Assessment of corticospinal and somatosensory conduction simultaneously during scoliosis surgery

Spinal Meningiomas: Diagnosis, Surgical Management, and Adjuvant Therapies

Meningiomas of the spinal canal are the most common intradural spinal canal tumors encountered in adults and account for 8% of all meningiomas. Patient presentation can vary considerably. Once diagnosed, these lesions are primarily treated surgically, but depending on location and pathological features, chemotherapy and radiosurgery may be required. Emerging modalities may represent adjuvant therapies. In this article, we review the current management of meningiomas of the spinal column.

Overview of intraoperative neuromonitoring

Intraoperative neuromonitoring (IONM) is used widely to reduce neurologic adverse postoperative outcomes. A variety of techniques are used. Initial techniques were used as far back as the 1930s, and the variety of methods expanded greatly since the 1980s. Many methods monitor baseline findings over time. Other methods test for neurologic function to identify nerves or eloquent cortex. Physicians trained in neurophysiology are key for interpretation of findings, supervision of staff, and making medical recommendations to the surgeon or anesthesiologist. Some neurophysiologists provide the services personally, and in other circumstances well-trained technologist staff help with the techniques. Much IONM is provided by the neurophysiology physician in the operating room, whereas in other cases, the physician may be on-line in real time from a remote site. When monitoring identifies changes, the IONM team must give a clear, timely, and compelling message to the surgeon and anesthesiologist.

Intraoperative evoked potential techniques

There are many recent advances in intraoperative evoked potential techniques for mapping and monitoring neural function during surgery. In particular, somatosensory evoked potential optimization speeds surgical feedback, motor evoked potentials provide selective motor system information, and new visual evoked potential methods promise reliable visual system monitoring. This chapter reviews these advances and provides a comprehensive background for understanding their context and importance.

Predictive value of intraoperative D-wave and m-MEP neurophysiological monitoring in patients with preoperative motor deficits in immediate and late postoperative period

Background:

Presence of preoperative motor deficits in patients poses a distinct challenge in monitoring the integrity of corticospinal tracts during spinal surgeries. The inconsistency of the motor-evoked potentials is such patients, limits its clinical utility. D-wave is a robust but less utilized technique for corticospinal tract monitoring. The comparative clinical value of these two techniques has not been evaluated in the patients with preoperative deficits.

Objectives:

The objective of the study was to compare the predictive utility of myogenic Motor Evoked Potentials (m-MEP) and D-wave in terms of recordability and their sensitivity and specificity in predicting transient and permanent new motor deficits.

Materials and Methods:

Thirty-one patients with preoperative motor deficit scheduled to undergo spinal surgery were included in the study. Intraoperative m-MEP and D-wave changes were identified and correlated with postoperative neurology in the immediate postoperative period and at the time of discharge.

Results:

The mean preoperative motor power of the patient pool in left and right lower limb was 2.97 ± 1.56 and 3.32 ± 1.49, respectively. The recordability of m-MEPs and D-wave was observed to be 79.4% and 100%, respectively. The m-MEP predicted the motor deterioration in immediate postoperative period with 100% sensitivity and 80% specificity, while D-wave had 14% sensitivity and 100% specificity. At the time of discharge, m-MEPs' specificity reduced to 61%, while D-wave demonstrated 100% specificity.

Conclusions:

D-wave has a better recordability than m-MEPs in neurologically compromised patients. D-wave predicts development of long-term deficits with 100% specificity, while m-MEPs have a high sensitivity for transient neurological deficit. A combination of D-wave and m-MEP is recommended for monitoring the integrity of the corticospinal tract in patients with preoperative motor deficits.

Multimodal Intraoperative Neurophysiological Monitoring in Spine Surgeries: The Experience at a Spine Centre through Years

Study Design

Retrospective observational study.

Purpose

To share our experience of multimodal intraoperative neurophysiological monitoring (IONM) used in Sakra World Hospital, Bengaluru in various spine surgeries.

Overview of Literature

The development of new onset postoperative neurological deficits can be completely avoided. In order to avoid these, IONM has become a standard of care in recent times for early detection and manipulation of the surgical procedure to prevent postoperative neurological deficits.

Methods

This retrospective study was performed on 408 patients who had undergone spine surgeries with IONM during April 2014 to March 2020 at a single center. The operative report, anesthesia record, and IONM were reviewed. All the patients were reassessed for postoperative neurological deficits in the postoperative period and followed up based on the intraoperative findings and neurological deficits for 4 weeks. Signal changes in IONM were reviewed, and the obtained results were further categorized into true positive, true negative, false positive, or false negative. If changes were observed during the IONM, the patients were managed as per the algorithm.

Results

Of the 408 patients being monitored continuously during the intraoperative period, 38 showed changes in recordings, 28 developed postoperative neurological deficits, and one developed neurological deficit without any change in the IONM. Nine patients had transient neurological deficits, and the other 20 had permanent neurological deficits. Overall, the multimodal IONM used in our study had a sensitivity of 96.6%, specificity of 97.4%, a positive predictive value of 73.7%, and a negative predictive value of 99.7%.

Conclusions

Use of decision algorithm and multimodal neuromonitoring consisting of motor evoked potentials, somatosensory evoked potentials, and electromyography complement each other in the detection of neurological injury during the course the surgery, improve intraoperative care, and prevent further damage and morbidity in patients.

Safe Intraoperative Neurophysiologic Monitoring During Posterior Spinal Fusion in a Patient With Cochlear Implants

OBJECTIVE

Cochlear implants are generally considered a contraindication for any procedure requiring electrical stimulation near the implant. We present a case of a patient undergoing intraoperative transcranial electrical motor-evoked potential monitoring with a cochlear implant without adverse outcomes.

PATIENT

A 12-year-old girl with a history of VACTERL presented with worsening congenital kyphosis and bilateral severe-to-profound hearing loss. Since age 7 the patient used a cochlear implant in the right ear and hearing aid in the left ear. Physical examination and magnetic resonance imaging in 2016 revealed a left-sided 66-degree thoracolumbar kyphosis at T11 making the patient a candidate for surgical correction.

INTERVENTIONS

She underwent a posterior spinal fusion surgery, performed with intraoperative transcranial electrical motor-evoked potential monitoring. Steps were taken to mitigate electrical stimulation of the patient's cochlear implant.

