Corticobulbar Motor Evoked Potentials Research Articles

Predictive value of intraoperative vagus nerve corticobulbar motor evoked potentials to assess the risk of dysphagia in fourth ventricle surgery.

Dysphagia is a significant complication in fourth ventricle surgery. Corticobulbar motor evoked potentials (CB-MEPs) of the lower cranial nerves may provide real-time information possibly correlating with postoperative swallowing dysfunction, and the vagus nerves may prove ideal for this purpose. However, the literature is heterogeneous, non-systematic, and inconclusive on this topic. The object of this retrospective study was to evaluate the correlation between CB-MEPs of the vagus nerve and postoperative worsening or new-onset swallowing deficits in intraaxial fourth ventricle surgery. In 21 consecutive patients undergoing surgery for fourth ventricle intraaxial tumors between February 2018 and October 2022, endotracheal tubes with two applied electrodes contacting the vocal cords were used to record vagus nerve MEPs including values at baseline, the end of surgery, and the minimum value during the operation. From the mean value of right and left vagus nerve MEP amplitudes, the minimum-to-baseline amplitude ratio (MBR) and final-to-baseline amplitude ratio (FBR) were calculated. These indexes were correlated with postoperative swallowing function. Given their clinical significance, receiver operating characteristic curves were obtained to evaluate the performance of these indexes in predicting postoperative swallowing function. The area under the curve (AUC) was 0.850 (p < 0.001) and the best cutoff for FBR was 67.55% for the worsening of swallowing in the postoperative period. The AUC was 0.750 (p = 0.026) and the best cutoff was 46.37% in MBR for the absence of a swallowing disorder at the late follow-up. This study confirmed that vagus nerve MEPs are reliable predictors of postoperative swallowing function in fourth ventricle surgery and can be feasibly used as an intraoperative monitoring technique.

Intraoperative Neurophysiologic Monitoring in Predicting Dysphagia After Brainstem and Fourth Ventricle Surgery

Dysphagia represents the main complication of posterior fossa neurosurgery. Adequate diagnosis of this complication is warranted to prevent untimely extubation with subsequent aspiration. Intraoperative neurophysiologic monitoring (IONM) modalities may be used for this purpose. However, it is not known which IONM modality may be significant for diagnosis. This study aimed to define the most significant IONM modality for dysphagia prognostication after posterior fossa neurosurgery. The analysis included 46 patients (34 with tumors of the fourth ventricle and 12 with brainstem localization) who underwent surgical excision of the tumor. Neurologic symptoms before and after neurosurgery were noted and magnetic resonance imaging with the subsequent volume estimation of the removed mass was performed, followed by an IONM findings analysis (mapping of the nucleus of the caudal cranial nerves [CN] and corticobulbar motor-evoked potentials [CoMEP]). Aggravation of dysphagia was noted in 24% of the patients, more often in patients with tumor localization in the fourth ventricle (26%) than in those with brainstem mass lesions (16%). Mapping of the caudal cranial nerve nuclei did not correlate with the dysfunction of these structures. CoMEP was significantly associated with the neurologic state of the CN. The decrease in CoMEP is a significant prognostic factor for postoperative bulbar symptoms appearance or aggravation. Mapping the CN is an important identification tool. The CoMEP modality should be used intraoperatively to determine the functional state of the CN and predict postoperative dysphagia.

Intraoperative Neuromonitoring for Thyroid Surgery in Children and Adolescents: A Single Center Experience.

