9.2 Anaphylaxis & Allergic Emergencies

Anaphylaxis & Allergic Emergencies

Defining Anaphylaxis: Diagnostic Criteria

Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction with rapid onset that can cause death through airway obstruction, bronchospasm, and/or circulatory collapse. The widely used clinical criteria (derived from National Institute of Allergy and Infectious Diseases/Food Allergy and Anaphylaxis Network consensus work) state that anaphylaxis is highly likely when any one of the following three criteria is met:

1. Acute onset (minutes to hours) of illness involving skin and/or mucosal tissue (hives, itch/flush, swollen lips-tongue-uvula) AND at least one of:

   • Respiratory compromise (dyspnea, wheeze/bronchospasm, stridor, hypoxemia)
   • Reduced blood pressure or associated end-organ dysfunction (hypotonia, syncope, incontinence)

2. Two or more of the following occurring rapidly after exposure to a likely allergen for that patient:

   • Skin/mucosal involvement
   • Respiratory compromise
   • Reduced blood pressure or associated symptoms
   • Persistent gastrointestinal symptoms (crampy pain, vomiting)

3. Reduced blood pressure after exposure to a known allergen for that patient (minutes to hours):

   • Infants and children: low systolic blood pressure for age, or a drop of more than 30% from the child's baseline
   • This criterion alone is sufficient even with no skin findings at all — a critical exam trap, because roughly 10-20% of anaphylaxis cases present without any cutaneous signs.

Exam trap: Skin findings are the most common presentation but are not required for the diagnosis. A child with a known peanut allergy who develops sudden hypotension and vomiting after accidental exposure — with no rash whatsoever — still meets criterion 3 and must be treated as anaphylaxis.

First-Line Treatment: Intramuscular Epinephrine

Epinephrine is the only first-line, life-saving treatment for anaphylaxis. No other drug class reverses the underlying mediator release and hemodynamic collapse. Delay in epinephrine administration is the single strongest modifiable risk factor for biphasic reactions and fatal outcomes tested on this exam.

• Dose: 0.01 mg/kg of the 1:1,000 (1 mg/mL) concentration, given intramuscularly (IM).
• Maximum single dose: 0.3 mg for most children (some protocols allow up to 0.5 mg in larger adolescents).
• Site: the mid-anterolateral thigh (vastus lateralis) — preferred over the deltoid because it achieves faster, more reliable peak plasma concentration.
• Repeat dosing: every 5-15 minutes if symptoms persist or recur; many patients need a second dose, and a meaningful minority need three or more.
• Fixed auto-injector doses commonly available in the region: 0.15 mg for children roughly 10-25 kg (junior auto-injector) and 0.3 mg for children/adults at or above roughly 25-30 kg.

Worked example: A 20 kg child (roughly 6 years old) develops facial angioedema, wheeze, and hypotension after a bee sting. The correct IM epinephrine dose is 0.01 mg/kg x 20 kg = 0.2 mg, drawn up as 0.2 mL of the 1:1,000 (1 mg/mL) solution and injected into the mid-anterolateral thigh. If a fixed-dose auto-injector is the only device available, the 0.15 mg junior device is the closer match for this weight and is an acceptable substitute for the adult 0.3 mg device.

Exam trap: Subcutaneous administration, intravenous (IV) push epinephrine outside a monitored resuscitation, and antihistamine-first management are all wrong answers the exam uses as distractors. IM epinephrine in the thigh, given immediately, is always the correct first step once anaphylaxis is suspected — do not wait to confirm with labs. Serum tryptase is a retrospective, confirmatory test and is never used to decide on initial treatment.

Biphasic Reactions and Observation Period

A biphasic reaction is recurrence of anaphylactic symptoms after apparent resolution, without any re-exposure to the trigger, typically within 1-4 hours but reported as late as 24-72 hours afterward. Reported incidence ranges roughly 1-20% depending on the population studied.

Risk factors for a biphasic reaction the exam expects you to recognize:

• Delayed initial epinephrine administration (more than 60 minutes from symptom onset)
• Need for more than one dose of epinephrine to control the initial reaction
• Severe initial presentation (for example, hypotension or respiratory compromise requiring intubation)
• Unknown trigger

Observation period guidance:

• Uncomplicated, mild reactions that fully respond to a single epinephrine dose: roughly 1-2 hours of observation may be sufficient.
• Reactions with any cardiorespiratory involvement, or requiring more than one epinephrine dose: at least 4-6 hours of observation is prudent, with some protocols recommending overnight admission for severe reactions.
• All patients should be discharged with a prescription for (or access to) an epinephrine auto-injector, a written action plan, and referral to allergy/immunology.

Adjunct Therapies (Never First-Line)

These treat associated symptoms but do not replace epinephrine and must never delay it:

Note: chronic allergy evaluation, allergen avoidance counseling, and long-term immunotherapy are covered in the outpatient chronic allergy chapter — this section focuses solely on acute emergency management.