Introduction

Beware the pitfalls of misinterpreting the following symptoms and signs in the context of anaphylaxis.

Pitfall: Wheeze = asthma

Differentiating asthma and anaphylaxis might be difficult clinically and made harder by the fact that in anaphylactic fatalities triggered by food, nearly all the cases had difficulty in breathing that led to respiratory arrest. [3]

It is, however, unlikely that newly diagnosed ‘asthma’ would present in such dramatic fashion; similarly, known asthmatics rarely deteriorate so abruptly. If in doubt, give adrenaline (IM), the key agent in anaphylaxis and formerly a standard therapy in asthma.

Pitfall: Stridor = croup/epiglottitis

Stridor secondary to infection (viral or bacterial) will have a slower onset and will usually be associated with fever.

Pitfall: ECG with ST depression = acute coronary syndrome

ST changes are recognised in anaphylactic patients with normal coronary arteries, with or without adrenaline; indeed, in the ischaemic heart disease population, these changes might even be expected in severe anaphylaxis. Do not withhold adrenaline when you suspect anaphylaxis, even if ECG changes are present.

Pitfall: Bradycardia rules out anaphylaxis

Bradycardia can occur in anaphylaxis, as a likely peri-arrest sign.

Pitfall: Vomiting, abdominal pain and diarrhoea = not anaphylaxis

Not so: anaphylaxis is a multisystem insult that may well involve the gut.

Pitfall: Anxiety misdiagnosed as a panic attack

Be careful. A sense of doom is well described in anaphylaxis and patients who have previously experienced anaphylaxis might well panic when they suffer another episode. Weigh up the clinical situation in the Resus room carefully.

Pitfall: Subtle rash only means allergy not anaphylaxis

No rash is found in 20% of anaphylactic cases.

Pitfall: Erythema is not compatible with anaphylaxis

Patchy or generalised erythema is recognised. Urticaria is not essential.

Pitfall: Pallor is not compatible with anaphylaxis

Beware adrenaline-induced pallor via its vasopressor action.

Note too that mucosal rather than skin changes may exist i.e. angioedema. This most commonly presents as swelling of the eyelids and lips, sometimes the mouth and throat (with potential stridor in the latter).

Learning bite

• Skin and mucosal changes can be subtle or absent in up to 20% of anaphylactic cases.

Other conditions mimicking anaphylaxis include:

• ACE inhibitor-induced angioedema (which may indeed be more common than anaphylaxis)
• Hereditary angioedema
• Scombroid poisoning