Spinal cord monitoring: current state of the art

A Modified Method of Triggered Electromyography Monitoring in Minimally Invasive Spine Surgery: Comparison to Conventional Techniques and Correlation with Body Mass Index

PURPOSE

Conventional triggered electromyography (EMG) in percutaneous pedicle screw (PPS) systems may be unreliable due to the interaction between the insertion apparatus and patient's soft tissue. Our aim was 1) to describe a modified technique of triggered EMG monitoring using insulated Kirschner wire (K-wires), 2) to compare EMG potentials with conventional techniques, and 3) to demonstrate the relationship between patient body mass index (BMI) and triggered EMG potentials.

METHODS

This was a prospective cross-sectional study of 50 patients undergoing minimally invasive PPS placement. Triggered EMG measurements using K-wires before and after insulation were compared. The difference between EMG measurements before and after insulation was correlated with patient BMI.

RESULTS

A total of 50 patients, 22 females and 28 males, underwent triggered EMG testing using K-wires prior to final PPS placement in the thoracic and lumbosacral spine for a total of 472 triggered EMG measurements. When compared to standard triggered EMG monitoring, insulated triggered EMG monitoring demonstrated an average 55.4% decrease in EMG values (P < 0.001). Increasing BMI correlated to increasing % decrease in EMG values (r-coefficient, 0.376; P < 0.01).

CONCLUSIONS

We describe a cost-effective, efficient, and reliable technique for triggered EMG during PPS placement which may help ensure accurate screw placement and minimize potentially devastating complications.

Constructing 2D maps of human spinal cord activity and isolating the functional midline with high-density microelectrode arrays

Intraoperative neuromonitoring (IONM) is a widely used practice in spine surgery for early detection and minimization of neurological injury. IONM is most commonly conducted by indirectly recording motor and somatosensory evoked potentials from either muscles or the scalp, which requires large-amplitude electrical stimulation and provides limited spatiotemporal information. IONM may inform of inadvertent events during neurosurgery after they occur, but it does not guide safe surgical procedures when the anatomy of the diseased spinal cord is distorted. To overcome these limitations and to increase our understanding of human spinal cord neurophysiology, we applied a microelectrode array with hundreds of channels to the exposed spinal cord during surgery and resolved spatiotemporal dynamics with high definition. We used this method to construct two-dimensional maps of responsive channels and define with submillimeter precision the electrophysiological midline of the spinal cord. The high sensitivity of our microelectrode array allowed us to record both epidural and subdural responses at stimulation currents that are well below those used clinically and to resolve postoperative evoked potentials when IONM could not. Together, these advances highlight the potential of our microelectrode arrays to capture previously unexplored spinal cord neural activity and its spatiotemporal dynamics at high resolution, offering better electrophysiological markers that can transform IONM.

Neurophysiological monitoring of spinal cord function during spinal deformity surgery: 2020 SRS neuromonitoring information statement

The Scoliosis Research Society has developed an updated information statement on intraoperative neurophysiological monitoring of spinal cord function during spinal deformity surgery. The statement reviews the risks of spinal cord compromise associated with spinal deformity surgery; the statement then discusses the various modalities that are available to monitor the spinal cord, including somatosensory-evoked potentials, motor-evoked potentials, and electromyographic (EMG) options. Anesthesia considerations, the importance of a thoughtful team approach to successful monitoring, and the utility of checklists are also discussed. Finally, the statement expresses the opinion that utilization of intraoperative neurophysiological spinal cord monitoring in spinal deformity surgery is the standard of care when the spinal cord is at risk.

Transcranial Motor Evoked Potentials during Spinal Deformity Corrections-Safety, Efficacy, Limitations, and the Role of a Checklist

Introduction

Intraoperative neuromonitoring (IONM) has become a standard of care in spinal deformity surgeries to minimize the incidence of new onset neurological deficit. Stagnara wake up test and ankle clonus test are the oldest techniques described for spinal cord monitoring, but they cannot be solely relied upon as a neuromonitoring modality. Somatosensory evoked potentials monitor only dorsal tracts and give high false positive and negative alerts. Transcranial motor evoked potentials (TcMEPs) monitor the more useful motor pathways. The purpose of our study was to report the safety, efficacy, limitations of TcMEPs in spine deformity surgeries, and the role of a checklist.

Study design

Retrospective review of all spinal deformity surgeries performed with TcMEPs from 2011 to 2015.

Materials and methods

All patients were subjected to IONM by TcMEPs during the spinal deformity surgery. Patients were included in the study only if complete operative reports and neuromonitoring data and postoperative neurological data were available for review. An alert was defined as 80% or more decrement in the motor evoked potential amplitude, or increase in threshold of 100 V or more from baseline. The systemic and surgical causes of IONM alerts and the postoperative neurological status were recorded.

Results

In total, 61 patients underwent surgery for spinal deformities with TcMEPs. The average age was 12.6 years (6–36 years) and male:female ratio was 1:1.3. Diagnoses included idiopathic scoliosis (n = 35), congenital scoliosis (n = 13), congenital kyphosis (n = 7), congenital kyphoscoliosis (n = 4), post-infectious kyphosis (n = 1), and post-traumatic kyphosis (n = 1). The average kyphosis was 72° (45°–101°) and the average scoliosis was 84° (62°–128°). There were in total 33 alerts in 22 patients (36%). The most common causes were hypotension (n = 7), drug induced (n = 5), deformity correction (n = 5), osteotomies (n = 3), tachycardia (n = 1), screw placement (n = 2), and electrodes disconnection (n = 1). Reversal of the inciting event cause resulted in complete reversal of the alert in 90% of the times. Three patients showed persistent alerts, out of whom one had a positive wake up test and woke up with neurodeficit, which recovered over few weeks, while the other patients showed persistent alerts but woke up without any deficit. Sensitivity and specificity of TcMEP in deformity correction surgery were 100 and 96.6%, respectively, in our study.

Conclusion

IONM alerts are frequent during spinal deformity surgery. In our study, more than 50% of the alerts were associated with anesthetic management. IONM with TcMEPs is a safe and effective monitoring technique and wake up test still remains a valuable tool in cases of a persistent alert.

Sugammadex to reverse neuromuscular blockade and provide optimal conditions for motor-evoked potential monitoring

Sugammadex is a novel pharmacologic agent, which reverses neuromuscular blockade (NMB) via a mechanism that differs completely from acetylcholinesterase inhibitors. By encapsulating rocuronium, sugammadex can provide recovery of neuromuscular function even when there is a profound degree of NMB. We report anecdotal experience with the use of sugammadex to reverse NMB to facilitate intraoperative neurophysiological monitoring (motor evoked potentials) in an adolescent with scoliosis during posterior spinal fusion. Its potential application in this unique clinical scenario is discussed, and potential dosing schemes are reviewed.

