A 28-year-old male presents to the emergency department with a 6-hour history of severe abdominal pain, vomiting, and blurred vision after consuming a bottle of homemade liquor at a local gathering in rural Maharashtra. On examination, he is confused, has a respiratory rate of 28/min with deep breathing, and his breath smells fruity. Arterial blood gas shows pH 7.22, HCO₃⁻ 12 mEq/L, and PaCO₂ 28 mmHg. Serum osmolality is 320 mOsm/kg (normal 280–295). What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The triad of visual disturbances (blurred vision), severe metabolic acidosis with high anion gap, and elevated osmolal gap in the context of homemade liquor consumption is pathognomonic for methanol poisoning. ### Why Methanol? Methanol is a common adulterant in illicit spirits, particularly in rural India. It is metabolized by alcohol dehydrogenase to formaldehyde and then formic acid, which causes: 1. **High anion gap metabolic acidosis** — formic acid accumulation 2. **Elevated osmolal gap** — unmetabolized methanol in serum 3. **Visual toxicity** — formic acid damages the optic nerve, causing blurred vision, scotomata, and potential blindness 4. **CNS depression** — confusion, altered sensorium 5. **Fruity breath** — due to ketones from metabolic acidosis ### Osmolal Gap Calculation $$\text{Osmolal gap} = \text{Measured osmolality} - \text{Calculated osmolality}$$ $$\text{Calculated osmolality} = 2[Na^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8} + \frac{\text{Ethanol}}{4.6}$$ In this case, osmolal gap ≈ 25 mOsm/kg (abnormal; normal <10), indicating presence of unmeasured osmoles (methanol). ### Metabolic Consequences | Feature | Methanol | Ethanol | Isopropanol | Ethylene Glycol | |---------|----------|---------|-------------|------------------| | **Visual toxicity** | Yes (optic nerve damage) | No | No | No | | **Metabolite** | Formic acid | Acetaldehyde | Acetone | Oxalic acid | | **Acidosis** | High anion gap | Lactic (if ketoacidosis) | Minimal | High anion gap | | **Fruity breath** | No (but acidosis) | Yes | Yes | Yes | | **Renal crystals** | No | No | No | Yes (calcium oxalate) | **High-Yield:** Methanol poisoning = **visual symptoms + high anion gap acidosis + elevated osmolal gap**. The visual disturbances are the clinical hallmark that distinguish methanol from other toxic alcohols. ### Management Principles 1. **Fomepizole** (4-methylpyrazole) — inhibits alcohol dehydrogenase, preventing conversion to formic acid 2. **Ethanol** — alternative if fomepizole unavailable (competes for ADH) 3. **Sodium bicarbonate** — alkalinization enhances formic acid excretion 4. **Hemodialysis** — removes methanol and formic acid; indicated if severe acidosis or visual symptoms 5. **Folate/folinic acid** — enhances formic acid metabolism **Clinical Pearl:** In rural India, always suspect methanol in cases of "alcohol poisoning" with visual symptoms and unexplained high anion gap acidosis. Homemade spirits are the commonest source. [cite:Forensic Medicine & Toxicology, Reddy 33e Ch 22]