Functional vitamin B12 deficiency is a consistent feature in hospital admissions for neurological disorders due to the use of nitrous oxide.

Introduction: Misuse of inhaled nitrous oxide is a growing concern in France. It is known to alter concentrations of vitamin B12, which is required as a cofactor for methionine synthase and methylmalonyl-CoA mutase activity. Hence, measurement of the concentrations of cobalamin metabolism biomarkers, including vitamin B12, homocysteine, and methylmalonic acid, could assist in the management of patients with a complex clinical presentation or in those who deny the consumption of nitrous oxide.Methods: We retrospectively collected clinical and biological data in patients hospitalized for nitrous oxide use in a university hospital in southern France between January 2020 and October 2022.Results: Thirty-one patients were identified during 34 months; 79% were men with a median age of 23.7 years. Most (97%) presented with peripheral polyneuropathy and/or myelopathy. The median vitamin B12 concentration was 134.6 pmol/L, with 17 of 31 patients having values less than 145 pmol/L (the lower limit of the normal range). The median plasma folate concentration was 20.1 nmol/L, which is within the normal range. The median plasma homocysteine concentration was 87.7 µmol/L (normal range <15 µmol/L), and the median plasma methylmalonic acid concentration was 3.8 µmol/L (normal range <0.5 µmol/L).Conclusion: Nitrous oxide use is an emerging public health problem in France, as shown by the number of patients admitted to our hospital. The presence of a functional vitamin B12 deficiency was a consistent feature that could be helpful in diagnosis in complex cases.