Introduction
Methemoglobinemia occurs when an imbalance arising from either increased methemoglobin production or decreased methemoglobin reduction is present. Methemoglobin is a derivative of hemoglobin in which the ferrous (Fe2+) irons are oxidized to the ferric (Fe3+) state. Methemoglobin is formed spontaneously at a slow rate by the autoxidation of hemoglobin. In the release of oxygen from Fe2+-oxyhemoglobin, one electron is partially transferred to a small portion of released oxygen, generating superoxide and eventually forming other ROS, which convert the iron to the ferric state and form Fe3+-hemoglobin, that is, methemoglobin (Fig1). Methemoglobin may also be formed from the oxidation of hemoglobin in other reactions with endogenous and exogenous compounds. The ferric hemes of methemoglobin are unable to bind oxygen and, additionally, if a ferriheme subunit is a part of a hemoglobin tetramer, the oxygen affinity of the accompanying ferrous hemes in the hemoglobin tetramer is increased. As a result, the oxygen dissociation curve is left-shifted and oxygen tissue delivery is impaired. Methemoglobin is formed continuously, but reducing mechanisms keep the met hemoglobin level at about 1% of the total hemoglobin. The only physiologically important mechanism is the NADH-dependent cytochrome b5 reductase (b5R). b5R contains a prosthetic FAD group that acts as an electron acceptor. NADH reduces FAD to FADH2, which then reduces the heme protein cytochrome b5. Electrons from the reduced cytochrome b5 are in turn transferred to methemoglobin, reducing iron back to the ferrous state (see Fig.1)
Fig1. METHEMOGLOBINEMIA. Formation of methemoglobin and its physiologic (open space) and therapeutic (shaded space) reduction. Iron is in the ferrous state (Fe2+) in oxygenated and deoxy-hemoglobin (deoxyHb). When oxygen is released, a small proportion of oxygen released from hemoglobin is converted to ROS. ROS then oxidizes bivalent ferrous iron (Fe2+); to the ferric state (Fe3+) (i.e., methemoglobin). NADH generated in glycolysis is a cofactor for methemoglobin reduction mediated by cytochrome b5 reductase (b5R) depicted in the open space, keeping methemoglobin at low levels (<1%). When this physiologic reduction of NADH-dependent methemoglobin reduction is either insufficient because of excessive ROS or decreased b5R activity, methemoglobin reduction can be achieved therapeutically. Exogenously administered methylene blue utilizing NADPH produced by G6PD in the pentose shunt can nonenzymatically convert methemoglobin to Fe2+ hemoglobin (shaded space). b5R, Cytochrome b5 reductase; NAD, oxidized form of nicotinamide adenine dinucleotide phosphate; NADH, reduced form of NAD; NADP, oxidized form of nicotinamide adenine dinucleotide phosphate; NADPH, reduced form of NADP; ROS, reactive oxygen species.
An alternative pathway mediated by NADPH-diaphorase uses NADPH generated by G6PD as a source of electrons to reduce redox dyes, such as methylene blue and flavin. Reduced methylene blue then reduces methemoglobin. Since these electron acceptors are not physiologic, this pathway is only of importance as it is the mechanism by which methylene blue treats acute toxic methemoglobinemia.
Epidemiology
Most cases of methemoglobinemia are acquired, resulting from increased methemoglobin formation by various exogenous agents. Acute or toxic methemoglobinemia may occur in the setting of over dose or poisoning, but also at standard doses of drugs. Acute methemoglobinemia occurs equally between males and females and over a wide range of ages; however, infants are more susceptible because their erythrocyte b5R activity is normally ~50% of adult activity.
Hereditary methemoglobinemia is most commonly caused by deficiency of b5R. b5R deficiency is an autosomal recessive condition and occurs in all racial and ethnic groups, but is endemic in certain populations, including Navajo and Athabasca Native Americans and natives of Yakutsk, Siberia and is also common among Puerto Ricans. Other causes of hereditary methemoglobinemia are the autosomal dominant inheritance of an abnormal hemoglobin in hemoglobin M disease and, very rarely, deficiency of cytochrome b5.
