methaemoglobinemia

Methaemoglobinemia is a blood disorder characterized by an abnormally high level of methaemoglobin. Methaemoglobin is a form of haemoglobin that cannot bind and transport oxygen effectively. Haemoglobin, the protein in red blood cells responsible for carrying oxygen from the lungs to the body's tissues, contains iron in its ferrous (Fe2+) state. In methaemoglobin, this iron is oxidized to its ferric (Fe3+) state, rendering it incapable of binding oxygen.

This condition can be congenital or acquired. Congenital methaemoglobinemia is typically caused by genetic defects in the enzymes responsible for reducing methaemoglobin back to haemoglobin, such as cytochrome b5 reductase. Acquired methaemoglobinemia is more common and results from exposure to certain drugs, chemicals, or toxins. Examples include nitrates, nitrites, certain local anaesthetics like prilocaine, and aniline dyes. Ingesting contaminated well water with high nitrate levels is a notable cause, particularly in infants.

Symptoms vary depending on the severity of the methaemoglobin level. Mild cases may be asymptomatic or present with cyanosis, a bluish discoloration of the skin and mucous membranes, due to lack of oxygen. More severe cases can lead to fatigue, headache, dizziness, shortness of breath, rapid heart rate, confusion, seizures, and even coma or death. Diagnosis is made through blood tests that measure the percentage of methaemoglobin. Treatment depends on the cause and severity, often involving the administration of methylene blue, which acts as a reducing agent to convert methaemoglobin back to functional haemoglobin. Removal of the offending agent is crucial for acquired cases.