Transient Monoplegia as a Result of Unilateral Femoral Artery Ischemia Detected by Multimodal Intraoperative Neuromonitoring in Posterior Scoliosis Surgery: A Case Report

This is to report a case of 16-year-old girl with transient right lower limb monoplegia as a result of femoral artery ischemia detected by multimodal intraoperative spinal cord neuromonitoring (MISNM) during posterior correction surgery of adolescent idiopathic scoliosis.A patient with a marfanoid body habitus and LENKE IA type scoliosis with the right thoracic curve of 48° of Cobb angle was admitted for posterior spinal fusion from Th6 to L2. After selective pedicle screws instrumentation and corrective maneuvers motor evoked potentials (MEP) began to decrease with no concomitant changes in somato-sensory evoked potentials recordings.The instrumentation was released first partially than completely with rod removal but the patient demonstrated constantly increasing serious neurological motor deficit of the whole right lower limb. Every technical cause of the MEP changes was eliminated and during the wake-up test the right foot was found to be pale and cold with no popliteal and dorsalis pedis pulses palpable. The patient was repositioned and the pelvic pad was placed more cranially. Instantly, the pulse and color returned to the patient's foot. Following MEP recordings showed gradual return of motor function up to the baseline at the end of the surgery, whereas somato-sensory evoked potentials were within normal range through the whole procedure.This case emphasizes the importance of the proper pelvic pad positioning during the complex spine surgeries performed in prone position of the patient. A few cases of neurological complications have been described which were the result of vascular occlusion after prolonged pressure in the inguinal area during posterior scoliosis surgery when the patient was in prone position. If incorrectly interpreted, they would have a significant impact on the course of scoliosis surgery.

A preliminary study of volatile agents or total intravenous anesthesia for neurophysiological monitoring during posterior spinal fusion in adolescents with idiopathic scoliosis

STUDY DESIGN

A prospective randomized controlled trial.

OBJECTIVE

The purpose of this study was to prospectively compare the efficacy of neurophysiological monitoring during general anesthesia with either a total intravenous technique or with the volatile anesthetic agent, desflurane.

SUMMARY OF BACKGROUND DATA

A total intravenous anesthetic technique is generally chosen when neurophysiological monitoring is used as it has been shown to facilitate such monitoring. Despite this, with prolonged infusions of propofol, prolonged awakening times may be seen, which may impact the time required for postoperative neurological assessment or more importantly result in significant delays, should a wake-up test become necessary. To date, there are no prospective trials comparing intravenous techniques with a volatile agent-based anesthetic technique and its effects on neurophysiological monitoring.

METHODS

This prospective study compares somatosensory evoked potential and motor evoked potential monitoring during posterior spinal fusion in 30 adolescents. The patients were randomized to receive a total intravenous technique with propofol-remifentanil or a volatile agent-based technique with desflurane-remifentanil.

RESULTS

The groups were similar with regard to age, weight, height, body mass index, Cobb angle, and distribution of Lenke classifications. No differences were noted in anesthesia time, surgery time, intraoperative fluids, or estimated blood loss between the 2 groups. Time to eye opening, time to following commands, and time to tracheal extubation were shorter in the volatile anesthesia group than the total intravenous anesthesia group. No clinically significant difference was noted in the amplitude or latency of somatosensory evoked potential monitoring. Although statistically significantly greater voltage amplitude was required to generate a motor evoked potential, the voltage amount was within a clinically acceptable range.

CONCLUSION

Our data demonstrate that a volatile agent-based anesthetic regimen is feasible even during neurophysiological monitoring. Advantages include a more rapid awakening and the feasibility of a rapid wake-up test (<5 min) in the event that irreversible changes in neurophysiological monitoring are noted.

LEVEL OF EVIDENCE

2.

Complications and outcomes of complex spine reconstructions in poliomyelitis-associated spinal deformities: a single-institution experience

STUDY DESIGN

Retrospective case series.

OBJECTIVE

To share our institutional experience with spinal reconstruction for deformity correction in patients with a history of poliomyelitis.

SUMMARY OF BACKGROUND DATA

Polio and postpolio syndrome are not uncommonly related to a paralytic spinal deformity. Limited modern data exist regarding outcomes and complications after spinal reconstruction in this population.

METHODS

A clinical database was reviewed for patients undergoing spinal reconstruction for polio-associated spinal deformity at our institution from 1985 to 2012. Relevant demographic, medical, surgical, and postoperative information were collected from medical records and analyzed. Preoperative and last follow-up Scoliosis Research Society-22 Questionnaire scores were recorded.

RESULTS

A total of 22 patients with polio who underwent surgical deformity correction were identified. Mean age was 49 years (range, 12-74 yr), and 15 patients (68%) were female. Preoperative motor deficit was present in 14 of 22 (64%) patients. All patients underwent instrumented spinal fusion (mean, 13 vertebral levels, range, 3-18). Ten (10/22, 45%) patients developed major complications, and 4 patients (4/22, 18%) developed new postoperative neurological deficits. Neurological monitoring yielded a 13% false-negative rate. At 2-year follow-up, 20 of 22 patients maintained an average coronal correction of 25° (33%, P = 0.001) and sagittal correction of 25° (34%, P = 0.003). Minimum 2-year follow-up data were available for 11 of 22 (50%) patients. At an average of 72 months of follow-up (range, 28-134 mo), the mean Scoliosis Research Society-22 Questionnaire pain subscore improved from a mean of 2.75 to 3.6 (P = 0.012); self-image from 2.8 to 3.7 (P = 0.041); function from 3.1 to 3.8 (P = 0.036); satisfaction from 2.1 to 3.9 (P = 0.08); and mental health from 3.7 to 4.5 (P = 0.115).

CONCLUSION

Spine reconstruction for poliomyelitis-associated deformity was associated with high complication rates (54%) and sometimes unreliable neurological monitoring data. Despite this, patients undergoing spine reconstructions had significantly improved outcome scores. These data may help surgeons to appropriately counsel this complicated patient population.

Transcranial motor evoked potential monitoring outcome in the high-risk brain and spine surgeries: Correlation of clinical and neurophysiological data - An Indian perspective

Objective:

The objective of this study is to assess the safety, feasibility and clinical value of transcranial motor evoked potential (MEP) monitoring by electrical stimulation.

Setting:

Clinical neurophysiology department of tertiary reach hospital.

Materials and Methods:

MEP monitoring was attempted in 44 “high risk” patients. Intraoperative surgical, anesthesia and neurophysiological findings were documented prospectively. MEP monitoring results were correlated with motor outcome.

Results:

The success for reliable MEP recording from the lower limbs was 75%. Incidence of new permanent post-operative motor deficit was zero. Nearly, 76.5% of the cases (13 out of 17 cases) who showed unobtainable and unstable MEP outcome had lesion location in the spine as compared with 23.5% (4 out of 17 cases) that had lesion location in the brain. Chi-square test demonstrated a statistically significant difference between these two groups (P = 0.0020). Out of these 13 spine surgery cases, 8 (62%) were operated for deformity. Seven out of 12 (60%) patients less than 12 years of age had a poor MEP monitoring outcome suggesting that extremes of age and presence of a spine deformity may be associated with a lesser incidence of successful MEP monitoring. No complications related to the repetitive transcranial electrical stimulation for eliciting MEP were observed.

Conclusion:

MEP monitoring is safe. The protocol used in this study is simple, feasible for use and has a fairly high success rate form the lower limbs. Pediatric age group and spine lesions, particularly deformities have an adverse effect on stable MEP recording.

Significant change or loss of intraoperative monitoring data: a 25-year experience in 12,375 spinal surgeries

STUDY DESIGN

Retrospective.

