Loss Of Motor Evoked Potentials Research Articles

Sudden Loss of Motor Evoked Response Followed by Clinical Recovery After Resection of an Intradural Dorsal Meningioma: A Unique Case of a ClinicalNeurophysiological Dissociation

In the neuromonitoring era, resection surgery of spinal intradural lesions is guided by Intraoperative Neurophysiological Monitoring (IONM) of Motor Evoked Potentials (MEP). We describe a single case of a 70s-year-old patient who underwent surgery as a treatment for a voluminous dorsal intradural juxtamedullary meningioma: during surgery, we observed a sudden loss of bilateral lower limbs muscle MEPs, followed by a rapid recurrence of normal responses exclusively in the right side, but not in the left one. After awakening, the patient presented a transitory complete monoplegia of the left inferior limb for 30 minutes, and then it was resolved with rapid restoration of the neurological status.

Repeated L5 Nerve Root Compromise Detected with Motor Evoked Potentials (MEP), but Not Electromyography (EMG): A Case Report

ABSTRACT We report a case where neuromonitoring, using motor evoked potentials (MEP), detected an intraoperative L5 nerve root deficit during a lumbosacral decompression and instrumented fusion procedure. Critically, the MEP changes were not preceded nor accompanied by any significant spontaneous electromyography (sEMG) activity. Presumptive L5 innervated muscles, including tibialis anterior (TA), extensor hallucis longus (EHL) and gluteus maximus, were targets for nerve root surveillance using combined MEP and sEMG techniques. During a high-grade spondylolisthesis correction procedure, attempts to align a left-sided rod resulted in repeated loss and recovery cycles of MEP from the TA and EHL. No accompanying EMG alerts were associated with any of the MEP changes nor were MEP variations seen from muscles innervated above and below L5. After several attempts, the rod alignment was achieved, but significant MEP signal decrement (72% decrease) remained from the EHL. Postoperatively, the patient experienced significant foot drop on the left side that recovered over a period of 3 months. This case contributes to a growing body of evidence that exclusive reliance on sEMG for spinal nerve root scrutiny can be unreliable and MEP may provide more dependable data on nerve root patency.

Importance of Multimodal Spinal Cord Monitoring and Hemodynamic Augmentation during High Thoracic Ventral Dural Tear Repair Using the Posterior Approach

AbstractCerebrospinal fluid (CSF) leakage due to large ventral dural tears (VDT) often requires surgical intervention. Surgical closure of a high thoracic VDT is challenging and associated with high morbidity, especially if it is performed after multiple epidural blood patch (EBP)/fibrin glue injections. A 44-year-old woman was diagnosed with spontaneous intracranial hypotension due to VDT at T1-T2, causing CSF leakage. Multiple EBP and fibrin glue injections failed to treat her symptoms; hence, the patient underwent surgical closure using the posterior approach. The patient was anesthetized using standard anesthetic drugs and was maintained under total intravenous anesthesia to facilitate continuous motor-evoked potential (MEP) monitoring. The surgical course was complicated by bleeding, hypotension, and MEP loss. Continuous MEP monitoring, effective team communication, quick restoration of blood pressure(BP) and BP augmentation with fluid, blood, and vasopressor helped to restore the MEP back to baseline. Hence, the patient recovered without neurological morbidity. This case report highlights the importance of adequate vascular access, multimodal spinal cord monitoring, and BP augmentation during a high thoracic VDT repair.

