Allergic Reaction
ABCs & initial assessment
assume anaphylaxis until proven otherwise, give epinephrine early
- airway: stridor, voice change, tongue / lip swelling → basic maneuvers, BVM if needed, call ICU / ENT early, prepare for intubation
- breathing: wheeze, dyspnea, hypoxia → high-flow O₂, nebulized salbutamol, consider epinephrine
- circulation: hypotension, tachycardia → IV access, 1–2 L crystalloid, continuous monitoring
- disability: confusion, dizziness → suggests hypoperfusion / hypoxia, reassess frequently
- exposure: urticaria, angioedema → look for systemic involvement
red flags
escalate early
- persistent hypotension despite fluids + epinephrine
- worsening airway compromise
- biphasic reaction → monitor 6–24 hr
- on discharge: prescribe an adrenaline auto-injector + allergy / immunology referral
- ACEi angioedema → often non-responsive; airway focus + ICU early
diagnostic criteria
- acute onset + ANY of the following:
- 1: known allergen + hypotension (SBP < 90 mmHg or ↓ ≥ 30% baseline)
- 2: skin / mucosa + ≥ 1 system (cardio: hypotension, syncope; resp: wheeze, stridor, dyspnea)
- 3: suspected allergen + ≥ 2 systems
- skin → urticaria, angioedema
- respiratory → wheeze, stridor
- GI → vomiting, diarrhea
- cardiovascular → hypotension, collapse
HPI & examination
- history: trigger (food, drugs, insect stings, environment); previous reactions / allergies; medications (antibiotics, NSAIDs, contrast)
- examination: skin (urticaria, angioedema); airway (hoarseness, stridor, swelling); respiratory (wheeze, distress); cardio (hypotension, tachycardia); neuro (dizziness, syncope)
management
- immediate treatment
- IM Epinephrine 0.5 mg (1:1000) mid-thigh; repeat every 5–15 min if needed
- lay flat, legs raised; do not sit or stand up (sudden upright posture can precipitate arrest)
- high-flow oxygen; IV fluids 1–2 L NS bolus
- adjuncts
- nebulized salbutamol (bronchospasm)
- antihistamines: Cetirizine 10 mg PO or Chlorpheniramine 10 mg IV
- steroids: Hydrocortisone 100–200 mg IV or Methylprednisolone 1–2 mg/kg
- adjuncts do not prevent biphasic reactions and must never delay or replace epinephrine
- supportive: stop offending agent; continuous monitoring; watch for a biphasic reaction
investigations (do not delay treatment)
| test | why |
|---|---|
| serum tryptase | confirms diagnosis (1–3 hr peak) |
| ABG / VBG | respiratory distress |
| ECG ± troponin | persistent hypotension |
| bloods / crossmatch | if unstable / preparing for escalation |
by severity
- mild (isolated symptoms): observe; oral antihistamines: Cetirizine 10 mg, Loratadine 10 mg
- moderate (more symptoms, not anaphylaxis): add steroids; nebulized bronchodilators if needed
escalation
- refractory → adrenaline infusion + ICU (± vasopressors); IV glucagon 1–2 mg if on a beta-blocker
- airway compromise → early intubation
- unclear diagnosis → treat as anaphylaxis
studying for the IM exam? the IM Rapid Review covers anaphylaxis in the same format. see the sample chapter.
Last reviewed · June 2026