Toxic Alcohols — Methanol & Ethylene Glycol

Overview & definition

The toxic alcohols — the methanol (the wood alcohol, the windshield washer, the antifreeze, the illicit spirits) and the ethylene glycol (the antifreeze, the coolant) — are the dangerous ingestions defined by the dual gap: the high anion-gap metabolic acidosis PLUS the high osmolar gap. The parent alcohol is the benign; the TOXICITY comes from the metabolites (the formic acid, the glycolate and the oxalate), formed by the alcohol dehydrogenase. The fomepizole (the ADH inhibitor) and the haemodialysis are the life-saving.[1][1]

The metabolism and the toxicity

Both methanol and ethylene glycol are the metabolised by the alcohol dehydrogenase (ADH) to the toxic metabolites — the parent alcohols themselves are the relatively benign (the mild intoxication). The toxicity is the metabolite-driven.[1][1]

The methanol — the ADH → the formaldehyde → the formic acid (the formate). The formate is the toxic — the retinal and the optic-nerve toxicity (the blindness), and the basal-ganglia (the putaminal) necrosis.[2]

The ethylene glycol — the ADH → the glycolaldehyde → the glycolate (the main acid) → the glyoxylate → the oxalate. The oxalate precipitates as the calcium oxalate crystals in the kidney (the acute kidney injury) and the tissue (the hypocalcaemia).[1][1]

The isopropanol (the distinction) — the ADH → the acetone. The HIGH osmolar gap but the NO anion gap (the acetone is the not an acid) and the ketonuria. The less toxic — the CNS depression (the haemorrhagic gastritis occasionally).[1]

The clinical features

• The latent period — the asymptomatic (the parent alcohol is the benign), the symptoms develop as the metabolism to the toxic metabolite proceeds (the 12 to 24 hours, the longer if the co-ingested ethanol).
• The methanol — the visual disturbance (the blurred vision, the photophobia, the "the snowstorm vision", the blindness), the headache, the nausea, the abdominal pain, the coma, the seizures.
• The ethylene glycol — the CNS depression, the inebriation, the flank pain (the renal), the hypocalcaemia (the tetany, the QT prolongation), the acute kidney injury.
• The inebriation the disproportionate to the ethanol level (the no ethanol detected, yet the drunken state).[1][1]

The investigation: the dual gap

• The high anion-gap metabolic acidosis — the formate (the methanol), the glycolate (the ethylene glycol).
• The high osmolar gap (the calculated osmolar gap = the measured minus the calculated osmolality; the normal below 10). The parent alcohol raises the osmolar gap.
• The dual gap (the high AG + the high osmolar gap) is the signature of the toxic alcohol in the undifferentiated acidosis. (The late presentation — the all-metabolised — may show the high AG with the normalised osmolar gap.)[1][1]
• The serum methanol / ethylene glycol levels (the confirmatory, the quantitative).
• The urine (the ethylene glycol → the calcium oxalate crystals — the envelope and the needle/birefringent; the fluorescence under the Wood's lamp from the antifreeze fluorescein).[1][1]
• The lactate gap (the false-high lactate on some analysers with the glycolate).

Treatment

1. The fomepizole (the ADH inhibitor) — the preferred.[1][3]

• The 15 mg/kg IV loading, then the 10 mg/kg every 12 hours for the 4 doses, then the 15 mg/kg every 12 hours — until the level below 20 mg/dL AND the acidosis resolved.
• The mechanism: the inhibits the ADH — the parent alcohol is the not metabolised to the toxic metabolite, and the eliminated unchanged (the kidney, the lung).
• The preferred over the ethanol (the easier, the no CNS depression, the no monitoring of the ethanol level).[3]

2. The ethanol (the competitive ADH substrate) — the alternative.[1]

• The IV or the oral, the target the blood-ethanol concentration of the 100 to 150 mg/dL (the ADH has the 100x greater affinity for the ethanol than the methanol/EG — the ethanol the outcompetes). The continuous infusion, the level monitoring. The ICU, the CNS depression, the more-laborious.[1]

