Anaphylaxis vs Vasovagal syncope
How to tell Anaphylaxis and Vasovagal syncope apart when a board question makes them look alike.
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Hypotension with concurrent urticaria, airway edema, or wheeze after exposure identifies anaphylaxis.
Side by side
Rapid multisystem reaction after exposure with hypotension plus skin or airway findings that requires immediate intramuscular epinephrine.
Transient reflex syncope with pallor, nausea, diaphoresis, and relative bradycardia that improves quickly when supine.
IgE-mediated mast-cell degranulation causes vasodilation, capillary leak, bronchospasm, and mucosal edema.
A sudden vagal surge and sympathetic withdrawal produce brief hypotension and bradycardia with cerebral hypoperfusion.
Clinical diagnosis is supported by acute hypotension or bronchospasm after exposure; serum tryptase may rise above baseline within 1-3 hours.
ECG is usually sinus bradycardia or normal; symptoms resolve rapidly with recumbency and there are no persistent urticaria, wheeze, or hypoxemia findings.
The trap
Mistaking early anaphylaxis for benign fainting delays epinephrine; isolated syncope improves supine, but persistent respiratory or cutaneous findings demand immediate epinephrine.
The repair move
Next best step: give IM epinephrine 0.3 to 0.5 mg of 1 mg/mL solution in the lateral thigh now, repeat every 5 to 15 minutes as needed, place the patient supine, start high-flow oxygen and IV fluids, then add H1 and H2 blockers and steroids after epinephrine.
Try a free question
CoreA 19-year-old man faints after seeing blood during a blood draw. BP is 88/54 mm Hg, HR 52/min, skin is cool and pale, and symptoms resolve within 2 minutes when supine. What is the most likely diagnosis?
Show answer and explanation
Answer: B. Vasovagal syncope. Brief hypotension with bradycardia and prompt recovery after recumbency is classic vasovagal syncope.
Why the other choices are tempting
- A. Incorrect because there is no urticaria, bronchospasm, or exposure-linked multisystem allergic reaction.
- C. Incorrect because septic shock usually causes persistent fever and tachycardia, not immediate resolution.
- D. Incorrect because pulmonary embolism typically causes ongoing dyspnea and tachycardia rather than reflex bradycardia.
- E. Incorrect because hypoglycemia would require low glucose and neuroglycopenic signs, not this classic trigger-response pattern.
Flashcards
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After allergen exposure, which feature distinguishes anaphylaxis from vasovagal syncope most reliably?
Multisystem involvement such as urticaria plus airway compromise or wheeze with hypotension indicates anaphylaxis.
What hemodynamic pattern is typical of vasovagal syncope at onset?
Transient hypotension with relative bradycardia that resolves when the patient is laid supine is typical.
Quick answers
What is the fastest way to tell Anaphylaxis from Vasovagal syncope?
Hypotension with concurrent urticaria, airway edema, or wheeze after exposure identifies anaphylaxis.
What is the key clue for Anaphylaxis?
Rapid multisystem reaction after exposure with hypotension plus skin or airway findings that requires immediate intramuscular epinephrine.
What is the key clue for Vasovagal syncope?
Transient reflex syncope with pallor, nausea, diaphoresis, and relative bradycardia that improves quickly when supine.
What is the common trap with Anaphylaxis vs Vasovagal syncope?
Mistaking early anaphylaxis for benign fainting delays epinephrine; isolated syncope improves supine, but persistent respiratory or cutaneous findings demand immediate epinephrine.