SPINE Volume 30, Number 18, pp 2094 –2103 ©2005, Lippincott Williams & Wilkins, Inc. Increased Risk of Postoperative Neurologic Deficit for Spinal Surgery Patients With Unobtainable Intraoperative Evoked Potential Data Earl D. Thuet, BS, Anne M. Padberg, MS, Barry L. Raynor, BA, Keith H. Bridwell, MD, K. Daniel Riew, MD, Brett A. Taylor, MD, and Lawrence G. Lenke, MD 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 poten- tial (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 compre- hensive review of this patient population has not been reported. Methods. A total of 4,310 consecutive orthopedic spi- nal surgeries at one institution from January 1994 through December 2003 were reviewed. Cases lacking sufficient monitoring data, despite functional neural in- tegrity (ambulators, intact sensation), were identified. Di- agnoses 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 differ- ence 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 neuro- logic deficits. Multiple Stagnara wake-up tests are strongly recommended when evoked potential data can- not be obtained. Key words: somatosensory evoked potential, neuro- genic mixed evoked potential, Stagnara wake-up test, functional neurologic integrity. Spine 2005;30:2094 –2103 The use of multimodality evoked potential monitoring has become a standard of care during spinal surgery in the last decade. 1–8 Before that time, monitoring was used in a majority of medical centers (88%) as reported in a large multisite survey by Nuwer et al. 9 Neurophysiologic information obtained during a surgical procedure is, in essence, an early warning system designed to protect the nervous system. These data are continuously collected, giving the surgeon ongoing information, particularly during periods of high neurologic risk. Surgeons using monitoring during spinal surgery depend on this contin- uous feedback to maximize patient safety and surgical outcome. The ability to obtain intraoperative evoked potential data is dependent on several factors. Functional neuro- logic status is foremost, along with anesthetic and tech- nical considerations. However, despite meeting all of the above criteria, a subgroup of patients exist for whom evoked potentials cannot be obtained intraoperatively. Although not completely normal neurologically, these patients may be functional ambulators and/or have largely intact sensation. Contained within this subgroup is another, quite small set of patients with apparently normal neurologic status but no usable evoked potential data. Protection of existing neurologic function is critical for all these patients. Previous reports have cited the in- cidence of unobtainable data in patients with spinal cord pathology and various systemic neuromuscular dis- eases. 10 –17 To our knowledge, no report of patients grouped solely by a lack of usable monitoring data has been previously published. The surgeon’s alternative, when faced with unobtain- able evoked potential data, is the Stagnara wake-up test. Although its risks have been well documented, the wake-up test has long been considered the gold standard for intraoperative assessment of spinal cord motor func- tion. 18 The most significant shortcoming is that the neu- rologic information is provided in short duration, “sin- gle snapshot” format. Sensitive timing is necessary to maximize the utility of an intraoperative wake-up test, and multiple wake-up tests may be required. The purpose of this paper was to evaluate those patients with absent monitoring data from the outset of their spinal surgery, despite functional neurologic status, at one in- stitution over the last 10 years. Cases were categorized by diagnosis, any underlying neurologic pathology or condition, and the extent of missing data (Table 1). The From the Washington University Medical Center, Department of Orthopaedic Surgery, BJC Health Systems, St. Louis, MO. Acknowledgment date: October 5, 2004. Revision date: February 3, 2005. Acceptance date: March 8, 2005. The device(s)/drug(s) is/are FDA-approved or approved by correspond- ing national agency for this indication. No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Lawrence G. Lenke, MD, #1 Barnes Hospital Plaza, Suite 11300, St. Louis, MO 63110; E-mail: lenkel@msnotes.wustl.edu 2094