Continuous electromyographic monitoring to detect nerve root injury during thoracolumbar scoliosis surgery

Optimal "Low" Pedicle Screw Stimulation Threshold to Predict New Postoperative Lower-Extremity Neurologic Deficits During Lumbar Spinal Fusions

OBJECTIVE

Previous studies have shown that pedicle screw stimulation thresholds ≤6-8 mA yield a high diagnostic accuracy of detecting misplaced screws. Our objective was to determine the optimal "low" stimulation threshold to predict new postoperative neurologic deficits and identify additional risk factors associated with deficits.

METHODS

We included patients with complete pedicle screw stimulation testing who underwent posterior lumbar spinal fusion surgeries from 2010-2012. We calculated the diagnostic accuracy of pedicle screw responses of ≤4 mA, ≤6 mA, ≤8 mA, ≤10 mA, ≤12 mA, and ≤20 mA to predict new postoperative lower-extremity (LE) neurologic deficits. We used multivariate modeling to determine the best logistic regression model to predict LE deficits and identify additional risk factors. Statistics software packages used were Python3.8.5, NumPy 1.19.1, Pandas 1.1.1, and SPSS26.

RESULTS

We studied 1179 patients who underwent 8584 pedicle screw stimulations with somatosensory evoked potential and free-run electromyographic monitoring for posterior lumbar spinal fusion. Twenty-five (2.1%) patients had new LE neurologic deficits. A stimulation threshold of ≤8 mA had a sensitivity/specificity of 32%/90% and a diagnostic odds ratio/area under the curve of 4.34 [95% confidence interval: 1.83, 10.27]/0.61 [0.49, 0.74] in predicting postoperative deficit. Multivariate analysis showed that patients who had pedicle screws with stimulation thresholds ≤8 mA are 3.15 [1.26, 7.83]× more likely to have postoperative LE deficits while patients who have undergone a revision lumbar spinal fusion surgery are 3.64 [1.38, 9.61]× more likely.

CONCLUSIONS

Our results show that low thresholds are indicative of not only screw proximity to the nerve but also an increased likelihood of postoperative neurologic deficit. Thresholds ≤8 mA prove to be the optimal "low" threshold to help guide a correctly positioned pedicle screw placement and detect postoperative deficits.

Diagnostic Accuracy of Thresholds Less Than or Equal to 8 mA in Pedicle Screw Testing During Lumbar Spine Procedures to Predict New Postoperative Lower Extremity Neurological Deficits

STUDY DESIGN

Retrospective observational study.

OBJECTIVE

It has been shown that pedicle screw stimulation thresholds less than or equal to 8 mA yield a very high diagnostic accuracy of detecting misplaced screws in spinal surgery. In our study, we determined clinical implications of low stimulation thresholds.

SUMMARY OF BACKGROUND DATA

Posterior lumbar spinal fusions (PSF), using pedicle screws, are performed to treat many spinal pathologies, but misplaced pedicle screws can result in new postoperative neurological deficits.

METHODS

Patients with pedicle screw stimulation testing who underwent PSF between 2010 and 2012 at the University of Pittsburgh Medical Center (UPMC) were included in the study. We evaluated the sensitivity, specificity, and diagnostic odds ratio (DOR) to determine how effectively low pedicle screw responses predict new postoperative lower extremity neurological deficits.

RESULTS

One thousand one hundred seventy nine eligible patients underwent 8584 pedicle screw stimulations with lower extremity somatosensory evoked potentials (LE SSEP) monitoring for lumbar fusion surgery. One hundred twenty one of these patients had 187 pedicle screws with a stimulation response at a threshold less than or equal to 8 mA. Smoking had a significant correlation to pedicle screw stimulation less than or equal to 8 mA (P = 0.012). A threshold of less than or equal to 8 mA had a sensitivity/specificity of 0.32/0.90 with DOR of 4.34 [1.83, 10.27] and an area under the ROC curve (AUC) of 0.61 [0.49, 0.74]. Patients with screw thresholds less than or equal to 8 mA and abnormal baselines had a DOR of 9.8 [95% CI: 2.13-45.17] and an AUC of 0.73 [95% CI: 0.50-0.95].

CONCLUSION

Patients with pedicle screw stimulation thresholds less than or equal to 8 mA are 4.34 times more likely to have neurological clinical manifestations. Smoking and LE deficits were shown to be significantly correlated with pedicle screw stimulation thresholds less than or equal to 8 mA. Low stimulation thresholds result in a high specificity of 90%. Pedicle screw stimulation less than or equal to 8 mA can serve as an accurate rule in test for postoperative neurological deficit, warranting reevaluation of screw placement and/or replacement intraoperatively.Level of Evidence: 3.

