Emergency Medicine · Hypersensitivity Reactions
The facts most likely to be tested
Press 1–5 to rate · ↑↓ to navigate
Intramuscular epinephrine in the mid-outer thigh is the first-line treatment and must be administered immediately upon suspicion of anaphylaxis.
Anaphylaxis is a clinical diagnosis that usually requires involvement of two or more organ systems (skin, respiratory, cardiovascular, or gastrointestinal) after exposure to a likely allergen.
Biphasic reactions, characterized by the recurrence of symptoms after the initial resolution, occur in a subset of patients and necessitate a period of observation.
Hypotension or end-organ dysfunction following allergen exposure is sufficient for a diagnosis of anaphylaxis even in the absence of cutaneous manifestations.
H1 and H2 antihistamines are considered adjunctive therapies and should never delay the administration of epinephrine.
Patients on beta-blockers may exhibit refractory anaphylaxis due to blunted response to epinephrine, requiring the use of glucagon for hemodynamic support.
Serum tryptase levels can be measured to confirm the diagnosis retrospectively, but the acute level must be drawn within 2 hours of symptom onset and compared with a later baseline tryptase — an acute value exceeding baseline by 20% + 2 ng/mL supports mast cell degranulation.
Vignette unlocked
A 24-year-old male presents to the emergency department shortly after eating a meal containing peanuts. He complains of diffuse pruritic rash, throat tightness, and abdominal cramping. Upon arrival, his blood pressure is 88/50 mmHg and his heart rate is 124 bpm. Physical examination reveals inspiratory stridor and generalized urticaria.
What is the most appropriate next step in management?
Intramuscular epinephrine
The patient meets clinical criteria for anaphylaxis involving the skin, respiratory, and cardiovascular systems; immediate administration of IM epinephrine is the definitive first-line intervention.
Full handout
High yield triage
Etiology / Epidemiology
Systemic IgE-mediated hypersensitivity. Food, medications, and insect stings are the most common triggers.
Clinical Manifestations
Multisystem involvement including urticaria, angioedema, and hypotension. Look for stridor.
Diagnosis
Clinical diagnosis. No laboratory testing is required for acute management.
Treatment
Epinephrine (IM) is the first-line treatment. Do not delay for diagnostic testing.
Prognosis
Risk of biphasic reaction in roughly 5% of patients (range 1-20%). Observation is risk-stratified: ~1 hour after complete symptom resolution may suffice in low-risk patients, with extended observation for severe reactions or >1 epinephrine dose.
Full handout
Epidemiology & Etiology
Common triggers include penicillins, Hymenoptera stings, and foods like peanuts or shellfish. Onset is typically rapid, occurring within minutes to 2 hours of exposure. Prior sensitization is required for IgE-mediated reactions.
Pertinent Anatomy
Airway involvement via laryngeal edema is the primary cause of mortality. Systemic vasodilation and increased capillary permeability lead to distributive shock.
Pathophysiology
Antigen exposure triggers cross-linking of IgE on mast cells and basophils. This causes massive release of histamine, tryptase, and leukotrienes. Resulting vasodilation and bronchoconstriction lead to the classic anaphylactic shock presentation.
Clinical Manifestations
Patients present with skin findings like urticaria or angioedema in 90% of cases. Respiratory distress, stridor, and wheezing indicate airway compromise. Cardiovascular collapse manifests as hypotension and tachycardia.
Diagnosis
Diagnosis is strictly clinical. Serum tryptase should be drawn as early as possible, ideally within 2 hours of symptom onset, with a second (baseline) tryptase obtained later for comparison; an acute value exceeding baseline by 20% + 2 ng/mL supports mast cell activation, but this is retrospective. Do not delay treatment for any diagnostic testing.
Treatment
Epinephrine 1:1000 (1 mg/mL) administered intramuscularly in the mid-outer thigh is the gold standard. Avoid subcutaneous administration due to poor absorption. Adjunctive therapy includes IV fluids and H1/H2 blockers, all secondary to epinephrine; corticosteroids are no longer recommended routinely, as they do not prevent biphasic reactions.
Prognosis
Most patients recover fully with prompt intervention. Monitor for biphasic reactions—a recurrence of symptoms without re-exposure—which can occur up to 72 hours later (median ~11 hours). Discharge requires an epinephrine autoinjector prescription.
Differential Diagnosis
Vasovagal syncope: bradycardia instead of tachycardia
Hereditary angioedema: absence of urticaria or pruritus
Scombroid poisoning: mimics anaphylaxis but follows ingestion of spoiled fish
Carcinoid syndrome: chronic flushing and diarrhea without acute shock
Panic attack: tachycardia and dyspnea may occur, but blood pressure is normal and urticaria/angioedema are absent