OBJECTIVE

The purpose of this study was to report the spectrum of intraoperative events responsible for a loss or significant change in intraoperative monitoring (IOM) data.

SUMMARY OF BACKGROUND DATA

The efficacy of spinal cord/nerve root monitoring is demonstrated in a large, single institution series of patients, involving all levels of the spinal column (occiput to sacrum) and all spinal surgical procedures.

METHODS

Multimodality IOM included somatosensory-evoked potentials, descending neurogenic-evoked potentials, neurogenic motor-evoked potentials, and spontaneous and triggered electromyography. A total of 12,375 patients who underwent surgery for spinal pathology between January 1985 and December 2010 were reviewed. There were 59.3% female patients (7178) and 40.7% male patients (5197). Procedures by spinal level were as follows: cervical 29.7% (3671), thoracic/thoracolumbar 45.4% (5624), and lumbosacral 24.9% (3080). Age at the time of surgery was as follows: older than 18 years, 72.7% (242/8993) and younger than 18 years, 27.3% (144/3382). A total of 77.8% (9633) patients underwent primary surgical procedures and 22.2% (2742) patients underwent revision surgical procedures.

RESULTS

A total of 406 instances of IOM data change/loss occurred in 386 of 12,375 (3.1%) patients. Causes for data degradation/loss included the following: instrumentation (n = 131), positioning (n = 85), correction (n = 56), systemic (n = 49), unknown (n = 24), and focal spinal cord compression (n = 15). Data loss/change was seen in revision (6.1%/167 patients) surgical procedures more commonly than in primary procedures (2.3%/219 patients; P < 0.0001). Data improvement was demonstrated by 88.7% (n = 360) after intervention versus 11.3% (n = 46) with no improvement in IOM data. One patient with improved data after intervention versus 14 with no improvement despite intervention had a permanent neurological deficit (P < 0.0001).

CONCLUSION

IOM data identified 386 (3.1%) patients with loss/degradation of data in 12,375 spinal surgical procedures. Fortunately, in 93.3% of patients, intervention led to data recovery and no neurological deficits. Reduction from a potential (worst-case scenario) 3.1% (386) of patients with significant change/loss of IOM data to a permanent neurological deficit rate of 0.12% (15) patients was achieved (P < 0.0001), thus confirming efficacy of IOM.

Neurologic risk in growing rod spine surgery in early onset scoliosis: is neuromonitoring necessary for all cases?

STUDY DESIGN

Retrospective case series from a multicenter database.

OBJECTIVE

To evaluate the risk of neurologic injury during growing rod surgeries and to determine whether intraoperative neuromonitoring is necessary for all growing rod procedures.

SUMMARY OF BACKGROUND DATA

Although the use of growing rod constructs for early-onset spinal deformity has become a commonly accepted treatment, the incidence of neurologic events during growing rod surgeries remains unknown.

METHODS

We reviewed data from a multicenter database on 782 growing rod surgeries performed in 252 patients. VEPTR devices and any constructs with rib attachments were excluded. A questionnaire was sent to all surgeons contributing cases requesting detailed information about all neurologic events associated with any growing rod surgery.

RESULTS

There were 782 growing rod surgeries performed on 252 patients including 252 primary growing rod implantations, 168 implant exchanges, and 362 lengthenings. Five hundred sixty-nine of 782 (73%) cases were performed with neuromonitoring. Only one clinical injury occurred in the series, resulting in an injury rate of 0.1% (1/782). This deficit occurred during an implant exchange while attempting pedicle screw placement, and resolved within 3 months. There were 2 cases with neuromonitoring changes during primary implant surgeries (0.9%, 2/231), 1 change during implant exchanges (0.9%, 1/116), and 1 neuromonitoring change during lengthenings (0.5%, 1/222). The single monitoring change that occurred during a lengthening was in a child with an intracanal tumor who also had a monitoring change during the primary surgery. There are anecdotal cases (outside this study group series) of neuromonitoring changes during simple lengthenings in children with uneventful primary implantations.

CONCLUSION

Based on our study, the largest reported series of growing rod surgeries, the rate of neuromonitoring changes during primary growing rod implantation (0.9%) and exchange (0.9%) justifies the use of intraoperative neuromonitoring during these surgeries. As there were no neurologic events in 361 lengthenings in patients with no previous neurologic events, the question may be raised as to whether intraoperative neuromonitoring is necessary for simple lengthenings in these patients. However, caution should be maintained when interpreting our results as anecdotal cases of neurologic changes from simple lengthenings do exist outside of this series.

Sciatic nerve injury associated with acetabular fractures

Sciatic nerve injuries associated with acetabular fractures may be a result of the initial trauma or injury at the time of surgical reconstruction. Patients may present with a broad range of symptoms ranging from radiculopathy to foot drop. There are several posttraumatic, perioperative, and postoperative causes for sciatic nerve palsy including fracture-dislocation of the hip joint, excessive tension or inappropriate placement of retractors, instrument- or implant-related complications, heterotopic ossification, hematoma, and scarring. Natural history studies suggest that nerve recovery depends on several factors. Prevention requires attention to intraoperative limb positioning, retractor placement, and instrumentation. Somatosensory evoked potentials and spontaneous electromyography may help minimize iatrogenic nerve injury. Heterotopic ossification prophylaxis can help reduce delayed sciatic nerve entrapment. Reports on sciatic nerve decompression are not uniformly consistent but appear to have better outcomes for sensory than motor neuropathy.

Effects of dexmedetomidine on propofol and remifentanil infusion rates during total intravenous anesthesia for spine surgery in adolescents

INTRODUCTION

Total intravenous anesthesia with propofol and a synthetic opioid is a frequently chosen anesthetic technique for posterior spinal fusion. Despite its utility, adverse effects may occur with high or prolonged propofol dosing regimens including delayed awakening. The current study investigated the propofol-sparing effects of the concomitant administration of the alpha(2)-adrenergic agonist, dexmedetomidine, during spinal fusion surgery in adolescents.

METHODS

The surgical database of the department of orthopedic surgery was searched and patients (12-21 years of age) were identified who had undergone spinal fusion for either idiopathic or neuromuscular scoliosis during the past 24 months. Patients were assigned to two groups. Group 1 included patients anesthetized with propofol and remifentanil and group 2 included patients anesthetized with dexmedetomidine, propofol, and remifentanil. In the latter group, dexmedetomidine was administered as a continuous infusion of 0.5 microg.kg(-1).h(-1) started after the induction of anesthesia without a loading dose. Propofol was adjusted to maintain the bispectral index (BIS) number at 40-50 and remifentanil was adjusted to maintain the mean arterial pressure (MAP) at 50-65 mmHg. Labetolol or hydralazine was used if the MAP could not be maintained at 50-65 mmHg with remifentanil up to a maximum dose of 0.6 microg/kg/min. Statistical analysis included a nonpaired t-test for parametric data (age, weight, remifentanil/propofol infusion requirements, and heart rate/blood pressure values). A nonparametric statistical analysis (Dunn) was used to compare BIS numbers. Parametric data are presented as the mean +/- SD while nonparametric data are presented as the median and the 95th percentile confidence intervals.

