Background
Methanol, also known as wood alcohol, is a commonly used organic solvent that, because of its toxicity, can cause metabolic acidosis, neurologic sequelae, and even death, when ingested. It is a constituent of many commercially available industrial solvents and of poorly adulterated alcoholic beverages. Methanol toxicity remains a common problem in many parts of the developing world, especially among members of lower socioeconomic classes. (See Etiology and Pathophysiology and Presentation.)
Sophisticated imaging techniques have enabled a better understanding of the clinical manifestations of methanol intoxication. Additionally, neurologic complications are recognized more frequently. This is possible because of early recognition of the toxicity and because of advances in supportive care. Hemodialysis and better management of acid-base disturbances remain the most important therapeutic improvements. (See Workup, Treatment, and Medication.)
According to a study by Jaff et al, methanol intoxication can lead to several ECG changes, with sinus tachycardia and non-specific T-wave changes being the most common. In the study, the changes were more prominent in cases of severe acidosis. A retrospective chart review of 9 patients between 2006 and 2011 revealed that lower pH and higher plasma methanol concentration were associated with multiple ECG changes. On admission, ECG changes included sinus tachycardia (44%), PR prolongation (11%), QTc prolongation (22%), and non-specific T-wave changes (66%). One patient developed a type-1 Brugada ECG pattern.
According to Zakharov et al, S-formate measurement can help in the laboratory diagnosis and clinical management of acute methanol poisoning. In their study of 38 patients from a Czech methanol mass poisoning in 2012, S-formate levels ≥3.7 mmol/L were seen to lead to the first clinical signs of visual toxicity, indicating hemodialysis. S-formate ≥11-12 mmol/L was associated with visual/CNS sequelae and a lethal outcome. The probability of a poor outcome (death or survival with sequelae) was higher than 90% in patients with S-formate levels ≥17.5 mmol/L, S-lactate levels ≥7.0 mmol/L, and/or pH < 6.87.
Complications
Vision loss
The mechanism by which the methanol causes toxicity to the visual system is not well understood. Formic acid, the toxic metabolite of methanol, is responsible for ocular toxicity in animal models and is rightly presumed to be responsible in human studies. (See Etiology and Pathophysiology.)
Serum methanol levels of greater than 20 mg/dL correlate with ocular injury. Funduscopic changes are notable within only a few hours after methanol ingestion and range from retinal edema in the perimacular region to the entire fundus. Optic disc edema and hyperemia are observed within 48 hours.
Visual injury may be prevented with prompt antidote therapy or via elimination of the metabolites from the system with hemodialysis; however, this is not always the case. (See Treatment and Medication.)
Movement disorders
Parkinsonian motor impairment has been described in some long-term survivors of methanol poisoning. This is thought to be due to formic acid’s predilection for accumulating in high concentrations within the putamen, but the reasons for this phenomenon are unclear. One proposed reason is that formic acid has the ability to impair dopaminergic pathways and increase enzymatic activity of dopa-B-hydroxylase. (See Etiology and Pathophysiology.)
Symptom onset is usually delayed several weeks after methanol exposure. Common parkinsonian symptoms, such as tremor, cogwheel rigidity, stooped posture, shuffling gait, and hypokinesis, have been well described. In addition, the development of dystonia and corticospinal tract signs has been established.
Several case reports have indicated symptom response to standard antiparkinsonian agents, particularly levodopa, amantadine, and bromocriptine.
Muscle spasms have also been reported in methanol poisoning. As expected, this symptom responds poorly to traditional therapy.
Rarely, lesions in the lobar regions of the cerebrum and cerebellum have been observed.
Prognosis
The prognosis in methanol poisoning correlates with the amount of methanol consumed and the subsequent degree of metabolic acidosis; more severe acidosis confers a poorer prognosis. The prognosis is further dependent on the amount of formic acid that has accumulated in the blood, with a direct correlation existing between the formic acid concentration and morbidity and mortality. Little long-term improvement can be expected in patients with neurologic complications. (See Treatment and Medication.)
The minimal lethal dose of methanol in adults is believed to be 0.3-1 g/kg of body weight.
The exact rates of morbidity and mortality from methanol intoxication are not available.