What are the clinical features of methanol toxicity?

Clinical Features of Methanol Toxicity

Methanol toxicity presents with a characteristic biphasic clinical course: initial mild CNS depression followed by a latent period of 12-24 hours, after which severe metabolic acidosis, visual disturbances ranging from blurred vision to complete blindness, and gastrointestinal symptoms develop. 1, 2

Initial Presentation (First Few Hours)

• Mild central nervous system depression with nausea, vomiting, and abdominal pain occur in the early phase 2
• Ataxia may be present as an early neurological manifestation 3
• Patients may appear relatively well during this initial period, which can lead to delayed recognition of the severity of poisoning 1

Latent Period

• A characteristic delay of approximately 12-24 hours occurs between ingestion and development of severe toxicity 1, 2
• The duration of this latent period depends on the methanol dose ingested and whether ethanol was co-ingested (which delays metabolism) 1, 2
• This delay reflects the time necessary for accumulation of toxic metabolites, primarily formic acid 4

Late Manifestations (After Latent Period)

Metabolic Derangements

• Severe high anion gap metabolic acidosis develops as formic acid accumulates 4, 5, 6
• Anion gap >27 mmol/L indicates severe toxicity requiring hemodialysis even with fomepizole treatment 3
• Elevated osmolar gap (>10-50 mOsm/kg) may be present, though this can be absent depending on timing of presentation and coingestion of ethanol 3

Visual Disturbances

• Ophthalmologic abnormalities range from blurred vision and altered visual fields to complete blindness 2
• Decreased visual acuity and potential permanent blindness result from formic acid interaction with the optic nerve 1
• Visual symptoms can appear anywhere from a few hours to a few days after exposure 4

Respiratory Manifestations

• Breathlessness occurs due to metabolic acidosis and compensatory hyperventilation 1
• Deficiency of oxygen in arterial blood may develop 1

Gastrointestinal Symptoms

• Abdominal pain persists or worsens in the late phase 1, 2
• Continued nausea and vomiting 2

Neurological Deterioration

• Permanent neurologic dysfunction can occur if treatment is delayed 5, 6
• Severe cases may progress to coma and death 1

Critical Diagnostic Pitfalls

• Both osmolar gap and anion gap can be absent depending on the timing of presentation, nature of exposure, and coingestion of ethanol 3
• The diagnosis is often elusive and requires a high index of suspicion based on history 5
• By the time patients present with severe symptoms, life-threatening metabolic acidosis may already be irreversible 1
• Methanol concentrations diminish in blood as they are metabolized, making diagnosis difficult if presentation is delayed 7

Multi-Organ Involvement

• Renal failure may develop, particularly with severe or prolonged toxicity 7
• Seizures and stupor can occur in severe cases 7
• The long-term prognosis for survivors is often poor, particularly in elderly males with comorbidities 1