Evaluation of combined use of transcranial and direct cortical motor evoked potential monitoring during unruptured aneurysm surgery

Does persistent crossover (ipsilateral) motor evoked potential (MEP) responses represent a technical failure for intracranial motor tract monitoring? A case example and practical solution

PURPOSE

Attention to motor evoked potential (MEP) stimulation intensity is necessary to avoid false negative MEP results during intracranial procedures. Observing ipsilateral (crossover) MEP responses has been hypothesized to indicate inappropriately strong stimulation intensity. We describe a case where persistent crossover MEP responses falsely suggested that stimulus intensity was too high and describe an alternative method to guide the selection of MEP stimulation intensity.

METHODS

A patient undergoing a suboccipital craniotomy for tumor resection had bilateral transcranial electrical MEP monitoring under total intravenous anesthesia. MEP results were obtained from left and right hand using C4-Cz and C3-Cz stimulation montages respectively. Selection of an appropriately superficial stimulus intensity was guided using MEP onset latencies.

RESULTS

MEP acquisition proceeded normally for contralateral left hand (C4-Cz montage). However, using the C3-Cz montage, persistent crossover responses were noted at stimulation intensities as low as threshold for contralateral right hand MEP (94 V/166 mA). Appropriate MEP stimulus intensity for subsequent monitoring (approximately 96 V/172 mA) was determined utilizing onset latency measurements from contralateral hand MEP responses. The stimulus intensity chosen was predicated on onset latency being ≥ 2 ms longer than latency at maximal stimulus level (shortest latency). A stimulus intensity-latency plot was generated offline to illustrate this important relationship for intracranial MEP use. MEP acquisition proceeded without incident and gross total resection was achieved without postoperative motor deficits.

CONCLUSION

Despite crossover appearance contralateral hand MEP were quantitatively validated for intraoperative application using onset latency guidance.

Transcranial motor-evoked potentials disappear with pons transection

Motor-evoked potential (MEP) monitoring by transcranial electrical stimulation (TES) is important for intraoperative motor function assessment in neurosurgery; however, false-negative results sometimes occur, and these findings should be interpreted with caution. Herein, we report an interesting MEP change resulting from a pons transection. The patient was a boy aged 5 years and 2 months. He underwent multiple craniotomies for cerebellar anaplastic ependymoma and was already paralyzed in the right upper and lower limbs. Therefore, we decided to remove the recurrent lesion from the left anterior pons. MEPs were recorded on both the right and left sides after the start of surgery but disappeared 1 h 30 min after the start of surgery in the TES on the operative side, even when the stimulation intensity was increased. The contralateral TES consistently recorded stable MEPs throughout the surgery. The tumor was completely resected on imaging. Immediately postoperatively, the patient experienced flaccid paralysis on the right side of the body, which recovered to preoperative levels over time. A transcranial MEP cannot be derived if the corticospinal tract is transected at the pons. Transcranial MEP findings may accurately reflect the corticospinal tract function if the injury is caudal to the pons.

Case series: Intraoperative neuromonitoring and angiography in the surgical treatment of vascular malformations

In the surgical treatment of cerebral vascular malformations, e.g., aneurysms and arteriovenous malformations, the risk of ischemic complications is 6.7%, and a residual aneurysm is possible in 5.2% of these cases. Ischemic lesions can result in permanent neurological deficits, and a residual aneurysm can lead to the recurrence of the aneurysm in 2% of cases. In this article, we present five cases (two cases of ruptured aneurysms, two cases of non-ruptured aneurysms, and a case of arteriovenous malformation) in which we reduced the aforementioned risks with the use of intraoperative neuromonitoring and angiography. Intraoperative neuromonitoring (IONM) is used to measure motor and sensory-evoked potentials to detect brain hypoperfusion. Intraoperative angiography with the dye indocyanine green (ICG-A), which fluoresces in a vessel under a microscope after intravenous administration, helps to identify residual aneurysm sacs and distal blood flow. With the use of IONM and ICG-A, we identified abnormalities and adjusted our interventions and treatments. IONM and ICG-A can lead to a better outcome after surgical treatment of cerebral vascular abnormalities.

Usefulness of Transcranial Motor Evoked Potential in Clipping Surgery for Cerebral Aneurysms-Introduction of a New Protocol for Stable Monitoring

The usefulness of transcranial motor evoked potentials (Tc-MEPs) in clipping surgery has been reported. However, numerous false positive and false negative cases were reported. We report the usefulness of a new protocol compared with direct cortical MEP (Dc-MEP).Materials were 351 patients who underwent aneurysmal clipping under simultaneous monitoring of Tc- and Dc-MEPs. A total of 337 patients without hemiparesis and 14 with hemiparesis were separately analyzed. Intraoperative changes of Tc-MEP thresholds were examined in the first 50 patients without hemiparesis. The stimulation strength of Tc-MEP was set at +20% of the stimulation threshold. As thresholds changed intraoperatively, thresholds were examined every 10 min and changed stimulation strength.Stimulation thresholds of Tc-MEP were significantly decreased after craniotomy and significantly increased after CSF aspiration. The recording ratios of Tc- and Dc-MEPs were 98.8% and 90.5%, respectively. Out of 304 patients without MEP change, 5 patients developed transient or mild hemiparesis with infarction of the territory of the perforating artery arising from the posterior communicating artery. Out of 31 patients whose MEP transiently disappeared, 3 patients developed transient or mild hemiparesis. The other two patients without MEP recovery manifested persistent hemiparesis. In 14 patients with preoperative hemiparesis, 3 patients whose healthy/affected ratio of Tc-MEP was large developed severe persistent hemiparesis.We clarified the intraoperative changes of Tc-MEP thresholds for the first time. A new protocol of Tc-MEP that followed thresholds and changed stimulation strength to +20% of thresholds is useful for stable monitoring. The usefulness of Tc-MEP is the same as that or better than that of Dc-MEP.

Cathodal Genesis of Ipsilateral Hand (Crossover) Motor Evoked Potentials: Evidence from a Patient with Previous Stroke

A case is described where baseline transcranial electrical motor evoked potentials (TcMEP) and somatosensory evoked potentials (SSEP) results were unilaterally absent in a patient with previous hemispheric stroke undergoing a right-sided carotid endarterectomy. SSEP data confirmed right cortical pathology and excluded a technical rationale for absent motor evoked responses. Attempts at generating left-hand (contralateral) TcMEP from right cortical anodal stimulation failed despite high stimulus intensities. However, TcMEP responses from anodal stimulation of the right cortex were recorded from the right-hand (ipsilateral) which were attributed to "crossover." Ipsilateral TcMEP onset latencies derived from the stimulus-response data supports the idea that crossover is a product of cathodal stimulation initially acting on pericortical motor pathways.

Short and long-term prognostic value of intraoperative motor evoked potentials in brain tumor patients: a case series of 121 brain tumor patients

PURPOSE

Iatrogenic neurologic deficits adversely affect patient outcomes following brain tumor resection. Motor evoked potential (MEP) monitoring allows surgeons to assess the integrity of motor-eloquent areas in real-time during tumor resection to lessen the risk of iatrogenic insult. We retrospectively associate intraoperative transcranial and direct cortical MEPs (TC-MEPs, DC-MEPs) to early and late post-operative motor function to prognosticate short- and long-term motor recovery in brain tumor patients undergoing surgical resection in peri-eloquent regions.