Transcranial motor evoked potentials electrically elicited by multi-train stimulation can reflect isolated nerve root injury more precisely than those by conventional multi-pulse stimulation: an experimental study in rats

Nerve root injury can occur in complex spine surgeries. Recording transcranial motor-evoked potentials (TcMEPs) has been the most popular method to monitor motor function during surgery. However, TcMEPs cannot detect single nerve root injury satisfactorily. Recently, multi-train stimulation (MTS) was demonstrated to effectively enhance TcMEPs. The aim of this study was to investigate the utility of TcMEPs elicited by MTS for intraoperative nerve root monitoring. TcMEPs were recorded from the quadriceps femoris (QF) and gastrocnemius (GC) muscles in the hindlimbs of 20 rats before and after transection of the nerve root at L6 (dominant root innervating the GC). For MTS, a multipulse (train) stimulus was delivered repeatedly at 5 Hz. The change ratio of the amplitude after transection of the nerve root was compared between MTS and conventional single-train stimulation (STS). The change in TcMEP amplitudes for QF after transection of the nerve root at L6 was 97.8 ± 12.2% with MTS and 100.1 ± 7.2% with STS (p = 0.496), whereas that for GC was 40.6 ± 11.5% with MTS and 64.8 ± 8.8% with STS (p < 0.001). MTS could improve the ability to detect isolated nerve root injury in intraoperative TcMEP monitoring.

Monitoring reduced scattering coefficient in pedicle screw insertion trajectory using near-infrared spectroscopy

Pedicle screw (PS) implantation is an ideal treatment for severe multilevel vertebra instabilities. The accuracy of fixating PS is a key factor of spinal surgery. We developed a near-infrared spectroscopy device with a needlelike optical fiber probe to monitor optical parameters (reduced scattering coefficient) of vertebra models in real time. The fresh-frozen cadaver, cats and porcine vertebras were first studied in the experiments. Moreover, the reduced scattering coefficient (μ' s) along the different trajectories of PS insertion was obtained. In the fresh-frozen cadavera experiment, μ' s values could be used to distinguish the different compositions of the thoracic vertebra. In cat vertebra experiment, μ' s values of vertebrae bones, including cortical bone (15.30 ± 0.18 cm(-1)), cancellous bone (7.84 ± 1.11 cm(-1)) and spinal cord (19.46 ± 0.21 cm(-1)), were different in vivo. In the pig vertebrae experiment, there were obvious differences between the normal and abnormal PS puncture curves based on μ' s values. Thus, μ' s values measured by using the proposed device could be used as the pattern factor in spinal fusion surgery. Our studies demonstrate that near-infrared spectroscopy method may be potentially used for assisting the PS insertion.

Determination of detection depth of optical probe in pedicle screw measurement device

BACKGROUND

There is a high probability of accidental perforation of the vertebral pedicle wall in pedicle screw insertion surgery. A pedicle screw (PS) measurement device with an optical probe has been reported to send out a warning signal before the PS tip breaking the vertebral pedicle wall.

METHODS

In this study, we explored the detection depth of optical probe in this measurement device, which was closely related to the effective alarm distance. In the boundary, the vertebrae tissues could be treated as 2-layer models including spongy bones and compact bones. The Monte Carlo simulation and phantom models were performed to analyse and define the detection depth. Then the porcine vertebrae models were performed to obtain optical spectrum and reduced scattering coefficient, based on which the detection depths were deduced. Moreover, a comparison was made to explore the most significant pattern factor from the experiment results.

RESULTS

According to the pattern factor, an alarm threshold was successfully deduced to define the alarm distance during pedicle screw monitoring.

CONCLUSIONS

Thus, the proposed alarm standard based on detection depth provides a potential for guiding pedicle screw in surgery.

Monitoring superficial peroneal nerve somatosensory evoked potential during L4-5 lumbar root decompression

BACKGROUND CONTEXT

Posttibial nerve somatosensory evoked potential (PTN-SSEP) often fails to detect individual nerve root dysfunction, whereas dermatomal somatosensory evoked potential (DSSEP) can be difficult to obtain in anesthetized patients.

PURPOSE

The main aim of this study is to investigate whether the superficial peroneal nerve SSEP (SPN-SSEP), a cutaneous nerve SSEP derived from L4-5 roots, can be relied on as an intraoperative neurophysiological assessment for L4-5 roots.

STUDY DESIGN/SETTING

Retrospective review of consecutive case series.

PATIENT SAMPLE

Thirty-five consecutive patients who underwent lumbar root microdecompression were reviewed. The control group consisted of 11 patients who underwent anterior cervical discectomy and fusion (ACDF).

OUTCOME MEASURES

Intraoperative SSEP measures defined outcome measures.

METHODS

Data from 35 patients undergoing SPN-SSEP, PTN-SSEP, and sural nerve SSEP (SN-SSEP) monitoring during lumbar surgeries were analyzed and compared with the control group of patients underwent ACDF surgery.

RESULTS

Cortical SPN-SSEP could be readily recorded in more than 90% patients from both groups. The waveforms of SPN-SSEP were about 30% smaller than PTN-SSEP, but very similar to SN-SSEP. Importantly, SPN-SSEP was capable of detecting intraoperative L4-5 root dysfunction without the corresponding PTN-SSEP or SN-SSEP alterations. Overall, SPN-SSEP displayed twofold greater sensitivity over PTN-SSEP or SN-SSEP.

