P491: Reversible intraoperative neurophysiologic monitoring changes associated with surgical retraction

Abstract

s of Poster Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339 S181 tion of the oculomotor nerve (onset-latency, 1.59±0.04 ms), the polyphasic responses were by the trachlear nerve (onset-latency, 3.2±0.5 ms) and the positive monophasic responses were by the abducens nerve (onset-latency, 1.5ms). The trigeminal nerve evoked potentials were elcited by the stimulation of the supraorbital nerve (peak-latency, 2.81±0.22 ms) the infraorbital nerve (peak-latency, 2.6 ms) and the mental nerve (peak-latency, 3.9±0.6 ms). Conclusions: The EOG responses were useful to identify the extraocular muscle-innervating nerves and to monitor its functions. The trigeminal nerve evoked potentials can be recorded on the trigeminal nerve by the supraorbital, infraorbital and mental nerves. P491 Reversible intraoperative neurophysiologic monitoring changes associated with surgical retraction L. Lee1, S. Cho1, V. Nguyen1, G. Steinberg2, S. Chang2, R. Dodd2, S. Ryu3, J. Lopez1 1Stanford University Medical Center, Neurology, Stanford, United States; 2Stanford University Medical Center, Neurosurgery, Stanford, United States; 3Palo Alto Medical Foundation, Neurosurgery, Palo Alto, United States Question: Inadvertent retraction-related injuries are a known risk of intracranial surgical procedures. Retraction in the vicinity of critical neural tissue and vascular structures may result in compression, stretch, or steno-occlusive ischemic injuries that are unexpected, and may not be recognized until the postoperative period. The critical role of multimodality intraoperative neurophysiologic monitoring (IONM) in helping to prevent retraction-related injuries is highlighted. Methods: We present a series of ten intracranial surgical cases where IONM changes occurred in association with retractor placement and positioning, procedures that include resection of tumor (4) and vascular malformations (2), and aneurysm clipping (4). Multimodality IONM monitoring was employed in all cases, including transcranial motor evoked potentials (tcMEPs), somatosensory evoked potentials (SSEPs), and electroencephalography (EEG). Results: In the four aneurysm cases, critical IONM changes occurred following retractor placement, but prior to any planned intervention. In all remaining cases changes occurred during the interventional period that also coincided with the placement or positioning of retractors. Most commonly transcranial motor evoked potentials were primarily affected. In all cases prompt identification of these IONM changes led to rapid surgical assessment and eventual removal or repositioning of retractors, which resolved neurophysiologic changes and correlated with no new sustained postoperative deficits. Conclusions: This case series highlights the importance of IONM in the early identification of potentially reversible changes that may correlate with impending retraction-related injuries. In particular, the pattern of IONM changes represented in the majority of these cases emphasizes the critical and distinct role of tcMEPs in identifying early signs of evolving cerebral injury. This further illustrates the utility and necessity of multimodality intraoperative neurophysiologic monitoring in a diversity of intracranial surgical cases for the entire duration of the procedure, including presumably non-interventional periods. P492 Critical intraoperative neurophysiologic monitoring (IONM) changes associated with patient positioning maneuvers L. Lee1, S. Cho1, V. Nguyen1, J. Ratliff2, J. Park2, J. Lopez1 1Stanford University Medical Center, Neurology, Stanford, United States; 2Stanford University Medical Center, Neurosurgery, Stanford, United States Question: Positioning maneuvers during surgical cases can place the patient at risk for spinal cord and/or peripheral nerve injury. Initial transition of the patient from the supine to prone position, as well as passive neck flexion or extension, are potentially high risk portions of the procedure, especially during spine surgeries. In addition, sudden or gradual shifts in head, neck, shoulder, and/or arm position may occur unexpectedly during the course of the surgical procedure, which can also result in central or peripheral nervous system injury. The role of IONM in helping to prevent such injuries is emphasized. Methods: We present a series of six cases where critical IONM changes were identified and resolved following modification of patient positioning in cervical spine procedures. Multimodality IONM monitoring was employed in all cases, including transcranial motor evoked potentials (tcMEPs), somatosensory evoked potentials (SSEPs), and electromyography (EMG). Results: Critical IONM changes were observed during the initial prone positioning onto the surgical table in three spine cases, while in one case changes occurred with passive neck extension while supine. In the remaining two cases, significant IONM changes occurred during exposure or the interventional period, findings which correlated with unexpected shifts in body or limb position over the course of the surgical procedure. In all cases prompt identification of IONM changes enabled rapid assessment and repositioning of the patient, which largely resolved all neurophysiologic changes and correlated with no new sustained postoperative deficits. Conclusions: This series highlights the importance of appropriately instituting IONM early, prior to initial patient positioning, to facilitate prompt identification of potentially reversible changes that may indicate impending positioning-related injuries. P493 Retrospective waveform analysis of transcranial motor evoked potentials (MEP) to identify early predictors of impending motor deficits in spinal surgeries S. Le1, A. Ekwueme2, S.C. Cho1, L. Lee1, V. Nguyen1, J. Lopez1 1Stanford University Medical Center, Neurology, Stanford, United States; 2University of California, Davis, Davis, United States Objective: This study aims to identify neurophysiologic parameters of motor evoked potentials (MEPs) that predict early compromise during spinal surgeries with the ultimate goal of providing real-time intra-operative neurophysiologic monitoring (IONM) feedback to surgeons to prevent irreversible post-operative neurologic motor deficits. Background: Although a 50% amplitude decrease in SSEPs correlates with potentially reversible physiologic ischemia, there are no corresponding standard warning criteria for MEPs [1]. Surgeons are not alerted until MEPs are no longer obtainable, by which time irreversible motor injury may have already occurred. Methods: From 2011-2013 at Stanford University and Lucile Packard Children’s Hospitals, we retrospectively identified 15 true positive cases of intraoperative loss of MEPs and post-operative motor deficits. The following MEP parameters were measured: latency, amplitude, duration, turns, phases, area under the curve (AUC) and an intraoperative spinal cord index (ISCI) [2] was calculated for each involved muscle group and for 5 traces prior to the complete MEP loss. Results: Out of 26 muscle groups in 15 cases, latency increased or had no change in 23 MEPs, duration decreased in 16 MEPs, amplitude decrease in 15 MEPs, AUC decreased in 14 MEPs, and ISCI decreased in 16 MEPs. In 12/15 cases (80%), there was more than >50% drop in ISCI in at least one monitored muscle group before the MEPs were completely lost. In 14 cases with concurrent SSEPs monitoring, 9 cases had >50% decrease in SSEPs: 2/9 changed before MEPs, 5/9 changed simultaneously with MEPs, and 2/9 changed after MEPs. Conclusions: In cases of impending irreversible motor injury, there is a trend toward smaller and simpler waveforms detected by IONM before complete MEP loss occurs. A 50% drop in ISCI could potentially be used an early warning parameter for motor compromise. In the future, MEPs need to be obtained more frequently to increase the sensitivity of impending injury. We hope further research will help identify a reliable index to alert surgeons before motor injury becomes irreversible.