Methemoglobinemia

Methemoglobinemia is a blood disorder characterized by an increased level of methemoglobin, a form of hemoglobin that contains iron in an oxidized state. Unlike normal hemoglobin, methemoglobin is unable to bind oxygen and carry it to body tissues, leading to tissue hypoxia.

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Etiology

• Methemoglobinemia can be either acquired or congenital:
• Acquired Methemoglobinemia: This is the most common type and is usually caused by exposure to certain drugs (such as dapsone, local anesthetics like benzocaine, and some antibiotics) and chemicals (like nitrates and nitrites).
• Congenital Methemoglobinemia: This type is less common and is caused by a genetic deficiency of the enzyme methemoglobin reductase (also known as cytochrome b5 reductase), which is responsible for converting methemoglobin back to hemoglobin.

Pathophysiology

Under normal conditions, a small amount of methemoglobin is produced in the body but is quickly reduced back to hemoglobin by methemoglobin reductase. However, when this enzyme is deficient or overwhelmed by the presence of certain drugs or chemicals, methemoglobin can accumulate, leading to methemoglobinemia.

Clinical Features

The primary symptom of methemoglobinemia is cyanosis, a bluish discoloration of the skin and mucous membranes, that does not improve with oxygen administration. Other symptoms can include headache, fatigue, shortness of breath, and at higher levels, confusion, seizures, and loss of consciousness.

Diagnosis

Methemoglobinemia is diagnosed through blood tests that measure the level of methemoglobin in the blood. In cases of congenital methemoglobinemia, genetic testing may be performed to identify the specific enzyme deficiency.

Treatment

Treatment for methemoglobinemia aims to reduce the level of methemoglobin and treat any underlying causes. In severe cases, treatment may include the administration of methylene blue, a medication that accelerates the conversion of methemoglobin back to hemoglobin. In cases of drug-induced methemoglobinemia, the offending drug must be discontinued.

Prognosis

With appropriate treatment, the prognosis for individuals with methemoglobinemia is generally good. However, repeated or prolonged exposure to methemoglobin-inducing agents should be avoided to prevent recurring episodes.

See Also

• Hemoglobin
• Cyanosis
• Oxygen transport
• Methylene blue

References

• Ash-Bernal R, Wise R, Wright SM. (2004). Acquired methemoglobinemia: a retrospective series of 138 cases at 2 teaching hospitals. Medicine (Baltimore), 83(5), 265-273.
• Coleman MD, Coleman NA. (1996). Drug-induced methaemoglobinaemia. Treatment issues. Drug Saf, 14(6), 394-405.
• Mansouri A, Lurie AA. (1993). Concise review: methemoglobinemia. Am J Hematol, 42(1), 7-12.

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