Methanol serum osmolality

Table 58-2 lists the concentration and expected contribution to the serum osmolality in ethanol, methanol, ethylene glycol, and isopropanol poisonings. 

Intoxication may present as inebriation and drowsiness similar to ethanol use. Other symptoms are vomiting, diarrhea, delirium and agitation, back and abdominal pain, and clammy skin. Toxic effects usually follow a latent period of several hours. Formate inhibits mitochondrial cytochromes resulting in neurotoxicity. Ocular signs include blurred vision, dilated pupils, and direct retinal toxicity with optic disc hyperemia and ultimately permanent blindness [91]. Cerebral hemorrhagic necrosis has been reported [92]. Severe poisoning may result in Kussmaul respiration, inspiratory apnea, coma, and death. Urine samples may have the characteristic smell of formaldehyde. An elevated serum osmolal gap from methanol will be evident early in presentation but may disappear after approximately 12 hours. At this time, an elevated anion gap metabolic acidosis from retained formate may be evident. 

Glasser DS. Utility of the serum osmol gap in the diagnosis of methanol or ethylene glycol ingestion. Ann Emerg Med 1996 27 343-6. 

LeGatt DF, Audette RJ, Blakney G, Vaughan D. Excess serum osmolality after ingestion of methanol The exception, not the rule. Clin Chem 1991 37 1802-4. 

An important clinical difference between the vapor pressure technique and the freezing point depression osmometer is the failure of the former to include in its measurement of total osmolality any volatile solutes present in the serum. Substances such as ethanol, methanol, and iso-propanol are volatile, and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent (water). This makes use of vapor pressure osmometers impractical for identifying osmolal gaps in acid-base disturbances (see Chapter 46). Thus use of this type of osmometer cannot be recommended for most clinical laboratories. 

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