Subcortical motor ischemia can be detected by intraoperative MRI within 1 ​h – A feasibility study

IntroductionTo achieve a maximum extent of resection, intraoperative MRI (ioMRI) scan is frequently performed. Intraoperative diffusion-weighted imaging (DWI) is not standardly performed and has been described to be inferior to early postoperative MRI regarding the detection of ischemia. Research questionThis feasibility study evaluates the detection of ischemia by ioMRI and its clinical relevance in patients with motor-eloquent gliomas. Material and methodsOf 262 glioma patients, eight patients (3.1%) showed an amplitude loss of continuous motor evoked potential (MEP) monitoring during resection before the ioMRI scan (group loss of MEP ​= ​LOM). In these patients and a matched-pair cohort (MPC) of glioma resections without MEP loss, we performed additional ioMRI sequences including turbo-spin-echo (TSE)- and echo-planar-imaging (EPI)-DWI and perfusion-weighted imaging (PWI). The clinical outcome was measured 5 days and 3 months after surgery. ResultsThe mean ​± ​standard time between loss of MEPs and ioMRI was 63.0 ​± ​8.7 ​min (range: 40–84). Ischemia within the motor system could be detected by ioMRI in group LOM in 100% EPI-DWIs, 75% TSE-DWIs, and 66.7% PWIs. No sequence showed motor ischemia in the MPC group. All patients of group LOM and no patient of group MPC suffered from permanent motor deficit. Discussion and conclusionThe current results provide data on the time sequence of ischemia apparent in MRI sequences which is superior to previous data on symptomatic stroke patients on this topic. The early detection of ischemia adds an additional predictor for the long-term outcome of patients and shows the reason of an intraoperative loss of MEPs. Thereby the performance of intraoperative EPI-DWI might be justified after confirmation of the present data in a larger cohort.Subcortical ischemia can be detected by ioMRI after MEP loss during the resection of motor-eloquent gliomas and was clinically relevant in all cases.

Feasibility of intraoperative motor evoked potential monitoring during tethered cord surgery in infants younger than 12 months

PurposeFeasibility, reliability, and safety assessment of transcranial motor evoked potentials (MEPs) in infants less than 12 months of age.MethodsA total of 22 patients with a mean age of 33 (range 13–49) weeks that underwent neurosurgery for tethered cord were investigated. Data from intraoperative MEPs, anesthesia protocols, and clinical records were reviewed. Anesthesia during surgery was maintained by total intravenous anesthesia (TIVA).ResultsMEPs were present in all patients for the upper extremities and in 21 out of 22 infants for the lower extremities. Mean baseline stimulation intensity was 101 ± 20 mA. If MEPs were present at the end of surgery, no new motor deficit occurred. In the only case of MEP loss, preoperative paresis was present, and high baseline intensity thresholds were needed. MEP monitoring did not lead to any complications. TIVA was maintained with an average propofol infusion rate of 123.5 ± 38.2 µg/kg/min and 0.46 ± 0.17 µg/kg/min for remifentanil.ConclusionIn spinal cord release surgery, the use of intraoperative MEP monitoring is indicated regardless of the patient’s age. We could demonstrate the feasibility and safety of MEP monitoring in infants if an adequate anesthetic regimen is applied. More data is needed to verify whether an irreversible loss of robust MEPs leads to motor deficits in this young age group.

Intraoperative MRI-based elastic fusion for anatomically accurate tractography of the corticospinal tract: correlation with intraoperative neuromonitoring and clinical status.

Tractography is a useful technique that is standardly applied to visualize subcortical pathways. However, brain shift hampers tractography use during the course of surgery. While intraoperative MRI (ioMRI) has been shown to be beneficial for use in oncology, intraoperative tractography can rarely be performed due to scanner, protocol, or head clamp limitations. Elastic fusion (EF), however, enables adjustment for brain shift of preoperative imaging and even tractography based on intraoperative images. The authors tested the hypothesis that adjustment of tractography by ioMRI-based EF (IBEF) correlates with the results of intraoperative neuromonitoring (IONM) and clinical outcome and is therefore a reliable method. In 304 consecutive patients treated between June 2018 and March 2020, 8 patients, who made up the basic study cohort, showed an intraoperative loss of motor evoked potentials (MEPs) during motor-eloquent glioma resection for a subcortical lesion within the corticospinal tract (CST) as shown by ioMRI. The authors preoperatively visualized the CST using tractography. Also, IBEFs of pre- and intraoperative images were obtained and the location of the CST was compared in relation to a subcortical lesion. In 11 patients (8 patients with intraoperative loss of MEPs, one of whom also showed loss of MEPs on IBEF evaluation, plus 3 additional patients with loss of MEPs on IBEF evaluation), the authors examined the location of the CST by direct subcortical stimulation (DSCS). The authors defined the IONM results and the functional outcome data as ground truth for analysis. The maximum mean ± SD correction was 8.8 ± 2.9 (range 3.8-12.0) mm for the whole brain and 5.3 ± 2.4 (range 1.2-8.7) mm for the CST. The CST was located within the lesion before IBEF in 3 cases and after IBEF in all cases (p = 0.0256). All patients with intraoperative loss of MEPs suffered from surgery-related permanent motor deficits. By approximation, the location of the CST after IBEF could be verified by DSCS in 4 cases. The present study shows that tractography after IBEF accurately correlates with IONM and patient outcomes and thus demonstrates reliability in this initial study.

