Weekly Article: Anaphylaxis Biphasic Reactions

Originally sent by Kristen

This is an older article (2000) but I’ve discussed it with a few people recently, so thought I’d get it out and read it again.   This is a retrospective chart review of 108 patients admitted to the hospital for a diagnosis of anaphylaxis from 1985-1999.  Of the 105 patients with resolution of symptoms, admitted for observation, 6 had biphasic reactions.  The asymptomatic interval before the biphasic reaction ranged from 1.3 to 28.4 hours (1.3, 5.8, 5.9, 8.0, 11.4, and 28.4 hours).  Three of those were considered significant and required a second dose of epi: one patient, triggered by nuts, was asymptomatic for 5.8 hours, but then developed angioedema, stridor, and required intubation and pressors.  The other two patients had asymptomatic intervals of 11.4 hours and 28.4 hours.  All three of these patients received steroids initially.

Time to initial epi dose was significantly longer among those who had a biphasic reaction compared with those who did not, but no other clinical risk factors were identified.  

In terms of initial treatment, 89% received epi, 83% received steroids, 94% received diphenhydramine, and 13% received H1 blockers; 7% were intubated and 7% received pressors.

I’ve seen widely varied practices on how long to observe patients after an anaphylactic episode.  In this study, 1 patient benefited from 6 hours of observation, 1 patient benefited from 12 hours of observation, and 1 patient benefited from 29 hours of observation.  Even after a period of observation, families should get careful discharge instructions on what to watch for, and they should be reminded to keep the epi-pen available at all times.  Parents and older children should also understand the importance of using the epi-pen as soon as serious symptoms develop, and make sure an epi-pen (and someone to give it) is available at school.