Introduction to Methanol Poisoning

Overview

Methanol is a highly toxic alcohol and, like ethylene glycol, is commonly found in and around the home where it may be ingested, resulting in serious poisoning. Common sources of methanol include windshield washer solvent, gas line antifreeze, fuels for small stoves and paint strippers. ² Occasionally, methanol is intentionally used to fortify low-quality liquor, leading to large outbreaks of methanol poisoning. ⁷ Methanol poisoning usually results from oral ingestion but, because of its volatility, toxicity by inhalation can also occur. ⁸ Similar to ethylene glycol, methanol poisoning must be aggressively treated to prevent serious morbidity and mortality. Prompt treatment of early-presenting, asymptomatic patients generally results in complete recovery. ³

topOccurrence

Like ethylene glycol, the actual number of methanol exposures may be greater than the number of cases reported to poison control centers each year. In 2005, more than 2,300 exposures to methanol were reported in the U.S., resulting in 13 fatalities. ¹

topMechanism of Toxicity

While methanol itself has a relatively low degree of toxicity, it is metabolized to formic acid, which is responsible for the acidosis and blindness that characterizes methanol poisoning. The initial step in the metabolism of methanol occurs by the action of alcohol dehydrogenase (ADH). ADH slowly oxidizes methanol to formaldehyde, which is then rapidly oxidized by formaldehyde dehydrogenase to yield formic acid. Finally, formic acid is metabolized to carbon dioxide and water, which are excreted by the kidneys and lungs. Metabolic acidosis results from the accumulation of formic acid as well as lactic acidosis which is caused by the inhibition of cellular respiration by formic acid. ⁶

The ocular injury caused by methanol may result from intra-retinal metabolism of methanol and the accumulation of formic acid or from the inhibition of normal metabolism in optic nerve cells. ⁶

topClinical Course

Although initial symptoms of methanol poisoning may appear as early as 12 hours, they usually do not occur until 24 hours after ingestion and may resemble ethanol intoxication: visual disturbances, drowsiness, confusion, ataxia, weakness, headache, nausea, vomiting and abdominal pain. Collectively, these symptoms may mimic an alcohol hangover and are caused by methanol and its toxic metabolites. ⁹

As methanol metabolism proceeds, severe anion gap metabolic acidosis develops. Metabolic acidosis combined with visual disturbances represents the primary features of methanol poisoning. Visual changes described by patients include blurred or misty vision, double vision or changes in color perception, constricted visual field and, occasionally, total loss of vision. Signs of ocular toxicity include papillary dilation and loss of pupillary reflex. ¹⁰

As the severity of metabolic acidosis increases, there may be mild to profound memory loss, confusion and agitation, progressing to stupor and coma. ¹⁰ Other signs and symptoms of methanol poisoning consist of shallow respiration, cyanosis, tachypnea, coma, seizures, electrolyte disturbances and various hemodynamic changes including bradycardia and profound hypotension. Surviving patients may be left with permanent blindness or other neurological deficits. ⁹