Intraoperative Detection of a Lateralized S2 Nerve Root Conduction Block Using Hemisphincter Bulbocavernosus Reflex...

Sacral reflex monitoring using bulbocavernosus reflex (BCR) and external urethral sphincter reflex (EUSR) is used to assess neural pathways responsible for bowel and bladder function during spine surgery. However, these techniques are generally considered limited in their ability to determine laterality due to bilateral afferent activation and shared sacral reflex circuitry. An 85-year-old male with progressive lower extremity weakness and worsening urinary incontinence underwent resection of a thoracic epidural tumor and separate sacral tumor involving the left S2 nerve root. BCR responses were recorded from the left and right external anal sphincter hemisphincters, and EUSR responses were recorded from the external urethral sphincter. Baseline recordings were stable. During manipulation of the thecal sac near the left S2 root, global sacral reflex attenuation occurred with complete loss of the left hemisphincter BCR response; release of retraction resulted in prompt bilateral recovery. Later, during targeted dissection adjacent to the left S2 nerve root, an isolated loss of the left hemisphincter BCR response occurred without changes in contralateral BCR or EUSR. Removal of the left S2 root retraction led to immediate return of the ipsilateral BCR response, though at a reduced amplitude. This case provides physiologic support that hemisphincter BCR recordings can reflect unilateral sacral nerve root dysfunction. A maneuver-dependent, reversible loss of the left hemisphincter BCR during S2 root retraction supports interpretation of hemisphincter changes as a marker of lateralized sacral reflex pathway dysfunction rather than global reflex suppression.