Methemoglobinemia is a congenital or acquired blood disorder caused by an abnormal amount of methemoglobin which is a form of hemoglobin.
is the protein in red blood cells (RBCs) responsible for carrying and distributing oxygen through the body. Methemoglobinemia occurs when the RBCs contain methemoglobin at levels higher than 1%. When this happens, there is a decreased availability of oxygen to the body tissues.
If methemoglobin levels rise above 15%, neurological and cardiac issues occur due to hypoxia. Levels above 70% are usually fatal.
What are the Causes of Methemoglobinemia?
Methemoglobinemia can be inherited, congenital or acquired (exposure to certain drugs, chemicals or foods).
There are two forms of inherited methemoglobinemia namely autosomal recessive methemoglobinemia and autosomal dominant methemoglobinemia
1, In autosomal recessive methemoglobinemia
both the parents carry the gene but do not express the condition. The enzyme cytochrome b5 reductase is responsible for this form.
There are two types of autosomal recessive methemoglobinemia:
- Type 1 (erythrocyte reductase deficiency) occurs when red blood cells are deficient in the enzyme
- Type 2 (generalized reductase deficiency) occurs when the enzyme does not work in the body
2, In autosomal dominant methemoglobinemia
one parent is the carrier. The disease is caused by defects in the protein hemoglobin.Autosomal dominant methemoglobinemia is also known as hemoglobin M disease.
Acquired methemoglobinemia is usually the most common form. It happens when people are exposed to chemicals, drugs and certain foods. Some of the causes include:
- Anesthetics like benzocaine or lidocaine (in excess)
- Antibiotics like chloroquine
What are the Symptoms of Methemoglobinemia?
Symptoms of inherited Type 1 methemoglobinemia and hemoglobin M disease usually manifest in bluish skin color
Symptoms of inherited Type 2 methemoglobinemia include:
- Failure of infants and children to thrive
- Developmental delays
- Intellectual disability
- Epileptic seizures
Symptoms of acquired methemoglobinemia include:
- Bluish skin color
- Breathing trouble
- Low energy
How to Diagnose Methemoglobinemia?
Some of the diagnostics used include:
- Complete blood counts (CBC), reticulocyte counts
- Liver function tests, electrolytes
- Hemoglobin electrophoresis for hemoglobin M disease. In some cases, an exome mapping may be required
- Enzyme assays
- Nitrite levels
To determine how efficiently oxygen is carried in the blood, other tests like arterial blood gas (ABG), co-oximetry and pulse oximetry
may be done.
Usually a potassium cyanide test is also done to distinguish between methemoglobin and sulfhemoglobin.
How do you Treat Methemoglobinemia?
Early clinical intervention is important to avoid fatalities. Severe methemoglobinemia is usually an emergency requiring immediate clinical management. There is no specified pharmacologic treatment for the hereditary methemoglobinemia.
The first step in managing methemoglobinemia should include:
- Supplementary oxygen
- Diagnosing the causative factor
- Tackling the main cause
The main treatment modalities include:
- Methylene blue which is usually the first line of treatment. However this is contraindicated in G6PD deficiency and usually ineffective for hemoglobin M
- Exchange transfusion (removing the patientís blood and replacing with donated blood) is useful for those patients who do not respond to methylene blue therapy
- Hyperbaric oxygen therapy where the patient breathes pure oxygen in a pressurized room
- IV rehydration to replenish electrolyte loss
- Other medications which include ascorbic acid, riboflavin
- Dietary control
Prognosis of Methemoglobinemia
People with Type 1 and hemoglobin M usually sustain and thrive well. Type 2 is more serious and fatal resulting in early death.