CONCLUSIONS

SPN-SSEP was a more sensitive measure for intraoperative L4-5 nerve root dysfunction in comparison with PTN-SSEP and SN-SSEP.

Electromyographic monitoring and its anatomical implications in minimally invasive spine surgery

STUDY DESIGN

Literature review.

OBJECTIVE

The objective of this article is to examine current intraoperative electromyography (EMG) neurophysiologic monitoring methods and their application in minimally invasive techniques. We will also discuss the recent application of EMG and its anatomic implications to the minimally invasive lateral transpsoas approach to the spine.

SUMMARY OF BACKGROUND DATA

Minimally invasive techniques require that the same goals of surgery be achieved, with the hope of decreased morbidity to the patient. Unlike standard open procedures, direct visualization of the anatomy is decreased. To increase the safety of minimally invasive spine surgery, neurophysiological monitoring techniques have been developed.

METHODS

Review of the literature was performed using the National Center for Biotechnology Information databases using PUBMED/MEDLINE. All articles in the English language discussing the use of intraoperative EMG monitoring and minimally invasive spine surgery were reviewed. The role of EMG monitoring in special reference to the minimally invasive lateral transpsoas approach is also described.

RESULTS

In total, 76 articles were identified that discussed the role of neuromonitoring in spine surgery. The majority of articles on EMG and spine surgery discuss the use of intraoperative neurophysiological monitoring (IOM) for safe and accurate pedicle screw placement. In general, there is a paucity of literature that pertains to intraoperative EMG neuromonitoring and minimally invasive spine surgery. Recently, EMG has been used during minimally invasive lateral transpsoas approach to the lumbar spine for interbody fusion. The addition of EMG to the lateral approach has contributed to decrease the complication rate from 30% to less than 1%.

CONCLUSION

In minimally invasive approaches to the spine, the use of EMG IOM might provide additional safety, such as percutaneous pedicle screw placement, where visualization is limited compared with conventional open procedures. In addition to knowledge of the anatomy and image guidance, directional EMG IOM is crucial for safe passage through the psoas muscle during the minimally invasive lateral retroperitoneal approach.

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.

Relative efficacy of transcranial motor evoked potentials, mechanically-elicited electromyography, and evoked EMG to assess nerve root function during sustained retraction in a porcine model

STUDY DESIGN

This is an animal experiment using transcranial motor evoked potentials (TcMEP), mechanically elicited electromyography (EMG), and evoked EMG during spinal nerve root retraction in a pig model.

OBJECTIVE

To compare the sensitivity of these 3 electrophysiological measures for a constant retraction force applied to an isolated lumbar nerve root for a specific duration of time.

SUMMARY OF BACKGROUND DATA

The incidence of nerve root injury during lumbar spine surgery ranges from 0.2% to 31%. Direct retraction of spinal nerve roots may cause these injuries, but the amount and duration of force that may safely be applied is not clear. Using an established porcine model, we examined the changes occurring to multimyotomal TcMEPs, mechanically elicited EMGs, and evoked EMGs during continuous retraction of a nerve root at a constant force applied over 10 minutes.

METHODS

TcMEP, mechanically elicited EMG, and evoked EMG responses were recorded from the tibialis anterior (TA) muscle in 10 experiments. The dominant root innervating the TA was determined with evoked EMG; preretraction TcMEP and nerve root stimulation threshold (NRT) was obtained. The dominant root was retracted at 2 Newton (N) for 10 minutes. TcMEP trials were elicited every minute during retraction. NRT was measured immediately after retraction. TcMEP and NRT were measured after 10 minutes of recovery. RESULTS.: During the 10 minutes of retraction at 2 N, the amplitude of the TA muscle progressively decreased in all trials in a highly significant curvilinear fashion. The mean TcMEP amplitude decreased 59% +/- 14% from baseline values. The mean NRT after 10 minutes of retraction at 2 N rose to 1.8 +/- 0.7 mA (P < 0.01 vs. baseline). The NRT increase after retraction strongly correlated with the decrease in motor evoked potentials amplitude in the TA (R = 0.90, P < 0.001). EMG activity was variable; tonic EMG was observed in only 2 nerve roots (20%).

CONCLUSION

Three electrophysiologic methods were used intraoperatively to assess neural function during retraction of a single nerve root. Retraction produced consistent changes in TcMEPs and evoked EMG. These 2 methods show promise for assessing the limits on the force and duration of nerve root retraction during spine surgery. Mechanically elicited EMG was not sensitive to the amount and duration of nerve root retraction.

Rate of complications in scoliosis surgery - a systematic review of the Pub Med literature

Background

Spinal fusion surgery is currently recommended when curve magnitude exceeds 40–45 degrees. Early attempts at spinal fusion surgery which were aimed to leave the patients with a mild residual deformity, failed to meet such expectations. These aims have since been revised to the more modest goals of preventing progression, restoring 'acceptability' of the clinical deformity and reducing curvature.

