What is Methemoglobinemia?
Methemoglobin is a type of hemoglobin. Methemoglobinemia is a blood condition where the patient has abnormal quantities of methemoglobin. Hemoglobin is the molecule that is present in the red blood cells and helps in distributing the oxygen all over the body; whereas, methemoglobin does not have the ability to release oxygen. When a patient suffers from methemoglobinemia, the hemoglobin in the body is not able to release oxygen in an effective manner to the body tissues.
Methemoglobinemia is also known as erythrocyte reductase deficiency, Hemoglobin M disease or generalized reductase deficiency.
Pathophysiology of Methemoglobinemia
In human body, the normal levels of methemoglobin are less than 1%. Increased methemoglobin levels in the blood occur when the body mechanism, which is responsible for defending itself against oxidative stress inside the RBCs, gets swamped and the ferrous ion in the oxygen gets oxidized to the ferric state; thus converting hemoglobin to methemoglobin. This results in decreased ability of the body to release the oxygen to its tissues and leads to hypoxia. Because of this, the blood takes on a chocolate-brown or bluish color. The methemoglobin that is formed spontaneously gets reduced normally, i.e. normal hemoglobin gets regenerated. This happens with the help of protective enzyme systems and also glutathione enzyme and ascorbic acid. Any disruption in these enzyme systems causes methemoglobinemia. Hypoxia results from the decreased oxygen-binding capacity of methemoglobin along with increased oxygen-binding affinity in the subunits present in the same molecule of hemoglobin. This prevents oxygen release at normal tissue oxygen levels.
Causes & Types of Methemoglobinemia
Methemoglobinemia can be inherited or congenital, i.e. it can be passed from the parents/families. Methemoglobinemia can also occur from exposure to certain chemicals, drugs, or foods; where it is referred to as acquired methemoglobinemia.
There are two types of inherited methemoglobinemia. Methemoglobinemia which is inherited by both parents is the first type. The parents often do not have this condition themselves; however, they carry the gene which causes methemoglobinemia. This happens when there is a problem with an enzyme known as cytochrome b5 reductase.
This form of methemoglobinemia has two types:
Type 1: Erythrocyte Reductase Deficiency Methemoglobinemia: This occurs when there is deficiency of this enzyme in the red blood cells.
Type 2: Generalized Reductase Deficiency Methemoglobinemia: This occurs when the enzyme is not able to work anywhere in the body.
Hemoglobin M disease is the second form of inherited methemoglobinemia that is caused due to defects present in the hemoglobin molecule. The abnormal gene can be passed to the child through one parent only for this disease to be inherited.
Acquired methemoglobinemia is more commonly found than inherited methemoglobinemia. Acquired methemoglobinemia develops in some individuals after they get exposed to certain drugs and chemicals, such as anesthetics (benzocaine), benzene, certain antibiotics (chloroquine, dapsone) and nitrites.
Signs & Symptoms of Methemoglobinemia
Symptoms of Hemoglobin M disease which is another type of inherited methemoglobinemia include: Bluish coloring of the skin.
The symptoms of type 1 methemoglobinemia, i.e. erythrocyte reductase deficiency methemoglobinemia are: There is bluish coloring of the skin.
The symptoms of type 2 methemoglobinemia, i.e. generalized reductase deficiency methemoglobinemia are:
- Developmental delays in the child.
- Intellectual disability.
- Failure to thrive in the child.
Symptoms of Acquired Methemoglobinemia are:
Diagnosis of Methemoglobinemia
A blood test helps with the diagnosis of methemoglobinemia. If a baby is born with this condition, then he/she will have cyanosis, i.e. bluish skin color at birth or after sometime. Other tests, such as pulse oximetry tests and arterial blood gases, can be done.
Treatment of Methemoglobinemia
Treatment of methemoglobinemia consists of methylene blue, which is a medicine used for treating severe methemoglobinemia. It is important to note that methylene blue can be dangerous and should not be used in individual who are suffering from or who are at risk for G6PD deficiency, which is a blood disease.
Ascorbic acid can also be used for decreasing the level of methemoglobin. Hyperbaric oxygen therapy and exchange transfusions can be done as alternative treatment for methemoglobinemia.
No treatment is needed in majority of the cases of mild acquired methemoglobinemia; but patient should avoid the specific chemical or medicine which has caused methemoglobinemia. Treatment may be needed in severe cases of acquired methemoglobinemia including blood transfusion.
Prognosis of Methemoglobinemia
The prognosis of hemoglobin M disease and type 1 methemoglobinemia is very good. The prognosis of Type 2 methemoglobinemia is not good, as it is more serious and patient usually dies within the first few years of his/her life.
The prognosis of acquired methemoglobinemia usually is also good and patients improve once the offensive food, drug, chemical is identified and stopped.
Potential Complications of methemoglobinemia include: seizures, shock and death.
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