What are the signs and symptoms of methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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Symptoms are proportional to the fraction of methemoglobin. A normal methemoglobin fraction is about 1% (range, 0-3%).

At methemoglobin levels of 3-15%, a slight discoloration (eg, pale, gray, blue) of the skin may be present.

Patients with methemoglobin levels of 15-20% may be relatively asymptomatic, apart from mild cyanosis. Signs and symptoms at levels of 25-50% include the following:

  • Headache

  • Dyspnea

  • Lightheadedness, even syncope

  • Weakness

  • Confusion

  • Palpitations, chest pain

Methemoglobin levels of 50-70% can cause the following:

  • Cardiovascular - Abnormal cardiac rhythms

  • CNS - Altered mental status; delirium, seizures, coma

  • Metabolic - Profound acidosis

At methemoglobin fractions exceeding 70%, death usually results.

Infants and children can develop methemoglobinemia in association with metabolic acidosis that is caused by prolonged dehydration and diarrhea. Sources of accidental toxin exposure that must be considered in infants and children include ingestion of water from wells contaminated with excess nitrates and exposure to local anesthetics in teething gels. [19] These factors can sometimes be elicited in a thorough history.

Any known family history of methemoglobinemia or glucose-6-phosphate dehydrogenase (G6PD) deficiency is important to clarify. Even patients who are heterozygous for methemoglobin reductase enzyme deficiencies are susceptible to low doses of oxidant drugs with resultant methemoglobinemia.

The presence of gastrointestinal (GI) symptoms (eg, nausea, vomiting, or diarrhea) may suggest the possibility of ingestion of a toxic substance.

The clinical effects of methemoglobinemia are exacerbated in the presence of anemia.

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