Methanol-Induced Toxic Optic Neuropathy (TON), formic acid (formate) toxicity, blindness

👁 Methanol-Induced Toxic Optic Neuropathy (TON)

Methanol is an industrial solvent used in windshield washer fluids, fuels, and antifreeze.
Even small amounts can lead to severe visual loss or permanent blindness.

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🧪 Pathophysiology

• Methanol is metabolized in the liver to formic acid (formate)
• Formate accumulates in the optic nerve and retina
• It causes mitochondrial dysfunction, leading to:
  • Axonal degeneration
  • Optic nerve swelling
  • Eventual optic atrophy

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🧠 Symptoms & Course

1. Early systemic signs:
   • Nausea, vomiting, abdominal pain
   • Visual scintillations (flashing lights)
2. Ophthalmic features:
   • Visual acuity decline, red-green color vision loss
   • Central/cecocentral scotomas, bitemporal hemianopia
3. Fundus findings:
   • Optic disc swelling & hyperemia early
   • Later: optic disc pallor, retinal arterial narrowing
   • Progression to optic atrophy → legal blindness

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📋 Diagnostic Tests

• OCT: Thinning of retinal nerve fiber layer
• FA (Fluorescein Angiography): Loss of optic disc vasculature
• VEP: Delayed P100 latency
• Color vision tests: Red-green dyschromatopsia

 

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⚠ Treatment & Prognosis

InterventionPurpose

Ethanol/Fomepizole	Inhibit methanol metabolism
Sodium Bicarbonate	Correct metabolic acidosis
Hemodialysis	Remove methanol and formate
High-dose IV steroids	Partial vision recovery in some cases
❌ No established treatment for optic neuropathy

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✅ Summary

• Methanol toxicity is a vision-threatening emergency
• Formic acid accumulation causes optic nerve damage
• Early diagnosis and metabolic treatment are key
• Ophthalmic recovery is often incomplete or poor