Methemoglobinemia

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

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

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

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