Intraoperative neuromonitoring (IONM) of the recurrent laryngeal nerve (RLN) has been shown in adults to minimize nerve palsy after thyroid surgery, but only few studies on its efficacy in a pediatric population have been reported. We conducted a retrospective study on patients operated for thyroid lesions from 2016 to 2022. The analyzed population was divided in two groups: patients treated from 2016 to 2020, when the identification of the RLN was performed without IONM (Group A); and patients treated since 2021, when IONM was implemented in every surgical procedure on the thyroid (Group B). Intraoperative Neurophysiological Monitoring was performed by using corticobulbar motor-evoked potentials and continuous electromyography. Twentyfive children underwent thyroid resection, 19 (76%) of which due to thyroid carcinoma. Each patient's recurrent nerve was identified; IONM was used in 13 patients. In Group A, one temporary nerve palsy was identified postoperatively (8.3%), while in group B one nerve dysfunction occurred (7.7%). No statistically significant difference was found between the two groups in terms of post-operative RLN palsy. No surgical complication due to the use of IONM was reported. In children and teenagers, intraoperative neuromonitoring of the recurrent laryngeal nerve is a safe and accurate method, minimizing the risk of nerve damage.

Monitoring of corticobulbar motor evoked potentials during surgical remov¬al of brainstem and fourth ventricle tumors in pediatric patients

Introduction. Corticobulbar motor evoked potentials (CB-MEP) are a modality of intraoperative monitoring (IOM) for assessment of the functional state of caudal cranial nerves. CB-MEPs can be used during fossa posterior surgery, when there is a risk of damaging the caudal cranial nerves and their nuclei. The possibility of using this modality in pediatric patients is rarely discussed in the literature. Objective. To assess the CB-MEP validity in the prognosis of neurological symptoms from caudal cranial nerves in the pediatric group of patients after surgery in the brainstem and fourth ventricle. Material and methods. The study included 30 children aged from 11 months to 17 years who underwent brainstem and the fourth ventricle tumor surgery with IOM at the N.N. Burdenko National Medical Research Center for Neurosurgery from January 2020 to November 2021. Results. We obtained statistically significant difference in the dynamics of the CB-MEP amplitude in the groups of patients with and without an increase in neurological symptoms (p&lt;0.05). Conclusion. CB-MEP can be used during brainstem surgery in pediatric patients for assessing the functional state of caudal cranial nerves.

Intraoperative monitoring of facial corticobulbar motor evoked potentials: Methodological improvement and analysis of 100 patients

ObjectiveA) To describe an improved methodology for continuously monitoring the functional integrity of facial nerve by eliciting facial corticobulbar motor-evoked potentials (FCoMEP) and B) To establish the prognosis of facial nerve function based on changes in FCoMEP during skull base surgery. MethodsIntraoperative monitoring of FCoMEP performed in 100 patients. Previously published methodology has been improved upon by a) doing preoperative mapping of the facial nerve, b) facilitating the corticobulbar tract (CBT) by continuous transcranial electrical stimulation (TES) at 2 Hz repetition rate, c) recording from multiple facial nerve innervated muscles, and d) eliciting blink reflex (BR). We analyzed changes in FCoMEP, comparing them with the clinical facial nerve outcome scored with the House-Brackman (HB) scale. ResultsThe monitorability rate was 100%. Out of 100 patients, nine presented a new facial deficit after surgery. Eight of these showed significant changes in FCoMEP. In four patients FCoMEPs were lost; they presented a complete facial paralysis from which they did not recover. To discriminate the prognosis of patients, ROC analysis identified a cut-off at 65% for FCoMEPs amplitude decrease with a sensitivity of 89% and specificity of 99%. In four patients FCoMEP showed a decrease in amplitude greater than 65%, and they presented mild/moderate facial paresis that was transient. One patient did not present changes in FCoMEP but had a mild facial paresis from which the patient recovered. ConclusionsThe improved methodology allows the maximum rate of monitorability and minimizes false positive and false negative results. This study shows that prognosis of facial nerve may be reliably established based on FCoMEP parameters. SignificanceWe improved the previously described methodology for continuously monitoring the functional integrity of the facial nerve by increasing the monitorability rate, and we describe the impact of FCoMEP intraoperative management of facial nerve. This method may permit establishing the short-term and long-term prognosis of facial nerve function in skull base surgery.

