Background

Nitrous oxide (NO), commonly known as 'laughing gas' is an anaesthetic agent and recreational drug, usually inhaled using a balloon, which can cause a myelopathy due to functional B12 deficiency. This in turn can lead to subacute combined degeneration of the cord which can be seen on imaging as posterior cord T2 hyperintensity. We describe a case series of acute nitric oxide (NO) neurotoxicity, highlighting key clinical, laboratory and imaging features of this poorly recognized cause of subacute sensory ataxia.

Objective

We aimed to perform a retrospective review of patients with MR imaging of the spine due to suspected NO induced myelopathy and describe common clinical, biochemical and imaging features.

Methods

Retrospective review of 9 cases presenting at Wexham park hospital from March 2019 February 2022 who had MR imaging of part of or the whole of the spine. All imaging was re-reviewed by a neuroradiologist in light of the patient's NO abuse. Patient clinical details are aggregated and any identifiable patient details are removed to ensure identification of an individual patient is not possible.

Results

9 male patients, mean age 21.7 years (SD 2.7), presented with subacute ataxia after inhalation of NO. 7 out of 9 patients had signal change within the spinal cord, particularly affecting the posterior columns. We describe common features of NO induce myelopathy such as the 'inverted V sign' and 'binoculars sign'. We additionally explore individuals where signal change in the posterior cord was not appreciated on the initial report and pitfalls for why this may have happened, such as a history of NO abuse not being include in the clinical history.

Conclusions

Common findings in NO induced myelopathy of the cord on MRI include the 'inverted V sign' but may be much more subtle and easy to miss, especially if not guided by clinical information, so it is important to specifically ask whether patients have used NO when appropriate, to prevent unnecessary tests and treatments.