methemoglobinemia, decrease in the oxygen-carrying capacity of the red blood cells (erythrocytes) due to the presence of methemoglobin in the blood. The severity of the symptoms of methemoglobinemia is related to the quantity of methemoglobin present in the circulation and range from a bluish discoloration of the skin and mucous membranes to weakness, difficulty in breathing, and dizziness in the more severe cases. Mild cases may involve vague flu-like symptoms and headaches and therefore may remain undiagnosed.

The iron component of the hemoglobin of the red blood cells must be in the reduced (deoxidized) state to bind with oxygen; methemoglobin contains the oxidized form of iron and is useless for oxygen transport. Normally, various organic catalysts or enzymes are active in keeping the iron in the reduced form. Hereditary methemoglobinemia occurs when there is an inborn defect in this enzyme system or when the hemoglobin molecule is abnormally structured (hemoglobin M) and is thereby more susceptible to oxidation of the iron component. Acquired methemoglobinemia may arise as a result of contact with certain drugs and chemicals that produce oxidant compounds in the circulation, causing the oxidation of iron to occur faster than the enzyme system can keep it in the reduced state.

Treatment of hereditary methemoglobinemia usually includes the administration of reduction compounds such as methylene blue. Acquired methemoglobinemia usually disappears spontaneously when the cause is removed.

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The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers.
Also spelled:
haemoglobinopathy

hemoglobinopathy, any of a group of disorders caused by the presence of variant hemoglobin in the red blood cells. Variant-hemoglobin disorders occur geographically throughout the Old World in a beltlike area roughly the same as that of malaria. The presence of variant hemoglobin in moderate amounts may constitute a selective advantage in that it provides some protection from the lethal effects of malaria, thereby allowing more persons to reach reproductive age. The most important of the hemoglobinopathies are sickle-cell anemia and thalassemia. Hemoglobin C (Hb C) is relatively common among African blacks living north of the Niger River and is found in 2–3 percent of blacks in the United States. Hemoglobin C disease (occurring when the variant Hb C gene is inherited from both parents) produces such symptoms and signs as vague pain, jaundice, enlarged spleen, mild to moderate anemia, and some hemorrhaging. The life span of the individual, however, is normal, and the disease is much milder than the sickle-cell anemia found in the same geographic range. It is possible that Hb C is gradually replacing Hb S (variant hemoglobin of sickle-cell anemia) by a process of selection in Africa; Hb C does not produce early mortality in homozygotes (persons with two genes for Hb C), as does sickle-cell anemia, yet Hb C may afford some protection from malaria.

Hemoglobin D is found mainly in people of Afghan, Pakistani, and northwestern Indian descent, but it also occurs in those of European ancestry. Hemoglobin D disease (two genes for Hb D) may produce mild hemolytic anemia. Hemoglobin E is widespread in Southeast Asia, being found especially among Thai, Cambodian, Laotian, Malaysian, Indonesian, Vietnamese, and Burmese peoples. Hemoglobin E disease (two genes for Hb E) may result in a mild microcytic (small red blood cell) anemia. Hemoglobin E–thalassemia disease (one gene for Hb E, one gene for thalassemia) is severe and clinically closely resembles thalassemia major. Hemoglobin H, found in many groups in the Old World (e.g., Chinese, Thai, Malayans, Greeks, Italians), has almost always been identified in combination with thalassemia; symptoms resemble those of thalassemia.

Many other variant hemoglobins—such as types G, J, K, L, N, O, P, Q, and variants of Hb A (“normal” hemoglobin) and Hb F (fetal hemoglobin)—are known but typically do not produce clinical manifestations.

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