Pathobiology
Acute Methemoglobinemia
Many drugs and toxins have been implicated in acute methemoglobinemia. More common culprits include dapsone and local anesthetics (benzocaine, lidocaine, prilocaine). Exposure to nitrates and nitrites, widely used as food preservatives and found in well water, can also cause methemoglobinemia. Infants less than 6 months of age may have increased susceptibility to methemoglobinemia at least in part because of their lower b5R activity.
b5 Reductase Deficiency
In RBCs, b5R transfers electrons to methemoglobin to reduce it to hemoglobin. In other cells, b5R transfers electrons from cytochrome b5 to stearoyl-CoA in the endoplasmic reticulum, a reaction that has an important role in cholesterol biosynthesis, fatty acid elongation, desaturation, and drug metabolism. There are two types of b5R deficiency. The more common type I b5R deficiency is usually caused by missense mutations leading to decreased stability of the enzyme. Thus, although b5R is abnormal in all cells, only mature RBCs, which cannot synthesize proteins and replace the enzyme, are significantly affected in patients with type I b5R deficiency. Type II b5R mutations affect the catalytic site or lead to marked structural changes and all cells have decreased b5R activity.
Clinical Manifestations
Methemoglobinemia greater than 1.5 g/dL causes clinically discernible cyanosis. Methemoglobinemia should be clinically suspected when “cyanosis,” occurs in the presence of a normal arterial PaO2. However, cyanosis is not specific for methemoglobinemia as it can be caused by high levels of deoxygenated hemoglobin (above approximately 5 g/dL) and sulfhemoglobin (above approximately 0.5 g/dL). The hemoglobin level is also a factor in the development of cyanosis. For example, a person with an Hb of 15 g/dL and 10% methemoglobin would be cyanotic, but a person with an Hb of 10 g/dL would need at least 15% methemoglobin to become cyanotic.
Acute Methemoglobinemia
Acute methemoglobinemia is an acquired disorder caused by certain oxidant drugs and chemicals; some patients who have baseline nor mal methemoglobin levels but who are heterozygous for b5R deficiency (see above) are more likely develop acute methemoglobinemia. Symptoms develop in acute methemoglobinemia due to abruptly impaired tissue oxygenation. Early symptoms include headache, fatigue, dyspnea, and lethargy. At higher levels, respiratory depression, altered consciousness, shock, seizures, and death may occur. As methemoglobin levels rise above 20% to 30%, patients can experience progressive respiratory compromise, myocardial ischemia, seizures, and coma. Death typically ensues at methemoglobin levels above 70% but can occur at lower levels.
Chronic Methemoglobinemia
Individuals with type I b5R deficiency, which is limited to RBCs, have methemoglobin concentrations of 10% to 35% and may or may not be cyanotic but are usually asymptomatic, even with methemoglobin levels up to 40%. Life expectancy is not shortened and pregnancies occur normally. Compensatory erythrocytosis is at times observed.
In addition to methemoglobinemia and cyanosis, patients with the less common type II b5R deficiency exhibit mental retardation and developmental delay. Other neurologic symptoms may be present, including microcephaly, opisthotonus, athetoid movements, strabismus, seizures, and spastic quadriparesis. Life expectancy is significantly shortened and death in infancy is typical.
Laboratory Manifestations
The laboratory diagnosis of methemoglobinemia is based on analysis of its absorption spectra. A fresh specimen should always be obtained because methemoglobin levels tend to increase with storage. Traditional pulse oximetry is unreliable in the presence of methemoglobinemia because of its light absorbance properties; however, arterial or venous blood co-oximetry can determine the methemoglobin fraction along with all other substances with the optical density at 630 nm.
