Methemoglobinemia

Differential Diagnosis

• Antibiotics
  • Dapsone (most common)
  • Trimethoprim
  • Sulfonamides
• Local anesthetics
  • Articaine
  • Benzocaine
  • Prilocaine
• Aniline dyes
• Metoclopramide
• Rasburicase
• Chlorates
• Bromates
• Ingestion of nitrate compounds, e.g. bismuth nitrate

Clinical Presentation

• Cyanosis and symptoms of hypoxia
• Hypoxia that does not respond to supplemental oxygen
• Clinical cyanosis in the presence of a normal arterial pO2

Management

• Stop offending agent
• If otherwise well and metHb level < 20%, can monitor
• If unwell or metHb > 30%
  • Methylene blue 1-2 mg/kg over 2-5 minutes
    • Repeat dose in an hour if metHb still high (e.g. >20%)
    • Contraindicated in G6PD deficiency
    • May cause serotonin syndrome
  • Second-line
    • Ascorbic acid
    • Exchange transfusion