3. The haemodialysis — the severe.[1][2]

• The indications: the methanol or the EG level above 50 mg/dL, the severe metabolic acidosis (the pH below 7.3), the renal failure, the visual symptoms (the methanol), the end-organ damage, the deterioration despite the fomepizole.
• The removes the alcohol AND the metabolites AND the corrects the acidosis. The continue the fomepizole DURING the dialysis (the increased dosing frequency — the q4h).[2]

4. The cofactors — the metabolite clearance.[2][1]

• The methanol — the folinic acid (the folate) IV (the cofactor for the formate → the CO2 + the water); the speeds the formate clearance.
• The ethylene glycol — the thiamine and the pyridoxine (the cofactors that the shunt the glyoxylate to the non-toxic metabolites — the glycine, the hippurate — away from the oxalate).

5. The bicarbonate for the acidosis (the correct the pH, the reduce the tissue penetration of the weak acids).[1]

Prognosis

The toxic alcohol poisoning is the survivable with the early fomepizole and the dialysis. The poor-prognostic features: the severe acidosis, the delayed presentation, the visual symptoms (the methanol — the irreversible blindness), the renal failure (the EG), the brain injury (the putaminal necrosis).[1][2][1]

Red flags

Methanol poisoning — deep dive

Methanol (methyl alcohol, CH₃OH — "wood alcohol") is found in windscreen washer fluid, antifreeze, paint thinner, shellac, photocopying fluid, canned-heat ("Sterno"), and illicitly distilled ("moonshine") spirits — the latter responsible for most large outbreaks. Mass-poisoning outbreaks (Czech Republic 2012, Iran 2018, Norway 2002–2004) carry a 20–50% mortality and leave survivors permanently blind.[2][12]

Pathophysiology — formaldehyde → formic acid

The parent methanol is only mildly intoxicating (less potent than ethanol). The toxicity is generated entirely by the oxidative pathway: [1]

Methanol ──alcohol dehydrogenase (ADH)──▶ Formaldehyde ──aldehyde dehydrogenase (ALDH)──▶ Formic acid (formate)
   (parent)                                   (transient)                                 (THE TOXIN)

Formic acid (formate) is the lethal metabolite. It acts through two mechanisms:[2][4]

1. Mitochondrial cytochrome c oxidase (complex IV) inhibition — formate blocks the terminal electron-transport chain, producing cellular hypoxia that is biochemically indistinguishable from cyanide toxicity at the mitochondrial level.
2. Strong organic acid — contributes the bulk of the high anion gap metabolic acidosis. [1]

Formate has a striking tropism for the retina and optic nerve (retinal ganglion cells, optic disc) → optic disc oedema and hyperaemia → permanent blindness. It also injures the basal ganglia — putaminal necrosis is the classic radiological finding in severe cases, and surviving patients may develop a parkinsonian syndrome weeks to months later.[4][12]

Why humans are uniquely susceptible — folate

Most mammals (rats, dogs) clear formate rapidly and tolerate methanol well. Humans and other primates are folate-dependent for formate detoxification — formate must be oxidised to CO₂ + H₂O via 10-formyl-tetrahydrofolate dehydrogenase, a folate-dependent enzyme. Because humans are relatively folate-deficient compared with rodents, formate accumulates. This is the molecular rationale for folinic acid (leucovorin) as a specific cofactor antidote in methanol poisoning.[2][4]

Clinical features of methanol

• Latent period 12–24 h (longer with co-ingested ethanol, which competes for ADH and slows metabolism). The patient looks merely "drunk."
• Visual disturbance is the cardinal methanol sign: blurred vision, photophobia, the sensation of "walking in a snowstorm" or "in a blizzard," reduced visual acuity, and frank blindness. Fundoscopy shows optic disc hyperaemia/oedema.
• Headache, nausea, vomiting, abdominal and back pain (pancreatitis is described).
• CNS: confusion, lethargy → coma; seizures in severe cases.
• Basal ganglia injury: a delayed parkinsonian syndrome (bradykinesia, rigidity, tremor) weeks–months after recovery, with putaminal necrosis on CT/MRI.[12]

