Methanol toxicity

Background

• Found in antifreeze, windshield washer fluid
• Volatile (as opposed to ethylene glycol)
  • Respiratory tract absorption can result in toxicity, esp. in infants
• Sweet-tasting
• Lethal dose:
  • Adult ~ 15mL of 40% methanol (lowest lethal dose reported)
  • Ped ~ 1.5mL of 100% methanol
• Blinding dose ~ 4mL of 100% methanol

Pathophysiology

• Methanol + alcohol/aldehyde dehydrogenase > formic acid
  • Optic neuropathy + putaminal necrosis (parkinsonism)

Work-Up

• Chemistry
  • Anion gap acidosis
• Serum Osm
  • Osm gap (measured - calculated)
    • Calculated serum osm = 2Na + BUN/2.8 + glucose/18 + ethanol/4.6
    • Normal is < 10
• Ethanol level
• VBG

Note:

• Cannot rule out toxic ingestion with a "normal" osmol gap
  • Only parent alcohol is osmotically active
    • Delayed presentation may mean that much of it is already metabolized

Clinical Manifestations

Symptoms begin typically 12-24hr after ingestion

• May occur later if ethanol is co-ingested

1. Intoxication signs (confusion, ataxia, depressed mental status, sz, coma)

• • Less inebriating than ethanol and ethylene glycol)

2. Visual disturbances (50% of pts)

• Development may precede or parallel that of other clinical symptoms
  • Cloudy or blurry vision ("stepping out into a snowstorm)
  • Central scotoma, complete vision loss

3. Anion-gap acidosis

• May be severe (bicarb < 5, pH < 7)
• Compensatory tachypnea

Treatment

1. Correction of metabolic acidosis with bicarbonate

• Bicarbonate 1-2mEq/kg IV bolus to attain pH = 7.45-7.50
  • Follow by infusion of 150mEq/L in D5 @ 1.5-2 times maintenance fluid rate
• Monitor for worsening hypocalcemia

2. ADH enzyme blockade

• Fomepizole (4-MP)
  • 15mg/kg followed by 10mg/kg q12hr x 4 doses
• Ethyl alcohol
  • BAL of 100-150 completely saturates alcohol dehydrogenase

3. Removal of parent alcohol and metabolites via dialysis

• Indications (controversial):
  • Severe acidosis (pH <7.30)
  • Visual symptoms
  • Electrolyte imbalances unreponsive to conventional therapy
  • Anion gap > 20

4. Other

• Folinic acid 50mg IV q4hr
  • Enhances formic acid metabolism

Source

Rosen's