Methanol causes severe anion gap metabolic acidosis – a case study

This recently published case study report provides a reminder that blood gas analysis plays an important role in the emergency room assessment of unconscious patients and that disturbance of acid-base homeostasis can be attributable to ingestion of toxic chemicals.

The case concerns a 37-year old non-insulin dependant diabetic lady who had a history of alcohol abuse.

She was brought to the emergency department of her local hospital in a comatose, unresponsive state (Glasgow Coma Score 3/15) that prompted intubation and mechanical ventilation.

Initial blood testing, which included blood gas analysis, revealed a severe metabolic (lactic) acidosis (pH 6.72; bicarbonate 5 mmol/L; and lactate 9.2 mmol/L).

The lady’s diabetic history, severe acidosis, increased blood glucose (20 mmol/L) and anion gap (29 mmol/L) initially suggested to the clinical staff that her comatose state was due to diabetic ketoacidosis, but this was excluded by the finding of normal blood ketone concentration (-hydroxybutyrate 0.28 mmol/L).

A correct presumptive diagnosis of methanol intoxication was made following measurement and calculation of plasma osmolality. The difference between measured (416 mOsmol/kg) and calculated osmolality (300 mOsmol/kg) was 116 mOsmol/kg.

This massive osmolal gap suggests the presence in blood of a toxic osmotically active substance (e.g. alcohol) that is not accounted for in the calculation of osmolality.

Since ethanol was not detectable in the lady’s blood, the assumption was made that the cause of the increased osmolal gap was methanol.

Blood collected at this time but measured in a distant reference laboratory later confirmed the presence of methanol (concentration 237 mg/dL).

Without the benefit of this confirmatory evidence, drug treatment for methanol intoxication was administered.

Following this, and a 6 hour session of hemodialysis to correct severe acidosis, the lady soon recovered consciousness and was successfully extubated 2 days after admission. She admitted taking large amounts of windshield wiper fluid - which contains methanol - as a substitute for ethanol.

Although she had some residual neurological deficit attributable to the neurotoxic effect of methanol, she was discharged from hospital 3 days later.

In discussion of this case history, the authors outline the differential diagnosis of high anion gap metabolic acidosis, and describe the toxic affects of methanol and how its metabolic product (formic acid) gives rise to the high anion gap metabolic acidosis evident in their patient.

There is also a brief discussion of the mechanism of the drugs (fomepizole and folate) that were used to successfully treat methanol intoxication.