MONITORING OF MEDIAN NERVE SOMATOSENSORY-EVOKED POTENTIALS DURING CERVICAL SPINAL-CORD DECOMPRESSION

We evaluated the intraoperative utility of monitoring median nerve som atosensory evoked potentials (SEPs) in 31 consecutively hospitalized n eurosurgical patients (mean age 55.3 +/- 2.1 years) who underwent spin al cord decompression for cervical herniated disc, spondylosis, or tum or. Pre- and postoperative standard neurological examinations included evaluation of motor function, sensory responses, gait, tone, and refl exes. Evoked potentials were recorded from clavicular Erb's (N9) and c ontralateral cortical (N20) points. Intraoperatively, SEP measurements were obtained after the onset of anesthesia (baseline control) and we re repeated throughout the operative procedures. N20 and N9-N20 conduc tion latencies were measured for each SEP recording; disappearance of the SEP waveform was interpreted as a nonquantifiable increase in late ncy. Follow-up neurological examinations were made immediately after a pd up to 6 months following surgery. Postoperatively, the 31 study sub jects were assigned to one of two groups based on neurological evaluat ion: 27 group I subjects had either no change or improvement (good out come) whereas four group II subjects had postoperative neurological de terioration (poor outcome). Intraoperative N9-N20 interpeak latency wa s found to increase during cervical decompression in six of 27 group I and in two of four group II subjects. Only two of the Group I subject s exhibited increases >10% (14 and 19%, respectively). Intraoperative communication to the surgeon of a marked increase of N9-N20 latency du ring positioning for cervical traction clearly obviated a poor outcome in one group I subject; Upon removal of traction, latency decreased a nd significant changes in neurological function were not detected post operatively. The SEP waveform disappeared in two of the group II and i n none of the group I subjects. In the two group II subjects exhibitin g increases of N9-N20 latencies, increments were >20%. These findings indicate that in patients undergoing cervical spinal cord decompressio n, disappearance of SEPs or increases >20% in the N9-N20 interpeak lat ency are suggestive of operative conditions that place patients at ris k for poor neurological outcome.