red man syndrome

**Pathophysiology:** Vancomycin directly activates mast cells and basophils through the MRGPRX2 receptor (a Mas-related G protein-coupled receptor). This causes non-IgE-mediated histamine release. It is not a true type I hypersensitivity reaction, so prior sensitization is not needed and it can occur on the first dose. Severity depends on infusion rate and concentration. It is not a true allergy, and the patient can usually be re-challenged with a slower infusion.

**Clinical presentation:** Onset is typically during the infusion or within minutes to hours after it. Findings include: - Pruritus - Erythematous, flushing rash of the face, neck, and upper torso (the "red man" distribution) - Sometimes angioedema, muscle pain or spasm of the chest and back, dyspnea, and hypotension - Rarely, severe hypotension or cardiac arrest

**Differentiation from true anaphylaxis:** Red man syndrome is rate-dependent and non-IgE-mediated, and it tends to be milder. Anaphylaxis is IgE-mediated and can occur at any infusion rate. It usually involves bronchospasm, stridor, urticaria, and more profound hypotension. Features such as wheezing or airway compromise should prompt treatment as anaphylaxis (epinephrine).

**Management:** - Stop the infusion. - Give an H1 antihistamine (diphenhydramine). Add an H2 blocker as needed. - For hypotension, give IV fluids. - Once symptoms resolve, restart at a slower rate, for example half the prior rate, and/or at a lower concentration.

**Prevention:** - Infuse vancomycin at ≤10 mg/min, or over at least 1–2 hours for doses above 1 g. - Pretreat with an antihistamine before the infusion if the reaction recurs.

**Key associations and high-yield points:** - Vancomycin: a glycopeptide that inhibits cell wall synthesis by binding D-Ala-D-Ala and blocking peptidoglycan polymerization. It is used for MRSA, *C. difficile* (oral), and serious gram-positive infections. - Other vancomycin adverse effects: nephrotoxicity, ototoxicity, thrombophlebitis, and DRESS. The mnemonic is "NOT": Nephrotoxicity, Ototoxicity, Thrombophlebitis. - Resistance: VRE results from a D-Ala-D-Lac substitution. - Similar rate-related, histamine-mediated reactions: amphotericin B ("shake and bake"), opioids (morphine), and fluoroquinolones. Rifampin can cause orange-red discoloration of body fluids, which is a separate entity. - Do not confuse it with other "red" syndromes: "red man" refers specifically to the flushing of vancomycin infusion reaction. It is also not the same as DRESS, SJS/TEN, or scarlet fever.

**Board pearl:** A patient on IV vancomycin develops flushing and itching of the face and upper body. The next step is to stop the infusion, give diphenhydramine, and restart more slowly. Do not label the patient as vancomycin-allergic.