A Study of Critical Events That Lead to Spinal Cord Injury and the Importance of Rapid Reversal of Surgical Steps in Improving Neurological Outcomes: A Porcine Model.

Porcine model. To quantify critical vascular and mechanical events that occur before and during an evolving spinal cord injury. Spinal cord injuries are one of the most devastating complications in spine surgery. Intraoperative neuromonitoring changes can occur as a secondary event of spinal cord compression and decrease in spinal cord blood flow (SCBF). Laser Doppler flowmetry has been well validated for measuring blood flow. Seventeen pigs were studied, 14 of which completed the experiment. Multilevel, midthoracic laminectomies were performed. Laser Doppler flowmetry electrodes were placed on the dura to measure SCBF. Spinal cord injury was induced by incremental balloon inflation in the epidural space. The animals were separated into two groups. After motor-evoked potential (MEP) loss, group A underwent medical interventions and then balloon decompression approximately 20 minutes later. Group B underwent immediate balloon decompression followed by medical interventions. After interventions, wake-up test was performed and computed tomography scan measured thoracic spinal canal volume. Median SCBF changes were seen 15.8 (5.4-25.1) minutes before MEP loss. However, the 20% threshold interval was often reached before. At the 20% threshold, median pressure was 7 psi, balloon volume was 0.5 cm, and 50% of the spinal canal was compromised. In group A, no pigs moved and all had pathology indicating ischemia. In group B, 9 of 10 were found to be moving their hind legs with 7 indicating ischemia. Compression spinal cord injury is the end of a cascade involving increasing intracanal pressure, decreasing canal volume, and hypoperfusion. Rapid relief of compression leads to MEP return. SCBF monitoring can detect ischemia preinjury, giving surgeons an opportunity for early intervention. 4.

Neuroanesthesia Guidelines for Optimizing Transcranial Motor Evoked Potentials Neuromonitoring During Deformity and Complex Spinal Surgery: A Delphi Consensus Study

Abstract INTRODUCTION Intraoperative neurophysiological monitoring of transcranial motor evoked potentials (MEPs) provides the most reliable method for assessing spinal cord functional integrity during deformity and other complex spinal surgeries. MEPs are affected by pharmacological and physiological parameters. It is the responsibility of the spine surgeon and neuroanesthesia team to understand how they can best maintain high quality MEP signals throughout surgery. Nevertheless, varying approaches to neuroanesthesia are seen in clinical practice. METHODS We identified 19 international spinal deformity expert teams for participation in our study. A modified Delphi process utilizing 2 rounds of surveying was performed. Greater than 50% and 75% agreement on the final statements was considered achieving “agreement” and “consensus,” respectively. RESULTS Anesthesia regimens and protocols were obtained from the expert centers. A large amount of variability in these centers was witnessed. Two rounds of consensus surveying were then performed, and all centers participated in both rounds of the surveying. Consensus was obtained in 12 of 15 statements and majority agreement in 2 of the remaining. Agreement on specific safe neuroanesthesia practices in the setting of MEP monitoring was obtained. Total intravenous anesthesia (TIVA) was identified as the optimal method of maintenance with few centers allowing for low MAC concentrations of inhaled anesthetic. While no strict cutoff values of propofol concentrations or opioid doses were identified, most centers advocated for less than 150 mcg/kg/min of propofol with titration to the lowest dose that maintains appropriate anesthesia depth based on bispectral index or electroencephalography awareness monitoring. Utilization of adjuvant intravenous anesthetics, including ketamine and lidocaine, may help to reduce propofol and opioid requirements without negatively impacting MEP signals. Low-dose dexmedetomidine was also routinely used with the same purpose, but with knowledge that higher doses may be suppressive. Maintenance of blood pressure parameters near the patient's preoperative baseline or with mean arterial pressure greater than 80 mmHg ensures appropriate spinal cord perfusion and prevents loss of MEPs. CONCLUSION Spine surgeons and their neuroanesthesia teams should be familiar with the methods for optimizing IOM of MEPs during deformity and complex spinal cases. While variability in practices exist, consensus exists among international deformity centers regarding best practices.

