8.3 Ethylene Glycol, Methanol, and Isopropanol: Metabolism, Osmolal Gaps, and Fomepizole

Toxic alcohols—ethylene glycol, methanol, and isopropanol—are readily accessible commercial liquids found in automotive antifreezes, windshield washer fluids, de-icers, solvents, and rubbing alcohols. When ingested, their low molecular weight and structural similarity to ethanol produce mild, early central nervous system intoxication. However, hepatic biotransformation by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) generates highly cytotoxic organic acids that trigger life-threatening metabolic acidosis, irreversible blindness, and acute renal failure. Rapid laboratory analysis, understanding gap kinetics, and early antidotal intervention are essential for survival.

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Comparative Metabolic Pathways and Pathophysiology

Each toxic alcohol undergoes distinct hepatic biotransformation, dictating its specific clinical toxidrome and target organ injury.

Ethylene Glycol ──(ADH)──> Glycolaldehyde ──(ALDH)──> Glycolic Acid ──> Glyoxylic Acid ──> Oxalic Acid (+Ca2+ ──> Ca-Oxalate)
Methanol        ──(ADH)──> Formaldehyde   ──(ALDH)──> Formic Acid (Formate) ──> Cytochrome Oxidase Block (Retina/Putamen)
Isopropanol     ──(ADH)──> Acetone (Excreted Renally & Pulmonarily; Ketosis WITHOUT Acidosis)

1. Ethylene Glycol: Glycolic Acid and Calcium Oxalate Crystalluria

Ethylene glycol (CH₂OH-CH₂OH) is a colorless, odorless, sweet-tasting diol utilized predominantly in automotive cooling systems and aircraft de-icing fluids.

• Biotransformation: Ethylene glycol is oxidized by alcohol dehydrogenase (ADH) to glycolaldehyde, which is rapidly converted by aldehyde dehydrogenase (ALDH) to glycolic acid.
• Glycolic Acid (The Acidosis Driver): Glycolic acid has a relatively slow clearance rate and accumulates in large quantities. It is the primary metabolite responsible for the profound high anion gap metabolic acidosis and systemic cellular toxicity.
• Oxalic Acid and Calcium Chelation: Glycolic acid is oxidized via glyoxylic acid into oxalic acid. Oxalic acid rapidly chelates free ionized calcium (Ca²⁺) in the circulation and tubular fluid to form insoluble calcium oxalate:
  • Calcium Oxalate Monohydrate: Needle-, spindle-, or dumbbell-shaped crystals.
  • Calcium Oxalate Dihydrate: Characteristic octahedral, envelope-shaped crystals.
• Target Organ Destruction: Calcium oxalate crystals precipitate heavily within renal proximal tubular lumens and parenchymal tissue, causing mechanical tubular obstruction, intense interstitial inflammation, acute tubular necrosis, hematuria, flank pain, and acute oliguric/anuric renal failure. Concurrently, systemic chelation precipitates acute hypocalcemia, leading to tetany, prolongation of the QTc interval, ventricular dysrhythmias, and myocardial depression.
• Fluorescein Diagnostics (Limitations): Many commercial antifreezes incorporate sodium fluorescein dye to assist in mechanical leak detection. While examining freshly voided urine under a Wood's lamp (ultraviolet light) for bright yellow-green fluorescence is historically described, it is plagued by high false-positive rates (from plastic container luminescence, riboflavin, or drugs) and false-negative rates (low dye concentrations). A negative Wood's lamp exam should never be used to rule out an ingestion.

2. Methanol: Formic Acid, Ocular Toxicity, and Putaminal Necrosis

Methanol (CH₃OH, methyl alcohol, wood alcohol) is an aliphatic alcohol found in windshield wiper fluids, gas line de-icers, carburetor cleaners, model airplane fuels, canned cooking fuels (Sterno), and illicitly distilled spirits.