MAIN OUTCOME MEASURES

Postoperative impedance of individual channels, audiometry, and neural response testing were compared with preoperative measurements.

RESULTS

Significant (>10%) impedance changes were observed postoperatively in channels 1, 2, 4, and 6; however, the net variation across all the channels was low (3%). The patient reported no hearing changes, and no significant changes in hearing threshold were seen in postoperative audiometric testing or neural response testing.

CONCLUSION

We present a case of successful posterior spinal fusion with intraoperative neurophysiological monitoring via transcranial electrical stimulation, in a patient with a cochlear implant. With proper precautions, motor-evoked potential monitoring can be safely performed in a patient with a cochlear implant.

Intraoperative Neurophysiological Monitoring : A Review of Techniques Used for Brain Tumor Surgery in Children

Intraoperative monitoring (IOM) utilizes electrophysiological techniques as a surrogate test and evaluation of nervous function while a patient is under general anesthesia. They are increasingly used for procedures, both surgical and endovascular, to avoid injury during an operation, examine neurological tissue to guide the surgery, or to test electrophysiological function to allow for more complete resection or corrections. The application of IOM during pediatric brain tumor resections encompasses a unique set of technical issues. First, obtaining stable and reliable responses in children of different ages requires detailed understanding of normal ageadjusted brain-spine development. Neurophysiology, anatomy, and anthropometry of children are different from those of adults. Second, monitoring of the brain may include risk to eloquent functions and cranial nerve functions that are difficult with the usual neurophysiological techniques. Third, interpretation of signal change requires unique sets of normative values specific for children of that age. Fourth, tumor resection involves multiple considerations including defining tumor type, size, location, pathophysiology that might require maximal removal of lesion or minimal intervention. IOM techniques can be divided into monitoring and mapping. Mapping involves identification of specific neural structures to avoid or minimize injury. Monitoring is continuous acquisition of neural signals to determine the integrity of the full longitudinal path of the neural system of interest. Motor evoked potentials and somatosensory evoked potentials are representative methodologies for monitoring. Free-running electromyography is also used to monitor irritation or damage to the motor nerves in the lower motor neuron level : cranial nerves, roots, and peripheral nerves. For the surgery of infratentorial tumors, in addition to free-running electromyography of the bulbar muscles, brainstem auditory evoked potentials or corticobulbar motor evoked potentials could be combined to prevent injury of the cranial nerves or nucleus. IOM for cerebral tumors can adopt direct cortical stimulation or direct subcortical stimulation to map the corticospinal pathways in the vicinity of lesion. IOM is a diagnostic as well as interventional tool for neurosurgery. To prove clinical evidence of it is not simple. Randomized controlled prospective studies may not be possible due to ethical reasons. However, prospective longitudinal studies confirming prognostic value of IOM are available. Furthermore, oncological outcome has also been shown to be superior in some brain tumors, with IOM. New methodologies of IOM are being developed and clinically applied. This review establishes a composite view of techniques used today, noting differences between adult and pediatric monitoring.

Intraoperative Neurophysiologic Monitoring for Degenerative Cervical Myelopathy

Multimodal intraoperative neurophysiologic monitoring is a reliable tool for detecting intraoperative spine injury and is recommended during surgery for degenerative cervical myopathy (DCM). Somatosensory evoked potential (SEP) can be used to monitor spine and peripheral nerve injury during positioning in surgery for DCM. Compensation technique for transcranial evoked muscle action potentials (tcMEPs) should be adopted in intraoperative monitoring during surgery for DCM. Free-running electromyography is a useful real-time monitoring add-on modality in addition to SEP and tcMEP.

Efficacy of Intraoperative Neurophysiologic Monitoring for Pediatric Cervical Spine Surgery

STUDY DESIGN

Clinical case series.

OBJECTIVE

To investigate the efficacy of intraoperative neuromonitoring in pediatric cervical spine surgery.

SUMMARY OF BACKGROUND DATA

Intraoperative neuromonitoring (IONM) consisting of somatosensory-evoked potentials (SSEP) and transcranial motor-evoked potentials (tcMEP) has been shown to effectively prevent permaneny neurologic injury in deformity surgery. The role of IONM during pediatric cervical spine surgery is not well documented. Advances in cervical spine instrumentation have expanded the surgical options in pediatric populations. The goal of this study is to report the ability of IONM to detect neurologic injury during pediatric cervical spine instrumentation.

METHODS

A single institution database was queried for pediatric-aged patients who underwent cervical spine instrumentation and fusion between 2011 and 2014. Age, diagnosis, surgical indication, number of instrumented levels, and a complete IONM were extracted. Sensitivity and specificity for the detection of neurologic deficits were calculated with exact 95% confidence intervals. Positive and negative predictive values were calculated with estimated 95% confidence intervals.

RESULTS

Sixty-seven patients who underwent cervical spine instrumentation were identified with a mean age of 11.6 years (range 1-18). Diagnoses included instability (27), congenital (11), kyphosis (8), fracture (7), tumor (7), arthritis (4), and basilar invagination (3). Mean number of vertebral levels fused was 4 (range 2-7). All patients underwent cervical instrumentation with SSEP and tcMEP monitoring. A significant change in tcMEP monitoring was observed in 7 subjects (10%). There were no corresponding SSEP changes in these patients. The sensitivity of combined IONM was 75% [95% CI = 24.9, 98.7] and the specificity was 98.5% [92.7, 99.9].

CONCLUSION

tcMEP is a more sensitive indicator to spinal cord injury than SSEP, which is consistent with previous studies. IONM changes in 10% of a patient population are significant enough to warrant intraoperative determination if true SCI has occurred or is underway and intervene accordingly.

LEVEL OF EVIDENCE

4.

Management of Spinal Meningiomas

Spinal meningiomas are the most common spinal tumors encountered in adults, and account for 6.5% of all craniospinal tumors. The treatment for these lesions is primarily surgical, but emerging modalities may include chemotherapy and radiosurgery. In this article, the current management of spinal meningiomas and the body of literature surrounding conventional treatment is reviewed and discussed.

