Several recent studies have reported subacute combined degeneration (SCD) induced by nitrous oxide (N₂O) abuse. However, the association between the evolution of dynamic neuroimaging and clinical manifestations has not been reported in patients with N₂O-induced SCD.

We described the case of a 24-year-old man who developed SCD with inverted V-sign hyperintensities over the posterior aspect of the spinal cord caused by frequent, excessive N₂O inhalation. One month after treatment, his weakness and paresthesia resolved and serum vitamin B₁₂ levels exceeded the normal levels. However, the hyperintensities had extended horizontally and longitudinally on T2-weighted magnetic resonance imaging (MRI), compared to those on the initial scan. Two months after treatment, the patient experienced some residual numbness in the distal limbs, and his serum homocysteine levels were normal, but the abnormal signals seen on cervical T2-weighted MRI had decreased only slightly compared to those seen on the one-month follow-up MRI. The evolution of conventional MRI findings lagged compared to the clinical manifestation, which was suggestive of a clinical-radiological dissociation.

Clinical-radiological dissociation might have occurred in this case because T2-weighted imaging was not sensitive enough to reveal cytotoxic edema. Moreover, the serum vitamin B₁₂ level is not a good indicator of cellular vitamin B₁₂. Thus, clinicians should recognize this phenomenon, comprehensively assess the condition of patients with N₂O-induced SCD, and avoid terminating treatment based on the resolution of clinical symptoms and serological results.