Clinically evident cyanosis is often due to increased levels of reduced hemoglobin in the blood caused by cardiac or pulmonary diseases. Recently a 23 year old woman presented with cyanosis of the tongue and finger tips. She was not anemic or icteric. She did not have any cardiac or pulmonary disease. Her capillary oxygen saturation was 93 percent. When a few drops of her blood (dark brown in colour) were spilled on a white filter paper, no change in colour was noted after adequate exposure to the atmosphere.
This provoked a discussion on the causes of cyanosis without hypoxia.
We discussed methaemoglobinemia as a possible cause. Methaemoglobinemia is a condition where the hemoglobin molecule has a ferric iron instead of the normal ferrous iron. This condition can occur following exposure to drugs like nitrites, sulphonamides and some chemical dyes. When iron is in the ferric state, hemoglobin does not easily release its oxygen to tissues. Methaemoglobinemia can also be due to inherited disorders: deficiency of the enzyme within red cells that normally converts iron from a ferric state to a ferrous state - an enzyme called methaemoglobin reductase - can result in methaemoglobinemia. Another reason is the presence of an abnormal hemoglobin called Hemoglobin M.
Our patient however did not have methaemoglobin reductase deficiency. Her hemoglobin electrophoresis showed the presence of the hemoglobin variant called Hemoglobin M. And that explained why she had cyanosis without hypoxia.
