B-301 Deciphering Neuropathy features in the Context of Nitrous Oxide abuse: An Integrated Analysis of Electrophysiology and Metabolism
Authors: Gernez E , Scuccimarra M , Plasse L , Cassim F , Touze B , Girot M , Bossaert C , Tard C , Grzych G 
Published: October 2024

Abstract

Background
Nitrous oxide (N2O), used medically for its analgesic and anxiolytic properties, has emerged as a common recreational substance, leading to a rise in chronic toxicity cases. N2O abuse induces peripheral neurological damage with a spectrum of clinical presentations, ranging from mild sensory disturbances to severe motor deficits. Neurological complications from N2O include axonal and demyelinating neuropathies, leading to diagnostic and prognostic challenges. The pathophysiology of N2O toxicity is linked to functional inactivation of vitamin B12, resulting in biochemical and electrophysiological alterations. This study aims to enhance understanding of N2O pathophysiology by examining metabolic changes in consumers with axonal and demyelinating patterns.

Methods
This study included 35 patients with recent N2O abuse (last declarative consumption within the past week) who were treated at Lille University Hospital between March 2020 and April 2023 and underwent electromyography. The collected clinical features encompassed sensory symptoms, motor weakness, gait abnormalities and deep tendon reflexes. Patients were classified based on clinical phenotypes, distinguishing between length-dependent and non-length dependent, sensory or sensory-motor involvement and their peripheral neuropathy disability (PND) score (ranging from I to IV). Neurophysiological data analysis followed the criteria of Hadden et al. to identify patients with demyelinating criteria, while those lacking demyelinating criteria were categorized as “axonal” in the neuropathy group. Simultaneously, biochemical evaluations of serum vitamin B12 and B9 by CLIA methods (Roche), along with plasma homocysteine by LC/MS/MS and CLIA methods (Snibe), methylmalonic acid (MMA), and methionine by LC/MS/MS, were systematically performed for all patients.

Results
40% of the selected patients exhibited demyelination criteria. The demyelinated group showed a significantly higher disability PND score compared to the axonal group (3.0 and 2.2, respectively; p-value = 0.028). Elevated levels of homocysteine and methylmalonic acid were found in all patients. Interestingly, plasma concentrations of homocysteine were significantly higher in the group of patient with axonal outcomes compared to the demyelinating group (90 µM and 114 µM; p-value = 0.04), suggesting an involvement of homocysteine in the type of lesion related to N2O abuse.

Conclusions
This study highlights the diverse electrophysiological manifestations of N2O-induced neuropathy and highlight the potential role of metabolic parameters as biomarkers to understand the pathophysiology. Indeed, lower hyperhomocysteinemia levels were observed in demyelinating patterns, highlighting a potentially biochemical difference between the groups: plasma homocysteine could also be used as a marker of neurological severity. The homocysteine assay used in these conditions must enable the patient to be managed rapidly: it would therefore be preferable to use rapid methods such as immunoanalysis. On the other hand, plasma methylmalonic acid did not differ between groups, which also suggests that other mechanisms have yet to be identified in the pathophysiology of N2O abuse.