Risk factors for false positive transcranial motor evoked potential monitoring alerts during surgical treatment of cervical myelopathy

How to make the best use of intraoperative motor evoked potential monitoring? Experience in 1162 consecutive spinal deformity surgical procedures

STUDY DESIGN

A retrospective study of 1162 consecutive patients who underwent spinal deformity surgical procedures at our spine center from January 2010 to December 2013.

OBJECTIVE

To develop and evaluate a protocol of intraoperative motor evoked potential (MEP) monitoring with the warning criteria we had established on the basis of our clinical experiences and the review of previous literature.

SUMMARY OF BACKGROUND DATA

Though MEPs monitoring have become widely used in spinal deformity surgery, different alarm criteria and response protocol used in different studies compromised their comparability; Furthermore, high false-positive rate of MEP reported by previous studies has become an increasingly prominent problem that will limit its clinical use and development.

METHODS

The intraoperative monitoring data of 1162 consecutive patients who underwent spinal deformity surgical procedures at our spine center were retrospectively analyzed. Age, sex, diagnosis, preoperative neurological status, intraspinal anomalies, baseline MEP, and MEP change were collected. The protocol with the warning criteria we had established was used. The false-positive rate, false-negative rate, and positive predictive value were calculated.

RESULTS

Significant intraoperative changes were seen in the MEP data in 52 (4.4%) of all the cases. In 25 cases among which, significant MEP changes were synchronously and logically associated with high-risk surgical maneuver (pedicle screw insertion, osteotomy, correction, etc.). The false-positive rate of MEP monitoring was 0.26% (3/1140), whereas the sensitivity and specificity of MEP for detection of clinically significant intraoperative cord injury were 100% and 99.7%, respectively. The positive predictive value of a MEP alert in terms of a new postoperative neurological deficit was 83.3%.

CONCLUSION

Our study indicates that the appropriate use of MEP monitoring based on our protocol is able to obtain satisfying sensitivity and specificity and thus provide important information for intraoperative decision making.

Intraoperative neuromonitoring in single-level spinal procedures: a retrospective propensity score-matched analysis in a national longitudinal database

STUDY DESIGN

Retrospective propensity score-matched analysis on a national database (MarketScan) between 2006 and 2010.

OBJECTIVE

To compare rates of neurological deficits after elective single-level spinal procedures with and without intraoperative neuromonitoring, as well as associated payment differences and geographic variance.

SUMMARY OF BACKGROUND DATA

Intraoperative neurophysiologic monitoring is a technique that may contribute to avoiding permanent neurological injury during some spine surgery procedures. However, it is unclear whether all patients undergoing spine surgery benefit from neuromonitoring.

METHODS

An identified 85,640 patients underwent single-level spinal procedures including anterior cervical discectomy and fusion (ACDF), lumbar fusion, lumbar laminectomy, or lumbar discectomy. Neuromonitoring was identified with appropriate Current Procedural Terminology (CPT) codes. Cohorts were balanced on baseline comorbidities and procedure characteristics using propensity score matching. Trauma and spinal tumors cases were excluded.

RESULTS

Patients (12.66%) received neuromonitoring intraoperatively. Lumbar laminectomies had reduced 30-day neurological complication rate with neuromonitoring (0.0% vs. 1.18%, P=0.002). Neuromonitoring did not correlate with reduced intraoperative neurological complications in ACDFs (0.09% vs. 0.13%), lumbar fusions (0.32% vs. 0.58%), or lumbar discectomy (1.24% vs. 0.91%). With the addition of neuromonitoring, payments for ACDFs increased 16.24% ($3842), lumbar fusions 7.84% ($3540), lumbar laminectomies 24.33% ($3704), and lumbar discectomies 22.54% ($2859). Significant geographic variation was evident. Some states had no recorded single-level spinal cases with concurrent neuromonitoring. Rates for ACDFs and lumbar fusions, laminectomies, and discectomies ranged as high as 61%, 58%, 22%, and 21%, respectively.

CONCLUSION

With intraoperative neurological monitoring in single-level procedures, neurological complications were decreased only among lumbar laminectomies. No difference was observed in ACDFs, lumbar fusions, or lumbar discectomies. There was a significant increase in total payments associated with the index procedure and hospitalization. We demonstrate significant geographic variation in neuromonitoring.

LEVEL OF EVIDENCE

3.

Differential rates of false-positive findings in transcranial electric motor evoked potential monitoring when using inhalational anesthesia versus total intravenous anesthesia during spine surgeries

BACKGROUND CONTEXT

False-positive loss of transcranial electrical motor evoked potentials (TCe-MEPs) limits the efficacy of motor tract monitoring during spine surgery. Although total intravenous anesthesia (TIVA) is widely regarded as the optimal regimen for TCe-MEPs, inhalational anesthesia is an alternative regimen.

PURPOSE

To compare the rates of false-positive TCe-MEPs during spine surgery for patients anesthetized with TIVA and inhalation anesthesia.

STUDY DESIGN

A retrospective analysis of data collected from consecutive patients undergoing TCe-MEP monitoring during spinal surgery.

PATIENT SAMPLE

Consecutive adult patients from multiple surgical centers undergoing spine surgery inclusive of cervical or thoracic spinal levels during 2008-2009 who received TIVA or inhalation anesthesia.

OUTCOME MEASURES

The primary outcome measure was the rate of false-positive alerts using TCe-MEPS, defined as a persistent loss of 90% or greater of the amplitude of TCe-MEP in one or more muscles not attributed to technical or transient systemic factors (hypotension or hypoxia) and not associated with any postoperative neurologic deficit.

