anaphylaxis

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.

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:
- commence iv adrenaline infusion at 0.05-1.0mcg/kg/min (see adrenaline / epinephrine for infusions)
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

may be given, especialy if bronchospasm present, although otherwise, no clear benefits demonstrated
methylprednisolone iv 1mg/kg or hydrocortisone

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
- see Allergy.org.au guidelines for prescribing 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
consider MedicAlert bracelet

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