Methanol toxicity

Summary

• AGMA with osmolar gap
• Formic acid is the toxic metabolite

Pathophysiology

graph TD
methanol["Methanol"] --> |Alcohol dehydrogenase|formaldehyde
formaldehyde["Formaldehyde"] --> |Aldehyde dehydrogenase|formicacid["Formic acid"]
formicacid --> |Folate|co2h2o["CO2 and H2O"]

Sources

• Wood alcohol
• Moonshine
• Fuels
• Cleaners
• Antifreeze
• Windshield fluid

Clinical Manifestations

• Decreased LOC
• Retinal injury
  • Blindness
  • Afferent pupillary defect
  • Mydriasis
  • Retinal sheen
  • Optic disc hyperemia
• Coma, seizures, hyperpnea, and hypotension are late findings

Investigations

• Elevated anion gap metabolic acidosis with an osmolar gap
  • As metabolism progresses the initial osmolar gap is replaced by an anion gap
• Also get acetaminophen and salicylate levels
• Follow renal function, including electrolytes and creatinine

Management

• ABCs
• NG aspiration if within 60 minutes
• Sodium bicarb if pH <7.3, with infusion to target pH ≥ 7.35
• Folinic acid (or folic acid), to help metabolise formic acid
• Fomepizole or ethanol, which competitively inhibit alcohol dehydrogenase
  • Serum methanol >6.2 mmol/L, or
  • Documented toxic ingestion and an osmolar gap > 10, or
  • Suspected toxic ingestion and 2 of:
    • pH < 7.3
    • Bicarb < 20
    • Gap (which one?) > 10
• Hemodialysis if ongoing acidosis and evidence of end-organ damage
  • Visual loss is an indication, as well