A case report of nitrous oxide-induced subacute combined degeneration complicated with neurosyphilis in a 23-year-old female

Background Recreational nitrous oxide (N₂O) abuse has become a common health issue in young adults, which can induce subacute combined degeneration (SCD) of the spinal cord. Neurosyphilis has highly similar clinical and imaging manifestations with SCD. The coexistence of N₂O-induced SCD and asymptomatic neurosyphilis is extremely rare and easily overlooked. We herein report such a case to improve clinical diagnosis.Case presentation A 23-year-old female presented with 3-month progressive limb numbness and gait instability, with a 6-month history of N₂O inhalation. Spinal MRI showed typical "inverted V-sign" of SCD. Further examinations confirmed asymptomatic neurosyphilis, severe vitamin B12 deficiency and hyperhomocysteinemia. After standard combined treatment, her neurological function was significantly improved with no recurrence during follow-up.Conclusion For young patients with spinal cord lesions, screening for N₂O abuse, vitamin B12 and syphilis should be performed simultaneously. Early diagnosis and combined intervention can effectively avoid permanent neurological damage.