Anaphylaxis is basically a severe allergic reaction. It may include rapidly developing hives, airway obstruction (wheezing or throat swelling), vomiting or diarrhea, drop in blood pressure, and/or loss of consciousness. People with severe food or bee allergies, for example, are at high risk for this type of reaction, and are generally advised to carry an adrenaline auto-injector for emergency use. This can be life saving, since early treatment will often reverse symptoms and allow enough time to get to a nearby hospital for further care.
However, about 11% of these reactions have a second phase occurring 1-20 hours after initial recovery (usually about 8 hours after the first reaction). It is important to be able to predict who is likely to have a second reaction, so that overnight in-hospital close observation can be arranged.
A return of symptoms is more likely if more than one adrenaline injection was needed at the beginning, or if there was a drop in blood pressure requiring IV fluid resuscitation. However, treatment with Benadryl or steroids (Prednisone or Medrol) does not change the likelihood of a second anaphylactic reaction.
Clin Exp Allergy 2009: 39: 1390-6. Clinical predictors for biphasic reactions in children presenting with anaphylaxis. Mehr S, Liew WK, Tey D, Tang ML.
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