Methemoglobinemia overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1], Associate Editor(s)-in-Chief: Aksiniya Stevasarova, M.D.
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. The levels of MetHb though, never exceed more than 1%, if the protective reduction enzyme systems in the RBCs are working properly. 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 to the tissues.
Classification
Methemoglobinemia may be classified into two groups, based on the mechanism of its formation- acquired and congenital methemoglobinemia.
Pathophysiology
Methemoglobin (MetHb) refers to the state of hemoglobin (Hb) in which the [[iron atom)] is oxidized or in ferric state (Fe3+). In this state the iron is incapable of creating a bond with the oxygen, thus it neither can bind, nor deliver oxygen to the tissues.The formation of methemoglobin can be a result of a normal physiologic process of losing an electron from the iron atom, after releasing the oxygen to the tissues, and we can detect methemoglobin in the blood of healthy people, but the normal levels should always be less than 1%. These levels are maintained by several enzyme systems that work to reduce the iron to its ferrous state (Fe2+).
Causes
Methemoglobinemia may be caused by either congenital or acquired conditions.
Differentiating Methemoglobinemia overview from Other Diseases
Methemoglobinemia must be differentiated from other diseases that cause hypoxia and cyanosis, such as heart failure, pulmonary embolism, polycythemia, anemia, etc.
Epidemiology and Demographics
The incidence of congenital methemoglobinemia in the United States is very low. There is no racial predilection to methemoglobinemia. The highest prevalence of G6PD deficiency is observed in the malaria-endemic regions: Sub-Saharan Afria, West Asia and Arabian Peninsula, as well as in people of Mediterranean descent.
Risk Factors
Screening
Methemoglobinemia screening is not routinely done in the United States.
Natural History, Complications, and Prognosis
Natural History
Depending on the causes that have led to methemoglobin formation, different complications and prognosis are expected respectively.
Complications
Death is the most serious complications of methemoglobinemia especially when MetHb levels approach 70%. In severely sick patients death may occur even with lower levels of MetHb. Other complications include myocardial infarction, seizure and coma. <ref name="pmid14579544">{{cite journal| author=Bradberry SM| title=Occupational methaemoglobinaemia. Mechanisms of production, features, diagnosis and management including the use of methylene blue.
Prognosis
Depending on the anoxic end-organ damage caused by MetHb, the prognosiss varies between mild and fatal.
Diagnosis
Diagnostic Criteria
History and Symptoms
Methemoglobinemia will present with different signs and symptoms depending on the methemoglobin levels in the blood.
Physical Examination
Laboratory Findings
Methemoglobinemia can be diagnosed with several laboratory findings such as ABG analysis, co-oximetry and pulse oximetry.
Imaging Findings
Other Diagnostic Studies
Treatment
Medical Therapy
Methemoglobinemia should be promptly treated once diagnosed.
Surgery
Surgery does not play a role in the treatment of Methemoglobinemia.