Symptomatic manifestation of a cauda equina arteriovenous fistula after resection of a coexisting spinal schwannoma: illustrative case.

backgroundSpinal arteriovenous fistulas (AVFs) arising below the conus medullaris, particularly those involving the cauda equina or filum terminale, are rare. The coexistence of spinal AVFs and spinal tumors is exceedingly uncommon. The authors report a case of a cauda equina AVF that became clinically evident after resection of a coexisting cauda equina schwannoma. OBSERVATIONS: A 72-year-old man presented with progressive bilateral lower extremity weakness, sensory disturbance, gait impairment, and constipation. MRI demonstrated multiple spinal schwannomas, and the symptoms were attributed to a cauda equina schwannoma at the L5-S1 level. Resection of the schwannoma resulted in neurological improvement. However, 17 months later, the patient developed gradual neurological deterioration without tumor recurrence. Further evaluation revealed a cauda equina AVF, which was retrospectively identifiable on the initial MRI examination. Microsurgical interruption of the AVF led to neurological improvement, with no recurrence during follow-up. LESSONS: This rare case of a cauda equina AVF concomitant with a schwannoma emphasizes two key considerations. Careful preoperative imaging evaluation is essential to identify occult vascular lesions. Additionally, resection of a spinal tumor can potentially alter local hemodynamics and render a previously silent AVF symptomatic, underscoring the importance of comprehensive imaging interpretation and surgical planning. https://thejns.org/doi/10.3171/CASE251017.