Published: August 2009

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Methaemoglobinaemia - signs and risk factors

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Prescriber Update 30(3): 20
August 2009

Prescribers are reminded that a number of medicines have been associated with the development of methaemoglobinaemia.

Methaemoglobin is formed when the iron in haemoglobin is oxidised to its ferric state. This reduces the oxygen carrying capacity of haemoglobin. Methaemoglobin is normally present in the blood at concentrations below 2%; symptoms are only likely to develop when methaemoglobin levels exceed 20% to 30%.

Signs and symptoms of methaemoglobinaemia include headache, fatigue, cyanosis, tachypnoea, dyspnoea, tachycardia, altered levels of consciousness, myocardial infarction and diffuse hypoxic brain injury. Severe cases have resulted in death.

In confirming the diagnosis of methaemoglobinaemia, pulse oximetry is not considered a reliable measurement¹. Diagnosis of methaemoglobinaemia requires co-oximetry testing on arterial blood samples.

Risk factors for developing methaemoglobinaemia include:²

• Topical or injectable administration of local anaesthetics such as prilocaine, benzocaine, lidocaine (lignocaine) and tetracaine (amethocaine).
• Use of amyl nitrite (and other nitrites and nitrates), chloroquine, primaquine, dapsone or sulphonamides.
• Age – children under three months of age have a higher risk due to being more susceptible to oxidant stress.
• Systemic infection.
• Anaemia.
• Presence of congenital methaemoglobinaemia or G6PD deficiency.

References

1. Sharma V, Haber A, (2002) Acquired methaemoglobinaemia: a case report of benzocaine-induced methaemoglobinaemia and a review of the literature, Clinical Pulmonary Medicine, 9(1): 53 – 8.
2. Kane G, Hoehn S, Behrenbeck T et al (2007) Benzocaine-induced methaemoglobinaemia based on the Mayo Clinic experience from 28,478 transoesophageal echocardiograms, Archives of Internal Medicine, 167(18): 1977 – 82.