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Methemoglobin is formed when iron in the hemoglobin molecule becomes oxidized from the ferrous to the ferric state. In this form, hemoglobin cannot bind oxygen. Methemoglobin levels are normally kept below 1.5% by an erythrocyte enzyme, methemoglobin reductase. Methemoglobinemia can be acquired or congenital. Acquired or toxic methemoglobinemia is much more common than congenital disease. It occurs following exposure to nitrites in food, nitrate in well water, or drugs containing an aniline ring such as sulfonamides, dapsone, Pyridium, local anesthetics, phenacetin, and acetaminophen. Household items containing aniline dyes include colored crayons, flavoring essences, furniture polish, ink, perfumes, and shoe polish. These agents oxidize ferrous hemoglobin at a rate that overwhelms the reductase enzyme. Signs and symptoms of methemoglobinemia may be delayed for several hours, because some chemicals do not directly oxidize hemoglobin but require hepatic metabolism to a toxic metabolite. The most common congenital methemoglobinemia is deficiency of erythrocyte methemoglobin reductase. It is inherited as an autosomal recessive trait and occurs with increased frequency in Inuit and Alaskan Native Americans. Homozygotes have chronic cyanosis and methemoglobin levels of 15 to 20%, while heterozygotes are predisposed to toxic methemoglobinemia. Another cause of congenital methemoglobinemia is autosomal dominant inheritance of an abnormal M-hemoglobin. Chronic methemoglobinemia is not accompanied by erythrocytosis. Acute methemoglobinemia is a confusing clinical problem. PaO2 may be normal despite cyanosis. Cyanosis occurs when the methemoglobin level is >1.5 g/dL and is usually central rather than peripheral. Symptoms of anoxia develop at methemoglobin levels of 30 to 45%. These symptoms include tachycardia, dyspnea, muscle cramps, headache, lethargy, and altered mental state. Vomiting, circulatory failure and death may occur at concentrations of 70%. Acute toxic cases may be associated with hemolytic anemia. Blood samples usually appear chocolate brown. Reference range is < 1.5%. Specimen requirement is one green top (lithium heparin) tube or one blood gas syringe. The tube should be transported in wet ice. |
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