Antifreeze toxicosis (Proceedings)
Methanol (also known as methyl alcohol or wood alcohol) is found most commonly in "antifreeze" windshield washer fluid and varies in concentration from 20-100% (with 20-30% being the most common form). Methanol's metabolite, formaldehyde, is rapidly oxidized by the enzyme aldehyde dehydrogenase to formic acid, which can cause metabolic acidosis if significant quantities of methanol are ingested. Formic acid is subsequently converted to carbon dioxide and water. In humans and some non-human primates, formic acid is inefficiently metabolized and accumulates causing injury to the retina and central nervous system. Even very small amounts of methanol can be extremely dangerous to humans, but this is not an issue in non-primates. The minimum lethal dose in dogs is 8.0 ml/kg of 100% methanol.
According to the ASPCA Animal Poison Control Center database, the most common methanol exposures occur in dogs and usually involve chewing on containers or lapping up spills. In general, alcohols are rapidly absorbed from gastrointestinal tract, so decontamination needs to be done within the first 20-30 minutes of an oral exposure to be effective. With small exposures in dogs and cats, only mild gastric upset may be seen. Larger exposures result in vomiting, diarrhea, ataxia, disorientation (inebriation), depression, hypothermia, tremors and dyspnea; in severe cases seizures, bradycardia or ventricular premature contractions, metabolic acidosis, coma, and respiratory depression may occur. Death is usually due to respiratory depression, hypothermia, or aspiration.A recent small ingestion should be treated with dilution (milk or water) to minimize gastric upset. Large ingestions warrant decontamination via emesis if less than 30 minutes have passed and the animal is asymptomatic. Activated charcoal is of questionable value in binding small alcohols and is generally not used. Symptomatic animals should be monitored until recovered. Treatment may entail IV fluids to enhance elimination of methanol, thermoregulation, correction of acid/base imbalances, management of cardiac abnormalities, ventilatory support, and seizure control (anticonvulsants should be used with care as they can worsen the CNS depression). Anecdotally, yohimbine has been used to aid in reversal of coma in alcohol-intoxicated dogs. Given prompt and aggressive care, most dogs and cats will recover within 4-36 hours.
Propylene glycol is the primary ingredient in some forms of automotive antifreezes/coolants, as well as many recreational vehicle antifreezes (be sure to check the label). Propylene glycol is also present in a variety of food, pharmaceutical, and cosmetic agents, generally at low concentrations (generally less than 12-15%). Propylene glycol in injectable medications or activated charcoal formulations may interfere with ethylene glycol test kits (i.e. result in false-positives, see ethylene glycol below). Products containing less than 20-30% propylene glycol should not pose an acute toxicity hazard to pets if ingested. Newer automotive antifreezes containing 50% or more propylene glycol (i.e. Prestone's Low Tox and Sierra) are indeed "safer" than ethylene glycol-based antifreezes in that they will not cause the serious kidney damage that is seen with ethylene glycol toxicosis. However, it is important to remember that ingestion of propylene glycol-containing antifreezes may result in serious intoxication similar to that seen with other alcohols. A dog given propylene glycol at 10 ml/kg displayed no clinical signs; however the LD50 (dose at which 50% of experimental dogs died) in dogs is 22 ml/kg. Therefore, doses over 10 ml/kg of 100% propylene glycol should be considered potentially toxic and justify veterinary intervention.
Like methanol and ethylene glycol, propylene glycol is rapidly absorbed from the gastrointestinal tract. Decontamination must therefore be accomplished within the first 20-30 minutes to be of much benefit. Clinical signs can occur as early as 20 minutes following ingestion. Due to the risk of aspiration of vomitus, induction of vomiting is NOT recommended in animals that are already symptomatic. As with other alcohols, propylene glycol can cause severe intoxication, ataxia, inebriation, and metabolic acidosis (see methanol clinical signs above). Treatment is be similar to that described for methanol toxicosis—fluids to promote excretion, treatment of CNS effects (seizures, coma) as needed, thermoregulation (hypothermia is a significant risk in comatose patients), and management of metabolic acidosis. As with methanol, the overall prognosis is generally good provided the animal receives prompt and appropriate veterinary care. Recovery usually occurs within 24-36 hours of ingestion.