Relationships among nitrous oxide exposure, neurological injury and biomarkers

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.