METHODS

We reviewed 121 brain tumor patients undergoing craniotomies with DC-MEP and/or TC-MEP monitoring. Motor function scores were recorded at multiple time-points up to 1 year postoperatively. Sensitivity, specificity, and positive and negative predictive values (PPV, NPV) were calculated at each time point.

RESULTS

The sensitivity, specificity, PPV, and NPV of TC-MEP in the immediate postoperative period was 17.5%, 100%, 100%, and 69.4%, respectively. For DC-MEP monitoring, the respective values were 25.0%, 100%, 100%, and 68.8%. By discharge, sensitivity had increased for both TC-MEP and DC MEPs to 43.8%, and 50.0% respectively. Subset analysis on patients without tumor recurrence/progression at long term follow-up (n = 62 pts, 51.2%) found that all patients with stable monitoring maintained or improved from preoperative status. One patient with transient intraoperative TC-MEP loss and permanent DC-MEP loss suffered a permanent deficit.

CONCLUSION

Brain tumor patients who undergo surgery with intact MEP monitoring and experience new postoperative deficits likely suffer transient deficits that will improve over the postoperative course in the absence of disease progression.

Deliberate Parent Artery Sacrifice Guided by Intraoperative Neurophysiological Monitoring During Complex Surgical Clipping of a Ruptured Anterior Communicating Artery Aneurysm

Aneurysms arising from the anterior communicating artery (ACOA) are the most common intracranial aneurysms encountered. Most aneurysms can be treated with surgical clipping or endovascular coiling; however, there are times when parent vessel sacrifice (PVS) is necessary such as aneurysms with fragile necks or large/giant aneurysms. Application of intraoperative neurophysiological monitoring (IONM) can assist in guiding permissive temporary vessel occlusion during complex aneurysm clippings. However, to-date there is no literature that describes how IONM can be used as a predictor of post-operative neurological status when PVS is employed or as a guide to determine whether PVS is safe. We present a case where IONM guided the sacrifice of the A1 and anterior communicating arteries after 2 hours and 25 min of temporary vessel occlusion. No attenuation was noted in the IONM at any point during the procedure, and the IONM predicted the patient would awake neurologically intact.

Temporary vessel occlusion in cerebral aneurysm surgery guided by direct cortical motor evoked potentials

BACKGROUND

Temporary clipping is an important tool in the vascular neurosurgeon's armamentarium. We routinely utilize intraoperative neurophysiological monitoring (IONM) for complex brain aneurysm surgery cases, relying on direct cortical motor evoked potential (DCMEP) alerts to guide the duration of temporary clipping. Previous studies have argued for relatively short and intermittent temporary clipping strategies. In this study, we sought to assess the maximal permissive temporary clipping time during complex aneurysm surgery. To do this, we assessed patient outcome in relation to temporary clip duration guided by DCMEP.

METHODS

We queried our prospectively collected neuromonitoring database for anterior circulation aneurysm cases where temporary clipping was utilized by a single cerebrovascular surgeon between 2018 and 2021. Operative and IONM reports were reviewed. Patients in whom the duration of temporary clipping could not be determined were excluded. The operative strategy permissively allowed continuous temporary clipping as long as no neuromonitoring alerts were encountered. Maximal permissive parent artery occlusion time (Clip_max) was recorded as the longest duration of tolerated temporary vessel clipping without decrement in DCMEP.

RESULTS

A total of 41 complex anterior circulation aneurysm clipping cases met criteria for this study. The mean Clip_max for all cases was just over 19 min and did not differ between ruptured and unruptured aneurysms. Initial alert times were not found to be predictive of final permissive temporary clip duration after re-perfusion. In 100% (41/41) of cases, the aneurysm was completely clip occluded without residual on catheter angiogram. Stable or improved modified Rankin Score was achieved in 98% (40/41) of cases at 3-month follow-up.

CONCLUSIONS

This study demonstrates that using DCMEP can facilitate relatively long but safe temporary clipping durations in complex anterior circulation aneurysm surgery. In the endovascular era with only a limited subset of technically challenging aneurysms needing open surgical treatment, extended permissive temporary clipping guided by DCMEPs can significantly enhance a surgeon's ability to achieve excellent technical and clinical outcomes.

Diagnostic Impact of Monitoring Visual Evoked Potentials to Prevent Visual Complications During Endovascular Treatment for Intracranial Aneurysm

Introduction

The present study aimed to determine the incidence of intraprocedural visual-evoked potential (VEP) changes and to identify correlations with intraprocedural ischemic complications during endovascular treatment in patients with intracranial aneurysm related to visual function.

Methods

This study analyzed data from 104 consecutive patients who underwent endovascular coil embolization to treat intracranial aneurysms related to visual function under VEP and transcranial motor evoked potential (MEP) monitoring. We analyzed associations between significant changes in MEP and VEP, defined as a >50% decrease in amplitude, and both intraprocedural complications and postoperative neurological deficits. Factors associated with postoperative neurological deficits were also assessed.

Results

Treated aneurysms were predominantly located in the internal carotid artery (95%). Five (5%) were located in the posterior cerebral artery (PCA). Significant decreases in intraprocedural VEP occurred in four patients (4%), although one of those four patients did not show concomitant MEP decreases during procedures. Immediate salvage procedures avoided postoperative visual disturbances. All VEP decreases were transient and not associated with postoperative visual impairment. One of three cases who underwent intraoperative balloon occlusion test showed tolerance to balloon occlusion of the proximal PCA under VEP assessment; parent artery occlusion was performed without postoperative visual disturbance in that case.

Conclusion

Although significant VEP decreases occurred 4% during neuro-endovascular aneurysm treatment related to visual function, intraprocedural VEP monitoring identifies ischemic changes associated with visual pathways and facilitates prompt initiation of salvage procedures.

Motor Evoked Potential Warning Criteria in Supratentorial Surgery: A Scoping Review

During intraoperative monitoring of motor evoked potentials (MEP), heterogeneity across studies in terms of study populations, intraoperative settings, applied warning criteria, and outcome reporting exists. A scoping review of MEP warning criteria in supratentorial surgery was conducted in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Sixty-eight studies fulfilled the eligibility criteria. The most commonly used alarm criteria were MEP signal loss, which was always a major warning sign, followed by amplitude reduction and threshold elevation. Irreversible MEP alterations were associated with a higher number of transient and persisting motor deficits compared with the reversible changes. In almost all studies, specificity and Negative Predictive Value (NPV) were high, while in most of them, sensitivity and Positive Predictive Value (PPV) were rather low or modest. Thus, the absence of an irreversible alteration may reassure the neurosurgeon that the patient will not suffer a motor deficit in the short-term and long-term follow-up. Further, MEPs perform well as surrogate markers, and reversible MEP deteriorations after successful intervention indicate motor function preservation postoperatively. However, in future studies, a consensus regarding the definitions of MEP alteration, critical duration of alterations, and outcome reporting should be determined.