RESULTS

Twelve patients received propofol-remifentanil-dexmedetomidine and 24 received propofol-remifentanil. There were no differences in the demographic data, BIS numbers or hemodynamic parameters between the two groups. There was a reduction in the propofol infusion requirements in patients who also received dexmedetomidine (71 +/- 11 microg.kg(-1).min(-1)) compared with those receiving only propofol-remifentanil (101 +/- 33 microg.kg(-1).min(-1), P = 0.0045). No difference was noted in the remifentanil infusion requirements or the use of supplemental agents (hydralazine and labetolol) to maintain controlled hypotension.

CONCLUSION

The concomitant use of dexmedetomidine in patients undergoing spinal fusion reduces propofol infusion requirements when compared with those patients receiving only propofol and remifentanil.

Effects of dexmedetomidine on intraoperative motor and somatosensory evoked potential monitoring during spinal surgery in adolescents

BACKGROUND

Dexmedetomidine may be a useful agent as an adjunct to an opioid-propofol total intravenous anesthesia (TIVA) technique during posterior spinal fusion (PSF) surgery. There are limited data regarding its effects on somatosensory (SSEPs) and motor evoked potentials (MEPs).

METHODS

The data presented represent a retrospective review of prospectively collected quality assurance data. When the decision was made to incorporate dexmedetomidine into the anesthetic regimen for intraoperative care of patients undergoing PSF, a prospective evaluation of its effects on SSEPs and MEPs was undertaken. SSEPs and MEPs were measured before and after the administration of dexmedetomidine in a cohort of pediatric patients undergoing PSF. Dexmedetomidine (1 microg x kg(-1) over 20 min followed by an infusion of 0.5 microg x kg(-1) x h(-1)) was administered at the completion of the surgical procedure, but prior to wound closure as an adjunct to TIVA which included propofol and remifentanil, adjusted to maintain a constant depth of anesthesia as measured by a BIS of 45-60.

RESULTS

The cohort for the study included nine patients, ranging in age from 12 to 17 years, anesthetized with remifentanil and propofol. In the first patient, dexmedetomidine was administered in conjunction with propofol at 110 microg x kg(-1) x min(-1) which resulted in a decrease in the bispectral index from 58 to 31. Although no significant effect was noted on the SSEPs (amplitude or latency) or the MEP duration, there was a decrease in the MEP amplitude. The protocol was modified so that the propofol infusion was incrementally decreased during the dexmedetomidine infusion to achieve the same depth of anesthesia. In the remaining eight patients, the bispectral index was 52 +/- 6 at the start of the dexmedetomidine loading dose and 49 +/- 4 at its completion (P = NS). There was no statistically significant difference in the MEPs and SSEPs obtained before and at completion of the dexmedetomidine loading dose.

CONCLUSION

Using the above-mentioned protocol, dexmedetomidine can be used as a component of TIVA during PSF without affecting neurophysiological monitoring.

Intraoperative somatosensory evoked potential monitoring during anterior cervical discectomy and fusion in nonmyelopathic patients--a review of 1,039 cases

BACKGROUND CONTEXT

Intraoperative somatosensory evoked potential (SSEP) monitoring has been shown to reduce the incidence of new postoperative neurological deficits in scoliosis surgery. However, its usefulness during cervical spine surgery remains a subject of debate.

PURPOSE

To determine the utility of intraoperative SSEP monitoring in a specific patient population (those with cervical radiculopathy in the absence of myelopathy) who underwent anterior cervical discectomy and fusion (ACDF) surgery.

STUDY DESIGN

Retrospective review.

PATIENT SAMPLE

A total of 1,039 nonmyelopathic patients who underwent single or multilevel ACDF surgery. The control group (462 patients) did not have intraoperative SSEP monitoring, whereas the monitored group (577 patients) had continuous intraoperative SSEP monitoring performed.

OUTCOME MEASURE

A new postoperative neurological deficit.

METHODS

SSEP tracings were reviewed for all 577 patients in the monitored group and all significant signal changes were noted. Medical records were reviewed for all 1,039 patients to determine if any new neurological deficits developed in the immediate postoperative period.

RESULTS

None of the patients in the control group had any new postoperative neurological deficits. In the monitored group there were six instances of transient SSEP changes (1 due to suspected carotid artery compression; 5 thought to be due to transient hypotension) which resolved with the appropriate intraoperative intervention (repositioning of retractors; raising the arterial blood pressure). Upon waking up from anesthesia, one patient in the monitored group had a new neurological deficit (partial central cord syndrome) despite normal intraoperative SSEP signals.

CONCLUSIONS

ACDF appears to be a safe surgical procedure with a low incidence of iatrogenic neurological injury. Transient SSEP signal changes, which improved with intraoperative interventions, were not associated with new postoperative neurological deficits. An intraoperative neurological deficit is possible despite normal SSEP signals.

Intraoperative neurophysiologic monitoring: focus on cervical myelopathy and related issues

BACKGROUND CONTEXT

The use of neurophysiologic monitoring during surgical procedures for cervical spondylotic myelopathy (CSM) is controversial.

PURPOSE

The aim of this article is to review the literature regarding various monitoring techniques as applied to the patient with CSM.

STUDY DESIGN/METHODS

A systematic literature review.

CONCLUSIONS

Neurophysiologic monitoring is a diagnostic tool for assessment of neurologic function during cervical spine surgery. Recording of somatosensory evoked potentials (SSEPs), transcranial electrical motor evoked potentials (tceMEPs), and electromyograms (EMGs) may be useful as these monitoring modalities provide complementary information.

Spinal cord monitoring in scoliosis surgery using an epidural electrode. Results of a prospective, consecutive series of 191 cases

STUDY DESIGN

Retrospective analysis of a prospectively accrued series of 191 consecutive patients who underwent intraoperative neurophysiologic monitoring during scoliosis corrective surgery.

OBJECTIVES

To compare the monitoring outcome of idiopathic and neuromuscular scoliosis. To demonstrate the usefulness of the epidural electrode. To report sensitivity and specificity of the monitoring method employed at a single institution.

SUMMARY OF BACKGROUND DATA

Reports in the literature emphasized the difficulty to obtain data in neuromuscular patients. Multimodality spinal cord monitoring has been recommended. Despite their still debated composition, neurogenic motor-evoked potentials have proven their validity in clinical practice.

METHODS

Somatosensory and neurogenic evoked potentials were attempted in all patients presenting for scoliosis correction between 1999 and 2005. Study patients were divided into 3 groups: group 1, idiopathic; group 2, neuromuscular; and group 3, miscellaneous origins.

RESULTS

The use of the epidural electrode demonstrated significant usefulness in the ability of monitoring otherwise nonmonitored patients, especially in group 2. Inability to obtain any evoked potentials occurred in 4 cases (2.1%). Five cases were found to be true positives. An adapted and rapid intervention permitted to avoid new postoperative deficit in all cases. There was no instance of false-negative data. The overall method sensitivity was 100%, and specificity was 52.69%.

CONCLUSIONS

The use of a single epidural electrode allowing somatosensory evoked potentials recording and spinal cord stimulation alternately is a safe and valid method of intraoperative monitoring.

Optimal end-tidal concentration of sevoflurane to test an ankle clonus in children

STUDY DESIGN

A prospective randomized study on the end-tidal concentrations of sevoflurane at which ankle clonus existed.

