Nitrous Oxide Induced Posterior Cord Myelopathy: Beware of the Methyl Folate Trap.

Posterior cord myelopathy and subacute combined degeneration of the spinal cord are well-known complications of nitrous oxide abuse. Supplementation with vitamin B12 is an effective treatment strategy to correct low serum B12 levels or normal serum levels of dysfunctional vitamin B12 associated with this pathology. In this case, we report a patient with a one-year history of heavy nitrous oxide consumption; anywhere from eight to 30 canisters of 8 grams of nitrous oxide daily. The patient's symptoms worsened after the institution of megadoses of vitamin B12, despite normal serum folate levels. This can be explained by the phenomenon of the methyl folate trap and warrants prompt supplementation with synthetic folate. We discuss the biochemistry of nitrous oxide toxicity and the underlying mechanisms contributing to the methyl folate trap.