Specific Poisonings

Methanol

Poisoning usually follows ingestion of contaminated alcohol beverages or 'methylated spirits'. Intoxication in industrial settings follows absorption across the skin or lung. It is metabolised by alcohol dehydrogenase to formaldehyde, which is itself rapidly oxidised to the principle toxin, formic acid.

Presentation

• Signifcant ingestion causes nausea, vomiting and abdo pain. Its effects on the CNS resemble those of ethanol although in low doses it does not have a euphoric effect.
• Visual symptoms present with falling visual acuity, photophobia and the sensation of 'being in a snow storm'.

Complications

• Up to 2/3rds of patients have a raised amylase (haemorrhagic pancreatitis has been reported at post mortem)
• Seizures are seen in severe intoxication. CT scanning usually shows cerebral oedema or even necrosis in the basal ganglia (a Parkinsonian-like state is sometimes seen with recovery).
• Patients with visual symptoms may develop irreversible visual impairment even with aggressive intervention.

Prognostic features

• Ingestion of only 10ml can cause blindness and 30ml can be fatal.
• Peak plasma methanol is useful although in practice the 'peak' is often unclear; >0.2g/1 (6.25 mmol/l) indicates significant ingestion and 0.5g/1 (15.6 mmol/l) is severe (c.f. driving limit for plasma ethanol of 17.4 mmol/L).
• Arterial pH seems to correlate best with formate levels and pH (< 7.2 is a severe intoxication).

Management

• Ipecac/gastric lavage is useful only if presenting within 2 hours of ingestion. Activated charcoal is not indicated.
• Seizures are probably best treated with phenytoin (250mg IV over 5 mins) since this will have less of a CNS depressant effect than diazepam NB exclude hypoglycaemia before giving any anticonvulsant.
• Specific antidote is ethanol and this should be given IV as a 10% solution in 5% dextrose A loading dose of 0.6g/kg should be given followed by an lVI of 0.07g/kg/hr for non-drinkers (regular drinkers should receive 0. 16g/kg/hr):
  
  Ethanol levels should be checked to ensure they are within the treatment range 1-1.5g/L (21.7-32.6 mmol/L). Ethanol should be given to: (1) all patients pending MeOH levels and then those >0.2g/L (6.25 mmol/L); (2) acidotic patients; (3) anyone needing haemodialysis.
  
  
• Methanol is most effectively cleared by haemodialysis (>peritoneal dialysis) and is reserved for those patients with renal failure, ANY visual impairment or a MeOH level of >0.5g/1 (15.6 mmol/L). The ethanol lVI rate should be doubled during HD or ethanol may be added directly to the dialysis fluid.
• 4-methylpyrazole (10-20mg/kg/d orally) has also been used as an inhibitor of alcohol dehydrogenase and has the advantage that unlike ethanol it does not cause CNS depression. It is not, however, generally available.
• Correct the metabolic acidosis with IV NaHCO3.

NB Because ethanol and severe MeOH overdose together prolong the half- life of MeOH to >30 hrs, the ethanol lVI should be continued for at least 48hrs.

ISOPROPANOL as a cause of poisoning with alcohols it is second after ethanol. It has twice the potency of ethanol on the CNS (its major metabolite, acetone, compounds this) and isopropanol-induced coma can last >24 hrs. Effects seen within 30-60mins of ingestion and large overdoses cause coma and hypotension as the major effect. HD is indicated if the hypotension fails to respons to IV fluids, vital signs decline or blood levels are >4g11 (66.7 mmol/l). Monitor for hypoglycaemia and myoglobinuria.