Methemoglobinemia: The presence in the blood of methemoglobin, a form of hemoglobin that is useless for carrying oxygen and delivering it to tissues throughout the human body. Since hemoglobin is the key carrier of oxygen in the blood, its replacement by methemoglobin can cause cyanosis (a slate gray-blueness) due to lack of oxygen. A small amount of methemoglobin is normally present in blood but the conversion of a larger fraction of hemoglobin into methemoglobin, which does not function reversibly as an oxygen carrier, results in perceptible cyanosis.

(In more technical terms, methemoglobin is a transformation product of normal oxyhemoglobin. It is created by the oxidation of the ferrous iron present in the heme part of hemoglobin to ferric iron . Ferric iron, in firm union with water, is chemically useless for respiration .)

Methemoglobinemia may be acquired anytime in life by exposure to a number of different chemical agents such as nitrites or it may be congenital due a genetic condition:

• Hb M disease -- the production of any one of a group of mutant hemoglobins known collectively as hemoglobin M ("M" for methemoglobin), or
• Deficiency of the enzyme methemoglobin reductase which is required for the reduction of methemoglobin to normal oxyhemoglobin.

In both kinds of congenital methemoglobinemia, the cyanosis starts in early infancy with no history of lung or heart disease to account for it. However, the patterns of inheritance deficiency are distinctively different -- autosomal dominant with Hb M disease versus autosomal recessive with the enzyme deficiency. The cyanosis from Hb M disease is resistant to treatment with reducing agents such as ascorbic acid or methylene blue, whereas with deficiency of the enzyme, the cyanosis is typically improved by ascorbic acid or methylene blue therapy .

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