• Biotransformation: Methanol is oxidized by ADH to formaldehyde (elimination half-life of only 1 to 2 minutes), which is rapidly oxidized by ALDH to formic acid (formate).
• Mechanism of Cytotoxicity: Formic acid is an exceptionally potent cellular toxin. Formate binds to and potently inhibits mitochondrial cytochrome c oxidase (Complex IV) of the electron transport chain. This halts aerobic ATP production, precipitating histotoxic cellular hypoxia, energetic collapse, severe lactic acidosis, and an elevated anion gap metabolic acidosis.
• Target Organ Toxicity:
  1. Retina and Optic Nerve: Retinal pigment epithelial cells and optic nerve axonal bundles are exquisitely sensitive to histotoxic hypoxia. Patients experience photophobia, visual blurring, constricted visual fields, and characteristically describe their vision as 'standing in a blinding snowstorm'. Fundoscopy reveals optic disc hyperemia, peripapillary retinal edema, fixed dilated pupils, and subsequent optic atrophy with permanent, irreversible blindness.
  2. Basal Ganglia (Putamen): Formate selectively targets the basal ganglia, precipitating bilateral putaminal necrosis, liquefaction, and secondary intracranial hemorrhage. Survivors frequently exhibit permanent parkinsonian extrapyramidal deficits (tremor, cogwheel rigidity, masked facies).

3. Isopropanol: Acetone and Ketosis Without Acidosis

Isopropanol (CH₃-CHOH-CH₃, isopropyl alcohol) is a secondary alcohol commonly found in 70% rubbing alcohol, hand sanitizers, and glass cleaners.

• Biotransformation: Isopropanol is oxidized chiefly by ADH into acetone (a smaller fraction is excreted unchanged). Acetone is a ketone, NOT an organic acid. Acetone undergoes no further oxidation into carboxylic acids and is eliminated slowly via pulmonary exhalation and renal filtration.
• Clinical Manifestations: Isopropanol is a potent central nervous system depressant—producing inebriation roughly twice as potent and lasting twice as long as ethanol. Ingestion induces severe chemical gastritis, characterized by severe abdominal pain, nausea, vomiting, hematemesis, and upper gastrointestinal bleeding.
• THE DIAGNOSTIC HALLMARK (KETOSIS WITHOUT ACIDOSIS): Acetone generates marked ketonemia and ketonuria (strongly positive nitroprusside ketone reactions) accompanied by a distinct fruity, sweet breath odor. Crucially, because no acid is produced, patients exhibit a completely normal arterial blood pH, normal serum bicarbonate, and normal anion gap! The osmolal gap is markedly elevated. Toxicity is benign compared to ethylene glycol and methanol; treatment is purely supportive (hydration, antiemetics, mucosal protection).

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Diagnostic Evaluation and Gap Kinetics

Accurately interpreting laboratory parameters requires an understanding of how osmotic and metabolic gaps fluctuate over time.

The Osmolal Gap Calculation

Serum osmolality reflects the total number of dissolved solute particles per kilogram of solvent. The calculated serum osmolality accounts for the primary endogenous physiological osmoles:

$\text{Calculated Osmolality } (\text{mOsm/kg}) = 2 \times [\text{Na}^+] + \frac{[\text{Glucose}]}{18} + \frac{[\text{BUN}]}{2.8}$

If ethanol is present on laboratory screening, it must be accounted for in the calculation:

$\text{Calculated Osmolality with Ethanol} = 2 \times [\text{Na}^+] + \frac{[\text{Glucose}]}{18} + \frac{[\text{BUN}]}{2.8} + \frac{[\text{Ethanol}]}{4.6}$

$\text{Osmolal Gap} = \text{Measured Osmolality} - \text{Calculated Osmolality}$

• Normal Osmolal Gap: Baseline physiological range is typically -10 to +10 mOsm/kg H₂O.
• An osmolal gap > 10 to 15 mOsm/kg strongly suggests the presence of an exogenous, unmeasured low-molecular-weight osmotically active solute.

