## Ethylene Glycol Poisoning: Antidote Selection ### Pathophysiology of Ethylene Glycol Toxicity Ethylene glycol itself is relatively non-toxic, but its metabolites are highly toxic: ```mermaid flowchart TD A[Ethylene glycol ingestion]:::outcome --> B[Alcohol dehydrogenase]:::action B --> C[Glycoaldehyde]:::outcome C --> D[Aldehyde dehydrogenase]:::action D --> E[Glycolic acid]:::outcome E --> F[Glyoxylate pathway]:::action F --> G[Oxalic acid]:::outcome G --> H[Calcium oxalate crystals]:::outcome H --> I[Acute kidney injury]:::urgent E --> J[Metabolic acidosis]:::urgent J --> K[CNS depression, seizures]:::urgent ``` **Key Point:** The toxic metabolites (glycolic acid and oxalic acid) cause: - **Metabolic acidosis** (from glycolic acid accumulation) - **Acute kidney injury** (from calcium oxalate crystal deposition in renal tubules) - **CNS toxicity** (from glycoaldehyde) - **Hypocalcemia** (from oxalate binding calcium) ### Antidote Strategy: Block Metabolism The goal is to **inhibit alcohol dehydrogenase** and prevent conversion of ethylene glycol to toxic metabolites. Two agents can do this: | Feature | Fomepizole | Ethanol | |---------|-----------|--------| | **Mechanism** | Competitive inhibitor of alcohol dehydrogenase (Ki ~0.1 μM) | Competitive substrate for alcohol dehydrogenase (Km ~1 mM) | | **Selectivity** | Highly selective; minimal off-target effects | Non-selective; metabolized to acetaldehyde, causes intoxication | | **Loading dose** | 15 mg/kg IV | 10 mL/kg of 10% solution (target 100–150 mg/dL blood level) | | **Maintenance** | 10 mg/kg IV every 12 hours (or 15 mg/kg if on dialysis) | Continuous infusion to maintain level; requires frequent monitoring | | **Monitoring** | Minimal; no therapeutic drug monitoring needed | Frequent blood ethanol levels; risk of hypoglycemia, CNS depression | | **Adverse effects** | Rare; headache, nausea | Intoxication, hypoglycemia, lactic acidosis, respiratory depression | | **Cost** | Higher | Lower | | **First-line status** | YES — preferred in modern practice | Alternative if fomepizole unavailable | **High-Yield:** Fomepizole is now the **preferred antidote** in most developed countries because it is more selective, easier to dose, and safer than ethanol. ### Why Fomepizole in This Case 1. **Early presentation (6 hours):** Fomepizole is most effective when given early, before extensive metabolism occurs. At 6 hours with serum level 180 mg/dL, fomepizole can still prevent further toxic metabolite formation. 2. **Oliguric renal failure:** The patient already has acute kidney injury from oxalate crystals. Fomepizole will prevent further metabolite accumulation while dialysis is arranged. 3. **Metabolic acidosis:** The acidosis is from glycolic acid. Stopping further metabolism with fomepizole is more effective than bicarbonate alone. 4. **Safety:** Fomepizole does not cause intoxication or hypoglycemia, unlike ethanol. ### Complete Management Algorithm ```mermaid flowchart TD A[Ethylene glycol poisoning suspected]:::outcome --> B[Check serum/urine osmolal gap]:::decision B -->|Osmolal gap > 10| C[Confirm with ethylene glycol level]:::decision C -->|Level > 20 mg/dL| D[Give fomepizole 15 mg/kg IV]:::action D --> E[Maintenance: 10 mg/kg IV q12h]:::action E --> F[Correct metabolic acidosis with NaHCO3]:::action F --> G[Arrange hemodialysis]:::action G --> H[Monitor for hypocalcemia, AKI]:::action H --> I[Continue fomepizole until ethylene glycol undetectable]:::action ``` ### Supportive and Adjunctive Measures 1. **Sodium bicarbonate:** Correct metabolic acidosis (target pH > 7.25) to: - Reduce CNS toxicity - Promote urinary excretion of glycolate - Prevent cardiac dysrhythmias 2. **Hemodialysis:** Indicated when: - Serum ethylene glycol > 50 mg/dL (some guidelines say > 20 mg/dL) - Severe metabolic acidosis (pH < 7.25) - Acute kidney injury (as in this case) - Dialysis removes both ethylene glycol and toxic metabolites 3. **Calcium supplementation:** If hypocalcemia develops (from oxalate binding), give IV calcium gluconate 4. **Thiamine and pyridoxine:** May reduce oxalate formation (though evidence is limited) **Clinical Pearl:** The **osmolal gap** is a useful screening tool: $$\text{Osmolal gap} = \text{Measured osmolality} - \text{Calculated osmolality}$$ $$\text{Calculated osmolality} = 2[Na^+] + \frac{[Glucose]}{18} + \frac{[BUN]}{2.8}$$ An osmolal gap > 10 mOsm/kg suggests presence of unmeasured osmoles (ethylene glycol, methanol, isoniazid, etc.). **Mnemonic for toxic alcohols:** **MEAD** — Methanol, Ethylene glycol, Antifreeze, Diethylene glycol. All require alcohol dehydrogenase inhibition (fomepizole or ethanol) as first-line antidote. [cite:Harrison 21e Ch 473; KD Tripathi 8e Ch 28]
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