Methemoglobinemia

Methemoglobinemia is a condition in which hemoglobin iron is oxidized from ferrous to ferric form, reducing oxygen-carrying capacity and impairing oxygen release to tissues.

Methemoglobin: Hemoglobin containing ferric iron that cannot bind oxygen effectively. It is the oxidized hemoglobin that accumulates in this condition.
Congenital methemoglobinemia: An inherited disorder in which methemoglobin remains abnormally elevated from birth. Inherited enzyme defects or hemoglobin variants can cause persistent forms of the condition.
Co-oximetry: A blood-analysis method that measures multiple hemoglobin species by their distinct light-absorption spectra. It directly quantifies methemoglobin, unlike standard pulse oximetry.
Cytochrome b5 reductase deficiency: An inherited deficiency of the enzyme that normally reduces methemoglobin in red blood cells. This defect can produce lifelong methemoglobinemia, with severity depending on the affected tissues.
Cyanosis: A bluish or slate-colored discoloration of skin or mucous membranes associated with abnormal blood oxygenation or pigments. Methemoglobin can cause cyanosis even when dissolved oxygen in arterial blood is adequate.
Hemoglobin: The iron-containing protein in red blood cells that transports oxygen and some carbon dioxide. Oxidation alters hemoglobin’s iron and changes its oxygen-binding behavior.
Acquired methemoglobinemia: An elevated methemoglobin level caused by oxidizing exposures or medications. This is the common acute form, often following exposure to oxidizing agents.
Methylene blue: A redox-active drug used to treat significant methemoglobinemia by promoting methemoglobin reduction. It can restore functional hemoglobin, but may be ineffective or harmful in some settings.
Hemoglobin M: A group of inherited hemoglobin variants that stabilize iron in its oxidized ferric state. These variants cause methemoglobinemia by resisting the usual enzymatic reduction.
Tissue hypoxia: A state in which tissues receive or use insufficient oxygen for their needs. Reduced oxygen carriage and release can impair tissue oxygenation as methemoglobin rises.

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