Chapter 9 - Mapping and monitoring of brainstem surgery

The surgical morbidity of brainstem lesions is higher than in other areas of the central nervous system because the compact brainstem is highly concentrated with neural structures that are often distorted or even unrecognizable under microscopic view. Intraoperative neurophysiologic mapping helps identify critical neural structures to avoid damaging them. With the trans-fourth ventricular floor approach, identifying the facial colliculi and vagal and hypoglossal triangles enables incising and approaching the brainstem through the safe entry zones, the suprafacial or infrafacial triangle, with minimal injury. Corticospinal tract mapping is adopted in the case of brainstem surgery adjacent to the corticospinal tract. Intraoperative neurophysiologic monitoring techniques include motor evoked potentials (MEPs), corticobulbar MEPs, brainstem auditory evoked potentials, and somatosensory evoked potentials. These provide real-time feedback about the functional integrity of neural pathways, and the surgical team can reconsider and correct the surgical strategy accordingly. With multimodal mapping and monitoring, the brainstem is no longer "no man's land," and brainstem lesions can be treated surgically without formidable morbidity and mortality.

WS5.2. Intraoperative Monitoring of Motor Evoked Potentials: Physiology and Applications

Intraoperative monitoring (IOM) of motor evoked potentials (MEP) represents a significant advance for evaluation of descending motor tract integrity. For spinal decompression, there is increasing evidence of its efficacy, but ongoing research is performed to improve sensitivity and specificity. To that end, there is also ongoing debate for warning criteria of MEP changes from transcranial electrical stimulation (TES). It is presently unclear if a cutoff for ensuing neurological deficit should be MEP disappearance or a value between this and 50% amplitude reduction. Interim adjunctive strategies include use of ipsilateral MEPs, modified scalp stimulating positions and multimodality monitoring. Some investigators have proposed a more directed motor threshold to obtain supramaximal MEP voltage in an individual subject. The indications of MEP monitoring have expanded from spinal surgery to central nervous system procedures involving cortical, subcortical and brainstem regions. Corticobulbar MEPs can provide localization and evaluate functional integrity of cranial nerve nuclei. Awake craniotomy can combine monopolar MEP mapping with other modalities to preserve speech and motor functions. The evidence of MEP monitoring for scoliosis to date has been encouraging for the prevention of neurological compromise, as a result of deformity correction. IOM of MEPs is pertinent here as a non-invasive surgical adjunct for these neurologically intact patients. Recently, a study has suggested that in susceptible individuals, intraoperative suppression of MEPs may rarely occur unpredictably, independent of surgical or anesthetic intervention, but anesthetic factors may be overall more significant. Side effects of TES are rare, but seizures, bite injury, lip laceration, mandibular fracture, scalp burns and cardiac complications should be anticipated. Improved knowledge in the use of intravenous anesthetic agents and pulse train cortical stimuli has allowed for more efficacious MEP monitoring. However, recent studies suggest that often, suboptimal MEPs were elicited, especially during IOM of diseased spinal cord. Various central and peripheral facilitatory techniques will be discussed.

Intraoperative Neuromonitoring of Blink Reflex During Posterior Fossa Surgeries and Its Correlation With Clinical Outcome

Blink reflex (BR) under general anesthesia as an intraoperative neuromonitoring method was used to monitor facial nerves in few studies. This study aimed to test the utility of intraoperative BR during cerebellopontine angle and skull base surgeries, assess its prognostic value for facial nerve functions, and compare it with facial corticobulbar motor evoked potentials (CoMEPs). Blink reflex and facial CoMEPs were recorded from 40 patients undergoing skull base surgeries. Subdermal needles were placed in the supraorbital notch for stimulation and in the orbicularis oculi muscle for recording the BR. A double train of 20 to 40 V intensity with an intertrain interval of 40 to 60 milliseconds, an interstimulus interval of 2.5 milliseconds, and a stimulus duration of 0.5 milliseconds were applied. Facial nerve functions were assessed with the House-Brackmann grading system in the postoperative day 1 and third-month period and correlated with intraoperative BR and CoMEPs measurements. Of 40 patients, BR was recordable on the affected side in 32 (80%) and contralateral side in 35 (87.5%) patients. According to our statistical results, BR had a slightly better sensitivity than facial CoMEPs in predicting impairment of facial nerve functions for both postoperative and third-month time points. Blink reflex showed better accuracy for predicting postoperative nerve functions, whereas CoMEPs correlated better in predicting third-month outcome. We suggest that BR is a valuable intraoperative neuromonitoring method that can be used in addition to facial CoMEPs during skull base surgeries to assess real-time facial nerve integrity and predict prognosis.