The value of multimodality intraoperative neurophysiological monitoring in treating pediatric Chiari malformation type I

INTRODUCTION

Chiari malformation type I is defined as a descent of cerebellar tonsils below the level of the foramen magnum. The traditional treatment for symptomatic patients is foramen magnum decompression (FMD) surgery. Intraoperative neurophysiological monitoring (INM) is an established surgical adjunct, which is proposed to reduce the potential risk of various surgical procedures. Though INM has been suggested as being helpful in patient positioning and in determining the optimal surgical extent of FMD (i.e., duroplasty, laminectomy, tonsillectomy), its shortcomings include prolongation of anesthesia and surgery as well as monetary costs. Multimodality INM including transcranial-electric motor evoked potential (TcMEP) is not routinely employed in most practices. This study evaluates efficacy of multimodality INM during FMD.

METHODS

This work is a retrospective analysis of prospectively collected data. Twenty-two FMD surgeries in 21 pediatric patients (aged 1-18 years) were performed at our center utilizing multimodality INM. All patients presented Chiari malformation type I, 18 of which had presented with syringomyelia, underwent posterior fossa decompression (FMD + C1 laminectomy), accompanied in some with additional cervical laminectomies, duroplasty, and partial tonsillectomies. TcMEP and somatosensory evoked potentials (SSEP) were monitored throughout the procedure including before and after positioning. INM alarms were correlated with perioperative and long-term patient outcomes.

RESULTS

INM data remained stable during 19 operations. Three cases displayed significant attenuation in the monitoring signals, all concomitant with patient positioning on the surgical table. One case showed attenuation in SSEP data only, which remained attenuated following repositioning. Another displayed altered TcMEP concomitant with positioning which partially stabilized following repositioning and resolved following bony decompression. The third case showed unilateral attenuation of both TcMEP and SSEP data, which did not rectify until closure. In each of these three cases, no new neurological deficits were observed post operatively.

CONCLUSIONS

Multimodality INM can be useful in FMD surgery, particularly during patient positioning. TcMEP attenuations may occur independent of SSEPs. The clinical implications of these monitoring alerts have yet to be defined. There is a need to establish an optimal, cost-effective monitoring protocol for FMD.

Predicting intraoperative feasibility of combined TES-mMEP and cSSEP monitoring during scoliosis surgery based on preoperative neurophysiological assessment

BACKGROUND CONTEXT

Combined monitoring of muscle motor evoked potentials elicited by transcranial electric stimulation (TES-mMEP) and cortical somatosensory evoked potentials (cSSEPs) is safe and effective for spinal cord monitoring during scoliosis surgery. However, TES-mMEP/cSSEP is not always feasible. Predictors of feasibility would help to plan the monitoring strategy.

PURPOSE

To identify predictors of the feasibility of TES-mMEP/cSSEP during scoliosis surgery.

STUDY DESIGN/SETTING

Prospective cohort study in a clinical neurophysiology unit and pediatric orthopedic department of a French university hospital.

PATIENT SAMPLE

A total of 103 children aged 2 to 19 years scheduled for scoliosis surgery.

OUTCOME MEASURES

Feasibility rate of intraoperative TES-mMEP/cSSEP monitoring.

METHODS

All patients underwent a preoperative neurological evaluation and preoperative mMEP and cSSEP recordings at both legs. For each factor associated with feasibility, we computed sensitivity, specificity, positive predictive value (PPV), and negative predictive value. A decision tree was designed.

RESULTS

Presence of any of the following factors was associated with 100% feasibility, 100% specificity, and 100% PPV: idiopathic scoliosis, normal preoperative neurological findings, and normal preoperative mMEP and cSSEP recordings. Feasibility was 0% in the eight patients with no recordable mMEPs or cSSEPs during preoperative testing. A decision tree involving three screening steps can be used to identify patients in whom intraoperative TES-mMEP/cSSEP is feasible.

CONCLUSIONS

Preoperative neurological and neurophysiological assessments are helpful for identifying patients who can be successfully monitored by TES-mMEP/cSSEP during scoliosis surgery.

Intraoperative motor evoked potential monitoring - a position statement by the American Society of Neurophysiological Monitoring

The following intraoperative MEP recommendations can be made on the basis of current evidence and expert opinion: (1) Acquisition and interpretation should be done by qualified personnel. (2) The methods are sufficiently safe using appropriate precautions. (3) MEPs are an established practice option for cortical and subcortical mapping and for monitoring during surgeries risking motor injury in the brain, brainstem, spinal cord or facial nerve. (4) Intravenous anesthesia usually consisting of propofol and opioid is optimal for muscle MEPs. (5) Interpretation should consider limitations and confounding factors. (6) D-wave warning criteria consider amplitude reduction having no confounding factor explanation: >50% for intramedullary spinal cord tumor surgery, and >30-40% for peri-Rolandic surgery. (7) Muscle MEP warning criteria are tailored to the type of surgery and based on deterioration clearly exceeding variability with no confounding factor explanation. Disappearance is always a major criterion. Marked amplitude reduction, acute threshold elevation or morphology simplification could be additional minor or moderate spinal cord monitoring criteria depending on the type of surgery and the program's technique and experience. Major criteria for supratentorial, brainstem or facial nerve monitoring include >50% amplitude reduction when warranted by sufficient preceding response stability. Future advances could modify these recommendations.

Intraoperative neurophysiologic monitoring during syringomyelia surgery: lessons from a series of 13 patients

BACKGROUND

Avoiding iatrogenic neurological injury during spinal cord surgery is crucially important. Intraoperative neurological monitoring (INM) has been widely used in a variety of spinal surgeries as a means of reducing the risk of intraoperative neurological insults. This study evaluates the benefits of INM specifically in spinal procedures for treatment of syringomyelia.

METHODS

Thirteen patients who underwent surgery for syrinx drainage with the assistance of INM were included in this study. In all patients both somatosensory-evoked potentials (SSEP) and motor-evoked potentials (MEP) were monitored. INM data and perioperative neurological evaluations were both recorded and analyzed.

RESULTS

Eleven patients underwent syringo-subarachnoid shunt (SSAS) surgery. One patient underwent syrinx drainage and foramen magnum decompression (FMD). One patient underwent syringo-pleural shunt (SPA) surgery. Baseline MEP and SSEP were recordable at the beginning of surgery in 11 patients (>84 %). In the other two cases, baseline data from specific INM modalities were absent, correlating with the antecedent neurologic symptomotology. Two patients exhibited significant intraoperative changes in MEP data that influenced the course of surgery and prompted removal or re-insertion of the shunt. Mild and transient worsening of preoperative symptoms was reported in these instances. No new postoperative neurological deficits were reported in the other 11 patients in whom INM data were preserved throughout surgery.

