Contributor: Aaron Lessen, MD Educational Pearls:

What is anaphylaxis and what are its treatments?

Anaphylaxis is a broad term for potentially life threatening allergic reactions that can progress to cardiovascular collapse (anaphylactic shock).
It is triggered by IgE and antigen cross-linking on mast cells to induce degranulation and the release of histamines, which can cause diffuse vasodilation and respiratory involvement with end-organ hypoperfusion.
First line treatment is the immediate administration of epinephrine at 0.01 mg/kg (max dose for pediatrics is 0.3 mg and for adults is 0.5 mg) as well as removal of the offending agent causing the reaction.
Additional pharmacologic treatments such as anti-histamines and steroids should be considered but not used instead of epinephrine when anaphylactic shock is evident as the sole therapy.

What is biphasic anaphylaxis and what is its occurrence?

Biphasic anaphylaxis is the return of anaphylactic symptoms after the initial anaphylactic event. Previous studies have reported an incidence ranging from 1-20% of patients having an initial anaphylactic reaction having biphasic anaphylaxis, at a range of time from 1-72 hours.
The mechanism of biphasic anaphylaxis is not completely known, but can be contributed to by initial interventions wearing off (and why patients will be monitored for 2-4 hours after initial symptoms and treatment), or delayed immune mediators beginning to take effect.
Recent studies show that the rate of biphasic anaphylaxis may be closer to 16% occurrence with a median time of occurrence being around 10 hours.

What is the key take away and patient education on biphasic anaphylaxis?

After patients have been observed for the initial 2-4 hours in the emergency room, they are generally safe to go home.
Patients should be informed of the need to carry an Epi-Pen for similar anaphylactic reactions, and informed that there is a chance within the next day (10-20 hours) that they may have the symptoms occur once again.
The biphasic reaction may be more mild, and patients should be educated on how to treat it and to seek immediate emergency care if the symptoms do not improve.

References

Golden DBK, Wang J, Waserman S, et al. Anaphylaxis: A 2023 practice parameter update. Annals of Allergy, Asthma & Immunology. 2024;132(2):124-176. doi:10.1016/j.anai.2023.09.015
Rubin S, Drowos J, Hennekens CH. Anaphylaxis: Guidelines From the Joint Task Force on Allergy-Immunology Practice Parameters. afp. 2024;110(5):544-546.
Weller KN, Hsieh FH. Anaphylaxis: Highlights from the practice parameter update. CCJM. 2022;89(2):106-111. doi:10.3949/ccjm.89a.21076
Gupta RS, Sehgal S, Brown DA, et al. Characterizing Biphasic Food-Related Allergic Reactions Through a US Food Allergy Patient Registry. The Journal of Allergy and Clinical Immunology: In Practice. 2021;9(10):3717-3727. doi:10.1016/j.jaip.2021.05.009

Summarized by Dan Orbidan OMS2 | Edited by Dan Orbidan & Jorge Chalit OMS4

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Podcast 994: Biphasic Anaphylaxis