T99. Assessing lower brainstem and vagal nerve integrity during posterior fossa surgery by using the laryngeal adductor reflex. Escalating intraoperative neuromonitoring of the lower brainstem

Surgery-related to lesions in or around the brainstem constitutes a challenge due to the extreme concentration of critical neural structures and the high risk of iatrogenic damage during surgery. Intraoperative neurophysiological monitoring (IONM) have increased the feasibility and safety of brainstem surgery. We present a case of a patient who underwent posterior fossa surgery in where the laryngeal adductor reflex (LAR) was used to assess both lower brainstem and vagal nerve integrity. A 62-year-old female scheduled for microsurgical resection of large left-Vestibular -Schwannoma with IOM. Anesthesia was maintained with TIVA. We applied LAR as a technique that provides functional information of brainstem and vagal nerve integrity, combined with other techniques currently used: Motor evoked potentials (MEP), somatosensory evoked potentials (SSEP), corticobulbar motor-evoked potentials (Co-MEPs), brainstem auditory-evoked potentials (BAER), blink reflex (BR) and cranial nerve mapping. LAR was elicited by a single electrical stimulus of the laryngeal mucosa (contralateral side to the operative field) using the electrodes attached to the endotracheal tube. Surface electrodes ipsilateral to the surgical field attached to the ETT were used to record the contralateral R1 (cR1) and R2 (cR2) responses of the LAR. Significant changes in LAR amplitude responses were the critical parameters for IONM interpretation. All MEP - SSEP – CoMEP – LAR - BR responses, were elicited bilaterally at the onset of surgery. During resection, the left trigeminal nerve was stretched, patient’s blood pressure (BP) and heart rate (HR) dropped drastically and concomitantly left blink reflex responses mildly decreased. The surgeon was informed immediately, surgical stimulation was discontinued. To counteract the bradycardia, atropine sulfate was administered; following with HR increased. Soon later, left blink reflex responses returned to baseline. There were no other IONM changes related to the event. Later, during the dissection of tumor adjacent to the left vagus nerve, severe BP and HR drop noticed. Once again, surgical stimulation was discontinued. Sinus rhythm, returned immediately after the procedure was stopped. The patient was given ephedrine again to counteract the bradycardia. At that time, bilateral LAR and ipsilateral vocal CoMEP responses decreased, with improvement when hemodynamic parameters returned to normal. During the end of resection, there were several trains of neurotonic discharges observed in the VII cranial nerve. Due to several IONM events and hemodynamic changes during the procedure, the surgical intervention was discontinued. LAR is a novel method for assessing lower brainstem and vagus nerve integrity that can be useful for IONM during brainstem surgery; it shows consistent results and adds a new tool in the armamentarium of posterior fossa IONM where vagal nerve function may be compromised.