METHODS

Patients were divided into two groups according to anesthetic regimen: those anesthetized with one or more inhalational agents (n=1,303) and patients anesthetized with TIVA (n=511). The Fisher exact test and unpaired t test were used to compare group characteristics and false-positive rates. Each group was further subdivided by spinal region (cervical, thoracic, and thoracolumbar) and by presence of preoperative motor deficit. A Pearson chi-squared test was used to identify differences according to spinal region. This study was not supported by any financial sources nor do the authors have any financial relationships to disclose.

RESULTS

Patient with inhaled anesthesia showed significantly higher rates of false-positive TCe-MEP changes (15.0% vs. 3.2%) compared with the TIVA group. These differences were significant across all surgical subgroups. The inhaled group had a larger number of patients with preoperative motor deficits compared with TIVA (45.0% vs. 37.4%), a potential confounder for false-positive results. However, a significantly higher rate of false-positive TCe-MEP changes was still observed in the inhaled group (11.4% vs. 0.6% for TIVA) when analyzing only those patients without preoperative motor deficits.

CONCLUSIONS

Use of inhalation anesthesia during adult spinal surgery is associated with significantly higher rates of false-positive changes compared with TIVA during TCe-MEP monitoring. This relationship appears independent of preoperative motor status. Further study and multivariate analysis of anesthetic agents, diagnosis, and symptoms is necessary to elucidate the impact of these variables. The potential confounding effects of inhalational anesthesia on TCe-MEP monitoring should be considered when determining anesthetic regimen.

Intraoperative changes in transcranial motor evoked potentials and somatosensory evoked potentials predicting outcome in children with intramedullary spinal cord tumors

OBJECT

Intraoperative dorsal column mapping, transcranial motor evoked potentials (TcMEPs), and somatosensory evoked potentials (SSEPs) have been used in adults to assist with the resection of intramedullary spinal cord tumors (IMSCTs) and to predict postoperative motor deficits. The authors sought to determine whether changes in MEP and SSEP waveforms would similarly predict postoperative motor deficits in children.

METHODS

The authors reviewed charts and intraoperative records for children who had undergone resection for IMSCTs as well as dorsal column mapping and TcMEP and SSEP monitoring. Motor evoked potential data were supplemented with electromyography data obtained using a Kartush microstimulator (Medtronic Inc.). Motor strength was graded using the Medical Research Council (MRC) scale during the preoperative, immediate postoperative, and follow-up periods. Reductions in SSEPs were documented after mechanical traction, in response to maneuvers with the cavitational ultrasonic surgical aspirator (CUSA), or both.

RESULTS

Data from 12 patients were analyzed. Three lesions were encountered in the cervical and 7 in the thoracic spinal cord. Two patients had lesions of the cervicomedullary junction and upper spinal cord. Intraoperative MEP changes were noted in half of the patients. In these cases, normal polyphasic signals converted to biphasic signals, and these changes correlated with a loss of 1-2 grades in motor strength. One patient lost MEP signals completely and recovered strength to MRC Grade 4/5. The 2 patients with high cervical lesions showed neither intraoperative MEP changes nor motor deficits postoperatively. Dorsal columns were mapped in 7 patients, and the midline was determined accurately in all 7. Somatosensory evoked potentials were decreased in 7 patients. Two patients each had 2 SSEP decreases in response to traction intraoperatively but had no new sensory findings postoperatively. Another 2 patients had 3 traction-related SSEP decreases intraoperatively, and both had new postoperative sensory deficits that resolved. One additional patient had a CUSA-related SSEP decrease intraoperatively, which resolved postoperatively, and the last patient had 3 traction-related sensory deficits and a CUSA-related sensory deficit postoperatively, none of which resolved.

CONCLUSIONS

Intraoperative TcMEPs and SSEPs can predict the degree of postoperative motor deficit in pediatric patients undergoing IMSCT resection. This technique, combined with dorsal column mapping, is particularly useful in resecting lesions of the upper cervical cord, which are generally considered to be high risk in this population. Furthermore, the spinal cord appears to be less tolerant of repeated intraoperative SSEP decreases, with 3 successive insults most likely to yield postoperative sensory deficits. Changes in TcMEPs and SSEP waveforms can signal the need to guard against excessive manipulation thereby increasing the safety of tumor resection.

Intraoperative neuromonitoring with MEPs and prediction of postoperative neurological deficits in patients undergoing surgery for cervical and cervicothoracic myelopathy

OBJECT

The use of intraoperative neurophysiological monitoring (IONM) in surgical decompression surgery for myelopathy may assist the surgeon in taking corrective measures to reduce or prevent permanent neurological deficits. We evaluated the efficacy of IONM in cervical and cervicothoracic spondylotic myelopathy (CSM) cases.

METHODS

The authors retrospectively reviewed 140 cases involving patients who underwent surgery for CSM utilizing IONM during 2011 at the University of California, San Francisco. Data on preoperative clinical variables, intraoperative changes in transcranial motor evoked potentials (MEPs), and postoperative new neurological deficits were collected. Associations between categorical variables were analyzed with the Fisher exact test.

