Neurogenic Motor Evoked Potentials Research Articles

Upper extremity neuromonitoring changes are more common than lower extremity during spinal fusion for Scheuermann's kyphosis.

The purpose of this study is to determine the incidence of intraoperative neuromonitoring (IONM) changes and postoperative neurologic deficit in patients with Scheuermann's Kyphosis (SK) undergoing posterior spinal fusion (PSF). Single-center, retrospective chart review of the clinical, surgical and IONM data (somatosensory evoked potential (SSEP) and neurogenic motor evoked potential (NMEP) or transcranial motor evoked potential (TcMEP)) from patients with SK undergoing PSF at our center from 1993 to 2021. One hundred and four SK patients (mean 16.4 ± 1.9years) underwent PSF with correction of kyphosis from mean 79.4 ± 10.8° to 35.4 ± 13.9°. MEP data were obtained using either NMEP in 34.6% of patients) or TcMEP in 65.4% of patients. Only 3.8% of cases had lower extremity (LE) IONM changes during surgery, with no postoperative neurologic deficits in those patients. IONM changes occurred more frequently in the upper extremities (UE) with 14 (13.4%) patients having changes in UE SSEPs. Patients with UE IONM changes had significantly longer surgical times (p = 0.0096) and higher number of levels fused (p = 0.003) compared to patients without changes. Their weight, but not BMI, was also significantly higher (p = 0.036). These UE IONM changes resolved with arm repositioning in all but one patient who had a postoperative UE neurapraxia that resolved by 6weeks. There was 1 postoperative transient femoral nerve palsy without IONM changes thought to be due to patient positioning. The incidence of critical LE IONM changes during PSF for SK is 3.4%, which is similar to that reported in AIS. UE IONM changes are significantly more common at 13.4%, revealing that these patients are vulnerable to malpositioning of the arms during surgery.

Use of transcranial motor-evoked potentials to provide reliable intraoperative neuromonitoring for the Charcot-Marie-Tooth population undergoing spine deformity surgery.

Intraoperative neuromonitoring (IONM) has historically been difficult to obtain in patients with Charcot-Marie-Tooth (CMT) disease. Transcranial motor-evoked potentials (TcMEPs) have been found to be safe and effective for other spinal deformity patients. Our objective was to determine the effectiveness of TcMEP monitoring in patients with CMT. An IRB-approved, retrospective review of CMT patients undergoing spinal deformity surgery assessing TcMEP, somatosensory-evoked potential (SSEP), and neurogenic motor evoked potential (NMEP) IONM was performed. A 2:1 matched cohort control group of idiopathic spinal deformity patients was used. A waveform grading system was applied to review baseline TcMEP reliability and quality, which was validated via intraclass correlation coefficient amongst five raters. Twenty-three CMT patients (26 surgical cases) were identified. The use of TcMEP improved the ability to obtain baseline IONM when compared to SSEP (83% vs. 20%; p < 0.001) and NMEP (83% vs. 18%; p = 0.003). Baseline monitoring was obtained less often for CMT patients using SSEP (20% vs. 100%; p < 0.001) and TcMEP (83% vs. 100%; p = 0.111) compared to idiopathic patients. Sweep length (time from stimulation waveform evaluation) and maximum stimulation voltage were higher in the CMT group (289ms vs. 111ms p = 0.007 and 740V vs. 345V p = 0.089, respectively). TcMEP monitoring significantly improves the ability to provide IONM for CMT patients undergoing spinal deformity surgery. Utilizing longer sweep lengths enhances the ability to attain baseline TcMEP readings, allowing surgeons to more safely proceed with surgery for these complex patients. Therapeutic-Level III.

Intraoperative Neuromonitoring in Pediatric and Adult Spine Deformity Surgery.

