What is Methaemoglobinaemia?

condition of elevated methemoglobin (Fe3+) in the blood which is unable to bind to oxygen

What are the main clinical features of Methaemoglobinaemia?

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

What is the management of Methaemoglobinaemia?

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

What is the pathophysiology of Methaemoglobinaemia?

Methemoglobin 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

What are the Congenital causes of Methaemoglobinaemia?

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

What are the Acquired causes of Methaemoglobinaemia?

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