A Challenging Case of Adult-Onset Still's Disease Complicated by Macrophage Activation Syndrome With Multiorgan Failure.

Adult-onset Still's disease (AOSD) is a rare systemic inflammatory disease that involves the excessive production of proinflammatory cytokines. Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening immune-mediated disorder that can be primary or secondary to malignancy, infections, and autoimmune diseases. We present an interesting case of a young female with adult-onset Still's disease that commenced during pregnancy. Whole-body fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) scan showed diffuse uptake in the spleen and bone marrow with widespread lymphadenopathy. During the delayed diagnostic process to exclude lymphoproliferative malignancy, she developed severe HLH/macrophage activation syndrome (MAS) with multiorgan failure. In this case report, we described the challenges faced during the diagnosis of AOSD. We also highlighted the importance of using clinical criteria to aid in the early diagnosis and management of AOSD patients and the role of FDG-PET/CT scans in patients with AOSD. Additionally, we discussed the management aspects for patients with macrophage activation syndrome.