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Methanol Poisoning

Methanol is the simplest of alcohols, with the chemical formula CH3OH.  It is also known as wood alcohol, wood spirit, wood naphtha, carbinol, or methylhydrate.  It is a common cause of poisoning, with approximately 1000 cases per year reported to the American Association of Poison Control Centers. Methanol resembles ethanol in taste, odor and intoxicating properties and is less expensive to purchase. It is readily available because it is a major constituent of many household and commercial projects including windshield washing fluid, duplicating fluid, nonpermanent antifreeze, glass cleaners, solvents, paint remover and embalming fluid. 

Ingested methanol is completely absorbed, reaching peak blood concentration about 30 to 60 minutes after ingestion. After absorption, it is distributed in total body water (0.6 L/kg) and may be concentrated in the vitreous humor and cerebrospinal fluid. Toxic exposure can also arise from inhalation or prolonged skin contact. 

The toxic dose of methanol is highly variable. The fatal dose is commonly stated to be 100 mL, but some patients have died after drinking 6 mL and others have survived after drinking 500 mL. Usually there is a lag time of 12 hours between ingestion and onset of symptoms.  Symptoms include nausea, vomiting, abdominal pain, visual disturbances, headache, generalized weakness, seizures, CNS depression and coma.  The most worrisome long-term complication in survivors is blindness. 

Approximately 20% of methanol is eliminated unchanged by the lungs and kidneys.  The remainder is metabolized in the liver.  Alcohol dehydrogenase converts methanol to formaldehyde, which is almost immediately converted to formic acid by aldehyde dehydrogenase.  Most of the pathologic effects of methanol ingestion are due to accumulation of formic acid.  It inhibits aerobic metabolism and increases anaerobic glycolysis and lactate production, causing a severe metabolic acidosis. Blood pH often ranges between 6.8 and 7.3. Poorer outcomes are associated with more severe acidosis.

Most hospital laboratories do not measure methanol levels. However, other laboratory tests are helpful in making the diagnosis of methanol poisoning.  Electrolytes reveal a low bicarbonate and elevated anion gap, consistent with metabolic acidosis.  Measurement of serum osmolality and calculation of the osmolal gap are also useful.  An osmolal gap greater than 10 mOsm/kg is consistent with ingestion of a foreign substance (see Osmolality for further details). Osmolal gap progressively decreases as methanol is metabolized to formaldehye and formic acid but the anion gap increases.

According to the American Academy of Clinical Toxicology guidelines, hemodialysis should be considered in patients with metabolic acidosis  (pH <7.3), methanol level of 50 mg/dL or higher, visual disturbences, renal failure or refractory electrolyte imbalance. Other treatment modalities include competitive inhibition of alcohol dehydrogenase using ethanol or fomepizole. 

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