and coagulopathy or encephalopathy develops. ESR can fall because fibrinogen falls.

Aetiology

01

Innate immune dysregulation

Excess activation of macrophage and cytokine pathways, especially interleukin-1 and interleukin-6, drives systemic JIA without a single autoantibody.

02

Polygenic susceptibility

Multiple immune variants and environmental interactions create risk, but most children have no identifiable inherited monogenic cause.

03

Infection as trigger or mimic

Viral and bacterial illness may precede inflammation or trigger macrophage activation while also requiring exclusion as primary disease.

04

Cytotoxic pathway vulnerability

Impaired natural-killer and cytotoxic T-cell control contributes to uncontrolled macrophage activation during systemic JIA or infection.

Pathophysiology

  1. 1
    Cytokine fever cycling

    Interleukin-1, interleukin-6 and other mediators reset temperature and drive neutrophilia, acute-phase response, rash and organ inflammation.

  2. 2
    Systemic endothelial inflammation

    Serositis, liver involvement, lymphoid enlargement and capillary effects reflect inflammation beyond synovium and can dominate before arthritis.

  3. 3
    Haemophagocytic amplification

    Uncontrolled macrophages and cytotoxic lymphocytes consume blood cells and release interferon-driven inflammation, causing liver, coagulation and neurological failure.

  4. 4
    Fibrinogen and ESR paradox

    Macrophage activation consumes fibrinogen, so ESR can fall despite worsening ferritin, CRP, cytopenia and organ injury.

Clinical features and red flags

Quotidian fever

Temperature spikes to 39°C or above once or twice daily and may return toward normal between peaks.

Evanescent rash

Salmon-pink macules appear with fever, warmth or rubbing and fade as temperature falls, unlike fixed purpura.

Systemic inflammation

Lymphadenopathy, hepatosplenomegaly, sore throat, pleuritis, pericarditis and neutrophilic inflammation can precede clear arthritis.

Evolving arthritis

Persistent synovitis may emerge after weeks of systemic symptoms and eventually become the dominant source of disability.

Macrophage activation change

Sustained fever, cytopenia, liver dysfunction, coagulopathy, neurological change and paradoxically falling ESR marks dangerous hyperinflammatory escalation.

Red flags requiring action

  • Fever with shock, focal infection, positive cultures or severe immunocompromise requires sepsis treatment while inflammatory disease remains under consideration.
  • Night pain, bruising, cytopenia, blasts, disproportionate lactate dehydrogenase, lymphadenopathy or organ enlargement requires urgent malignancy exclusion.
  • A quotidian fever pattern with evanescent salmon-pink rash, arthritis, nodes, hepatosplenomegaly or serositis supports systemic JIA but is not pathognomonic.

Investigations

  1. 01
    Serial FBC film CRP and ESRFirst step
    Why
    Track neutrophilia, thrombocytosis, cytopenia, inflammation and malignant blood-film warning features.
    Interpretation and limitations
    Falling counts and ESR during clinical worsening suggests MAS rather than recovery and requires immediate expanded testing.
  2. 02
    Ferritin AST triglycerides and fibrinogen
    Why
    Screen and trend macrophage activation in every concerning systemic-JIA deterioration.
    Interpretation and limitations
    Use absolute MAS thresholds and rate of change; very high or rapidly rising ferritin is concerning but not disease-specific.
  3. 03
    Cultures and targeted infection testing
    Why
    Identify sepsis, EBV and other infections that mimic or trigger systemic inflammation and MAS.
    Interpretation and limitations
    Do not withhold urgent antimicrobials or assume a positive infection excludes concurrent macrophage activation.
  4. 04
    Malignancy assessment
    Why
    Exclude leukaemia, lymphoma and neuroblastoma when pain, cytopenia, LDH, nodes or organ findings are atypical.
    Interpretation and limitations
    Blood film, imaging and bone-marrow examination are selected with oncology before major immune suppression when suspicion remains.
  5. 05

Differential diagnosis

01

Sepsis or endocarditis

Persistent fever, shock, focal infection, murmur or positive microbiology requires immediate antimicrobial treatment and source investigation.

02

Leukaemia or lymphoma

Night pain, bruising, cytopenia, blasts, disproportionate LDH and fixed organ or nodal enlargement requires marrow and oncological assessment.

03

Kawasaki disease

Prolonged fever with conjunctivitis, mucosal, extremity, rash and nodal changes, especially in younger children, requires coronary-risk treatment.

04

Systemic lupus or vasculitis

Nephritis, complement consumption, specific autoantibodies, purpura or vascular organ injury suggests an alternative multisystem immune disease.

05

Monogenic autoinflammation

Recurrent stereotyped attacks, family pattern, serositis, cold trigger or complete wellness between episodes suggests a periodic fever syndrome.

