Sally Mandour: Methemoglobinemia When Hemoglobin Cannot Carry Oxygen
Sally Mandour, Associate Professor, Head of Hematology Unit at Clinical Pathology Department of National Liver Institute, Monofeya University, shared a post on LinkedIn:
“When haemoglobin can’t carry oxygen!
Methemoglobin is haemoglobin with its iron oxidised to the ferric (Fe³plus) state and in that form, it simply cannot bind oxygen.
Normally, this it is kept below 1 percent of total haemoglobin owing to the NADH dependent methemoglobin reductase system, which reduces methemoglobin back to its functional form at roughly 250 times the rate it’s normally produced.
Methemoglobinaemia is diagnosed once methemoglobin exceeds 1percent and it happens when this balance breaks down.
Clinical types:
Acquired the most common cause.
Drugs or toxic substances oxidise haemoglobin faster than the reductase system can keep up, overwhelming an otherwise normal enzyme pathway.
Congenital inherited in two very different ways:
Recessive NADH diaphorase (methemoglobin reductase) deficiency.
The enzyme system itself is defective, so the small amount of methemoglobin formed daily simply can’t be reduced and gradually accumulates.
Dominant caused by Haemoglobin M (HbM), a structural variant.
Of the nine described HbM variants, all occur only in the heterozygous state.
Most arise from a tyrosine substitution for the proximal or distal histidine in the haem pocket tyrosine forms a covalent bond with the haem iron, locking it into the oxidised, ferric state and making it resistant to reduction.
Cyanosis that doesn’t improve with supplemental oxygen is a red flag chocolate brown drawn blood sample rather than bright red also a sign, even on exposure to air.
This happens because methemoglobin simply can’t bind oxygen, so the blood never regains that characteristic red colour.”
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