Definition of Methemoglobinemia:

Methemoglobinemia is a blood disorder in which the body cannot recycle hemoglobin after it is damaged. Hemoglobin is the oxygen-carrying molecule found in red blood cells. In some cases of methemoglobinemia, the hemoglobin is unable to carry oxygen effectively to body tissues.

Causes, incidence, and risk factors:

Methemoglobinemia may be passed down through families (inherited), or it may result from exposure to certain drugs, chemicals, or foods (acquired).

There are two forms of inherited methemoglobinemia. The first form is an autosomal recessive condition. This means it is passed on by both parents who usually do not have the condition themselves but carry the gene that causes the condition. It occurs when there is a problem with an enzyme called cytochrome b5 reductase. There are two subtypes of autosomal recessive methemoglobinemia:

• Type 1, also called erythrocyte reductase deficiency, occurs when red blood cells lack the enzyme.
• Type 2, also called generalized reductase deficiency, occurs when the enzyme doesn't work anywhere in the body.

The second form of inherited methemoglobinemia, called hemoglobin M disease, is caused by defects in the hemoglobin molecule itself. It is an autosomal dominant condition. That means only one parent needs to pass you the abnormal gene in order for you to inherit the disease.

Acquired methemoglobinemia is more common than the inherited forms. It occurs after exposure to certain chemicals and drugs, including:

• Anesthetics such as benzocaine and Xylocaine
• Benzene
• Certain antibiotics (including dapsone and chloroquine)
• Nitrites (used as additives to prevent spoilage of meat)

The condition may also occur in infants who are very ill or who are fed too many vegetables containing nitrates (such as beets).