Methemoglobinemia overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1], Associate Editor(s)-in-Chief: Template:Aksiniya K. Stevasarova
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. [1]
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 (Hb M)
Acquired or Acute Methemoglobinemia
The most common causes are different oxidant drugs, toxins and chemicals.
Pathophysiology
Causes
1. Drug Induced
• Anesthetics like benzocaine, lidocaine, prilocaine
• Methylene blue
• Nitric Oxide
• Amilnitrate
• Nitroglycerin
• Antimalarial drugs like Primaquine phosphate (in nicotinamide adenine dinucleotide (NADH) methemoglobin reductase deficient individuals)
• Rasburicase
• Sulfasalazine
• Dapsone
• Trimethoprim
• Sulfonamides
• Aniline dyes
• Metoclopramide
• Chlorates and Bromates
2. Contaminated well water (in premature infants and infants younger than 4 months)
3. Solid foods (not well cooked vegetables high in nitrates in premature infants and infants younger than 4 months)
Differentiating Methemoglobinemia overview from Other Diseases
Depending on the levels of MetHb in the blood we can observe different sympotoms as follows: MetHB 15 % presents with skin and blood color changes at levels, Levels above 15 % will result in hypoxia and levels above 70% can lead to death.
There are several conditions that can present similar to methemoglobinemia. Note that the hypoxia in methemogobinemia will be unresponsive to oxygen treatment, in comparison to cardiac and pulmonary cause. Worth mentioning is that high doses of antidotal treatment for methemogobinemia (Methylene blue) can also cause cyanotic discoloration of the skin, but this does not mean the the treatment is not working.
Differential Diagnoses
• Heart failure
• Pulmonary embolism
• Polycythemia
• Anemia
• Rasbirucase (Rx Tumor Lysis Syndrome)
• Metabolic Acidosis
• Methylene Blue treatment
• Sulfhemoglobin
• Asthma
Epidemiology and Demographics
Risk Factors
Screening
Natural History, Complications, and Prognosis
Natural History
Complications
Prognosis
Diagnosis
Diagnostic Criteria
History and Symptoms
Physical Examination
Laboratory Findings
Imaging Findings
Other Diagnostic Studies
Treatment
Medical Therapy
Surgery
Prevention
References
- ↑ {{Rev Bras Anestesiol. 2008 Nov-Dec;58(6):651-64. Methemoglobinemia: from diagnosis to treatment. [Article in English, Portuguese] do Nascimento TS1, Pereira RO, de Mello HL, Costa J. PMID: 19082413}}