In view of the fact that there is no evidence that health related signs and symptoms of scoliosis can be altered by spinal fusion in the long-term, a clear medical indication for this treatment cannot be derived. Knowledge concerning the rate of complications of scoliosis surgery may enable us to establish a cost/benefit relation of this intervention and to improve the standard of the information and advice given to patients. It is also hoped that this study will help to answer questions in relation to the limiting choice between the risks of surgery and the "wait and see – observation only until surgery might be recommended", strategy widely used. The purpose of this review is to present the actual data available on the rate of complications in scoliosis surgery.

Materials and methods

Search strategy for identification of studies; Pub Med and the SOSORT scoliosis library, limited to English language and bibliographies of all reviewed articles. The search strategy included the terms; 'scoliosis'; 'rate of complications'; 'spine surgery'; 'scoliosis surgery'; 'spondylodesis'; 'spinal instrumentation' and 'spine fusion'.

Results

The electronic search carried out on the 1^st February 2008 with the key words "scoliosis", "surgery", "complications" revealed 2590 titles, which not necessarily attributed to our quest for the term "rate of complications". 287 titles were found when the term "rate of complications" was used as a key word. Rates of complication varied between 0 and 89% depending on the aetiology of the entity investigated. Long-term rates of complications have not yet been reported upon.

Conclusion

Scoliosis surgery has a varying but high rate of complications. A medical indication for this treatment cannot be established in view of the lack of evidence. The rate of complications may even be higher than reported. Long-term risks of scoliosis surgery have not yet been reported upon in research. Mandatory reporting for all spinal implants in a standardized way using a spreadsheet list of all recognised complications to reveal a 2-year, 5-year, 10-year and 20-year rate of complications should be established. Trials with untreated control groups in the field of scoliosis raise ethical issues, as the control group could be exposed to the risks of undergoing such surgery.

Monitoring of nerve root injury using transcranial motor-evoked potentials in a pig model

STUDY DESIGN

Animal experiment using transcranial motor-evoked potentials (tcMEPs) in a pig model.

OBJECTIVE

To validate measurement of tcMEPs from multiple myotomes in a pig model and determine the capacity to detect injury to a single nerve root.

SUMMARY OF BACKGROUND DATA

The ability of intraoperative neuromonitoring methods to give information about a single nerve root remains poorly understood. Reports suggest that tcMEPs may be a reliable and accurate method to detect nerve root injury. An animal model to study the sensitivity and specificity of this technique has yet to be validated.

METHODS

Transcranial stimulation was delivered through customized electrodes placed in burr holes over the motor cortex in 7 pigs. Spontaneous and evoked muscle potential activity was recorded in 5 myotomes (rectus femoris, vastus lateralis, vastus medialis, tibialis anterior, and gastrocnemius) bilaterally. After unilateral exposure of the L3-S1 nerve roots, sequential ligations were performed. The tcMEP responses from all myotomes were measured after ligation of each nerve root.

RESULTS

Robust MEP responses (range, 37-1165 mV) were achieved in all monitored myotomes. Significant decreases in tcMEP amplitudes occurred in specific myotomes after ligation of the corresponding nerve root. Consistent and substantial decreases were observed after L3 and L5 ligations in rectus femoris (48%) and tibialis anterior (67%), respectively.

DISCUSSION

Our results validate monitoring of tcMEPs in multiple myotomes to detect nerve root injury in pigs. This model may be used for further study of the use of tcMEPs to detect predictors and risk factors of nerve root injury during spinal surgery.

The efficacy of motor evoked potentials in fixed sagittal imbalance deformity correction surgery

STUDY DESIGN

Retrospective analysis of transcranial motor evoked potential (TcMEP) responses and clinical outcome.

OBJECTIVE

To determine the sensitivity and specificity of TcMEPs to detect and predict isolated nerve root injury in selected patients having complex lumbar spine surgery.

SUMMARY OF BACKGROUND DATA

The surgical correction of fixed sagittal plane deformity involves posterior-based osteotomies and significant changes in the length of and space for the neural elements. The role of transcranial motor-evoked potential (TcMEP) monitoring in osteotomies below the conus has not been established. The purpose of this paper is to describe the relationship between neural complications from surgery and intraoperative TcMEP changes.

METHODS

We retrospectively studied 35 consecutive patients in a single center treated with posterior-based osteotomies for the correction of fixed sagittal plane deformity. Transcranial motor-evoked potentials, free-running and evoked electromyography data were assessed for each case. Analysis includes description of the intraoperative changes observed, and a correlation of changes with postoperative clinical findings.

RESULTS

Thirty-five consecutive patients underwent surgery for fixed sagittal plane deformity with complete neuromonitoring data. Twenty-five patients (71%) had an episode of greater than 80% reduction in MEP amplitude to at least 1 muscle. Fifteen of 25 had improvement of TcMEPs after repositioning of the legs (1), additional surgical decompression (4), or volume and pharmacologic resuscitation (10). All 15 of these awoke with no detectable neurologic injury. Ten patients (29%) had reduced TcMEP signals that did not improve despite further decompression and manipulation of the osteotomy site. All 10 had a greater than 67% drop in TcMEPs for at least 1 muscle persisting at the end of the case, and all had a postoperative neurologic deficit. The TcMEP changes in patients who demonstrated nerve injury postoperatively were observed most often during osteotomy closure or sustained dural retraction. 9 patients had weakness involving the iliopsoas or quadriceps; 1 patient had isolated unilateral dorsiflexion weakness. Monitoring TcMEPs in multiple muscle groups was both highly sensitive and specific for predicting injury. Nine patients had recovered motor function completely by discharge, and all but 1 patient (grade 4/5) had a normal motor examination at 6-week follow-up.