Specific management of methanol

1. Fomepizole (ADH inhibitor) — preferred.
2. Folinic acid (leucovorin) 1 mg/kg (up to 50 mg) IV every 4–6 h — drives formate → CO₂ + H₂O. Folic acid is a cheaper alternative if folinic acid unavailable.
3. Haemodialysis — mandatory and immediate for any visual symptom, severe acidosis (pH <7.3), renal failure, or methanol >50 mg/dL.
4. Bicarbonate for the acidosis.[2][8]

Ethylene glycol poisoning — deep dive

Ethylene glycol (1,2-ethanediol, HOCH₂CH₂OH) is the principal component of automotive antifreeze/coolant (sweet-tasting, which makes it attractive to children and a vehicle for deliberate self-harm), and is also found in brake/hydraulic fluids and industrial solvents.[7]

Pathophysiology — glycolate → oxalate

Ethylene glycol ──ADH──▶ Glycoaldehyde ──ALDH──▶ Glycolic acid ──▶ Glyoxylic acid ──▶ Oxalic acid
   (parent)                                         (acidosis)       (shunt point)      (renal toxin)

Glycolic acid (glycolate) is the dominant contributor to the metabolic acidosis (a strong organic acid). Oxalic acid is the renal toxin: it irreversibly chelates calcium to form calcium oxalate crystals, which precipitate in and obstruct the proximal renal tubules → acute tubular necrosis → AKI (frequently oliguric/anuric).[4][7]

The calcium consumption produces hypocalcaemia (prolonged QT, tetany, Chvostek/Trousseau signs). Glyoxylic acid sits at a metabolic branch point: with the cofactors thiamine (to α-hydroxy-β-ketoadipate) and pyridoxine (to glycine) it is diverted away from oxalate to non-toxic metabolites — the rationale for these two cofactors as specific adjuncts in EG poisoning.[4]

Clinical features of ethylene glycol (three classical stages)

• Stage 1 (1–12 h) — CNS stage: inebriation without the smell of ethanol, ataxia, slurred speech, progressing to stupor and coma. Nausea, vomiting. Hypocalcaemic tetany and seizures may occur early.
• Stage 2 (12–24 h) — metabolic/cardiorespiratory stage: severe high anion gap metabolic acidosis, tachycardia, hyperventilation (Kussmaul), hypocalcaemia, QT prolongation, and occasionally congestive cardiac failure or pulmonary oedema. The most dangerous stage.
• Stage 3 (24–72 h) — renal stage: flank pain, oliguria → anuric AKI from calcium oxalate crystal nephropathy. Creatinine rises; urinalysis shows crystals and haematuria/proteinuria. [1]

Urine findings — calcium oxalate crystals

Calcium oxalate crystals are the hallmark (ethylene glycol). Two morphologies:[1][1]

• Dihydrate — envelope-shaped / octahedral ("letterbox" or "envelope" crystals).
• Monohydrate — needle-shaped / prism-shaped (monoclinic) — often the more abundant form in poisoning. [1]

Wood's lamp (UV) fluorescence of urine — antifreeze often contains fluorescein (added so mechanics can detect radiator leaks), which may fluoresce under UV. This is unreliable: many preparations lack fluorescein, and a negative result never excludes EG poisoning.[1][1]

"Lactate gap" — glycolate is misread as lactate by some blood-gas analysers (glycolate cross-reacts with the L-lactate oxidase on certain platforms). A large discrepancy between lactate measured on two different platforms (e.g., blood gas vs laboratory) in an acidotic patient is a strong clue to EG poisoning.[4]

Specific management of ethylene glycol

1. Fomepizole (ADH inhibitor) — preferred.
2. Thiamine 100 mg IV and pyridoxine (vitamin B6) 50 mg IV every 6 h — divert glyoxylate to glycine, away from oxalate.
3. Haemodialysis for severe acidosis (pH <7.3), renal failure/oliguria, or EG >50 mg/dL.
4. Bicarbonate for the acidosis; correct calcium cautiously (only for symptomatic hypocalcaemia — over-correction can worsen crystallisation).[6][7]

Methanol vs ethylene glycol — the two patterns

[1]

