Synopsis
Distinguish allergic mechanisms and common mimics, recognise anaphylaxis clinically, give age-appropriate intramuscular adrenaline without delay, and plan observation, referral and prevention.
- Anaphylaxis is a life-threatening systemic hypersensitivity reaction identified clinically by sudden airway, breathing or circulation compromise, usually with skin or mucosal change; skin signs can be absent.
- Call for help, remove the trigger if feasible, lie the patient flat with legs raised unless breathing requires supported sitting, and do not let them stand or walk.
- Give intramuscular adrenaline 1 mg/mL into the anterolateral thigh promptly: 500 micrograms for adults and children over 12 years, with age-specific smaller paediatric doses.
Key red flags
Hoarse voice, tongue swelling, stridor or rapidly progressive upper-airway symptoms.
Wheeze, hypoxaemia, exhaustion, cyanosis or silent chest after a plausible exposure.
Hypotension, collapse, confusion or shock, with or without visible skin signs.
Persistent ABC compromise five minutes after intramuscular adrenaline or deterioration despite repeated doses.
Hoarse voice, tongue or pharyngeal swelling, stridor, drooling or rapidly progressive throat symptoms require immediate adrenaline and airway help.
Wheeze, tachypnoea, hypoxaemia, exhaustion or reduced air entry can be the dominant anaphylactic feature even without hypotension.
Hypotension, collapse, altered consciousness, pallor and shock reflect vasodilation, capillary leak or arrhythmia and need immediate treatment.
Reasoning priorities
Identify and treat airway, breathing and circulation compromise immediately.
Diagnosis is clinical; no laboratory result should delay intramuscular adrenaline when life-threatening features are present.
Worked reasoning
Minutes after an intravenous medicine, a 34-year-old develops generalised flushing, wheeze, hoarse voice and hypotension while still conscious.
- Call the resuscitation team, stop the suspected trigger, lie the patient flat with legs raised while protecting the airway, and begin ABCDE assessment with oxygen and monitoring.
- Recognise sudden skin change plus airway, breathing and circulation compromise as anaphylaxis; do not wait for a rash to progress, a tryptase result or diagnostic certainty.
- Give 500 micrograms IM adrenaline using 1 mg/mL solution (0.5 mL) into the anterolateral thigh. For hypotension, establish IV access and give 500–1,000 mL isotonic crystalloid rapidly, reassessing blood pressure, perfusion, breathing and overload before further fluid.
- Reassess continuously and repeat IM adrenaline after 5 minutes if ABC problems persist; persistent respiratory or cardiovascular compromise after two appropriate doses is refractory and needs expert critical-care management. Obtain adult timed tryptase samples once emergency treatment has started without delaying resuscitation; after resolution, apply NG258 observation criteria and offer allergy referral. If assessment confirms an easily avoided medicine as the cause, document why the usual two-auto-injector discharge provision is excepted; otherwise supply two with training.
A stable adult develops localised urticaria after contact exposure, with normal voice, breathing, perfusion and observations.