Trending algorithm discriminates hemodynamic from injury related TcMEP amplitude loss.

Jasiukaitis and Lyon (J Clin Monit Comput, https://doi.org/10.1007/s10877-018-0181-9, 2018) described an motor evoked potential (MEP)amplitude trending system to detect MEP amplitude loss against a background of MEP variability. They found that the end of case value of a running R2 triggered by a set MEP amplitude loss criterion appeared to discriminate new injury from non-injury in a small sample of three patients. The present study examines the predictive capability of the running R2 in a larger sample of patients (21 injured and 19 non-injured). It also varies the amplitude loss criterion (50%, 65% and 80%) for triggering the running R2 and the numbers of points used in the moving linear regression (8, 12 and 16). 40 patients who had undergone correction for lumbar deformity were retrospectively examined. 21 of these woke up with a newly acquired radicular injury, 19 did not but were characterized by hypovolemic hemorrhage. All 40 patients had sufficient MEP amplitude loss sometime during their procedure to cause the monitoring specialist to report this to the surgeon and anesthesia. End-of-case running R2s were significantly larger in the injury group. Using an 80% amplitude loss criterion to trigger the running R2 proved to be too stringent, causing reduced sensitivity. The running R2 appeared to have equivalent sensitivity to that of conventional MEP amplitude loss ratios, but superior specificity within this monitoring challenged sample. The different number of points for the moving regressions did not have any significant effect. End-of-case R2 values greater than 60% appeared to be highly predictive of new post-operative deficit, while values less than 40% appeared to insure no new deficit. The proposed trending system can discriminate injury from non-injury outcomes when compressive radicular injury during correction for lumbar deformity is involved. This discrimination appears to be successful even when MEP amplitude loss for non-iatrogenic reasons (i.e., hemorrhage) is also occurring.

The reliability of motor evoked potentials to predict dorsiflexion injuries during lumbosacral deformity surgery: importance of multiple myotomal monitoring.

STUDY DESIGN: Case-control analysis of transcranial motor evoked potential (MEP) responses and clinical outcome. OBJECTIVE: To determine the sensitivity and specificity of MEPs to predict isolated nerve root injury causing dorsiflexion weakness in selected patients having complex lumbar spine surgery. SUMMARY OF BACKGROUND DATA: The surgical correction of distal lumbar spine deformity involves significant risk for damage to neural structures that control muscles of ankle and toe dorsiflexion. Procedures often include vertebral translation, interbody fusion, and posterior-based osteotomies. The benefit of using MEP monitoring to predict dorsiflexion weakness has not been well-established. The purpose of this paper is to describe the relationship between neural complications from lumbar surgery and intraoperative MEP changes. METHODS: Included were 542 neurologically intact patients who underwent posterior spinal fusion for the correction of distal lumbar deformity. Two myotomes, including tibialis anterior (TA) and extensor hallucis longus (EHL), were monitored. MEP and free-running electromyography data were assessed in each patient. Cases of new dorsiflexion weakness noted postoperatively were identified. Data in case and control patients were compared. There was no direct funding for this work. The Department of Anesthesiology and Perioperative Care provides salary support for authors one and six. Authors two and three report employment in the field of intraoperative neurophysiological monitoring as a study-specific conflict of interest. RESULTS: Twenty-five patients (cases) developed dorsiflexion weakness. MEP amplitude decreased in the injured myotomes by an average of 65 ± 21% (TA) and 60±26% (EHL), which was significantly greater than the contralateral uninjured side or for control subjects. (p < .01) Receiver operator characteristic (ROC) curves showed high sensitivity, specificity, and predictive value for changes in MEP amplitude using either the TA or EHL. Analysis of MEP changes to either TA or EHL yielded a superior ROC curve. Net reclassification improvement analysis showed assessing MEP changes to both TA and EHL improved the predictability of injury. CONCLUSIONS: The use of MEP amplitude change is highly sensitive and specific to predict a new postoperative dorsiflexion injury. Monitoring two myotomes (both TA and EHL) is superior to relying on MEP changes from a single myotome. Electromyography activity was less accurate but compliments MEP use. Additional studies are needed to define optimal intraoperative MEP warning thresholds.