RESULTS

Of the 140 patients, 16 (11%) had significant intraoperative decreases in MEPs. In 8 of these cases, the MEP signal did not return to baseline values by the end of the operation. There were 8 (6%) postoperative deficits, of which 6 were C-5 palsies and 2 were paraparesis. Six of the patients with postoperative deficits had demonstrated persistent MEP signal change on IONM. There was a significant association between persistent MEP changes and postoperative deficits (p < 0.001). The sensitivity of intraoperative MEP monitoring was 75%, the specificity 98%, the positive predictive value 75%, and the negative predictive value 98%. Due to higher rates of false negatives, the sensitivity decreased to 60% in the subgroup of patients with vascular disease comorbidity. The sensitivity increased to 100% in elderly patients and in patients with preoperative motor deficits. The sensitivity and positive predictive value of deltoid and biceps MEP changes in predicting C-5 palsy were 67% and 67%, respectively.

CONCLUSIONS

The authors found a correlation between decreased intraoperative MEPs and postoperative new neurological deficits in patients with CSM. Sensitivity varies based on patient comorbidities, age, and preoperative neurological function. Monitoring of MEPs is a useful adjunct for CSM cases, and the authors have developed a checklist to standardize their responses to intraoperative MEP changes.

Efficacy of biphasic transcranial electric stimulation in intraoperative motor evoked potential monitoring for cervical compression myelopathy

STUDY DESIGN

Retrospective analysis of prospectively collected data from consecutive patients undergoing 2 methods of transcranial electrical motor evoked potential (TCE-MEP) monitoring during cervical spine surgery.

OBJECTIVE

To investigate the efficacy of biphasic transcranial electric stimulation, the deviation rate, amplitude of TCE-MEPs, complications, and sensitivity and specificity of TCE-MEP monitoring were compared between the biphasic and conventional monophasic stimulation methods.

SUMMARY OF BACKGROUND DATA

With biphasic stimulation, unlike monophasic stimulation, measurement time can be reduced considerably because a single stimulation elicits bilateral responses almost simultaneously. However, no study has yet reported a detailed comparison of the 2 methods.

METHODS

Examination 1: Amplitude and derivation rate of TCE-MEPs was compared for monophasic and biphasic stimulation in the same 31 patients with cervical compression myelopathy. Examination 2: Sensitivity, specificity, and complications of TCE-MEP monitoring were compared in 200 patients with cervical compression myelopathy who received monophasic or biphasic stimulation (100 patients each) during intraoperative monitoring.

RESULTS

Examination 1: Derivation rates of biphasic stimulation in the deltoid, biceps brachii, abductor digiti minimi, and flexor hallucis brevis muscles were the same or higher than for monophasic stimulation. TCE-MEP amplitudes elicited by biphasic stimulation compared with monophasic stimulation were significantly larger in the biceps (paired t, P < 0.0001), but similar in the other 3 muscles. Examination 2: In the biphasic and monophasic stimulation groups, warnings were issued to surgeons in 10 and 11 cases, for a sensitivity of 100% for both groups and specificity of 97.8% and 96.7%, respectively. No complications related to stimulation were observed in any of the 200 patients.

CONCLUSION

Biphasic stimulation had similar or higher derivation rates and equivalent sensitivity and specificity than monophasic stimulation. No complications were observed for either stimulation method. Biphasic stimulation is an effective TCE-MEP monitoring method for cervical spine surgery that may also reduce measurement time.

LEVEL OF EVIDENCE

4.

A new alarm point of transcranial electrical stimulation motor evoked potentials for intraoperative spinal cord monitoring: a prospective multicenter study from the Spinal Cord Monitoring Working Group of the Japanese Society for Spine Surgery and Related Research

OBJECT

Although multimodal intraoperative spinal cord monitoring provides greater accuracy, transcranial electrical stimulation motor evoked potential (TcMEP) monitoring became the gold standard for intraoperative spinal cord monitoring. However, there is no definite alarm point for TcMEPs because a multicenter study is lacking. Thus, based on their experience with 48 true-positive cases (that is, a decrease in potentials followed by a new neurological motor deficit postoperatively) encountered between 2007 and 2009, the authors set a 70% decrease in amplitude as the alarm point for TcMEPs.

METHODS

A total of 959 cases of spinal deformity, spinal cord tumor, and ossification of the posterior longitudinal ligament (OPLL) treated between 2010 and 2012 are included in this prospective multicenter study (18 institutions). These institutions are part of the Japanese Society for Spine Surgery and Related Research monitoring working group and the study group on spinal ligament ossification. The authors prospectively analyzed TcMEP variability and pre- and postoperative motor deficits. A 70% decrease in amplitude was designated as the alarm point.

RESULTS

There were only 2 false-negative cases, which occurred during surgery for intramedullary spinal cord tumors. This new alarm criterion provided high sensitivity (95%) and specificity (91%) for intraoperative spinal cord monitoring and favorable accuracy, except in cases of intramedullary spinal cord tumor.

CONCLUSIONS

This study is the first prospective multicenter study to investigate the alarm point of TcMEPs. The authors recommend the designation of an alarm point of a 70% decrease in amplitude for routine spinal cord monitoring, particularly during surgery for spinal deformity, OPLL, and extramedullary spinal cord tumor.

Analysis of Intraoperative Neuromonitoring Events During Spinal Corrective Surgery for Idiopathic Scoliosis

STUDY DESIGN

Multicenter review of prospectively collected data.

OBJECTIVE

To determine the frequency and outcome of intraoperative neuromonitoring (IONM) alerts during surgical correction of idiopathic scoliosis (IS).

SUMMARY OF BACKGROUND DATA

Providing corrective action after identifying IONM events will ideally prevent neurologic complication.

