History
A careful history should be taken in high-risk patients who report typical symptoms of methanol poisoning.
Time course
Initial symptoms generally occur 12-24 hours after ingestion. The interval between ingestion and the appearance of symptoms correlates to the volume of methanol ingested and the amount of ethanol concomitantly ingested; competitive inhibition exists between the 2 compounds. [10]
Methanol blood levels peak at 30-90 minutes following ingestion and often do not correlate to the time to symptom appearance.
Neurologic manifestations
Initially, the symptoms of methanol intoxication are similar to those of ethanol intoxication, often with disinhibition and ataxia. Following a latent period, patients may develop headache, nausea, vomiting, or epigastric pain. In later stages, drowsiness may rapidly progress to obtundation and coma.
Seizures may occur, generally as a complication of the metabolic derangement or as a result of damage to the brain parenchyma.
Cases of axonal polyneuropathy in association with chronic exposure have been reported. [15] Further, motor neuron disease resembling amyotrophic lateral sclerosis has been documented in a case report. [16] It is likely that neuropathies and spinal cord dysfunction are underestimated.
Vision loss
Blindness from methanol inhalation was described as early as 1910. Formic acid accumulates within the optic nerve, which results in the classic visual symptoms of flashes of light and blurring.
Patients initially may present with diminished visual acuity, which can progress to scotomata and scintillations. The frank blindness that develops sometimes responds to immediate therapy; however, complete loss of vision is a common sequela.
Physical Examination
Physical examination helps to rule out other causes of altered mental status and visual dysfunction, the 2 most common presenting signs of methanol intoxication. In cases of altered mental status and intentional overdose, the diagnosis of methanol intoxication may be difficult without a high clinical index of suspicion.
General physical examination
During the initial phase of methanol poisoning, individuals may experience effects similar to inebriation with alcohol and thus may not seek medical attention. As symptoms develop, most signs are related to metabolic acidosis; these are manifested as tachycardia, tachypnea, hypertension, and altered mental status. Pulmonary edema and acute respiratory distress may ensue, requiring intubation.
With large ingestions of methanol, depressed cardiac contractility heralds circulatory collapse and leads to signs of heart failure, cardiac arrhythmias, or both.
Neurologic examination
In addition to the progression of symptoms from drowsiness to stupor to coma, ocular findings in patients with methanol poisoning are prominent during a careful neurologic examination.
Visual symptoms necessitate a thorough examination of the fundi. Optic disc hyperemia occurs early in the course of methanol intoxication. Pupillary response to light is compromised and, subsequently, is lost. Little or no retinal damage is observed.