A motor evoked potential trending system may discriminate outcome: retrospective application with three cases.

This report presents a method for tracking Motor Evoked Potential (MEP) amplitudes over the course of a case using a moving least squares linear regression (LSMAs). During a case, newly obtained MEP amplitudes are compared to those predicted by a just previous linear regression (least squares moving average or LSMA). When detected by this comparison, a set criterion amplitude loss will then trigger linear regression of ensuing MEP amplitudes on an expanding step function which tracks the persistence of the amplitude loss for the remainder of the case. Three cases are presented. One in which the patient woke up with a newly acquired weakness in the left tibialis anterior and another in which MEP amplitudes were suddenly lost from the right foot, but after intervention, they were restored again. In a third case the patient again woke up with a new post-operative deficit, but MEP trial sampling had been more limited and variable than in the first two cases. When the linear trending method was applied to the affected myotome in the first case, the expanding step function regression was triggered after the moment of MEP loss and remained at a high level until the end of case. In the second case, the expanding step function regression was also triggered in the relevant myotome at the time of the reported MEP change, but diminished by end of case. In the third case the tracking method again successfully triggered a predictive R-Square despite the limited number of pre-event trials. The R-Square value of the expanding step function regression appears to have discriminative capability with regard to new post-op deficit. Given the importance of the intra-operative MEP for monitoring motor functioning and the high degree of variability that can affect it, the development of new quantitative, statistical methods to detect real from apparent MEP change will be necessary.

Correction to: Changes in transcranial motor evoked potentials during hemorrhage are associated with increased serum propofol concentrations.

In the original publication of the article, the corresponding author inadvertently omitted one of the co-authors in the author group. The corrected author group is given in this erratum.

Changes in transcranial motor evoked potentials during hemorrhage are associated with increased serum propofol concentrations.

Transcranial motor evoked potentials (TcMEPs) monitor the integrity of the spinal cord during spine surgery. Propofol-based anesthesia is favored in order to enhance TcMEP quality. During intraoperative hemorrhage, TcMEP amplitudes may be reduced. The serum concentration of propofol may increase during hemorrhage. No study has determined whether changes in TcMEPs due to hemorrhage are related to changes in propofol blood levels. We monitored TcMEPs, mean arterial pressure (MAP), and cardiac output (CO) and hemoglobin in pigs (n = 6) undergoing controlled progressive hemorrhage during a standardized anesthetic with infusions of propofol, ketamine, and fentanyl. We recorded TcMEPs from the rectus femoris (RF) and tibialis anterior (TA) muscles bilaterally. A pulmonary artery catheter was placed to measure CO. Progressive hemorrhage of 10% blood volume increments was done until TcMEP amplitude decreased by >60% from baseline. Serum propofol levels were also measured following removal of each 10% blood volume increment. TcMEP responses were elicited every 3 min using constant stimulation parameters. We removed between 20 and 50% of total blood volume in order to achieve the >60% reduction in TcMEP amplitude. MAP and CO decreased significantly from baseline. At maximum hemorrhage, TcMEP amplitude decreased in the RF and TA by an average of 73 and 62% respectively from baseline (P < 0.01). Serum propofol levels varied greatly among animals at baseline (range 410-1720 ng/mL) and increased in each animal during hemorrhage. The mean propofol concentration rose from 1190 ± 530 to 2483 ± 968 ng/mL (P < 0.01). The increased propofol concentration correlated with decreased CO. Multivariate analysis using hierarchical linear models indicated that the decline of TcMEP amplitude was primarily associated with rising propofol concentrations, but was also independently affected by reduced CO. We believe that the decrease in blood volume and CO during hemorrhage increased the serum concentration of propofol by reducing the volume of distribution and/or rate of hepatic metabolism of the drug. Despite wide acceptance of propofol as the preferred anesthetic when using TcMEPs, intravenous anesthetics are vulnerable to altered pharmacokinetics during conditions of hemorrhage and could contribute to false-positive TcMEP changes.

