From the Guidelines
Methemoglobinemia should be treated with intravenous methylene blue at a dose of 1-2 mg/kg administered over 5 minutes for symptomatic patients or those with methemoglobin levels above 20%. This recommendation is based on the most recent and highest quality study available, which suggests that methylene blue is the first-line treatment for methemoglobinemia 1. The treatment depends on the severity and cause of the condition.
Treatment Options
- For severe cases unresponsive to methylene blue, exchange transfusion or hyperbaric oxygen therapy may be necessary, as suggested by the American Heart Association guidelines 1.
- Patients with G6PD deficiency should not receive methylene blue as it can cause hemolysis; however, there is no strong evidence to support the use of alternative treatments like ascorbic acid (vitamin C) in this context 1.
- For mild cases (methemoglobin <20% in asymptomatic patients), removing the offending agent and providing supportive care is often sufficient, with monitoring and oxygen supplementation as needed 1.
Mechanism of Action
Methylene blue works by providing an artificial electron carrier to reduce methemoglobin back to hemoglobin, restoring oxygen-carrying capacity. The expected time by which the methemoglobin level should normalize is 1 hour after the administration of intravenous methylene blue 1.
Prevention
Prevention involves avoiding triggering substances like certain medications (dapsone, benzocaine, nitrates), chemicals, and foods high in nitrates, especially in susceptible individuals. It is essential to note that the main morbidity in methemoglobinemia is due to the hypoxic state, and thus, if the patient is asymptomatic, they can be monitored without additional treatment 1.
From the Research
Definition and Causes of Methemoglobinemia
- Methemoglobinemia is a disorder characterized by an abnormal amount of methemoglobin, a form of hemoglobin, in the blood 2, 3, 4.
- Methemoglobin is the oxidized form of hemoglobin, which does not bind oxygen and increases the affinity of oxygen for the partially oxidized portion of hemoglobin 2.
- The causes of methemoglobinemia can be congenital or acquired, with acquired causes including exposure to certain chemicals, medications, and toxins 2, 3, 4, 5.
Clinical Characteristics and Diagnosis
- The main clinical characteristic of methemoglobinemia is cyanosis, which is unresponsive to oxygen therapy, and blood that is chocolate-colored when drawn 2, 3.
- Co-oximetry is the gold standard for diagnosis, but arterial blood gas paired with pulse oximetry and serum methemoglobin levels can also confirm the diagnosis clinically 2, 3, 5.
- Diagnosis can be complex, as the resulting biological abnormalities may go undetected 4.
Treatment and Management
- Treatment is aimed at removal of the offending agent, if medication-induced, and is directed at aggressive oxygen therapy and treatment with the antidote, methylene blue 2, 3, 4, 5.
- Ascorbic acid is an effective alternative in the management of acquired methemoglobinemia if methylene blue is unavailable 6, 5.
- Riboflavin may also be used as a treatment for methemoglobinemia 5.
- It is essential to identify the etiology and restore the heme molecule to its physiological state 4.