5. Vocal cord paralysis in children: A new neurophysiological approach for diagnosis and functional prognosis

To establish the possibility to use the methodology of recording corticobulbar motor evoked potentials (CoMEPs) from cricothyroid muscles (CTHY) and vocal cord elicited by transcranial electrical stimulation (TES), as diagnostic tool for predicting function recovery in children with vocal fold immobility (VFI). We included six children (mean age: 4.7 years; range: 2–9 years) with diagnosis of cord immobility that underwent video-fibro-endoscopy procedure. Under general anaesthesia, we used TES by stimulation over C3/Cz or C4/Cz to elicit CoMEPs. To record CoMEPs from vocal muscles we used two hook wire electrodes (76 micron diameter) passing through a 27-gauge needle endotracheally placed in the vocal muscles after intubation. Recording of CoMEPs in the vocal muscles after TES was successfully performed bilaterally in 4 patients and monolaterally in two patients. TES over the right or left hemisphere elicited responses bilaterally. The mean onset latency for the vocal muscles was 16.4 ± 3 ms. The possibility to record CoMEP from vocal muscle was a positive prognostic feature for clinical improvement. This method shows the ability to evaluate the functional integrity of corticobulbar pathways in a reliable manner and adds an additional tool in the diagnosis and prognosis of VFI.

Neurophysiologic markers of primary motor cortex for laryngeal muscles and premotor cortex in caudal opercular part of inferior frontal gyrus investigated in motor speech disorder: a navigated transcranial magnetic stimulation (TMS) study.

Transcranial magnetic stimulation studies have so far reported the results of mapping the primary motor cortex (M1) for hand and tongue muscles in stuttering disorder. This study was designed to evaluate the feasibility of repetitive navigated transcranial magnetic stimulation (rTMS) for locating the M1 for laryngeal muscle and premotor cortical area in the caudal opercular part of inferior frontal gyrus, corresponding to Broca's area in stuttering subjects by applying new methodology for mapping these motor speech areas. Sixteen stuttering and eleven control subjects underwent rTMS motor speech mapping using modified patterned rTMS. The subjects performed visual object naming task during rTMS applied to the (a) left M1 for laryngeal muscles for recording corticobulbar motor-evoked potentials (CoMEP) from cricothyroid muscle and (b) left premotor cortical area in the caudal opercular part of inferior frontal gyrus while recording long latency responses (LLR) from cricothyroid muscle. The latency of CoMEP in control subjects was 11.75±2.07ms and CoMEP amplitude was 294.47±208.87µV, and in stuttering subjects CoMEP latency was 12.13±0.75ms and 504.64±487.93µV CoMEP amplitude. The latency of LLR in control subjects was 52.8±8.6ms and 54.95±4.86 in stuttering subjects. No significant differences were found in CoMEP latency, CoMEP amplitude, and LLR latency between stuttering and control-fluent speakers. These results indicate there are probably no differences in stuttering compared to controls in functional anatomy of the pathway used for transmission of information from premotor cortex to the M1 cortices for laryngeal muscle representation and from there via corticobulbar tract to laryngeal muscles.

Implementation of Intraoperative Neurophysiological Monitoring during Endovascular Procedures in the Central Nervous System

Background and Objective: Intraoperative monitoring (IOM) has been used in different surgical disciplines since the 1980s. Nonetheless, regular routine use of IOM in interventional neuroradiology units has only been reported in a few centers. The aim of this study is to report our experience, 1 year after deciding to implement standardized IOM during endovascular treatment of vascular abnormalities of the central nervous system. Methods: Basic recordings included somatosensory-evoked potentials (SEPs) and motor-evoked potentials (MEPs). Corticobulbar motor-evoked potentials and flash-visual-evoked potentials were also recorded depending on the topography of the lesion. Intra-arterial provocative tests (PTs) with amobarbital and lidocaine were also performed. All patients except 1 were under total intravenous anesthesia. Clinical outcome was assessed prospectively and correlated with IOM events. Results: Twelve patients and 15 procedures were monitored during the inclusion period. Significant IOM events were detected during 3 of the 15 procedures (20%). We observed temporary MEP changes in 2 cases which resolved after interruption of the embolization or application of corrective measures, leaving no postoperative neurological deficits. In 1 case, persistent SEP and MEP deterioration was detected secondary to a frontal hematoma, resulting in mild sensory-motor deficit in the right upper extremity after the procedure. Overall, 12 PTs (4 spinal cord and 8 brain abnormalities) were performed using lidocaine and sodium amytal injections. One positive result occurred after the injection of lidocaine. No false negatives were detected. Conclusions: IOM may provide continuous real-time data about the functional status of eloquent areas and pathways of the central nervous system in patients under general anesthesia. It therefore allows us to detect early neurological damage in time to perform specific actions that may prevent irreversible neurological deficits.

