Can systemic inflammatory response syndrome in a 5 year old with juvenile rheumatoid arthritis cause macrophage activation syndrome?

Not exactly: Macrophage activation syndrome has been observed in children with systemic onset jra. Macrophages are activated to engulf various blood cells resulting in low hemoglobin, platelet, and fibrinogen level.

SIRS + JRA = MAS?: There is controversy as to whether macrophage activating syndrome is a real entity, versus it being confused for hemophagocytic lymphohistiocytosis (hlh). Hlh has much more defined criteria and can make a patient very sick (and requires chemo for treatment). A question i would have is could a patient with jra who develops sirs really have hlh. I would consult peds hem/onc (rheum mds may disagree).

Cause - no: Mas has sirs as part of its clinical features. Sirs is just a description of some signs and symptoms of excessive inflammatory mediator release that is found as part of several clinical syndromes. Discuss this with your pediatric rheumatologist.

Yes but it depends: Children with systemic-onset jra can definitely get mas which may be triggered by a flare of disease or an infection such as mononucleosis. Kids with mas can become extremely ill; mas can be life-threatening. A ferritin level should be checked and if it is very high or the child looks very ill, they should be evaluated in a children's hospital where a board certified pediatric rheumatologist works.