Topical Benzocaine and Methemoglobinemia

Area of Science:

• Toxicology
• Emergency Medicine
• Pharmacology

Background:

• Methemoglobinemia (MetHb) presents as hypoxia, cyanosis, and dyspnea unresponsive to oxygen.
• Topical benzocaine is a known cause of iatrogenic MetHb.
• Uncommon exposure routes, like topical vulvar application, can lead to MetHb.

Observation:

• A patient with vulvar cancer and HIV presented with severe hypoxia and cyanosis.
• Initial treatment with methylene blue was insufficient due to continued, undisclosed topical benzocaine use on the vulva.

Findings:

• Methemoglobin (MetHb) levels reached 49% and later 56%, indicating significant methemoglobinemia.
• Diagnosis was confirmed after detailed patient interviews revealed occult use of over-the-counter benzocaine (Vagicaine).
• Vaginal lavage and methylene blue treatment ultimately resolved symptoms and lowered MetHb levels.

Implications:

• This case underscores the critical need for comprehensive medication histories, including non-prescription drugs, in emergency settings.
• Healthcare providers should be aware of uncommon exposure routes for methemoglobinemia, such as topical benzocaine application.
• Patient education regarding the potential risks of over-the-counter medications is essential to prevent adverse events.