Anaphylaxis
Anaphylaxis is the sudden onset of life-threatening airway and/or breathing and/or circulation problems after exposure to an allergen. Skin rash, wheezing and inspiratory stridor and hypotension are the cardinal signs and symptoms.
Image: “Signs and symptoms of anaphylaxis” by Mikael Häggström, CC0, via Wikimedia Commons.
It is a severe, generalised hypersensitivity reaction, caused by the release of immune and inflammatory mediators from basophils and mast cells. Its lifetime prevalence is anywhere between 0.05% and 2% of the total population. Risk factors include:
- atopy/asthma
- History of anaphylaxis
- Exposure to common sensitizer
- Female sex
The cause of anaphylaxis is the re exposure to a pre-sensitized allergen. Common allergens include foods, drugs and insect stings, sometimes a combination of factors is required to trigger anaphylaxis (e.g. alcohol, or NSAIDs). Allergens are introduced to the body by various means, and on first exposure the person forms IgE antibodies specific to the antigen, which then attach to high affinity Fc receptors on basophils and mast cells. On subsequent exposure, the binding of antigen to the IgE antibodies leads to bridging and triggers the degranulation of mast cells.
Anaphylaxis can also be non immunologically mediated, this is instead known as an anaphylactoid reaction, although it presents in the same manner.
The cardinal symptom of shock (severe hypotension) is caused by immune mediators which act in a number of ways:
- Depress myocardial functionality
- Cause vasodilation
- Increase vessel permeability
Airway obstruction is caused by:
- Fluid leaking from capillary blood vessels and causing tissue swelling
- Altered smooth muscle tone causing bronchospasm
Urgent management - ABCDE
- Secure airway, give 100% oxygen and intubate if obstruction imminent
- Remove the cause - raise feet to help circulation
- Give IM 0.5mg of 1:1000 adrenaline (repeat every 5 min if needed guided by BP, pulse and RR)
- Secure IV access
- Give chlorphenamine 10mg IV and hydrocortisone 200mg IV
- IV fluids (0.9% saline 500mL over 15 min up to 2L)
- If still hypotensive admit to ICU and consider IV adrenaline with or without aminophylline and nebulized salbutamol