We look at evaluating the patient with encephalopathy and unexplained anion gap, including the workup and treatment of toxic alcohol poisoning, with guest Dr. Jerry Snow (@ToxicSnowEM), medical toxicologist and director of the toxicology fellowship at Banner University Medical Center in Phoenix.
- A toxicologic exposure should be suspected, even without a clear story, based on the prehospital scene. EMS or family reports of chemicals, pill bottles, etc should be elicited. Prescribed medications should be questioned, as well as any other meds that could be available to the patient, such as older meds, current and older meds prescribed to family members, and supplements.
- Physical exam maneuvers high-yield for tox diagnosis include the pupillary exam, skin exam (diaphoretic vs dry), and examination of muscle tone and deep tendon reflexes.
- Laboratory clues of tox diagnoses include an elevated anion gap in the absence of common causes (lactate, ketones, uremia), as most of the remaining causes of a gap are toxins.
- Elevated osmolal gaps should also be investigated, although considered an insensitive test for most toxins. A serum chemistry, as well as salicylate and acetaminophen levels, should be sent routinely. An ECG should be checked for findings like interval prolongation and morphology changes.
- “Normal” osmolality varies too much for a low osm gap to be useful, but a clearly elevated gap is diagnostically helpful, particularly when its presence/absence is compared with the presence/absence of an anion gap.
- The most common source of methanol ingestion in the US is windshield wiper fluid; it’s also present in poorly-distilled homemade moonshine, hand sanitizer, model car fuel, food-warmer fuel, lacquer and paint thinner, and many others. For ethylene glycol, the most common US source is automotive antifreeze. In both cases, these are usually intentional ingestions.
- Toxic alcohol levels, namely methanol and ethylene glycol levels, are send-out tests in most centers and result too slowly to be useful in the early stages. You will need to treat empirically based on suspicion and perhaps based on osmolar gap.
- Urine tox screens rarely change management, and may lead to missed diagnoses due to anchoring. Many substances are not tested, and positive tests (e.g. for opioids or benzodiazepines)—even for substances that may explain the clinical picture—can be false positives. Even true positives do not rule out the presence of another medical or even a second toxicologic cause. Correlate cautiously with the clinical picture (e.g. opioid toxicity may not explain encephalopathy in a patient with normal pupils and hyperventilation), or simply don’t send it to begin with.
- Acute iron overdose can cause anion gap acidosis, GI symptoms including bleeding, and shock and an overal critically ill presentation.
- Ethanol has fallen out of favor for treatment of toxic alcohols, although it does work; it is logistically challenging, requiring frequent lab checks to ensure therapeutic levels, central venous access, and other fuss; complications are much higher than with fomepizole. It’s good for low-resource settings that may not have the more expensive fomepizole, however, and co-ingestion of ethanol with toxic alcohols provides some fortuitous initial protection until the ethanol level falls.
- Ethylene glycol and methanol are not themselves toxic, but as the parent alcohols are metabolized, they turn into toxic acids. The goal of fomepizole or ethanol is therefore to block this conversion (by alcohol dehydrogenase). This also means that if checked early after ingestion, osmolar gap will be high, but anion gap is low, as only the parent compounds are active osmoles. As metabolism continues, osmolar gap falls, but the anion gap increases. One upside of treatment with hemodialysis is that it clears both the parent alcohol and the toxic metabolites, so it’s helpful even in late presentations.
- Toxic alcohols may confuse testing for lactate. Some methods, mainly used on blood gas analyzers that report lactate, can be fooled by glycolate—a metabolite of ethylene glycol—and report a falsely elevated lactate. The same sample tested in the lab using another method may show a lower lactate. This “lactate gap” can be diagnostically useful if understood.
- A normal fomepizole course is two days, dosed every twelve hours, but monitoring should be done of either methanol/ethylene glycol levels (if lab turnaround is fast), or monitoring the pH, anion gap, and osm gap for response. If not resolved, a longer treatment course may be needed, and dose may need to be increased, as it induces its own metabolism.
- Hemodialysis may be used in the sickest patients, as a rescue, if pH is severely deranged, or if there is severe kidney injury, since renal clearance is needed to clear ethylene glycol. Fomepizole should usually still be given to temporize until treatment is completed, and may need to be dosed more frequently during dialysis as it is a dialyzable compound. A single prolonged HD session (eg 8 hours) is often adequate, and HD is superior to CRRT.
- Thiamine and pyridoxine (vitamin B6) can be given to help shunt toxic alcohols to benign metabolites, although evidence for this is fairly poor. Other supportive care is as routine.
- If acute toxicity is survived, ethylene glycol patients usually do well, although they occasionally have calcium crystal deposition in nerves and develop cranial nerve palsies or peripheral neuropathy. Methanol patients tend to do worse, sometimes developing permanent blindness and CNS pathology like delayed intracranial hemorrhage or Parkinsonism.
- Every hospital in the US has a poison control center available to them as a resource, which includes an on-call medical toxicologist who can discuss cases if needed. They are available even to review med lists and assist with diagnostic mysteries. The most common error in tox cases is the failure to consider a tox diagnosis!