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Methaemoglobinaemia Congenital

HbM a or b globin mutation in vicinity of Fe

Fe becomes stabilised in Fe3+ form Heterozygote has 25% HbM MetHb reductase def. Due to deficiency of NADH-cytochrome b5 reductase. Autosomal recessive inheritance; symptoms mainly in homozygote Clinical features Cyanosis from infancy. PaO2 is normal

General health is good

Acquired Occurs when RBCs are exposed to oxidising agents, producing HbM. Implicated agents include: phenacetin, local anaesthetics (e.g. lignocaine), inorganic nitrates (NO2). Patients may experience severe tissue hypoxia. HbM binds O2 tightly and fails to release to tissues

►► HbM >60% requires urgent medical attention.

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