CONCLUSION

The use of TcMEPs is sensitive and specific to change in neural function. No patients had a false negative test. The rate of neural deficits is consistent with previous literature, suggesting that TcMEP monitoring may not prevent neural injury. However, there were several cases in which intraoperative intervention resulted in recovery of TcMEPs, and none of these patients sustained any postoperative neural deficit. The severity of neural deficits in this series was minor and the duration was limited. TcMEPs may contribute to calling attention to the need for intraoperative corrections including widening decompressions, improving perfusion, and limiting deformity correction so that more severe neural compromise may be prevented.

Questionnaire Study of Neuromonitoring Availability and Usage for Spine Surgery

STUDY DESIGN

Questionnaire study presented to practicing spine surgeons.

OBJECTIVE

To evaluate surgeon preference and availability of selected electrophysiologic neuromonitoring for different spine surgeries.

SUMMARY OF BACKGROUND DATA

Maximizing the safety of spinal procedures and limiting potential iatrogenic neurologic injury has made intraoperative neuromonitoring an attractive option.

METHODS

We distributed a questionnaire to 180 orthopedic spine surgeons and neurosurgeons at a clinically oriented spine meeting asking surgeon preference and availability of various types of intraoperative neuromonitoring modalities for different types of surgical procedures. Demographic data were also gathered.

RESULTS

Somatosensory evoked potentials (SSEPs) were the most available neuromonitoring modality, followed by electromyographies and motor-evoked potentials. In both anterior and posterior cervical surgery, SSEPs were the most preferred modality. MEPs were frequently preferred in myelopathic cervical cases. Almost 70% preferred some neuromonitoring for anterior thoracic/thoracolumbar cases and 55% for posterior thoracic/thoracolumbar cases. Surgeon satisfaction was related to the number of available neuromonitoring modalities. No significant differences were found between orthopedist and neurosurgeon preferences. Fellowship-trained surgeons were more likely to use neuromonitoring for specific indications.

CONCLUSIONS

SSEPs remains the most widely available and preferred type of neuromonitoring for spine surgeons. The type of case and neurologic status of patient (eg, presence of myelopathy) affects these choices. Surgeons were more satisfied with greater neuromonitoring availability, and were more likely to use neuromonitoring if they had a fellowship background.

Impact of spine surgery on signs and symptoms of spinal deformity

Paediatric scoliosis is associated with signs and symptoms including reduced pulmonary function, increased pain and impaired quality of life, all of which worsen during adulthood, even when the curvature remains stable. Spinal fusion has been used as a treatment for nearly 100 years. In 1941, the American Orthopedic Association reported that for 70% of patients treated surgically, outcome was fair or poor: an average 65% curvature correction was reduced to 27% at >2 year follow-up and the torso deformity was unchanged or worse. Outcome was worse in children treated surgically before age 10, despite earlier intervention. Today, a reduced magnitude of curvature obtained by spinal fusion in adolescence can be maintained for decades. However, successful surgery still does not eliminate spinal curvature and it introduces irreversible complications whose long-term impact is poorly understood. For most patients there is little or no improvement in pulmonary function. Some report improved pain after surgery, some report no improvement and some report increased pain. The rib deformity is eliminated only by rib resection which can dramatically reduce respiratory function even in healthy adolescents. Outcome for pulmonary function and deformity is worse in patients treated surgically before the age of 10 years, despite earlier intervention. Research to develop effective non-surgical methods to prevent progression of mild, reversible spinal curvatures into complex, irreversible structural deformities, is long overdue.

Intraoperative neurophysiologic monitoring: focus on cervical myelopathy and related issues

BACKGROUND CONTEXT

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

PURPOSE

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

STUDY DESIGN/METHODS

A systematic literature review.

CONCLUSIONS

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

Intraoperative Electrophysiologic Monitoring: Considerations for Complex Spinal Surgery

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

Intraoperative Monitoring of Segmental Spinal Nerve Root Function with Free-Run and Electrically-Triggered Electromyography and Spinal Cord Function with Reflexes and F-Responses

BACKGROUND CONTEXT

Orthodromic ascending somatosensory evoked potentials and antidromic descending neurogenic somatosensory evoked potentials monitor spinal cord sensory function. Transcranial motor stimulation monitors spinal cord motor function but only activates 4-5% of the motor units innervating a muscle. Therefore, 95-96% of the motor spinal cord systems activating the motor units are not monitored. To provide more comprehensive monitoring, 11 techniques have been developed to monitor motor nerve root and spinal cord motor function. These techniques include: 1. neuromuscular junction monitoring, 2. recording free-run electromyography (EMG) for monitoring segmental spinal nerve root function, 3. electrical stimulation to help determine the correct placement of pedicle screws, 4. electrical impedance testing to help determine the correct placement of pedicle screws, 5. electrical stimulation of motor spinal nerve roots, 6. electrical stimulation to help determine the correct placement of iliosacral screws, 7. recording H-reflexes, 8. recording F-responses, 9. recording the sacral reflex, 10. recording intralimb and interlimb reflexes and 11. recording monosynaptic and polysynaptic reflexes during dorsal root rhizotomy.

