Methemoglobinemia in a renal transplant patient: case report.

cyanosis develops in the absence of any car. diac or respiratory abnormalities.1 Cyanosis is a result of the accumulation of methemoglobin, which imparts a bluish color to the blood. This condition has two forms: a hereditary and an acquired form. Hereditary methemoglobinemia is an extremely rare inborn error of metabolism resulting in the accumulation of methemoglobin. 2 The acquired form results from exposure to various environmental toxins or administration of certain medications. Cohen and Bovasso3 include aniline derivatives (found in crayons, inks, shoe polish, dyes, and drugs), benzene derivatives, nitrates, certain local anesthetics, and sulfonamides as compounds known to produce methemoglobinemia. Exposure may be through transdermal absorption, inhalation, ingestion, or parental administration. Hemoglobin normally contains iron in its ferrous (Fe÷3) or reduced state. Methemoglobin contains the oxidized form, ferric (Fe÷2), significantly altering the tertiary structure of the heme moiety making it incapable of releasing oxygen. The ferrous cation is inherently unstable and, in the normal biological system, oxidizes to the ferric form.4 Methemoglobin reductase, normally present in erythrocytes, is absent or decreased in hereditary methemoglobinemia. This enzyme system ensures that 99% of the hemoglobin is maintained in the more functional ferrous state. An absence or decreased level of the enzyme alters homeostasis to favor accumulation of the ferric cation. Agents that cause acquired methemoglobinemia may increase production of methemoglobin or may inhibit the methemoglobin reductase system or both, resulting in a preponderance of ferric iron and thus the nonfunctional methemoglobin. Blood Level (g/dl) Signs and Symptoms