METHODS

Prospectively gathered clinical data and IONM reports recorded during spinal surgery for IS were analyzed. Patients were divided into 2 groups: those with clear IONM changes and those with no IONM events. A neuromonitoring event was defined as a change from baseline transcranial motor evoked potentials or somatosensory evoked potentials in the lower extremities. The risk, rate, cause, and outcome of each neuromonitoring alert were assessed.

RESULTS

A total of 579 IS cases were analyzed. Twenty-one cases (3.6%) had an IONM event. In 18 of the 21 cases (86%), the potentials returned to baseline after corrective action. Of the 3 with remaining abnormalities, 2 (0.3%) awoke with deficits (1 unilateral weakness and 1 unilateral sensory change). In 12 cases, the changes were thought to result from low blood pressure; all responded to elevation of the mean arterial pressure. Five changes were associated with misplaced screws (in 4, the monitoring returned after removal or redirection; all 5 were normal postoperatively). Traction was the cause in 2 cases, both of which responded to reduction in traction. In 4 cases, the cause was unclear. Two remained abnormal and awoke with neurologic deficit. There were no postoperative neurologic deficits in any cases with normal neuromonitoring. Surgical time (416 vs. 291 minutes; p < .001) and percent estimated blood volume lost (64% ± 47% vs. 31% ± 26%; p < .001) were significantly increased in cases in which intraoperative events were observed.

CONCLUSIONS

The rate of neuromonitoring changes was 3.6% when using both transcranial motor evoked potential and somatosensory evoked potential monitoring. When a cause was identified and corrective action was taken, there were no postoperative neurologic deficits.

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.

A CT-based study investigating the relationship between pedicle screw placement and stimulation threshold of compound muscle action potentials measured by intraoperative neurophysiological monitoring

PURPOSE

Neurophysiological monitoring aims to improve the safety of pedicle screw placement, but few quantitative studies assess specificity and sensitivity. In this study, screw placement within the pedicle is measured (post-op CT scan, horizontal and vertical distance from the screw edge to the surface of the pedicle) and correlated with intraoperative neurophysiological stimulation thresholds.

METHODS

A single surgeon placed 68 thoracic and 136 lumbar screws in 30 consecutive patients during instrumented fusion under EMG control. The female to male ratio was 1.6 and the average age was 61.3 years (SD 17.7). Radiological measurements, blinded to stimulation threshold, were done on reformatted CT reconstructions using OsiriX software. A standard deviation of the screw position of 2.8 mm was determined from pilot measurements, and a 1 mm of screw-pedicle edge distance was considered as a difference of interest (standardised difference of 0.35) leading to a power of the study of 75 % (significance level 0.05).

RESULTS

Correct placement and stimulation thresholds above 10 mA were found in 71 % of screws. Twenty-two percent of screws caused cortical breach, 80 % of these had stimulation thresholds above 10 mA (sensitivity 20 %, specificity 90 %). True prediction of correct position of the screw was more frequent for lumbar than for thoracic screws.

CONCLUSION

A screw stimulation threshold of >10 mA does not indicate correct pedicle screw placement. A hypothesised gradual decrease of screw stimulation thresholds was not observed as screw placement approaches the nerve root. Aside from a robust threshold of 2 mA indicating direct contact with nervous tissue, a secondary threshold appears to depend on patients' pathology and surgical conditions.

The design, development, and implementation of a checklist for intraoperative neuromonitoring changes

OBJECT

The purpose of this study was to provide an evidence-based algorithm for the design, development, and implementation of a new checklist for the response to an intraoperative neuromonitoring alert during spine surgery.

METHODS

The aviation and surgical literature was surveyed for evidence of successful checklist design, development, and implementation. The limitations of checklists and the barriers to their implementation were reviewed. Based on this review, an algorithm for neurosurgical checklist creation and implementation was developed. Using this algorithm, a multidisciplinary team surveyed the literature for the best practices for how to respond to an intraoperative neuromonitoring alert. All stakeholders then reviewed the evidence and came to consensus regarding items for inclusion in the checklist.

RESULTS

A checklist for responding to an intraoperative neuromonitoring alert was devised. It highlights the specific roles of the anesthesiologist, surgeon, and neuromonitoring personnel and encourages communication between teams. It focuses on the items critical for identifying and correcting reversible causes of neuromonitoring alerts. Following initial design, the checklist draft was reviewed and amended with stakeholder input. The checklist was then evaluated in a small-scale trial and revised based on usability and feasibility.

CONCLUSIONS

The authors have developed an evidence-based algorithm for the design, development, and implementation of checklists in neurosurgery and have used this algorithm to devise a checklist for responding to intraoperative neuromonitoring alerts in spine surgery.

The cutoff amplitude of transcranial motor-evoked potentials for predicting postoperative motor deficits in thoracic spine surgery

STUDY DESIGN

Prospective clinical study of intraoperative transcranial motor-evoked potentials (TcMEP) amplitudes and postoperative motor deficits.

OBJECTIVE

To determine the cutoff amplitude during intraoperative TcMEP monitoring for predicting postoperative motor deficits after thoracic spine surgery.

SUMMARY OF BACKGROUND DATA

Several alarm points when monitoring with TcMEP have been advocated, but there have been no reports on an actual cutoff amplitude of TcMEP for predicting the occurrence of postoperative motor deficits.