Delayed loss of motor evoked potentials during cerebral aneurysm clipping surgery

The surgical outcomes of patients with cerebral aneurysm may be severely impaired by ischemic complications resulting from inadvertent vascular occlusion or compromise. We report a case of delayed loss of motor evoked potentials (MEPs) during cerebral aneurysm clipping surgery, in which aneurysm clip rotation after the release of brain retraction might have disrupted blood flow in the branching arteries near the aneurysm. Although an indocyanine green angiogram confirmed the patency of all adjacent vessels after aneurysmal clipping, the loss of MEPs was observed during dural closure. We emphasize that surgeons and intraoperative neurophysiological monitoring (IONM) personnel should not only exercise caution in the clip placement itself, but also be aware of the necessity of continuing IONM until the end of surgery, because major changes may occur even during the non-key portions of surgery.

Intraoperative neurophysiological monitoring for intradural extramedullary spinal tumors: predictive value and relevance of D-wave amplitude on surgical outcome during a 10-year experience.

OBJECTIVEThe purpose of this study was to evaluate the technical feasibility, accuracy, and relevance on surgical outcome of D-wave monitoring combined with somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) during resection of intradural extramedullary (IDEM) spinal tumors.METHODSClinical and intraoperative neurophysiological monitoring (IONM) data obtained in 108 consecutive patients who underwent surgery for IDEM tumors at the Institute for Scientific and Care Research "ASMN" of Reggio Emilia, Italy, were prospectively entered into a database and retrospectively analyzed. The IONM included SSEPs, MEPs, and-whenever possible-D-waves. All patients were evaluated using the modified McCormick Scale at admission and at 3, 6, and 12 months of follow-up .RESULTSA total of 108 patients were included in this study. A monitorable D-wave was achieved in 71 of the 77 patients harboring cervical and thoracic IDEM tumors (92.2%). Recording of D-waves in IDEM tumors was significantly associated only with a preoperative deeply compromised neurological status evaluated using the modified McCormick Scale (p = 0.04). Overall, significant IONM changes were registered in 14 (12.96%) of 108 patients and 9 of these patients (8.33%) had permanent loss of at least one of the 3 evoked potentials. In 7 patients (6.48%), the presence of an s18278 caudal D-wave was predictive of a favorable long-term motor outcome even when the MEPs and/or SSEPs were lost during IDEM tumor resection. However, in 2 cases (1.85%) the D-wave permanently decreased by approximately 50%, and surgery was definitively abandoned to prevent permanent paraplegia. Cumulatively, SSEP, MEP, and D-wave monitoring significantly predicted postoperative deficits (p = 0.0001; AUC = 0.905), with a sensitivity of 85.7% and a specificity of 97%. Comparing the area under the receiver operating characteristic curves of these tests, D-waves appeared to have a significantly greater predictive value than MEPs and especially SSEPs alone (0.992 vs 0.798 vs 0.653; p = 0.023 and p < 0.001, respectively). On multiple logistic regression, the independent risk factors associated with significant IONM changes in the entire population were age older than 65 years and an anterolateral location of the tumor (p < 0.0001).CONCLUSIONSD-wave monitoring was feasible in all patients without severe preoperative motor deficits. D-waves demonstrated a statistically significant higher ability to predict postoperative deficits compared with SSEPs and MEPs alone and allowed us to proceed with IDEM tumor resection, even in cases of SSEP and/or MEP loss. Patients older than 65 years and with anterolateral IDEM tumors can benefit most from the use of IONM.

