Definition and epidemiology
Macrophage activation syndrome (MAS) is a life-threatening cytokine storm — a form of secondary haemophagocytic lymphohistiocytosis (HLH) — driven by uncontrolled activation and proliferation of T cells and well-differentiated macrophages. It classically complicates systemic juvenile idiopathic arthritis (sJIA), where roughly 10% develop overt MAS and a further 30–40% show subclinical disease, but it also arises in systemic lupus erythematosus, Kawasaki disease, and infections. Mortality remains substantial (historically 8–22%), and the danger is that early MAS mimics a flare of the underlying disease or sepsis. This is a can’t-miss emergency: the trajectory can collapse within hours.
Clinical features
Suspect MAS in a child with sJIA who deviates from the expected course. The hallmark is persistent non-remitting high fever (loss of the usual quotidian spike), hepatosplenomegaly, lymphadenopathy, and rapidly progressive multi-organ involvement — encephalopathy (irritability, lethargy, seizures), bleeding/purpura from consumptive coagulopathy, and hepatic dysfunction. Paradoxically the patient may look like they are improving (arthritis and rash settle) while the laboratory picture deteriorates. Late features include hypotension, DIC, and multi-organ failure.
Diagnosis
MAS is recognised by a falling-counts paradox in an acutely inflamed child: an acute-phase response would normally raise platelets, fibrinogen, and counts, so a fall against a backdrop of high ferritin is the alarm. The 2016 EULAR/ACR/PRINTO classification criteria apply to a febrile patient with known or suspected sJIA: classify as MAS when ferritin > 684 ng/mL plus any two of — platelets ≤ 181 × 10⁹/L, AST > 48 U/L, triglycerides > 156 mg/dL, fibrinogen ≤ 360 mg/dL. Note the cut-offs are deliberately near-normal — a fibrinogen of 350 is abnormal in a child who should be hyperfibrinogenaemic. Serial trends beat single values: a rapidly rising ferritin and falling platelets/fibrinogen is more telling than any one threshold. Bone-marrow haemophagocytosis supports but is not required and may be absent early. The HLH-2004 criteria are less sensitive in this setting.
See the full criteria: Macrophage Activation Syndrome (MAS) Criteria
Red flags
- Sustained high fever losing its quotidian pattern in a child with sJIA — treat as MAS until proven otherwise.
- Rising ferritin with falling platelets and falling fibrinogen — the defining biochemical signature; do not wait for all criteria.
- New encephalopathy, bleeding, or hepatic dysfunction — impending multi-organ failure.
- Falling ESR with rising D-dimer (fibrinogen consumption) — a deceptive “improving” ESR.
- Hypotension, DIC, or hyperferritinaemia in the thousands — escalate to critical care immediately.
Management overview
MAS is a medical emergency requiring urgent paediatric rheumatology and intensive-care involvement. The cornerstone is prompt high-dose systemic corticosteroids (pulse glucocorticoids). Refractory or fulminant disease is escalated with IL-1 inhibition (anakinra), calcineurin inhibitors (ciclosporin), and, in severe cytokine storm, IL-6 or interferon-γ–directed biologics; etoposide-based HLH protocols are reserved for refractory cases. Identify and treat any triggering infection, and provide aggressive supportive care for coagulopathy, organ support, and the metabolic consequences of the cytokine storm. Doses are deliberately omitted here — protocols are weight- and severity-specific; verify against current guidance. Early recognition and treatment are the single biggest determinants of survival.
References
- Ravelli A, et al. 2016 EULAR/ACR/PRINTO Classification Criteria for MAS complicating sJIA. Arthritis Rheumatol. 2016;68(3):566-576.
- Henderson LA, Cron RQ. MAS and secondary HLH in childhood inflammatory disorders. Paediatr Drugs. 2020;22(1):29-44.
- IAP Rheumatology Chapter — MAS recognition and management.
- Nelson Textbook of Pediatrics, 21st ed.
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.