METHODS

Among 80 consecutive surgical cases, 28 had a deterioration in TcMEP amplitude in at least 1 monitored muscle during surgery. We examined intraoperative electrophysiological changes and postoperative motor deficits in 270 monitorable muscles in those 28 patients. Through receiver operating characteristic curve analysis, we identified the cutoff amplitudes at the intraoperative point of deterioration and at the end of surgery for predicting postoperative motor deficits in both relative and absolute values.

RESULTS

The relative and the absolute cutoff amplitudes of TcMEP at the intraoperative point of deterioration and at the end of thoracic spine surgery were 12% of control amplitude and 1.9 μV and 25% of control amplitude and 3.6 μV, respectively. Sensitivity/specificity for those cutoff points are 88%/64%, 69%/83%, 90%/64%, and 70%/82%, respectively.

CONCLUSION

We determined the cutoff amplitude for predicting postoperative motor deficits in thoracic spine surgery. The results may help establish the alarm criteria for thoracic spine surgery.

The usefulness of intraoperative neurophysiological monitoring in cervical spine surgery: a retrospective analysis of 200 consecutive patients

The usefulness of intraoperative neurophysiological monitoring (IONM), including somatosensory-evoked potential (SSEP) and transcranial electrical motor-evoked potentials (TcMEPs) in cervical spine surgery still needs to be evaluated. We retrospectively reviewed 200 cervical spine surgery patients from 2008 to 2009 to determine the role of IONM in cervical spine surgery. Total intravenous anesthesia was used for all patients. IONM alerts were defined as a 50% decrease in amplitude, a 10% increase in latency, or a unilateral change for SSEP and an increase in stimulation threshold of more than 100 V for TcMEP. Three patients had SSEP alerts that were related to arm malposition (2 patients) and hypotension (1 patient). Five patients had TcMEP alerts: 4 alerts were caused by hypotension and 1 by bone graft compression of the spinal cord. All alerts were resolved when causative reasons were corrected. There was no postoperative iatrogenic neurological injury. The sensitivities of SSEP and TcMEP alerts for detecting impending neurological injury were 37.5% and 62.5%, respectively. The sensitivity of both SSEP and TcMEP used in combination was 100%. No false-positive and false-negative alerts were identified in either SSEP or TcMEP (100% specificity). The total intravenous anesthesia technique optimizes the detection of SSEP and TcMEP and therefore improves the sensitivity and specificity of IONM. SSEP is sensitive in detecting alerts in possible malposition-induced ischemia or brachial plexus nerve injury. TcMEP specifically detects hypotension-induced spinal functional compromises. Combination use of TcMEP and SSEP enhances the early detection of impeding neurological damage during cervical spine surgery.

A new criterion for the alarm point for compound muscle action potentials

Object

The purpose of this study was to review the present criteria for the compound muscle action potential (CMAP) alert and for safe spinal surgery.

Methods

The authors conducted a retrospective study of 295 patients in whom spinal cord monitoring had been performed during spinal surgery. The waveforms observed during spinal surgery were divided into the following 4 grades: Grade 0, normal; Grade 1, amplitude decrease of 50% or more and latency delay of 10% or more; Grade 2, multiphase pattern; and Grade 3, loss of amplitude. Waveform grading, its relationship with postoperative motor deficit, and CMAP sensitivity and specificity were analyzed. Whenever any wave abnormality occurred, the surgeon was notified and the surgical procedures were temporarily suspended. If no improvements were seen, the surgery was terminated.

Results

Compound muscle action potential wave changes occurred in 38.6% of cases. With Grade 1 or 2 changes, no paresis was detected. Postoperative motor deficits were seen in 8 patients, all with Grade 3 waveform changes. Among the 287 patients without postoperative motor deficits, CMAP changes were not seen in 181, with a specificity of 63%. The false-positive rate was 37% (106 of 287). However, when a Grade 2 change was set as the alarm point, sensitivity was 100% and specificity was 79.4%. The false-positive rate was 20% (59 of 295).

Conclusions

Neither the Grade 1 nor the Grade 2 groups included patients who demonstrated a motor deficit. All pareses occurred in cases showing a Grade 3 change. Therefore, the authors propose a Grade 2 change (multiphasic waveform) as a new alarm point. With the application of this criterion, the false-positive rate can be reduced to 20%.

Cervical decompression and reconstruction without intraoperative neurophysiological monitoring

Object

The primary goal of this study was to review the immediate postoperative neurological function in patients surgically treated for symptomatic cervical spine disease without intraoperative neurophysiological monitoring. The secondary goal was to assess the economic impact of intraoperative monitoring (IOM) in this patient population.

Methods

This study is a retrospective review of 720 consecutively treated patients who underwent cervical spine procedures. The patients were identified and the data were collected by individuals who were not involved in their care.

Results

A total of 1534 cervical spine levels were treated in 720 patients using anterior, posterior, and combined (360°) approaches. Myelopathy was present preoperatively in 308 patients. There were 185 patients with increased signal intensity within the spinal cord on preoperative T2-weighted MR images, of whom 43 patients had no clinical evidence of myelopathy. Three patients (0.4%) exhibited a new neurological deficit postoperatively. Of these patients, 1 had a preoperative diagnosis of radiculopathy, while the other 2 were treated for myelopathy. The new postoperative deficits completely resolved in all 3 patients and did not require additional treatment. The Current Procedural Terminology (CPT) codes for IOM during cervical decompression include 95925 and 95926 for somatosensory evoked potential monitoring of the upper and lower extremities, respectively, as well as 95928 and 95929 for motor evoked potential monitoring of the upper and lower extremities. In addition to the charge for the baseline [monitoring] study, patients are charged hourly for ongoing electrophysiology testing and monitoring using the CPT code 95920. Based on these codes and assuming an average of 4 hours of monitoring time per surgical case, the savings realized in this group of patients was estimated to be $1,024,754.

