Allergy & Immunology

Anaphylaxis

A rapidly developing systemic hypersensitivity reaction in which mast-cell mediator release obstructs the airway, closes the small airways and empties the circulation, reversed by intramuscular adrenaline given within minutes.

In a nutshell

Anaphylaxis is a rapidly developing hypersensitivity reaction causing life-threatening airway, breathing or circulation compromise, with or without a rash. Lie the patient flat and give adrenaline 500 micrograms intramuscularly (0.5 mL of 1 in 1000) into the anterolateral thigh, repeated every 5 minutes until they improve.

Classic presentation

Minutes after a nut, an antibiotic or a wasp sting, a patient develops throat tightness, wheeze and dizziness with urticaria and facial swelling, although the skin can look entirely normal.

Key points

Skin or mucosal changes are subtle or absent in 10 to 20% of reactions, and skin changes alone, without airway, breathing or circulation compromise, are not anaphylaxis.
Adrenaline treats all three problems at once: alpha-1 vasoconstriction restores blood pressure and shrinks airway oedema, beta-2 bronchodilates and beta-1 supports the heart.
Biphasic anaphylaxis is recurrence within 72 hours of full recovery with no further allergen exposure, and is treated exactly like the first reaction.
In hospital, draw adrenaline from an ampoule rather than firing an auto-injector: auto-injector strengths cannot deliver an age-appropriate dose for most patients.
Idiopathic anaphylaxis is a diagnosis of exclusion, reached only after every recognised cause, including delayed reactions such as alpha-gal allergy, has been ruled out.
A reaction during a supervised allergy challenge can still be discharged at 2 hours even if two adrenaline doses were needed.

First-line investigation

None before treatment: anaphylaxis is a clinical airway, breathing, circulation, disability and exposure (ABCDE) diagnosis. Timed serum mast-cell tryptase is taken afterwards and never changes acute management.

Management

Recognise it and give adrenaline

Airway, breathing or circulation compromise after a likely trigger is anaphylaxis, rash or no rash. Call for help, stop the trigger, and give adrenaline now.^1,2
Adult or child over 12 years: adrenaline 500 micrograms IM (0.5 mL of 1 in 1000) into the anterolateral middle third of the thigh, repeated every 5 minutes until improvement.^1,3
Lie the patient flat with legs raised. Sitting or standing up suddenly can cause fatal collapse within minutes; allow semi-recumbent only if breathing is the dominant problem.¹
Paediatric IM adrenaline, 1 in 1000: under 6 months 100 to 150 micrograms (0.1 to 0.15 mL), 6 months to 6 years 150 micrograms (0.15 mL), 6 to 12 years 300 micrograms (0.3 mL).^1,3

Oxygen, fluids, and what is no longer given

Highest available oxygen, then titrate to saturations of 94 to 98%. IV crystalloid bolus 500 to 1000 mL in an adult, or 10 mL/kg in a child, repeated by response.¹
Nebulised salbutamol 5 mg for persisting wheeze, never instead of more adrenaline. Attach pulse oximetry, non-invasive blood pressure and a 3-lead ECG once treatment has started.^1,11
Chlorphenamine and hydrocortisone were dropped from routine emergency treatment in 2021. Cetirizine 10 mg orally treats residual skin symptoms after stabilisation; corticosteroids are not routine.^1,12,13

Refractory anaphylaxis

Persisting symptoms after two appropriate IM doses is refractory anaphylaxis. Call critical care and an anaesthetist: they run an adrenaline infusion, 1 mg in 100 mL of sodium chloride 0.9%.^1,3
Do not give IV adrenaline boluses to a patient with a pulse. In cardiac arrest start cardiopulmonary resuscitation (CPR), because intramuscular absorption is then unreliable.¹

Confirm, then observe from resolution

Mast-cell tryptase: one sample once treatment has started, a second 1 to 2 hours (no later than 4 hours) after onset, and a baseline at least 24 hours later.^7,1
Discharge at 2 hours only after a single IM dose given within 30 minutes worked in 5 to 10 minutes, with full resolution, two auto-injectors held and supervision available.⁶
Otherwise observe at least 6 hours (two doses, or previous biphasic reaction) or at least 12 hours (more than two doses, severe asthma, out-of-hours, poor access to care).⁶

Prevent the next one

Discharge with two in-date auto-injectors, EpiPen or Jext at 150 or 300 micrograms, plus a brand-specific trainer demonstration and advice to carry both at all times.^3,8,15
Teach: use at the first sign, dial 999 and say anaphylaxis, lie flat with legs raised, and use the second device after 5 minutes if no better.¹⁵
Offer referral to an age-appropriate specialist allergy service. Admit under-16s who do not meet the 2-hour discharge criteria under a paediatric team.^9,14

Exam traps

Sitting a hypotensive patient up, or walking them to a trolley, can kill within minutes: lie them flat with legs raised and keep them there.
Do not wait for a rash. Skin changes are subtle or absent in 10 to 20% of reactions, and airway, breathing or circulation compromise alone is enough.
The BNF still lists chlorphenamine 10 mg and hydrocortisone 100 to 300 mg for anaphylaxis; Resuscitation Council UK removed both from routine emergency treatment in 2021.
Adrenaline goes intramuscularly into the thigh, not into the deltoid and not intravenously. Intravenous boluses are for cardiac arrest, or clinicians who titrate vasopressors routinely.
Adrenaline may fail in a patient taking a beta-blocker; glucagon is the next step once an infusion and adequate fluids have not worked.
A normal mast-cell tryptase does not exclude anaphylaxis, and a raised one means little without the clinical picture and the sampling times.
Improvement is not a discharge criterion: the observation clock runs from complete resolution and its length depends on how many adrenaline doses were needed.

Illustrations

Anaphylaxis treatment algorithmA flow diagram of the Resuscitation Council UK anaphylaxis algorithm: recognise airway, breathing or circulation compromise, lie the patient flat, then give adrenaline 500 micrograms intramuscularly into the thigh. Repeat at 5 minutes, add oxygen and fluids, and escalate refractory cases to a specialist-run adrenaline infusion.PassFinals · original

Clinical features of anaphylaxisA body map of anaphylaxis by system: airway, breathing, circulation, disability and skin or mucosal changes. Skin findings can be subtle or absent in 10 to 20% of reactions; airway, breathing or circulation compromise defines the emergency.PassFinals · original

Pathophysiology of anaphylaxisA diagram of mast-cell activation and mediator release causing airway oedema, bronchospasm, vasodilatation, capillary leak, urticaria and angioedema. The alpha-1, beta-1 and beta-2 actions of adrenaline are shown at each site.PassFinals · original

Allergic angioedemaA patient photograph showing periorbital angioedema, a mucosal feature that may accompany anaphylaxis but does not by itself define it.James Heilman, MD, Wikimedia Commons · CC-BY-SA-3.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.