A 25-year-old presents 18 hours after ingesting methanol. Arterial blood gas shows severe metabolic acidosis with high anion gap. The earliest specific vision-related symptom expected in methanol poisoning is:
- A Total painless bilateral blindness
- B Blurring of vision and appearance of 'snow-field' (seeing snowflakes) ✓
- C Monocular vision loss with painful eye movement
- D Hemianopia with preserved central vision
Explanation
Methanol is oxidised by alcohol dehydrogenase to formaldehyde and then to formic acid, which selectively poisons the cytochrome oxidase system in the optic nerve. The early ophthalmological symptom is blurred vision described as looking through a 'snow field' or 'ground glass', representing bilateral toxic optic neuropathy. If untreated, permanent total blindness ensues. The latent period of 12–24 hours accounts for time needed for metabolite accumulation. Treatment includes fomepizole or ethanol to competitively inhibit alcohol dehydrogenase, along with haemodialysis to remove formic acid.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.