Monitoring cerebral ischemia during cerebrovascular surgery.

Patients undergoing intracranial cerebrovascular surgery under general anesthesia are at risk of cerebral ischemia due to the nature of the surgery and/or the underlying cerebrovascular occlusive disease. It is thus imperative to reliably and continuously monitor cerebral perfusion during this type of surgery to timely reverse ischemic processes. The aim of this review is to discuss the techniques currently available for monitoring cerebral ischemia during cerebrovascular surgery with a focus on the advantages and disadvantages of each technique.

Neurological outcomes and surgical complications in 221 spinal nerve sheath tumors.

OBJECTIVE Among all primary spinal neoplasms, approximately two-thirds are intradural extramedullary lesions; nerve sheath tumors, mainly neurofibromas and schwannomas, comprise approximately half of them. Given the rarity of these lesions, reports of surgical complications are limited. The aim of this study was to identify the rates of new or worsening neurological deficits and surgical complications associated with the resection of spinal nerve sheath tumors and the potential factors related to these outcomes. METHODS Patients were identified through a search of an institutional neuropathology database and a separate review of current procedural terminology (CPT) codes. Age, sex, clinical presentation, presence of neurofibromatosis (NF), tumor type, tumor location, extent of resection characterized as gross total or subtotal, use of intraoperative neuromonitoring, surgical complications, presence of neurological deficit, and clinical follow-up were recorded. RESULTS Two hundred twenty-one tumors in 199 patients with a mean age of 45 years were identified. Fifty-three tumors were neurofibromas; 163, schwannomas; and 5, malignant peripheral nerve sheath tumors (MPNSTs). There were 70 complications in 221 cases, a rate of 32%, which included 34 new or worsening sensory symptoms (15%), 12 new or worsening motor deficits (5%), 10 CSF leaks or pseudomeningoceles (4%), 11 wound infections (5%), 5 cases of spinal deformity (2%), and 6 others (2 spinal epidural hematomas, 1 nonoperative cranial subdural hematoma, 1 deep venous thrombosis, 1 case of urinary retention, and 1 recurrent laryngeal nerve injury). Complications were more common in cervical (36%) and lumbosacral (38%) tumors than in thoracic (18%) lesions (p = 0.021). Intradural and dumbbell lesions were associated with higher rates of CSF leakage, pseudomeningocele, and wound infection. Complications were present in 18 neurofibromas (34%), 50 schwannomas (31%), and 2 MPNSTs (40%); the differences in frequency were not significant (p = 0.834). Higher complication rates were observed in patients with NF than in patients without (38% vs 30%, p = 0.189), although rates were higher in NF Type 2 than in Type 1 (64% vs 31%). There was no difference in the use of intraoperative neuromonitoring when comparing cases with surgical complications and those without (67% vs 69%, p = 0.797). However, the use of neuromonitoring was associated with a significantly higher rate of gross-total resection (79% vs 66%, p = 0.022). CONCLUSIONS Resection is a safe and effective treatment for spinal nerve sheath tumors. Approximately 30% of patients developed a postoperative complication, most commonly new or worsening sensory deficits. This rate probably represents an inevitable complication of nerve sheath tumor surgery given the intimacy of these lesions with functional neural elements.

Efficacy of transcranial motor evoked potentials, mechanically elicited electromyography, and evoked electromyography to assess nerve root function during sustained compression in a porcine model.

