Methemoglobinemia medical therapy

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Aksiniya Stevasarova, M.D.

Methemoglobinemia should be promptly treated once diagnosed.

• Initial management of methemoglobinemia patients includes administration of supplemental oxygen and if the toxin is still present on the skin, the clothes should be promptly removed and the skin washed. If the patient cannot provide information on the cause of the presenting symptoms, we could check the blood levels of MetHb and also perform gastric lavage. In asymptomatic patients, we can measure the serum levels of MetHb multiple times until the levels normalize, which usually happens.^{[1] [2]}

• Patients who are symptomatic and have MetHb levels above 20% or less than 20% but with multiple comorbidities, should be admitted to the hospital and treated with intravenous (IV) methylene blue as this is the first-line antidotal agent, 1% solution (10mg/ml) 1-2mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen-carrying state. This is achieved through the enzyme inducing effect of methylene blue on levels of diaphorase II (NADPH methemoglobin reductase). Diaphorase II normally contributes only a small percentage of the red blood cells reducing capacity but is pharmacologically activated by exogenous cofactors, such as methylene blue, to 5 times its normal level of activity.^[3]

• Methylene blue should not be used in patients with G6PD deficiency, as the antidote requires G6PD to work. The use of methylene blue in G6PD deficient patients can lead to hemolysis. In G6PD deficient patients we can use exchange transfusion as treatment. Exchange transfusion as the name implies, replaces abnormal hemoglobin with normal hemoglobin. We can also use it to treat patients presenting with severe symptoms and unresponsive to methylene blue treatment.^[4]

• The use of methylene blue is contraindicated in patients concurrently taking serotonergic psychiatric drugs, as the risk for inducing serotonin syndrome increases. Furthermore, methylene blue is not used in patients with Hemoglobin M (Hb M), [[nicotinamide adenine dinucleotide phosphate (NADPH) methemoglobin reductase]] (ie, diaphorase II deficiency]] and sulfhemoglobinemia, due to lack of effect. In such cases we can use hyperbaric oxygen treatment, that allows tissue oxygenation via dissolving oxygen directly in the plasma. ^[5]

• Genetically induced chronic low-level methemoglobinemia may be treated with oral methylene blue daily, ascorbic acid or riboflavin in order to decrease cyanosis.

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