Review of techniques and description of institutional clinical experience. To provide a historical review and description of key neuromonitoring concepts, focusing on neurogenic motor-evoked potentials and descending neurogenic evoked potentials, and to review the authors' experience with neuromonitoring techniques in children and adults undergoing spinal deformity surgery. The original form of neuromonitoring, the Stagnara wake-up test, remains the "gold standard" for detecting true neurological deficits. Multiple newer modalities involving cortical and muscular monitoring, such as somatosensory evoked potentials and motor evoked potentials, have been developed and are widely used. Descending and neurogenic evoked potentials are becoming more common for neuromonitoring in patients undergoing spinal deformity surgery. A PubMed search for literature related to "neuromonitoring" was performed, and recent, as well as historical, articles were reviewed. Clinical experience regarding the use of neuromonitoring in adult and pediatric spinal deformity surgery was obtained from institutional experts. Although not regularly used, the Stagnara wake-up test remains the gold standard for detecting neurological injury. Somatosensory evoked potentials measure signals transmitted from the periphery to the cortex and have historically been widely used but are limited by delay, poor localization, and the inability to detect damage to motor tracts. Motor evoked potentials continue to be used widely and measure muscular activity after cortical stimulation, but they are difficult to interpret in patients with underlying motor disorders and cannot be continuously monitored. Newer techniques such as descending neurogenic evoked potentials and neurogenic motor evoked potentials monitoring are used at some high-volume centers. Familiarity with the history of neuromonitoring in spinal deformity surgery and an understanding of the physiological systems used for neuromonitoring provide a framework from which spine surgeons can select appropriate monitoring for their patients.

Significant Change or Loss of Intraoperative Monitoring Data

Retrospective. 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. 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. 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. 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). 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.

Multimodal intraoperative neuromonitoring in corrective surgery for adolescent idiopathic scoliosis: Evaluation of 354 consecutive cases

Background:Multimodal intraoperative neuromonitoring is recommended during corrective spinal surgery, and has been widely used in surgery for spinal deformity with successful outcomes. Despite successful outcomes of corrective surgery due to increased safety of the patients with the usage of spinal cord monitoring in many large spine centers, this modality has not yet achieved widespread popularity. We report the analysis of prospectively collected intraoperative neurophysiological monitoring data of 354 consecutive patients undergoing corrective surgery for adolescent idiopathic scoliosis (AIS) to establish the efficacy of multimodal neuromonitoring and to evaluate comparative sensitivity and specificity.Materials and Methods:The study group consisted of 354 (female = 309; male = 45) patients undergoing spinal deformity corrective surgery between 2004 and 2008. Patients were monitored using electrophysiological methods including somatosensory-evoked potentials and motor-evoked potentials simultaneously.Results:Mean age of patients was 13.6 years (±2.3 years). The operative procedures involved were instrumented fusion of the thoracic/lumbar/both curves, Baseline somatosensory-evoked potentials (SSEP) and neurogenic motor-evoked potentials (NMEP) were recorded successfully in all cases. Thirteen cases expressed significant alert to prompt reversal of intervention. All these 13 cases with significant alert had detectable NMEP alerts, whereas significant SSEP alert was detected in 8 cases. Two patients awoke with new neurological deficit (0.56%) and had significant intraoperative SSEP + NMEP alerts. There were no false positives with SSEP (high specificity) but 5 patients with false negatives with SSEP (38%) reduced its sensitivity. There was no false negative with NMEP but 2 of 13 cases were false positive with NMEP (15%). The specificity of SSEP (100%) is higher than NMEP (96%); however, the sensitivity of NMEP (100%) is far better than SSEP (51%). Due to these results, the overall sensitivity, specificity and positive predictive value of combined multimodality neuromonitoring in this adult deformity series was 100, 98.5 and 85%, respectively.Conclusion:Neurogenic motor-evoked potential (NMEP) monitoring appears to be superior to conventional SSEP monitoring for identifying evolving spinal cord injury. Used in conjunction, the sensitivity and specificity of combined neuromonitoring may reach up to 100%. Multimodality monitoring with SSEP + NMEP should be the standard of care.