Conclusions

With the continuing increase in health care costs, it is our responsibility as providers to minimize expenses when possible. This should be accomplished without compromising the quality of care to patients. This study demonstrates that decompression and reconstruction for symptomatic cervical spine disease without IOM may reduce the cost of treatment without adversely impacting patient safety.

Pattern-specific changes and discordant prognostic values of individual leg-muscle motor evoked potentials during spinal surgery

OBJECTIVE

The aim of the study is to evaluate the efficacy of muscle motor evoked potentials (mMEPs) in individual leg muscles for spinal surgery monitoring.

METHODS

Data were obtained from 209 patients who underwent spine surgery with intra-operative mMEP monitoring in the tibialis anterior (TA) and abductor hallucis (AH) muscles. The mMEP generation, pattern-specific mMEP loss and recovery, and the accuracy of individual mMEP changes in predicting postoperative motor deficit were assessed.

RESULTS

Generation rate of mMEPs was higher in the AH than in the TA (p<0.001). The mMEP in the TA was more sensitive in detecting mMEP loss than in the AH (p<0.001); however, mMEP in the AH was more sensitive in detecting mMEP recovery (p<0.001). The mMEPs in the TA had high sensitivity in predicting sustained postoperative motor deficits. By contrast, mMEPs in the AH showed a high positive predictive value.

CONCLUSIONS

Although mMEPs were generated at a high rate in the AH, mMEP in the TA can play an important complementary role in intra-operative mMEP monitoring, because mMEP in the TA can be more sensitive to potential neural damage.

SIGNIFICANCE

Using a combination of muscles with individual sensitivities and clinical significances will improve intra-operative mMEP monitoring strategies.

A role for motor and somatosensory evoked potentials during anterior cervical discectomy and fusion for patients without myelopathy: Analysis of 57 consecutive cases

BACKGROUND

Although the usage of combined motor and sensory intraoperative monitoring has been shown to improve the surgical outcome of patients with cervical myelopathy, the role of transcranial electric motor evoked potentials (tceMEP) used in conjunction with somatosensory evoked potentials (SSEP) in patients presenting with radiculopathy but without myelopathy has been less clear.

METHODS

We retrospectively reviewed all patients (n = 57) with radiculopathy but without myelopathy, undergoing anterior cervical decompression and fusion at a single institution over the past 3 years, who had intraoperative monitoring with both tceMEPs and SSEPs.

RESULTS

Fifty-seven (100%) patients presented with radiculopathy, 53 (93.0%) with mechanical neck pain, 35 (61.4%) with motor dysfunction, and 29 (50.9%) with sensory deficits. Intraoperatively, 3 (5.3%) patients experienced decreases in SSEP signal amplitudes and 4 (6.9%) had tceMEP signal changes. There were three instances where a change in neuromonitoring signal required intraoperative alteration of the surgical procedure: these were deemed clinically significant events/true positives. SSEP monitoring showed two false positives and two false negatives, whereas tceMEP monitoring only had one false positive and no false negatives. Thus, tceMEP monitoring exhibited higher sensitivity (33.3% vs. 100%), specificity (95.6% vs. 98.1%), positive predictive value (33.3% vs. 75.0%), negative predictive value (97.7% vs. 100%), and efficiency (91.7% vs. 98.2%) compared to SSEP monitoring alone.

CONCLUSIONS

Here, we present a retrospective series of 57 patients where tceMEP/SSEP monitoring likely prevented irreversible neurologic damage. Though further prospective studies are needed, there may be a role for combined tceMEP/SSEP monitoring for patients undergoing anterior cervical decompression without myelopathy.

Rates of new neurological deficit associated with spine surgery based on 108,419 procedures: a report of the scoliosis research society morbidity and mortality committee

STUDY DESIGN

Retrospective review of a prospectively collected, multicenter database.

OBJECTIVE

To assess rates of new neurologic deficit (NND) associated with spine surgery.

SUMMARY OF BACKGROUND DATA

NND is a potential complication of spine surgery, but previously reported rates are often limited by small sample size and single-surgeon experiences.

METHODS

The Scoliosis Research Society morbidity and mortality database was queried for spinal surgery cases complicated by NND from 2004 to 2007, including nerve root deficit (NRD), cauda equina deficit (CED), and spinal cord deficit (SCD). Use of neuromonitoring was assessed. Recovery was stratified as complete, partial, or none. Rates of NND were stratified based on diagnosis, age (pediatric < 21; adult ≥ 21), and surgical parameters.

RESULTS

Of the 108,419 cases reported, NND was documented for 1064 (1.0%), including 662 NRDs, 74 CEDs, and 293 SCDs (deficit not specified for 35 cases). Rates of NND were calculated on the basis of diagnosis. Revision cases had a 41% higher rate of NND (1.25%) compared with primary cases (0.89%; P < 0.001). Pediatric cases had a 59% higher rate of NND (1.32%) compared with adult cases (0.83%; P < 0.001). The rate of NND for cases with implants was more than twice that for cases without implants (1.15% vs. 0.52%, P < 0.001). Neuromonitoring was used for 65% of cases, and for cases with new NRD, CED, and SCD, changes in neuromonitoring were reported in 11%, 8%, and 40%, respectively. The respective percentages of no recovery, partial, and complete recovery for NRD were 4.7%, 46.8%, and 47.1%, respectively; for CED were 9.6%, 45.2%, and 45.2%, respectively; and for SCD were 10.6%, 43%, and 45.7%, respectively.