SUMMARY OF BACKGROUND DATA

The ankle clonus reflex is a transient neurologic abnormality, which normally occurs in patients during emergence from general anesthesia.

OBJECTIVE

To determine the optimal end-tidal concentration of sevoflurane to test an ankle clonus in children during emergence from general anesthesia.

METHODS

We studied 30 children (aged 5-15 years). General anesthesia was induced with thiopental sodium. Anesthesia was maintained with sevoflurane, oxygen, and air. At completion of surgery, 3% volume of the end-tidal sevoflurane concentration was maintained for at least 10 minutes. Ankle clonus was checked at 1.0% to 0.1% volume of the end-tidal sevoflurane concentration with an interval of 0.05% volume.

RESULTS

Of children, 80% had bilateral ankle clonus, which was found most frequently when patients responded poorly to verbal commands. The median of the end-tidal sevoflurane concentration for a reactive ankle clonus was 0.45% volume (interquartile range 0.5% to 0.4% volume).

CONCLUSIONS

The ankle clonus should be tested at 0.45% volume of end-tidal sevoflurane concentration in children undergoing scoliosis surgery during emergence from the general anesthesia.

Long-term changes in spinal cord evoked potentials after compression spinal cord injury in the rat

1. After traumatic spinal cord injury (SCI), histological and neurological consequences are developing for several days and even weeks. However, little is known about the dynamics of changes in spinal axonal conductivity. The aim of this study was to record and compare repeated spinal cord evoked potentials (SCEP) after SCI in the rat during a 4 weeks' interval. These recordings were used: (i) for studying the dynamics of functional changes in spinal axons after SCI, and (ii) to define the value of SCEP as an independent outcome parameter in SCI studies. 2. We have used two pairs of chronically implanted epidural electrodes for stimulation/recording. The electrodes were placed below and above the site of injury, respectively. Animals with implanted electrodes underwent spinal cord compression injury induced by epidural balloon inflation at Th8-Th9 level. There were five experimental groups of animals, including one control group (sham-operated, no injury), and four injury groups (different degrees of SCI). 3. After SCI, SCEP waveform was either significantly reduced or completely lost. Partial recovery of SCEPs was observed in all groups. The onset and extent of recovery clearly correlated with the severity of injury. There was good correlation between quantitated SCEP variables and the volumes of the compressing balloon. However, sensitivity of electropohysiological parameters was inferior compared to neurological and morphometric outcomes. 4. Our study shows for the first time, that the dynamics of axonal recovery depends on the degree of injury. After mild injury, recovery of signal is rapid. However, after severe injury, axonal conductivity can re-appear after as long as 2 weeks postinjury. In conclusion, SCEPs can be used as an independent parameter of outcome after SCI, but in general, the sensitivity of electrophysiological data were worse than standard morphological and neurological evaluations.

Spinal cord monitoring in patients with spinal deformity and neural axis abnormalities: a comparison with adolescent idiopathic scoliosis patients

STUDY DESIGN

A retrospective review of spinal cord monitoring (SCM) results of patients with neural axis abnormalities (NAA) as compared with a control group of adolescent idiopathic scoliosis (AIS) patients.

OBJECTIVE

To analyze SCM on a group of patients who had a NAA undergoing spinal deformity surgery.

SUMMARY OF BACKGROUND DATA

To our knowledge, only 1 report in the literature has analyzed the accuracy and reliability of SCM in patients with NAA.

METHODS

Over a 10-year period, 41 patients with NAA had SCM while undergoing surgery for spinal deformity. These patients were retrospectively compared with a group of 136 AIS patients.

RESULTS

The average ages were similar (14.4 vs. 14.6 years), but there were more males (48.8% vs. 19.1%) and greater preoperative curve magnitude in the NAA group (65.9 degrees vs. 59.8 degrees ) (P < 0.05). Good baseline values were achieved less often in the NAA group for somatosensory-evoked potentials (SSEP) (85.4% vs. 98.5%) and motor-evoked potentials (MEP) (82.6% vs. 100%) (P < 0.05). Significant deviations from baseline values were seen more often in the NAA group for SSEP (8.6% vs. 1.5%) and MEP (5.3% vs. 2.5%). There were no false negatives in either group.

CONCLUSIONS

SCM in patients who have NAA can be more difficult to obtain than in AIS but results in few false positives and does not miss neurologic injury.

Intraoperative Electrophysiologic Monitoring: Considerations for Complex Spinal Surgery

Intraoperative neurophysiologic monitoring techniques have evolved as the complexity of spinal surgery has increased and the limitations of individual modalities have become apparent. Current monitoring strategies include a combination of techniques directed toward detecting changes in sensory, motor, and nerve root function. Close coordination and communication between the monitoring personnel, surgeon, and anesthesiologist is essential to effective intraoperative monitoring.

Intraoperative somatosensory evoked potential monitoring during cervical spine corpectomy surgery: experience with 508 cases

STUDY DESIGN

Retrospective review.

OBJECTIVES

To review consecutive cases of cervical spine corpectomy surgery performed with intraoperative somatosensory-evoked potential (SSEP) monitoring.

SUMMARY OF BACKGROUND DATA

There is controversy about the utility of SSEP monitoring during anterior cervical spine surgery. There is no study in the literature that has specifically evaluated the utility of SSEP monitoring for cervical spine corpectomy surgery.

METHODS

Intraoperative SSEP tracings for 508 patients (average age, 55.7 years; 268 male, 240 female) who underwent anterior cervical fusion with single-level or multilevel corpectomies were reviewed. Intraoperative and postoperative records were analyzed to determine if any new neurologic deficits developed when the patients woke up from anesthesia.

RESULTS

The overall incidence of a new postoperative neurologic deficit in this series of patients was 2.4% (11 with nerve root injury, 1 with quadriplegia). The incidence of significant SSEP changes was 5.3% (27 of 508 patients). The most common identifiable cause of SSEP changes was hypotension, and the most common neurologic deficit was deltoid (C5) weakness. One patient had irreversible SSEP changes, and he woke up with new-onset quadriplegia. The calculated sensitivity and specificity of intraoperative SSEP monitoring for detecting impending or resultant intraoperative iatrogenic neurologic injury were 77.1% and 100%, respectively. However, if the isolated nerve root injuries are removed from the analysis, then both the calculated sensitivity and the negative predictive values were 100%.

CONCLUSIONS

Intraoperative SSEP monitoring can alert the surgeon to adverse iatrogenic intraoperative events with potential for neurologic injury. Most SSEP signal changes are reversible and do not result in a clinical deficit. Isolated nerve root injury appears to be the most common iatrogenic intraoperative injury during cervical spine corpectomy surgery.

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.

Somatosensory evoked potential monitoring during anterior cervical discectomy and fusion

STUDY DESIGN

A retrospective, multicenter clinical review was conducted.

OBJECTIVE

To examine our experience using somatosensory evoked potential (SSEP) monitoring during anterior cervical discectomy and fusion (ACDF) to determine if monitoring of the spinal cord with SSEPs was helpful in identifying reversible causes of neurologic impairment while performing the procedure.

SUMMARY OF BACKGROUND DATA

Recent studies have strongly supported the use of SSEP monitoring during complicated and upper-cervical spine surgery.