OBJECTIVE

This paper is the position statement of the American Society of Neurophysiological Monitoring. It is the practice guideline for the intraoperative use of these 11 techniques.

METHODS

This statement is based on information presented at scientific meetings, published in the current scientific and clinical literature, and presented in previously-published guidelines and position statements of various clinical societies.

RESULTS

These 11 techniques when used in conjunction with somatosensory and transcranial motor evoked potentials provide a multiple-systems approach to spinal cord and nerve root monitoring.

CONCLUSIONS

The techniques reviewed in this paper may be helpful to those wishing to incorporate these techniques into their monitoring program.

Value of dermatomal somatosensory evoked potentials in detecting acute nerve root injury: an experimental study with special emphasis on stimulus intensity

STUDY DESIGN

Dermatomal somatosensory-evoked potentials (D-SSEPs) in rats were recorded at the spinal level after L2-, L4-, and L5-dermatome stimulation. Pre- and post-transection patterns and rates of change of corresponding nerve roots were compared to determine accuracy.

OBJECTIVE

To investigate characteristics and normal values of D-SSEP elicited from lower limb dermatomes; to determine specificity, sensitivity, and utility of D-SSEP in detecting single-nerve root injury; and to determine optimal stimulation intensity.

SUMMARY OF BACKGROUND DATA

D-SSEP allows assessment of single nerve root-specific pathways, electrodiagnosis of lumbosacral radiculopathy, and intraoperative neuromonitoring. Unacceptably low sensitivity and specificity make its value suspect. D-SSEP is insufficiently documented.

METHODS

Eight rats were used to specify a standard D-SSEP waveform and its characteristics, evaluate stimulation sites and strengths, and determine appropriate stimulation and recording techniques. The L4 nerve root was transected in one group of 8 rats and the L5 in another. D-SSEPs were recorded at the thoracolumbar junction following submaximal and supramaximal stimulation at the L2, L4, and L5 dermatomal fields. Potentials recorded before transection, and immediately, 1 hour, and 1 week post-transection were compared.

RESULTS

Reproducible spinal responses were obtained in all rats on all tests. Stimulation intensity, but not rates, affected amplitude. Relative amplitude reductions in transected-root D-SSEP were larger using submaximal than supramaximal intensity. D-SSEP elicited by submaximal than supramaximal intensity produced fewer false negatives and false positives.

CONCLUSIONS

D-SSEP is valuable for detecting acute single nerve root injury. In clinical settings, submaximal dermatomal stimulation identifies conduction abnormalities more consistently and with fewer false negatives and false positives than does supramaximal stimulation. We recommend submaximal stimulation.

Real-time continuous intraoperative electromyographic and somatosensory evoked potential recordings in spinal surgery: correlation of clinical and electrophysiologic findings in a prospective, consecutive series of 213 cases

STUDY DESIGN

Retrospective analysis of a prospectively accrued series of 213 consecutive patients who underwent intraoperative neurophysiologic monitoring with electromyography and somatosensory-evoked potentials during thoracolumbar spine surgery.

OBJECTIVES

To study the incidence of significant intraoperative electrophysiologic changes and new postoperative neurologic deficits.

SUMMARY OF BACKGROUND DATA

Continuous intraoperative electromyography and somatosensory-evoked potentials are frequently used in spinal surgery to prevent neural injury. However, only limited data are available on the sensitivity, specificity, and predictive values of intraoperative electrophysiologic changes with regard to the occurrence of new postoperative neurologic deficits.

METHODS

We examined data on patients who underwent intraoperative monitoring with continuous lower limb electromyography and somatosensory-evoked potentials. The analysis focused on the correlation of intraoperative electrophysiologic changes with the development of new neurologic deficits.

RESULTS

A total of 213 patients underwent surgery on a total of 378 levels; 32.4% underwent an instrumented fusion. Significant electromyograph activation was observed in 77.5% of the patients and significant somatosensory-evoked potential changes in 6.6%. Fourteen patients (6.6%) had new postoperative neurologic symptoms. Of those, all had significant electromyograph activation, but only 4 had significant somatosensory-evoked potential changes. Intraoperative electromyograph activation had a sensitivity of 100% and a specificity of 23.7% for the detection of a new postoperative neurologic deficit. Somatosensory-evoked potentials had a sensitivity of 28.6% and specificity of 94.7%.