CRITICAL LABORATORY DIRECTIVE (FREEZING POINT OSMOMETRY): Serum osmolality MUST be determined using a freezing-point depression osmometer. Vapor pressure osmometers heat the sample to measure vapor condensation; volatile alcohols (methanol, isopropanol, ethanol) vaporize into the headspace during testing and are not registered, falsely generating a completely normal measured osmolality and masking a fatal exposure!

The Dynamic Gap Evolution (Gap Dissociation)

The relationship between the osmolal gap and the anion gap ([Na⁺] - ([Cl⁻] + [HCO₃⁻])) changes continuously as the parent alcohol is metabolized into toxic acids:

Early Post-Ingestion (0–4 h):   Parent Alcohol High ──> High Osmolal Gap | Normal Anion Gap
Mid Metabolic Phase (6–12 h):   Conversion Underway ──> Decreasing Osmolal Gap | Rising Anion Gap
Late Phase (>12–24 h):          Parent Alcohol Gone ──> Normal Osmolal Gap | SEVERE High Anion Gap Acidosis

• Early Presentation: The uncharged parent alcohol molecules circulate in high molar concentrations, generating a massive osmolal gap. Because toxic acids have not yet been produced, the anion gap and arterial blood pH remain entirely normal.
• Late Presentation: Hepatic ADH and ALDH convert the parent alcohol into organic acid metabolites (glycolate, formate). As parent molecules vanish, the osmolal gap progressively closes. Meanwhile, accumulating acid anions consume bicarbonate and expand the unmeasured anion pool. The patient presents with a severe high anion gap metabolic acidosis and a completely normal osmolal gap.
• Clinical Rule: A normal osmolal gap never rules out toxic alcohol poisoning if the patient presents late after ingestion.

The False-Positive 'Lactate Gap' in Ethylene Glycol

Point-of-care (POC) blood gas analyzers (e.g., Radiometer, i-STAT) utilize the enzyme L-lactate oxidase in their electrochemical sensor cartridges. Because the chemical structure of glycolic acid (HO-CH₂-COOH) is nearly identical to L-lactic acid (CH₃-CHOH-COOH), lactate oxidase cannot distinguish between them.

Consequently, severe ethylene glycol toxicity causes an astronomical 'pseudo-lactic acidosis' reading on point-of-care analyzers (e.g., POC lactate reported as >15 to 20 mmol/L), whereas a formal core laboratory assay (utilizing lactate dehydrogenase) demonstrates a normal or only mildly elevated true lactate level. This marked discrepancy—the lactate gap—is virtually pathognomonic for severe ethylene glycol poisoning.

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Targeted Antidote Pharmacotherapy: Fomepizole Protocols

Blocking alcohol dehydrogenase halts the conversion of parent alcohols into toxic acids, allowing the non-toxic parent compounds to be safely cleared by the kidneys or removed via hemodialysis.

Fomepizole (4-Methylpyrazole, Antizol)

Fomepizole is a potent, competitive inhibitor of alcohol dehydrogenase with an affinity for ADH > 1,000 to 8,000 times greater than that of ethanol.

Dosing and Administration Protocol

• Loading Dose: Administer 15 mg/kg IV reconstituted in at least 100 mL of normal saline or D5W infused over 30 minutes.
• Maintenance Dosing: Administer 10 mg/kg IV every 12 hours for 4 doses.
• Extended Maintenance Dosing: After 48 hours (4 maintenance doses), increase the dose to 15 mg/kg IV every 12 hours. This increase is mandatory because fomepizole auto-induces its own hepatic clearance via cytochrome P450 isoenzymes (primarily CYP2E1).
• Dosing During Hemodialysis: Fomepizole has a low molecular weight (82.1 Da), low plasma protein binding, and a volume of distribution (Vd ≈ 0.6 to 1.0 L/kg) that makes it highly dialyzable. During intermittent hemodialysis:
  • Administer fomepizole every 4 hours throughout the dialysis run, or
  • Initiate a continuous IV infusion of 1.0 to 1.5 mg/kg/hour during dialysis.
  • At the end of dialysis (per the product label): if less than 1 hour has passed since the last dose, give no extra dose; if 1 to 3 hours have passed, give half of the next scheduled dose; if more than 3 hours have passed, give the next scheduled dose.

