Unusual clinical and MRI manifestations of nitrous oxide intoxication: a case report and literature review

Abstract

Nitrous oxide (N₂O) abuse can lead to functional vitamin B 12 deficiency and hypoxia, resulting in neurological dysfunction and subacute combined degeneration (SCD) of the spinal cord. SCD typically involves the posterior and lateral columns of the spinal cord and may also affect peripheral nerves. In most reported cases, serum vitamin B 12 levels are decreased. We report the case of a 22-year-old female who developed progressive numbness and weakness in all four limbs following N₂O abuse. She also exhibited clumsiness of the upper limbs and postural instability, without urinary or fecal incontinence. On neurological examination, muscle strength was graded as 5 − in both upper limbs and 4 in both lower limbs, with normal muscle tone. A right-sided Babinski sign was present. Tendon reflexes were absent in all extremities, and both deep and superficial sensations were impaired. Electromyography revealed a sensorimotor polyneuropathy with features suggestive of combined axonal and demyelinating injury, with features suggestive of combined axonal and demyelinating injury. Laboratory tests revealed a markedly elevated serum vitamin B 12 level (> 2,000 pg/mL) and an increased methylmalonic acid level (602.0 nmol/L). To further clarify the diagnosis, cervical spine MRI was performed, which demonstrated extensive abnormal signals in the spinal cord from C2 to C6, simultaneously involving the anterior, posterior, and lateral columns. The patient was diagnosed with SCD secondary to N₂O intoxication and was treated with mecobalamin and vitamin B 1 supplementation. Her symptoms improved, and she was subsequently discharged. At the 3-year follow-up, cervical MRI showed no significant abnormalities. Simultaneous involvement of the anterior, posterior, and lateral columns in SCD is extremely rare. A characteristic “butterfly sign” may be observed on axial PD-weighted MRI. Methylmalonic acid and homocysteine may serve as more reliable diagnostic biomarkers than serum vitamin B 12 levels in SCD.