Corneal nerve loss associated with nitrous oxide neurotoxicity.

Introduction: Nitrous oxide is an inhaled anaesthetic used recreationally that is neurotoxic. Corneal nerve parameters were assessed in patients with nitrous oxide toxicity.

Methods: Four individuals underwent clinical, electrophysiological and corneal confocal microscopy assessment. Corneal images were analysed with automated software and compared to healthy controls.

Results: Two patients had prolonged heavy use of nitrous oxide ≥ two years. In both patients, there was a reduction in inferior whorl corneal nerve fibre length (6.3 mm/mm² of cornea, 9.8 mm/mm² of cornea, respectively versus healthy controls: mean 19.4 ± 9.0 mm/mm² of cornea), and reduced inferior whorl corneal nerve fibre density (8.3 fibres/mm² cornea, 2.5 fibres/mm² of cornea, respectively versus healthy controls: mean 27.5 ± 11.1 fibres/mm² of cornea. Conversely, patients three and four had shorter, intense exposure and normal corneal nerve parameters (inferior whorl corneal nerve fibre density 20.8 fibres/mm² of cornea and 23.4 fibres/mm² of cornea, and corneal nerve fibre length 14.5 fibres/mm² of cornea and 19.6 fibres/mm² of cornea_, respectively).

Discussion: We demonstrated corneal nerve changes with long-term use of nitrous oxide, but preservation of corneal nerve indices with shorter duration of exposure.

Conclusion: Corneal nerve assessment may be a marker of neurological injury from nitrous oxide.