Fomepizole vs. Ethanol Therapy

Historically, intravenous ethanol was the standard treatment. While inexpensive, ethanol carries significant clinical complications: variable zero-order clearance requiring invasive hourly blood ethanol draws (targeting 100 to 150 mg/dL), severe CNS depression, respiratory depression, risk of profound hypoglycemia (especially in children), and chemical phlebitis requiring central venous catheterization for 10% IV solutions. Fomepizole is universally preferred as the first-line antidote.

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Essential Metabolic Cofactors

Adjuvant vitamin cofactors stimulate alternative, non-toxic metabolic pathways to clear intermediate acid metabolites:

Ethylene Glycol Cofactors: Thiamine and Pyridoxine

• Thiamine (Vitamin B1): Administer 100 mg IV every 6 hours. Thiamine pyrophosphate is an obligate coenzyme for alpha-ketoglutarate-glyoxylate carboligase, which converts glyoxylic acid into the non-toxic, water-soluble metabolite alpha-hydroxy-beta-ketoadipate.
• Pyridoxine (Vitamin B6): Administer 50 to 100 mg IV every 6 hours. Pyridoxal phosphate acts as a cofactor for alanine-glyoxylate transaminase, which shunts glyoxylic acid into glycine.
• Goal: Together, thiamine and pyridoxine actively shunt glyoxylate away from the terminal pathway that forms destructive oxalic acid.

Methanol Cofactors: Folinic Acid (Leucovorin) or Folic Acid

• Folinic Acid (Leucovorin): Administer 50 mg IV every 4 to 6 hours (or Folic Acid 50 mg IV q4-6h if leucovorin is unavailable).
• Goal: Formic acid elimination is mediated by 10-formyltetrahydrofolate dehydrogenase, a folate-dependent enzyme that oxidizes toxic formate into harmless carbon dioxide (CO₂) and water (H₂O). Administering exogenous folate accelerates formate clearance and protects against retinal necrosis.

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Indications for Emergent Hemodialysis

Intermittent hemodialysis rapidly removes both parent alcohols and their small, water-soluble toxic metabolites (glycolate, formate), while simultaneously correcting severe acid-base derangements and electrolyte imbalances.

Older guidance used a single "50 mg/dL" trigger for both alcohols. Current indications come from the EXTRIP workgroup (methanol, 2015; ethylene glycol, 2023), which ties the decision to the antidote in use, the acid-base picture, and end-organ injury.

Methanol (EXTRIP)

ECTR is recommended for any of the following:

• Coma, seizures, or new visual deficits
• Blood pH 7.15 or lower, persistent metabolic acidosis despite antidotes and supportive care, or an anion gap above 24 mmol/L
• Methanol concentration above 70 mg/dL with fomepizole, above 60 mg/dL with ethanol, or above 50 mg/dL with no ADH blocker
• Impaired kidney function

Ethylene Glycol (EXTRIP)

The EXTRIP ethylene glycol guideline calculates the anion gap as Na + K − Cl − HCO3, so its thresholds run slightly higher than a gap calculated without potassium.

Key teaching point: a patient on fomepizole with normal kidney function and no significant acidosis can often clear ethylene glycol without dialysis, even at levels that once triggered automatic hemodialysis. Continue fomepizole (and folate for methanol) during ECTR.

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Clinical Poison Center Case Scenario: Antifreeze Ingestion

A 36-year-old mechanic is brought to the emergency department after being found stuporous in his garage next to an open container of antifreeze. On arrival, the patient is obtunded, tachypneic (RR 28 breaths/min), and hyperventilating. Initial arterial blood gas: pH 7.14, pCO2 22 mmHg, pO2 96 mmHg, HCO3 7 mEq/L. Basic metabolic panel: Na 140 mEq/L, K 5.2 mEq/L, Cl 98 mEq/L, HCO3 7 mEq/L, BUN 22 mg/dL, Cr 2.4 mg/dL, Glucose 108 mg/dL. Point-of-care blood gas lactate reads 18.2 mmol/L. Measured serum osmolality is 320 mOsm/kg. Urinalysis reveals numerous spindle- and envelope-shaped crystals.

