Methemoglobinemia

Methemoglobin unlike conventional hemoglobin contains no reduced iron (Fe ²⁺ ), and oxidized (Fe ³⁺ ) in the process of reversible oxygenation oxyhemoglobin (Hb O ₂ ) is partially oxidized to methemoglobin (Mt Hb). Per day in vivo oxidizes 0.5-3% Hb O ₂ of total hemoglobin, but under the influence metgemoglobinreduktazy (NADH-dependent reductase) iron Mt Hb quickly recovers and Mt Hb level in the blood does not exceed 0.5-2%. The activity of methaemoglobin reductase is significantly reduced in healthy full-term newborns and is very low in premature infants. Increase in activity to adult levels occurs by 4 months of life. When the level of Mt Hb is 15% of total hemoglobin, the skin becomes swarthy and the blood acquires a brown, chocolate color.

[1], [2], [3], [4], [5], [6],

Classification of methemoglobinopathies

1. Hereditary
   • M-hemoglobinopathy - there is a synthesis of abnormal globins containing a- or beta-chains in oxidized form (due to the replacement of either proximal or distal histidines in the abnormal chain of tyrosine)
   • fermentopathy - there is a very low activity (or absence) of methaemoglobin reductase.
2. Acquired.

With all forms of methemoglobinemia, generalized gray-gray cyanosis is observed in the absence of changes in the nail phalanges of the fingers in the form of "drum sticks" and cardiopulmonary lesions. In severe cases, when more than 50% of hemoglobin has the form of methemoglobin, patients complain of shortness of breath during physical exertion, fatigue and severe headaches. Stagnation in the veins of the conjunctiva and the retina can also be observed.

[7], [8]

M-hemoglobinemia

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

Hereditary fermentopenic methemoglobinemia

It is inherited autosomally-recessively, the frequency of heterozygous carriage is about 1% in the population. Among some people, the frequency of heterozygous carriage of the methaemoglobin reductase deficiency is much higher, in particular among the Yakuts - 7%. In homozygotes, cyanosis of the skin and visible mucous membranes, especially noticeable in the region of the lips, nose, ear lobes, nail bed, and oral cavity, are detected from birth. The color spectrum varies from gray-green to dark-violet. The disease is benign. The number of erythrocytes and hemoglobin level in a unit of blood is increased, but because Mt Hb is not capable of oxygenation, tissues are hypoxic, that is, there is a "hidden" anemia.

In hereditary variants of methemoglobinemia, hemolysis is not present, but it can occur with acquired methemoglobinemia.

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

Diagnosis of methemoglobinemia

It is based on the exclusion of congenital heart defects of the blue type, diseases of the lungs and adrenals that occur with cyanosis. The presence of Mt Hb in the blood is confirmed by a fairly simple breakdown - a drop of patient's blood is applied to the filter paper and a drop of blood of a healthy person is next. Conventional venous blood in the air becomes red due to HbO ₂, whereas with methemoglobinemia, the blood remains brown. With a positive sample, the examination is expanded: the determination of the Mt Hb level by the cyanhaemoglobin method, the activity of methaemoglobin reductase, the electrophoresis of hemoglobins.

[9], [10], [11], [12], [13], [14]

Treatment of methemoglobinemia

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