Variability of somatosensory-evoked potentials in different stages of scoliosis surgery.

STUDY
DESIGN: A comparison of the latencies and amplitudes of the somatosensory-evoked potential in different stages of scoliosis surgery.
OBJECTIVES: To investigate the normal variability of the somatosensory-evoked potential at different stages of scoliosis surgery and to improve the reliability of spinal cord monitoring. SUMMARY OF BACKGROUND DATA: False alarms in intra-operative spinal cord monitoring have been reported when the somatosensory-evoked potential measured before skin incision was used as the reference baseline value. However, the normal variability during different stages of surgery and the specificity of somatosensory-evoked potential is not well documented.
METHODS: The somatosensory-evoked potential of 90 patients who underwent surgical correction for idiopathic scoliosis was monitored intraoperatively, with stimuli applied to the posterior tibial nerve. The amplitudes and latencies of the somatosensory-evoked potential were recorded over Cz'-Fz and Cv-Fz. The values of latencies and amplitudes and their percentage variabilities were analyzed.
RESULTS: The values of latencies and their percentage variabilities did not differ significantly from each other in different stages of surgery. The values of amplitudes between different stages showed statistically significant differences. In particular, the amplitude of the somatosensory-evoked potential was found to be significantly decreased after exposure of the spine when compared with the amplitude before skin incision.
CONCLUSION: Some variability in latencies and amplitudes between different stages of scoliosis surgery is normal. These results suggest that the somatosensory-evoked potential when the spine is exposed may be used as the reference baseline to determine whether somatosensory-evoked potentials are subnormal at the subsequent stages of surgery.