A motor evoked potential trending system may discriminate outcome: retrospective application with three cases

This report presents a method for tracking Motor Evoked Potential (MEP) amplitudes over the course of a case using a moving least squares linear regression (LSMAs). During a case, newly obtained MEP amplitudes are compared to those predicted by a just previous linear regression (least squares moving average or LSMA). When detected by this comparison, a set criterion amplitude loss will then trigger linear regression of ensuing MEP amplitudes on an expanding step function which tracks the persistence of the amplitude loss for the remainder of the case. Three cases are presented. One in which the patient woke up with a newly acquired weakness in the left tibialis anterior and another in which MEP amplitudes were suddenly lost from the right foot, but after intervention, they were restored again. In a third case the patient again woke up with a new post-operative deficit, but MEP trial sampling had been more limited and variable than in the first two cases. When the linear trending method was applied to the affected myotome in the first case, the expanding step function regression was triggered after the moment of MEP loss and remained at a high level until the end of case. In the second case, the expanding step function regression was also triggered in the relevant myotome at the time of the reported MEP change, but diminished by end of case. In the third case the tracking method again successfully triggered a predictive R-Square despite the limited number of pre-event trials. The R-Square value of the expanding step function regression appears to have discriminative capability with regard to new post-op deficit. Given the importance of the intra-operative MEP for monitoring motor functioning and the high degree of variability that can affect it, the development of new quantitative, statistical methods to detect real from apparent MEP change will be necessary.

Retrospective Waveform Analysis of Transcranial Motor Evoked Potentials (MEP) to Identify Early Predictors of Impending Motor Deficits in Spinal Surgeries

ABSTRACTPurpose: Although there are guidelines analyzing transcranial motor evoked potentials (MEP) waveform criteria, they vary widely and are not applied universally during intraoperative neurophysiologic monitoring (IONM). The objective is to generate hypotheses to identify early and reliable MEP waveform characteristics prior to complete loss of MEP to predict impending motor spinal cord injuries during spinal surgeries. The ultimate goal is to enhance real-time feedback to prevent injury or detect reversible spinal cord damage.Methods: Fifteen true positive cases of persistent intraoperative MEP loss and new postoperative motor deficits were retrospectively identified from 2011 to 2013. Waveform characteristics of latency, amplitude, duration, phases, and area-under-the-curve (AUC) were measured, and an intraoperative spinal cord index (ISCI) was calculated for 5 traces prior to complete MEP loss. ISCI = [amplitude x duration x (phases+1) x AUC]/latency.Results: Out of 22 muscles in 15 cases, latency increased in 2, duration decreased in 12, amplitude decreased in 13, AUC decreased in 13, and ISCI decreased in 14. In 11 out of 15 cases (73%), ISCI dropped > 40% in at least one muscle before MEP were completely lost. Thirteen cases had concurrent somatosensory evoked potentials (SSEP) changes, 9 out of 13 had > 50% decrease in SSEP: 2 out of 9 changed before MEP, 5 out of 9 simultaneously, and 2 out of 9 after.Conclusions: In these cases of motor injury, smaller and simpler MEP waveforms preceded complete loss of signal. An ISCI 40% drop could be tested as a warning threshold for impending motor compromise in future prospective studies and lead to eventual standardization to predict irreversible postoperative deficits.

Prognostic Values of Motor Evoked Potentials in Insular, Precental, or Postcentral Resections.

