Arterial blood gas analysis revealed an MHg level of 18.8% (normal 0.0% to 1.5%). A total of 50 mg (approximately 1 mg/kg) of methylene blue was administered by direct intravenous push over 5 min. Repeat blood gases 90 min later showed resolution of the methemoglobinemia (MHg 1.2%), and she was completely asymptomatic. She was transferred back to the medical ward 3.5 h following the administration of methylene blue and her vital signs were monitored closely. Further investigations into her nausea and vomiting were completed and were unrevealing. Her nausea and vomiting spontaneously improved and she was discharged after one week in hospital with instructions to avoid benzocaine-containing compounds in the future.
Benzocaine has been associated with methemoglobinemia in a wide variety of different preparations and concentrations (5% to 20%), including over-the-counter preparations used for the relief of pain and itching associated with vaginal irritation , oral sores and in its use as a topical anesthetic spray during intubation, bronchoscopy, transesophogeal electrocardiography and endoscopy . MHg is formed when an iron moiety of hemoglobin is oxidized from a ferrous ion to a ferric ion (Fe3+).