Key Facts

Anaphylaxis is a medical emergency requiring immediate recognition and treatment with adrenaline. It is an IgE-mediated (or non-IgE mediated) systemic allergic reaction that can be fatal within minutes. All healthcare professionals should be able to recognise and treat anaphylaxis.

Epidemiology

Anaphylaxis incidence is approximately 50-100 per 100,000 person-years in the UK and is increasing, particularly food-triggered reactions. Fatal anaphylaxis accounts for approximately 20 deaths per year in the UK. Food is the most common trigger in children; drugs and insect stings in adults.

Aetiology

• Food: Peanuts, tree nuts, shellfish, milk, eggs, sesame, wheat (most common trigger in children)
• Drugs: Antibiotics (penicillin), NSAIDs, anaesthetic agents
• Insect venom: Bee and wasp stings
• Latex: Healthcare workers, patients with spina bifida
• Exercise-induced: Particularly food-dependent exercise-induced anaphylaxis
• Idiopathic: No trigger identified in ~20%

Pathophysiology

Anaphylaxis is primarily a type I (IgE-mediated) hypersensitivity reaction. Cross-linking of IgE on mast cells and basophils triggers degranulation and release of histamine, tryptase, leukotrienes, and prostaglandins. This causes vasodilation (hypotension), increased vascular permeability (oedema, urticaria), bronchospasm, and upper airway oedema. Non-IgE-mediated (anaphylactoid) reactions produce identical clinical features via direct mast cell activation.

Diagnostic Criteria (Any ONE of the following with acute onset)

• Airway: Swelling of tongue, throat, laryngeal oedema, stridor, hoarseness
• Breathing: Bronchospasm, wheeze, increased work of breathing, cyanosis, SpO2 <92%
• Circulation: Hypotension, tachycardia, pale, clammy, reduced consciousness, cardiac arrest
• Skin/mucosal: Urticaria, erythema, angioedema (present in ~80% but may be absent)
• GI: Abdominal pain, vomiting, diarrhoea

Rapid Onset (Usually Within Minutes)

• Skin changes may precede or follow systemic features
• Cardiovascular collapse may be the ONLY feature (especially drug-triggered)
• GI symptoms common in food-triggered anaphylaxis

Red Flags

• Stridor or tongue swelling — imminent airway obstruction
• Wheeze with desaturation — severe bronchospasm
• Hypotension unresponsive to initial adrenaline — repeat dose at 5-min intervals
• Previous fatal/near-fatal anaphylaxis — high-risk patient
• Asthma — increases severity of anaphylaxis (respiratory arrest risk)

Bedside

• Clinical diagnosis: Do NOT delay treatment for investigations
• Observations: SpO2, BP, HR — continuous monitoring
• Peak flow: If bronchospasm predominant

Bloods

• Serum tryptase: Sample 1 — as soon as feasible after treatment; Sample 2 — 1-2 hours; Sample 3 — >24 hours (baseline). Elevated tryptase confirms mast cell degranulation. May be normal in food-triggered anaphylaxis.
• ABG: If respiratory compromise

Imaging

• CXR: If respiratory distress — exclude pneumothorax, aspiration

Special Tests

• Specialist allergy testing (6 weeks post-event): Skin prick tests, specific IgE for suspected triggers, component-resolved diagnostics
• Serum IgE and specific IgE: To suspected allergens
• Drug provocation testing: In selected cases with specialist supervision

Non-pharmacological

• Remove trigger: Stop drug infusion, remove insect sting (scrape, don't squeeze)
• Position: Supine with legs raised (if breathing allows); seated if respiratory distress predominant
• Call for help: 999 or 2222
• High-flow oxygen: 15L/min via non-rebreather mask

Pharmacological

• Adrenaline IM (1:1000): Adult 500mcg (0.5mL); Child 6-12yr 300mcg; Child <6yr 150mcg — anterolateral thigh. Repeat every 5 min if no improvement.
• IV fluids: 500mL-1L crystalloid bolus (20mL/kg in children) — repeat as needed for hypotension
• Chlorphenamine IV: Adult 10mg; Child 6-12yr 5mg; <6yr 2.5mg
• Hydrocortisone IV: Adult 200mg; Child 6-12yr 100mg; <6yr 50mg (prevents biphasic reaction)
• Bronchospasm: Salbutamol 5mg nebulised; ipratropium 500mcg nebulised
• Refractory anaphylaxis: IV adrenaline 50mcg boluses (1mL of 1:10,000); adrenaline infusion 0.05-1mcg/kg/min; glucagon 1-2mg IV if on beta-blockers

Surgical/Interventional

• Emergency intubation/cricothyroidotomy: If complete airway obstruction from angioedema

Referral Criteria

• ALL patients with anaphylaxis — refer to specialist allergy clinic
• Prescribe 2 adrenaline auto-injectors (EpiPen/Jext) with training
• Provide personalised anaphylaxis action plan
• MedicAlert bracelet recommended
• If drug-triggered — report via Yellow Card Scheme

• Mortality: <1% overall; ~20 deaths/year in UK from anaphylaxis
• Food anaphylaxis: Most common cause of fatal anaphylaxis in young people
• Drug anaphylaxis: Most common cause of fatal anaphylaxis in adults
• Delayed adrenaline: Single most important factor in fatal anaphylaxis — every patient should carry 2 auto-injectors
• Biphasic reactions: 5-20% at 4-12 hours — all patients observed for minimum 6-12 hours
• Long-term: Excellent prognosis with allergen avoidance and emergency action plan; venom immunotherapy is 95% effective for insect sting allergy

Adrenaline Auto-Injector Doses

Resuscitation Council UK Anaphylaxis Algorithm (Simplified)