Optimal Use of Temporary Clip Application during Aneurysm Surgery - In Search of the Holy Grail

Temporary clips are invaluable safety tools during the clipping of an aneurysm. Controversies regarding maximum permissible duration and safety, however, remain unanswered. This descriptive narrative attempts to review the literature to provide valuable insights on controversies clouding the use of temporary clips among neurosurgeons. Popular databases, including Pub Med, Medline/Medscape, Scopus, Cochrane, Embase, Google Scholar, were searched to find available literature on temporary clips. The searched MeSH terms were "Temporary Clip," "Temporary Clipping," "Cerebral Aneurysm," and "Aneurysm." Temporary clips have been in use since 1928 and have undergone considerable structural and technical modifications. A temporary clip's optimal safety limit is not yet defined with literature evidence ranging from immediate to 93 min. It is not yet definite whether temporary clips application aggravates vasospasm, but emergency temporary clips application, especially in poor-grade aneurysmal subarachnoid hemorrhage patients, is associated with poor outcomes. A temporary clip needs to be applied with caution in patients treated earlier by endovascular technique and having indwelling stents. Nitinol Stent is feasible, while a Cobalt-Chromium alloy stent does not get occluded and gets deformed under the closing pressure of a temporary clip. Although a temporary clip application is a fundamental strategy during the clipping of an aneurysm; the exact safe duration remains to be decided in randomized control trials. Their utility for the shorter duration is beneficial under un-conclusive evidence of neuroprotective agents and intraoperative monitoring. Neurosurgeons need to consider all aspects of their pros and cons for optimal use.

Loss of Motor Evoked Potentials Due to Carotid Artery Retraction in an Exoscopic Clipping of a Basilar Tip Aneurysm

The internal carotid artery (ICA) may inhibit visualization of a basilar tip aneurysm during an orbitozygomatic craniotomy. Retraction of the ICA may be warranted for better visualization; however, it may lead to impending neurological sequelae. Impending neurological injury due to ICA retraction may be mitigated if multi-modal neuromonitoring techniques are employed. The authors present a case report showing the contemporaneous loss of transcranial motor evoked potentials and direct cortical motor evoked potentials during an exoscopic clipping of a basilar tip aneurysm due to ICA retraction and subsequent loss of perfusion to the vascular territory supplied by ICA. The motor evoked potentials immediately returned after retraction was removed and the patient awoke neurologically intact.

Diagnostic impact of monitoring transcranial motor-evoked potentials to prevent ischemic complications during endovascular treatment for intracranial aneurysms

The present study aimed to determine the incidence of intraprocedural motor-evoked potential (MEP) changes and to correlate them with intraprocedural ischemic complications and postprocedural neurological deficits in patients after endovascular intracranial aneurysm treatment. This study analyzed data from 164 consecutive patients who underwent endovascular coil embolization to treat intracranial aneurysms under transcranial MEP monitoring. We analyzed associations between significant changes in MEP defined as > 50% decrease in amplitude, and intraprocedural complications as well as postoperative neurological deficits. Factors associated with postprocedural neurological deficits were also assessed. The treated aneurysms were predominantly located in the anterior circulation (71%). Fourteen (9%) were located at perforators or branches that supplied the pyramidal tract. Intraprocedural complications developed in eight (5%) patients, and four of eight (50%) patients occurred postprocedural neurological deficits. Significant intraprocedural MEP changes occurred during seven of eight endovascular procedures associated with intraprocedural complications and salvage procedures were performed immediately. Among these changes, four transient MEP changes, recovered within 10 min, were not associated with postprocedural neurological deficits, whereas three permanent MEP changes were associated with postprocedural neurological deficits and mRS ≥ 1 at discharge. Aneurysms located at perforators/branches supplying the pyramidal tract, and permanent intraprocedural MEP changes were associated with postprocedural neurological deficits. We conclude that intraprocedural transcranial MEP monitoring can reliably identify ischemic changes and can initiate prompt salvage procedures during endovascular aneurysm treatment.

Transient Augmentation of Intraoperative Motor Evoked Potentials During Middle Cerebral Artery Aneurysm Surgery

OBJECTIVE

To study clinical significance of augmentation of intraoperative motor evoked potentials (MEPs) during direct open surgery for middle cerebral artery (MCA) aneurysms.

METHODS

Between 2009 and 2017, 134 MCA aneurysm surgeries were performed with intraoperative MEP monitoring. The frequency and cause of augmentation with >50% increase of MEP amplitude from baseline were studied. Factors associated with MEP augmentation were investigated.

RESULTS

MEP augmentation was demonstrated in 9 patients. All 9 events were observed just after application of the temporary clip to the parent artery. The ratio of the maximum amplitude to baseline was 2.6 ± 1.1 at an mean of 2.4 ± 1.1 minutes after parent artery occlusion. Ten patients who did not show MEP augmentation after parent artery occlusion were compared with the patients showing MEP augmentation. The distance of the temporary clip point from the midline was smaller in patients with MEP augmentation compared with patients without MEP augmentation (P = 0.033).

CONCLUSIONS

MEP augmentation was thought to be an early ischemic sign preceding a significant decrease in MEPs during MCA aneurysm surgery. Transient augmentation of MEPs was more frequently observed in cases with a temporary clip applied to the more proximal part of the MCA.

Comparison of Intraoperative Motor Evoked Potentials Monitoring with Direct Cranial Stimulation by Peg-Screw and Transcranial Stimulation by Corkscrew for Supratentorial Surgery

OBJECTIVE

Corkscrew (CS) electrodes are usually used for transcranial electrical stimulation (TES) in the intraoperative monitoring of motor evoked potentials (MEP). Direct cranial stimulation with peg-screw (PS) electrodes can elicit MEP. The present study investigated the difference in the initial threshold between PS and CS electrodes for intraoperative MEP monitoring.

METHODS

We retrospectively analyzed TES-MEP monitoring for supratentorial surgery in 72 patients. Of these 72 patients, 44 were monitored with PS and CS electrodes (PS/CS group) and 28 were monitored with CS and CS electrodes (CS/CS group). TES was used to deliver electrical stimulation by a train of 4-pulse anodal constant current stimulation. The initial threshold in each electrode was checked and analyzed.

RESULTS

In the PS/CS group, the initial threshold with the PS electrode was 38.3 ± 15.1 mA (mean ± standard deviation) on the affected side, and the initial threshold with the CS electrode was 51.4 ± 13.9 mA on the unaffected side. The initial threshold with the PS electrode was significantly lower than that with the CS electrode (P = 0.0001). In the CS/CS group, the initial threshold was 56.2 ± 16.5 mA on the affected side and 62.1 ± 18.6 mA on the unaffected side, with no statistically significant difference (P = 0.23).

CONCLUSION

The initial threshold to elicit MEP was significantly lower with the PS electrode than with the CS electrode. A PS electrode can be used as a feasible stimulation electrode for TES-MEP.