CONCLUSIONS

Intraoperative electromyographic activation has a high sensitivity for the detection of a newpostoperative neurologic deficit but a low specificity. In contrast, somatosensory-evoked potentials have low sensitivity but high specificity. Combined intraoperative neurophysiologic monitoring with electromyography and somatosensory-evoked potentials is helpful for predicting and possibly preventing neurologic injury during thoracolumbar spine surgery.

The effects from lumbar nerve root transection in rats on spinal somatosensory and motor-evoked potentials

STUDY DESIGN

Spinal somatosensory-evoked potentials (SSEPs), elicited by mixed nerve and dermatomal stimulation, and compound evoked muscle potentials (CMAPs), elicited by lower thoracic interspinous space stimulation, were recorded in rats that underwent single nerve root transection.

OBJECTIVES

To investigate and compare the effects of single nerve root transection using various electrophysiological models and to define a monitoring strategy that can easily and accurately predict compromise caused by a single lumbosacral nerve root injury.

SUMMARY OF BACKGROUND DATA

Neuromonitoring is an important and effective preventive measure against neurologic complications during spinal surgery, but monitoring the lumbosacral nerve roots is not well established.

METHODS

Rats received (Group I) a sham operation or had the (Group II) left L4, (Group III) L5, or (Group IV) L6 nerve root transected. SSEP were recorded at the thoracolumbar junction following stimulation of the sciatic nerve (M-SSEP) and the L5 dermatome (D-SSEP). CMAP was recorded at the intrinsic muscles of the foot by electrical stimulation in the lower thoracic spinal cord. Potentials recorded before and after a single nerve root transection were compared and correlated with clinical status by walking-track analysis 1 week later.

RESULTS

Relative amplitudes in Group II were 54.1% (M-SSEP), 84.6% (D-SSEP), and 85.5% (CMAP); 25.2% (M-SSEP), 66.5% (D-SSEP), and 85.8% (CMAP) in Group III; and 66.5% (M-SSEP), 95.5% (D-SSEP), and 23.2% (CMAP) in Group IV. M-SSEP is sensitive but not specific to single nerve root injury. D-SSEP and CMAP are less sensitive but more specific.

CONCLUSIONS

Injury to a single lumbosacral nerve root is diagnosed more easily with M-SSEP. With D-SSEP and CMAP, it was possible to differentiate the lesioned nerve root by stimulating the primary dermatome or recording from the innervated muscle. M-SSEP is an easy-sampling and appropriate tool for screening nerve root injury; its poor specificity may be overcome by using D-SSEP and CMAP in addition. Optimal monitoring of the lumbosacral nerve roots during lumbar spinal surgery requires the administration of M-SSEP, D-SSEP, and CMAP to provide independent verification of lumbosacral nerve root integrity and to allow detection of the occasional injuries that selectively affect either the sensory or motor system.

Intraoperative electromyography

Intraoperative electromyography (EMG) provides useful diagnostic and prognostic information during spine and peripheral nerve surgeries. The basic techniques include free-running EMG, stimulus-triggered EMG, and intraoperative nerve conduction studies. These techniques can be used to monitor nerve roots during spine surgeries, the facial nerve during cerebellopontine angle surgeries, and peripheral nerves during brachial plexus exploration and repair. However, there are a number of technical limitations that can cause false-positive or false-negative results, and these must be recognized and avoided when possible. The author reviews these basic electrophysiologic techniques, how they are applied to specific surgical situations, and their limitations.

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

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

Electrical thresholds for biomechanical response in the ankle to direct stimulation of spinal roots L4, L5, and S1. Implications for intraoperative pedicle screw testing

STUDY DESIGN

A comparison of electrical thresholds for biomechanical response in the ankle and for evoked electromyographic signals from specific leg muscles during intraoperative extradural direct stimulation of roots L4, L5, and S1.

OBJECTIVE

To determine whether a biomechanical response in the ankle to direct root stimulation occurs before evoked electromyographic signals and to determine differences in electrical excitability of the roots circumferentially.

SUMMARY OF BACKGROUND DATA

Stimulus intensities of 1.2-5.7 mA are reported to evoke electromyographic response in corresponding muscles to direct stimulation of normal roots. Stimulus intensities of 6-8 mA were suggested to detect bony pedicular compromise by stimulation of a hole or a screw during pedicle instrumentation. Electrical thresholds of three-dimensional torque response in the ankle to direct root stimulation have not yet been evaluated and compared with thresholds of evoked electromyogram.

METHODS

Direct monopolar stimulation of the surgically exposed roots L4, L5, and S1 was performed from different sites around the root by a cuff multielectrode. Biomechanical response was measured as an isometric torque in the ankle at each of three orthogonal axes. Compound muscle action potentials (CMAPs) from root-specific muscles were detected by a pair of surface or wire electrodes.

RESULTS

Mean threshold for biomechanical response in the ankle to stimulation of roots L4, L5, and S1 was 0.72 +/- 0.39 mA and for CMAP response was 1.09 mA +/- 0.36 (N = 13). Thresholds for biomechanical responses were significantly lower than for CMAP responses (P = 0.0004; paired t test). Nerve roots were electrically most excitable on their ventral aspects.

