Anaphylaxis
Emergency Medicine
Illness script · Emergency Medicine
Anaphylaxis
Life-threatening systemic hypersensitivity reaction causing distributive shock via massive mast cell/basophil mediator release.
This illness script for Anaphylaxis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Foods (peanuts, tree nuts, shellfish) — most common in children
- Medications (beta-lactams, NSAIDs, contrast) — most common in adults
- Insect venom (Hymenoptera stings)
- Prior sensitization required for IgE-mediated reactions
- Atopy/asthma increases severity risk
- Mastocytosis predisposes to severe reactions
02
Presentation
- Onset within minutes to 2 hours of exposure
- Urticaria/flushing/pruritus (skin involved in ~90%)
- Bronchospasm: wheezing, stridor, dyspnea
- Hypotension, tachycardia → distributive shock
- GI symptoms: nausea, vomiting, cramping
- Biphasic reaction in ~5%: recurrence 8–72 hours later without re-exposure
03
Pathophysiology
- Re-exposure to antigen → cross-linking of IgE on mast cells/basophils
- Massive histamine, tryptase, leukotrienes, prostaglandins released
- Vasodilation + increased vascular permeability → distributive shock
- Bronchoconstriction + airway edema → respiratory compromise
04
Diagnostics
- Clinical diagnosis — do NOT delay treatment for labs
- Serum tryptase peaks 1–3 hrs post-reaction (confirms mast cell activation)
- Tryptase may be normal in food-triggered anaphylaxis
- Skin prick testing/specific IgE for allergen ID after recovery
- No single test required; ≥2 organ systems + exposure = diagnosis
05
Management
- IM epinephrine 0.3–0.5 mg (1:1000) into lateral thigh — FIRST LINE
- Supine positioning with legs elevated (unless respiratory distress)
- High-flow O2, IV access, NS bolus for hypotension
- Adjuncts: H1 antihistamines, H2 blockers, corticosteroids (do NOT replace epi)
- Observe ≥4–6 hours for biphasic reaction; discharge with epinephrine auto-injector
- Beta-blockers blunt epinephrine response — use glucagon if on beta-blocker
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Vasovagal syncope
Bradycardia + pallor, no urticaria/bronchospasm, no exposure history
Angioedema (hereditary)
No urticaria, no hypotension, C4 low, bradykinin-mediated — epinephrine less effective
Carcinoid syndrome
Episodic flushing without urticaria or shock; elevated urinary 5-HIAA
Systemic mastocytosis
Recurrent episodes, Darier sign (urticaria pigmentosa), chronically elevated tryptase
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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.