Methemoglobinemia

Key Features

• Oxidation of hemoglobin to methemoglobin can be caused by

  • Benzocaine

  • Aniline

  • Propanil

  • Nitrites

  • Nitrogen oxide gases

  • Nitrobenzene

  • Dapsone

  • Phenazopyridine

  • Many other oxidants

• Dapsone has a long half-life and may produce prolonged or recurrent methemoglobinemia

• Amyl nitrite and isobutyl nitrite ("poppers") are inhaled as sexual stimulants but can result in methemoglobinemia

Clinical Findings

• Severity of symptoms depends on the percentage of hemoglobin oxidized to methemoglobin

  • Severe poisoning is usually present when methemoglobin fractions are > 40–50%

  • Even at low levels (15–20%), victims appear cyanotic because of the chocolate-brown color of methemoglobin

• Dizziness

• Nausea

• Headache

• Dyspnea

• Confusion

• Seizures

• Coma

Diagnosis

• PO₂ results are normal on arterial blood gas determinations; conventional pulse oximetry gives inaccurate oxygen saturation measurements

• Severe metabolic acidosis

• Hemolysis may occur, especially in patients with glucose-6-phosphate dehydrogenase deficiency

Treatment

• Administer high-flow oxygen

• Activated charcoal

  • Administer 60–100 g orally or via gastric tube, mixed in aqueous slurry for ingestions within 1 h

  • Repeat-dose may enhance dapsone elimination

• For symptomatic patients, administer methylene blue, 1–2 mg/kg (0.1–0.2 mL/kg of 1% solution) intravenously

  • The dose may be repeated once in 15–20 min if necessary

  • Patients with hereditary methemoglobin reductase deficiency or glucose-6-phosphate dehydrogenase deficiency may not respond to methylene blue

• In severe cases where methylene blue is not available or is not effective, exchange blood transfusion may be necessary