Intraoperative use of transcranial motor/sensory evoked potential monitoring in the clipping of intracranial aneurysms: evaluation of false-positive and false-negative cases

OBJECTIVE

Somatosensory and motor evoked potentials (SEPs and MEPs) are often used to prevent ischemic complications during aneurysm surgeries. However, surgeons often encounter cases with suspicious false-positive and false-negative results from intraoperative evoked potential (EP) monitoring, but the incidence and possible causes for these results are not well established. The aim of this study was to investigate the efficacy and reliability of EP monitoring in the microsurgical treatment of intracranial aneurysms by evaluating false-positive and false-negative cases.

METHODS

From January 2012 to April 2016, 1514 patients underwent surgery for unruptured intracranial aneurysms (UIAs) with EP monitoring at the authors' institution. An EP amplitude decrease of 50% or greater compared with the baseline amplitude was defined as a significant EP change. Correlations between immediate postoperative motor weakness and EP monitoring results were retrospectively reviewed. The authors calculated the sensitivity, specificity, and positive and negative predictive values of intraoperative MEP monitoring, as well as the incidence of false-positive and false-negative results.

RESULTS

Eighteen (1.19%) of the 1514 patients had a symptomatic infarction, and 4 (0.26%) had a symptomatic hemorrhage. A total of 15 patients showed motor weakness, with the weakness detected on the immediate postoperative motor function test in 10 of these cases. Fifteen false-positive cases (0.99%) and 8 false-negative cases (0.53%) were reported. Therefore, MEP during UIA surgery resulted in a sensitivity of 0.10, specificity of 0.94, positive predictive value of 0.01, and negative predictive value of 0.99.

CONCLUSIONS

Intraoperative EP monitoring has high specificity and negative predictive value. Both false-positive and false-negative findings were present. However, it is likely that a more meticulously designed protocol will make EP monitoring a better surrogate indicator of possible ischemic neurological deficits.

Intraoperative evoked potential monitoring for detecting cerebral injury during adult aneurysm clipping surgery: a systematic review and meta-analysis of diagnostic test accuracy

OBJECTIVES

We aim to evaluate the diagnostic test accuracy (DTA) of intraoperative evoked potential (EP) monitoring to detect cerebral injury during clipping of cerebral aneurysms.

DESIGN

Systematic review.

DATA SOURCES

Major electronic databases including MEDLINE, EMBASE, LILACS.

ELIGIBILITY CRITERIA

We included studies that reported the DTA of intraoperative EP monitoring during intracranial aneurysm clipping procedures in adult patients.

DATA EXTRACTION AND SYNTHESIS

After quality assessment, we performed a meta-analysis using the bivariate random effects model, and calculated the possible range of DTA point estimates using a new best-case/worst-case scenario approach to quantify the impact of rescue intervention on DTA.

RESULTS

A total of 35 studies involving 4011 patients were included. The quality of the primary studies was modest and the heterogeneity across studies was high. The pooled sensitivity and specificity for predicting postoperative neurological deficits for the somatosensory evoked potential (SSEP) monitoring was 59% (95% CI: 39% to 76%; I²: 76%) and 86% (95% CI: 77% to 92%; I²: 94%), for motor evoked potential (MEP) monitoring was 81% (95% CI: 58% to 93%; I²: 54%) and 90% (95% CI: 86% to 93%; I²: 81%), and for combined SSEP and MEP monitoring was 92% (95% CI: 62% to 100%) and 88% (95% CI: 83% to 93%). The best-case/worst-case range for the pooled point estimates for sensitivity and specificity for SSEP was 50%-63% and 81%-100%, and for MEP was 59%-74% and 93%-100%, and for combined SSEP and MEP was 89%-94% and 83%-100%.

CONCLUSIONS

Due to the modest quality and high heterogeneity of the existing primary studies, it is not possible to confidently support or refute the diagnostic value of EP monitoring in cerebral aneurysm clipping surgery. However, combined SSEP and MEP appears to provide the best DTA for predicting postoperative stroke. Contrary to popular assertion, the modest sensitivity of SSEP monitoring is not explained by the use of rescue intervention.

PROSPERO REGISTRATION NUMBER

CRD42015016884.

Limitation of Intraoperative Transcranial Electrical Stimulation-Motor Evoked Potential Monitoring During Brain Tumor Resection Adjacent to the Primary Motor Cortex

Transcranial electrical stimulation-motor evoked potential (TES-MEP) is a valuable intraoperative monitoring technique during brain tumor surgery. However, TES can stimulate deep subcortical areas located far from the motor cortex. There is a concern about false-negative results from the use of TES-MEP during resection of those tumors adjacent to the primary motor cortex. Our study reports three cases of TES-MEP monitoring with false-negative results due to deep axonal stimulation during brain tumor resection. Although no significant change in TES-MEP was observed during surgery, study subjects experienced muscle weakness after surgery. Deep axonal stimulation of TES could give false-negative results. Therefore, a combined method of TES-MEP and direct cortical stimulation-motor evoked potential (DCS-MEP) or direct subcortical stimulation should be considered to overcome the limitation of TES-MEP.

Predictive value of neurophysiologic monitoring during neurovascular intervention for postoperative new neurologic deficits

PURPOSE

Forms of intraoperative neurophysiologic monitoring (IONM), including somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs), have been widely used in the field of neurosurgery. This study aimed to evaluate the diagnostic efficacy of IONM in identifying intraoperative events and predicting postoperative neurologic deficits in neurovascular intervention.

METHODS

From January 2013 to December 2016, we retrospectively reviewed patients who underwent neurovascular intervention under general anesthesia with the use of IONM. Associations between significant changes in MEPs or SSEPs which were defined as a decrease more than 50% in amplitude and/or an increase more than 10% in latency and any identifiable intraoperative events and/or postoperative neurologic deficits were determined. The sensitivity and specificity values for both MEPs and SSEPs were calculated.

RESULTS

In total, 578 patients (175 men and 403 women) were included. Their mean age was 59.5 years. SSEP changes occurred in 1% (n = 6), and MEP changes occurred in 1.2% (n = 7). Four patients suffered postoperative neurologic deficits, and identifiable intraoperative events were observed in seven patients. Both SSEP and MEP changes were significantly associated with identifiable intraoperative events and/or postoperative neurologic deficits (p < 0.001, Fisher's exact test). The calculated sensitivity and specificity of MEP monitoring were 50 and 99.5%, respectively. The sensitivity and specificity of SSEP monitoring were both 100%.

CONCLUSION

Intraoperative SSEP monitoring might be a reliable and sensitive method to surveil neurologic complications during neurovascular intervention. Intraoperative MEP monitoring appears to be feasible. However, it is unclear whether MEP monitoring has any additive benefit over SSEP monitoring.

Bite injuries caused by transcranial electrical stimulation motor-evoked potentials' monitoring: incidence, associated factors, and clinical course

PURPOSE

The incidence of bite injuries associated with transcranial electrical stimulation motor-evoked potentials monitoring reportedly ranges from 0.13 to 0.19%. However, in clinical practice, bite injuries appear to occur more frequently than previously reported. Our aim was to identify the incidence of and perioperative risk factors associated with bite injuries caused by transcranial electrical stimulation motor-evoked potential monitoring.

