Methemoglobinemia


Methemoglobinemia

Description, Causes and Risk Factors:

Methemoglobinemia is a blood disorder in which an abnormal amount of methemoglobin -- a form of hemoglobin -- is produced. Hemoglobin is the molecule in red blood cells (RBCs) that distributes oxygen to the body. Methemoglobin cannot release oxygen.

In Methemoglobinemia, the hemoglobin is unable to release oxygen effectively to body tissues.

Methemoglobinemia may be passed down through families (inherited or congenital) or it may be caused by exposure to certain drugs, chemicals, or foods (acquired). There are two forms of inherited methemoglobinaemia. The first form is passed on by both parents. The parents usually do not have the condition themselves, but they carry the gene that causes the condition. It occurs when there is a problem with an enzyme called cytochrome b5 reductase.

There are two types of this form of 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 is called hemoglobin M disease. It is caused by defects in the hemoglobin molecule itself. Only one parent needs to pass on the abnormal gene for the child to inherit the disease.

Acquired methemoglobinemia is more common than the inherited forms. It occurs in some people after they are exposed to certain chemicals and drugs, including:

    Anesthetics such as benzocaine.

  • Benzene.

  • Certain antibiotics (including dapsone and chloroquine).

  • Nitrites (used as additives to prevent meat from spoiling).

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

Symptoms:

Signs and symptoms of methemoglobinemia (methemoglobin >1%) may include:

    Shortness of breath.

  • Cyanosis.

  • Mental status changes.

  • Headache.

  • Fatigue.

  • Exercise intolerance.

  • Dizziness and loss of consciousness.

Severe methemoglobinemia (methemoglobin >50%) patients may have:

    Dysrhythmias.

  • Seizures.

  • Coma and death.

Healthy people may not have many symptoms with methemoglobin levels < 15%, however patients with co-morbidities such as anemia, cardiovascular disease, lung disease, sepsis, or presence of other abnormal hemoglobin species (e.g. carboxyhemoglobin, sulfehemoglobin or sickle hemoglobin) may experience moderate to severe symptoms at much lower levels (as low as 5-8%).

Diagnosis:

Pattern recognition is key: cyanosis, low hemoglobin oxygen saturation on pulse oximetry(typically 85-89% representing the absorbance spectrum of methemoglobin), normal PaO2 onABG, “chocolate blood”. Co-oximetry ofABG will quantify the percent of methemoglobinemia in a fresh arterial sample.

Treatment:

Methemoglobinemia can be treated with supplemental oxygen and methylene blue 1% solution (10 mg/mL) 1 to 2 mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen-carrying state.

This is achieved by providing an artificial electron acceptor (such as methylene blue, or flavin) for NADPH methemoglobin reductase (RBCs usually don't have one; the presence of methylene blue allows the enzyme to function at 5x normal levels) The NADPH is generated via the hexose monophosphate shunt.

Diaphorase II normally contributes only a small percentage of the red blood cells reducing capacity but is pharmacologically activated by exogenous cofactors, such as methylene blue, to 5 times its normal level of activity. Genetically induced chronic low-level methemoglobinemia may be treated with oral methylene blue daily. Also, vitamin C can occasionally reduce cyanosis associated with chronic methemoglobinemia but has no role in treatment of acute acquired methemoglobinemia.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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