Spinal Cord Monitoring in Scoliosis Surgery Using an Epidural Electrode. Results of a Prospective, Consecutive Series of 191 Cases

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

BS3 How to preserve the functional integrity of the corticospinal tracts during spine and spinal cord surgeries

Up to the 1990’s, motor evoked potentials (MEPs) were not yet a part of intraoperative monitoring (IOM) methodologies. Therefore, conclusions about the integrity of the corticospinal tracts (CTs) within the spinal cord were indirectly drawn from monitoring the dorsal columns with somato sensory evoked potentials (SEPs). It is understandable that such a correlation cannot always be correct and discrepancies between the functional integrity of the dorsal columns and of the CTs was reported. These reports, together with the observation that the disappearance of SEPs during myelotomy for spinal cord tumor surgery is not necessarily followed by motor deficits, further questioned IOM’s usefulness. Furthermore, it was shown that ‘‘neurogenic MEPs’’ method, which electrically stimulates the spinal cord and records elicited activity from the peripheral nerves does not represent motor evoked potentials, but antidromic activity from the stimulated dorsal columns. Further development of motor evoked potentials recorded from the spinal cord as a D and I waves as well as from the limb muscle after eliciting them with a short train of transcranialy applied stimuli, has brought the full confidence of (neuro)surgeons in the ability of IOM to predict and prevent intraoperative injury to the spinal cord’s motor system. Combination that uses the D wave and muscle MEPs’ to monitor spinal cord surgeries has proven to be realiable tool for preventing intraoperative injury to the spinal cord’s motor system. By contrast, the D wave – but not muscle MEPs – has shown false positive data, but only during scoliosis surgeries. This is due to the new anatomical relationship between recording electrode and the spinal cord after correction of the scoliotic curvature. Further development of IOM techniques to neurophysiologically identify (map) dorsal and lateral columns of the exposed spinal cord, giving us a new tools to prevent injury to those structures.

What does intraoperative monitoring of motor evoked potentials bring to the neurosurgeon?

The neurosurgeons’ attitude towards intraoperativeneurophysiology is similar to the attitude the villagershad towards the shepherd boy who falsely cried ‘‘wolf’’.When the wolf really showed up and he cried ‘‘wolf’’the villagers did not believe him and did not help him.In a time when somatosensory evoked potentials(SEPs) monitoring was the only available method to in-directly monitor the corticospinal tracts (CTs), the onlydifference between the shepherd boy and a neurophys-iologist is that a neurophysiologist really thinks that awolf is coming whenever a significant change in theparameters of SEPs appears.Unfortunately, in some instances the wolf truly ar-rived and the neurophysiologist did not recognize him.This is exemplified by a recent report that describesintact SEPs throughout surgery, with the patient awakingwith a new paraplegia [4, 6, 7]. Gradually the neurophys-iological and neurosurgical communities have realizedthat SEPs can not always evaluate the functional integ-rity of the CTs intraoperatively.The short marriage between neurophysiologists andneurosurgeons has endured some set backs. One fol-lowed the introduction of ‘‘neurogenic motor evokedpotentials’’, (neurogenic MEPs). This intraoperative neu-rophysiological method consists of electrical stimula-tion of the spinal cord via two needles inserted in theproximity of the vertebral lamina with recording elicitedactivity over the peripheral nerves [8]. In the beginningof this method it was not clear what neural elementswere being stimulated and to what structures activityrecorded from the peripheral nerves could be attributedto [2].Subsequently, a neurophysiological study performed byR. Toleikis, provided evidence that ‘‘neurogenic MEPs’’belonged to the antidromic activation of the dorsal col-umns recorded from the sensory part of the peripheralnerves [12]. Finally R. E. Minahan described two para-plegic patients with preserved neurogenic MEPs togetherwith SEPs from the lower extremities [7]. At the sametime this showed that neurogenic MEPs remained a reli-able method to intraoperatively evaluate the functionalintegrity of the dorsal columns.For many years we have been waiting for a methodthat can directly evaluate the functional integrity of themotor pathways and can also be highly specific andsensitive.Based on Patton and Amassian’s work in animals [9],Boyd et al. made the first intraoperative recording of Dwaves in patients with scoliosis using a catheter elec-trode inserted epidurally close to the spinal cord [1]. Thevalue of this method as a new intraoperative tool hasbeen confirmed during surgery of the spinal cord [5] andbrain [13].Unfortunately, a number of factors may prohibit theneurophysiologist’s ability to monitor the D wave. For anumber of reasons, surgeons may be unwilling to use anepi- or subdural catheter during the procedure. Further-more, a D wave can be absent caudal to the spinal cordtumor, or in patients who underwent radiation of the spi-nal cord. This is due to the extreme desynchronization