To determine the predictive values of motor evoked potentials (MEPs) on the muscle strength of patients undergoing insular and/or around precentral or postcentral neurosurgeries. Neurosurgeries were performed in 104 patients with continuous MEPs elicited with direct cortical and/or transcranial stimulation (DCS, TES; alarm criterion: reproducible 50% decrease in the MEP amplitudes). Muscle strength was evaluated with the British Medical Research Council Scale, before and postoperatively (1-3 days, 3 months). Permanent decreases in DCS-MEPs identified all new upper limbs deficits (sensitivity: 100%; TES-MEPs: 82.3% upper, 78.6% lower limbs). All MEP decreases predicted deficits (positive predictive value: 100%). None of the stable limbs were associated with MEP decrease (specificity: 100%). All unchanged DCS-MEPs predicted unchanged strength (negative predictive values: 100%; TES-MEPs: 95.3% upper, 95.5% lower limbs). The risks of paresis at 3 months were of 0% with none or MEP deterioration <50%; 1.1% (1/91 patients) with MEP deterioration (50%-90%); 7.7% (7/91 patients) with MEP loss. Deficits at 3 months were due to ischemia detected intraoperatively (4.8%, 5/104 patients) or consequent to postoperative hemorrhage (5.8%, 6/104 patients) or to disease progression (2.9%, 3/104 patients). The primary motor cortex and corticospinal pathway can reliably be monitored to protect motor strength during insular, precentral, and postcentral resections under general anesthesia. Nevertheless, MEPs did not prevent subcortical ischemias that might be reduced with continuous subcortical mapping. For the preservation of complex motor functions, for example, bimanual coordination, not evaluated here, insular surgeries can be performed with awake surgeries for which decision to undergo remains to the patient, aware of possible shorter survival.

Continuous distraction-induced delayed spinal cord injury on motor-evoked potentials and histological changes of spinal cord in a porcine model.

Experimental study. This study evaluated distraction-induced delayed spinal cord injury in a porcine model. Department of Orthopedics, Korea University Guro Hospital, Seoul, Korea. Global osteotomy of three columns was performed on the thirteenth thoracic vertebrae with 13 pigs. The osteotomized vertebrae were distracted to 57-103% of segmental vertebral height (SVH) length, which was less than the distraction length that induces prompt SCI. The vertebral height was maintained until the loss of motor-evoked potential (MEP) signals with continuous distraction. The distraction distance and the time at which SCI occurred were measured, and distraction was then released to observe MEP recovery patterns. We found delayed SCI in 8 of the 12 pigs, with a mean 20.9 mm (range 19-25 mm) and 10.7 min (range 8-12 min) of continuous spinal distraction, which was equivalent to 74.3% (68-84%) of SVH and 3.63% (3.42-4.31%) of thoracolumbar spinal length. A continuous 74.3% SVH distraction over an average of 10.7 min caused a delayed SCI, which was indicated by mild histologic changes in the spinal cord. Recovery patterns from SCI after distraction release were compatible with the degree of histological change; however, these patterns differed from the previously investigated prompt type of SCI. Late onset injury due to continuous spinal distraction, which is comparable to iatrogenic SCI in spinal correction surgery, is important for understanding the impact of corrective surgery.

Intraoperative motor evoked potential monitoring to patients with preoperative spinal deficits: judging its feasibility and analyzing the significance of rapid signal loss

