What are the phenotypic varieties of hemoglobin M (Hb M) in methemoglobinemia?

Updated: Dec 09, 2018
  • Author: Mary Denshaw-Burke, MD, FACP; Chief Editor: Emmanuel C Besa, MD  more...
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The inheritance pattern for Hb M variants is autosomal dominant, whereas that for methemoglobinemia due to cytochrome b5 reductase deficiency is autosomal recessive. Patients with Hb M appear cyanotic but are otherwise generally asymptomatic. There are three phenotypic varieties of Hb M, corresponding to the globulin gene affected (alpha, beta, or gamma) as follows [6, 9, 10] :

  • Alpha-chain variants cause neonatal cyanosis that is persistent

  • Beta-chain variants do not cause cyanosis until several months after birth, when the level of fetal hemoglobin has decreased

  • Gamma-chain variants cause transient neonatal cyanosis that resolves once the level of fetal hemoglobin decreases

Another hemoglobin variant, hemoglobin E (Hb E), is associated with methemoglobinemia as well. In a study from the National Thalassemia Center in Sri Lanka, 45 patients who had a diagnosis of Hb E beta-thalassemia were found to have significantly higher median methemoglobin levels that normal control subjects and patients with other hemoglobinopathies (2.7% vs 0.3%.) Furthermore, methemoglobin levels were significantly elevated in patients who had undergone a previous splenectomy. [11]

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