METHODS

The complete medical records of 163 patients who underwent ACDF, and who were monitored with SSEPs during the procedure between 1995 and 2002 were retrospectively reviewed. A single observer who was uninvolved with patient care abstracted these medical records. Demographic data, length of symptoms, workers' compensation status, primary diagnosis, preoperative neurologic status, number of levels fused, bone graft type, implants used, SSEP findings, postoperative neurologic status, complications, and recovery from complications were recorded. Final neurologic status was determined through phone contact with patients or outpatient charts of patients who could not be contacted personally.

RESULTS

There were 3 false positive (1.8%) intraoperative SSEP findings in which SSEP changes intraoperatively did not reflect a neurologic deterioration after surgery. There was 1 false negative (0.6%) in which a new neurologic deficit occurred after surgery, despite no change in SSEP amplitudes during the operation. There were 2 true negatives (1.2%) in which SSEP monitoring showed a preexisting neurologic deficit, which did not change during the operation.

CONCLUSION

In no instance were positive SSEP findings clinically useful in alerting the surgeon to potential intraoperative complications. Intraoperative SSEP monitoring is not helpful to the surgeon when performing routine ACDF. Additionally, ACDF is a safe procedure with a low rate of neurologic complications.

Nitrous oxide with propofol reduces somatosensory-evoked potential amplitude in children and adolescents

STUDY DESIGN

A repeat measures design.

OBJECTIVES

Nitrous oxide significantly reduces cortical somatosensory-evoked potential amplitude in adults; however, its effect on children and adolescents is unknown. This study evaluates the effect of nitrous oxide combined with propofol on the amplitude of the cortical response in children and adolescents.

SUMMARY OF BACKGROUND DATA

Somatosensory-evoked potential amplitude measurements are used to determine spinal cord function during spine surgery. A significant decrease in amplitude of the cortical response may indicate either a compromise of neural conduction in the spinothalamic tracts of the dorsal column medial lemniscus system or the effect of anesthesia, an innocuous event.

METHODS

Ten consecutive participants were evaluated. The study group comprised 4 boys and 6 girls, ages 10 to 18 years, undergoing corrective spinal surgery at an orthopedic children's hospital in northwestern Pennsylvania. The tibial nerve was stimulated and responses recorded from the somatosensory cortex when anesthesia was administered with and without nitrous oxide. Results were compared using a paired t test (alpha = 0.05)

RESULTS

A significant decrease of 42 +/- 17% (P < 0.05) occurred in the cortically recorded somatosensory-evoked potential amplitude from 1.63 +/- 0.59 uV without nitrous oxide to 0.92 +/- 0.47 uV with nitrous oxide. Responses were similar in right and left extremities.

CONCLUSIONS

In our study, nitrous oxide use during low-dose isoflurane anesthesia supplemented with titrated propofol infusion caused a significant reduction in cortically recorded somatosensory-evoked potentials that was comparable with the 50% decrease observed in adults.

Probing for thoracic pedicle screw tract violation(s): is it valid?

BACKGROUND

Preparation of the thoracic pedicle screw tract is a critical step prior to the placement of screws. The ability to detect pedicle wall violation(s) by probing prior to insertion of thoracic pedicles screws, however, has not been studied. The purpose of this study was to evaluate the inter- and intraobserver agreement and the accuracy in detecting thoracic pedicle screw tract violation(s) among surgeons at various levels of training.

METHODS

With use of a straightforward trajectory, under direct visualization, 108 thoracic pedicle screw tracts (54 cadaveric thoracic vertebrae) were prepared in a standard fashion, followed by tapping with a 4.5-mm cannulated tap. A deliberate pedicle violation was randomly created by an independent investigator in either the anterior, the medial, or the lateral wall in 65 pedicles. Following this, four blinded, independent surgeons at various levels of training probed the specimens on three separate occasions to determine if a breach was present (1,296 discrete data points). Surgeon findings were then recorded as breach present or absent and, if present, breach location. The Cohen kappa correlation coefficient (kappa a) and 95% confidence interval were used to assess the accuracy of the observers and the inter- and intraobserver agreement.

RESULTS

The mean accuracy over three iterations, the validity in detecting the breach location, and the intraobserver agreement varied by level of training and experience, with the most experienced observer (observer 1) scoring the best and the least experienced observer (observer 4) scoring the worst. The three most senior surgeons had good intraobserver agreement. Interobserver agreement was low between the four observers.

CONCLUSIONS

An observer's ability to accurately detect the presence or absence of a pedicle tract violation and the breach location, if present, is dependent on the surgeon's level of training. Probing the pedicle tract prior to placement of pedicle screws in the thoracic spine is likely a learned skill that improves with repetition and experience.

The effects of isoflurane and propofol on intraoperative neurophysiological monitoring during spinal surgery

OBJECTIVES

To compare the effects of isoflurane and propofol on intraoperative neurophysiological monitoring (IONM) during spinal surgery.

METHODS

Thirty-five patients were randomly assigned to receive isoflurane (n = 17) or propofol (n = 18) anesthesia. Somatosensory evoked potentials (SEPs) following posterior tibial nerve stimulation were recorded before induction as baselines. Isoflurane concentrations and propofol infusions were adjusted to obtain four pre-determined BIS ranges: 65-55, 55-45, 45-35 and 35-25. For each range, a stable state was maintained for at least 10 min to perform IONM. The SEP latency P40 and amplitude P40-N50, the onset latency and amplitude of transcranial motor evoked potentials (tcMEPs), and threshold intensity of triggered electromyographic activity (EMG) following pedicle screw stimulation were statistically analyzed.

RESULTS

Compared with baseline values, P40 latency increased and P40-N50 amplitude decreased after anesthesia with isoflurane or propofol. Isoflurane caused a dose-dependent depression of SEPs, but propofol did not. TcMEPs were recordable and stable in all patients receiving propofol in each BIS range, but only recordable in 10 (58.8%) receiving isoflurane with BIS >55, and 3 (17.8%) with BIS <55. No difference was noted in triggered EMG.

CONCLUSIONS

Isoflurane inhibited IONM more than propofol. Propofol is recommended for critical spinal surgery, particularly when motor pathway function is monitored.

Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery

BACKGROUND

There has been little enthusiasm for somatosensory evoked potential monitoring in cervical spine surgery as a result, in part, of the increased risk of motor tract injury at this level, to which somatosensory monitoring may be insensitive. Transcranial electric motor evoked potential monitoring allows assessment of the motor tracts; therefore, we compared transcranial electric motor evoked potential and somatosensory evoked potential monitoring during cervical spine surgery to determine the temporal relationship between the changes in the potentials demonstrated by each type of monitoring and neurological sequelae and to identify patient-related and surgical factors associated with intraoperative neurophysiological changes.

METHODS

Somatosensory evoked potential and transcranial electric motor evoked potential data recorded for 427 patients undergoing anterior or posterior cervical spine surgery between January 1999 and March 2001 were analyzed. All patients who showed substantial (at least 60%) or complete unilateral or bilateral amplitude loss, for at least ten minutes, during the transcranial electric motor evoked potential and/or somatosensory evoked potential monitoring were identified.

