The frequency of biphasic anaphylaxis is a subject of some controversy, with most estimates derived from retrospective chart review. The frequency may be as high as 20%, as low as 3%, or those may yet be gross overestimations based on partial symptom recurrence.
For these folks, the answer was: 14.7%.
This is yet another evaluation of Emergency Department visits for anaphylaxis, as collected by retrospective chart review. Looking at one year’s worth of data collected at two pediatric hospitals in Canada, these authors identified 484 visits for anaphylaxis with adequate data for analysis. Of these visits, 71 met their criteria for a biphasic reaction: a period of full symptom resolution lasting at least an hour, followed by recurrence of symptoms requiring additional pharmacologic intervention. They subsequently reviewed features of the initial reaction to determine any potential predictors of biphasic manifestations.
Some of their features make sense, and some – none. Independent predictors included delayed ED presentation, wide pulse pressure, multiple doses of epinephrine to treat the initial episode, and administration of beta-agonists in the initial episode. Essentially, those patients with the most severe, multi-system involvement. However, their strongest odds ratio for predicting return of symptoms was for patients simply aged 6-9 years of age – and the authors do not address the aberration in their discussion.
So, ultimately, this study doesn’t reliably alter our management. Chances are, you’ve already been observing the mildest anaphylaxis for the shortest time, and the most severely ill for longer. Thus, as seen in this cohort, most of these severely ill patients were still undergoing observation in the ED when the biphasic reaction occurred – 3 to 6.5 hours later. All told, 18 patients were discharged from the ED and returned with biphasic symptoms – with a median time of 18.5 hours to return. So, unfortunately, there’s no reasonably useful clinical endpoint to observation that would catch all revisits – and the best course of action is simply to ensure patients have epinephrine for home use at discharge, and inform them of the small likelihood of recurrence.
“Epidemiology and clinical predictors of biphasic reactions in children with anaphylaxis”
http://www.ncbi.nlm.nih.gov/pubmed/26112147
Hey Ryan — As someone interested in CoI, you might find this piece from Harper's I read several years ago interesting in the context of research on anaphylaxis/allergic emergencies, particularly research using definitions of anaphylaxis derived from an organization like FAAN (now FARE, I guess?):
http://harpers.org/archive/2008/01/everyones-gone-nuts/
Middle paragraph on the second page, specifically– the definition they're using in this paper was published by the FAAN medical advisory member mentioned in the Harper's piece and Ms. Munoz-Furlong, and includes recommendations for prescribing Epi Pens for anyone with respiratory/cardiovascular symptoms who thinks they were exposed to an allergen, prompt referral to an allergist, referral to helpful websites/advocacy organizations like FAAN. 🙂
This is obviously not meant as a criticism of the article or authors–the study appears to be well done and the reputation of the group who produced it obviously speaks for itself. I also don't think Big Epipen has an interest in the epidemiology of biphasic reactions, but in the broader context of anaphylaxis research and public awareness/fear of this outcome, it's always interesting to think about how these relationships might play a role. Also, I went to a Giants game the other night and found myself in a "No Peanut Zone", complete with sponsor-backed t-shirts being handed out, so maybe this was just on my mind a bit. 🙂
Big Epipen! They are, at least, surprisingly expensive. Don't discount the chance of COI playing a role, by any stretch of imagination ….