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Clinical features of anaphylaxis

  • anaphylaxis is a rapid onset, systemic, type 1 hypersensitivity reaction which is an IgE antibody mediated allergic reaction which results in histamine release and release of other mediators from mast cells which occurs in some 1 in 1000 people but thankfully fatal cases are rare with an annual incidence of only 1 per 5 million population
  • it is defined clinically by an acute reaction involving the presence of either1):
    • respiratory difficulty, which may be due to laryngeal oedema or bronchospasm (asthma), and/or
    • hypotension, which may present as fainting, collapse, or loss of consciousness
  • it should be distinguished from benign non-allergic urticaria (the far majority of urticaria with or without angioedema is non-IgE mediated and NOT anaphylaxis) and angioedema (which is at least 100 times more common), and that adrenaline is used only in those patients that truly require it.
    • whilst it is critical to avoid delaying adrenaline in a patient with true anaphylaxis, giving a patient a diagnosis of anaphylaxis inappropriately can not only lead to unnecessary prescriptions of adrenaline but may also lead to increased anxiety levels for patients and family.

Differential diagnosis

Brief overview of Mx

early adrenaline as 1st line Rx

  • 1:1000 im 0.01ml/kg to maximum 0.5ml (Adult 0.5ml) im into lateral thigh, repeat after 5min if no improvement
    • do not give s/c as absorption is unreliable
    • do not give iv adrenaline / epinephrine bolus doses unless arrest imminent
  • if hypotensive:
    • elevate legs, lie patient flat, although may need to have head up at 45 deg if short of breath
    • iv fluid boluses of 10-20ml/kg 0.9% saline
  • if upper airway obstruction, consider nebulised adrenaline as for croup (below) in addition to im adrenaline
  • if inadequate response to repeat im adrenaline dose:
  • if airway obstruction not settling, consider early intubation and rapid sequence induction (RSI) for emergency intubation
  • if requires 2 or more adrenaline doses or more than 20ml/kg saline, contact ICU

adjunctive Rx

if wheezing
  • may help pruritus
  • preferably non-sedating second generation antihistamines (H1)
  • avoid promethazine as it is sedating and may cause hypotension


  • all patients with anaphylaxis should be observed for 6-12 hours or overnight as risk of rebound if either:
    • more than 1 dose of adrenaline needed
    • if iv fluid bolus needed
    • if lives too far from hospital to return ASAP
  • consider ED Short Stay Observation Unit if suitable
  • discharge advice
  • consider EpiPen
    • a GP cannot prescribe an EpiPEN under PBS as they will need to refer to a paediatrician or immunologist which will delay access, or prescribe a non-PBS more expensive script (~$100)
    • an ED doctor CAN prescribe an EpiPEN as follows:
      • contact PBS Authority 1800 888 333 to get an authority number (you will need patient's Medicare number)
        • authority requirement:
          • “Initial sole PBS-subsidised supply for anticipated emergency treatment of acute allergic reactions with anaphylaxis in a patient who has been discharged from hospital or an emergency department after treatment with adrenaline for acute allergic reaction with anaphylaxis.”
      • write the PBS Authority script
      • AND give the patient a ASCIA anaphylaxis action plan available from here
      • AND explain how to use the epiPEN

potential precipitants

patient information sheets

references and other resources

anaphylaxis.txt · Last modified: 2021/10/26 01:30 by gary1

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