methemoglobinemia

Introduction

A condition characterized by blood methemoglobin levels of > 1-1.5%.

Etiology

• hereditary
  • defects in activity of NADH cytochrome-B5 reductase -type 4 associated with defects in CYB5A
  • hemoglobinopathy predisposing to hemoglobin oxidation
• acquired: oxidation of hemoglobin by drugs or toxins
  • nitrates
  • nitrites
  • chlorates
  • quinones
  • dapsone
  • acetanilid
  • phenacetin
  • sulfonamides
  • aniline dyes
  • large doses of ferrous sulfate

Pathology

• methemoglobinemia can compromise oxygen delivery to tissues

Clinical manifestations

• cyanosis when methemoglobin levels exceed 1.5 g/dL (> 10% of total hemoglobin, normal is < 1.5%)
  • degree of cyanosis comparable to hemoglobin < 5 g/dL
• at blood methemoglobin levels > 15%, neurologic & cardiac symptoms develop due to hypoxia
• blood methemoglobin levels > 70% are generally fatal

Laboratory

• blood methemoglobin is quantified spectrophotometrically
• arterial blood gas (ABG), pulse oximetry
  • normal paO2
  • low SaO2
• complete blood count
• reticulocyte count
• serum electrolytes
• liver function tests
• renal function tests
• hemoglobin electrophoresis
• cyanide in serum/plasma, cyanide in blood

Radiology

• ref suggests head, chest & cardiac imaging studies

Management

• discontinue offending medications
• supplemental oxygen, IV hydration
• methylene blue 2 mg/kg IV for severe toxic methemoglobinemia
• exchange transfusion
• hyperbaric oxygen

More general terms

• sign/symptom

More specific terms

• hereditary methemoglobinemia

Additional terms

• methemoglobin (MetHb, Hi)
• methemoglobin in blood