Lucan · Conditions

Anaphylaxis

Australian guidance: ASCIA Guidelines — Acute Management of Anaphylaxis (2024), cited by the captured PCH and RCH pages; WA Health PCH ED Anaphylaxis guideline (reviewed Sep 2025)

1. Definition

Anaphylaxis is an immunoglobulin E (IgE) mediated, potentially life-threatening severe allergic reaction that may progress to shock and severe airway compromise.

It is highly likely when either of two criteria is met (World Allergy Organization criteria, carried by sources/rch-cpg/Anaphylaxis.md):

1. Acute onset, minutes to hours, with skin or mucosal involvement (generalised hives, pruritus, flushing, swollen lips, tongue or uvula) and at least one of: - respiratory compromise (dyspnoea, wheeze or bronchospasm, stridor, reduced peak expiratory flow, hypoxaemia); - reduced blood pressure or end-organ dysfunction (collapse, syncope, incontinence); - severe gastrointestinal symptoms (severe crampy abdominal pain, repetitive vomiting), especially after a non-food allergen.
2. Acute onset of hypotension, bronchospasm or laryngeal involvement after exposure to a known or highly probable allergen, minutes to hours, even without skin involvement.

The second arm is the one that matters clinically: anaphylaxis can occur without any skin signs.

2. Pathophysiology

IgE-mediated mast cell and basophil activation on re-exposure to an allergen, with release of histamine and other mediators producing vasodilatation, increased vascular permeability, bronchoconstriction and mucosal oedema. Asthma is a risk factor for a more severe respiratory course, and the severity of past reactions does not reliably predict the severity of the next one. Non-IgE-mediated and idiopathic variants are not carried by either captured source.

3. Clinical features

The captured guideline sorts features into an allergic reaction and anaphylaxis.

Mild to moderate allergic reaction: swelling of the lips, face or eyes; tingling of the mouth; hives or welts; abdominal pain; vomiting.

Anaphylaxis, and only one feature may be present: swelling or tightness in the throat; swelling of the tongue; difficulty speaking or a hoarse voice; difficulty breathing or stridor; wheeze or persistent cough; persistent dizziness; collapse; pallor, floppiness or drowsiness in very young children; abdominal pain and vomiting in insect sting or bite allergy.

Timing depends on the route: parenteral allergens such as IV medication, an intramuscular injection or a sting act in 5 to 15 minutes; ingested food allergens typically by 30 minutes; oral medication reactions more slowly, in 1 to 6 hours.

4. Investigations

Neither captured source carries a laboratory criterion, and serum mast cell tryptase is not mentioned in either, so it is deliberately absent here rather than remembered into the note.

5. Management

The captured sequence:

1. Remove the allergen.
2. Lay the patient flat. If there is respiratory distress they may sit upright. Do not allow them to stand or walk.
3. ABCDE assessment, high-flow oxygen, continuous cardiac monitoring and pulse oximetry, observations every 15 to 30 minutes until stable.
4. Intramuscular adrenaline 1:1000 (1 mg/mL) immediately: 10 microg/kg for all ages, maximum 500 microg. Repeat after 5 minutes if symptoms of anaphylaxis persist.
5. Antihistamines are not indicated in the treatment of anaphylaxis, stated in the source in terms.
6. Shock: two large IV cannulae and sodium chloride 0.9% 20 mL/kg, repeated as needed; an adrenaline infusion if still hypotensive after 40 mL/kg, with critical care referral.
7. Upper airway obstruction: nebulised adrenaline 1:1000, 0.5 mg/kg (0.5 mL/kg) to a maximum of 5 mL undiluted; consider intubation and critical care.
8. Persistent wheeze: salbutamol metered dose inhaler via spacer, 6 puffs under 6 years and 12 puffs at 6 years or over; consider repeating the intramuscular adrenaline and an adrenaline infusion.
9. Observe for at least 4 hours after the last dose of adrenaline. Nobody stands or walks until haemodynamically stable, usually at least 1 hour after one dose and 4 hours after more than one.
10. Before discharge: two adrenaline injector devices prescribed, education and a written anaphylaxis action plan, and follow up with immunology or allergy.

The adult dose is derived, not carried. The captured dose is weight-based at 10 microg/kg with a 500 microg maximum, which for an adult of 50 kg or more is 500 microg, the familiar 0.5 mg. Both captured sources are paediatric, so this note does not present an adult-only regimen or an adrenaline infusion rate.

6. Pharmacology

Adrenaline 1:1000 (1 mg/mL) intramuscularly is the first-line drug. Its alpha-1 effect reverses vasodilatation and mucosal oedema, beta-1 supports cardiac output, and beta-2 bronchodilates and reduces further mast cell mediator release.

Adrenaline injector doses carried by the capture, from the ASCIA recommendations:

7. Complications and prognosis

Fatal anaphylaxis is rare in Australia, approximately 1 per million population, and is mainly triggered by food in children. A biphasic reaction is not described in either capture and is not asserted here.

8. Source list and scope

• sources/pch-ed/Anaphylaxis.md — Perth Children's Hospital Emergency Department guideline, reviewed September 2025. Paediatric scope. Carries the definition, the feature list, the full management sequence, the intramuscular and nebulised doses, the observation period, the injector table and the statement that antihistamines are not indicated.
• sources/rch-cpg/Anaphylaxis.md — Royal Children's Hospital Melbourne Clinical Practice Guideline. Paediatric scope. Carries the World Allergy Organization criteria, the timing by route, and the fatal anaphylaxis prevalence.

Not captured, therefore not asserted: the adult ASCIA acute management guideline in full, any adrenaline infusion regimen, serum tryptase criteria, biphasic reaction incidence, and any paediatric-specific observation nuance beyond the 4 hour rule. Capture the ASCIA guideline before adding an adult regimen.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.