Anaphylaxis

Overview

Sudden onset, rapidly progressive reaction with airway and/or breathing and/or circulation compromise, usually with skin/mucosal changes. Absence of skin changes does not exclude the diagnosis.

•Urticaria/erythema - pruritic wheals; present in up to 80%

•Angioedema - swelling of lips, tongue, face, throat

•Stridor - laryngeal oedema, upper airway obstruction; greater immediate threat to life than bronchoconstriction

•Wheeze/dyspnoea - lower airway bronchoconstriction

•Hypotension and tachycardia - distributive shock; SBP <90 mmHg is a red flag

🚨

Stridor = laryngeal oedema = upper airway obstruction. This takes absolute priority - nebulised bronchodilators do NOT address upper airway oedema and must never delay IM adrenaline.

Investigations

•Anaphylaxis is a clinical diagnosis - do not delay treatment to await results

•Serum mast cell tryptase - draw at 1-2 hours after onset and again at 24 hours (baseline); raised level supports mast cell activation; normal tryptase does NOT exclude anaphylaxis (especially food-triggered)

•Gold standard (post-acute): skin prick testing and allergen-specific IgE - performed by allergy specialist after recovery

Management

🥇 First-line

•IM adrenaline - given immediately into the anterolateral aspect of the middle third of the thigh

•Repeat after 5 minutes if no improvement; if two doses fail, seek senior help for IV adrenaline (specialist only)

•High-flow oxygen, IV access, fluid resuscitation for shock

•Antihistamines and corticosteroids are not first-line and must not delay adrenaline (corticosteroids no longer routinely recommended per Resuscitation Council UK 2021)

Age group	Dose	Volume (1:1,000)
<6 months	100-150 micrograms	0.1-0.15 mL
6 months - 5 years	150 micrograms	0.15 mL
6-11 years	300 micrograms	0.3 mL
≥12 years / adults	500 micrograms	0.5 mL

🎯

Always use 1:1,000 (1 mg/mL) for IM anaphylaxis dosing. 1:10,000 (0.1 mg/mL) is for IV cardiac arrest dosing only. 0.5 mL of 1:1,000 = 0.5 mg = 500 micrograms.

⚠️

IV adrenaline is reserved for specialists experienced in titrating vasopressors (e.g. anaesthetists, intensivists) or refractory anaphylaxis with cardiovascular collapse - not routine emergency management.

Follow-up

•Observation - minimum 6-12 hours for significant anaphylaxis due to risk of biphasic reaction (second wave 1-12 hours after initial reaction, without re-exposure)

•Prescribe two adrenaline auto-injectors (e.g. EpiPen/Jext) on discharge with training for patient and carers

•Written anaphylaxis action plan on discharge

•Refer to specialist allergy clinic for allergen identification (skin prick testing, specific IgE)

•Venom immunotherapy referral if systemic reaction to insect sting

•Review beta-blockers and ACE inhibitors - blunt response to adrenaline and worsen severity