CONCLUSION

These data support routine use of INM in syringomyelia surgery. INM can alert the surgeon to potential intraoperative threats to the functional integrity of the spinal cord, providing a useful adjunct to spinal cord surgeries for the treatment of syringomyelia.

Spinal Cord Monitoring With Transcranial Motor Evoked Potentials in Patients With Neural Axis Abnormalities Undergoing Spinal Deformity Surgery

STUDY DESIGN

Retrospective, case-control study.

OBJECTIVES

To report the effectiveness of transcranial motor evoked potentials (TcMEPs) in patients undergoing scoliosis surgery with neural axis abnormalities (NAAs).

SUMMARY OF BACKGROUND DATA

Transcranial motor evoked potentials are a safe and sensitive modality to identify impending spinal cord injury in adolescent idiopathic scoliosis (AIS). Previous studies have analyzed somatosensory evoked potentials (SSEPs) and neurogenic motor evoked potentials in NAA patients, but to our knowledge, no study has addressed the use of TcMEPs in these patients.

METHODS

We performed an institutional review board-approved retrospective review of a consecutive series of patients with NAA at a single institution and compared them with a consecutive series of AIS patients undergoing scoliosis surgery with spinal cord monitoring using TcMEP and SSEP. We compared the ability to obtain baseline data and the incidence of critical changes in TcMEPs and SSEPs between groups and examined a correlation with postoperative neurologic deficits.

RESULTS

We compared 38 patients with NAA (15 Chiari malformations, 12 syrinx, 7 tethered cords, and 4 spinal cord tumors) with 184 patients with AIS. The age was similar and preoperative curve magnitude was greater in the NAA group. Good baseline data were obtained less frequently in the NAA group for TcMEPs (94.7% vs. 100%; p < .001) and SSEPs (89.5% vs. 100%; p < .001). There was no statistical difference in critical deviation from baseline in the NAA group for TcMEPs (3 of 38 [7.9%] vs. 5 of 184 [2.7%]; p = .120) or SSEPs (0 of 38 vs. 3 of 184 [1.6%] (p = .430). There were no postoperative neurologic deficits in the NAA or AIS group.

CONCLUSIONS

The ability to obtain baseline spinal cord monitoring in patients with NAA approaches that of an AIS group and accurately identifies impending neurologic deficits with high sensitivity. Surgeons should be confident that TcMEP baseline data can be obtained in patients with spinal cord pathology and should trust critical changes in TcMEPs intraoperatively to prevent spinal cord injury.

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.

Relevance of intraoperative D wave in spine and spinal cord surgeries

PURPOSE

The combined recordings of epidural-(D wave) and muscle motor evoked potentials (m-MEPs) have been proposed in many studies in intramedullary spinal cord tumour (IMSCT) surgery, although not all agree. Furthermore, the usefulness of the intraoperative monitoring of motor systems using these methods in other types of spine surgery has not yet been clearly confirmed. The aim of this study is to test the impact of intraoperative D wave on the monitorability and motor outcome in spine surgery.

METHODS

Intraoperative recording of posterior tibial nerve somatosensory potentials, lower limb m-MEPs (LLm-MEPs) and epidurally recorded D wave caudally to the surgical level was attempted in a total of 103 spine and spinal cord surgeries (23 IMSCT, 55 extramedullary spinal cord tumours and 25 myelopathies).

RESULTS

There was a 97.1 %, overall monitorability where at least 1 of the 3 modalities was applicable in 100 surgical procedures. Baseline LLm-MEPs were recorded bilaterally in 85 cases and unilaterally in 11. A caudal D wave was recorded in 97 cases. Transient, or persistent intraoperative modifications occurred in 14/23 IMSCT, 5/55 extramedullary spinal cord tumours and in 2/25 myelopathies. The presence of a persistent stable caudal D wave was predictive of a good motor outcome even when the LL-MEPs were absent and/or when lost during surgery.

CONCLUSIONS

Not only is intraoperative D wave recording to be considered mandatory in IMSCT surgery but it should also be attempted in other types of spine/spinal cord surgeries.

Motor-evoked potential monitoring for intramedullary spinal cord tumor surgery: correlation of clinical and neurophysiological data in a series of 100 consecutive procedures

Resection of intramedullary spinal cord tumors carries a high risk for surgical damage to the motor pathways. This surgery is therefore optimal for testing the performance of intraoperative motor evoked potential (MEP) monitoring. This report attempts to provide evidence for the accurate representation of patients' pre- and postoperative motor status by combined epidural and muscle MEP monitoring during intramedullary surgery. The authors used transcranial electrical motor cortex stimulation to elicit MEPs, which were recorded from the spinal cord (with an epidural electrode) and from limb target muscles (thenar, anterior tibial) with needle electrodes. The amplitude of the epidural MEPs and the presence or absence of muscle MEPs were the parameters for MEP interpretation. A retrospective analysis was performed on data from the resection of 100 consecutive intramedullary tumors and MEP data were compared with the pre- and postoperative motor status. Intraoperative monitoring was feasible in all patients without severe preoperative motor deficits. Preoperatively paraplegic patients had no recordable MEPs. The sensitivity of muscle MEPs to detect postoperative motor deficits was 100% and its specificity was 91%. There was no instance in which a patient with stable MEPs developed a motor deficit postoperatively. Intraoperative MEPs adequately represented the motor status of patients undergoing surgery for intramedullary tumors. Because deterioration of the motor status was transient in all cases, it can be considered that impairment of the functional integrity of the motor pathways was detected before permanent deficits occurred.

Intraoperative decrease in amplitude of somatosensory-evoked potentials of the lower extremities with interbody fusion cage placement during lumbar fusion surgery

STUDY DESIGN

A retrospective analysis was performed.

OBJECTIVE

To characterize neurophysiological data of patients who had a decrease in amplitude of somatosensory-evoked potentials (SSEP) of the lower extremities secondary to interbody fusion cage placement during lumbar fusion surgery with no alert of the electromyography (EMG).

SUMMARY OF BACKGROUND DATA

The most consistently used and studied modalities of neurophysiological monitoring during spine surgery are SSEPs, motor-evoked potentials (MEPs), and EMG. In general, it is accepted that MEPs along with SSEPs are used to detect spinal cord injury and EMGs are used to detect nerve root injury.

