Methaemoglobinaemia

Methaemoglobinaemia

Methaemoglobinaemia describes haemoglobin which has been oxidised from Fe2+ to Fe3+.

Methaemoglobinaemia = oxidation of Fe2+ in haemoglobin to Fe3+

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Importance: 87

This is normally regulated by NADH methaemoglobin reductase, which transfers electrons from NADH to methaemoglobin resulting in the reduction of methaemoglobin to haemoglobin. There is tissue hypoxia as Fe3+ cannot bind oxygen, and hence the oxidation dissociation curve is moved to the left

Congenital causes

• haemoglobin chain variants: HbM, HbH
• NADH methaemoglobin reductase deficiency

Acquired causes

• drugs: sulphonamides, nitrates (including recreational nitrates e.g. amyl nitrite 'poppers')
  nitrates (including recreational nitrates e.g. amyl nitrite 'poppers')
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  , dapsone
  Dapsone + dyspnoea --> methaemoglobinaemia
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  Importance: 87
  , sodium nitroprusside, primaquine
• chemicals: aniline dyes

Features

• 'chocolate' cyanosis
• dyspnoea, anxiety, headache
• severe: acidosis, arrhythmias, seizures, coma
• normal pO2 but decreased oxygen saturation
  Normal pO2 but decreased oxygen saturation is characteristic of methaemoglobinaemia
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  Importance: 60

Management

• NADH methaemoglobinaemia reductase deficiency: ascorbic acid
  ascorbic acid
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• IV methylthioninium chloride (methylene blue
  methylene blue
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  ) if acquired

+PassMedicine Notes

Methaemoglobinaemia describes haemoglobin which has been oxidised from Fe2+ to Fe3+. This is normally regulated by NADH methaemoglobin reductase, which transfers electrons from NADH to methaemoglobin resulting in the reduction of methaemoglobin to haemoglobin. There is tissue hypoxia as Fe3+ cannot bind oxygen, and hence the oxidation dissociation curve is moved to the left

Congenital causes

• haemoglobin chain variants: HbM, HbH
• NADH methaemoglobin reductase deficiency

Acquired causes

• drugs: sulphonamides, nitrates (including recreational nitrates e.g. amyl nitrite 'poppers'), dapsone, sodium nitroprusside, primaquine
• chemicals: aniline dyes

Features

• 'chocolate' cyanosis
• dyspnoea, anxiety, headache
• severe: acidosis, arrhythmias, seizures, coma
• normal pO2 but decreased oxygen saturation

Management

• NADH methaemoglobinaemia reductase deficiency: ascorbic acid
• IV methylthioninium chloride (methylene blue) if acquired