Which of the following is the characteristic ocular manifestation of methanol poisoning that distinguishes it from ethanol toxicity?

Explanation

## Ocular Manifestations of Methanol Poisoning **Key Point:** The **hallmark ocular finding in methanol poisoning is optic neuritis leading to blindness**, caused by **formic acid (formate)** damaging the optic nerve through demyelination and mitochondrial dysfunction. ### Mechanism of Optic Nerve Damage 1. Formic acid crosses the blood-brain barrier 2. Accumulates in the **optic nerve** (high metabolic demand, rich in mitochondria) 3. Inhibits **cytochrome c oxidase** in mitochondrial complex IV 4. Causes energy depletion, demyelination, and axonal loss 5. Results in **optic neuritis** → vision loss → permanent blindness **Clinical Pearl:** Blindness in methanol poisoning may be **reversible if treated early** with ethanol or fomepizole to prevent formic acid formation, but becomes permanent if formic acid accumulates. ### Differential: Methanol vs. Ethanol Toxicity | Feature | Methanol | Ethanol | |---------|----------|----------| | **Ocular findings** | Optic neuritis, blindness | None (except from CNS depression) | | **Metabolic acidosis** | Severe (formic acid) | Mild or absent | | **Lethal dose** | 30–100 mL | >300 mL | | **Specific antidote** | Ethanol, fomepizole | Supportive care only | **High-Yield:** **Formic acid, not formaldehyde**, is the primary culprit for optic nerve toxicity. Formaldehyde causes systemic toxicity but formate specifically targets the optic nerve. **Mnemonic:** **BLIND from FORMATE** — **B**lindness is **L**ess likely with **I**ncreased **N**APD (relative afferent pupillary defect) **D**ue to **FOR**mic acid **ATE** the optic nerve. [cite:Forensic Medicine & Toxicology Ch Methanol Poisoning]