METHODS

The medical records of a consecutive series of 115 patients who had undergone a transforaminal lumbar interbody fusion (TLIF) procedure in which SSEPs, MEPs, and EMGs were utilized for neurophysiological monitoring were retrospectively reviewed.

RESULTS

One hundred fifteen cases of TLIF procedures were reviewed. The follow-up was 2 years after the last procedure. A total of 5 cases that demonstrated intraoperative SSEP changes were found. The age range for these cases was from 39 to 81 years (mean age, 61 yr). All 5 patients developed SSEP changes that were secondary to interbody fusion cage placement. All 5 cases demonstrated reversal of the SSEP changes to baseline after removal of the interbody cage. Three of these cases had no new postoperative neurological findings. However, given that these 3 cases of SSEP change were associated with a surgical event that improved secondary to an intervention (in this case removal of the interbody cage), those cases were classified as presumed positive. Two of the 5 cases were in fact associated with a new postoperative neurological deficit.

CONCLUSION

To our knowledge this study demonstrates the first reported SSEP alerts that were associated with a posterior lumbar interbody cage placement without a corresponding EMG alert.

A review of intraoperative monitoring for spinal surgery

BACKGROUND

Intraoperative neurophysiologic monitoring (IONM) is a technique that is helpful for assessing the nervous system during spine surgery.

METHODS

This is a review of the field describing the basic mechanisms behind the techniques of IONM. These include the most often utilized trancranial motor evoked potentials (Tc-MEPs), somatosensory evoked potentials (SSEPs), and stimulated and spontaneous EMG activity. It also describes some of the issues regarding practices and qualifications of practitioners.

RESULTS

Although the anatomic pathways responsible for the Tc-MEP and SSEP are well known and these clinical techniques have a high sensitivity and specificity, there is little published data showing that monitoring actually leads to improved patient outcomes. It is evident that IONM has high utility when the risk of injury is high, but may be only marginally helpful when the risk of injury is very low. The monitoring team must be well trained, be able to provide the surgeon feedback in real time, and coordinate activities with those of the surgical and anesthesia teams.

CONCLUSIONS

Although IONM is a valuable technique that provides sensitive and specific indications of neurologic injury, it does have limitations that must be understood. Maintaining a high quality of practice with appropriately trained personnel is critical.

Transcranial electric motor evoked potential monitoring during spine surgery: is it safe?

STUDY DESIGN

Retrospective review.

OBJECTIVE

To report on the safety of repetitive transcranial electric stimulation (RTES) for eliciting motor-evoked potentials during spine surgery.

SUMMARY OF BACKGROUND DATA

Theoretical concerns over the safety of RTES have hindered broader acceptance of transcranial electric motor-evoked potentials (tceMEP), despite successful implementation of spinal cord monitoring with tceMEPs in many large spine centers, as well as their apparent superiority over mixed-nerve somatosensory-evoked potentials (SSEP) for detection of spinal cord injury.

METHODS

The records of 18,862 consecutive patients who met inclusion criteria and underwent spine surgery with tceMEP monitoring were reviewed for RTES-related complications.

RESULTS

This large retrospective review identified only 26 (0.14%) cases with RTES-related complications; all but one of these were tongue lacerations, most of which were self-limiting.

CONCLUSIONS

The results demonstrate that RTES is a highly safe modality for monitoring spinal cord motor tract function intraoperatively.

Intraoperative electrophysiological monitoring in spine surgery

STUDY DESIGN

Review of the literature with analysis of pooled data.

OBJECTIVE

To assess common intraoperative neuromonitoring (IOM) changes that occur during the course of spinal surgery, potential causes of change, and determine appropriate responses. Further, there will be discussion of appropriate application of IOM, and medical legal aspects. The structured literature review will answer the following questions: What are the various IOM methods currently available for spinal surgery? What are the sensitivities and specificities of each modality for neural element injury? How are the changes in each modality best interpreted? What is the appropriate response to indicated changes? Recommendations will be made as to the interpretation and appropriate response to IOM changes.

SUMMARY OF BACKGROUND DATA

Total number of abstracts identified and reviewed was 187. Full review was performed on 18 articles.

METHODS

The MEDLINE database was queried using the search terms IOM, spinal surgery, SSEP, wake-up test, MEP, spontaneous and triggered electromyography alone and in various combinations. Abstracts were identified and reviewed. Individual case reports were excluded. Detailed information and data from appropriate articles were assessed and compiled.

RESULTS

Ability to achieve IOM baseline data varied from 70% to 98% for somatosensory-evoked potentials (SSEP) and 66% to 100% for motor-evoked potentials (MEP) in absence of neural axis abnormality. Multimodality intraoperative neuromonitoring (MIOM) provided false negatives in 0% to 0.79% of cases, whereas isolated SSEP monitoring alone provided false negative in 0.063% to 2.7% of cases. MIOM provided false positive warning in 0.6% to 1.38% of cases.

CONCLUSION

As spine surgery, and patient comorbidity, becomes increasingly complex, IOM permits more aggressive deformity correction and tumor resection. Combination of SSEP and MEP monitoring provides assessment of entire spinal cord functionality in real time. Spontaneous and triggered electromyography add assessment of nerve roots. The wake-up test can continue to serve as a supplement when needed. MIOM may prove useful in preservation of neurologic function where an alteration of approach is possible. IOM is a valuable tool for optimization of outcome in complex spinal surgery.

False negative and positive motor evoked potentials in one patient: is single motor evoked potential monitoring reliable method? A case report and literature review

STUDY DESIGN

A case report and literature review.

OBJECTIVE

To report a false negative and delayed positive motor-evoked potential (MEP) in 1 patient.

SUMMARY OF BACKGROUND DATA

An unreliable MEP can result in fatal outcomes because surgeons have recently begun to depend on the MEP for intraoperative decision-making.

METHODS

We report a case of a false MEP during scoliosis surgery that showed false negative and positive MEPs during a series of operations.

RESULTS

A 23-year-old man with a history of spondyloepiphyseal dysplasia presented with severe kyphoscoliosis. The initial neurologic examination did not reveal any neurologic abnormalities. Surgical correction and fusion were performed with transcranial MEP monitoring. During the entire procedure, the MEP did not reveal any signs of cord injury. However, lower limb paralysis and paresthesia was observed when the patient awakened. After 2 additional surgical procedures to recover the neurologic deficit, the MEP did not show any signs of cord injury but the patient's neurologic status had recovered slightly. At postoperative day 8, the neurologic status recovered, and a third operation was performed to fix the long rods. However, there were abnormal amplitudes in both lower limbs but the patient's neurologic status was almost normal.

