Anaphylaxis is the quintessential medical emergency. While the vast majority of anaphylaxis cases are relatively benign, about 1 percent of these patients die from anaphylactic shock quickly—within about five to 30 minutes of onset. Many of these deaths occur because the anaphylaxis was misdiagnosed and/or the treatment of anaphylaxis and anaphylactic shock was inappropriate.

Anaphylaxis is not simply an acute onset of an itchy rash with hypotension. To the contrary, up to 20 percent of patients do not manifest a rash. Moreover, anaphylaxis can present with isolated hypotension, making the diagnosis even more challenging.

Criteria

In order for time-sensitive lifesaving treatment to be initiated promptly, it’s imperative that emergency medicine providers understand the diagnostic criteria for anaphylaxis:

1. Acute illness with skin, mucosal tissues (or both) involvement, and at least one of the following:
   • Respiratory compromise
   • Reduced blood pressure or associated symptoms of end-organ dysfunction
2. Two or more of the following that occur rapidly after exposure to a likely antigen:
   • Skin-mucosal tissue involvement
   • Respiratory compromise
   • Reduced blood pressure or associated symptoms of end-organ dysfunction
   • Gastrointestinal symptoms
3. Reduced blood pressure after exposure to a known allergen

Anaphylaxis can present with isolated hypotension, hypotension plus vomiting, or hypotension plus wheezing, without rash. Not recognizing this in a timely manner can lead to misdiagnosis and death.

Anaphylaxis is not simply an acute onset of an itchy rash with hypotension. To the contrary, up to 20 percent of patients do not manifest a rash. Moreover, anaphylaxis can present with isolated hypotension, making the diagnosis even more challenging.

Epinephrine: Timing, Location, and Dose

All patients who fulfill the criteria for anaphylaxis require epinephrine. Epinephrine is the only drug to show a mortality benefit in the management of anaphylaxis.

Epinephrine should be administered as soon as possible intramuscularly (IM) in the anterolateral thigh. Administering epinephrine IM in the deltoid muscle or subcutaneously is not recommended.

The most common cause of death in anaphylaxis is not giving epinephrine at the right time at the correct dose. The correct dose of epinephrine for the treatment of anaphylaxis is 0.01 mg/kg (to a max of 0.5 mg) IM, repeated after five minutes if there’s no clinical improvement. It’s common practice to underdose epinephrine in this setting.

The combination of 50 mg of diphenhydramine plus 50 mg of ranitidine compared to diphenhydramine plus placebo as a second-line agent for anaphylaxis was shown in one study to be significantly more likely to result in absence of urticaria at two hours. However, there’s a paucity of evidence for the efficacy of steroids in patients with allergic reactions or anaphylaxis, and recent evidence suggests that steroids have little effect on preventing the dreaded biphasic reaction. Nonetheless, steroids are standard care in many jurisdictions. My practice is that if the patient fulfills the diagnostic criteria for anaphylaxis, I give epinephrine and steroids.