Relationships among nitrous oxide exposure, neurological injury and biomarkers.

Higher cumulative recreational nitrous oxide (N2O) exposure is the strongest predictor of severe neurological outcomes, showing a dose-dependent relationship with neurotoxicity. In a retrospective cohort study of 81 individuals hospitalized across six Sydney hospitals from 2020 to 2025, greater cumulative N2O exposure was associated with worse neurological impairment, including subacute combined degeneration of the spinal cord and peripheral neuropathy, and correlated with greater neuropathy severity. Median homocysteine was 50 μmol/L and median methylmalonic acid (MMA) was 0.68 μmol/L. Homocysteine and MMA were sensitive markers associated with neurological toxicity, making them useful screening biomarkers, but their limited specificity reduces utility as confirmatory tests. B12 and holotranscobalamin showed high specificity but low sensitivity for detecting cases with spinal cord degeneration.

Study at a glance

Abstract

Recreational nitrous oxide (N2O) misuse is an increasing public health problem associated with functional cobalamin (B12) deficiency and severe neurological complications. Despite growing recognition of its harms, the dose-response relationship and clinical value of biomarkers remain unclear. This study aimed to characterize the demographics, patterns of use, outcomes, and biomarker utility in patients hospitalized with N2O toxicity, as well as to examine the relationship between exposure and toxicity. This retrospective cohort study included hospitalized individuals with N2O toxicity (2020-2025) across six Sydney hospitals. Weekly and cumulative N2O exposure volumes were estimated. Exposure volumes were compared with clinical outcomes and biomarker results to assess correlations and biomarker utility. Eighty-one individuals across 100 presentations were included. Higher cumulative N2O exposure was associated with worse neurological impairment, including subacute combined degeneration of the spinal cord (SCD, p = .006) and peripheral neuropathy (PN, p = .036), and correlated with greater neuropathy severity (Spearman's ⍴ = 0.43, p = .001). Median homocysteine was 50 μmol/L (IQR:32-114;normal 5-15) with a median half-life of 14.59 h. Median methylmalonic acid (MMA) was 0.68 μmol/L (IQR:0.18-3.16;normal<0.27). B12 and holotranscobalamin showed high specificity (87% and 98%) but low sensitivity (22% and 5%) for detecting cases associated with SCD, while homocysteine and MMA showed high sensitivity (95% and 93%) but lower specificity (13% and 45%). Cumulative N2O exposure is the strongest predictor of severe neurological outcomes, demonstrating a dose-dependent relationship with neurotoxicity. Homocysteine and MMA were sensitive markers associated with neurological toxicity, making them useful screening biomarkers; however, their limited specificity reduces their utility as confirmatory tests. These findings reinforce the importance of clinical assessment and exposure history in diagnosing toxicity.