Methemoglobinemia - two case histories

In health no more than 1-2 % of total hemoglobin is present as methemoglobin. This is a dysfunctional form of hemoglobin that is unable to bind and transport oxygen because the iron atom of the heme moiety is in the ferrous rather then the normal ferric state. Abnormal increase in methemoglobin, a condition called methemoglobinemia, compromises oxygen delivery and thereby causes tissue hypoxia. 

The condition may be inherited but much more commonly is acquired as a result of ingestion of oxidant chemicals, including a range of therapeutic drugs. Two case histories of acquired methemoglobinemia were recently reported, one the result of deliberate ingestion of an aniline-containing fluid used in the print industry and the other due to a prescribed drug, dapsone. 

Together they provide a wealth of information relating to the pathogenesis, varied clinical presentation, diagnosis and treatment of methemoglobinemia. Of particular interest is the significance of results derived from arterial blood gas analysis, pulse oximetry and CO-oximetry for assessment and diagnosis. In the second case, failure to respond to the standard methylene blue treatment led to an additional diagnosis: glucose-6-phosphate dehydrogenase deficiency.