Methanol Toxicity

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Alcohol dehydrogenase inhibition is needed as quickly as possible by giving alcohol or fomepizole to prevent the metabolism of Methanol to its toxic metabolites formic acid and formaldehyde.

About

• Interesting as they are treatable and cause characteristic blood chemistry
• Methanol has been added to alcoholic drinks.
• Even a mouthful can cause irreversible toxicity

Aetiology

• Methanol is harmless until metabolised
• Methanol ? (Alcohol Dehydrogenase) ? Formaldehyde
• Formaldehyde ? (Formaldehyde dehydrogenase) ? Formic acid
• The metabolites cause renal failure and a raised anion gap Metabolic acidosis
• Formic acid metabolizes slowly in the human body, with a half-life of about 20 hours.

Source

• Windscreen washer fluid, Cleaning solvents
• Antifreeze, "Moonshine"

Clinical

• Dilated or fixed pupils, Coma and Convulsions, blindness
• Lethargy, Kussmaul respiration, Renal failure(ATN), Parkinsonism
• Abdominal pain, Nausea, Vomiting and late stages can cause respiratory arrest
• Crystals forming within the eye can lead to so-called 'snow field' cataract formation.

Investigations

• FBC, U&E - may show AKI. ECG
• Lactate is typically high
• ABG: Metabolic acidosis + increased anion gap and serum osmolol gap
• Hypoglycaemia: monitor and treat
• Hypocalcaemia
• Methanol: Definitive diagnosis of methanol intoxication presently requires measurement with gas or liquid chromatography, a laborious and expensive procedure.
• CK: if there is concern of rhabdomyolysis

Management: Criteria

• Criterion for treatment (patients blood gases):
• A. Asymptomatic patients, normal blood gas: Observe.
• B. pH>7.2, HCO₃>20: Observe minimum 24 hours. Give bicarbonate if necessary (increasing acidosis)
• C. pH 7.0-7.2, HCO₃ 10-20: Give bicarbonate, ethanol (or fomepizole), consider HD
• D. pH<7.2, HCO₃<10: Give bicarbonate, fomepizole (or ethanol), HD, folinic acid

Ethanol (double infusion rate if regular drinker)

• 5% Ethanol start at 15 mL/kg and then 2 ml/kg/hr
• 10% Ethanol start at 7.5 mL/kg and then 1 ml/kg/hr
• 20% Ethanol start at 4 mL/kg and then 0.5 ml/kg/hr
• 40% Ethanol start at 2 mL/kg and then 0.25 ml/kg/hr

Management: Take prompt poisons advice

• Advice on the treatment of ethylene glycol and methanol poisoning should be obtained from the National Poisons Information Service
• ABC, close monitoring, Oxygen to give sats of 94-98%. IV fluids to maintain good diuresis. Consider IV bicarbonate to correct severe acidosis (pH level <7.2)
• Alcohol loading dose: of 50 g of ethanol (conveniently given as approximately 125 ml of gin, whisky, or vodka)* should be administered immediately orally. It has an affinity in the order of 100 (some say 8,000) times that of methanol for alcohol and aldehyde dehydrogenases
• IV (Ethyl) Alcohol infusion: intravenous infusion of ethanol usually 10ml/kg in 5% Dextrose to provide blood ethanol concentrations of 500 mg to 1 g/l. The infusion should be continued until ethylene glycol is no longer detectable in the blood. Keep a close watch on pH and Anion gap as well as ABG and renal function.
• Fomepizole loading dose 15 mg/kg then 10 mg/kg every 12 hours. if it can be quickly obtained: give on suspicion of methanol ingestion and/or anion gap metabolic acidosis, increased osmolar gap, visual disturbances, serum methanol concentration greater than 20 mg/dL
• Consider FOLINIC (OR FOLIC) ACID: 50 mg iv. or orally (e.g. 10 tablets of 5 mg) every 6 hours for 24-48 hrs.
• Consider Bicarbonate IV
• Consider haemodialysis as a treatment method and contact local renal unit early and dialyse until no ethylene glycol is detectable in the blood. Indications include pH < 7.25, HCO₃ < 10, Acute renal failure, Visual disturbance >50, Serum glycolic acid >8

References

• Methanol intoxication approach
• methanol poisoning