Motor Evoked Potentials Responses Research Articles

Transspinal stimulation decreases corticospinal excitability and alters the function of spinal locomotor networks.

Locomotion requires the continuous integration of descending motor commands and sensory inputs from the legs by spinal central pattern generator circuits. Modulation of spinal neural circuits by transspinal stimulation is well documented, but how transspinal stimulation affects corticospinal excitability during walking in humans remains elusive. We measured the motor evoked potentials (MEPs) at multiple phases of the step cycle conditioned with transspinal stimulation delivered at sub- and suprathreshold intensities of the spinally mediated transspinal evoked potential (TEP). Transspinal stimulation was delivered before or after transcranial magnetic stimulation during which summation between MEP and TEP responses in the surface EMG was absent or present. Relationships between MEP amplitude and background EMG activity, silent period duration, and phase-dependent EMG amplitude modulation during and after stimulation were also determined. Ankle flexor and extensor MEPs were depressed by suprathreshold transspinal stimulation when descending volleys were timed to interact with transspinal stimulation-induced motoneuron depolarization at the spinal cord. MEP depression coincided with decreased MEP gain, unaltered MEP threshold, and unaltered silent period duration. Locomotor EMG activity of bilateral knee and ankle muscles was significantly depressed during the step at which transspinal stimulation was delivered but fully recovered at the subsequent step. The results support a model in which MEP depression by transspinal stimulation occurs via subcortical or spinal mechanisms. Transspinal stimulation disrupts the locomotor output of flexor and extensor motoneurons initially, but the intact nervous system has the ability to rapidly overcome this pronounced locomotor adaptation. In conclusion, transspinal stimulation directly affects spinal locomotor centers in healthy humans.NEW & NOTEWORTHY Lumbar transspinal stimulation decreases ankle flexor and extensor motor evoked potentials (MEPs) during walking. The MEP depression coincides with decreased MEP gain, unaltered MEP threshold changes, and unaltered silent period duration. These findings indicate that MEP depression is subcortical or spinal in origin. Healthy subjects could rapidly overcome the pronounced depression of muscle activity during the step at which transspinal stimulation was delivered. Thus, transspinal stimulation directly affects the function of spinal locomotor networks in healthy humans.

Characterization of stimulus response curves obtained with transcranial magnetic stimulation from bilateral tibialis anterior muscles post stroke

BackgroundStimulus response curves (SR curves), measured using transcranial magnetic stimulation (TMS) induced motor evoked potentials (MEP), yield important information regarding corticomotor connectivity. Limited understanding of SR curve analyses techniques for leg muscles after stroke may limit the utility of TMS data for walking recovery. ObjectiveTo compare linear and non-linear curve fitting for MEP responses from the non-paretic and paretic tibialis anterior (TA) muscles. MethodsAccuracy of fit was measured using coefficient of determination (R2). Similarities of the fit were compared using slopes and area under the curve (AUC). ResultsThe non-linear function demonstrated higher R2 and slopes. The AUC was not significantly different between the two analyses approaches. The non-linear non-paretic SR slopes and paretic AUC had significant associations with walking speed. ConclusionOur results highlight the differences between non-linear and linear approaches to best fit the SR curves from bilateral TA muscles in stroke survivors. Although the linear function can appropriately fit the SR curve of the paretic and non-paretic TA, the non-linear function estimated a higher slope. We found the AUC to be a more robust measure that was not affected by the type of curve-fitting approach and only the AUC of the paretic TA showed significant association with walking speeds. A better understanding of SR curve fitting approaches for the TA muscles in individuals with chronic stroke allows for their optimal use in interpretation of TMS data and literature.

Survivals of the Intraoperative Motor-evoked Potentials Response in Pediatric Patients Undergoing Spinal Deformity Correction Surgery: What Are the Neurologic Outcomes of Surgery?