Differentiation from ethanol and other causes of a high anion gap + high osmolar gap

Not every combined gap is a toxic alcohol. The bedside task is to separate the toxic alcohols (which need fomepizole ± dialysis) from the mimics (which do not).[1][4]

[1]

The isopropanol trap

Isopropanol (isopropyl alcohol, rubbing alcohol) is the classic exception. ADH converts it to acetone, which is osmotically active but not an acid. The result is a HIGH osmolar gap with a NORMAL anion gap and NO metabolic acidosis, plus ketonuria/ketonaemia. Isopropanol also causes haemorrhagic gastritis (unique among the alcohols) and CNS depression. Treatment is supportive — do NOT give fomepizole (blocking ADH prevents formation of the harmless acetone and would prolong exposure to the parent isopropanol, which is more CNS-toxic than its metabolite). Haemodialysis is reserved for massive ingestion with refractory shock.[1][4]

Propylene glycol — the ICU iatrogenic cause

Propylene glycol is the solvent vehicle for several IV drugs — lorazepam, diazepam, phenobarbital, etomidate, nitroglycerin, and esmolol. Patients on high-dose or prolonged infusions (e.g., lorazepam infusion for status epilepticus or sedation) develop a high anion gap metabolic acidosis (lactic) + high osmolar gap. Recognise the cause, switch the sedative (midazolam, propofol — not in propylene glycol), and stop the offending drug; haemodialysis is rarely needed.[4]

The "lactate gap" and the osmol gap

In ethylene glycol poisoning, glycolate cross-reacts with the L-lactate oxidase on certain point-of-care blood gas analysers, producing a spuriously high lactate that is much lower when re-measured on the laboratory analyser. A large discrepancy ("lactate gap") between platforms in an acidotic patient is a strong pointer to EG. The "osmole gap" itself can also be confounded by ethanol, acetone, mannitol, or ketones, so an isolated modestly elevated gap is non-specific — it is the combination with a high anion gap and a compatible history that drives the diagnosis.[4]

The osmolar gap FALLS as the anion gap RISES

This is the single most commonly examined — and most commonly missed — concept in toxic alcohol poisoning. The parent alcohol generates the osmolar gap; the metabolites generate the anion gap. As metabolism proceeds, osmoles are converted into acids.[1][4]

[1]

A normal osmolar gap NEVER excludes toxic alcohol poisoning. A late presenter, or a chronic drinker of contaminated spirits, may have a completely normal osmolar gap because all the parent alcohol has been converted to (non-osmotic) organic acids. If the anion gap is high and the history fits, treat empirically with fomepizole — do not be reassured by a "normal" osmolar gap.[1][4]

Fomepizole — the ADH inhibitor of choice

Fomepizole (4-methylpyrazole) is a potent competitive inhibitor of alcohol dehydrogenase with approximately 500–10000 times the affinity of ethanol for ADH. By blocking ADH it halts formation of every downstream toxin (formate, glycolate, oxalate), and the parent alcohol is then cleared unchanged by the kidney (~zero-order) and lung.[5][6]

Fomepizole dosing

[1]

The dose escalation after 48 h is the most commonly examined pharmacokinetic point: fomepizole auto-induces its own hepatic metabolism, so its half-life falls and the dose must rise. During haemodialysis the clearance of fomepizole roughly doubles, so dosing intervals shorten to q4h.[5][8]

Why fomepizole is preferred over ethanol

[1]

Ethanol — the alternative ADH inhibitor

Ethanol has approximately 100× the affinity of methanol or ethylene glycol for ADH, so a therapeutic blood-ethanol concentration saturates the enzyme and competitively blocks metabolism of the toxic alcohol. It is effective, cheap, and universally available, but laborious.[1][8]

