Methemoglobinemia

Unlike normal hemoglobin, methemoglobin contains oxidized iron (Fe ^{3+ ) rather than reduced iron (Fe2+} ), and during reversible oxygenation, oxyhemoglobin (Нb О ₂ ) is partially oxidized to methemoglobin (Mt Hb). In vivo, 0.5-3% of Нb О ₂ of the total hemoglobin is oxidized per day, but under the influence of methemoglobin reductase (NADH-dependent reductase), the iron in Mt Hb is quickly restored and the level of Mt Hb in the blood does not exceed 0.5-2%. The activity of methemoglobin reductase is significantly reduced in healthy full-term newborns and is very low in premature infants. Activity increases to adult levels by the age of 4 months. When the level of Mt Hb is 15% of the total hemoglobin, the skin becomes dark and the blood acquires a brown, chocolate color.

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Classification of methemoglobinopathies

1. Hereditary
   • M-hemoglobinopathy - the synthesis of abnormal globins occurs, containing a- or beta-chains in an oxidized form (caused by the substitution of either the proximal or distal histidines in the abnormal chain for tyrosine)
   • enzymopathy - very low activity (or absence) of methemoglobin reductase is observed.
2. Acquired.

All forms of methemoglobinemia are characterized by generalized slate-gray cyanosis without clubbing of the fingertips and cardiopulmonary lesions. In severe cases, when more than 50% of hemoglobin is in the form of methemoglobin, patients complain of shortness of breath during physical exertion, fatigue, and severe headaches. Congestion in the veins of the conjunctiva and retina may also be observed.

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M-hemoglobinemia

With a mutation in the alpha chain of globin, children are cyanotic from birth; with a mutation in the beta chain, cyanosis appears at 3-6 months of life, when the main part of Hb F is replaced by Hb A.

Hereditary fermentopenic methemoglobinemia

It is inherited in an autosomal recessive manner, the frequency of heterozygous carriage is about 1% in the population. Among some peoples, the frequency of heterozygous carriage of methemoglobin reductase deficiency is significantly higher, in particular among the Yakuts - 7%. Homozygotes have cyanosis of the skin and visible mucous membranes from birth, especially noticeable in the area of the lips, nose, earlobes, nail beds, and oral cavity. The color spectrum varies from gray-green to dark purple. The disease is benign. The number of erythrocytes and the level of hemoglobin in a unit of blood are increased, but due to the fact that Mt Hb is not capable of oxygenation, tissues experience hypoxia, that is, there is a "latent" anemia.

In hereditary variants of methemoglobinemia, there is no hemolysis, but it can occur in acquired methemoglobinemia.

Acquired methemoglobinemia develops in heterozygotes of hereditary fermentopenic methemoglobinemia, individuals with hemoglobinopathies. When using long-stored drugs, including phenacetin, sulfonamides, aniline and its derivatives, as well as water and products with a large amount of nitrates and nitrites (well water, and sometimes tap water, sausage, canned meat, vegetables, fruits grown using nitrite and nitrate fertilizers).

Diagnosis of methemoglobinemia

Based on the exclusion of congenital blue-type heart defects, lung and adrenal diseases occurring with cyanosis. The presence of Mt Hb in the blood is confirmed by a fairly simple test - a drop of the patient's blood is applied to filter paper and a drop of a healthy person's blood is placed next to it. Normal venous blood turns red in the air due to Hb O ₂, while with methemoglobinemia the blood remains brown. With a positive test, the examination is expanded: determination of the Mt Hb level by the cyanhemoglobin method, methemoglobin reductase activity, hemoglobin electrophoresis.

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Treatment of methemoglobinemia

In M-hemoglobinopathies, treatment is usually not required. In case of enzymopathy in newborns and infants, ascorbic acid is prescribed at 0.1-0.15 x 3 times a day orally or riboflavin at 0.01 x 2-3 times a day orally. It is possible to administer methylene blue (the drug "Chromosmon") at a dose of 1 mg/kg intravenously. The Mt Hb level normalizes after 1 hour, but increases again after 2-3 hours, so the drug is given in the same dose 3 times a day. After the first months of life, treatment is usually not required.