To the Editor: Subacute combined degeneration (SCD) of the spinal cord affects the lateral and posterior columns, primarily through demyelination. It can be caused by inadequate oral intake of vitamin B12, poor absorption of vitamin B12, or using medications such as metformin, proton pump inhibitors, and nitrous oxide (N2O). 1 It is clinically characterized by symmetric dysesthesia, disturbance of the position sense, and spastic paraparesis or tetraparesis. 2 N2O is an odorless, colorless gas that is mainly used as an inhaled anesthetic. However, recreational use is rapidly increasing. 3 N2O irreversibly inactivates vitamin B12 by oxidation; converting the vitamin from the active monovalent form to the inactive bivalent form. Because vitamin B12 is an essential coenzyme for methionine synthase, its deficiency causes irreversible inhibition of the enzyme. This leads to defective myelin and lipid membrane formation. 4, 5 Here, we present a case of SCD secondary to vitamin B12 deficiency due to nitrous oxide abuse.

A 22-year-old right-handed women without any past medical history presented with symmetrical bilateral upper and lower extremity numbness, tingling, and paresthesia of 3 weeks duration. She reported recreational N2O use daily for three months before presentation that peaked during a skiing trip 3 weeks before presentation, after which she felt tightness in her limbs and numbness that remained constant since then. Symptoms were limited to the distal two-thirds of her fingers and from the knee down to the toes bilaterally. She felt weakness in both legs that only manifested as reduced exercise tolerance. She denied any other neurological symptoms, including headache, visual disturbances, neck stiffness, diplopia, photophobia, urinary or stool incontinence, saddle region numbness, or swallowing problems. She did not have any fever, chills, sick contacts, recent trauma, rash, or