Recognition and Diagnosis

Diagnosis

The more rapidly ethylene glycol or methanol poisoning can be diagnosed, the more quickly antidotal treatment can be initiated and the greater the likelihood that the therapy will be beneficial.

Unfortunately, many of the signs and symptoms associated with ethylene glycol poisoning, such as nausea, vomiting and CNS depression, are highly nonspecific and similar to those of many other poisonings or illnesses. Patients are often unwilling or unable to provide a history of ethylene glycol poisoning as they may be confused, distressed or even comatose. ⁶ Similarly, the early signs and symptoms of methanol poisoning are non-specific and include drowsiness, confusion, headache, nausea and vomiting. When a specific history is unavailable because the patient is unable or unwilling to report having ingested methanol, diagnosis may be difficult. ⁹

topDiagnosis of ethylene glycol poisoning

The direct measurement of plasma ethylene glycol concentration represents the most reliable method of diagnosing ethylene glycol poisoning; however, it must be remembered that there is an inverse relationship between plasma ethylene glycol concentrations and the severity of clinical symptoms. That is, an asymptomatic patient may present with very high plasma ethylene glycol concentrations soon after ingestion. In contrast, the plasma ethylene glycol concentration may be very low or absent in a critically ill patient because the ethylene glycol has been metabolized, but the concentration of toxic metabolites is high. ⁴

A plasma ethylene glycol concentration >20 mg/dL soon after ingestion generally indicates the need for antidotal therapy; however, any concentration of ethylene glycol in the presence of systemic toxicity should be treated in late-presenting patients. ⁴ Unfortunately, plasma ethylene glycol determinations are not routinely available in most hospitals and may result in a significant delay in diagnosis and initiation of treatment. In those situations, other laboratory indicators such as metabolic acidosis ⁶ or changes in the anion or osmolal gap ¹¹ may provide valuable information for diagnosis and management.

As many as 50% of ethylene glycol-poisoned patients exhibit urinary calcium oxalate crystals (oxaluria) upon admission. ⁶ If other analyses are not available, microscopic examination of the urine may be useful in the differential diagnosis of metabolic acidosis of unknown origin.

The indications for the treatment of ethylene glycol poisoning are summarized in the diagnostic chart.

topDiagnosis of methanol poisoning

Ideally, any suspicion of methanol poisoning would be confirmed by measuring the serum methanol concentration. A serum methanol concentration greater than 20 mg/dL soon after ingestion generally indicates the need for antidotal therapy; however, in late-presenting patients, any concentration of methanol in the presence of systemic toxicity should be treated.⁹ Unfortunately, serum methanol determinations are sometimes unavailable in hospital laboratories, resulting in a significant delay in diagnosis and initiation of treatment. In those situations, calculation of the anion gap and osmolal gaps may be beneficial.

The decision to perform a serum methanol determination may be based on the presence of an anion gap metabolic acidosis. The presence of an osmolal gap may further support the diagnosis of methanol poisoning; however, its absence does not rule it out, as the osmolal gap will diminish as methanol metabolism proceeds.^* Other diagnostic clues include ophthalmic changes such as hyperemia, edema or pallor of the optic disc. ⁶

The indications for the treatment of methanol poisoning are summarized in the following diagnostic chart.

* An anion gap metabolic acidosis is not immediately seen following ingestion of methanol. This may be due to other types of poisonings including iron, salicylates, ethylene glycol or other alcohols. In addition, disease states such as diabetic ketoacidosis or uremia will produce an osmolal gap as well. Finally, the coingestion of ethanol may produce a confusing clinical picture as the toxic effects of methanol may be masked or delayed. As with ethylene glycol poisonings, the clinical complexity of toxic exposure to methanol emphasizes the importance of consulting a regional poison control center regarding the diagnosis and treatment of these poisonings.