Is that rash to vancomycin an allergic reaction?

You may hear patients say sometimes, “I got vancomycin once…but I’m allergic to it. I got itchy and my skin turned completely red!”

Being able to diagnose “red man syndrome” (RMS) is a classic Step 1 question. The rash is described as erythematous and usually involving the upper torso, arms, and face. Patients may have pruritis, flushing, and even muscle spasms and dyspnea. Although it can feel like an allergic reaction, it is actually a “pseudoallergic” drug reaction. Other examples of pseudoallergic drug reactions include:

  • contrast “allergy” (although true allergic reactions happen, patients can also get a vasovagal response that can cause bradycardia and hypotension or RMS-esque chemotoxic response)
  • opiates (patients who report a “rash” to morphine, for example)
  • NSAIDs
  • rash in response to immunomodulators and biologics

How should I treat RMS?

This reaction is thought to be a rate-related phenomenon: you just have to run the vanc at <10 mg/min or half the previous rate. Giving benadryl and/or an H2 blocker can reduce discomfort.

Of course, vancomycin can also cause anaphylaxis! If you are concerned about a severe RMS (muscle pain, dyspnea, hypotension), or anaphylaxis (which will usually present with hives, wheezing, tachycardia, etc.) then give epinephrine and the usual, including H2 blockers, antihistamines, and steroids.

How can I prevent RMS? Should I premedicate my patients?

Premedication is not necessary unless the patient is going to get a huge dose of vanc bolused, or they have a history of severe RMS and there are no alternative medications. Premedication can be accomplished with oral diphenhydramine or an H2 blocker.

Interestingly, RMS may be more likely when given with other meds that can cause mast cell degranulation, like opioids or contrast dye–so it would be best to space out these meds if possible.

Can I give contrast to a patient with a shellfish allergy?

The short answer: yes!

There is a longstanding theory that because shellfish has a lot of iodine in it, patients who have anaphylactic reactions to shellfish should not be given iodinated radiocontrast because that will set off anaphylaxis as well. This is kind of hand-wavy pathophysiology, as Jennifer Gunter notes on KevinMD: the substance patients do react to in contrast is the hyperosmolar agent in contrast, which tends to be irritating, and there are now low-osmolar alternatives. Iodine itself is an element in the periodic table that is in fact, essential for life, so saying you have an iodine allergy is like saying you’re allergic to water. Which, hopefully, you’re not.

This is an entertaining piece on Clinical Correlations that gives a great evidence-based explanations for why it is absolutely okay to give contrast. A review in the Journal of Emergency Medicine reports that severe reactions (the kind we care about) only occur in 0.02-0.5% of patients, and that patients with higher risk of any kind of reaction tend to be more atopic, that is, have asthma, multiple food allergies, etc.