Excitability of contralateral and ipsilateral projections of corticobulbar pathways recorded as corticobulbar motor evoked potentials of the cricothyroid muscles

ObjectiveThe objective of this study was to evaluate the excitability of contralateral and ipsilateral corticobulbar pathways, using the methodologies of navigated transcranial magnetic stimulation (nTMS) and transcranial electrical stimulation (TES). MethodsIn 11 healthy subjects, the primary motor cortex (M1) for laryngeal muscles was mapped with nTMS in both hemispheres and the corticobulbar motor evoked potentials (CoMEPs) were recorded from the right cricothyroid muscle. In 15 patients undergoing left craniotomy, CoMEPs were obtained from cricothyroid muscles bilaterally, using TES over C3/Cz and C4/Cz. ResultsIn five out of 11 healthy subjects, both contralateral and ipsilateral CoMEPs were recorded from the right cricothyroid muscle. In eight out of 15 patients, contralateral and ipsilateral CoMEPs were elicited with TES over C3/Cz, while in five out of 15 patients contralateral and ipsilateral CoMEPs were elicited with TES over C4/Cz. Contralateral CoMEP amplitude responses were significantly larger compared to ipsilateral CoMEP amplitudes in both groups. ConclusionWe obtained significantly larger amplitude responses of contralateral CoMEPs from cricothyroid muscles compared to ipsilateral CoMEP amplitude using nTMS in healthy subjects and TES in patients. This confirms the bilateral nature of corticobulbar pathway projections for laryngeal muscles, with contralateral domination. SignificanceThese findings will influence recording of CoMEPs during preoperative and intraoperative mapping of M1 for laryngeal muscle representation, and they facilitate the pathophysiologic research of motor speech disorders.

P40. Intraoperative monitoring of corticobulbar motor evoked potentials in children

The motor function of cranial nerves (CN) can be continuously monitored by transcranial corticobulbar motor evoked potentials (CoMEPs) during neurosurgical interventions. Despite their advantages, CoMEPs are not yet widely used. While there are several publications of CoMEPs in adults (Sarnthein et al., 2013), the feasibility and safety of CoMEPs in children has not yet been documented. We included 13 consecutive procedures involving 12 patients (median age 5 y, range 1–15 y, 4 female) that were operated since 2013 and in whom CoMEPs were monitored. While most authors use a 50% reduction of CoMEP response amplitudes as a warning criterion, our approach was to keep the response amplitude constant by increasing the stimulation intensity and to establish a warning criterion based on the “threshold-level” method. For the facial nerve, a threshold increase greater than 20 mA for eliciting CoMEPs in the most reliable facial nerve target muscle was considered a prediction of reduced postoperative facial nerve function, and subsequently a warning was issued to the surgeon. Preoperative and early postoperative function was documented using the House-Brackmann (HB) grading system. Monitoring of CoMEPs was feasible in all 13 surgeries in at least one facial nerve target muscle. The mentalis muscle yielded the best result (92% of trials), followed by orbicularis oris (91%) and orbicularis oculi muscles (86%). The median stimulation threshold was initially 60 mA (range 40–100 mA) for CoMEPs and 60 mA (15–95 mA) for MEP of the thenar muscles. The initial CoMEP threshold exceeded the MEP threshold in 7/13 cases. CoMEP deterioration showed specificity for HB deterioration of 83%. Intraoperative CoMEP monitoring is feasible and safe also in young children. Stable facial nerve CoMEP predict stable HB grade. We found no evidence that procedures and thresholds should differ from CoMEP in adults. CoMEP monitoring is a valid indicator of CN function in skull base surgery. It should be used as an adjunct to direct electrical CN stimulation and continuous EMG monitoring of CN target muscles.

