Methaemoglobinaemia

Methaemoglobinaemia occurs when red blood cells contain methaemoglobin at levels higher than 1%. Methaemoglobinaemia results from the presence of iron in the ferric form instead of the usual ferrous form. The ferric form is unable to bind oxygen, and its presence results in a decreased availability of oxygen to the tissues. Methaemoglobinaemia can be congenital or acquired.

Causes

Methaemoglobinaemia can be congenital or acquired.

Congenital causes of methaemoglobinaemia include:

Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Pyruvate kinase deficiency
Cytochrome b5 oxidase deficiency
NADH methaemoglobin reductase deficiency

Acquired methaemoglobinaemia is usually due to exposure to a drug or oxidising agent. These include:

Sodium nitroprusside
Nitroglycerin
Amyl nitrate
Local anaesthetics, e.g. benzocaine, prilocaine
Antimalarials, e.g. primaquine, chloroquine
Cyclophosphamide
Paracetamol
Dapsone
Sulfonamide antibiotics

Clinical features

The clinical features of methaemoglobinaemia are proportional to the methaemoglobin level:

<15%: Skin (grey-blue cyanosis) and blood (chocolate-brown) blood colour changes only
15-30%: Anxiety, headache, weakness, tachycardia, dizziness
30-70%: Myocardial ischaemia, arrhythmias, seizures, coma
>70%: Usually fatal

Pulse oximetry

Pulse oximetry is usually falsely elevated in the early stages and cannot be relied upon. Once levels reach 30%, the oxygen saturations generally fall to around 80-85%, which is due to the combined light absorption of both oxyhaemoglobin and deoxyhaemoglobin. The oxygen saturations will not be responsive to supplemental oxygen.

Diagnosis

All patients should have a serum methaemoglobin level taken to confirm the diagnosis and guide management.

Management

Patients should be treated if they are symptomatic or if they are asymptomatic with methaemoglobin levels >25%. Management of methaemoglobinaemia typically involves: