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- Methemoglobin is dysfunctional hemoglobin in which the deoxygenated heme moiety has been oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state.
- Methemoglobinemia is an undue accumulation of methemoglobin within the blood.
- Toxic methemoglobinemia, resulting from exposure to oxidant chemicals or drugs, is the most common cause of methemoglobinemia among children older than 6 months of age.
- Enteritis-associated methemoglobinemia is the most common cause among children younger than 6 months of age:
- As many as 2/3 of infants with severe diarrhea have methemoglobinemia.
- Hemoglobin in the allosteric configuration of methemoglobin cannot carry oxygen.
- Methemoglobin increases the oxygen affinity of normal heme moieties in the blood and results in impaired oxygen delivery to tissues.
- NADH-dependent cytochrome b5 methemoglobin reductase is the major source of physiologic reduction of methemoglobin.
- A normally dormant NADPH-dependent methemoglobin reductase is the site of action for antidotal methylene blue therapy.
- Toxic methemoglobinemia
- Dietary or environmental chemicals: chlorates, chromates, copper sulfate fungicides, naphthalene, nitrates, and nitrites
- Industrial chemicals: aniline and other nitrogenated organic compounds
- Drugs: amyl nitrite, benzocaine, dapsone, lidocaine, metoclopramide, nitric oxide, nitroprusside, phenazopyridine, prilocaine, many others
- Methemoglobinemia is a common iatrogenic complication of drug therapy.
- Enteritis-associated methemoglobinemia is multifactorial in origin:
- Intestinal nitrate and nitric oxide promotes methemoglobin formation.
- Innate enzymatic methemoglobin reduction systems may be underdeveloped during infancy.
- Acidemia further inhibits enzymatic methemoglobin reduction systems.
- Methemoglobinemia is also reported with nitrite-producing bacterial infections of the intestines or urinary tract.
- Congenital methemoglobinemia (rare)
- Hemoglobin M: Heterozygotes for autosomal dominant hemoglobin M will exhibit lifelong cyanosis.
- NADH-dependent methemoglobin reductase deficiency: Homozygotes for this autosomal recessive enzyme will have lifelong cyanosis; heterozygotes may have increased susceptibility to oxidative hemoglobin injury.
Commonly Associated Conditions
- Heinz body hemolytic anemia
- Oxidant stress on the globin protein may cause hemolysis.
- Oxidant stress on the hemoglobin porphyrin ring may cause sulfhemoglobinemia.