Sarah McCrea discusses the symptoms and signs of methaemoglobinaemia, a relatively rare condition which can result in a dramatic cyanosis

Methaemoglobin is a form of altered haemoglobin, which can be acquired or congenital, impairs oxygen transport, and can usually be converted to normal haemoglobin by the administration of methylene blue. Oxygen is vital for all bodily activities, and although some is dissolved in the blood plasma the ‘useful’ oxygen is carried by haemoglobin, which is a constituent of red blood cells. Haemoglobin is made up of four protein chains, each chain has a haem molecule buried within it, and each haem molecule has an atom of iron at its centre (Price 1998). Haemoglobin can only transport oxygen when this iron is in its reduced, ferrous, state. Methaemoglobin is haemoglobin in an oxidized form, it is incapable of oxygen transport. Also, if the ferric iron in methaemoglobin is not quickly reduced to its original (ferrous) state, the methaemoglobin molecules become denatured and precipitate within the red blood cell, causing haemolytic anaemia (Sullivan 2001). The presence of an oxidized haemoglobin monomer group will interfere with the oxygen carrying behaviour of others, increasing their oxygen affinity and making it more difficult for them to unload their oxygen to the tissues (Park and Nagel 1984).