CONCLUSION

Our data demonstrate that, even among skilled spinal deformity surgeons, new neurologic deficits are inherent potential complications of spine surgery. These data provide general benchmark rates for NND with spine surgery as a basis for patient counseling and for ongoing efforts to improve safety of care.

Surgical Approaches to Intramedullary Cavernous Malformations of the Spinal Cord

Background:

Resection of intramedullary spinal cord cavernous malformations is associated with a significant risk of morbidity because of the high density of eloquent tissue within the spinal cord. Despite this risk, surgery remains the definitive treatment for symptomatic lesions.

Objective:

To review the clinical aspects of surgical approaches for spinal cord cavernous malformations.

Methods:

This article reviews the epidemiology, pathophysiology, clinical and imaging characteristics, and indications for surgical resection. Surgical issues and operative approaches by anatomical location are also detailed, drawing from evidence in the literature and from the senior author’s clinical experience.

Results:

The 3 primary approaches to spinal cord cavernous malformations—the posterior, posterolateral, and lateral approaches—are described and illustrated. Magnetic resonance imaging and intraoperative photographs of representative cases are included.

Conclusion:

Intramedullary spinal cord cavernous malformations are complex entities, and it is our hope that this article will improve readers’ understanding of their clinical characteristics, their indications for treatment, and the surgical pathways through which these lesions can be safely resected.

Intraoperative neuromonitoring: lessons learned from 32 case events in 2095 spine cases

STUDY TYPE

 Restrospective chart review Introduction:  Intraoperative neuromonitoring is becoming the standard of care for many more spinal surgeries, especially with deformity correction and instrumentation. We reviewed our institution's neuromonitored spine cases over the past 4 years to see the immediate intraoperative and postoperative clinical findings when an intraoperative neuromonitoring event was noted.

OBJECTIVE

 The main question addressed in this review is how multimodality intraoperative neuromonitoring has affected our ability to avoid potential neurological injury during spine surgery.

METHODS

 We retrospectively reviewed 2,095 neuromonitored spine cases at one institution performed over a period of 4 years. Data from the single neuromonitoring provider (Impulse Monitoring, Inc.) at our institution was collected and any cases with possible intraoperative events were isolated. The intraoperative and immediate postoperative clinical documentation of these 32 cases were reviewed (Table 1). [Table: see text]

RESULTS

 There were 17 cases where changes noted on EMG, SSEP, and/or MEPs affected the course of the surgery, and prevented possible postoperative neurological deficits. Of these 17, five were related to hypotension, seven due to deformity correction, one screw had a low triggered EMG threshold and was repositioned, and four cases had changes related to patient positioning and external pressure (ie, brachial plexus stretch). None of the 17 cases had postoperative motor or sensory deficits (Figure 1). Figure 1 DURING THE INSERTION OF THE CONVEX ROD: decrease of the MEP amplitude in left foot by 80% amplitude (yellow arrow). The baseline recording is in blue, the current recording in purple. The right side (non represented) will remain normal. Four cases consisted of intradural cord biopsies or tumor resections that had various positive neuromonitoring findings that essentially serve as controls. These cases confirm that the expected changes were seen on neuromonitoring. Four cases had false-positive neuromonitoring findings due to one technical issue requiring needle repositioning, one low threshold with triggered EMG without a pedicle breach, one case had decreased MEP responses with stable SSEPs, and one case had decreased SSEPs after positioning the patient prone. None of these four cases had any postoperative deficits. Four cases showed improved SSEPs after decompression; three cervical corpectomies, and one thoracic discectomy. Three cases of lumbar instrumentation with spontaneous EMGs each had a medial screw breach without intraoperative findings (Figure 2). They all had a postoperative motor deficit (foot drop). None of these three cases had triggered EMGs performed with the index procedure. Figure 2 Left L4 pedicle screw medial breach. Triggered EMGs were not performed during the index procedure. Postoperative foot drop required a second surgery to reposition the screw.

CONCLUSIONS

 Overall, this review reinforces the importance of multimodality neuromonitoring for spinal surgery. The incidence of possible events in our series was 1.5%. It is difficult to determine the true incidence, since it is impossible to know of any missed events due to lack of complete documentation. In a majority of the cases with events, possible postoperative neurologic deficits were avoided by intraoperative intervention, but the possible outcomes without intervention are not known. Clearly, in the three cases with lumbar pedicle screw malposition, triggered EMGs would have likely shown low thresholds. This would allow for screw reposition, and thus avoid a postoperative lumbar radiculopathy and revision surgery. The incidence of false-positive findings was very low in this review, and unfortunately the true incidence of false-negative findings is not able to be elucidated with this database.

The evidence for intraoperative neurophysiological monitoring in spine surgery: does it make a difference?

OBJECTIVE

The objective of this article was to undertake a systematic review of the literature to determine whether IOM is able to sensitively and specifically detect intraoperative neurologic injury during spine surgery and to assess whether IOM results in improved outcomes for patients during these procedures.

SUMMARY AND BACKGROUND DATA

Although relatively uncommon, perioperative neurologic injury, in particular spinal cord injury, is one of the most feared complications of spinal surgery. Intraoperative neuromonitoring (IOM) has been proposed as a method which could reduce perioperative neurologic complications after spine surgery.