CONCLUSION

From our experience of false negative and positive MEP in 1 patient, it is concluded that undesirable events can occur with use of MEP in scoliosis or other spinal surgery. Therefore, we warn the surgeons too heavily rely on the MEP monitoring, and propose a further prospective study as well as use of alternative method that can improve the reliability of single MEP.

Intraoperative neurophysiological monitoring during spine surgery: a review

Spinal surgery involves a wide spectrum of procedures during which the spinal cord, nerve roots, and key blood vessels are frequently placed at risk for injury. Neuromonitoring provides an opportunity to assess the functional integrity of susceptible neural elements during surgery. The methodology of obtaining and interpreting data from various neuromonitoring modalities-such as somatosensory evoked potentials, motor evoked potentials, spontaneous electromyography, and triggered electromyography-is reviewed in this report. Also discussed are the major benefits and limitations of each modality, as well as the strength of each alone and in combination with other modalities, with regard to its sensitivity, specificity, and overall value as a diagnostic tool. Finally, key clinical recommendations for the interpretation and step-wise decision-making process for intervention are discussed. Multimodality neuromonitoring relies on the strengths of different types of neurophysiological modalities to maximize the diagnostic efficacy in regard to sensitivity and specificity in the detection of impending neural injury. Thorough knowledge of the benefits and limitations of each modality helps in optimizing the diagnostic value of intraoperative monitoring during spinal procedures. As many spinal surgeries continue to evolve along a pathway of minimal invasiveness, it is quite likely that the value of neuromonitoring will only continue to become more prominent.

Transcranial electric stimulation for intraoperative motor evoked potential monitoring: Stimulation parameters and electrode montages

OBJECTIVE

To evaluate the efficacy of constant current transcranial electric stimulation (TES) parameters for eliciting muscle motor evoked potentials (MEPs) in the abductor pollicis brevis muscles (APB) and the tibialis anterior muscles (TA). The following parameters were tested intraoperatively: interstimulus interval (ISI), individual stimulation pulse duration within a train of five stimuli. Different montages of stimulating electrodes were assessed for effectiveness and focality. Further, reference values for APB and TA motor thresholds in neurosurgical patients with normal motor status under total intravenous anesthesia were determined.

METHODS

Motor thresholds of contralateral muscle MEPs were determined at 0.1, 0.2, 0.4, and 0.5 ms pulse duration and ISIs of 2, 3, 4, and 5 ms using a train of five monophasic constant current pulses with C3/C4 (27 patients). The stimulating electrodes were positioned at C1, C2, C3, C4, Cz, and Cz+6 cm. Different montages were used to determine the most effective and the most focal stimulation montages for the APB and TA muscles (30 patients). Eighty-six patients with clinically normal motor function were studied for motor threshold reference values.

RESULTS

The prolongation of the pulse duration has the strongest effect to decrease the motor threshold, which proportionally increases the delivered charge. The lowest stimulation threshold to elicit muscle MEPs in the APB and TA muscles is achieved with a train of stimuli consisting of an individual stimulus pulse duration of 0.5 ms. An ISI of 4 ms gave the lowest motor thresholds, but did not reach statistical significance compared to 3 ms. The stimulating electrode montage C3/C4 (C4/C3) allows for the lowest stimulation thresholds, but the vigorous muscle contractions it has is a disadvantage. The most focal stimulating electrode montages for the contralateral APB muscles are C3/Cz and C4/Cz, respectively, and for the TA muscles Cz/Cz+6 cm.

CONCLUSIONS

In adult neurosurgical patients with a normal motor status under total intravenous anesthesia, an individual pulse duration of 0.5 ms and an ISI of 4 ms provide the lowest motor thresholds. Pragmatically, C1/C2, resp., C2/C1 montage provides monitorable responses in both APB and TA muscles at reasonable stimulation thresholds without inducing movements disturbing surgery and especially microdissection. If the most focal hemispheric stimulation for the distal upper extremity muscles is required, the use of C3 or C4 referenced to Cz is recommended.

SIGNIFICANCE

The stimulation parameters within a train of five pulses with an individual pulse duration of 0.5 ms and an ISI of 4 ms provide the lowest motor threshold. These data confirm not only studies for D wave recovery but also provide optimal stimulation parameters for intraoperative near threshold stimulation.

Intraoperative Motor Evoked Potential Monitoring: Overview and Update

Amidst controversy about methodology and safety, intraoperative neurophysiology has entered a new era of increasingly routine transcranial and direct electrical brain stimulation for motor evoked potential (MEP) monitoring. Based on literature review and illustrative clinical experience, this tutorial aims to present a balanced overview for experienced practitioners, surgeons and anesthesiologists as well as those new to the field. It details the physiologic basis, indications and methodology of current MEP monitoring techniques, evaluates their safety, explores interpretive controversies and outlines some applications and results, including aortic aneurysm, intramedullary spinal cord tumor, spinal deformity, posterior fossa tumor, intracranial aneurysm and peri-rolandic brain surgeries. The many advances in motor system assessment achieved in the last two decades undoubtedly improve monitoring efficacy without unduly compromising safety. Future studies and experience will likely clarify existing controversies and bring further advances.

Somatosensory- and motor-evoked potential monitoring during spine and spinal cord surgery

STUDY DESIGN

Prospective, observational study.

SETTING

Regional Trauma Center, Torino, Italy.

OBJECTIVES

Complex spinal surgery carries a significant risk of neurological damage. The aim of this study is to determine the reliability and applicability of multimodality motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SEPs) monitoring during spine and spinal cord surgery in our institute.

METHODS

Recordings of MEPs to multipulse transcranial electrical stimulation (TES) and cortical SEPs were made on 52 patients during spine and spinal cord surgery under propofol/fentanyl anaesthesia, without neuromuscular blockade.

RESULTS

Combined MEPs and SEPs monitoring was successful in 38/52 patients (73.1%), whereas only MEPs from at least one of the target muscles were obtained in 12 patients (23.1%); both MEPs and SEPs were absent in two (3.8%). Significant intraoperative-evoked potential changes occurred in one or both modalities in five (10%) patients. Transitory changes were noted in two patients, whereas three had persistent changes, associated with new deficits or a worsening of the pre-existing neurological disabilities. When no postoperative changes in MEP or MEP/SEP modalities occurred, it was predictive of the absence of new motor deficits in all cases.

