title: Variability of somatosensory evoked potential monitoring during scoliosis surgery.
abstract: OBJECTIVE
  Somatosensory evoked potentials (SEPs) of 65 patients undergoing scoliosis surgery were monitored by stimulation of posterior tibial nerve to observe variations in latencies and amplitudes.
  
  
  METHODS
  Monitoring was divided into five stages: pre incision, spine exposure, instrumentation loading, deformity correction, and wound closure (stages 1-5, respectively).
  
  
  RESULTS
  We found the latency showed significant increase and the amplitude significant reduction from stages 1 to 2. There was no significant variability from stages 2, 3, and 4, but both latency and amplitude recovered significantly from stage 4 to 5. This variability correlated with the changes in mean arterial pressure and end-tidal concentrations of isoflurane and was not dependent on the type of surgical procedure. If either 50% amplitude reduction or 10% latency prolongation of SEP compared with baseline recordings at stage 1 (pre incision) was used as warning criterion, the overall false-positive rate was 23.1%. It was significantly reduced to 7.7% if stage 2 (spine exposure) recordings were used as the baseline (P < 0.05). The false-positive rate decreased to 0% if a combined 50% amplitude reduction and 10% latency prolongation of SEP compared with the stage 2 baseline were used (P < 0.001).
  
  
  CONCLUSIONS
  Based on these findings, we concluded that the time to obtain SEP baseline data should be adjusted to be post incision instead of pre incision.
authors.affiliation: Department of Orthopaedic Surgery, National University Hospital, Singapore.
authors.forename: Zheng-Yong
authors.initials: ZY
authors.lastname: Chen
authors.forename: Hee-Kit
authors.initials: HK
authors.lastname: Wong
authors.forename: Yiong-Huak
authors.initials: YH
authors.lastname: Chan
mesh_terms: D000293:Adolescent; D000328:Adult; D001794:Blood Pressure; D001831:Body Temperature; D002648:Child; D005073:Evoked Potentials, Somatosensory; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007530:Isoflurane / Q000032:analysis; D008297:Male; D019637:Orthopedic Procedures / Q000379:methods; D012016:Reference Values; D012189:Retrospective Studies; D012600:Scoliosis / Q000503:physiopathology* / Q000601:surgery*
chemical_list: D007530:Isoflurane