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Anaphylaxis

Key Points ⚡

  • Anaphylaxis is a severe, rapid-onset, potentially fatal systemic hypersensitivity reaction caused by IgE-mediated mast cell degranulation.
  • Incidence: 50-112 episodes per 100,000 person-years; lifetime prevalence 0.3-5.1%.
  • Common triggers: food (peanuts, tree nuts, milk), medications (antibiotics, neuromuscular blockers), insect stings (bees, wasps), latex, exercise, and idiopathic causes.
  • Risk factors: previous anaphylaxis, allergies, asthma, atopic dermatitis, allergen exposure, age, cardiovascular disease, mast cell disease, beta-blocker/ACE inhibitor use.
  • Symptoms: sudden onset, rapid progression; airway compromise (throat closing, stridor), breathing difficulties (dyspnoea, wheeze), circulatory collapse (hypotension, tachycardia), skin changes (urticaria, angioedema).
  • Diagnosis is clinical: sudden onset with airway/breathing/circulatory compromise plus skin/mucosal changes; supported by exposure to known allergen.
  • Investigations include routine observations, ECG, blood tests (FBC, U&Es, CRP, LFTs, coagulation), ABG if hypoxic, and serum mast cell tryptase (elevated but not definitive).
  • Management: ABCDE approach, immediate IM adrenaline (0.5ml 1:1,000) repeated every 5 minutes if needed, oxygen, remove trigger, positioning; refractory cases may require IV adrenaline infusion.
  • Adjuncts: antihistamines for skin symptoms post-stabilization, corticosteroids for refractory or ongoing symptoms.
  • Discharge: referral to allergy specialists, education, adrenaline auto-injector prescription, biphasic reaction warning.
  • Complications: anaphylactic shock, respiratory failure, biphasic reactions, myocardial infarction, death.

Introduction

Anaphylaxis is an extreme form of allergy—an inappropriate exaggerated immune response to harmless environmental substances that rapidly progresses and can be fatal if untreated. It is a type I hypersensitivity reaction mediated by IgE antibodies triggering mast cell and basophil degranulation.

Aetiology

  • IgE antibodies bind allergens and trigger mast cells/basophils to release histamine, tryptase, chymase → vasodilation, increased permeability, bronchospasm, mucus secretion.
  • Anaphylactoid reactions mimic anaphylaxis but are non-IgE mediated.

Triggers

  • Food: peanuts, tree nuts, cow’s milk most common.
  • Medications: penicillins, neuromuscular blockers, chlorhexidine.
  • Insect stings: bees, wasps.
  • Latex, exercise (rare), idiopathic causes.

Risk Factors

  • Prior anaphylaxis episodes (1 in 12 risk/year).
  • Allergic diseases (asthma, rhinitis, eczema).
  • Repeated allergen exposure.
  • Age, cardiovascular diseases, mast cell disorders.
  • Medications: beta-blockers, ACE inhibitors.

Clinical Features

History

  • Sudden onset post-exposure to allergen.
  • Airway: throat tightness, hoarseness, stridor, swollen tongue/lips.
  • Breathing: dyspnoea, tachypnoea, wheeze, cyanosis.
  • Circulation: tachycardia, hypotension, weak pulse, cold clammy skin.
  • Skin/mucosa: urticaria, flushing, pruritus, angioedema.

Differential Diagnosis

  • Simple allergic reaction (skin only).
  • Other causes of sudden SOB: foreign body, epiglottitis, asthma, PE, pneumothorax, panic attack.
  • Oral allergy syndrome (mild oropharyngeal symptoms).
  • Other rash causes: carcinoid syndrome, vancomycin reaction, scombroid poisoning.

Investigations

  • Bedside: vital signs, ECG to exclude cardiac causes.
  • Labs: FBC, U&Es, CRP, LFTs, coagulation; ABG if hypoxic.
  • Serum mast cell tryptase: taken within 2-4 hours of onset; elevated supports but does not exclude diagnosis.
  • Chest X-ray post-stabilization if indicated.

Diagnosis

  • Clinical: sudden onset, airway/breathing/circulation compromise, skin/mucosal involvement.
  • Skin signs alone are insufficient for diagnosis.
  • If in doubt, treat as anaphylaxis immediately.

Management

Emergency Treatment

  • Call for help.
  • Remove trigger.
  • ABCDE assessment and support.
  • Positioning: sitting if airway/breathing problems; supine with legs raised if circulatory shock.
  • Administer IM adrenaline 0.5ml of 1:1000 into mid-thigh; repeat every 5 minutes if needed.
  • Oxygen 100%.
  • IV fluids for hypotension.
  • Bronchodilators for bronchospasm (nebulised salbutamol).
  • Consider airway support/intubation early.

Refractory Anaphylaxis

  • Persistent symptoms after 2 doses of IM adrenaline.
  • Admit expert help; consider low-dose IV adrenaline infusion under specialist supervision.
  • IV hydrocortisone for ongoing symptoms/asthma (not routine initial treatment).
  • Antihistamines for skin symptoms only post-stabilization.

Pregnancy

  • Similar management; position left lateral to avoid vena cava compression.
  • Manual uterine displacement if >20 weeks gestation during CPR.

Cardiac Arrest in Anaphylaxis

  • Follow ALS protocol.
  • Use IV/intraosseous adrenaline as per ALS guidelines.

Post-emergency Treatment and Discharge

  • Observe in hospital for risk of biphasic reactions.
  • Referral to allergy specialist.
  • Provide education on trigger avoidance.
  • Prescribe and train in adrenaline auto-injector use (2 devices).
  • Inform about biphasic reaction risk and patient support groups.

Complications

  • Anaphylactic shock (major cause of death).
  • Respiratory failure.
  • Biphasic anaphylaxis (symptom recurrence within 72 hours).
  • Myocardial infarction (due to hypotension or adrenaline side effects).
  • Death, usually rapid post-exposure.

References

  • Aldakheel FM, Int J Environ Res Public Health, 2021.
  • Cardona V et al., World Allergy Org J, 2020.
  • NICE Clinical Knowledge Summaries, 2022.
  • BMJ Best Practice, 2023.
  • Reber LL et al., J Allergy Clin Immunol, 2017.
  • Resuscitation Council UK, 2021 Guidelines.
  • Additional references per source.

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