Ethanol protocol

• Target blood-ethanol concentration: 100–150 mg/dL (22–33 mmol/L).
• Loading: 10% ethanol 7.5–10 mL/kg IV over 30 min (or 0.8 g/kg oral/NG), adjusted for chronic alcoholics (higher) and naïve patients (lower).
• Maintenance: 10% ethanol ~1–1.5 mL/kg/h (continuous infusion), titrated to hourly ethanol levels.
• Increase during haemodialysis to ~2–3 mL/kg/h (HD clears ethanol).
• Monitor: hourly ethanol level, CNS depression, blood glucose (hypoglycaemia — especially children), LFTs.
• Continue until toxic alcohol level <20 mg/dL AND anion gap normal AND asymptomatic.[1][8]

Haemodialysis — indications and endpoints

Haemodialysis removes the parent alcohol AND the metabolites and rapidly corrects the acidosis. Both the EXTRIP methanol (2015) and ethylene glycol (2023) workgroups endorse extracorporeal treatment in severe poisoning.[9][10]

Indications for haemodialysis (any one)

• Severe metabolic acidosis: pH <7.25–7.3 or HCO₃⁻ <15–18 mmol/L despite bicarbonate.
• Visual symptoms (methanol) — immediate, regardless of level.
• Renal failure / oliguria (especially ethylene glycol).
• Methanol or ethylene glycol level >50 mg/dL (some centres use >25 mg/dL).
• Deterioration despite adequate fomepizole.
• Severe electrolyte disturbance (e.g., refractory hypocalcaemia in EG). [1]

Endpoint for haemodialysis

Stop when pH and anion gap have normalised AND the toxic alcohol level is <20 mg/dL, then recheck the level 2–4 h after stopping to detect rebound (ongoing absorption or redistribution may raise the level again). Increase fomepizole to q4h during HD because dialysis removes fomepizole itself.[9][10]

Cofactors — the metabolite-clearance adjuncts

Cofactors are cheap, safe, and essential. They accelerate clearance of the toxic metabolite once it has formed, and they are given empirically alongside fomepizole.[2][4]

[1]

Management protocol

[1]

Key trials and evidence

[1] [1] [1] [1] [1] [1] [1]

Clinical pearls

Additional red flags

Exam-style questions and answers

1. "Describe the metabolism of methanol and explain why folinic acid is a specific antidote." Methanol → (ADH) → formaldehyde → (ALDH) → formic acid (formate). Formate inhibits mitochondrial cytochrome c oxidase (cellular hypoxia) and is a strong organic acid (high AG acidosis), with tropism for the retina/optic nerve and basal ganglia. Humans clear formate via the folate-dependent enzyme 10-formyl-tetrahydrofolate dehydrogenase (formate → CO₂ + H₂O); we are relatively folate-deficient compared with rodents, so formate accumulates. Folinic acid (leucovorin) provides the folate cofactor and accelerates formate detoxification — hence it is specific to methanol (not EG).[2][4]

2. "How does the osmolar gap change over time in toxic alcohol poisoning, and what is the practical implication?" The parent alcohol generates the osmolar gap; the metabolites generate the anion gap. Early (0–12 h) the OG is high and the AG near normal; late (>24 h) the OG has normalised (parent metabolised away) while the AG is high (organic acids remain). Implication: a normal osmolar gap never excludes toxic alcohol poisoning. Treat on the anion gap and the history.[1][4]

3. "Compare fomepizole and ethanol as ADH inhibitors." Both competitively inhibit ADH and halt toxic metabolite formation. Fomepizole is a more potent ADH inhibitor (5000–10000× ethanol's affinity), requires no level titration (fixed intermittent dosing), does not intoxicate, does not cause hypoglycaemia or hepatotoxicity, and does not confound the neuro exam. Ethanol is cheap and universally available but requires hourly level monitoring and a titrated infusion, intoxicates the patient, and risks hypoglycaemia (children) and gastritis. Fomepizole is preferred wherever available.[5][6][13]

4. "What is the fomepizole dosing regimen, and why does it change after 48 h and during haemodialysis?" 15 mg/kg IV loading, then 10 mg/kg q12h for 4 doses, then 15 mg/kg q12h thereafter (the increase is because fomepizole auto-induces its own hepatic CYP metabolism, raising clearance). During haemodialysis, dose q4h because HD removes fomepizole. Continue until the toxic alcohol level is <20 mg/dL AND the anion gap is normal, then recheck at 2–4 h for rebound.[5][8]