METHODS

A systematic review of the English language literature was undertaken for articles published between 1990 and March 2009. MEDLINE, EMBASE, and Cochrane Collaborative Library databases were searched, as were the reference lists of published articles examining the use of IOM in spine surgery. Two independent reviewers assessed the level of evidence quality using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) criteria, and disagreements were resolved by consensus.

RESULTS

A total of 103 articles were initially screened and 32 ultimately met the predetermined inclusion criteria. We determined that there is a high level of evidence that multimodal IOM is sensitive and specific for detecting intraoperative neurologic injury during spine surgery. There is a low level of evidence that IOM reduces the rate of new or worsened perioperative neurologic deficits. There is very low evidence that an intraoperative response to a neuromonitoring alert reduces the rate of perioperative neurologic deterioration.

CONCLUSION

Based on strong evidence that multimodality intraoperative neuromonitoring (MIOM) is sensitive and specific for detecting intraoperative neurologic injury during spine surgery, it is recommended that the use of MIOM be considered in spine surgery where the spinal cord or nerve roots are deemed to be at risk, including procedures involving deformity correction and procedures that require the placement of instrumentation. There is a need to develop evidence-based protocols to deal with intraoperative changes in MIOM and to validate these prospectively.

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

BACKGROUND

Multimodal intraoperative neuromonitoring is recommended during corrective spinal surgery, and has been widely used in surgery for spinal deformity with successful outcomes. Despite successful outcomes of corrective surgery due to increased safety of the patients with the usage of spinal cord monitoring in many large spine centers, this modality has not yet achieved widespread popularity. We report the analysis of prospectively collected intraoperative neurophysiological monitoring data of 354 consecutive patients undergoing corrective surgery for adolescent idiopathic scoliosis (AIS) to establish the efficacy of multimodal neuromonitoring and to evaluate comparative sensitivity and specificity.

MATERIALS AND METHODS

The study group consisted of 354 (female = 309; male = 45) patients undergoing spinal deformity corrective surgery between 2004 and 2008. Patients were monitored using electrophysiological methods including somatosensory-evoked potentials and motor-evoked potentials simultaneously.

RESULTS

Mean age of patients was 13.6 years (+/-2.3 years). The operative procedures involved were instrumented fusion of the thoracic/lumbar/both curves, Baseline somatosensory-evoked potentials (SSEP) and neurogenic motor-evoked potentials (NMEP) were recorded successfully in all cases. Thirteen cases expressed significant alert to prompt reversal of intervention. All these 13 cases with significant alert had detectable NMEP alerts, whereas significant SSEP alert was detected in 8 cases. Two patients awoke with new neurological deficit (0.56%) and had significant intraoperative SSEP + NMEP alerts. There were no false positives with SSEP (high specificity) but 5 patients with false negatives with SSEP (38%) reduced its sensitivity. There was no false negative with NMEP but 2 of 13 cases were false positive with NMEP (15%). The specificity of SSEP (100%) is higher than NMEP (96%); however, the sensitivity of NMEP (100%) is far better than SSEP (51%). Due to these results, the overall sensitivity, specificity and positive predictive value of combined multimodality neuromonitoring in this adult deformity series was 100, 98.5 and 85%, respectively.

CONCLUSION

Neurogenic motor-evoked potential (NMEP) monitoring appears to be superior to conventional SSEP monitoring for identifying evolving spinal cord injury. Used in conjunction, the sensitivity and specificity of combined neuromonitoring may reach up to 100%. Multimodality monitoring with SSEP + NMEP should be the standard of care.

Evaluation of reliability of post-tetanic motor-evoked potential monitoring during spinal surgery under general anesthesia

STUDY DESIGN

A prospective research.

OBJECTIVE

Compare the reliability of post-tetanic motor-evoked potential (p-MEP) monitoring in the detection of motor injury during spinal surgery with that of conventional MEP (c-MEP).

SUMMARY OF BACKGROUND DATA

Myogenic MEPs are sensitive to suppression by anesthetics and neuromuscular blockade. Recently, we reported a new technique for MEP recording, called "p-MEP" in which MEP amplitude can be enlarged by tetanic stimulation of peripheral nerve before transcranial stimulation in comparison with that of c-MEP. The purpose of this study is to compare the reliability of p-MEP monitoring in the detection of motor injury during spinal surgery with that of c-MEP.

METHODS

Eighty patients undergoing elective spinal surgery were enrolled in the study. Both c-MEP and p-MEP monitoring were performed throughout the operation in each patient. For recording c-MEPs, transcranial electrical train of five pulses stimulation with an interstimulus interval of 2 milliseconds was performed and compound muscle action potentials were bilaterally recorded from abductor pollicis brevis, abductor hallucis, tibialis anterior, and soleus muscles. For recording p-MEPs, tetanic stimulation (50 Hz, 50 mA, 5 sec) was applied to the left median nerve and bilateral tibial nerves 1 second before transcranial stimulation and compound muscle action potentials were recorded from the same muscles. The false positive, false negative, and accuracy of MEP monitoring in the detection of change in motor function were compared between p-MEP and c-MEP.

RESULTS

At the baseline, success rates of baseline c-MEP and p-MEP recording were 66.3% (53/80) and 92.5% (74/80), respectively. The false positive, false negative, and accuracy of p-MEP monitoring were 0%, 0%, and 100%, respectively, whereas c-MEP were 4%, 20%, and 95%, respectively.

CONCLUSION

The results indicate that p-MEP is a more reliable method to detect changes in motor function during spinal surgery under general anesthesia in comparison with c-MEP.