CONCLUSION

Intraoperative combined SEP and MEP monitoring is a safe, reliable and sensitive method to detect and reduce intraoperative injury to the spinal cord. Therefore, the authors suggest that a combination of SEP/MEP techniques could be used routinely during complex spine and/or spinal cord surgery.

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.

Effects of Anesthetic Agents and Physiologic Changes on Intraoperative Motor Evoked Potentials

Motor evoked potentials (MEPs) have shown promise as a valuable tool for monitoring intraoperative motor tract function and reducing postoperative plegia. MEP monitoring has been reported to contribute to deficit prevention during resection of tumors adjacent to motor structures in the cerebral cortex and spine, and in detecting spinal ischemia during thoracic aortic reconstruction. Many commonly used anesthetic agents have long been known to depress MEP responses and reduce MEP specificity for motor injury detection. Although new stimulation techniques have broadened the spectrum of anesthetics that can be used during MEP monitoring, certain agents continue to have dose-dependent effects on MEP reliability. Understanding the effects of anesthetic agents and physiologic alterations on MEPs is imperative to increasing the acceptance and application of this technique in the prevention of intraoperative motor tract injury. This review is intended as an overview of the effects of anesthetics and physiology on the reproducibility of intraoperative myogenic MEP responses, rather than an analysis of the sensitivity and specificity of this monitoring method in the prevention of motor injury.

Safety of intraoperative transcranial electrical stimulation motor evoked potential monitoring

This article reviews intraoperative transcranial electrical stimulation (TES) motor evoked potential (MEP) monitoring safety based on comparison with other clinical and experimental brain stimulation methods and clinical experience in more than 15000 cases. Comparative analysis indicates that brain damage and kindling are highly unlikely. There have been remarkably few adverse events. Pulse train TES-induced or coincidental seizures (n = 5) are rare, probably because of very brief (<0.03 second) stimuli, anesthesia, and the general absence of predisposing cerebral conditions. Soft bite blocks may prevent tongue or lip laceration (n = 29) or mandibular fracture (n = 1). Rare cardiac arrhythmia (n = 5) and intraoperative awareness (n = 1) may be coincidental. Minor scalp burns (n = 2) are rare. Although possible, no spinal epidural recording electrode complications or injuries resulting from TES-induced movement were found. There have been no recognized adverse neuropsychological effects, headaches, or endocrine disturbances. Comprehensive relative contraindications include epilepsy, cortical lesions, convexity skull defects, raised intracranial pressure, cardiac disease, proconvulsant medications or anesthetics, intracranial electrodes, vascular clips or shunts, and cardiac pacemakers or other implanted biomedical devices. Otherwise unexplained intraoperative seizures and possibly arrhythmias are indications to abort TES. With appropriate precautions in expert hands, the well-established benefits of TES MEP monitoring decidedly outweigh the associated risks.

Combined spinal cord monitoring using neurogenic mixed evoked potentials and collision techniques

STUDY DESIGN

Neurogenic mixed evoked potentials are used routinely to monitor the spinal cord during spine surgery. This study investigates the differential sensory-motor contribution by using collision techniques.

OBJECTIVE

To demonstrate that neurogenic mixed evoked potentials do contain a motor component.

SUMMARY OF BACKGROUND DATA

Spinal cord monitoring is now routinely used during spine deformity surgery. Neurogenic mixed evoked potentials (i.e., potentials recorded from lower limb nerves after spinal cord stimulation) represent a reliable and sensitive technique. However, their specificity (sensory and motor spinal pathways) remains debated.

METHODS

Neurogenic mixed evoked potentials and collisions were performed in 24 consecutive patients during scoliosis surgery. Neurogenic mixed evoked potentials were elicited by a high thoracic spinal test stimulation and recorded from the tibial nerve at the ankle. A peripheral conditioning stimulation was delivered at the popliteal fossa 15 ms before spinal stimulation, inducing an ascending volley. The antidromic ascending motor component stops at the anterior horn cell level, whereas the orthodromic sensory component reaches the dorsal columns. The 15-ms interstimulus interval between peripheral conditioning and spinal test stimulation makes the collision with descending volleys occur in the spinal cord. The descending sensory volley is blocked, whereas the descending motor volley is unaffected.

RESULTS

Reproducible evoked potentials were recorded from the tibial nerve in all the patients studied when the conditioning stimulation was performed. These conditioned neurogenic mixed evoked potentials consist of a small and polyphasic wave whose amplitude represents approximately 26% that of the wave of unconditioned neurogenic mixed evoked potentials. It is likely that they correspond to motor spinal pathway activation.

CONCLUSION

Both standard and conditioned neurogenic mixed evoked potentials are proposed to provide combined sensory and motor spinal pathway monitoring.

Monitoring of motor evoked potentials with high intensity repetitive transcranial electrical stimulation during spinal surgery

OBJECTIVE

Clinical utility of high voltage repetitive transcranial electrical stimulation (TES) was investigated in 46 patients undergoing spine surgery.

METHODS

During spinal surgery, motor evoked potentials (MEPs) were recorded from upper or lower limb muscles following high voltage repetitive TES of motor cortex under propofol and opioid/N2O anesthesia.

RESULTS

The number of responses evoked by the double pulse stimulation was significantly higher than the single pulse stimulation. A similar finding was obtained when repetitive and single pulse stimulation was compared. Compound muscle action potentials (CMAPs) were recorded from upper and lower limbs in 4 patients with cervical spine myclopathy. The CMAP was absent on the affected side in 1 patient, which improved slightly after decompression. Radiculopathy was clinically present in 6 patients undergoing posterior lumbar decompression and fusion. No improvement of MEP was noted intraoperatively after spinal decompression and instrumentation.

CONCLUSION

The findings suggest that intraoperative MEP monitoring is feasible method, however, its immediate prognostic value for adequacy of neuronal decompression and improvement requires further studies with larger patient population.

