Methanol (methyl wood, wood alcohol)

•used as solvent in paint removers and windshield washer fluids

• Also in gasoline antifreeze fluids (together with ethylene glycol), and as a denaturant

Toxicology

•Readily absorbed from the GIT

•Fatal dose by mouth is 60-250mls of pure methanol

•Ingestion of 30mls is reported to cause ocular toxicity in adults.

•Inhalation of methanol may also cause death.

•Prognosis improves if treatment is started before visual disturbances appear.

•Characteristic signs and symptoms develop in rapid succession after a latency period of 6 to 30 hours (usually 12 to 18 hours) following initial intake of pure methanol.

•Latency period is probably the time during which transformation of methanol to toxic metabolites occurs

MOA of toxicity

•Methanol metabolised to formaldehyde, which is further oxidised to formic acid.

•After initial inebriation, metabolic acidosis occurs and is due to formic acid. CNS depression is probably due to acidosis and cerebral oedema.

•Visual disturbances (dilated uncreative pupils and dimness of vision) occur about the time of acidosis and are due possibly to formate.

•Retinal damage occurs with inflammation followed by atrophy of predominantly ganglion cells.

•The result is bilateral blindness, unless treatment is prompt and energetic.

•Even if blindness is avoided, residual scotomata are common.

•Ingestion of methanol together with ethanol prolongs the latency period

Signs and symptoms

•Following acute poisoning:

•Mild symptoms include: fatigue, headache, weakness, nausea, blurred vision.

•Moderate symptoms include severe headache, dizziness, depression of CNS and temporary or permanent loss of vision.

•Severe symptoms occur with high doses and include; worsening of the above symptoms, occasional vomiting and diarrhoea, violent abdominal pain (due to pancreatitis), shallow respiration (due to acidosis), cyanosis, fall in blood pressure, coma and death in respiratory failure which occurs in a quarter of people who are severely poisoned

Management

•Induce emesis with ipecac syrup if within 2 hours of ingestion.

•Perform gastric lavage with 2-4l of water containing 20g/l of sodium bicarbonate; leave some in the stomach.

•Antidote; Ethanol, loading dose to achieve blood level of 100mg/dl

•4-Methylpyrazole (4-MP) is another alcohol dehydrogenase inhibitor  

Management :general measures

–Keep the patient warm.

–Administer fluids orally and intravenously to maintain good urine output.

–Ensure adequate nutrition by giving regular meals.

–Treat acidosis with sodium bicarbonate 5-10g orally, every hour until urine is alkaline.

–Oxygen and artificial respiration.

•Extracoporeal haemodialysis is a successful measure and recommended when large doses are ingested, when acidosis becomes severe (pH < 7.25 to 7.3), when visual disturbances commence, during renal failure, when blood formate levels are > 50mg/100ml or when methanol level exceeds 50mg/100ml. Where haemodialysis is not available, peritoneal dialysis may be considered

Laboratory monitoring

Monitor the following

•Serum amylase (rises due to pancreatitis).

•Blood alcohol and arterial PH

•Osmolal gap

•Serum K + (hypokalaemia may occur and be severe)

•Serum methanol (t1/2=8 hours)