Methaemo- globinaemia is an unusual but readily treatable cause of unexpected cyanosis and collapse

Oxidation of the iron in haemoglobin from the ferrous to the ferric state yields methaemoglobin, which does not carry oxygen and imparts a chocolate hue to the blood. The diagnosis should be suspected when a blood sample is brown coloured and does not redden on exposure to air. Blood concentrations of methaemoglobin do not normally exceed 2%. The aetiology of methaemoglobinaemia may be congenital due to deficiency in cytochrome b₅ reductase and to structural abnormalities in the haemoglobin molecule (haemoglobin M disorders) or be acquired as the result of drug treatment or the presence of toxins—most commonly nitrites or nitrates. For example, methaemoglobinaemia is not uncommon in patients receiving inhaled nitric oxide.1 Concentrations of methaemoglobin up to 20% are generally well tolerated, while values above 40% are associated with cardiorespiratory symptoms. Life threatening methaemoglobinaemia is rare and is usually the result of acute poisoning by drugs or toxins.2

We report life threatening methaemoglobinaemia associated with sodium nitrite in three previously healthy siblings, 4 year old twin boys (cases 1 and 2) and their sister aged 2 years (case 3).