First report of a patient meeting criteria for both multisystem inflammatory syndrome in children and adult onset Still's disease.

Background: COVID-19 is associated with a postinfectious hyperinflammatory disorder, multisystem inflammatory syndrome in children (MIS-C), that shares characteristics with still's disease, known as systemic juvenile idiopathic arthritis (SJIA) in children younger than 16, and adult onset Still's disease (AOSD) in children 16 and older. Both MIS-C and SJIA/AOSD can be complicated by macrophage activation syndrome (MAS), a potentially fatal condition of cytokine storm.

Case Presentation: We present a 16 year-old male who developed quotidian fever, headache, conjunctival injection, sore throat, nausea and vomiting, diarrhea, rash, and symmetrical polyarticular arthralgia/arthritis 4 weeks after exposure to SARS-CoV-2 and 2 weeks after his first vaccination against COVID-19. Our patient's laboratory results were significant for elevated inflammatory markers and acute phase reactants. He met criteria for diagnosis with both MIS-C and AOSD. After receiving first-line treatment for both diseases, IVIG and methylprednisolone, our patient improved.

Conclusion: MAS is a life-threatening rheumatological emergency, and physicians must be able to identify diseases, like MIS-C and AOSD, that may be complicated by MAS. Our patient's distinguishing feature on presentation was symmetrical polyarticular arthralgia/arthritis, which has not been associated with MIS-C. Simultaneously, AOSD-which is associated with polyarticular arthralgia/arthritis-is only now being recognized as a possible post-infectious entity in the aftermath of COVID-19 infection. In patients like our own, who meet criteria for both MIS-C and AOSD, administering first line treatment for both diseases may be best practice.