Secondary Haemophagocytic Lymphohistiocytosis Triggered by Adult-Onset Still's Disease in Mid-trimester Pregnancy Initially Presenting as an Upper Respiratory Tract Infection: A Case Report.

Adult-onset Still's disease (AOSD) is a rare systemic inflammatory disorder, characterised by fever, arthritis, and an evanescent rash. A severe complication is macrophage activation syndrome (MAS), also known as secondary haemophagocytic lymphohistiocytosis (HLH), a life-threatening state of uncontrolled immune activation that may lead to multiorgan failure. Diagnosis during pregnancy is particularly challenging because of overlapping clinical and biochemical features. We report a 32-year-old African female primigravida at 16 weeks' gestation, presenting with sore throat, dysphagia, chest pain, fever, rash, and progressive polyarthralgia for three weeks. The atypical presentation delayed diagnosis. Further evaluation confirmed AOSD, complicated by secondary HLH, with marked hyperferritinaemia, hypertriglyceridaemia, and persistent high inflammatory markers. Her clinical course was further complicated by pericardial effusion and suspected myopericarditis, requiring tertiary multidisciplinary care. Treatment with high-dose corticosteroids and an interleukin-1 (IL-1) antagonist resulted in clinical improvement, although she was later readmitted with an HLH flare, requiring escalation of immunosuppressive therapy. This case highlights the diagnostic difficulty of AOSD-associated HLH in pregnancy and emphasises the importance of early recognition and timely immunosuppressive treatment to improve maternal outcomes.