A 46 year old woman arrives at the ED with hives, hypotension, difficulty breathing, and stridor after eating dinner with her family.  She is allergic to shrimp, and thought she had avoided it when she ordered from her favorite restaurant.  However, shrimp dishes are on the menu and she wonders if there was cross contamination.  Unfortunately, she couldn’t find her epinephrine auto-injector at home so her family drove her to the ED.  Immediately recognizing anaphylaxis, you give her a dose of IM epinephrine and she improves within several minutes.  After seeing she has stabilized, you wonder how long should she be observed and what the evidence is behind the use of antihistamine and glucocorticoid therapy.

An updated guideline on the treatment of anaphylaxis was published in the Journal of Allergy and Clinical Immunology.  It reaffirms the definition of anaphylaxis as the following:

1. Sudden onset of an illness (minutes to hours) with involvement of the skin, mucosal tissue, or both (e.g. generalized hives, itching or flushing, swollen lips-tongue-uvuvla) AND at least one of the following: sudden respiratory symptoms and signs or sudden reduced BP or symptoms of end organ dysfunction.

OR

2. Two or more of the following that occur suddenly after exposure to a likely allergen or other trigger for that patient (minutes to hours): Sudden skin or mucosal symptoms and signs, sudden respiratory symptoms and signs, sudden reduced BP or symptoms of end organ dysfunction, or sudden gastrointestinal symptoms.

OR

3. Reduced blood pressure after exposure to a known allergen for that patient.  Infants or children: Low age systolic BP or greater than 30% decrease in systolic BP.  Adults: systolic BP of less than 90mmHg or greater than 30% decrease from that person’s baseline.

The guideline goes on to reaffirm the mainstay of treatment of anaphylaxis is epinephrine at a dose of 0.01mg/kg up to a maximum of 0.5mg in adults and 0.3mg for children, delivered at the anterolateral aspect of the thigh.  Unfortunately, the guidance notes, epinephrine remains underutilized for anaphylaxis presentations.

Being aware of and watchful for biphasic anaphylaxis is important and a common teaching in emergency medicine.  In order to prevent such reactions, medication adjuncts such as antihistamines (H1 and H2 blockers) and glucocorticoids have been used.   Further, extended periods of observation are used when there is a higher concern for biphasic reactions.  This guideline recommends against the routine use of antihistamines and glucocorticoids to prevent biphasic anaphylaxis but notes that this is with low certainty evidence.  

Regarding the decision to use extended observation of these patients, the guideline recommends first risk stratifying patients into having a high or low likelihood of having a biphasic reaction.  Risk factors associated with biphasic reactions were noted to include:

• A more severe anaphylactic reaction

• Repeated doses of epinephrine

• A wide pulse pressure on presentation

• Unknown trigger

• Cutaneous signs and symptoms

• Concern for medication reaction in pediatric patients

If the patient has factors raising their risk for biphasic reactions, the treating physician should consider observation for 6 hours (and possibly longer - including hospital admission).  However, if a patient does not have these characteristics, discharge after a much shorter period is felt to be safe.

All patients should be educated on the signs and symptoms of recurrent reaction, prescribed an epinephrine autoinjector prior to discharge, educated on how to use the autoinjector, and instructed to return to the ED if they experience further signs and symptoms of an allergic reaction.

Reference: Shaker MS, Wallace DV, Golden DBK, et al. Anaphylaxis – a 2020 Practice Parameter Update, Systematic Review and GRADE Analysis. J Allergy Clin Immun. 2020;145(4):1082-1123. doi:10.1016/j.jaci.2020.01.017