Subacute combined cord degeneration: a rare complication of nitrous oxide misuse. A case report.

We describe a severe case of subacute combined degeneration of the spinal cord precipitated by overuse of the analgesic Entonox (a 50:50 mixture of nitrous oxide and oxygen) in a 19-year-old female with previously undiagnosed vitamin B12 deficiency. MRI of the spine revealed symmetric abnormal signal in the dorsal and anterolateral columns of the cervical and thoracic cord, correlating with the patient's symptoms. Myeloneuropathy is caused by inactivation of vitamin B12 by nitrous oxide. The patient's condition slowly improved with cessation of nitrous oxide exposure and vitamin B12 replacement therapy.