Acquired methemoglobinemia: A case report of benzocaine-induced methemoglobinemia and a review of the literature

Benzocaine is widely used as a topical anesthetic and is also present in a number of over-the-counter preparations. Methemoglobinemia is a rare, but potentially serious, complication of its use; a fact that is not well documented in the Physician's Desk Reference or product inserts. Unfamiliarity with this complication may delay diagnosis and appropriate therapy. A case of methemoglobinemia occurring as a complication of using benzocaine during bronchoscopy is presented and is followed by a review of the literature and discussion of the pathophysiology, clinical presentation, diagnosis, and treatment of acquired methemoglobinemia. Methemoglobin is incapable of carrying oxygen and is formed when the ferrous iron in the heme molecule is oxidized to the ferric state. The normal mechanisms that convert methemoglobin back to hemoglobin can be overwhelmed by many oxidant drags, resulting in toxic methemoglobinemia. The diagnosis should be entertained when cyanosis, unresponsive to 100% oxygen therapy, appears suddenly, especially when exposure to an oxidant drug is established. Diagnosis is confirmed by multiple-wavelength cooximetry. Most cases require only decontamination and supportive care. Methylene blue is the specific antidote, but should be reserved for more severe cases or if comorbid conditions make mild hypoxia unadvisable. Exchange transfusion or hemodialysis may be indicated in patients who fail to respond to methylene blue.