STUDY DESIGN: This is an animal experiment using transcranial motor evoked potentials (TcMEPs), mechanically elicited electromyographic (EMG) responses, and evoked EMG responses during nerve root compression in a pig model. OBJECTIVE: To compare these 3 electrophysiological measures for compression applied to a lumbar nerve root. SUMMARY OF BACKGROUND DATA: Lumbar nerve root injury may result in motor weakness in up to 30% of spinal deformity cases. Compressive injury may occur during the surgical approach, decompression, and manipulation of the spine. Using an established porcine model, we examined the changes to TcMEPs, mechanically elicited EMG responses, and evoked EMG responses during varied compressive forces. METHODS: TcMEPs, mechanically elicited EMG responses, and evoked EMG responses were recorded for the tibialis anterior muscle in 16 experiments. Precompression TcMEP and nerve root stimulation threshold (NRT) were obtained. The dominant root was compressed at 1 N (n = 8) or 2 N (n = 8) for 10 minutes. TcMEP was recorded every minute during compression, and TcMEP and NRT were recorded after both compression and 10 minutes of recovery. RESULTS: After 10 minutes of 1-N compression, TcMEP amplitude of the tibialis anterior muscle decreased to 69% ± 13% of baseline (P < 0.02 vs. baseline). The mean NRT increased to 645% ± 433% (P < 0.02 vs. baseline NRT). After the recovery period, TcMEP in the 1-N group returned to 98% ± 11% of baseline (P = 0.36 vs. baseline). After 10 minutes of 2-N compression, TcMEPs from the tibialis anterior muscle decreased to 27% ± 15% of baseline (P < 0.02 vs. baseline). After the recovery period, TcMEP in the 2-N group returned to 30% ± 10% of baseline (P < 0.02 vs. baseline). Tonic EMG activity was observed in 3 nerve roots compressed at 2 N. CONCLUSION: Compression at 1 and 2 N produced consistent changes in TcMEPs and EMG responses. TcMEP monitoring is sensitive to an increase in compressive force. TcMEP amplitude change was correlated to the force applied and the ability of the nerve root to recover. Mechanically elicited EMG responses were not sensitive to nerve root compression. LEVEL OF EVIDENCE: N/A.

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.

Effect of hemorrhage and hypotension on transcranial motor-evoked potentials in swine.

BACKGROUND: Transcranial motor-evoked potentials (TcMEPs) monitor spinal cord motor tract integrity. Using a swine model, the authors studied the effects of vasodilatory hypotension, hemorrhage, and various resuscitation efforts on TcMEP responses. METHODS: Twelve pigs were anesthetized with constant infusions of propofol, ketamine, and fentanyl. Animals were incrementally hemorrhaged, until bilateral tibialis anterior TcMEP amplitude decreased to less than 40% of baseline or until 50% of the blood volume was removed. Mean arterial pressure (MAP), cardiac output (CO), and oxygen delivery (DO2) were examined. Resuscitation with phenylephrine, epinephrine, and colloid were evaluated. In seven animals, vasodilatory hypotension was examined. Paired comparisons and multivariate analysis were performed. RESULTS: Hemorrhage significantly reduced (as a percentage of baseline, mean±SD) TcMEPs (left, 33±29%; right, 26±21%), MAP (60±17%), CO (49±12%), and DO2 (43±13%), P value less than 0.001 for all. Vasodilation reduced MAP comparably, but TcMEPs, CO, and DO2, were not significantly lowered. After hemorrhage, restoration of MAP with phenylephrine did not improve TcMEPs, CO, or DO2, but similar restoration of MAP with epinephrine restored (to percentage of baseline) TcMEPs (59±40%), and significantly increased CO (81±17%) and DO2 (72±19%) compared with both hemorrhage and phenylephrine, P value less than 0.05 for all. Resuscitation with colloid did not improve TcMEPs. Multivariate analysis revealed that changes in TcMEPs were more closely associated with changes in CO and DO2 as compared with MAP. CONCLUSIONS: Hypotension from hemorrhage, but not vasodilation, is associated with a decrease in TcMEP amplitude. After hemorrhage, restoration of TcMEPs with epinephrine but not phenylephrine indicates that CO and DO2 affect TcMEPs more than MAP. Monitoring CO may be beneficial in major spine surgery when using TcMEP monitoring.

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.

Increases in voltage may produce false-negatives when using transcranial motor evoked potentials to detect an isolated nerve root injury.