Methemoglobin detected by co-oximeter is optimally confirmed by the specific Evelyn-Malloy method if available. This method involves direct spectrophotometric analysis and should be used when methemoglobinemia is suspected. In the Evelyn-Malloy method, blood is lysed in a slightly acidic buffer and the optical density is measured at 630 nm before and after adding a small amount of neutralized cyanide. Absorption of methemoglobin at this wavelength disappears when it is converted to cyanmethemoglobin. This method remains the most accurate technique for the estimation of methemoglobin concentration. It also rules out spurious methemoglobinemia from extrinsic substances with an absorbance at 630 nm.
An eight-wavelength pulse oximeter, Masimo Rad-57 (the Rainbow-SET Rad-57 Pulse CO-Oximeter, Masimo Inc, Irvine, CA) has been approved by the US Food and Drug Administration and appears to be accurate for the measurement of both carboxyhemoglobin and methemoglobin.
Distinguishing the hereditary forms of congenital methemoglobinemia requires interpretation of family pedigrees as well as biochemical analyses. Cyanosis in successive generations suggests autosomal dominant hemoglobin (Hb) M disease, whereas normal parents but possibly affected siblings implies autosomal recessive b5R deficiency. Incubation of blood with methylene blue also distinguishes b5R deficiency from Hb M disease, because this treatment results in the rapid reduction of methemoglobin through the NADPH-flavin reductase pathway in cases of b5R deficiency, but not in cases of Hb M dis ease. Types I and II b5R deficiency are distinguished by their clinical phenotype and by analysis of enzymatic activity in erythroid and non-erythroid cells. Because the enzyme defect is found in fibroblasts, analysis of b5R activity in cultured amniotic cells for prenatal diagnosis is possible.
Differential Diagnosis
Sulfhemoglobin in concentrations greater than 0.5 gm% also causes “cyanosis,” with a normal PaO2 and may be erroneously measured as methemoglobin. Some drugs and other pigments, including methylene blue, may also produce false-positive results when methemoglobin is measured by co-oximetry.
Prognosis
Mild acute methemoglobinemia usually resolves spontaneously in a few days. Acute methemoglobinemia that is more symptomatic, including those with respiratory or other significant compromises, generally resolves with treatment, provided if the offending cause is discontinued; however, severe cases may be fatal. Patients with type I b5R deficiency have cyanosis but a normal life expectancy, whereas the pan-deficient type II b5R patients usually succumb in childhood.
Therapy
Offending agents in cases of acquired methemoglobinemia should be discontinued. No other therapy may be required in an asymptomatic patient. However, if the patient is symptomatic or if methemoglobin levels are greater than 20%, specific therapy is indicated. Methylene blue, 1 to 2 mg/kg intravenously over 5 minutes, is an effective treatment for patients with methemoglobinemia because NADPH formed in the pentose shunt can rapidly reduce this dye to leukomethylene blue in a reaction catalyzed by NADPH diaphorase. Leukomethylene blue, in turn, nonenzymatically reduces methemoglobin to hemoglobin.
There are two exceptions to the efficacy of this treatment. In patients who are G6PD deficient, methylene blue would not only fail to give the desired effect on methemoglobin levels but might compound the impaired tissue oxygen delivery by inducing an acute hemolytic episode. Methylene blue also has monoamine oxidase inhibitor properties and thus can precipitate a potentially life-threatening serotonin syndrome in individuals receiving serotonergic agents such as selective serotonin reuptake inhibitors (SSRIs) and other serotonergic antidepressants. If methylene blue is contraindicated, ascorbic acid can be given. Patients with G6PD deficiency and acute methemoglobinemia have been successfully treated with exchange transfusion. Hyperbaric oxygen has also been used in severe cases of methemoglobinemia.
The cyanosis in hereditary b5R type I deficiency is of cosmetic significance only but can be treated with methylene blue or ascorbic acid, both of which facilitate the reduction of methemoglobin through alternate pathways. However, this therapy has no effect on the neuro logic and other systemic defects seen in type II b5R deficiency.
