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Emergency Medicine›Condition·Updated Apr 16, 2026·v1

Anaphylaxis

Anaphylaxis is a clinical diagnosis requiring rapid IM epinephrine. Management focuses on airway protection, circulatory support via positioning and fluids, and monitoring for biphasic recurrences.

High Evidence29 references·1,336 words·6 min read·v1
emergency medicineallergyimmunologyresuscitation

Quick Reference

RxDrug of choiceEpinephrine 0.3–0.5 mg IM (Adult) or 0.01 mg/kg IM (Pediatric)
AltAlternativesIntranasal epinephrine 2.0 mg (if available/approved); Glucagon for beta-blocked patients
⊘AvoidDelaying epinephrine to administer antihistamines; sudden upright positioning
DxTest of choiceClinical diagnosis; Serum Tryptase (drawn 0.5–2 hours after onset) for confirmation
ScKey score2023 AAAAI/ACAAI Diagnostic Criteria
→When to referAll patients should be referred to an allergist for trigger identification and venom immunotherapy if applicable
Early intramuscular epinephrine in the mid-outer thigh is the definitive treatment; never delay for adjuncts.
Anaphylaxis is a life-threatening, multi-systemic medical emergency characterized by rapid-onset respiratory compromise and/or cardiovascular collapse. The bottom line is that immediate administration of intramuscular epinephrine is the only life-saving intervention; there are no absolute contraindications to its use in a true emergency. Recognition relies on identifying acute involvement of at least two organ systems (skin/mucosa, respiratory, gastrointestinal, or cardiovascular) following allergen exposure, though hypotension alone after a known trigger is sufficient for diagnosis. While adjunctive therapies like antihistamines and corticosteroids are commonly used, they do not treat airway obstruction or shock and must never delay epinephrine. Management also requires strict adherence to supine positioning to prevent fatal venous collapse. All patients require an emergency action plan and referral to an allergist for trigger identification and long-term risk mitigation.

Overview and Recommendations

Background

  • •Recognize as a severe, potentially fatal systemic hypersensitivity reaction resulting from sudden and basophil degranulation.
  • •Identify common triggers including foods (peanuts, shellfish), medications (NSAIDs, antibiotics), and Hymenoptera venom.
  • •Anticipate in 0.4% to 20% of cases, typically occurring 1–48 hours after the initial resolution.
  • •Screen for high-risk comorbidities such as , atopic dermatitis, and underlying cardiovascular disease which increase reaction severity.

Evaluation

  • •Diagnose clinically based on the 2023 revised criteria: acute onset of skin/mucosal involvement PLUS respiratory distress, , or severe GI symptoms.
  • •Assess for cutaneous signs including , , or generalized flushing in the majority of patients.
  • •Monitor for respiratory compromise such as wheezing, stridor, or dyspnea.
  • •Evaluate for gastrointestinal distress, specifically severe crampy abdominal pain or recurrent vomiting, which may be the sole systemic sign in some cases.
  • •Identify atypical presentations in infants, such as sudden lethargy, inconsolable crying, or behavioral changes.
  • •Obtain a basal serum tryptase level if a mast cell disorder is suspected or in cases of severe venom-induced reactions.

Management

  • •Administer 0.3–0.5 mg IM (adults) or 0.01 mg/kg IM (pediatrics) in the mid-outer thigh immediately.
  • •Repeat epinephrine every 5–15 minutes if symptoms persist or recur.
  • •Maintain the patient in a supine position with legs elevated to prevent "empty ventricle syndrome"; avoid sudden standing or sitting.
  • •Provide high-flow oxygen and aggressive fluid resuscitation with Isotonic Crystalloids (1–2 L bolus for adults; 20 mL/kg for peds) for hypotension.
  • •Administer 1–5 mg IV over 5 minutes for patients on who are refractory to epinephrine.
  • •Utilize adjunctive (Cetirizine 10 mg) and (Prednisone 50 mg) only for symptomatic relief of cutaneous symptoms.
  • •Observe patients for 4–8 hours minimum; extend monitoring for those with severe initial symptoms or those requiring multiple epinephrine doses.
  • •Prescribe at least two epinephrine auto-injectors and provide a written emergency action plan upon discharge.

Board Review — High Yield

  • •Empty Ventricle Syndrome — Fatal cardiovascular collapse caused by suddenly standing or sitting during anaphylaxis due to reduced venous return.
  • •Biphasic Reaction — Recurrence of symptoms without further allergen exposure, most common within 12 hours but possible up to 48 hours.
  • •Beta-blocker Refractoriness — Patients on beta-blockers may not respond to epinephrine; Glucagon is the specific antidote.
  • •Uterine Contractions — A unique clinical manifestation of anaphylaxis in pregnant patients; may respond better to beta-2 agonists.
  • •Tryptase — A serum marker of mast cell degranulation; levels >11.4 ng/mL suggest an underlying mast cell activation syndrome.
  • •Mid-outer Thigh — The preferred site for IM epinephrine due to superior pharmacokinetic absorption compared to the deltoid.
  • •Venom Immunotherapy (VIT) — Highly effective long-term prevention for Hymenoptera-induced anaphylaxis.
  • •Kounis Syndrome — Acute coronary syndrome (vasospastic or thrombotic) occurring in the setting of an allergic reaction.

Deep Dive — Evidence Details

References

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