Neurogenic motor evoked potentials: role in brachial plexus surgery

✓ Peripheral nerve graft repair after severe brachial plexus injury is futile if there is degeneration of motor fibers in the proximal nerve stump to which the graft must be attached. Traditional intraoperative neurophysiological assessment methods like nerve action potential (NAP) and somatosensory evoked potential (SSEP) monitoring have been used to evaluate proximal nerve stump integrity, but these methods do not allow evaluation of the integrity of motor fibers back to the anterior horn cell. Consequently, the authors used transcranial electrical stimulation and recorded neurogenic motor evoked potentials (MEPs) directly from the brachial plexus in a patient undergoing surgical repair of a complete upper brachial plexus injury (Erb palsy) to assess the functional continuity of motor fibers. In addition, selected elements of the brachial plexus were directly stimulated, and NAPs were recorded. Finally, SSEPs were recorded from the scalp after stimulation of selected elements of the brachial plexus. Neurogenic MEPs were present from the medial cord of the brachial plexus, but not the middle or upper trunk; NAPs were present from the lateral and posterior cords after middle trunk stimulation, but absent after upper trunk stimulation; and SSEPs were present after medial cord stimulation but absent after stimulation of the upper and middle trunks. For the first time, neurogenic MEPs were coupled with NAPs and SSEPs to evaluate successfully the functional status of motor fibers back to the anterior horn cell for accurate localization of the lesion sites.

Neurogenic motor evoked potentials: role in brachial plexus surgery

Peripheral nerve graft repair after severe brachial plexus injury is futile if there is degeneration of motor fibers in the proximal nerve stump to which the graft must be attached. Traditional intraoperative neurophysiological assessment methods like nerve action potential (NAP) and somatosensory evoked potential (SSEP) monitoring have been used to evaluate proximal nerve stump integrity, but these methods do not allow evaluation of the integrity of motor fibers back to the anterior horn cell. Consequently, the authors used transcranial electrical stimulation and recorded neurogenic motor evoked potentials (MEPs) directly from the brachial plexus in a patient undergoing surgical repair of a complete upper brachial plexus injury (Erb palsy) to assess the functional continuity of motor fibers. In addition, selected elements of the brachial plexus were directly stimulated, and NAPs were recorded. Finally, SSEPs were recorded from the scalp after stimulation of selected elements of the brachial plexus. Neurogenic MEPs were present from the medial cord of the brachial plexus, but not the middle or upper trunk; NAPs were present from the lateral and posterior cords after middle trunk stimulation, but absent after upper trunk stimulation; and SSEPs were present after medial cord stimulation but absent after stimulation of the upper and middle trunks. For the first time, neurogenic MEPs were coupled with NAPs and SSEPs to evaluate successfully the functional status of motor fibers back to the anterior horn cell for accurate localization of the lesion sites.

Anterior spinal cord injury with preserved neurogenic ‘motor’ evoked potentials

Objective: To describe two cases in which intraoperative monitoring of neurogenic ‘motor’ evoked potentials (NMEPs) did not identify a spinal cord injury that resulted in paraplegia. Methods: Bilateral tibial nerve somatosensory evoked potential (SEP) and NMEP testing was performed in two patients during spinal deformity corrective surgery using standard stimulation and recording parameters. These potentials were obtained repetitively throughout the primary procedures and were performed again during a subsequent procedure that took place after the discovery of paraplegia. Results: SEP and NMEP signals were preserved in both patients and no adverse events were identified during the initial procedures. Postoperatively, paraplegia was identified immediately upon recovery from anesthesia and preserved posterior column function was apparent on clinical exam. In the procedures following the discovery of paraplegia, SEP and NMEP signals remained comparable with signals elicited in the initial surgeries. Conclusions: Based on these cases and previously published experimental evidence, we conclude that while ‘NMEPs’ remain a useful second test of spinal cord function, they are not reliable indicators of motor tract function. An alternate term, such as ‘spinally-elicited peripheral nerve responses’ should be used.

