severe allergic reaction with involvement of 2 or more organ systems

3 criteria -

1. Sudden onset skin +/- mucous membrane changes with resp or low BP
2. Sudden onset after likely allergen, at least 2 of skin/MM, decreased BP, sudden GI, Resp
3. exposure to known allergen + low BP or 30% drop

Aetiology

• RARELY from inhaled
• most commonly from injections (straight to blood), Foods, insect stings, drugs. can be from any allergen.

pathophysiology

• systemic whole body type I hypersensitivity
• secondary exposure of sensitised allergen, target cells degranulate within 15mins.
• Allergen can cross link multiple IgE molecules, causing multiple and enhanced effects
• Mast cells degranulation (histamine, 5-HT and cytokines) → multiple effects (inflammation, vasodilation (oedema), SMC contraction) depending on tissue. if degranulating in blood → systemic effects
• 2nd wave of cells recruited by degranulation products and cause further 'late phase' effects 4-24hrs

Clinical Features

• systemic vasodilation → hypotension +/- shock. Can be life threatening.
• acute deterioration: rash, bronchoconstriction, diarrhoea/vomitting, SOB, hypoxia, angioedema, tachycardia, swelling of lips tongue and larynx can comprimise airways → wheeze/stridor, confusion