In addition to laboratory studies, imaging can reveal diagnostic clues and help establish the extent of damage to the spinal cord. However, CT may not be sufficiently sensitive to detect abnormalities; thus, MRI should be ordered to confirm the extent of the damage and to rule out other differential diagnoses, such as multiple sclerosis. A systematic review found that hyperintensities on T2-weighted images of the spinal cord were detected in 68% of patients with nitrous oxide toxicity. Abnormalities tend to involve at least three spinal segments, with the cervical spinal cord most often affected. Upper thoracic levels are more commonly involved in adults. EMG studies may show a mix of axonal and demyelinating neuropathy, most often in the lower extremities.[2,5]
The core principles of treatment involve immediate cessation of nitrous oxide and vitamin B12 supplementation.[2,5,7,8] There are currently no standardized guidelines, however, and multiple types of regimens have been described in the literature. One protocol involves daily intramuscular injection of 1000 µg of vitamin B12 for 1 week, followed by once-weekly injections for 4-8 weeks and then once-monthly injections.[2,5] Studies have also demonstrated that high-dose oral administration (1000-2000 µg) has similar efficacy. If vitamin B12 supplementation does not improve clinical symptoms, some alternatives that have been shown to be efficacious are oral methionine and methylprednisolone.[13,14] Most neurologic symptoms, as well as any lesions identified on imaging, resolve completely with treatment.
The patient in this case was admitted to the inpatient unit and started receiving daily intramuscular injections of vitamin B12, along with physiotherapy. By day 7, his neurologic function had improved significantly. Once his levels of homocysteine and methylmalonic acid had normalized, the patient was discharged. He still had paresthesias and weakness in the lower extremities. His treatment plan included follow-up with occupational therapy on an outpatient basis, supplementation with 1000 µg of oral vitamin B12, and cessation of nitrous oxide use.
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