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We report the case of a young man who presented with high-spiking fever, erosive arthritis, and generalized lymphadenopathy. He was diagnosed as adult-onset Still's disease (AOSD), and his clinical course was complicated by reactive macrophage activation syndrome (MAS). The sudden precipitous drop in leukocyte and platelet counts proved a valuable clue to the diagnosis of MAS in the setting of AOSD. The new American College of Rheumatology/European League Against Rheumatism classification criteria for reactive MAS complicating systemic juvenile idiopathic arthritis were applied in this patient. When MAS is associated with AOSD, treatment could be challenging. We managed this patient successfully with a combination of systemic corticosteroids and cyclosporine in the acute phase.