OBJECTIVE: Transcranial Motor Evoked Potentials (TcMEPs) are sometimes used during lumbar spine surgery in order to detect and prevent an intraoperative nerve root injury. Typically, a fixed stimulus is applied, and one monitors for changes in response amplitude from several myotomes. Increased stimulating voltage may or may not alter the response after an acute injury. METHODS: We suture ligated the dominant root innervating the tibialis anterior (TA) muscle in 7 experiments in swine monitored with TcMEPs. Injury to the root was confirmed by an increase in threshold current needed to evoke an EMG response in the TA (from 0.32 ± 0.10 to 2.3 ± 0.9 mA, P < 0.01). We recorded TcMEPs at baseline, after injury, and with incremental 25 V increases in stimulation up to 100 V. RESULTS: After ligation, mean TcMEP amplitude in the TA decreased by 56% from baseline (P < 0.01). Adding voltage progressively restored mean amplitude to within 17% of baseline, but with wide variability in the response. In 1 experiment, there was no augmentation; 3 studies showed partial improvement toward baseline; and in 3 studies, the amplitude was augmented to levels above baseline. CONCLUSION: An acute nerve root injury may be detected by TcMEP monitoring. However, if the stimulating voltage is increased after injury, the response may or may not be affected. In complex spine procedures, adjustments to TcMEP stimulating parameters are often needed to maintain reproducible responses. However, if these changes are made during a period where injury might occur, this could mask the changes and lead to a false-negative interpretation.

Mixed-muscle electrode placement ("jumping" muscles) may produce false-negative results when using transcranial motor evoked potentials to detect an isolated nerve root injury in a porcine model.

INTRODUCTION: Placing EMG electrode pairs that span several muscles is sometimes used to enhance the efficacy of electromyographic recordings. This technique, often referred to as "jumping," has not been studied when using Motor Evoked Potentials (TcMEP) for detecting isolated spinal nerve root injury during spine surgery. METHODS: TcMEPs were obtained in seven pigs under general anesthesia. One pair of recording electrodes was placed entirely within the tibialis anterior (TA) muscle; a second pair had one lead in the TA and the other in the gastrocnemius muscle (TA-GAS). The dominant root innervating the TA was determined using evoked EMG. MEP amplitudes recorded by the TA and TA-GAS electrodes were compared before and after suture ligation of this root in 12 separate experiments. RESULTS: Mean baseline TcMEP amplitude was not significantly different for the TA vs. TA-GAS. After root ligation, mean amplitude dropped from baseline by 72 +/- 13% in the TA vs. 50 +/- 29% in the TA-GAS (p < 0.01). All amplitudes decreased by >50% in the TA group; half of the TA-GAS group had <50% decrease in amplitude. DISCUSSION: Mixed-myotomal recording electrodes did not consistently increase baseline TcMEP amplitude. The decrease in amplitude after ligation was both smaller and more variable in the "jumped" TA-GAS electrodes. Thus, this technique may allow someone relying on TcMEP monitoring to miss an otherwise detectable isolated nerve root injury (i.e., have a false-negative result).

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.

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.

The effect of age on motor evoked potentials in children under propofol/isoflurane anesthesia.

Intraoperative transcranial motor evoked potential (MEP) monitoring may help prevent neurologic injury during spine surgery. This type of monitoring may be difficult in the pediatric population under general anesthesia. We retrospectively reviewed data from 56 children, aged 2 to 18 yr, who were to undergo surgical correction of idiopathic scoliosis with MEP monitoring. Under combined isoflurane-propofol general anesthesia, before incision, we examined the minimum stimulating threshold voltage required to achieve a 50-microvolt or greater MEP response amplitude. Younger age was associated with an increase in the threshold voltage needed to elicit a sufficient MEP response. In addition, younger age was associated with longer stimulating pulse trains and greater need to adjust stimulating scalp electrodes. Body surface area, height, weight, and body mass index were also significant factors, but they were not independent predictors, after adjusting for age. Younger children received significantly lower levels of isoflurane and comparable doses of propofol, compared with older patients. Stronger stimulation needed to produce MEP responses in younger patients may reflect immaturity of their central nervous system, specifically conduction by the descending corticospinal motor tracts. Greater attention must be given to optimizing physiologic variables, limiting depressant anesthetics, and selecting the most favorable stimulating conditions in children, especially those <10 yr old.