5. "A patient with presumed EG poisoning has hypocalcaemia. How aggressively do you correct it?" Cautiously. Oxalate chelates calcium; symptomatic hypocalcaemia (tetany, prolonged QT, seizures) should be corrected with cautious IV calcium, but aggressive repletion can drive further calcium oxalate crystallisation and worsen AKI. The definitive therapy is fomepizole + thiamine + pyridoxine (shunt glyoxylate to glycine) + haemodialysis.[6][7]

6. "Why does isopropanol cause a high osmolar gap but NOT a high anion gap?" ADH converts isopropanol to acetone, which is osmotically active (raises the OG) but not an acid (no contribution to the AG). The result is ketosis without acidosis — plus haemorrhagic gastritis. Do NOT give fomepizole; treatment is supportive.[1][4]

One-line summary

The toxic alcohols (methanol, ethylene glycol) cause a high anion gap + high osmolar gap metabolic acidosis from ADH-derived organic acids (formate → blindness; glycolate/oxalate → AKI). Block ADH early with fomepizole 15 mg/kg IV (preferred over ethanol), add cofactors (folinic acid for methanol; thiamine + pyridoxine for EG), and haemodialyse for severe acidosis, visual symptoms, renal failure, or level >50 mg/dL. A normal osmolar gap never excludes the diagnosis — treat empirically on the anion gap and the history.[1][2][4]

References

1. [1]Ross JA, Borek HA, Holstege CP, et al. Toxic Alcohol Poisoning Emerg Med Clin North Am, 2022.PMID 35461626
2. [2]Kraut JA Approach to the Treatment of Methanol Intoxication Am J Kidney Dis, 2016.PMID 27180631
3. [3]Pohanka M Antidotes Against Methanol Poisoning: A Review Mini Rev Med Chem, 2019.PMID 30864518
4. [4]Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management Clin J Am Soc Nephrol, 2008.PMID 18045860
5. [5]Brent J, McMartin K, Phillips S, Aaron C, Kulig K. Fomepizole for the treatment of methanol poisoning N Engl J Med, 2001.PMID 11172179
6. [6]Brent J, McMartin K, Phillips S, Burkhart KK, Donovan JW, Wells M, Kulig K. Fomepizole for the treatment of ethylene glycol poisoning. Methylpyrazole for Toxic Alcohols Study Group N Engl J Med, 1999.PMID 10080845
7. [7]Barceloux DG, Krenzelok EP, Olson K, Watson W. American Academy of Clinical Toxicology Practice Guidelines on the Treatment of Ethylene Glycol Poisoning. Ad Hoc Committee J Toxicol Clin Toxicol, 1999.PMID 10497633
8. [8]Barceloux DG, Bond GR, Krenzelok EP, Cooper H, Vale JA. American Academy of Clinical Toxicology practice guidelines on the treatment of methanol poisoning J Toxicol Clin Toxicol, 2002.PMID 12216995
9. [9]Roberts DM, Yates C, Megarbane B, et al. Recommendations for the role of extracorporeal treatments in the management of acute methanol poisoning: a systematic review and consensus statement Crit Care Med, 2015.PMID 25493973
10. [10]Ghannoum M, Gosselin S, Hoffman RS, et al. Extracorporeal treatment for ethylene glycol poisoning: systematic review and recommendations from the EXTRIP workgroup Crit Care, 2023.PMID 36765419
11. [11]McMartin K, Jacobsen D, Hovda KE Antidotes for poisoning by alcohols that form toxic metabolites Br J Clin Pharmacol, 2016.PMID 26551875
12. [12]Zakharov S, Kurcova I, Diblik P, et al. Long-term visual damage after acute methanol poisonings: Longitudinal cross-sectional study in 50 patients Clin Toxicol (Phila), 2015.PMID 26364866
13. [13]Zakharov S, Pelclova D, Diblik P, et al. Fomepizole versus ethanol in the treatment of acute methanol poisoning: Comparison of clinical effectiveness in a mass poisoning outbreak Clin Toxicol (Phila), 2015.PMID 26109326