Studies on the Visual Toxicity of Methanol*

Folate as an Adjuvant Therapy in Methanol Poisoning

Folate and its derivatives have long been used as an adjunctive treatment in methanol poisoning. Methanol is ultimately metabolized to formate, the toxic compound. The accumulation of formate can lead to acidemia, retinal damage, visual impairment, and death. Formate is converted to carbon dioxide and water in a folate-dependent manner, and folate is often given in cases of methanol poisoning. In this paper, the evidence for folate as an adjunctive therapy in methanol poisoning is reviewed.

Comparative lethality of methanol, ethanol and mixtures in female rats

The lethalities of pure methanol and pure ethanol were compared to two mixtures of ethanol/methanol with the following percentages (95/5% and 65/35% v/v). This study was conducted to simulate situations of human exposure to denaturated alcohol (by 5% methanol) or adulterated alcohol (by 35% methanol). Four groups of female adult virgin albino rats were treated with the four mixtures. A fifth group was used as a vehicle control. Graded oral doses were given to eight animals per dose. Lethality over 24 h was used as an end-point. The LD50 was calculated for each of the four treatments on a molar basis. A dose-response function for each mixture was plotted of percentage lethality vs. mmol kg-1 equivalent to the given ml kg-1 dose. Results showed a significantly different LD50 estimates (P less than 0.03) for the four mixtures. The order of lethal toxicity was as follows: 95/5% methanol/ethanol, pure methanol, pure ethanol then 65/35% methanol/ethanol. Slope comparisons indicated two pairs: 65/35% ethanol/methanol and pure ethanol yielding a steep slope, and 95/5% ethanol/methanol and pure methanol yielding a shallow slope. These data indicated that the acute lethality of ethanol/methanol mixtures is a complex unpredictable function. This toxicity presumably depends in a complicated way on the differences in the effective molecular weights of the two alcohols in each of the mixtures.

The toxicity of inhaled methanol vapors

Methanol could become a major automotive fuel in the U.S., and its use may result in increased exposure of the public to methanol vapor. Nearly all of the available information on methanol toxicity in humans relates to the consequences of acute, rather than chronic, exposures. Acute methanol toxicity evolves in a well-understood pattern and consists of an uncompensated metabolic acidosis with superimposed toxicity to the visual system. The toxic properties of methanol are rooted in the factors that govern both the conversion of methanol to formic acid and the subsequent metabolism of formate to carbon dioxide in the folate pathway. In short, the toxic syndrome sets in if formate generation continues at a rate that exceeds its rate of metabolism. Current evidence indicates that formate accumulation will not challenge the metabolic capacity of the folate pathway at the anticipated levels of exposure to automotive methanol vapor.