Representation of cricothyroid muscles at the primary motor cortex (M1) in healthy subjects, mapped by navigated transcranial magnetic stimulation (nTMS)

ObjectivesTo establish a methodology for mapping of primary motor cortex (M1) for cricothyroid (CTHY) muscles in a group of healthy subjects using three-dimensional (3D) magnetic resonance imaging (MRI) navigated transcranial magnetic stimulation (nTMS). MethodsTwo independent measurements were performed. Twelve right-handed healthy subjects were included in the study. In the first measurement, mapping of the abductor pollicis brevis (APB) muscle was followed by mapping of the M1 for CTHY. This was performed in 11 subjects. Second, to avoid bias concerning using a hand knob as a landmark, mapping of M1 for CTHY muscle was followed by mapping of M1 for APB. This was performed in three healthy subjects.The nTMS was used, with selective recordings of motor evoked potentials (MEPs) from APB muscle and corticobulbar motor evoked potentials (CoMEPs) from the CTHY muscle. For recording the responses from the CTHY muscle two hook wire electrodes (the size of 76 μm of diametre passing through 27 gauge needle) were inserted in the muscle. For the recording of MEPs from APB muscle, surface electrodes were used. ResultsFirst measurement: Stimulation over the left M1 for APB muscles elicits MEPs in the contralateral APB muscle with a mean latency of 22.8±1.69ms. Stimulation over the left M1 for the CTHY muscle elicits CoMEPs in the contralateral CTHY muscle with a mean latency of 11.89±1.26ms. The distance between the cortical representation for APB and CTHY was 25.19±6.52mm, with CTHY muscle representation lateral to the APB muscle.Second measurement: The results of second measurement of the distance between M1 for CTHY and M1 for APB and their cortical localisation were comparable to the results of the first measurement. ConclusionThis is the first study with the aim to determine the exact cortical localisation of CTHY muscle with nTMS. Mapping of M1 for CTHY and APB muscles by nTMS was successfully performed in all healthy subjects. The exact location of the stimulating points over M1 muscles eliciting responses in CTHY and APB muscles was determined and superimposed over 3D MRI images. The data show that M1 for CTHY muscle is about 25mm more lateral with regard to M1 for the APB muscle. SignificanceMapping of M1 for CTHY muscle might represent an important neurophysiologic marker for facilitating preoperative mapping of motor speech-related cortical areas due to the proximity of motor cortical representation for laryngeal muscles and opercular part of the Broca area.

Continuous Monitoring of Corticobulbar Motor-Evoked Potentials of Vagus Nerve during Brainstem or Skull Base Surgery

Objective: Skull base surgery can lead to dysphagia as a result of intraoperative damage to brainstem centers and/or cranial nerves involved in swallowing.

Methodology for intra-operative recording of the corticobulbar motor evoked potentials from cricothyroid muscles

