Anaphylaxis
Anaphylaxis is a severe systemic type I hypersensitivity reaction that is potentially life-threatening. It usually has sudden onset and rapid progression, with airway, breathing or circulation problems, often alongside skin or mucosal changes. Immediate treatment with IM adrenaline is the key priority.
This updated UKMLA guide to anaphylaxis is based on RCUK and NICE NG258 guidelines, which covers causes, risk factors, symptoms, diagnosis and management.
Definition
| Term | Definition |
|---|---|
| Anaphylaxis | Severe systemic type I hypersensitivity reaction (IgE-mediated) that is potentially life-threatening |
| Refractory anaphylaxis | Persisting respiratory or cardiovascular symptoms despite 2 appropriate doses of IM adrenaline |
| Biphasic anaphylaxis | Recurrence of symptoms within 72 hours after complete recovery of anaphylaxis, in the absence of further exposure to the trigger |
Anaphylaxis should NOT be confused with anaphylactoid reactions (aka non-IgE mediated anaphylaxis, pseudoallergic reactions) [Ref]
- Anaphylactoid reactions arise from non-immune (non-IgE) mediated activation of mast cells
- Examples
- Vancomycin infusion reaction (‘Red-man syndrome’)
- N-acetylcysteine infusion reaction
- Most contrast media reactions
- Still, anaphylactoid reactions are clinically indistinguishable from anaphylaxis, and are treated the same as anaphylaxis (with adrenaline)
Causes and Risk Factors
Common triggers include:
- Food
- Most common trigger in children
- Nuts = leading cause
- Drugs
- Adults: most common trigger
- Most frequently
- Anaesthetics
- Antibiotics (esp. penicillin and cephalosporins)
- NSAIDs / aspirin
- Venoms (wasp & bee stings)
Other causes:
- Idiopathic (significant portion of cases)
- Exercise induced → rare (~2%)
Risk factors for severe anaphylaxis:
- Older age
- Concomitant mastocytosis
Complications and Prognosis
Complications
| Airway and respiratory |
|
| Cardiovascular |
|
| Death |
|
Prognosis
- Prognosis is generally good with prompt treatment (case fatality ratio <1%)
- Higher mortality risk if there is
- Pre-existing asthma (esp. poorly controlled)
- Delay in, or failure to, use adrenaline
- Recurrence risk → previous reactions increase risk of recurrence
Clinical Features and Diagnosis
The Resuscitation Council UK suggests that anaphylaxis is likely when ALL of the following are present:
- Sudden onset and rapid progression of symptoms
- Either one of the ABC problems
- Airway – angioedema, stridor, drooling
- Breathing – widespread wheezing, hypoxia, cyanosis
- Circulation – hypotension, tachycardia, weak thready pulse, reduced GCS
- Skin and/or mucosal changes (e.g. urticarial rash)
In the absence of A/B/C problems, anaphylaxis is unlikely.
If there is just urticaria, consider a simple allergic reaction.
Clinical suspicion of anaphylaxis is sufficient to initiate treatment.
If anaphylaxis is clinically suspected, measure serum mast cell tryptase:
- First sample to be taken ASAP after emergency treatment has started, and
- Second sample to be taken ideally 1-2 hours (but no later than 4 hours) from the onset of symptoms
IMPORTANT: serum mast cell tryptase can be used to help confirm anaphylaxis, but it should NOT delay life-saving treatment, and a normal result does NOT rule out anaphylaxis.
Management Algorithm
Apart from the ABCDE resuscitation measures, adrenaline is the single most important and high-priority management of anaphylaxis.
Initial Management
- Call for help
- Remove trigger if possible (e.g. stop any infusions)
- Reposition the patient
- Upright sitting position for patients with predominantly breathing symptoms
- Supine position +/- leg elevation for patients with predominantly circulatory symptoms
Adrenaline Therapy
Give 1 dose of IM adrenaline into the anterolateral aspect of the middle-third thigh.
If there is no response after 5 minutes after the 1st dose →
- Repeat IM adrenaline (2nd dose)
- Give IV fluid challenge with crystalloid
The Resuscitation Council UK guidelines recommend adrenaline should always be given via the IM route as the 1st line treatment for anaphylaxis, even if IV access is readily available.
This is due to the greater risk of potentially life-threatening adverse effects from dilution error or incorrect dosing. IV adrenaline should only be administered by trained and experienced physicians like anaesthetists and intensive care physicians.
Adrenaline doses:
Adrenaline dilution: 1 in 1,000 or 1 mg/mL
| Age | Dose | Volume |
|---|---|---|
| >12 y/o | 500 micrograms (0.5 mg) | 0.5 mL |
| 6 y/o – 12 y/o | 300 micrograms (0.3 mg) | 0.3 mL |
| 6 months – 6 y/o | 150 micrograms (0.15 mg) | 0.15 mL |
| <6 months | 100-150 micrograms (0.10-0.15 mg) | 0.10 – 0.15 mL |
Refractory Anaphylaxis Management
Seek specialist input to start low-dose IV adrenaline infusion
Before IV adrenaline infusion can be initiated, or if it’s not appropriate:
- IM adrenaline to be given every 5 min
- Rapid IV fluid boluses
IV adrenaline infusion should only be given by an experienced specialist (e.g. intensive care physician, anaesthetist) in an appropriate setting.
Post-Emergency Treatment
Adjunct Therapy
The following medications do NOT form part of the initial emergency management of anaphylaxis. They have no role in treating the life-threatening complications of anaphylaxis, they should only be considered once the patient has been stabilised:
- Oral antihistamines (2nd generation, non-sedating) can be used to treat cutaneous symptoms from allergic reactions
- Corticosteroids should only be considered for refractory reactions or ongoing asthma / shock
Discharge and Follow-Up
Prior to being considered for discharge, all patients should be kept for in-hospital observation for a minimum of 2-12 hours following resolution of symptoms. A risk-stratified approach is recommended to decide the length of in-hospital observation that is warranted (see below).
Prior to discharge, all patients should be offered the following:
- Referral to a specialist allergy service
- Information about anaphylaxis, including how to recognise anaphylaxis and how to avoid the suspected trigger (if known)
- 2 Adrenaline auto-injectors, with appropriate training (via a prescription if needed), unless anaphylaxis was due to a drug allergy and the drug can be easily avoided
- Educate on the self-management of anaphylaxis + safety netting (including when and how to seek help)
- Information on the risk of biphasic anaphylaxis
- Information about patient support groups
Full Risk-Stratified Approach for Period of Observation
Included for completeness, as the exact duration of in-hospital observation after anaphylaxis is unlikely to be examined in detail. The key point is that patients should not be discharged immediately after initial symptom resolution.
| Period of observation | Indications |
|---|---|
| 2 hours | ALL the following must be met:
|
| 6 hours (minimum) | ANY of the following:
|
| 12 hours (minimum) | ANY of the following:
|