RESULTS

Twelve of the 427 patients demonstrated substantial or complete loss of amplitude of the transcranial electric motor evoked potentials. Ten of those patients had complete reversal of the loss following prompt intraoperative intervention, whereas two awoke with a new motor deficit. Somatosensory evoked potential monitoring failed to identify any change in one of the two patients, and the change in the somatosensory evoked potentials lagged behind the change in the transcranial electric motor evoked potentials by thirty-three minutes in the other. No patient showed loss of amplitude of the somatosensory evoked potentials in the absence of changes in the transcranial electric motor evoked potentials. Transcranial electric motor evoked potential monitoring was 100% sensitive and 100% specific, whereas somatosensory evoked potential monitoring was only 25% sensitive; it was, however, 100% specific.

CONCLUSIONS

Transcranial electric motor evoked potential monitoring appears to be superior to conventional somatosensory evoked potential monitoring for identifying evolving motor tract injury during cervical spine surgery. Surgeons should strongly consider using this modality when operating on patients with cervical spondylotic myelopathy in general and on those with ossification of the posterior longitudinal ligament in particular.

Motor Evoked Potential Monitoring for the Surgery of Brain Tumours and Vascular Malformations

Brain surgery incurs a significant risk of a new motor deficit in lesions within or adjacent to the motor areas and pathways which, for the patient, presents one of the most disabling complications of such operations. It is a major concern of intracranial procedures to delineate and monitor motor regions in order to preserve their structural and functional integrity, while still achieving maximal cytoreduction. The technique of motor evoked potential recording has had to be adapted to intraoperative recording conditions under general anaesthesia, but has been available for clinical use now for almost ten years. This contribution summarizes the current technique and related methods, as well as our clinical experience in some 400 cases of MEP monitoring in supratentorial tumors, lesions in and around the brainstem, and aneurysm surgery. Intraoperative MEP recordings have been shown to reliably reflect an impending new motor deficit. Irreversible MEP deterioration heralds new paresis, and unaltered recordings predict preserved motor function. This is also true in aneurysm surgery where conventional SEP monitoring may yield false-negative results with regard to development of a new motor deficit. Moreover, if MEP deterioration can be reversed, or halted by early surgical intervention, the presence of only a transient motor deficit, or even the lack of a new postoperative deficit, indicates the success of the MEP monitoring method in the prevention of a significant motor impairment. Certain complicated lesions can only be operated on at all because MEP monitoring is available. In conclusion, intraoperative MEP monitoring is a useful aid in brain surgery with which to avoid a new motor deficit without compromise to the surgical result. Controlled prospective studies will be required to verify the clinical value of the method.

Multimodality monitoring of transcranial electric motor and somatosensory-evoked potentials during surgical correction of spinal deformity in patients with cerebral palsy and other neuromuscular disorders

STUDY DESIGN

This prospective, descriptive study determined the reliability of transcranial electric motor and posterior tibial nerve somatosensory-evoked potentials in children with neuromuscular scoliosis.

OBJECTIVE

To assess the applicability of transcranial electric motor and posterior tibial nerve somatosensory-evoked potentials during surgical correction of neuromuscular scoliosis, particularly with cerebral palsy-related deformity.

SUMMARY OF BACKGROUND DATA

During corrective spinal surgery for neuromuscular scoliosis, intraoperative multimodality spinal cord monitoring is recommended. There exist conflicting, retrospective studies regarding the reliability of spinal cord monitoring in patients with neuromuscular scoliosis.

METHODS

Transcranial electric motor potentials and posterior tibial nerve somatosensory-evoked potentials were monitored in all patients presenting for spinal fusion between 2000 and 2001. Anesthesia was standardized for all patients.

RESULTS

There were 68 patients subdivided into two subject groups. Group I consisted of 39 patients with neuromuscular scoliosis associated with cerebral palsy, and Group II consisted of 29 children with neuromuscular scoliosis due to a disease process other than cerebral palsy. Five of the 68 patients had significant amplitude changes in 1 or both monitoring methods during surgery relative to baseline. Of these, one had permanent neurologic deficit despite standard intervention. Somatosensory-evoked potentials were monitored successfully in 82% of the cerebral palsy and 86% of the noncerebral palsy patients. Transcranial electric motor-evoked potentials, on the other hand, were monitorable in 63% of patients with mild or moderate degrees of cerebral palsy and 39% of those with severe involvement. Eighty-six percent of those with noncerebral palsy-related neuromuscular scoliosis had recordable motor-evoked potentials at baseline.

CONCLUSION

Both transcranial electric motor and posterior tibial nerve somatosensory-evoked potentials can be monitored reliably in most patients with neuromuscular scoliosis. Those with severe cerebral palsy present the greatest challenge to successful neurophysiologic monitoring.

Transcranial electrical motor-evoked potential monitoring during surgery for spinal deformity: a study of 145 patients

STUDY DESIGN

A descriptive historic cohort study was conducted.

OBJECTIVES

To determine intraoperative response amplitude criteria for transcranial electrical motor-evoked potential monitoring that warn of neurologic damage, and to determine the additional value of monitoring six instead of two muscle sites.

SUMMARY OF BACKGROUND DATA

Transcranial electrical motor-evoked potential monitoring provides immediate and reliable information about the integrity of the motor pathways during spine surgery. Although this monitoring technique is more frequently used, criteria for interpretation of the amplitude responses have not been defined.

METHODS

The intraoperative monitoring outcomes were compared with the patient's clinical outcomes. The sensitivity, specificity, positive predictive value, and negative predictive value were determined for four different monitoring criteria.

RESULTS

Transcranial electrical motor-evoked potential monitoring was possible 142 of 145 patients undergoing corrective surgery. In this study, 16 patients had a neurologic event, and 11 patients showed recovery of response amplitude after a second surgical maneuver, whereas the remaining 5 patients had permanent partial neurologic damage. The criterion that at least one of six recordings must show an amplitude decrease of more than 80% was sufficiently strict to achieve a sensitivity of 1.0 and a specificity of 0.91. Less strict criteria, including recording at two instead of six sites, resulted in lower sensitivity, with the lower bound of the 95% confidence interval at 0.62.

CONCLUSIONS

Transcranial electrical motor-evoked potential monitoring allowed successful intraoperative monitoring. The criterion of one recording showing a response amplitude decrease of more than 80% during a surgical action can be considered a valuable warning criterion for neurologic damage. The authors also consider that monitoring at six instead of two muscles improves the value of neuromonitoring.

Traumatic atypical Brown-Sequard syndrome: case report and literature review

A man was kicked on the neck, 10 days after which he noted right-sided numbness. Clinically, he had diminished deep tendon reflexes over the left upper limb, absent left superficial abdominal reflexes, a left extensor plantar response, mild left hemiparesis, diminished pinprick, temperature and vibratory sensation up to the right T(4-5) dermatome and diminished proprioceptive sensation in the right upper and lower limbs. Cervicothoracic magnetic resonance imaging (MRI) revealed a left C(6-7) posterolateral disc prolapse with indentation and oedema of the cord at the same level. He was given intravenous dexamethasone, with mild resolution of his motor but not his sensory symptoms. Transcranial magnetic stimulation and evoked potentials performed 3 days later were unremarkable. He was discharged soon after, preferring to seek traditional therapy in his country. We attempt to explain the anatomical basis for his clinical signs, review the literature for similar cases, and examine the usefulness of available treatment and investigations.