METHODS

Patients who underwent elective surgery with transcranial electrical stimulation motor-evoked potential monitoring at a single tertiary hospital in Japan between June 2017 and December 2017 were included in this study. All patients were assessed by oral surgeons preoperatively and postoperatively. The associated factors with bite injuries were explored by the univariate analysis.

RESULTS

12 of 186 patients experienced 13 bite injuries, including three lip, six oral mucosa, and four tongue injuries. No patient required suture repair. 11 of 12 patients had uneventful postoperative courses and were cured within 12 postoperative days. One patient with a tongue ulcer and a hematoma had difficulty in oral intake and persistent dysgeusia. Patient severe movement during transcranial electrical stimulation motor-evoked potential monitoring was associated with bite injuries (p = 0.03).

CONCLUSIONS

The incidence of bite injuries assessed by oral surgeons was 6.5% in patients with transcranial electrical stimulation motor-evoked potential monitoring, and the patients with severe movement during the monitoring tended to incur bite injuries. In rare cases, transcranial electrical stimulation motor-evoked potential monitoring may cause difficulty in oral intake and dysgeusia.

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.

Effectiveness and Limitations of Intraoperative Monitoring with Combined Motor and Somatosensory Evoked Potentials During Surgical Clipping of Unruptured Intracranial Aneurysms

OBJECTIVE

Postoperative neurologic deficits are one of the devastating complications that can result from surgical clipping of unruptured intracranial aneurysms. Intraoperative monitoring (IOM) of motor and somatosensory evoked potentials (EPs) has been used to reduce neurologic sequelae. We evaluated the effectiveness and limitations of IOM in prevention of surgical complications during aneurysm clipping.

METHODS

A retrospective analysis was performed, involving 386 operations for 429 unruptured intracranial aneurysms in 386 patients with consecutively collected IOM data.

RESULTS

Significant EP changes were detected during clipping of 23 aneurysms in 23 patients (5.4% of aneurysms). Among them, 8 patients (accounting for 2.1% of operations and 1.9% of aneurysms) experienced postoperative motor deficits, including 3 permanent and 5 temporary motor deficits with corresponding radiologic lesions. In detecting postoperative motor deficits, the sensitivity and specificity of motor EP monitoring were 0.38 and 0.99, respectively, and those of somatosensory EP monitoring were 0.25 and 0.96, respectively. Seven patients (1.8% of operations) with unchanged EPs had other kinds of postoperative neurologic complications, including altered mentality in 5 cases, motor aphasia in 1, and gaze limitation in 1, with corresponding radiologic abnormalities. However, all 7 patients with other neurologic symptoms recovered within 6 months after surgery.

CONCLUSIONS

IOM of motor and somatosensory EPs was useful and reliable in predicting and preventing postoperative motor deficits. However, it also showed some limitations in the significance of positive EP changes and detection of neurologic deficits other than motor function.

The impact of several craniotomies on transcranial motor evoked potential monitoring during neurosurgery

OBJECTIVE

Transcranial motor evoked potential (tMEP) monitoring is popular in neurosurgery; however, the accuracy of tMEP can be impaired by craniotomy. Each craniotomy procedure and changes in the CSF levels affects the current spread. The aim of this study was to investigate the influence of several craniotomies on tMEP monitoring by using C3–4 transcranial electrical stimulation (TES).

METHODS

The authors used the finite element method to visualize the electric field in the brain, which was generated by TES, using realistic 3D head models developed from T1-weighted MR images. Surfaces of 5 layers of the head (brain, CSF, skull, subcutaneous fat, and skin layer) were separated as accurately as possible. The authors created 5 models of the head, as follows: normal head; frontotemporal craniotomy; parietal craniotomy; temporal craniotomy; and occipital craniotomy. The computer simulation was investigated by finite element methods, and clinical recordings of the stimulation threshold level of upper-extremity tMEP (UE-tMEP) during neurosurgery were also studied in 30 patients to validate the simulation study.

RESULTS

Bone removal during the craniotomy positively affected the generation of the electric field in the motor cortex if the motor cortex was just under the bone at the margin of the craniotomy window. This finding from the authors' simulation study was consistent with clinical reports of frontotemporal craniotomy cases. A major decrease in CSF levels during an operation had a significantly negative impact on the electric field when the motor cortex was exposed to air. The CSF surface level during neurosurgery depends on the body position and location of the craniotomy. The parietal craniotomy and temporal craniotomy were susceptible to the effect of the changing CSF level, based on the simulation study. A marked increase in the threshold following a decrease in CSF was actually recorded in clinical reports of the UE-tMEP threshold from a temporal craniotomy. However, most frontotemporal craniotomy cases were minimally affected by a small decrease in CSF.

CONCLUSIONS

Bone removal during a craniotomy positively affects the generation of the electric field in the motor cortex if the motor cortex is just under the bone at the margin of the craniotomy window. The CSF decrease and the shifting brain can negatively affect tMEP ignition. These changes should be minimized to maintain the original conductivity between the motor cortex and the skull, and the operation team must remember the fluctuation of the tMEP threshold.

[An experience of neurophysiological monitoring in neurosurgery]

AIM

To evaluate the efficacy of neuromonitoring methods in prevention of postoperative neurological complications and estimation of predictive power of intraoperative changes in monitored characteristics.

MATERIAL AND METHODS

Retrospective study examined 240 patients, operated in the years 2014-2015 using intraoperative neurophysiological monitoring. One hundred and seventy-three patients suffered from hemispheric lesions and 67 had lesions located in or near the brainstem. Somatosensory evoked potentials (SSEP) were monitored in 152 cases, visual in 32, brainstem acoustic in 22, transcranial motor in 36; stimulation mapping of motor cortex was performed in 69 surgeries, and cranial nerves identification in 27. EEG was recorded in 7 patients, and 3 of them were woke up during the surgery for speech mapping.

RESULTS

The sensitivity of the SSEP in motor dysfunction detection was low (33%), while the specificity was relatively high (82%). These characteristics for visual and motor evoked potentials were close to 100% provided that the parameters of anesthesia met the corresponding requirements. The most effective methods in respect of prevention of postoperative dysfunctions were the stimulation mapping of functionally significant areas (motor and speech) and motor pathways mapping.

CONCLUSION

Intraoperative neuromonitoring reduces a number of neurological complications after neurosurgical operations. The SSEP method is not sensitive enough in surgeries that could affect motor centers and/or pathways, and multimodal monitoring combining SSEP and motor responses recording during transcranial and/or direct electrical brain stimulation. Successful monitoring requires highly coordinated actions between neurophysiologists, neurosurgeons and anesthesiologists.

