A 35-year-old man is brought to the emergency department with altered mental status, metabolic acidosis (pH 7.18, HCO₃⁻ 12 mEq/L), and visual disturbances. A 28-year-old woman presents with similar altered mental status and metabolic acidosis but without visual symptoms; she has a normal fundoscopic examination. Which single feature best distinguishes methanol poisoning from ethanol poisoning in this comparison?

Explanation

## Distinguishing Methanol from Ethanol Poisoning ### Clinical Presentation Comparison | Feature | Methanol | Ethanol | |---------|----------|--------| | **Visual symptoms** | Pathognomonic (blurred vision, photophobia, blindness) | Absent | | **Optic nerve involvement** | Optic disc hyperemia, papillitis, optic atrophy | Not involved | | **Metabolic acidosis** | Severe, high anion gap | Variable, may be absent | | **CNS depression** | Present (altered mental status) | Present (altered mental status) | | **Osmolal gap** | Elevated early | Elevated early | | **Latent period** | 12–24 hours (delayed toxicity) | Immediate | ### Key Point: **Optic nerve involvement (hyperemia, visual disturbances, potential blindness) is the pathognomonic and best discriminating feature of methanol poisoning.** Ethanol does not cause direct optic nerve toxicity. ### High-Yield: Methanol's toxic metabolite **formic acid** preferentially accumulates in the optic nerve and retina, causing: - Optic disc hyperemia and swelling - Retinal edema - Visual blurring, photophobia - Permanent blindness if untreated Ethanol lacks this metabolite and does not produce optic nerve damage. ### Clinical Pearl: A patient with metabolic acidosis + visual symptoms = **suspect methanol poisoning** until proven otherwise. Fundoscopy is a critical bedside discriminator. ### Mnemonic: **METHANOL = Metabolic acidosis + Ethers (formic acid) + Toxicity + Hyperemia of optic nerve + Altered vision + Neurological deficit + Osmolal gap + Loss of sight** [cite:Park 26e Ch 25]