Regarding the clinical and biochemical features of ethanol and methanol poisoning, all of the following statements are correct EXCEPT:

Explanation

## Comparative Toxicology of Ethanol vs. Methanol ### Correct Answer Identification **Key Point:** Fomepizole is NOT contraindicated in ethanol poisoning. In fact, fomepizole is occasionally used in severe ethanol poisoning (though less commonly than in methanol poisoning) because it prevents further toxic metabolite formation. The statement that fomepizole "may potentiate CNS depression" is false — fomepizole has no CNS effects and does NOT worsen respiratory depression. ### Pathophysiology Comparison Table | Feature | Ethanol | Methanol | |---------|---------|----------| | **Toxic metabolite** | Acetaldehyde → Acetic acid | Formaldehyde → Formic acid | | **Acidosis mechanism** | Lactate ↑, Ketoacids ↑ | Formic acid ↑ | | **Osmolar gap** | Yes (early) | Yes (early) | | **Visual symptoms** | NO | YES (optic nerve damage) | | **Blindness** | Does not occur | Occurs if untreated | | **CNS depression** | Severe | Moderate (unless severe acidosis) | | **Hypoglycemia risk** | High | Low | | **Lethal dose (oral)** | ~300 mL (pure) | ~30 mL (pure) | **High-Yield:** Methanol is ~10× more toxic than ethanol on a per-mL basis. ### Metabolic Acidosis in Alcohol Poisoning **Mnemonic: ETHANOL ACIDOSIS = Ethanol → Acetaldehyde → Acetic acid + Lactate (from altered NAD/NADH) + Ketoacids (from lipolysis)** **Mnemonic: METHANOL ACIDOSIS = Methanol → Formaldehyde → Formic acid (the primary culprit)** ### Osmolar Gap in Both Poisonings 1. **Early phase** (first 12 hours): High osmolar gap due to unmeasured parent alcohol 2. **Late phase** (after 12+ hours): Osmolar gap normalizes as parent alcohol is metabolized 3. **Anion gap metabolic acidosis** develops as toxic metabolites accumulate **Clinical Pearl:** In methanol poisoning, the transition from osmolar gap to anion gap acidosis is a critical prognostic marker — it indicates conversion to formic acid and risk of blindness. ### Fomepizole in Ethanol Poisoning **Warning:** The statement that fomepizole is "contraindicated" in ethanol poisoning is FALSE. While fomepizole is primarily indicated for methanol and ethylene glycol poisoning, it is NOT contraindicated in ethanol poisoning. In fact: - Fomepizole has **no CNS depressant effects** (unlike ethanol) - It does NOT potentiate respiratory depression - It may be used in severe ethanol poisoning with concurrent methanol or ethylene glycol exposure - The main reason it is not routinely used for isolated ethanol poisoning is **cost** and the fact that supportive care (airway management, glucose, thiamine) is usually sufficient ### Half-Life Comparison | Alcohol | Normal Half-Life | With ADH Inhibitor (Fomepizole/Ethanol) | |---------|------------------|------------------------------------------| | **Ethanol** | 4–6 hours (zero-order kinetics) | ~17 hours (first-order kinetics) | | **Methanol** | 24–30 hours (zero-order kinetics) | ~24–48 hours (first-order kinetics) | **Key Point:** When alcohol dehydrogenase is inhibited, both ethanol and methanol shift from zero-order to first-order kinetics, and their half-lives INCREASE. Methanol's half-life becomes even longer because it is a poor substrate for ADH to begin with.