Causes and Risks: There are two forms of methemoglobinemia, both inheritable. The autosomal recessive condition results from an absence of an enzyme that normally converts methemoglobin back to normal hemoglobin thereby leading to an increase in methemoglobin and a decrease in the ability of blood to carry oxygen. This results in a bluish color to the skin (cyanosis).

The second type, called hemoglobin M disease, is an autosomal dominant trait characterized by the production of an abnormal methemoglobin, and is usually milder in its degree of cyanosis. Exposure to environmental chemicals causes an increase in production of methemoglobin in affected people. These chemicals are more commonly aniline dyes (in brightly colored cloths), nitrobenzene, and nitrites (used commonly to prevent spoilage of meat).

Prevention: Genetic counseling is recommended for prospective parents with a family history of methemoglobinemia. People with methemoglobinemia should avoid exposure to chemicals that could precipitate a cyanotic crisis.

Symptoms:

• a family history of methemoglobinemia
• a constant bluish discoloration to the skin
• shortness of breath on physical exertion
• mental retardation (only in cytochrome b5 reductase type II form)

Signs and Tests:

• increased methemoglobin in blood
• decreased blood oxygen (found on arterial blood gas analysis)
• abnormal methemoglobin (hemoglobin M disease)
• deficiency of NADH cytochrome b5 reductase in red blood cells

Treatment: Medications used to treat this disorder include:

• high doses of oral ascorbic acid (only in cytochrome b5 reductase deficiency)
• methylene blue (only in cytochrome b5 reductase deficiency)

Prognosis: With the exception of Type II cytochrome b5 reductase deficiency, cyanosis should be the only significant symptom barring exposure to chemicals that might cause a crisis.

Complications:

• mental retardation

Call to make an appointment with your health care provider if there is a family history of methemoglobinemia and you develop symptoms of this disorder.
Call to see your health care provider immediately if there is severe shortness of breath.