Spinally elicited peripheral nerve responses are sensory rather than motor

Objectives: Spinally elicited peripheral nerve responses, commonly called neurogenic motor evoked potentials (NMEPs), are widely used to monitor spinal cord motor function during surgery. However, numerous evidence suggests that these responses are primarily sensory rather than motor. The collision technique was utilized to address this issue. Methods: Collision studies were performed in 7 patients during surgery. An ascending volley of sensory (AS) and motor activity (AM) was elicited by posterior tibial nerve stimulation at the popliteal fossa. After a short time delay, high cervical spinal stimulation produced a descending volley of sensory (DS) and motor (DM) activity. The AM volley ascended only to the anterior horn cells whereas the AS and DS volleys collided in the spinal cord. The inter-stimulus delays were varied so as to affect the degree of spinal cord collision. The DS and DM activity which remained after collision was recorded from the posterior tibial nerves at the ankle. Results: Inter-stimulus delays of 18 ms or less resulted in no apparent peripheral descending volleys. These findings were consistent for all the patients studied. Conclusions: Spinally elicited peripheral nerve responses are primarily sensory rather than motor and are mediated by the same neural pathways as SEPs.

Antidromic corticospinal tract potential of the brain

Objective: To describe a novel potential component (antidromic corticospinal tract potential, ACSP) of the brain after translaminar spinal stimulation of a relaxed patient during scoliosis surgery. To study the origin of this component and to compare its source to known sources of the somatosensory evoked potentials (SEPs).Methods: We studied 17 consecutive patients during posterior scoliosis surgery. SEPs and ACSPs were elicited by translaminar spinal stimulation at the Th 2 and L 1 levels. ACSPs and SEPs were recorded on the scalp midline. Neurogenic motor evoked potentials (NMEPs) were recorded on the popliteal spaces. Preoperative tibial SEPs were also recorded.Results: ACSP was distinctly separated from the corresponding spinally evoked cortical SEP that showed longer latency than the ACSP. ACSPs decreased and disappeared when stimulation was moved to the caudal direction in the conus region while SEP persisted. In addition, the hemispheric origin of ACSP was confirmed with multichannel midline recordings of the scalp and neck. Thus there was no confusion to the response of nucleus gracilis, corresponding the P 31 response of the tibial nerve SEP.Conclusions: The origin of ACSP seemed to be in the rostral part of the corticospinal tract. ACSP diminished in the conus region when stimulation was moved caudally and it disappeared when the stimulus was given to the root level. This proves that ACSP is not a response of the somatosensory tract, instead ACSP represents antidromic response of the pyramidal tract. ACSP can be used in monitoring of the motor tracts during scoliosis surgery together with NMEPs.

Somatosensory- and motor-evoked potential monitoring without a wake-up test during idiopathic scoliosis surgery. An accepted standard of care.

This was a retrospective study of 500 patients undergoing corrective surgery between 1987 and 1997 for spinal deformity caused by idiopathic scoliosis. To report the sensitivity and specificity of somatosensory-evoked and neurogenic motor-evoked potentials monitoring and the requirements for an intraoperative wake-up test for all idiopathic scoliosis surgeries at a single institution. Intraoperative monitoring is recommended for use during corrective spinal surgery. Accepted monitoring standards and requirements for an intraoperative wake-up test are still debated. The study group consisted of 500 patients undergoing corrective surgery for idiopathic scoliosis between 1987 and 1997. All patients were monitored using somatosensory-evoked and neurogenic motor-evoked potential techniques, using a standard protocol developed at this institution. The false-positive rate (significant data change without postoperative neurologic deficit) was 0.014% (n = 7). The true-positive rate (degradation of data that met warning criteria, with a corresponding postoperative neurologic deficit) was 0.004% (n = 2). No false-negative results (normal data during with a postoperative neurologic deficit) were seen. The sensitivity of combined somatosensory-evoked and neurogenic motor-evoked potential data in predicting neurologic status was 98.6%, and the specificity of normal data predicting normal findings in a neurologic examination was 100%. Combined somatosensory-evoked and neurogenic motor-evoked potentials monitoring during idiopathic scoliosis surgery represents a standard of care that obviates the need for an intraoperative wake-up test when reliable data are obtained and maintained.

