Topical
benzocaine and
lidocaine are widely used in
general anesthesia to minimize the stimulation by awake intubation and in very rare occasion they may induce
methemoglobinemia. Although this complication is uncommon, it may be potentially lethal. Here we report a 29-year-old female who was scheduled to receive correction of
malocclusion and developed acute
methemoglobinemia soon after induction of
general anesthesia. Three weeks ago, she had received open reduction for fracture of mandible with intermaxillary fixation under
general anesthesia, for which awake fiberoptic intubation was smoothly performed after
premedication with 2% topical
lidocaine and intravenous
fentanyl. This time, trachomucosal block with 4 mL of 4% topical
lidocaine and spray of 20% topical
benzocaine over the oral cavity and nostrils were carried out before intubation. Awake blind intubation was performed because she could not open her mouth for more than 1 cm. A 6.5 mm-sized nasal endotracheal tube was smoothly placed in first attempt. About 10 min later, an unexplained
cyanosis occurred and SpO2 fell to about 70%. Based on a high
oxygen tension by arterial blood gas analysis (PaO2) with a contradictory fall of oxygen saturation by pulse oximetry (SpO2), acute
methemoglobinemia was highly suspected. The diagnosis was confirmed by multiple-wavelength CO-oximetry. The
methemoglobinemia was resolved gradually after
methylene blue was given. In conclusion, we must always take the possibility of
methemoglobinemia into consideration for differential diagnosis in case of unexplained
cyanosis, particularly when patients have prior exposure to
methemoglobin-inducing agents.