Methylene blue has FDA approval for the treatment of methemoglobinemia, a condition when Fe2+ of hemoglobin gets oxidized to Fe3+, reducing the oxygen-carrying capacity of hemoglobin, and typically presents with cyanosis of the lips and extremities, characteristic "chocolate-colored urine," and hypoxia. Methemoglobinemia results from exposure to certain drugs such as dapsone, a drug indicated for the treatment of Mycoplasma leprae and Pneumocystis jirovecii prophylaxis, benzocaine (a local anesthetic), high altitude water sources, and nitrites such as nitroglycerin or amyl nitrite used for treating coronary artery disease. The remaining indications are non-FDA approved. The first is for vasoplegic syndrome, a type of distributive shock that occurs during coronary procedures (specifically coronary artery bypass grafting), as a means to increase systemic vascular resistance when the use of epinephrine is refractory. Another indication for methylene blue is during either a lumpectomy/mastectomy with sentinel lymph node biopsy, as it applied as a dye to map out which lymph nodes, if any, have any signs of malignancy. It can also reduce post-injection pain when used 45 seconds before the administration of propofol. Indications for methylene blue also include the treatment of Plasmodium falciparum in areas that have shown resistance to chloroquine and pyrimethamine-sulfadoxine. Ifosfamide-induced encephalopathy is also treatable with methylene blue due to the reduction ability it has to prevent the neurotoxic metabolites that cause the encephalopathy from forming. Another indication for the use of methylene blue is for the identification of the parathyroid glands during parathyroidectomy procedures.
The main mechanism of action of methylene blue is reducing the oxidized form of hemoglobin Fe3+ when in a state of methemoglobinemia, to Fe2+. In turn, this will increase the oxygen-binding capacity of hemoglobin and thus increase oxygen delivery to tissues. This property is also exhibited by vitamin C, which is an alternative treatment option for methemoglobinemia. Methylene blue also inhibits the enzymes endothelial nitric oxide synthase (eNOS), inducible nitric oxide synthase (iNOS), and guanylate cyclase, decreasing the amount of cGMP needed for nitrous oxide to be released, therefore causing vasoconstriction of blood vessels via inhibition of vascular smooth muscle relaxation. Regarding treating malaria, methylene blue has been shown to improve the chloroquine response to malaria by decreasing its resistance and inhibiting Plasmodium falciparum glutathione reductase, an enzyme that prevents the formation of byproducts formed by the Plasmodium species that triggers the body's immune response.
Administration of methylene blue for both children and adults experiencing methemoglobinemia is done intravenously at a dose of 1 mg/kg of a 1% solution over 5 to 30 minutes. When using methylene blue for lymph node mapping, the most common two doses are 2 mL and 5 mL of a 1% solution t given intraparenchymal. When used to prevent post-injection pain when used 45 seconds before the administration of propofol, the dose is 50 mg given as a 2 mL bolus. Treating ifosfamide-induced encephalopathy with methylene blue is given intravenously at a dose of 50 mg every 4 hours until asymptomatic. Methylene blue is also infused for 20 minutes at a dose of 2 mg/kg for a patient that is experiencing vasoplegic syndrome and may be given pre-operatively and post-operatively to reduce mortality. For the identification of the parathyroid glands, methylene blue is administered 1 hour before the parathyroidectomy intravenously at a dose of 5mg/mL. Treating Plasmodium falciparum requires three days of treatment at a dose of 36 to 72 mg/kg of methylene blue, given as a supplement.
One of the most common adverse effects of methylene blue is the bluish-green discoloration of urine. Another common adverse effect is limb pain following IV administration. methylene blue may contribute to serotonin syndrome if combined with other serotonergic drugs such as SSRIs, SNRIs, MAOIs, and TCAs due to the MAOI activity that methylene blue has. Serotonin syndrome increases sympathetic and neuromuscular activity; therefore, symptoms include diaphoresis, clonus, and tremors. In adults, methylene blue can cause central nervous system-related symptoms such as dizziness, confusion, and headaches. Administration of methylene blue in neonates has led to hyperbilirubinemia, respiratory depression, pulmonary edema, phototoxicity, and hemolytic anemia.
Methylene blue is contraindicated due to any hypersensitivities or anaphylaxis after past administration and for those with glucose-6-phosphate dehydrogenase deficiency due to susceptibility of experiencing hemolytic anemia. Those experiencing hemolytic anemia under this situation will have jaundice and characteristic Heinz bodies on a peripheral blood smear. Methylene blue also is contraindicated in pregnant women with the FDA assigning a pregnancy class X due to intestinal atresia and fetal death that occurs following an intra-amniotic injection, specifically in the second trimester.
Methylene blue is a safe drug at a therapeutic dose of <2 mg/kg; however when levels are >7 mg/kg, many of the adverse effects it exhibits will occur.  Serotonin syndrome has been found to occur when combining serotonergic agents with methylene blue at a dose of 5 mg/kg. Methylene blue use also requires caution in patients with renal failure due to its ability to reduce renal blood flow. Also, as noted in adverse effects, patients taking any drug with serotonergic activity such as SSRIs should avoid the administration of methylene blue due to the risk of serotonin syndrome.
The research concludes that there has been no antidote for treating methylene blue toxicity to be found to exist. If anaphylaxis does occur, the administration of methylene blue should stop immediately and promptly follow by supportive care, although severe anaphylactic shock with methylene blue is quite rare. Methylene blue may become potentially fatal when used in combination with medications, which include or increase serotonin; this is because of its monoamine oxidase inhibiting properties. It may precipitate serotonin toxicity at doses of >5 mg/kg.
Clinicians primarily use methylene blue for methemoglobinemia and lymph node mapping during breast surgery; however, there are other serious life-threatening situations where it can be applied that are not widely recognized. Understanding and mastering the indications for the use of methylene blue is essential in many potentially life-threatening situations such as vasoplegic syndrome and ifosfamide-induced encephalopathy, that if not acted upon appropriately and promptly, these disorders could spell disaster for the patient. The entire interprofessional healthcare team must maintain open communication and information sharing to optimize the use of methylene blue for maximum patient benefit.
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