Neurogenic Motor Evoked Potentials

Neurogenic motor evoked potentials (NMEPs) elicited by spinal cord stimulation via the spinous processes (SP-NMEP) have been widely accepted as a sensitive method of monitoring motor tract function. SP-NMEP requires additional surgical dissection as well as electrodes within the wound, making the method somewhat inconvenient. A less invasive percutaneous method of spinal cord stimulation (PERC-NMEP) has more recently been described. We prospectively compared the SP-NMEP and PERC-NMEP methods in 184 patients undergoing 225 surgical procedures. Although SP-NMEP responses were more readily obtainable than PERC-NMEP, the reliability of the two methods was not significantly different. Both methods were found to be sensitive to neurologic deficit. The present study suggests that when responses are obtained, the percutaneous method is reliable enough to obviate the spinous process method of monitoring the motor function of the spinal cord.

Usefulness of neurogenic motor evoked potentials for spinal cord monitoring findings in 112 consecutive patients undergoing surgery for spinal deformity

Neurogenic motor evoked potential (NMEP) monitoring, which basically represents a monitoring of both motor and somatosensory tracts, has been proposed as a warning system in preventing neural damage during spinal surgery. The aim of this study was to report our clinical experience in 112 consecutive patients undergoing surgery for spinal deformity, and to emphasize the interest of NMEP monitoring. NMEPs were elicited in each patient by electrical stimulation of the spinal cord via needle electrodes placed by the surgeon in the rostral part of the surgical field, and recorded from the right and left sciatic nerves. Concomitantly, somatosensory evoked potentials (SSEPs) were obtained using a standard method. No false-negative cases of intra-operative spinal cord damage were reported. In 3 patients, both NMEPs and SSEPs suddenly disappeared during specific surgical manipulations of the vertebral implants. In these patients, the level of the lesion was easily recognized by moving the stimulating electrodes of NMEPs along the spinal cord, allowing the surgeon to perform laminectomy at the appropriate vertebral level. Spinal cord decompression was a success in two patients, the last unfortunately being paraplegic. Two additional patients exhibited transient reduction in NMEP amplitude at the insertion of a rod while SSEPs did not change significantly. In these two cases, the surgeon modified his procedure according to the NMEP changes, possibly avoiding a neurological complication. Each time evoked potentials were significantly altered, significant information was more rapidly acquired with NMEPs than with SSEPs. These results suggest that NMEPs can be used as primary choice for detecting impeding lesion of the spinal cord during critical steps of spinal surgery.

Neurogenic Motor Evoked-Potential Monitoring in Anterior Cervical Surgery

A prospective evaluation of a consecutive series of patients undergoing anterior cervical surgery monitored by the neurogenic motor evoked-potential technique. To evaluate the efficacy of neurogenic motor evoked-potential (NMEP) monitoring and its ability to predict clinical outcome in anterior cervical surgery. Forty-nine consecutive patients undergoing anterior cervical surgery were monitored using NMEP technique and were followed prospectively. Latency and amplitude data from the median and ulnar nerves were recorded and evaluated at baseline and at closure. Because existing injury or needle placement may affect absolute values, ratios of the data from both sides were used. All patients had their data evaluated for detection of intraoperative neurological injury. Thirty-eight patients had a monoradiculopathy on presentation and were followed for > 1 year and had either a good or excellent clinical result by the Odom criteria. The NMEP data from these patients were evaluated using the paired Student's test, looking for improvement during surgery. No patient had diminution of NMEP data nor a worsening or new neurological deficit. In patients with symptoms < 1 year before surgery, significant improvement was seen in the ipsilateral median nerve ratios (p = 0.05). No statistically significant results were seen in the ulnar nerve latencies or in the amplitude from either nerve. In patients with symptoms > 1 year, no statistically significant improvement was seen in any parameters. NMEP monitoring of anterior cervical surgery is a promising technique, allowing evaluation of the motor tracts. Duration of symptoms affects NMEP results. Intraoperative improvement of NMEP data after decompression may predict improved clinical outcome.

Effects of isoflurane and desflurane on neurogenic motor- and somatosensory-evoked potential monitoring for scoliosis surgery.

