Synopsis
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.
- 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.
- 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.
Key red flags
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.
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.
Investigation priorities
Track neutrophilia, thrombocytosis, cytopenia, inflammation and malignant blood-film warning features.
Management branches
Persistent fever and systemic inflammation remain unexplained.
- Obtain cultures and exposure-directed infection tests and start sepsis therapy when physiology requires.
- Review FBC film, LDH, urate, pain and organ findings with oncology when malignancy is plausible.
- Involve paediatric rheumatology early without waiting for arthritis to become established.