Topics

Acute allergic reactions

Overview and approach

Anaphylaxis is a potentially life-threatening systemic allergic reaction, usually rapid in onset: typical skin features (urticaria, flushing, angioedema) plus respiratory, cardiovascular or persistent severe GI involvement; or any acute hypotension or respiratory impairment (bronchospasm, upper airway obstruction) where anaphylaxis is considered possible, even without skin signs (1). Common triggers: foods, insect stings, drugs; less common: latex, tick bites, exercise, cold (1). Urticaria and angioedema alone are localised.

Assessment and clinical features
  • Mild to moderate: swelling of lips/face/eyes, hives, tingling mouth (abdominal pain/vomiting counts as anaphylaxis in insect allergy) (1).
  • Severe (any one): difficult/noisy breathing, swollen tongue, throat tightness, wheeze or persistent cough, hoarse voice, persistent dizziness or collapse, pale and floppy (young children) (1).
  • Persistent dizziness/collapse reflects hypotension; in children, persistent tachycardia is the first cardiovascular sign and hypotension is late (1).
Differentials and investigations

Differentials: asthma (sudden wheeze in a known allergic patient is anaphylaxis until proven otherwise), FPIES or gastroenteritis (severe vomiting in isolation is unlikely anaphylaxis), vasovagal syncope, panic, hereditary angioedema. Tryptase and allergist referral for all anaphylaxis (1). Document episodes and triggers in the preceding 6 to 8 hours (1).

Management and follow-up

Current ASCIA guidance (1):

  • Adrenaline IM first: 1:1000, 0.01 mg/kg up to 0.5 mg per dose (mid-thigh); repeat as needed (5-minute interval per action plan); do not withhold in pregnancy. If asthma with sudden breathing difficulty give adrenaline first, then reliever (1). Adrenaline devices: EpiPen Jr 150 mcg, EpiPen 300 mcg, Anapen 500, Jext, neffy nasal 1 mg/2 mg (1).
  • Lay flat (or sit with legs out if mainly respiratory); do not stand or walk; left lateral in pregnancy; call for help; oxygen and IV access with 20 mL/kg normal saline if persistent; refractory anaphylaxis uses IV adrenaline infusion (1).
  • Adjuncts only: salbutamol for persistent wheeze; nebulised adrenaline for upper airway obstruction; corticosteroids are unproven; non-sedating antihistamines only for skin symptoms after treatment; avoid sedating oral antihistamines and injectable promethazine (can worsen hypotension) (1).
  • Observe at least 4 hours after the last adrenaline dose; overnight if severe/refractory, previous severe or biphasic reaction, asthma, remote or late presentation (1). Biphasic reactions occur in about 3-20% (1).
  • Before discharge: prescribe adrenaline device, train patient, provide ASCIA Action Plan, review asthma control, refer to clinical immunology/allergy specialist (1).

Localised urticaria/angioedema: most hives resolve within 24 hours; treat with antihistamines and avoid triggers; call 000 for breathing difficulty or facial/throat swelling (2).

  • Multi-system
    • acute anaphylaxis
    • anaphylactic reactions
  • Localised
    • angioedema
    • urticaria

Anaphylaxis and anaphylactic reactions

Treatment (adults)

First line

  • Oxygen 6-8 L/min (by face mask)
  • Adrenaline 0.3-0.5mg (1:1000) SC or IM (more severe) best given in upper body (eg deltoid) (mg = mL of 1:1000 adrenaline)
  • If no rapid improvement: insert IV line-give adrenaline 1 in 10000: (dilute 1 mL 1:1000 in 9 mL saline) 5-10 mL IV over 2-5mins
  • Repeat adrenaline every 5-10mins as nec. (IM or IV)
  • Infuse colloid solution, eg Haemaccel (500 mL > 1 L) or crystalloid solution, eg N saline (1.5 L » 3 L), 1 part colloid = 3 parts crystalloid (by vol)
  • Salbutamol aerosol inhalation (or nebulisation if severe)
  • Promethazine 10mg IV slowly (or 25mg IM) or diphenhydramine 10mg IV (or 25mg IM)
  • Admit to hospital (observe at least 4h)
  • Discharge on promethazine 25mg tds + prednisolone 50mg/d for 2d

If not responding

Continue adrenaline every 5mins:

  • hydrocortisone 500mg IV (takes 3-4h for effect)
  • establish airway (oral airway or endotracheal intubation) if required

Treatment for children

  • Oxygen 6-8 L/min by mask
  • Adrenaline 1:1000 (0.01 mL/kg) IM or SC or adrenaline 1:10000 slow IV infusion in dose 0.1 mL/kg of 1:10000 or less if the desired effect is achieved before full dose given
  • Insert IV line
  • Admit to hospital

If necessary

  • Repeat adrenaline IM or IV every 5-10mins
  • Hydrocortisone: 8-10mg/kg IV
  • Hypotension: colloid solutions IV (eg Haemaccel, stable plasma protein solution (SPPS) or Dextran 70)
  • Bronchospasm: continuous nebulised salbutamol
  • Upper airways obstruction
  • mild to moderate: inhaled adrenaline (0.5mL/kg) 1:1000 (max. 4mL)
    dilute to 4 mL, with saline or water if nec.
  • severe: intubation may be nec.

Angioedema and acute urticaria

Acute urticaria and angioedema are essentially anaphylaxis limited to the skin, subcutaneous tissues and other specific organs. They can occur together.

Treatment

Uncomplicated cutaneous swelling

  • antihistamines, eg promethazine 25mg (o) tds or 25mg IM if more severe

Upper respiratory involvement

  • adrenaline 0.3mg SC or IM
  • antihistamine IM
Red flags and clinical pearls
  • Never stand or walk a patient with suspected anaphylaxis; fatalities occur within minutes (1).
  • Asthma plus allergen exposure with sudden breathing difficulty: adrenaline first (1).
  • Pregnant, infants and children: same first-line adrenaline; 150 mcg device may be used for infants 7.5 to 10 kg after considered assessment (1).
  • Hives with throat swelling or breathing difficulty: emergency (2).
References
  1. ASCIA: Acute management of anaphylaxis guidelines (2026)
  2. Healthdirect: Hives

Last updated from Sen’s notes on 9 October 2026.