Methemoglobinemia

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 - brown colored blood (off note: brown sputum is found in ABPA)
• dyspnoea, anxiety, headache
• severe: acidosis, arrhythmias, seizures, coma
• normal pO2 but decreased oxygen saturation

Management

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

Cyanosis can be seen when you have greater than 5 grams per cent of deoxyhemaglobinemia, or 1.5 grams per cent of methemaglobinemia, or 0.5 grams per cent of sulphemaglobinemia, all serious medical abnormalities.