Anaphylactic shock

Anaphylactic shock is life-threatening anaphylaxis with hypotension and signs of tissue hypoperfusion due to a systemic hypersensitivity reaction with massive mediator release from mast cells/basophils.

Etiology / triggers

Common triggers:

• Foods (e.g., peanuts/tree nuts, shellfish, milk, egg)
• Drugs (especially β-lactams; NSAIDs)
• Insect stings (Hymenoptera)
• Latex
• Radiocontrast media (often non-IgE mediated)

Risk factors for severe course: asthma, delayed adrenaline, older age, cardiovascular disease, mastocytosis, β-blockers/ACE inhibitors.

Pathogenesis / pathophysiology

Allergen exposure → mast cell/basophil activation → mediator release (histamine, leukotrienes, prostaglandins, cytokines) →

• systemic vasodilation + increased capillary permeability → relative/absolute hypovolemia → hypotension/shock
• bronchospasm + laryngeal edema → respiratory compromise
• mucosal edema + GI smooth muscle effects → abdominal symptoms

Clinical manifestations

Typical rapid onset (minutes–hours).

• Skin/mucosa: urticaria, flushing, pruritus, angioedema
• Respiratory: wheeze, stridor, dyspnea, hypoxemia
• Cardiovascular: hypotension, tachycardia, syncope/collapse
• GI: cramps, vomiting, diarrhea
• Neurologic: anxiety, confusion (from hypoperfusion/hypoxia)

Diagnosis

Clinical diagnosis (do not delay treatment). Suggested criteria (any one):

1. Acute onset with skin/mucosal involvement AND (respiratory compromise OR hypotension/end-organ symptoms)
2. Two or more after likely allergen: skin/mucosa, respiratory compromise, hypotension, persistent GI symptoms
3. Hypotension after known allergen exposure

Treatment (emergency)

First-line: intramuscular adrenaline (epinephrine).

• Call for help, stop exposure
• Lay patient supine and elevate legs (left lateral if pregnant); do not allow sudden standing/sitting
• Adrenaline IM into mid-anterolateral thigh:
  • Adults: 0.5 mg IM (0.5 mL of 1 mg/mL)
  • Children: 0.01 mg/kg IM (max usually 0.3 mg in small children; up to 0.5 mg in adolescents)
  • Repeat every 5 minutes if no improvement
• High-flow oxygen
• IV access + rapid fluids (crystalloids; adults often 500–1000 mL bolus, repeat as needed)
• Inhaled β2-agonist (e.g., salbutamol) for bronchospasm (adjunct)
• Antihistamines and corticosteroids are adjuncts (do not replace adrenaline)

Refractory anaphylactic shock:

• Consider adrenaline infusion/vasopressors in monitored setting
• If on β-blockers and poor response: consider IV glucagon (specialist/ICU setting)

Investigations (after stabilization)

• Serum tryptase (supports diagnosis; take soon after reaction and a baseline later)
• Identify trigger: allergy assessment/testing after recovery

Complications

• Biphasic reaction (recurrence after initial improvement)
• Hypoxic injury, arrhythmias
• Airway obstruction

Prevention

• Avoidance of confirmed trigger
• Adrenaline auto-injector prescription + patient education
• Allergy specialist referral

Summary

Anaphylactic shock is life-threatening anaphylaxis with hypotension/hypoperfusion; IM adrenaline is the first-line treatment and should not be delayed.

Diagram

Signs and symptoms of anaphylaxis

References

• RESUSCITATION COUNCIL UK. Emergency treatment of anaphylaxis: Guidelines for healthcare providers [online]. Resuscitation Council UK, 2021. [cited 18/12/2025]. Available from: https://www.resus.org.uk/sites/default/files/2021-05/Emergency%20Treatment%20of%20Anaphylaxis%20May%202021_0.pdf
• CARDONA, V., et al. World Allergy Organization Anaphylaxis Guidance 2020 [online]. World Allergy Organization Journal, 2020. [cited 18/12/2025]. Available from: https://www.worldallergyorganizationjournal.org/article/S1939-4551%2820%2930375-6/fulltext
• MURARO, A., et al. EAACI guidelines: Anaphylaxis (2021 update) [online]. Allergy, 2022. [cited 18/12/2025]. Available from: https://pubmed.ncbi.nlm.nih.gov/34343358/