Objective To establish the methodology for recording corticobulbar motor evoked potentials (CoMEPs) from cricothyroid muscles (CTHY) elicited by transcranial electrical (TES) and direct cortical stimulation (DCS). Methods Six healthy subjects and 18 patients undergoing brain surgery were included in the study. In six healthy subjects as well as in nine patients under general anaesthesia, CoMEP was obtained by TES. In three patients under general anaesthesia and in six patients during awake craniotomy, CoMEP was obtained by DCS. We used three methods of electrical stimulation for eliciting responses from CTHY muscles: (1) TES over C3/Cz or C4/Cz (in six healthy subjects and nine patients under general anaesthesia), (2) DCS with a strip electrodes placed over exposed cortex in two patients, and (3) DCS with a hand-held probe in seven patients. For recordings, we percutaneously placed hook-wire electrodes in CTHY muscle (76 μm diameter) passing through 27-gauge needles, guided by electromyogram (EMG) feedback. In anaesthetised patients, recording electrodes were placed before patients were put to sleep. Results Recordings of CoMEPs by TES was successfully performed in all but one healthy subject and all patients. Either method of stimulation resulted in recordings of short- and long-latency responses (SLRs and LLRs) in CTHY muscle. After DCS, SLRs were recorded in all patients but two, in whom only LLR was obtained; in patient No. 1 ( Table 3) LLR was elicited by a strip electrode, while in patient No. 4 ( Table 3), LLR was elicited by a hand-held electrode. The possible explanation for eliciting only LLR in patient No. 1 was that the strip electrode did not cover the primary motor cortex. In patient No. 4, only LLR but not SLR was recorded, due to the underlying pathology which displaced M1 for cricothyroid muscle not being accessible to the DCS with the hand-held electrode. As each CTHY muscle is innervated from both hemispheres, CoMEPs were recorded from the left or right CTHY muscle during TES or DCS over either the left or right hemisphere. In healthy subjects, TES elicited SLR with a latency of 13.25 ± 1.38 ms, while in patients the latency was 12.73 ± 0.64 ms. In healthy subjects, TES elicited LLR with a latency of 40.53 ± 4.66 ms, and in patients, it elicited LLR with a latency of 44.70 ± 4.61 ms. The group of patients in whom the responses were elicited by DCS, SLR and LLR had similar latencies as responses elicited by TES; in the patient’s group the values were 13.97 ± 1.11 ms versus 12.73 ± 0.64 ms and 45.91 ± 3.88 ms versus 44.70 ± 4.61 ms, respectively. Conclusions Together with the existing methodology for intra-operatively eliciting CoMEPs in the vocal muscles, a new methodology for eliciting CoMEPs in CTHY muscle has the potential to continuously monitor the functional integrity of the structures involved in conveying signals from the motor cortex to the CTHY muscle. It is highly probable that SLR is a neurophysiological marker for the primary motor cortex (M1) and it is equal to CoMEP for laryngeal muscle, while LLR is a marker for the opercular part of Broca’s area. Significance This new and rather simple method adds a new tool in exploration of the functional organisation of motor cortex and corticobulbar pathways for laryngeal muscles. Furthermore, it has great potential to intra-operatively monitor its functional integrity.

Methodology for intraoperatively eliciting motor evoked potentials in the vocal muscles by electrical stimulation of the corticobulbar tract

To establish a methodology for recording corticobulbar motor evoked potentials (CoMEPs) from vocal muscles after transcranial electrical stimulation (TES) and direct cortical stimulation (DCS). Twenty-four patients were included in this study (22 for TES, 2 for DCS, 3 for TES plus DCS) that underwent different surgical procedures. We used two methods to elicit CoMEPs: (a) TES by stimulation over C3/Cz or C4/Cz and (b) DCS with a strip electrode placed over the primary motor area (M1) for laryngeal muscles. To record CoMEPs from vocal muscles we used two hook wire electrodes 76 microm of diameter passing through 27 gauge needle endotracheally placed in the vocal muscles after intubation. Recording of CoMEPs in the vocal muscles after TES was successfully performed in 22 patients. TES over the right or left hemisphere elicit responses bilaterally. The onset latencies for the right vocal muscle was 12.4+/-3.1 ms (ipsilateral stimulation) and 12.7+/-2.2 ms (contralateral stimulation) while for the left vocal muscle, onset latency was 12.9+/-2.3 ms (ipsilateral stimulation) and 14.1+/-3.4 ms (contralateral stimulation). In five patients DCS elicited CoMEPs in right and left vocal muscle with latency of 16.6+/-4.7 and 15.6+/-3.7 ms, respectively. The method to elicit and record CoMEPs in vocal muscles shows reliable results and adds one more tool in the armamentarium of intraoperative neurophysiology. This method shows the ability to continuously monitor the functional integrity of corticobulbar pathways, vagal nucleus and laryngeal nerves.