Benzocaine-Induced Methemoglobinemia in a Patient with G6PD Deficiency

Benzocaine and other local anesthetics oxidize ferrous iron in heme to ferric iron, producing methemoglobin. Ferric heme cannot bind oxygen reversibly, and it also shifts the oxygen dissociation curve of remaining normal hemoglobin to the left, impairing tissue oxygen delivery. The result is functional anemia despite a normal PaO2. PaO2 measures dissolved oxygen and is normal or high because the lungs are oxygenating; pulse oximetry is misleading because methemoglobin absorbs light at both the 660 nm and 940 nm wavelengths. At low methemoglobin fractions the reading may be falsely low; as the fraction rises toward 30-40%, the SpO2 tends to plateau near 85%, and at very high levels it can be falsely reassuring. The saturation gap-normal PaO2 with low SpO2 and a chocolate-brown sample-is the clue. Co-oximetry is required for diagnosis and should be ordered specifically; standard blood gas analyzers that calculate saturation from PaO2 will not detect methemoglobin. The 38% level explains cyanosis, dyspnea, and agitation. Treatment is not based on SpO2 alone; it depends on the methemoglobin level, symptoms, and comorbidities.

Methylene blue is the first-line treatment for symptomatic methemoglobinemia, usually 1-2 mg/kg IV over 5 minutes, repeated if needed. It works by acting as an electron acceptor that is reduced to leukomethylene blue by NADPH-dependent methemoglobin reductase; leukomethylene blue then reduces methemoglobin back to hemoglobin. That pathway requires NADPH, which is generated largely by the pentose phosphate pathway and therefore depends on G6PD. In this patient with documented low G6PD activity, methylene blue may be ineffective and can precipitate hemolysis by generating oxidative stress in deficient red cells. The prior hemolytic episode and activity of 5.2 U/g Hb make this a real risk. Pregnancy adds another layer: methylene blue crosses the placenta, and there are concerns about fetal hemolysis and neonatal toxicity, especially at higher doses. It is not absolutely contraindicated in a life-threatening maternal emergency, but it should not be given reflexively at 38% in a stable patient. If the methemoglobin level is rising, or if there is chest pain, syncope, ischemic ECG change, or respiratory compromise, exchange transfusion is the definitive treatment because it removes methemoglobin and provides normal red cells and G6PD-replete cells.

Give high-flow oxygen and stop further benzocaine exposure. In a stable patient with 38% methemoglobin and no end-organ ischemia, supportive care plus ascorbic acid is reasonable. Ascorbic acid 1-2 g IV can reduce methemoglobin slowly by non-enzymatic reduction; it is much slower than methylene blue and should not be used as a single rescue therapy for severe disease. Monitor methemoglobin every 2-4 hours, along with hemoglobin, reticulocytes, bilirubin, LDH, and urine output, because hemolysis can worsen anemia. Escalate if methemoglobin exceeds 50%, if it continues to rise despite treatment, or if the patient develops chest pain, syncope, ischemic ECG changes, respiratory failure, or altered mental status. Exchange transfusion is the preferred escalation in G6PD deficiency and pregnancy because it removes the oxidized hemoglobin and corrects anemia without relying on the deficient NADPH pathway. Hyperbaric oxygen is not a standard treatment for methemoglobinemia. If methylene blue is ultimately judged necessary because of imminent collapse, use the lowest effective dose with close hematologic monitoring and involve obstetrics and a hematologist.