Pathophysiology
Anaphylaxis is a life-threatening type 1 hypersensitivity reaction. It occurs when IgE activation stimulates mast cells to degranulate and release histamine as well as other pro-inflammatory mediators leading to rapid onset ABC compromise.
This is the difference between anaphylaxis and allergy, as non-anaphylactic allergy does not compromise airway, breathing or circulation.
Some common allergens include:
- Food - such as nuts
- Medication - most commonly ß-lactams (such as penicillins) and NSAIDs. General anaesthetics are also a common cause of anaphylaxis.
- Insect stings - mainly wasp and bee stings.
However, sometimes it may be idiopathic.
😷 Presentation
Rapid onset of allergy symptoms:
- Urticaria - a raised rash
- Itching
- Angio-oedema - swelling of deeper layers under skin due to fluid buildup
- Abdominal pain
- Airway - swelling of throat and tongue causes
- Hoarse voice
- Stridor
- Breathing
- Wheeze
- SOB
- Circulation
- Hypotension
- Tachycardia
- Lightheadedness
- Syncope
Technically, if there are no ABC symptoms it is not considered anaphylaxis.
🧰 Management
B - Provide oxygen if needed and salbutamol nebuliser to aid dyspnoea. The nebuliser can be given back to back until symptoms resolve.
C - Provide a bolus of IV fluids.
D - Keep the patient flat to increase cerebral perfusion.
E - Assess for urticaria, flushing, etc. This is one of the few times when there is no time to look up the dose of medication. The Resuscitation Council UK has guidelines for anaphylaxis doses of IM adrenaline. IM adrenaline is by far the most important step in management and needs to be administered immediatel. It is usually given with an epipen auto-injector. It is best given in the anterolateral aspect of the middle 1/3rd of the thigh. It can be repeated every 5 minutes if need be. Recommended dosage is as follows:
Age | Dosage |
<6 months | 100-150mcg (0.1-0.15ml) |
6 months - 6 years | 150mcg (0.15ml) |
6 - 12 years | 300mcg (0.3ml) |
>12 years | 500mcg (0.5ml) |
→ If the patient had no complications and responded well to a single dose of IM adrenaline then they should be fast-tracked for discharge 2 hours after resolution.
→ If 2 doses of adrenaline were needed or the patient has had a previous biphasic reaction then they should remain admitted for at least 6 hours after symptom resolution.
→ If they require >2 doses of adrenaline, have severe asthma, have a possible ongoing reaction (e.g. due to slow release medication), present late at night, are in remote areas with poor access to healthcare, they need to be monitored for at least 12 hours after symptom resolution.
- Non sedating oral-antihistamines - such as cetirizine. Chlorphenamine may also be given.
- 2 adrenaline auto-injector to carry on their person.
- Referral to specialist allergy clinic.
🔍 Investigations
As it is a medical emergency there is no time to do investigations prior to administering IM adrenaline. However, we need to ensure that it was a true anaphylactic reaction, so serum tryptase can be assessed, as these are elevated in patients who have had anaphylaxis for up to 12 hours post-event. It should be done within 6 hours.
🚨 Complications
- Biphasic reaction - a second anaphylactic reaction after resolution of the first reaction. It occurs in 20% of anaphylactic cases.
- Refractory anaphylaxis - anaphylaxis that has failed to improve after two doses of IM adrenaline. In this situation, the Resuscitation Council guidelines (2021) for the management of anaphylaxis recommend that an appropriate expert should be contacted for consideration of low-dose IV adrenaline infusion. An IV fluid bolus should also be given.