A Case Report of Nitrous Oxide-Induced Subacute Combined Degeneration Complicated with Neurosyphilis in a 23-Year-Old Female

Introduction Recreational nitrous oxide (N₂O) abuse has become a prevalent public health concern among young adults, often triggering subacute combined degeneration (SCD) of the spinal cord by inactivating vitamin B12. Neurosyphilis, a late complication of Treponema pallidum infection invading the nervous system, also involves the spinal cord, with highly overlapping clinical and imaging features with SCD. Concurrent occurrence of the two disorders is extremely rare and prone to missed diagnosis and misdiagnosis. This paper aims to report a rare case of N₂O abuse-induced SCD complicated with asymptomatic neurosyphilis, summarize diagnosis and treatment experience, and improve clinical recognition. Case presentation A 23-year-old female, a clothing salesperson, was admitted with progressive numbness of limbs and unsteady gait for 3 months, with a clear 6-month history of N₂O inhalation. Cervicothoracic spinal magnetic resonance imaging (MRI) revealed symmetric hyperintensity in the posterior and lateral columns from C2 to T8, showing a typical "inverted V-sign". Lumbar puncture and serological tests confirmed concurrent neurosyphilis, and laboratory examinations indicated severe vitamin B12 deficiency and elevated homocysteine. Treatment and outcome The patient received standardized anti-syphilitic therapy, high-dose vitamin B12 supplementation, N₂O abstinence intervention and rehabilitation training. After 8 weeks of treatment, her neurological function improved significantly, and no recurrence occurred during 6 months of follow-up. Conclusion N₂O-induced SCD complicated with neurosyphilis is clinically rare. For young patients with spinal cord lesions, screening for N₂O exposure history, vitamin B12 level and syphilis indicators should be performed simultaneously. Early combined intervention can improve prognosis and avoid permanent neurological damage.