Methanol Toxicity

OVERVIEW

• industrial solvent and synthetic precursor
• highly variable lethal dose
• in itself if is not very toxic, however metabolised to formaldehyde -> formic acid (neurotoxic – retina and optic nerves)

CLINICAL FEATURES

• drunk: windshield fluid, antifreeze, paint, paint removers, gasoline, adhesives, glass cleaner, a solvent/cleaner.
• symptoms develop 12-24 hours post ingestion
• neurology: cerebral oedema, seizures, meningeal irritation, cerebral infarction, blurred vision -> blindness, scotomata, papilloedema, loss of light reflexes
• respiratory failure
• circulatory shock
• GI symptoms

INVESTIGATIONS

• raised osmolar gap
• severe metabolic acidosis (raised anion gap)
• methanol level (not useful acutely due to availability and turn around times)
• normal ionized Ca2+

MANAGEMENT

• decreased production of toxic metabolites:

-> ethanol (competitive inhibitor to alcohol dehydrogenase)
-> 4-methylapyrazole (same as ethanol but easier to titrate and no sedative effects)
-> sodium bicarbonate (undissociated formic acid more toxic than the dissociated product – give if pH < 7.3)
-> folinic acid: 50mg Q4hrly (decreases formate levels and decreases toxicity)

• increase elimination:

-> haemodialysis: effectively and rapidly removed including toxic metabolites