Methanol poisoning

Introduction

Poisons are chemical or physical agents that produce adverse responses in biological systems.
Poisoning on the other hand is the ingestion of, or exposure of a patient to excessive doses of a medicine or other substances that may cause harm.
Methanol is toxic to humans when ingested.
It is commonly found in many
industrial solvents and in adulterated
alcoholic beverages. Methylated spirit
contains 10% methanol.
Other sources are: automotive windshield washer fluids, paint thinners and removers, varnishes, copy machine fluids, and fuel additives (octane boosters).
Toxicity of methanol is attributed to its
metabolism to formaldehyde and formic
acids which are toxic and reactive
metabolites

Symptoms and clinical features of methanol poisoning

After methanol ingestion, there is usually an initial lag period of 12-24 hours before toxic manifestation occur.
The symptoms of the poisoning include the following:

  1. Visual disturbance:
    • dimming or blurring of vision
    • scintillations
    • photophobia
    • visual field defects and blindness may occur.
    • Fundoscopy may show enlarged blind spot
  2. Gastrointestinal tract symptoms:
    • Nausea
    • vomiting
    • epigastric pain, and
    • pancreatitis
  3. Others symptoms of methanol poisoning:
    • dizziness
    • headache
    • malaise
    • agitation
    • generalized weakness, and
    • sensorineural depression

Laboratory findings following methanol ingestion/poisoning

  1. Elevated anion gap
  2. Metabolic acidosis
  3. Elevated osmol gap
  4. Positive serum methanol assay
  5. CT scan may show characteristic finding of bilateral cerebral infarction selectively involving the putamenand adjacent areas

Treatment for methanol poisoning

1. Fomepizole

Fomepizole is the antidote of choice for methanol poisoning.

Adult:

Loading dose: 15 mg/kg intravenous infusion over 30 min,

then

10 mg/kg intravenously 12 hourly for 4 doses,

then

increase to 15 mg/kg 12 hourly.
Treatment should be continued until methanol serum levels are less than 20 mg/dL

Child:

Loading dose: 15 mg/kg intravenously,

then

10 mg/kg intravenously 12 hourly for 4 doses

then

15 mg/kg intravenously 12 hourly. Continue until methanol levels reduce to <20 mg/dL

2. Ethanol

Ethanol is used when it is impossible to use Fomepizole, due to unavailability, cost or hypersensitivity.

3. Hemodialysis

In severe poisoning, hemodialysis removes methanol. This can be combined with Fomepizole usage.