Changes in transcranial motor evoked potentials during intramedullary spinal cord tumor resection correlate with postoperative motor function.

OBJECTIVE: Intraoperative monitoring of transcranial motor evoked potentials (TcMEPs) has been investigated recently as a means of preventing motor deficits associated with resection of intramedullary spinal cord tumors (IMSCTs). In this study, we hypothesized that changes in the intraoperative MEPs during tumor resection correlate with postoperative motor function deficits. METHODS: A retrospective record review was conducted for 28 patients who underwent resection of an IMSCT using myogenic or muscle-recorded TcMEPs during a 44-month period. Intraoperative MEP recordings and results from preoperative, immediate postoperative, and subsequent follow-up neurological examinations were analyzed. RESULTS: Of the 28 patients who underwent resection of an IMSCT using TcMEPs, MEP changes occurred in 13 patients (46%). Impaired motor conduction was detected by changes in pattern and duration of the MEP waveform morphology (polyphasic to biphasic in 9 patients and polyphasic to biphasic to loss of MEP response in 5 patients, 1 patient demonstrated both changes) and by an increase in voltage threshold (median, 175 V; range, 100-225 V; n = 22 extremities). Alterations in morphology and reduction in duration of the MEP response persisted despite significant increases in stimulation voltage. In 12 patients, reductions in the complexity and/or loss of the TcMEP waveform correlated with motor grade loss in the immediate postoperative period (P < 0.0001), at discharge (P < 0.001), and at follow-up (P < 0.001). The decrease in the duration of the response correlated with motor grade loss immediately after surgery (P < 0.001), at discharge (P < 0.0001), and at follow-up (P < 0.005). CONCLUSION: These results support the application of distal muscle-recorded TcMEPs to predict the occurrence and severity of postoperative motor deficits during resection of IMSCTs. Attention to such quantitative intraoperative monitoring data may help to minimize postoperative motor deficits by avoiding or correcting excessive spinal cord manipulation and modifying surgical technique during tumor resection.

Intraoperative motor mapping of the cerebral peduncle during resection of a midbrain cavernous malformation: technical case report.

OBJECTIVE AND IMPORTANCE: Brainstem cavernous malformations that seem to come to a pial or ependymal surface on preoperative magnetic resonance imaging studies may, in fact, be covered by an intact layer of neural tissue. For cavernous malformations in the cerebral peduncle, intraoperative stimulation mapping with a miniaturized probe can determine whether this overlying tissue harbors fibers in the corticospinal tract. In addition, intermittent monitoring with transcranial motor evoked potentials (TcMEPs) helps to protect this vital pathway during resection of the lesion. CLINICAL PRESENTATION: A 20-year-old woman collapsed after a cavernous malformation in the left cerebral peduncle hemorrhaged into the pons, midbrain, and thalamus. She presented with right hemiparesis and left oculomotor palsy. INTERVENTION: The cavernous malformation was completely resected through a left orbitozygomatic craniotomy and transsylvian approach. Stimulation mapping of the cerebral peduncle with a Kartush probe (Medtronic Xomed, Inc., Jacksonville, FL) identified the corticospinal tract lateral to the lesion, and a layer of tissue over the lesion harbored no motor fibers. TcMEP monitoring helped to guide the resection, with increased voltage thresholds and altered waveform morphologies indicating transient impaired motor conduction. All TcMEP changes returned to baseline by the end of the procedure, and the patient's hemiparesis improved after surgery. CONCLUSION: Stimulation mapping of the corticospinal tract and intermittent TcMEPs is a safe and simple surgical adjunct. Expanded monitoring of the motor pathway during the resection of cerebral peduncle cavernous malformations may improve the safety of these operations.