Macrophage activation syndrome in a 13-month-old child with systemic juvenile idiopathic arthritis mimicking sepsis: a diagnostic challenge and case report.

Introduction: Macrophage activation syndrome (MAS) is a rare but life-threatening hyperinflammatory complication of systemic juvenile idiopathic arthritis (sJIA). Early diagnosis is challenging because MAS frequently mimics severe sepsis, particularly in young children. We report a rare case of MAS in a 13-month-old child with sJIA who presented with severe hyperferritinemia and coagulopathy without hemophagocytosis on bone marrow examination. Case presentation: A 13-month-old male with sJIA presented with high-grade fever, rash, lethargy, hepatosplenomegaly, and left knee arthritis 2 months after corticosteroid discontinuation. Initial investigations showed pancytopenia and elevated inflammatory markers, and he was treated empirically for sepsis. Persistent fever despite antibiotics together with worsening transaminitis, coagulopathy, hypofibrinogenemia, markedly elevated D-dimer, and hyperferritinemia (7442 ng/mL) raised suspicion of MAS. Blood cultures were negative. Bone marrow examination showed no hemophagocytosis. MAS secondary to sJIA was diagnosed using the 2016 ACR/EULAR criteria. High-dose intravenous methylprednisolone led to rapid clinical and laboratory improvement. The patient remained asymptomatic without relapse at 3-month follow-up. Clinical discussion: MAS in children younger than 2 years with sJIA is exceedingly rare. This case highlights the diagnostic challenge caused by the overlap between MAS and sepsis. Progressive coagulopathy, rising ferritin levels, and poor response to antibiotics were important clues favoring MAS. The absence of hemophagocytosis did not exclude the diagnosis. Conclusion: MAS should be considered in children with sJIA presenting with persistent fever, cytopenias, coagulopathy, and hyperferritinemia despite antimicrobial therapy. Early recognition and prompt corticosteroid treatment are critical and may be life-saving, even without bone marrow confirmation.