Modelling the response of scalp sensory receptors to transcranial electrical stimulation

Transcranial electrical stimulation of the brain causes considerable discomfort to the patient. The purpose of the study was to find out whether this could be affected by the choice of stimulation parameters. A spherical volume conductor model of the head and active compartmental models of a pyramidal motor nerve and scalp nociceptor were used in combination to simulate the scalp nociception to transcranial electrical stimulation. Scalp nociceptors were excited at distances of several centimetres from the electrodes. The size of the excited scalp area correlated with the length of the stimulation pulse. The area was 12.3, 20.4 and 26.0 cm2, for a 10 micros, 100 micros and 1 ms constant current pulse, respectively. With a 100 micros constant current pulse, the threshold for motor excitation was 205mA and, for nociception, it was 51 mA. There was no significant difference between constant current and capacitor discharge pulses or between electrodes of different sizes. The results imply that the use of very short stimulation pulses can reduce the pain. If a topical anaesthesia is used to reduce the pain, it has to be applied on a large area around the electrodes.

Corticospinal volleys and compound muscle action potentials produced by repetitive transcranial stimulation during spinal surgery

OBJECTIVES

To report our experience with neurophysiological monitoring of corticospinal function using compound muscle action potentials (CMAPs) produced by repetitive transcranial electrical stimulation in a large series of patients, after defining optimal stimulus parameters in a small group of patients.

METHODS

In 100 patients undergoing spinal surgery, corticospinal volleys were recorded using epidural electrodes, or CMAPs were recorded from innervated muscles, or both techniques were used to monitor spinal cord function. In subsets of patients, stimulus parameters were varied to determine the optimal parameters for CMAP recordings, using the corticospinal volleys to guide the initial choice.

RESULTS

Recordings of corticospinal volleys indicated that less energy was delivered to the cortex if the duration of each stimulus in the stimulus train was brief (e.g. 50 micros) and that there was attenuation of D and I waves in the corticospinal volley when the interstimulus interval in the train was <5 ms. An interstimulus interval of 5 ms proved significantly more effective than an interstimulus interval of 2 ms in evoking CMAPs, but resulted in a more complex, dispersed electromyographic (EMG) potential. The superiority of the 5 ms interval did not depend on stimulus intensity or the existence of pre-existing neurological deficit. Using trains of 5 pulses of duration 50 micros, interstimulus interval 5 ms and intensity 500 V, satisfactory CMAPs could be recorded in 55 of 82 patients, significantly less often in neurologically impaired patients than in neurologically normal subjects. Epidural recordings of the corticospinal volley were obtained in 61 of 69 patients, again more often in neurologically normal subjects.

CONCLUSIONS

When epidural recordings can be made, direct recordings of corticospinal activity are probably more reliable than recordings of CMAPs. However, epidural recordings are not suitable under all circumstances, and the ability to record CMAPs reliably represents an advance in intraoperative monitoring. Under the anaesthetic conditions used in the present study, the optimal stimulus parameters consist of a train of 5 stimuli of 50 micros duration at an interstimulus interval of 5 ms and an intensity of 500 V.

Threshold-level repetitive transcranial electrical stimulation for intraoperative monitoring of central motor conduction

OBJECT

The authors conducted a study to evaluate repetitive transcranial electrical stimulation (TES) to assess spinal cord motor tract function in individuals undergoing spine surgery, with emphasis on safety and efficacy.

METHODS

Somatosensory evoked potentials (SSEPs) were elicited using standard technique. Muscle electromyographic values were measured in response to a three- or four-pulse train of stimulation delivered to the motor cortex via subdermal electrodes. They also evaluated whether changes in the minimum stimulus intensity (that is, threshold level) needed to elicit a response from a given muscle predict motor status immediately postoperatively, as well as whether changes in SSEP response amplitude and latency predict sensory status immediately postoperatively. Anesthesia was routinely induced with intravenous propofol and remifentanil, supplemented with inhaled nitrous oxide. Use of neuromuscular block was avoided after intubation. Satisfactory monitoring of muscle response to threshold-level repetitive TES was achieved in all but nine of the 194 patients studied. In contrast, cortical SSEP responses could not be elicited in 42 of 194 individuals. In cases in which responses were present, TES-based evoked responses proved to be extremely accurate for predicting postoperative motor status. Somatosensory evoked potential monitoring was nearly as accurate for predicting postoperative sensory status. There were frequent instances of postoperative motor or sensory deficit that were not predicted by SSEP- and TES-based monitoring, respectively. There were no adverse events attributable to TES-based monitoring, although since this study ended we have had a single adverse event attributable to threshold-level repetitive TES.

CONCLUSIONS

Intraoperative threshold-level repetitive TES-based monitoring of central motor conduction has proven to be a simple, safe, and highly accurate technique for the prevention or minimization of inadvertent motor deficit during surgery involving the spine or spinal cord.

Intraoperative motor evoked potentials to transcranial electrical stimulation during two anaesthetic regimens

OBJECTIVES

To study motor evoked potentials (MEPs) to multi-pulse transcranial electrical stimulation (MP-TES) during orthopaedic spinal surgery under different anaesthetic regimens.

METHODS

MEPs to MP-TES were recorded from tibialis anterior and abductor hallucis bilaterally in 50 operations. Anaesthesia was maintained with propofol and nitrous oxide in 29 operations and isoflurane (0.78+/-0.17% end-tidal) and nitrous oxide in 23 (two patients received both regimens). Analgesia was provided with fentanyl or remifentanil.

RESULTS

Motor stimulation caused neither EEG changes nor seizures. MEPs were obtained in 97% of patients during propofol anaesthesia. The median amplitude and coefficient of variation (CV) at baseline (across all muscles) were 198 microV and 22%, respectively. Amplitudes throughout the operation paralleled the degree of neuromuscular block and were reduced after fentanyl bolus, isoflurane or morphine. Loss of MEPs or persistent amplitude decrements were associated with neurological complications in one patient and severe blood loss in another two patients. MEPs were obtainable in 61% of patients during isoflurane anaesthesia and became inconsistent for end-tidal concentrations >0.87+/-0.08%. Amplitudes were smaller (85 microV) and baseline variability higher (coefficient of variation 29%) than in the propofol group. The decrease in the number of recordings was greater for isoflurane than propofol when the number of pulses/train decreased from 4 to 2.

CONCLUSIONS

Muscle MEPs to MP-TES are a safe, sensitive and reliable method for monitoring motor pathways during propofol/nitrous oxide and fentanyl or remifentanil anaesthesia. MEPs are also obtainable in the majority of patients during isoflurane/nitrous oxide anaesthesia, but quantitative monitoring is not always possible with this regimen.