CONCLUSION

The biomechanical response in the joint to root stimulation can be used to test all root-related muscles crossing that joint at their individual innervation pattern and their residual innervation and to detect electrical excitation of the root at electric thresholds lower than those for detecting CMAP from single standard root-specific muscle. However, this method does not provide sufficient root specificity. It will be valuable in conjunction with multimodality neurophysiologic monitoring of the roots for earlier and more reliable detection of pedicle bone breakthrough or integrity. Further clinical investigations are suggested.

Spinal cord monitoring: current state of the art

The intraoperative application of evoked potential and electromyographic (EMG) monitoring has increased significantly over the last 2 decades. Cranial nerve monitoring is widely accepted and used by otologists, neurologic surgeons, and ophthalmologists. Direct and indirect techniques for assessing the peripheral nervous system are used by plastic and orthopedic surgeons when performing intraoperative nerve grafting. Myriad techniques and applications for monitoring the spinal cord and peripheral nervous system have been developed, evaluated, and used by orthopedic and neurologic surgeons involved in spinal surgery.

Subcortical strokes from intracranial aneurysm surgery: implications for intraoperative neuromonitoring

Forty-five patients underwent surgery for anterior circulation aneurysms using intraoperative neurophysiologic monitoring at the Johns Hopkins Hospital during 1996. There were seven intraoperative strokes. Two were cortical strokes associated with irreversible somatosensory evoked potential (SEP) changes during temporary arterial occlusion. The remaining five were subcortical strokes, one of which was associated with transient SEP changes during temporary arterial occlusion, but the other four occurred despite normal SEPs throughout surgery. Somatosensory evoked potential monitoring is not sensitive for the detection of subcortical ischemia and infarction in the distribution of the deep perforating arterial branches during intracranial aneurysm surgery. Although attenuation of loss of cortical SEP responses may indicate cerebral ischemia from inadequate collateral circulation during temporary arterial occlusion, normal SEPs can not exclude subcortical ischemia sufficient to cause significant postoperative deficits, and may therefore provide a false sense of security during these surgeries.

Intraoperative electromyography during thoracolumbar spinal surgery

Intraoperative electromyography can provide useful information regarding lumbosacral nerve root function during thoracolumbar spinal surgery. Free-running electromyography provides continuous feedback regarding the location and potential for surgical injury to the lumbosacral nerve roots within the operative field. Stimulus-evoked electromyography can confirm that transpedicular instrumentation has been positioned correctly within the bony cortex. However, electromyography has a number of potential limitations, which are discussed in this article along with improved methods to increase the overall efficacy of intraoperative electromyography, including: 1) Electromyography is sensitive to blunt lumbosacral nerve root irritation or injury, but may provide misleading results with "clean" nerve root transection. 2) Electromyography must be recorded from muscles belonging to myotomes appropriate for the nerve roots considered at risk from surgery. 3) Electromyography can be effective only with careful monitoring and titration of pharmacologic neuromuscular junction blockade. 4) When transpedicular instrumentation is stimulated, an exposed nerve root should be stimulated directly as a positive control whenever possible. 5) Pedicle holes and screws should be stimulated with single shocks at low-stimulus intensities when pharmacologic neuromuscular blockade is excessive. 6) Chronically compressed nerve roots that have undergone axonotmesis (wallerian degeneration) have higher thresholds for activation from electrical and mechanical stimulation. 7) Hence, whenever axonotmetic nerve root injury is suspected, the stimulus thresholds for transpedicular holes and screws must be specifically compared with those required for the direct activation of the adjacent nerve root (and not published guideline threshold values).

Higher electrical stimulus intensities are required to activate chronically compressed nerve roots. Implications for intraoperative electromyographic pedicle screw testing

STUDY DESIGN

A comparison of the electrical thresholds required to evoke myogenic responses from direct stimulation of normal and chronically compressed nerve roots.

OBJECTIVE

To determine whether intraoperative electromyographic testing to confirm the integrity of instrumented pedicles should be performed at higher stimulus intensities in cases where there is preoperative lumbosacral radiculopathy.

SUMMARY OF BACKGROUND DATA

Postoperative neurologic deficits may occur as a result of pedicle screw misplacement during spinal instrumentation. The failure to evoke myogenic responses from stimulation of pedicle holes and screws at intensities of 6-8 mA is commonly used to exclude bony pedicular wall perforation.

METHODS

Direct nerve root stimulation was used to compare the stimulus thresholds of normal and compressed nerve roots in six patients with limb weakness from chronic lumbosacral radiculopathy.

RESULTS

The stimulus thresholds of chronically compressed nerve roots significantly exceeded those of normal nerve roots, indicating partial axonal loss (axonotmesis). In most cases, the direct stimulus thresholds of compressed nerve roots exceeded 10 mA.

CONCLUSIONS

When instrumentation is placed at spinal levels where there is preexisting chronic lumbosacral radiculopathy, holes and screws may need to be stimulated at higher intensities to exclude pedicular perforation and prevent further iatrogenic nerve root injury.