Management

01ExcludeTreat infection and malignancy riskFirst stepPersistent fever and systemic inflammation remain unexplained.+
  1. 1Obtain cultures and exposure-directed infection tests and start sepsis therapy when physiology requires.
  2. 2Review FBC film, LDH, urate, pain and organ findings with oncology when malignancy is plausible.
  3. 3Involve paediatric rheumatology early without waiting for arthritis to become established.
02ControlSuppress systemic cytokine diseaseSystemic JIA is confirmed or strongly supported after parallel exclusion.+
  1. 1Use early IL-1 or IL-6 pathway treatment matched to organ and joint phenotype.
  2. 2Limit systemic glucocorticoids to severe disease or bridging with a written taper and adrenal plan.
  3. 3Measure fever, rash, joints, function and laboratory response against treat-to-target milestones.

Complications

01

Macrophage activation syndrome

Uncontrolled hyperinflammation causes cytopenia, coagulopathy, hepatitis, encephalopathy, shock and multiorgan failure requiring immediate specialist intensive therapy.

02

Cardiopulmonary inflammation

Pericarditis, myocarditis, pleuritis and emerging systemic-JIA-associated lung disease can cause hypoxia, chest pain and long-term morbidity.

03

Destructive arthritis

Persistent synovitis damages cartilage, restricts movement and impairs growth when systemic control does not suppress joint inflammation.

04

Amyloid A deposition

Rare prolonged uncontrolled inflammation can cause AA amyloidosis with proteinuria and organ dysfunction, emphasising sustained disease control.

05

Treatment toxicity

Glucocorticoids and immune therapy increase infection, growth, bone, metabolic, liver and blood-count risks requiring active surveillance.

Monitoring and follow-up

  • Trend temperature pattern, rash, active joints, function and systemic symptoms rather than CRP alone.
  • During possible MAS repeat FBC, ferritin, AST, triglycerides, fibrinogen and coagulation at urgency-driven intervals.
  • Monitor respiratory symptoms, saturation, echocardiography and lung imaging when cardiopulmonary involvement is suspected.
  • During anakinra or other biologics track infection, blood counts, liver tests, vaccination and injection or infusion reactions.
  • During glucocorticoids follow growth, weight, blood pressure, glucose, bone, eyes, infection and adrenal sick-day planning.
  • Assess school, fatigue, mood, family emergency confidence and rapid access to rheumatology.
  • Screen urine protein and sustained inflammatory burden when long-term AA amyloid risk is relevant.

Special situations

The fever may lead

Systemic JIA can present before arthritis, so waiting for swollen joints delays recognition of a dangerous cytokine syndrome.

Falling ESR can be bad

Fibrinogen consumption during MAS can lower ESR while ferritin, hepatitis, coagulopathy and organ failure worsen.

Infection and MAS coexist

An infection can trigger macrophage activation, making parallel antimicrobial and immunomodulatory treatment necessary rather than contradictory.

Ferritin needs a trend

One elevated value is nonspecific, while rapid rise alongside falling counts and fibrinogen carries stronger emergency meaning.

Cytokine blockade saves steroids

Early IL-1 or IL-6 therapy targets systemic biology and reduces cumulative glucocorticoid growth and bone toxicity.

Common pitfalls

  1. 01

    Waiting for arthritis before referring systemic inflammation.

  2. 02

    Calling every high ferritin macrophage activation syndrome.

  3. 03

    Reassuring from a falling ESR during clinical deterioration.

  4. 04

    Treating infection and MAS as mutually exclusive.

  5. 05

    Missing malignancy before major immune suppression.

  6. 06

    Using prolonged high-dose glucocorticoid without cytokine strategy.

  7. 07

    Delaying intensive care until organ failure is advanced.

Practice

Two practice questions

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Paediatrics and child healthOriginal SBA

Macrophage activation change

A child with systemic JIA develops sustained fever, confusion, falling platelets, rising AST and ferritin, high triglycerides and falling fibrinogen. What is the most likely complication?

Choose one answer, then submit
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Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • NICE TA685 Anakinra for Still DiseaseAnakinra commissioning for systemic JIA and adult-onset Still disease.
  • NHSGGC Juvenile Idiopathic Arthritis GuidelineSystemic JIA recognition, dangerous mimics and urgent specialist assessment.
  • Arthritis UK Systemic JIAUK family information on systemic and joint disease.
  • RCPCH Paediatric RheumatologyUK paediatric rheumatology scope and specialist-care context.
  • EULAR ACR PRINTO MAS Criteria DevelopmentExact laboratory classification thresholds and value of changing trends in systemic-JIA-associated MAS.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting review

Use the MAS criteria as support, not a gate. A febrile child with systemic JIA and ferritin above 684 micrograms/L meets classification with any two specified laboratory thresholds, but organ deterioration warrants treatment before all are present. Infection can trigger MAS and must be treated rather than assumed mutually exclusive.

