Macrophage Activation Syndrome (MAS): A Case Report and Narrative Review.

Hemophagocytic lymphohistiocytosis (HLH) is a rare and life-threatening syndrome of excessive inflammation and tissue destruction secondary to abnormal immune activation. The term macrophage activation syndrome (MAS) is used when HLH develops in the setting of systemic juvenile idiopathic arthritis (SJIA; formerly known as Still's disease), adult-onset Still's disease, or any other rheumatologic disorder. We present a case of a 21-year-old female with a known history of SJIA who presented to the hospital with fever, chills, myalgia, nausea, vomiting, and hypotension. Initial evaluation at the time of presentation suggested sepsis likely due to acute pyelonephritis, and the patient was started on antibiotics and intravenous fluid hydration. However, further workup suggested that her symptoms were non-infectious and were likely due to MAS, a rare complication of SJIA. We promptly diagnosed her, and she received a course of steroids and made an uneventful recovery.