Background ContextTranscranial motor evoked potential (MEP) monitoring has been widely adopted in spine surgery, but so far the useful monitoring data for patients with preoperative spinal deficits (PPSDs) are limited. Originally we thought that they seemed technically more difficult and less reliable in performing the MEP monitoring to PPSDs. PurposeOur objective was to study (1) the feasibility of MEP monitoring in PPSDs and the (2) the significance of rapid MEP loss. Study Design/SettingA retrospective case notes study from a prospective patient register was used as the study design. Patient SampleA total of 332 PPSDs who underwent posterior spine surgery with a reliable MEP monitoring were collected between September 2010 and December 2014. Outcome MeasuresRelevant MEP loss was identified as rapid amplitude reduction (more than 80% MEP) associated with high-risk surgical maneuvers or high-risk diagnoses. MethodThe muscles with higher strength were used to record the optimal MEP signal. MEP monitoring of these patients was considered to be feasible if reproducible signals had been obtained; moreover, sensitivity, specificity, positive predictive value (PPV), and negative predictive value were computed. The significance of the patients with rapid MEP loss was analyzed. ResultsFrom a total of 332 PPSDs, 27 cases showed significant MEP loss (23 true positive, 4 false positive), and 21 showed new spinal deficits. Invalid MEP baselines were found in 11 paralysis and 6 severely incomplete paraplegia patients, and success rate of reliable MEP was 95.1% in PPSDs. The congenital kyphoscoliosis, tuberculous kyphoscoliosis, and thoracic spinal stenosis are considered high-risk diagnoses to result in MEP loss. The sensitivity of intraoperative MEP monitoring was 100%, the specificity 98.7%, the positive predictive value 85.2%, and the negative predictive value 100%. ConclusionsIntraoperative MEP monitoring is feasible for most of the PPSDs. The rapid MEP loss during high-risk diagnoses and complicated surgical procedures may indicate new spinal deficits.

Change in body surface temperature as an ancillary measurement to motor evoked potentials.

Experimental study. To study the role of surface temperature as an adjunct to motor evoked potentials (MEPs) in rabbit spinal cord injury (SCI) model. Department of Orthopedics, Korea University Guro Hospital, Seoul, Korea. Rabbits (n =18) were divided into Complete (n = 9) and Incomplete (n = 9) SCI groups. Complete SCI was defined as being non-responsive to a wake-up test with loss of MEPs after transection of spinal cord. Incomplete SCI was defined as being responsive to a wake-up test with significant attenuation (⩾ 80%) of MEPs after impaction on spinal cord. Surface temperature of upper and lower extremities, core temperature and MEPs signals were checked before, during and after SCI for 20 min. A wake-up test was conducted and spinal cord was histologicaly evaluated. Experimental conditions between the two groups were statistically similar (P > 0.005 for all values). After SCI, upper extremity temperatures did not change in either group (P > 0.005); however, the surface temperature of the lower extremities in the Complete SCI Group elevated to 1.7 ± 0.5°C in comparison to 0.5 ± 0.1°C in the Incomplete SCI Group (P < 0.001). The scores of wake-up test in the Incomplete SCI Group were significantly different from that of the Complete SCI Group (P < 0.001), while white and gray matter damage was variable on histology. Monitoring of changes of body surface temperature of the lower extremities can be potentially used to identify the completeness of SCI in a rabbit model.

Methodology of motor evoked potentials in a rabbit model.

Spinal cord ischemia (SCI) is a devastating complication of aortic operations. Neuromonitoring using motor evoked potentials (MEPs) is a sensitive modality to detect SCI in humans. We describe a leporine SCI model using MEPs to test pharmaceutical therapeutics and other neuroprotective adjuncts. In 80 rabbits, methods to obtain MEPs in normotensive and ischemic rabbits were developed. The effects of isoflurane, propofol, apnea, and hypotension on lower extremity MEPs were studied. Lower extremity MEPs disappear upon SCI induction in 78 of 78 (100%) rabbits. Prior to SCI induction and during apneic episodes, lower extremity MEPs were lost in all (100%) and upper extremity MEPs in one (25%). Isoflurane was used in four experiments, with loss of lower extremity MEPs in all four (100%) and loss of upper extremity MEPs in zero. With propofol upper extremity, MEPs were obtainable in 80 of 80 rabbits (100%) and lower extremity MEPs in 78 of 80 rabbits (97.5%) prior to SCI induction. The presence of these lower extremity MEPs prior to SCI induction was not correlated with systolic or diastolic blood pressure. Disappearance of MEPs occurred in all 45 rabbits with postoperative lower extremity impairment. MEPs in the leporine model correlate closely with paraplegia. MEPs are influenced by inhaled anesthetics and apnea but not by hypotension alone. Propofol anesthesia provides reliable MEPs. This study provides the basis for a reproducible model of SCI to be used for novel therapeutic drug development.

