A 32-year-old male labourer in Delhi is brought to the emergency department with altered mental status, severe metabolic acidosis (pH 7.18, HCO₃⁻ 8 mEq/L), and an anion gap of 22. He consumed approximately 100 mL of a clear liquid from an unlabeled bottle 6 hours ago, thinking it was ethanol. On examination, he has visual blurring, photophobia, and fundoscopy reveals optic disc hyperaemia. His serum osmolality is elevated at 320 mOsm/kg. What is the most likely toxic agent he ingested?

Explanation

## Clinical Diagnosis: Methanol Poisoning ### Key Presentation Features **Key Point:** Methanol poisoning presents with a **latent phase (6–24 hours)** followed by a **metabolic acidosis phase** with characteristic visual symptoms and CNS involvement. The clinical triad in this case is pathognomonic: 1. **Severe metabolic acidosis** (pH 7.18, high anion gap 22) 2. **Visual symptoms** (blurring, photophobia, optic disc hyperaemia) 3. **Elevated serum osmolality** (320 mOsm/kg) ### Pathophysiology of Methanol Toxicity Methanol is metabolized via alcohol dehydrogenase (ADH) to formaldehyde, then to formic acid: $$\text{CH}_3\text{OH} \xrightarrow{\text{ADH}} \text{HCHO} \xrightarrow{\text{aldehyde dehydrogenase}} \text{HCOOH (formic acid)}$$ Formic acid causes: - **Severe high-anion-gap metabolic acidosis** - **Optic nerve toxicity** → visual loss, optic atrophy (pathognomonic) - **CNS depression** → altered mental status, coma - **Osmotic effect** → elevated serum osmolality ### Differential Features: Methanol vs. Ethylene Glycol | Feature | Methanol | Ethylene Glycol | |---------|----------|------------------| | **Latent period** | 6–24 hours | 1–12 hours | | **Visual symptoms** | Prominent (photophobia, blindness) | Absent | | **Optic nerve involvement** | Yes (hyperaemia, atrophy) | No | | **Metabolite** | Formic acid | Glycolic acid | | **Urinary findings** | Normal | Calcium oxalate crystals | | **Renal involvement** | Mild | Severe (acute kidney injury) | | **Cardiac involvement** | Rare | Myocarditis, arrhythmias | **High-Yield:** The **presence of visual symptoms + optic disc findings + metabolic acidosis** is virtually diagnostic of **methanol poisoning**. ### Management Principles 1. **Fomepizole** (4-methylpyrazole) — inhibits ADH, prevents formation of toxic metabolites - Loading dose: 15 mg/kg IV - Maintenance: 10 mg/kg IV every 12 hours 2. **Ethanol** (if fomepizole unavailable) — competitive substrate for ADH 3. **Sodium bicarbonate** — correct metabolic acidosis 4. **Hemodialysis** — remove methanol and formic acid in severe cases 5. **Supportive care** — mechanical ventilation, ICU monitoring **Clinical Pearl:** Fomepizole is now the preferred antidote over ethanol because it has fewer side effects and more predictable kinetics. ### Why Optic Nerve is Vulnerable Formic acid accumulates in the optic nerve and retina, causing: - Mitochondrial dysfunction - Selective toxicity to retinal ganglion cells - Irreversible blindness if treatment is delayed **Warning:** Delayed diagnosis and treatment can result in permanent blindness, even if the patient survives the acute acidosis.