Acute exposure Inhalation

The victim should be removed from exposure and all contaminated clothing removed. The respiratory function should be assessed. Further treatment is symptomatic and supportive. See below for management of systemic effects.

Dermal

Contaminated clothing should be removed and the skin thoroughly irrigated with water or saline. Further treatment is symptomatic and supportive. See below for management of systemic effects.

The eyes should be thoroughly irrigated with water or saline for 15 minutes and then stained with fluorescein. Referral to an ophthalmologist is recommended if there is any uptake of fluorescein.

Ingestion

Gastric lavage may be considered if ingestion was recent. Activated charcoal is of no benefit. See below for management of systemic effects.

Systemic effects

The electrolytes and blood gases should be monitored and corrected if required. Acidosis should be treated aggressively with bicarbonate; large doses may be required. Blood should be taken for determination of the anion and osmolal gap. As methanol has a latent phase before clinical effects are evident, all patients should be observed for at least 12 hours post exposure. If any clinical effects or biochemical abnormalities are noted, the patient needs to be admitted until asymptomatic.

While waiting for laboratory results, but after taking blood for determination of the methanol/formic acid concentration, anion and osmolal gap, all patients should receive a loading dose of ethanol (McCoy et al., 1979). Fomepizole may be used as an alternative to ethanol therapy (Brent et al., 2001). Folinic acid may be given to aid in the transformation of formic acid. Depending on the results of blood analyses, antidotal therapy may need to be continued. Blood ethanol concentrations of >1,000 mg/l are needed to prevent further formation of the toxic metabolites. The indications for continued antidotal therapy are a methanol concentration >200 mg/l, acidosis, increased osmolal gap (>10 mOsm/kg H2O) or a formic acid concentration >10 mg/l.

A formic acid concentration of >500 mg/l justifies administration of ethanol and possibly haemodialysis (Mahieu et al., 1989).

Haemodialysis is indicated if the patient has a severe or unresponsive acidosis, ocular signs, or methanol or formic acid concentrations >500 mg/l. Although less effective, peritoneal dialysis has been used and could be considered where haemodialysis facilities are not available. Dialysis should be continued until the methanol concentration is <200 mg/l. During haemodialysis, ethanol treatment should be continued, but as ethanol is also readily dialysed, it will be necessary to increase the dosage by 100 mg/kg/hour, or more, to maintain a blood ethanol concentration of 1,000-1,500 mg/l. It is preferable in this situation to add ethanol to the dialysate to achieve a concentration of 1,000 mg/l (Chow et al., 1997).

Osmolal and anion gap

Blood should be taken to estimate methanol and formic acid concentrations. In many situations an assay for methanol/formic acid (formate) is unavailable routinely. In these cases the calculation of the anion and osmolal gaps can provide an early guide to diagnosis. Formic acid concentrations in blood and urine correlate well with the severity of toxic effects as seen in experimental animals and clinical studies. This is not the case for methanol concentrations (Mahieu et al., 1989). Higher formic acid concentrations and wider anion gaps appear to correlate with the development of sequelae and may be indicative of a poorer prognosis (Barton-Burns et al., 1998). It has been suggested that a time interval between ingestion and treatment exceeding 10 hours and a blood formic acid concentration above 500 mg/l are predictive of severe methanol poisoning possibly leading to permanent sequelae (Mahieu et al., 1989).

The anion gap is a measure of organic acids. It is elevated in the presence of organic acids such as formic acid. If the measured osmolality is more than the calculated osmolality by greater than 10 mOsm/kg H2O, this indicates the presence of an unmeasured substance, which may be methanol (Church and Witting, 1997). Although the serum osmolal gap is useful if elevated, a normal osmolal gap does not rule out toxicity. The osmolal gap may be normal once the methanol has been converted to its metabolites. An elevated osmolal gap may indicate the presence of methanol, and the anion gap may indicate the presence of the metabolite, formic acid.

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