Poison Specialist Diagnostic and Management Calculation

1. Anion Gap Calculation: $\text{Anion Gap} = 140 - (98 + 7) = 35\text{ mEq/L} \quad (\text{Markedly Elevated})$
2. Osmolal Gap Calculation: $\text{Calculated Osmolality} = 2(140) + \frac{108}{18} + \frac{22}{2.8} = 280 + 6 + 7.9 = 293.9\text{ mOsm/kg}$ $\text{Osmolal Gap} = 320 - 293.9 = 26.1\text{ mOsm/kg} \quad (\text{Elevated})$
3. Interpretation: The combination of a high anion gap acidosis (35 mEq/L), elevated osmolal gap (26.1 mOsm/kg), and calcium oxalate crystalluria confirms ethylene glycol toxicity. The astronomical POC lactate of 18.2 mmol/L represents a classic lactate gap, driven by glycolic acid cross-reacting with the analyzer's lactate oxidase sensor.
4. Urgent Multimodal Interventions:
   • Fomepizole: Administer immediate loading dose of 15 mg/kg IV over 30 minutes to halt further glycolate and oxalate generation.
   • Metabolic Shunting: Order Thiamine 100 mg IV q6h and Pyridoxine 100 mg IV q6h.
   • Emergent Hemodialysis: Nephrology is consulted immediately for hemodialysis based on severe acidemia (pH 7.14), acute renal failure (Cr 2.4 mg/dL), and substantial toxic metabolite accumulation. Fomepizole dosing is adjusted to every 4 hours during the hemodialysis run.

Ethanol: The Alcohol Behind Most Calls

Ethanol deserves its own place on the topic list because it is the most commonly ingested alcohol, it changes the management of the other alcohols, and it behaves differently in children than in adults.

Kinetics and Effects

• Absorption is rapid, with peak concentrations 30 to 60 minutes after ingestion on an empty stomach.
• Elimination is zero-order in most adults at roughly 15 to 20 mg/dL per hour (faster, up to 25 to 35 mg/dL per hour, in chronic heavy drinkers; slower, about 15 mg/dL per hour, in naive drinkers and children).
• Each 1 mL of absolute ethanol per kilogram raises the blood concentration by roughly 100 to 130 mg/dL before distribution and elimination are accounted for.
• Ethanol contributes about 22 to 27 mOsm/kg to the osmolal gap for every 100 mg/dL of blood concentration (dividing by 4.6 gives about 22; the empiric divisor 3.7 gives about 27), which is why the measured ethanol level must be subtracted before attributing an osmolal gap to another alcohol.

Tolerance uncouples level from appearance: a chronic drinker can be conversant above 400 mg/dL, while a toddler can be comatose and hypoglycemic at 100 mg/dL.

Children Are Different

Young children develop hypoglycemia because ethanol blocks gluconeogenesis and their glycogen stores are small. The sources are rarely beverages: mouthwash (up to about 27% ethanol), hand sanitizer (60% to 70%), perfumes, colognes, vanilla extract, and some cough and cold syrups.

• Check a bedside glucose in every intoxicated child and treat hypoglycemia with dextrose, then a dextrose-containing infusion.
• Hypothermia, seizures from hypoglycemia, and airway compromise are the causes of death.
• An estimated ingestion above roughly 0.4 mL/kg of absolute ethanol (for example, about 1.5 mL/kg of a 27% mouthwash) warrants evaluation and a glucose check.

Related Clinical Problems

Pitfall: the presence of ethanol never rules out a second alcohol. Ethanol delays the metabolism of methanol and ethylene glycol, so the patient may present late with an intact osmolal gap and a still-normal anion gap, and then deteriorate as ethanol clears.