Anaphylaxis
Summary
Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction. It typically occurs within minutes of exposure to an allergen and can be rapidly fatal without treatment. Adrenaline is the first-line treatment and should be given intramuscularly without delay. Common triggers include foods, drugs, and insect stings.
Key Facts
- Definition: Severe systemic allergic reaction with airway/breathing/circulation compromise
- Incidence: 50-100 per 100,000 per year
- Pathognomonic: Multi-system involvement after allergen exposure
- Gold Standard Investigation: Clinical diagnosis; tryptase post-event
- First-line Treatment: IM Adrenaline 0.5mg (1:1000) mid-outer thigh
- Prognosis: Excellent if treated promptly; fatal if delayed
Clinical Pearls
Adrenaline Pearl: IM into mid-outer thigh - never IV except in cardiac arrest.
Repeat Pearl: Can repeat adrenaline every 5 min if no response.
Biphasic Pearl: 5-20% have biphasic reaction - observe 6-12 hours.
| System | Features |
|---|---|
| Airway | Swelling, stridor, hoarseness |
| Breathing | Wheeze, increased RR, cyanosis |
| Circulation | Hypotension, tachycardia, pale |
| Skin | Urticaria, flushing, angioedema |
Algorithm
Treatment
| Intervention | Details |
|---|---|
| Adrenaline | 0.5mg IM (1:1000), repeat every 5 min |
| Lie flat | Legs raised if hypotensive |
| Oxygen | High-flow |
| IV fluids | 500ml-1L bolus if hypotensive |
| Chlorphenamine | 10mg IV/IM (adjunct) |
| Hydrocortisone | 200mg IV (adjunct) |
Post-Event
- Serum tryptase (1-2h and 24h)
- Allergy clinic referral
- Adrenaline autoinjector prescription
-
Resuscitation Council UK. Emergency treatment of anaphylaxis. 2021.
-
NICE guideline CG134. Anaphylaxis: assessment and referral. 2020.
Viva Points
"Anaphylaxis: multi-system allergic reaction. IM adrenaline 0.5mg first-line into thigh - repeat every 5 min. Lie flat, O2, fluids. Antihistamine and steroids are adjuncts. Tryptase and allergy referral."
Last Reviewed: 2026-01-01 | MedVellum Editorial Team