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Methaemoglobinaemia may be congenital or acquired. The iron in the haem is in the inactive (ferric) Fe3+ form instead of the normal (ferrous) Fe2+ form. Approximately 3% of haemoglobin is methaemoglobin in the healthy individual.

A sign of methaemoglobinaemia is the chocolate brown colour of blood.

The congenital form is an autosomal recessive condition caused by deficiency of cytochrome b5 reductase which converts methaemoglobin back to normal haemoglobin. Clinically, it is characterized by cyanosis.

Acquired methaemoglobinaemia occurs when haemoglobin is reduced to methaemoglobin as may occur with large doses of prilocaine or application of silver sulphadiazine.

For all forms, methylene blue administered intravenously in a dose of 1mg/kg is an effective treatment.

Note that in normal oxygenation of haemoglobin, Fe2+ is not oxidised to Fe3+.

Last reviewed 01/2018