Increased risk of postoperative neurologic deficit for spinal surgery patients with unobtainable intraoperative evoked potential data

Study Design. This was a retrospective study of 4,310 patients undergoing spinal surgery between 1994 and 2003. Objectives. To examine the incidence and potential causality of unobtainable somatosensory evoked potential (SSEP) and neurogenic mixed evoked potential (NMEP) data for a population of spinal surgery patients. Summary of Background Data. Patients with absent or unobtainable evoked potential data may increase the risk of undetected neurologic injury. To date, a comprehensive review of this patient population has not been reported. Methods. A total of 4,310 consecutive orthopedic spinal surgeries at one institution from January 1994 through December 2003 were reviewed. Cases lacking sufficient monitoring data, despite functional neural integrity (ambulators, intact sensation), were identified. Diagnoses were divided into six general categories. The association between absent evoked potential data and associated neurologic and/or medical pathology was evaluated. Results. A total of 59 of 4,310 cases (1.37%) had absent SSEP and/or NMEP intraoperative data despite functional neural integrity (44 ambulators/15 nonambulators); 5.08% of study patients awoke with increased neurologic deficit (3 of 59), 2 global deficits, and 1 nerve root deficit. The incidence of postoperative neurologic deficit in the entire surgical population was 0.77% ( 33 of 4,310), 8 global (0.19%), and 25 nerve root deficits (0.058%). A Fisher's exact test demonstrated a statistically significant difference between the incidence in these two populations (P=0.0121) and the incidence of global paraplegic deficits (P=0.0075). Conclusion. Patients with unobtainable data pose a much higher risk (P=0.0121) for postoperative neurologic deficits. Multiple Stagnara wake-up tests are strongly recommended when evoked potential data cannot be obtained.