Discrepancy between intraoperative SSEP's and postoperative function. Case report.

The authors report a case in which midline myelotomy for the removal of a C4-T4 ependymoma was immediately followed by abolition of short-latency somatosensory evoked potentials (SSEP's) in response to bilateral posterior tibial nerve (PTN) stimulation which proved irreversible intraoperatively. Subsequent intraoperative testing also revealed obliteration of median nerve (MN)-elicited responses. Postoperatively, joint and vibration sensations deteriorated in the lower extremities and there was unchanged very mild impairment of light touch, pinprick, and temperature sensibilities without significant loss of muscle strength. Nearly 5 months after surgery, despite recovery of postural and vibratory senses in the lower limbs and of SSEP's in response to MN stimulation, no SSEP's could be demonstrated on PTN stimulation. The possibility of "false positive" results (that is, intraoperative SSEP abolition without postoperative motor deficits) and of dissociations between SSEP's and deep sensation should be taken into consideration when interpreting SSEP's intraoperatively.