Methemoglobinemia overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Methemoglobinemia is a blood disorder in which, due to increased production (congenital or acquired reasons), the red blood cells (RBCs) contain higher than normal levels of methemoglobin (MetHb) (>1%). Methemoglobin forms from the substitution of iron (Fe) in ferric/reduced (Fe2+) form, as found in normal hemoglobin, with iron in oxidized (Fe3+) form. The oxidation of Hb to MetHb ( (Fe2+) to (Fe3+) ) occurs naturally in healthy people, as a result of the interaction of Hb with oxygen free radicals, which are produced during normal cell metabolism. but the protective reduction enzyme systems in the red blood cells (RBCs), if working properly, will never allow the MetHb levels to be more than 1%.
Hemoglobin is the polypeptide protein in the RBCs, consisting of 2 alfa and 2 beta chains connected to an iron atom in ferric form, responsible for binding, carrying and distributing oxygen from the lungs to the tissues. MetHb is unable to bind oxygen, and in case of methemoglobinemia, the affinity of the remaining normal Hb, that has not been yet oxidized to MetHb, to oxygen is very high. This leads to leftward shift of the oxygen-hemoglobin dissociation curve, resulting in hypoxia and dyspnea, because no oxygen gets released or delivered to the tissues.
Historical Perspective
Classification
Congenital (Hereditary) Methemoglobinemia
There are three main congenital conditions that lead to methemoglobinemia:
1. Cytochrome b5 reductase deficiency and pyruvate kinase deficiency
2. G6PD deficiency
3. Presence of abnormal hemoglobin.
Acquired or Acute Methemoglobinemia
The most common causes are different oxidant drugs, toxins and chemicals.