Electrophysiological influence of temporal occlusion of the parent artery during aneurysm surgery

Intraoperative monitoring of the motor evoked potential (MEP) during cerebral aneurysm surgery has been widely used to confirm surgical safety. In this study, we retrospectively analyzed the influence of the MEP amplitude resulting from temporal occlusion of the parent artery, and appropriate judgement in the surgery was discussed. Ten patients underwent temporal occlusion of the parent artery during aneurysm surgery, and five of these patients showed a decrease in the MEP amplitude following temporal arterial occlusion. Clinical factors in patients with and without MEP decrease were compared. The time gap between the surgical procedure and the MEP change and recovery was then investigated. A decrease in the MEP amplitude caused by temporal occlusion had a significantly higher occurrence compared with permanent clip failure. The time from the release procedure to MEP amplitude recovery was relatively longer than the time from the occlusion procedure to the decrease in MEP amplitude. The time from release procedure to MEP amplitude recovery showed a weak correlation with the parent artery occlusion time. There is a time gap between releasing the temporal arterial occlusion and MEP recovery that is similar to temporal parent arterial occlusion and the MEP decrease. The cause of MEP amplitude should be judged carefully, and influence of parent artery temporal occlusion should be taken into consideration during aneurysm clipping.

The Diagnostic Accuracy of Evoked Potential Monitoring Techniques During Intracranial Aneurysm Surgery for Predicting Postoperative Ischemic Damage: A Systematic Review and Meta-Analysis

OBJECTIVE

To investigate the diagnostic accuracy of various evoked potential monitoring techniques in predicting postoperative neurologic deficit in intracranial aneurysm surgery.

METHODS

A literature search of the MEDLINE, Embase, and Cochrane databases was conducted for English language articles published between March 31, 1983 and March 31, 2016. Original studies that reported the use of evoked potential monitoring during intracranial aneurysm surgery in predicting postoperative neurologic damage were selected, and their relevant reference lists were hand searched. Test performance characteristics were summarized using hierarchic summary receiver operating characteristic (ROC) curves and bivariable random-effects models.

RESULTS

Thirteen qualifying studies (1597 patients; 1689 aneurysms) from 6 countries were identified. Eight studies investigated the use of the somatosensory evoked potential (SSEP) monitoring technique, 5 investigated transcranial motor evoked potential (TcMEP) and another 5 investigated direct cortical motor evoked potential (DMEP). Bivariable pooled sensitivity and specificity were 48% (95% confidence interval [CI], 30.7-65.0) and 92% (CI, 88%-94.4%), respectively, for SSEP; 73% (CI, 21.0%-96.7%) and 94% (CI, 87.1%-97.5%) for TcMEP; and 97% (CI, 74.43%-99.99%) and 89% (CI, 84.0%-94.5%) for DMEP. ROC curve analysis showed that TcMEP had the highest accuracy (area under ROC curve 0.95; 95% CI, 0.93-0.97), followed by DMEP (0.91, 0.89-0.94) and SSEP (0.88, 0.85-0.91).

CONCLUSIONS

TcMEP and DMEP have higher diagnostic accuracy than SSEP in predicting postoperative neurologic deficit. The type of anesthetic agent, the use of neuromuscular blocking drugs, and the choice of diagnostic criteria for significant change in cerebral blood flow during aneurysm surgery affect the diagnostic accuracy of evoked potential techniques in predicting postoperative neurologic deficit.

Predictive Value of Motor Evoked Potential Monitoring during Surgery of Unruptured Anterior Circulation Cerebral Aneurysms

Objective and Background:

Surgery of unruptured aneurysms is always a great challenge to neurovascular surgeons because no postoperative neurological deficits should be expected postoperatively as the patients are fully asymptomatic before the surgery. Here, we present our experience with selective motor evoked potential (MEP) monitoring of our patients in a 2-year time window.

Patients and Methods:

From 2012 to 2014, 27 patients with unruptured intracranial aneurysms were operated in our institute with the help of MEP monitoring. All patients underwent endoscope-assisted microsurgery with pre- and post-clipping indocyanine green angiography.

Results:

In this period, no mortality was observed, but 18.5% of the patients developed postoperative deficits which showed good recovery in all cases. Overall, MEP showed about 90% accuracy in predicting postoperative deficits.

Conclusions:

MEP as a part of multimodality monitoring of aneurysm surgeries is a valuable tool to improve the outcome. However, we should know its limitations as its results are not always consistent with the outcome.

Comparison of effectiveness between cork-screw and peg-screw electrodes for transcranial motor evoked potential monitoring using the finite element method

Background:

Intraoperative monitoring of motor evoked potentials by transcranial electric stimulation is popular in neurosurgery for monitoring motor function preservation. Some authors have reported that the peg-screw electrodes screwed into the skull can more effectively conduct current to the brain compared to subdermal cork-screw electrodes screwed into the skin. The aim of this study was to investigate the influence of electrode design on transcranial motor evoked potential monitoring. We estimated differences in effectiveness between the cork-screw electrode, peg-screw electrode, and cortical electrode to produce electric fields in the brain.

Methods:

We used the finite element method to visualize electric fields in the brain generated by transcranial electric stimulation using realistic three-dimensional head models developed from T1-weighted images. Surfaces from five layers of the head were separated as accurately as possible. We created the “cork-screws model,” “1 peg-screw model,” “peg-screws model,” and “cortical electrode model”.

Results:

Electric fields in the brain radially diffused from the brain surface at a maximum just below the electrodes in coronal sections. The coronal sections and surface views of the brain showed higher electric field distributions under the peg-screw compared to the cork-screw. An extremely high electric field was observed under cortical electrodes.

Conclusion:

Our main finding was that the intensity of electric fields in the brain are higher in the peg-screw model than the cork-screw model.

Effects of transcranial stimulating electrode montages over the head for lower-extremity transcranial motor evoked potential monitoring

OBJECTIVE The aim of this study was to determine the most effective electrode montage to elicit lower-extremity transcranial motor evoked potentials (LE-tMEPs) using a minimum stimulation current. METHODS A realistic 3D head model was created from T1-weighted images. Finite element methods were used to visualize the electric field in the brain, which was generated by transcranial electrical stimulation via 4 electrode montage models. The stimulation threshold level of LE-tMEPs in 52 patients was also studied in a practical clinical setting to determine the effects of each electrode montage. RESULTS The electric field in the brain radially diffused from the brain surface at a maximum just below the electrodes in the finite element models. The Cz-inion electrode montage generated a centrally distributed high electric field with a current direction longitudinal and parallel to most of the pyramidal tract fibers of the lower extremity. These features seemed to be effective in igniting LE-tMEPs. Threshold level recordings of LE-tMEPs revealed that the Cz-inion electrode montage had a lower threshold on average than the C3-C4 montage, 76.5 ± 20.6 mA and 86.2 ± 20.6 mA, respectively (31 patients, t = 4.045, p < 0.001, paired t-test). In 23 (74.2%) of 31 cases, the Cz-inion montage could elicit LE-tMEPs at a lower threshold than C3-C4. CONCLUSIONS The C3-C4 and C1-C2 electrode montages are the standard for tMEP monitoring in neurosurgery, but the Cz-inion montage showed lower thresholds for the generation of LE-tMEPs. The Cz-inion electrode montage should be a good alternative for LE-tMEP monitoring when the C3-C4 has trouble igniting LE-tMEPs.

