Methanol and Ethylene Glycol Poisoning
On this page
Direct answer
Methanol and ethylene glycol poisoning cause a high anion gap metabolic acidosis with an osmolar gap, because their toxic metabolites, formic acid and glycolic acid, accumulate. Methanol mainly injures the optic nerve and retina, while ethylene glycol damages the kidneys through calcium oxalate crystals. Fomepizole is the preferred antidote, and haemodialysis removes both the parent alcohol and its metabolites when acidosis or organ failure is severe.
What you must remember
- Osmolar gap: A raised gap between measured and calculated serum osmolality suggests a toxic alcohol, but a normal gap does not exclude poisoning, especially late.
- Methanol eyes: Formic acid causes optic neuritis with blurred vision and, in severe cases, blindness, sometimes described as snowfield vision.
- Ethylene glycol kidneys: Glycolic acid and calcium oxalate crystals cause acute kidney injury, with crystals in urine supporting the diagnosis.
- Fomepizole: Inhibits alcohol dehydrogenase, given as a loading dose of about 15 mg/kg intravenously followed by maintenance doses; ethanol is an alternative where it is unavailable.
- Cofactors: Folinic acid aids formate clearance in methanol poisoning, and thiamine with pyridoxine supports ethylene glycol metabolism.
- Dialysis indications: Severe acidosis, kidney failure, visual impairment or high toxic alcohol levels, since dialysis clears the parent compound rapidly.
A worked case of an unexplained anion gap
A 34-year-old man is found drowsy after drinking homemade liquor. His bicarbonate is low, the anion gap is about 26, and measured osmolality exceeds the calculated value by about 30.
- Act on the clues: The combination of acidosis, a raised anion gap and an osmolar gap points to a toxic alcohol, so treatment starts before the confirmatory level returns.
- Start fomepizole early: Enzyme blockade must precede formation of toxic metabolites, so fomepizole is given early. Sodium bicarbonate corrects the acidosis.
- Send confirmatory tests: Serum methanol or ethylene glycol levels are sent, along with calcium, renal function and a urine check for oxalate crystals.
- Plan dialysis: Haemodialysis is planned if he develops visual symptoms, kidney failure or persistent acidosis despite bicarbonate. Dialysis clears the parent compound rapidly, so it is used when acidosis or organ failure is severe.
- Judge the response: The acidosis should improve as metabolites clear, so persisting acidosis suggests inadequate treatment.
Where the exam tries to catch you
Four traps recur in toxic alcohol vignettes.
- Trap: trusting a normal osmolar gap Correct: late presenters have metabolised the parent alcohol, so a normal value does not exclude toxic alcohol ingestion.
- Trap: blaming a raised osmolar gap on toxic alcohol alone Correct: ethanol intoxication also raises the osmolar gap, so the rest of the metabolic profile is needed to avoid misattribution.
- Trap: assuming an apparently well patient is safe Correct: methanol's visual toxicity can be delayed by a day or more, so observation is still needed.
- Trap: missing the crystal in ethylene glycol poisoning Correct: oxalate crystals with renal failure suggest the diagnosis, and hypocalcaemia can cause seizures.
Ethanol is generally not preferred because it is less predictable, needs blood level monitoring and causes intoxication itself.
Frequently asked questions
Why does methanol poisoning cause blindness?
Methanol is converted by alcohol dehydrogenase to formaldehyde and then to formic acid, and formate is toxic to the optic nerve and retina. This explains the blurred vision, optic neuritis and occasional blindness seen in methanol poisoning.
What is the antidote for methanol and ethylene glycol poisoning?
Fomepizole, an alcohol dehydrogenase inhibitor, is the preferred antidote for both methanol and ethylene glycol poisoning. Ethanol is an older alternative because it competes for the same enzyme, but it has its own toxicity and is used where fomepizole is unavailable.
What is the osmolar gap and why is it useful?
The osmolar gap is the difference between measured serum osmolality and the calculated value, and a raised gap suggests a toxic alcohol. It is most useful early, because the gap narrows as the parent alcohol is metabolised.
When is haemodialysis indicated in toxic alcohol poisoning?
Haemodialysis is indicated for severe acidosis, kidney failure, visual impairment or high toxic alcohol levels, since it removes the parent alcohol and metabolites quickly. It is also used when fomepizole is unavailable or cannot be given promptly.