Most techniques used to monitor spinal cord tracts are sensitive to the effects of anesthesia, particularly to volatile anesthetic agents. The aim of this prospective study was to show that evoked potentials recorded from the peripheral nerves after spinal cord stimulation, so-called neurogenic motor evoked potentials, are resistant to clinical concentrations of isoflurane or desflurane, compared with somatosensory-evoked potentials. Twenty-three patients were studied during surgery to correct scoliosis. The background anesthetic consisted of a continuous infusion of propofol. Isoflurane (n = 12) or desflurane (n = 11) were then introduced to achieve 0.5 and 1.0 end-tidal minimum alveolar concentrations (MAC), both in 50% oxygen-nitrous oxide and in 100% oxygen. Somatosensory-evoked potentials were elicited and recorded using a standard method, defining cortical P40 and subcortical P29. Neurogenic motor-evoked potentials were elicited by electric stimulation of the spinal cord via needle electrodes placed by the surgeon in the rostral part of the surgical field. Responses were recorded from needle electrodes inserted in the right and left popliteal spaces close to the sciatic nerve. Stimulus intensity was adjusted to produce a supramaximal response; that is, an unchanged response in amplitude with subsequent increases in stimulus intensity. Measurements were obtained before introducing volatile agents and 20 min after obtaining a stable level of each concentration. Isoflurane and desflurane in both 50% oxygen-nitrous oxide and 100% oxygen were associated with a significant decrease in the amplitude and an increase in the latency of the cortical P40, whereas subcortical P29 latency did not vary significantly. Typical neurogenic motor-evoked potentials were obtained in all patients without volatile anesthetic agents, consisting of a biphasic wave, occurring 15 to 18 ms after stimulation, with an amplitude ranging from 1.3 to 4.1 microV. Latency or peak-to-peak amplitude of this wave was not significantly altered with isoflurane and desflurane, either in the presence or in the absence of nitrous oxide. Compared with cortical somatosensory-evoked potentials, neurogenic motor-evoked potential signals are well preserved in patients undergoing surgery to correct scoliosis under general anesthesia supplemented with isoflurane or desflurane in concentrations as great as 1 MAC.

Validity and Reliability of Spinal Cord Monitoring in Neuromuscular Spinal Deformity Surgery

Somatosensory evoked potentials (SSEPs) have become a standard of care in surgery for spinal deformity. Recent reports in the literature have suggested SSEP monitoring is not efficacious in surgeries for patients with neuromuscular disease. Electrophysiologic data were retrospectively analyzed from 74 patients with various neuromuscular disorders undergoing spinal-deformity surgery from 1989 through February 1995 at this medical center. The monitoring protocol included SSEPs recorded from multiple sites located cortically, subcortically, and peripherally. Neurogenic motor evoked potentials (NMEPs) were also employed. Anesthetic regimens were controlled for compatibility with evoked potential monitoring. Use of this intraoperative monitoring protocol resulted in reliable data for 95% of the patient population having baseline responses. Findings suggest that evoked potentials can be used effectively during surgery for neuromuscular spinal deformity. Use of a specific protocol allowed acquisition of reliable data intraoperatively, suggesting these methods are a valid means of monitoring neurologic status.

Direct spinal stimulation for intraoperative monitoring during scoliosis surgery

Intraoperative electrophysiological monitoring of the spinal cord has traditionally been done by recording somatosensory evoked potentials (SEP). There is a risk that SEPs can be unaltered when significant injury to the anterior spinal cord has occurred. The purpose of this report is to describe a simple technique for intraoperative spinal cord stimulation which monitors descending pathways in the anterior spinal cord. Stimulation occurs through needle electrodes inserted into spinous processes in the rostral surgical wound, and recordings are made from electrodes in the popliteal spaces. We report our experience in monitoring spinal instrumentation in 45 patients with idiopathic scoliosis and 20 with some form of neurological disease causing scoliosis. The neurogenic motor evoked potentials (NMEP) are stable and easily recorded from the popliteal spaces in the majority of patients. We describe the case of 1 patient with Friedreich's ataxia in whom no SEPs could be recorded, but NMEPs were used successfully for monitoring. We have fond that combining traditional SEP monitoring with NMEP recording provides a safe and effective method to monitor the spinal cord during surgical procedures where it is at risk.