Methaemoglobinæmia

Methemoglobinemia is a blood disorder in which the body has a higher than normal level of methemoglobin. Methemoglobin is a form of hemoglobin, the protein in red blood cells that carries oxygen. In methemoglobin, the iron in the heme group is in the ferric (Fe3+) state, rather than the usual ferrous (Fe2+) state. This altered iron state means that methemoglobin cannot bind to oxygen effectively, thus reducing the oxygen-carrying capacity of the blood.

This condition can be inherited or acquired. Congenital methemoglobinemia is usually caused by genetic defects in the enzymes involved in maintaining hemoglobin in its reduced state, most commonly deficiency of cytochrome b5 reductase. Acquired methemoglobinemia is more common and can be caused by exposure to certain drugs, chemicals, or toxins. Examples include nitrates, nitrites, certain anesthetics like benzocaine and prilocaine, and aniline dyes. These substances oxidize the iron in hemoglobin from the ferrous to the ferric state.

Symptoms of methemoglobinemia depend on the level of methemoglobin in the blood. Mild cases may be asymptomatic or cause a bluish discoloration of the skin and mucous membranes (cyanosis). As levels increase, symptoms can include headache, fatigue, shortness of breath, dizziness, and rapid heart rate. Severe methemoglobinemia, with very high levels, can lead to confusion, seizures, coma, and even death. Diagnosis is typically made by measuring methemoglobin levels in a blood sample. Treatment for acquired methemoglobinemia involves removing the offending agent and administering methylene blue, which acts as a reducing agent to convert methemoglobin back to functional hemoglobin. Congenital forms often require chronic treatment or management.