GPAtlas🧭 When to suspect
Anaphylaxis is a severe, life-threatening, generalised hypersensitivity reaction. It is a clinical diagnosis resting on three features: sudden onset, rapid progression, and a life-threatening Airway, Breathing or Circulation (ABC) problem. Skin and mucosal changes (urticaria, flushing, angio-oedema) accompany most cases but are absent in up to 20% – so their absence never excludes the diagnosis. Exposure to a known allergen supports it, yet no trigger is found in around 30% (idiopathic).
Angio-oedema is self-limiting deep dermal and subcutaneous swelling (lips, tongue, eyelids, genitalia, hands, feet), typically non-itchy and tight rather than the itchy weals of urticaria, and may take up to 72 hours to settle. The decisive question is mechanism: histaminergic (mast-cell / allergic, responds to antihistamines and adrenaline) versus bradykinin-mediated (ACE inhibitor or hereditary), which does not respond to antihistamines, steroids or adrenaline. The two primary-care priorities are to never miss evolving anaphylaxis – any ABC involvement means IM adrenaline now – and to identify and remove the trigger.
| Pattern | Recognise it by | First action |
|---|---|---|
| Anaphylaxis | Sudden onset, rapid progression, life-threatening Airway and/or Breathing and/or Circulation problem; skin changes usual but absent in up to 20% | IM adrenaline now + call 999 |
| Histaminergic angio-oedema (± urticaria) | Itchy swelling, often with weals; mast-cell / allergic; lips, eyelids, tongue | Non-sedating antihistamine; treat as anaphylaxis if any ABC involvement |
| Bradykinin-mediated angio-oedema (ACE inhibitor or hereditary) | Non-itchy, no weals, slower onset, face / tongue / larynx; poor response to antihistamine, steroid and adrenaline | Stop the ACE inhibitor; protect the airway; specialist (C1-INH / icatibant) if airway threatened |
Source: Resuscitation Council UK · NICE NG258
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