P491: Reversible intraoperative neurophysiologic monitoring changes associated with surgical retraction

P491: Reversible intraoperative neurophysiologic monitoring changes associated with surgical retraction

Paraplegia prevention by oral pretreatment with memantine in a rabbit model

To evaluate the role of memantine (N-methyl-d-aspartate receptor antagonist) pretreatment for the prevention of spinal cord ischemia after infrarenal aortic clamping in a rabbit model. Thirty New Zealand White rabbits were divided into 5 different groups of 6 rabbits. Groups 60-7 and 60-5 received oral memantine 60 mg once a day for 7 and 5 days, respectively, and groups 30-5 and 30-3 received oral memantine 30 mg once a day for 5 and 3 days, respectively, all before surgery. Group C (control) received normal feeds without memantine. A paraplegic model was created by clamping both the aorta and the inferior vena cava infrarenally and just proximal to their bifurcations for 45 minutes. The modified Tarlov score, motor evoked potential (MEP), serum memantine concentration, and histopathology of the spinal cord were evaluated. The mean modified Tarlov scores were 4.2±1.3, 4.3±1.0, 4.2±1.3, 4.3±1.2, and 0.8±1.6 in groups 60-7, 60-5, 30-5, 30-3, and C, respectively at 6, 24, 48, and 72 hours (P<.009 for individual groups vs control). Percentage amplitude loss of MEP by the end of surgery was 29.5%±46.3%, 11.9%±28.0%, 30.0%±46.8%, 16.7%±40.8%, and 81.8%±40.3% for the 5 groups, respectively (P=.049). After declamping, MEP reappeared in 83%, 100%, 83%, 83%, and 33% of cases in the 5 groups, respectively (P=.073). The serum memantine level was similar in the 4 memantine groups. Spinal cords were normal in most of the rabbits in groups 60-7, 60-5, 30-5, and 30-3, but severely ischemic in most of the rabbits in group C (P=.041). Oral memantine pretreatment is protective against spinal cord ischemia, and can be an additional strategy for the prevention of paraplegia during thoracoabdominal aortic surgeries.

47. Monitoring of motor evoked potentials during thoracoabdominal aortic aneurysm repair

Spinal cord ischemia is one of the most serious complications of thoracoabdominal aortic aneurysm surgery. Hypotermia, cerebrospinal fluid drainage and pressure monitoring, reimplantation of segmental arteries and staged clamping are usual methods of spinal cord protection. We use electrophysiological monitoring of motor evoked potentials (MEPs) to measure a functional status of the thoracolumbar spinal cord during thoracoabdominal aortic surgery. Between May 2011 and May 2013, MEPs monitoring-assisted thoracoabdominal repair was performed in 14 patients. Stable MEPs were reproducible in 10 of 14 patients (71%). Nine of them displayed no neurological deficit after postoperative awakening. One patient suffered light hemiparesis due to brain ischemia. Bilateral MEPs loss occurred in 4 of 14 patients. Two of them recovered after spinal-cord protecting interventions with no neurological deficit. One patient died early after surgical intervention in the ICU and it was not possible to assess neurological functions in this patient. The remaining patient suffered left-sided hemiparesis, which improved to a very light deficit within several days after the procedure. There was no case with paraparesis or paraplegia in our group. Neurophysiological monitoring of spinal cord function is necessary to detect spinal cord ischemia. An advantage of MEPs comparing to the somatosensory evoked potentials (SSEPs) is rapid response to spinal malperfusion and corrective steps to prevent or reduce the extent of paraplegia and paraparesis can be taken with little delay. False positive results, detected in two patients of our group, are well known and described in literature and were not detrimental for the patients.