The benefit of neurogenic mixed evoked potentials for intraoperative spinal cord monitoring during correction of severe scoliosis: a case study

STUDY DESIGN

A case report is presented.

OBJECTIVE

To present a case in which surgical correction of a severe scoliotic curve caused unilateral loss of neurogenic mixed evoked potential data despite unchanged somatosensory data.

SUMMARY OF BACKGROUND DATA

Surgical correction of large scoliotic curves presents a risk to the function of the spinal cord. Multimodality intraoperative neurophysiologic monitoring of the spinal cord is recommended during such procedures.

METHODS

A 13-year-old girl with severe double major scoliosis underwent a staged operative procedure for correction of her spine deformity. Intraoperative neurophysiologic monitoring using somatosensory-evoked potentials and neurogenic mixed evoked potentials was performed for each stage.

RESULTS

During the final stage (a T4-L5 posterior instrumentation and fusion) left neurogenic mixed evoked potential data were lost approximately 45 minutes after placement of the left-side, correcting rod. The surgeon was warned of the data change. Set bolts were loosened at all fixation points, and the data quickly returned to within normal limits of baseline. Somatosensory data never approached warning criteria at any point during surgery. The patient awakened with no neurologic deficit.

CONCLUSIONS

Neurophysiologic monitoring using both somatosensory-evoked potentials and neurogenic mixed evoked potentials is recommended when surgery is performed to correct spine deformity. The Stagnara wake-up test, somatosensory-evoked potentials, and neurogenic mixed evoked potentials are important components of spinal cord monitoring during surgery, and should be used together for optimal protection of neurologic function.

Effect of sevoflurane/nitrous oxide versus propofol anaesthesia on somatosensory evoked potential monitoring of the spinal cord during surgery to correct scoliosis

BACKGROUND

Use of intraoperative somatosensory evoked potential (SSEP) monitoring is helpful in spinal corrective surgery but may be affected by anaesthetic drugs. An anaesthetic technique that has less effect on SSEP or allows faster recovery is an advantage. We compared the effects on SSEP and the clinical recovery profiles of sevoflurane/nitrous oxide and propofol anaesthesia during surgery to correct scoliosis.

METHODS

Twenty adolescent patients were randomized into two groups of 10. One group received sevoflurane-nitrous oxide anaesthesia and the other received propofol i.v. anaesthesia. An alfentanil infusion was used for analgesia in both groups.

RESULTS

Changes in anaesthetic concentration produced little effect on the latency of SSEP, but the effect on the variability of SSEP amplitude was significant (P<0.05). Sevoflurane produced a faster decrease in SSEP and a faster recovery than propofol (P<0.05). On emergence, patients who received sevoflurane tended to have shorter recovery times to eye opening (mean 5.1 vs 20.6 min, P=0.09) and toe movement (mean 7.9 vs 15.7 min, P=0.22). Those who had received sevoflurane were significantly more lucid and cooperative in recovery.

CONCLUSIONS

Sevoflurane produces a faster decrease and recovery of SSEP amplitude as well as a better conscious state on emergence than propofol.

Near infrared spectroscopy for intraoperative monitoring of the spinal cord

STUDY DESIGN

Animal model study of three healthy commercial pigs was conducted.

OBJECTIVE

To determine whether near infrared spectroscopic monitoring of the spinal cord is feasible, and whether changes in near infrared spectroscopy correlate with changes in blood flow to the cord or operative maneuvers.

SUMMARY OF BACKGROUND DATA

Near infrared spectroscopy is a noninvasive continuous monitoring tool capable of measuring absolute changes in the concentration of three chromophores: oxygenated hemoglobin, deoxygenated hemoglobin, and cytochrome aa3, the terminal enzyme in the electron transfer chain and a measure of cellular energy equilibrium. Near infrared spectroscopy has been used to monitor the brain intraoperatively in multiple circumstances. The authors hypothesized that near infrared spectroscopy could be used to monitor the spinal cord's cellular energy equilibrium during spinal surgery (i.e., that vascular compromise could be identified before irreversible damage occurred).

METHODS

The posterior elements of the spine were exposed, and near infrared spectroscopy optodes (fiberoptic bundles) were sutured to either the lamina or the spinous processes of T9, T10, or both and directed toward the spinal cord. Interventions included manipulation of oxygen saturation and distraction of the T9-T10 disc space.

RESULTS

With reduced oxygen delivery (lower arterial oxygen saturation and blood flow), oxygenated hemoglobin concentration decreased and deoxygenated hemoglobin concentration increased. With distraction, blood volume (oxygenated hemoglobin plus deoxygenated hemoglobin) decreased, and cytochrome aa3 became more oxidized. Changes were apparent within 1 second of the intervention beginning, and recovery to the baseline of near infrared spectroscopy occurred with relief of each intervention.

CONCLUSIONS

This near infrared spectroscopy technique monitors changes in oxygenation of the spinal cord, and therefore appears capable of intraoperative warning about impending vascular compromise of the spinal cord.

Effect of stimulation parameters on intraoperative spinal cord evoked potential monitoring

The purpose of this study was to investigate the effects of the stimulus parameters on spinal cord evoked potential (SCEP) and to recommend a practical epidural stimulation protocol for intraoperative spinal cord monitoring. This prospective study compared the latencies and amplitudes of SCEP obtained on epidural stimulation of 30 patients with scoliosis under anesthesia using different stimulus pulse duration and stimulation rates. SCEP was found to be undetectable with shorter stimulus duration (<0.05 ms). The SCEP latencies did not show any significant difference among different stimulation parameters. However, the SCEP amplitude showed significant changes with differing stimulus durations. The SCEP amplitudes were found to significantly decrease when the pulse durations become shorter than 0.2 ms. Stimulus parameters showed significant effects on SCEP amplitude but not latency. Stimulus rates in the range of 21 to 61 Hz are equivalent for quick and reliable detection of SCEP. Considering the short latency of SCEP, a pulse duration of 0.2 ms is recommended for SCEP using epidural stimulation.

Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis

The author describes application of intraoperative neurophysiologic monitoring to surgical treatment of lumbar stenosis. Benefits of somatosensory and motor evoked potential studies during surgical correction of spinal deformity are well known and documented. Free-running and evoked electromyographic studies during pedicle screw implantation is an accepted practice at many institutions. However, the functional integrity of spinal cord, cauda equina, and nerve roots should be monitored throughout every stage of surgery including exposure and decompression. Somatosensory evoked potentials monitor overall spinal cord function. Intraoperative electromyography provides continuous assessment of motor root function in response to direct and indirect surgical manipulation. Electromyographic activities observed during exposure and decompression of the lumbosacral spine included complex patterns of bursting and neurotonic discharge. In addition, electromyographic activities at distal musculature were elicited by impacting a surgical instrument or graft plug against bony elements of the spine. All electromyographic events provided direct feedback to the surgical team and were regarded as a cause for concern. Simultaneously monitored evoked potential and electromyographic studies protect spinal cord and nerve roots during seemingly low-risk phases of a surgical procedure when neurologic injury may occur and the patient is placed at risk for postoperative myelopathy or radiculopathy.