Loss of Intraoperative Evoked Responses during Dorsal Column Surgery Associated with Isolated Postoperative Sensory Deficit

Two patients underwent surgery for the removal of dermoid tumors that involved the dorsal aspect of the spinal cord. Both patients were monitored with intraoperative posterior tibial nerve somatosensory evoked potentials (SSEPs). In each case, the surgical procedure was nearly complete when an abrupt and persistent loss of SSEPs occurred. Although minor recovery of waveforms was present by the end of the operation, the SSEP waveforms remained strikingly abnormal. Neither patient had a postoperative motor deficit, although both had evidence of dorsal column dysfunction. These observations suggest that, during dorsal column surgery, even dramatic SSEP loss may not be associated with motor pathway injury, but rather, may correlate better with postoperative dorsal column dysfunction. The possible implications for intraoperative monitoring of SSEPs during dorsal column surgical procedures are discussed.