Systemic treatment is increasingly cytokine directed. Anakinra or another IL-1 inhibitor and tocilizumab or another IL-6 pathway agent can control fever and arthritis and reduce steroid exposure. Short systemic glucocorticoid is used for severe organ disease or as a bridge, with a documented taper and adrenal plan. NSAIDs alone are inadequate for important systemic disease.

MAS requires hospital care with paediatric rheumatology, haematology and intensive care. High-dose IV methylprednisolone is common first treatment; anakinra and ciclosporin are added according to severity and response, while refractory disease may require other HLH-directed therapy. Support coagulation, liver, brain, heart, lung and infection physiology simultaneously.

Key points

  • Systemic JIA is an autoinflammatory disease with fever, evanescent rash and systemic or joint inflammation after infection, malignancy and other inflammatory causes are excluded.
  • Typical fever spikes once or twice daily to at least 39°C with return toward baseline, and the salmon-pink rash often appears with fever or rubbing.
  • Arthritis may be absent early, so persistent systemic features require rheumatology review rather than waiting for six weeks of joint swelling.
  • First-line investigation includes serial FBC and film, CRP, ESR, ferritin, liver tests, LDH, triglycerides, fibrinogen, coagulation, cultures and phenotype-directed infection and malignancy studies.
  • Macrophage activation warning is change: sustained fever, falling platelets and other counts, rapidly rising ferritin or AST, rising triglycerides and falling fibrinogen or ESR.
  • The 2016 MAS classification supports recognition in a febrile child with known or suspected systemic JIA: ferritin over 684 micrograms/L plus any two of platelets 181 or below, AST over 48 units/L, triglycerides over 1.76 mmol/L or fibrinogen 3.6 g/L or below.
  • Do not wait for formal thresholds when a child is deteriorating; criteria aid classification and can miss early or atypical MAS.
  • Treat active systemic JIA early with paediatric rheumatology, increasingly using interleukin-1 or interleukin-6 blockade to limit prolonged systemic glucocorticoids.
Change from spiking to sustained fever, falling platelets, falling ESR, rising ferritin or AST and new confusion strongly suggests macrophage activation syndrome.
  • Chest pain, dyspnoea or enlarged cardiac silhouette may reflect pericarditis, myocarditis or pleural inflammation and needs urgent cardiac-respiratory assessment.
  • Persistent high-dose corticosteroid exposure without a steroid-sparing plan creates infection, growth, bone, metabolic and adrenal harm.
    • Cardiac and lung evaluation
    Why
    Detect pericarditis, myocarditis, effusion, pulmonary hypertension or systemic-JIA-associated lung disease.
    Interpretation and limitations
    ECG, echocardiography, radiography, CT and respiratory testing follow symptoms and physiological severity.
    03MASEscalate hyperinflammation immediatelyEscalationSustained fever, falling counts, ferritin rise, hepatitis, coagulopathy or neurological change emerges.+
    1. 1Admit to monitored care and involve rheumatology, haematology and intensive care simultaneously.
    2. 2Give specialist high-dose corticosteroid and add anakinra or ciclosporin according to response.
    3. 3Treat infection triggers and support liver, coagulation, brain, respiratory and circulatory failure in parallel.
    04Long termPrevent relapse and toxicitySystemic inflammation is controlled and maintenance care begins.+
    1. 1Taper glucocorticoid, maintain effective cytokine blockade and monitor infection and laboratory toxicity.
    2. 2Assess arthritis, lung and heart disease, growth, puberty, bone, vaccination and school participation.
    3. 3Provide flare and MAS warning plans with rapid access to the specialist team.
    Key medicines and prescribing safety2 treatments · regimens, roles and cautions+
    Interleukin-1 receptor blockade rapidly controls fever and systemic inflammation and can reduce glucocorticoid exposure.

    Anakinra

    For systemic JIA, children under 50 kg commonly start 1–2 mg/kg subcutaneously once daily and those 50 kg or above 100 mg once daily; severe disease or MAS may use higher specialist regimens outside routine licensing.

    Screen and monitor infection, neutrophils, liver tests and injection reactions, adjust in severe renal impairment, avoid unplanned live vaccines and document off-label higher-dose governance.

    Rapid broad immunosuppression for macrophage activation syndrome or severe life-threatening systemic inflammation.

    Intravenous methylprednisolone for MAS

    A common emergency regimen is 30 mg/kg intravenously once daily, maximum 1 g, for 1–3 days before specialist taper or additional therapy according to response and organ failure.

    Treat infection in parallel and monitor glucose, blood pressure, electrolytes, mental state, gastrointestinal protection and infection; repeated courses require a steroid-sparing plan and adrenal precautions.

    JurisdictionUnited Kingdom
  • MAS treatment commonly starts with high-dose intravenous corticosteroid and often anakinra or ciclosporin under intensive specialist care while infection is treated in parallel.
  • Monitor joints, fever, rash, heart and lungs, growth, bone, infection, vaccination and psychological and school effects through one coordinated plan.