Detection of Cerebral Vasospasm Following Aneurysmal Subarachnoid Hemorrhage Using Motor Evoked Potentials

BACKGROUND

Early detection of vasospasm (VS) following aneurysmal subarachnoid hemorrhage (aSAH) is vital to trigger therapy and to prevent infarction and subsequent permanent neurological deficit. Although motor evoked potentials (MEPs) are a well-established method for intraoperative detection of cerebral VS and cerebral ischemia during aneurysm surgery, there are no studies investigating the diagnostic value of MEPs for detecting delayed VS following aSAH in an intensive care unit.

OBJECTIVE

A prospective study was conceived to assess the diagnostic accuracy of MEPs in comparison with digital subtraction angiography.

METHODS

MEP threshold changes were determined in patients both with and without angiographic VS following high-grade aSAHs. Sensitivity, specificity, and the positive and negative predictive values of significant MEP threshold increases, which indicate angiographic VS, were calculated.

RESULTS

In all patients experiencing VS of the arteries supplying cerebral motor areas, a minimal MEP threshold increase of 50 mA (mean 66.25 mA) was observed, whereas a maximum MEP threshold increase of 30 mA was observed in patients without VS. Therefore, an increase from a baseline of ≥50 mA was considered significant and resulted in a sensitivity of 0.83, a specificity of 0.92, a positive predictive value of 0.83, and a negative predictive value of 0.92.

CONCLUSION

VS following aSAH can be detected accurately by using MEPs. MEPs are a feasible bedside tool for online VS detection in an intensive care unit and, therefore, may complement existing diagnostic tools.

Real-Time Evaluation of Anterior Choroidal Artery Patency During Aneurysm Clipping

Inadvertent occlusion of the anterior choroidal artery during aneurysm clipping can cause a disabling stroke in minutes. We evaluate the clinical utility of direct cortical motor evoked potential (MEP) monitoring during aneurysm clipping, as a real-time assessment of arterial patency, prior to performing indocyanine green videoangiography.  

Direct cortical MEPs were recorded in seven patients undergoing surgery for aneurysms that involved or abutted the anterior choroidal artery. The aneurysms clipped in those seven patients included four anterior choroidal artery aneurysms and six posterior communicating artery aneurysms. Serial MEP recordings were performed during the intradural dissection, aneurysm exposure, and clip placement. A significant change in MEPs after clip placement would prompt immediate inspection and removal or repositioning of the clip. If the clip placement appeared satisfactory and MEP recordings were stable, then an intraoperative indocyanine green videoangiogram was performed to confirm obliteration of the aneurysm and patency of the arteries. 

Seven patients underwent successful clipping of anterior choroidal artery aneurysms and posterior communicating artery aneurysms using direct cortical MEP monitoring, with good clinical and radiographic outcomes. In six patients, no changes in MEP amplitudes were observed following permanent clip placement. In one patient, a profound decrease in MEP amplitude occurred 220 seconds after placement of a permanent clip on a large posterior communicating aneurysm. An inspection revealed that the anterior choroidal artery was kinked. The clip was immediately removed, and the MEP signals returned to baseline shortly thereafter. A clip was then optimally placed, and the patient awoke without neurologic deficit. 

Direct cortical MEPs are a useful adjunct to standard electrophysiologic monitoring in aneurysm surgery, particularly when the anterior choroidal artery or lenticulostriate arteries are at risk. When these arteries are occluded, infarction may occur before the occlusion is detected by indocyanine green videoangiography or intraoperative angiography. The use of MEPs allows real-time detection of ischemia to subcortical motor pathways.

Microvascular decompression for glossopharyngeal neuralgia using intraoperative neurophysiological monitoring: Technical case report

Background:

Glossopharyngeal neuralgia (GN) is a rare functional disorder representing around 1% of cases of trigeminal neuralgia. Lancinating throat and ear pain while swallowing are the typical manifestations, and are initially treated using anticonvulsants such as carbamazepine. Medically refractory GN is treated surgically. Microvascular decompression (MVD) is reportedly effective against GN, superseding rhizotomy and tractotomy.

Methods:

We encountered three patients with medically refractory GN who underwent MVD using intraoperative neurophysiological monitoring (IONM). The offending vessels were the posterior inferior cerebellar arteries, which were confirmed intraoperatively via a transcondylar fossa approach to be affecting the root exit zones of the glossopharyngeal and vagus nerves. As IONM, facial motor-evoked potentials (MEPs) and brainstem auditory-evoked potentials were monitored during microsurgery in all three patients. Pharyngeal and vagal MEPs were added for two patients to avoid postoperative dysphagia.

Results:

GN disappeared immediately after surgery with complete preservation of hearing acuity and facial nerve function. Transient mild swallowing disturbance was observed in 1 patient without pharyngeal or vagal MEPs, whereas the remaining two patients with pharyngeal and vagal MEPs demonstrated no postoperative dysphagia.

Conclusion:

Although control of severe pain is expected in surgical intervention for GN, lower cranial nerves are easily damaged because of their fragility, even in MVD. IONM including pharyngeal and vagal MEPs appears very useful for avoiding postoperative sequelae during MVD for GN.

Strategies and Pitfalls of Motor-Evoked Potential Monitoring during Supratentorial Aneurysm Surgery

BACKGROUND

The aims of this study were to reveal the strategies and pitfalls of motor-evoked potential (MEP) monitoring methods during supratentorial aneurysm surgery, and to discuss the drawbacks and advantages of each method by reviewing our experiences.

METHODS

Intraoperative MEP monitoring was performed in 250 patients. Results from 4 monitoring techniques using combinations of 2 stimulation sites and 2 recording sites were analyzed retrospectively.

RESULTS

MEP was recorded successfully in 243 patients (97.2%). Direct cortical stimulation (DCS)-spinal recorded MEP (sMEP) was used in 134 patients, DCS-muscle recorded MEP (mMEP) in 97, transcranial electrical stimulation (TES)-mMEP in 11 and TES-sMEP in 1. TES-mMEP during closure of the skull was used in 21 patients. DCS-mMEP was able to detect waveforms from upper and/or lower limb muscles. Alternatively, DCS-sMEP (direct [D]-wave) could accurately estimate amplitude changes. A novel "early warning sign" indicating ischemia was found in 21 patients, which started with a transiently increased amplitude of D-wave and then decreased after proximal interruption of major arteries. False-negative findings in MEP monitoring in 2 patients were caused by a blood insufficiency in the lenticulostriate artery and by a TES-sMEP recording, respectively.

CONCLUSIONS

The results of this study suggest that to perform accurate MEP monitoring, DCS-mMEP or DCS-sMEP recording should be used as the situation demands, with combined use of TES-mMEP recording during closure of the skull. DCS-sMEP is recommended for accurate analysis of waveforms. We also propose a novel "early warning sign" of blood insufficiency in the D-wave.

Visualization of the electric field evoked by transcranial electric stimulation during a craniotomy using the finite element method

BACKGROUND

Transcranial MEP (tMEP) monitoring is more readily performed than cortical MEP (cMEP), however, tMEP is considered as less accurate than cMEP. The craniotomy procedure and changes in CSF levels must affect current spread. These changes can impair the accuracy. The aim of this study was to investigate the influence of skull deformation and cerebrospinal fluid (CSF) decrease on tMEP monitoring during frontotemporal craniotomy.

METHODS

We used the finite element method to visualize the electric field in the brain, which was generated by transcranial electric stimulation, using realistic 3-dimensional head models developed from T1-weighted images. Surfaces of 5 layers of the head were separated as accurately as possible. We created 3 brain types and 5 craniotomy models.

RESULTS

The electric field in the brain radiates out from the cortex just below the electrodes. When the CSF layer is thick, a decrease in CSF volume and depression of CSF surface level during the craniotomy has a major impact on the electric field. When the CSF layer is thin and the distance between the skull and brain is short, the craniotomy has a larger effect on the electric field than the CSF decrease.

COMPARISON WITH EXISTING METHOD

So far no report in the literature the electric field during intraoperative tMEP using a 3-dimensional realistic head model.

CONCLUSION

Our main finding was that the intensity of the electric field in the brain is most affected by changes in the thickness and volume of the CSF layer.

Role of endoscopy in multi-modality monitoring during aneurysm surgery: A single center experience with 175 consecutive unruptured aneurysms

Objective and Background:

Unruptured aneurysm surgery is a challenge to all vascular neurosurgeons as the patient is asymptomatic and no even slight neurological deficits should be expected postoperatively. To this aim, multi-modality checking of the vessels during the surgery is highly recommended to assure of the patency of the parent and perforator arteries next to an aneurysm. In this paper, we present our experience in the last 1.5 years with emphasis on the role of endoscope assisted microsurgery.

Methods:

One hundred and seventy-five patients with unruptured intracranial aneurysms were operated in our institute in the last 1½ years. All patients underwent endoscope assisted microsurgery with pre- and post-clipping indocyanine green angiography. In selected cases, motor evoked potential monitoring was implemented.

Results:

No mortality was observed in this period, and only 6 patients (3.4%) suffered new permanent neurological deficits postoperatively. Our illustrative cases show how endoscopy may help the surgeon to visualize hidden vessels behind and medial to an aneurysm.

Conclusions:

Our results indicated that multi-modality monitoring during unruptured aneurysm surgeries is associated with excellent outcome. Endoscope is able to show blind corners during aneurysm surgery which cannot be routinely observed with microscope and its application in aneurysm surgery assists the surgeon to make certain of complete neck clipping and preservation of perforating arteries around the aneurysm.

The significance of intraoperative monitoring of muscle motor evoked potentials during unruptured large and giant cerebral aneurysm surgery

The goal of this study was to characterize the utility of muscle motor evoked potentials (MMEPs) elicited by direct cortical stimulation as a means of monitoring during unruptured large and giant cerebral aneurysm surgery. This analysis focused on intraoperative changes in MMEPs and their relationship to postoperative motor function. The study population consisted of 50 patients who underwent surgery for large (n = 31) or giant (n = 19) cerebral aneurysms. Intraoperative MMEPs were continuously and successfully obtained in muscles belonging to the vascular territory of interest. There was no postoperative motor paresis in 31 (62%) patients in whom intraoperative MMEPs remained unchanged. Transient MMEP change occurred in 15 (30%) of the 50 patients, but 9 of those patients had no postoperative motor deficits, 5 had transient motor deficits, and 1 suffered permanent motor deficits resulting from postoperative delayed blood flow insufficiency due to arteriosclerosis of the parent artery. Permanent MMEP loss occurred in 4 (8%) of 50 patients, all of whom developed severe and permanent postoperative motor deficits. MMEP is a useful monitoring modality in patients undergoing surgery for large or giant cerebral aneurysms. This strategy can help predict functional prognosis or guide the neurosurgeon intraoperatively in an effort to promote better outcomes.

Facial corticobulbar motor-evoked potential monitoring during the clipping of large and giant aneurysms of the anterior circulation

Surgical outcomes for large and giant intracranial aneurysms are suboptimal. Two important reasons for higher complication rates are either occlusion of perforators or parent arteries during aneurysm clipping, or prolonged temporary occlusion of the main arteries. Somatosensory-evoked potential (SSEP) monitoring and transcranial motor-evoked potential (TcMEP) monitoring are standard techniques for monitoring ischemia either during temporary arterial occlusion or after permanent clipping. In our study, facial corticobulbar motor-evoked potential (FCoMEP) monitoring was included to determine whether this modality improved intraoperative monitoring. FCoMEP were recorded intraoperatively in 21 patients undergoing surgical clipping of large and giant aneurysms of the anterior circulation. Valid TcMEP parameters were obtained for all patients. A correlation tending to significance between a prolonged temporary clipping time and TcMEP decrement was observed. In addition to this, the inclusion of FCoMEP improved the sensitivity of extremity muscle motor-evoked potential (ExMEP, which included TcMEP) monitoring (from 80% to 100%). In the long-term assessment, a favorable outcome was achieved in 16 of the 21 patients (76%). In conclusion, FCoMEP provides complementary corticobulbar tract information for detecting perforating vessel compromise that may lead to motor impairment and that is not identified by ExMEP.

Motor evoked potential monitoring of the vagus nerve with transcranial electrical stimulation during skull base surgeries

Object

Dysphasia is one of the most serious complications of skull base surgeries and results from damage to the brainstem and/or cranial nerves involved in swallowing. Here, the authors propose a method to monitor the function of the vagus nerve using endotracheal tube surface electrodes and transcranial electrical stimulation during skull base surgeries.

Methods

Fifteen patients with skull base or brainstem tumors were enrolled. The authors used surface electrodes of an endotracheal tube to record compound electromyographic responses from the vocalis muscle. Motor neurons were stimulated using corkscrew electrodes placed subdermally on the scalp at C3 and C4. During surgery, the operator received a warning when the amplitude of the vagal motor evoked potential (MEP) decreased to less than 50% of the control level. After surgery, swallowing function was assessed clinically using grading criteria.

Results

In 5 patients, vagal MEP amplitude permanently deteriorated to less than 50% of the control level on the right side when meningiomas were dissected from the pons or basilar artery, or when a schwannoma was dissected from the vagal rootlets. These 5 patients had postoperative dysphagia. At 4 weeks after surgery, 2 patients still had dysphagia. In 2 patients, vagal MEPs of one side transiently disappeared when the tumors were dissected from the brainstem or the vagal rootlets. After surgery, both patients had dysphagia, which recovered in 4 weeks. In 7 patients, MEP amplitude was consistent, maintaining more than 50% of the control level throughout the operative procedures. After surgery all 7 patients were neurologically intact with normal swallowing function.

Conclusions

Vagal MEP monitoring with transcranial electrical stimulation and endotracheal tube electrode recording was a safe and effective method to provide continuous real-time information on the integrity of both the supranuclear and infranuclear vagal pathway. This method is useful to prevent intraoperative injury of the brainstem corticobulbar tract or the vagal rootlets and to avoid the postoperative dysphagia that is often associated with brainstem or skull base surgeries.