This is a retrospective cases study from a prospective patient register. To clarify the clinical implication regard to the survivals of motor-evoked potential (MEP) response. Intraoperative neurophysiological monitoring has become an essential component for decreasing the incidence of neurological deficits during spine surgeries. Significant motor-evoked potential (MEP) loss but does not vanish completely is common especially in some high-risk and complicated pediatric spine deformity surgeries. A total of 1820 young patients (mean age = 12.2 years) underwent spinal deformity correction were mainly analyzed. Intraoperative monitoring (somatosensory-evoked potential, MEP, free-run electromyography, free-run electromyography) and postoperative neurologic outcomes were mainly analyzed in this study. All patients with monitoring alerts were divided into two groups: group 1, intraoperative MEP recovery group; and group 2, no obvious MEP recovery group. Moreover, the patients would be followed up strictly if he/she showed IOM alerting. The surviving MEP response was identified as significant monitoring alerts (80%-95% MEP Amp. loss) associated with high-risk surgical maneuvers. The results showed that there were 32 pediatric patients (group 1, 21 cases and group 2, 11 cases) presenting significant MEP monitoring alerts (80%-95% loss) relative to baseline. The patients in group 1 presented the partial/entire signal recovery from MEP alerts and they did not show spinal cord deficits postoperation. The patients in group 2 without obvious intraoperative MEP recovery showed different levels of new spinal deficits, no patient showed postoperative complete paraplegia or permanent spinal cord/nerve root deficits. When the intraoperative MEP changes significant and persistent but without totally disappeared, the rate of postoperative neural complication is relatively low. The chance of recovery of these neurological deficits is very high. Therefore, this phenomenon may be used to predictive of nonpermanent paraplegia. 3.

Superconditioning TMS for examining upper motor neuron function in MND.

We used transcranial magnetic stimulation (TMS) of motor cortex, including a novel four-pulse superconditioning (TMSsc) paradigm, in repeated examinations of motor-evoked potentials (MEPs) in eight subjects with motor neuron disease (MND), including seven with amyotrophic lateral sclerosis (ALS). The goals were: (1) to look for evidence of cortical hyperexcitability, including a reduction in short-interval intracortical inhibition (SICI); and (2) to examine the utility of using TMSsc for quantifying upper motor neuron function during MND progression. Testing of abductor pollicis brevis (APB) and tibialis anterior (TA) muscles bilaterally was carried out every 3months in MND subjects for up to 2years; results were compared to those from a cohort of 15 control subjects. Measures of SICI were not significantly different between control and MND subjects for either APB or TA muscles. Other measures of cortical excitability, including TMS threshold and MEP amplitude, were consistent with lowered cortical excitability in MND subjects. Certain combinations of superconditioning TMS were capable of causing stronger inhibition or facilitation of MEPs compared to dual-pulse TMS, for both APB and TA target muscles. Moreover, there were multiple cases in which target muscles unresponsive to strong single-pulse TMS, whether at rest or when tested with an active contraction, showed an MEP in response to TMSsc optimized for facilitation. Our findings suggest that a multi-faceted neurophysiologic protocol for examining upper motor neuron function in MND subjects might benefit from inclusion of TMSsc testing.

P19-F Variability of Intraoperation monitoring parameters in patients with degenerative cervical myelopathy

Background motor cortex transcranial electric stimulation in Intraoperative monitoring (IOM) is used to assess the corticospinal functional state by measuring the fast axons conduction (registration of D-wave), motor evoked potentials (MEPs) threshold and amplitude. While D-wave is considered to be a stable parameter independent of anesthesia, the informativeness of MEPs amplitude is controversial due to its instability, especially when in cervical spine surgery. Material and methods 20 patients with cervical degenerative myelopathy underwent surgical decompression with anterior cervical discectomy. In Group 1 (10 patients) the TIVA anaestiological protocol was used and in Group 2 (10 patients) the Sevoflurane 0.6–0.8 MAC was added. In all patients before intubation MEPs threshold for m. abd. pollicis brevis, m.add. digiti minimi, m. tibialis anterior, m. add. hallucis were assessed in response to a train of 5 pulses with a duration of 0.5 ms and inter-pulse interval 2 ms. Results In the course of operation MEPs threshold increase and amplitude decrease were detected in all patients Group2. In 6 patients of Group 1 (4 – unilaterally, 2 – bilaterally) and 4 patients of Group 2 (2– unilaterally, 2 – bilaterally) during surgery MEPs responses were lost; though there were observed no signs of additional neurological deficit after surgery. Unilateral MEP changes in patients operated on cervical spine can be attributed to compression of the plexus related to the position and patient fixation; bilateral – because of traction manipulations and spinal cord ischemia. Conclusions for detection of intraoperational ischemia Multimodal IOM will be preferred.

Multimodal Evaluation of TMS - Induced Somatosensory Plasticity and Behavioral Recovery in Rats With Contusion Spinal Cord Injury.

Introduction: Spinal cord injury (SCI) causes partial or complete damage to sensory and motor pathways and induces immediate changes in cortical function. Current rehabilitative strategies do not address this early alteration, therefore impacting the degree of neuroplasticity and subsequent recovery. The following study aims to test if a non-invasive brain stimulation technique such as repetitive transcranial magnetic stimulation (rTMS) is effective in promoting plasticity and rehabilitation, and can be used as an early intervention strategy in a rat model of SCI.Methods: A contusion SCI was induced at segment T9 in adult rats. An rTMS coil was positioned over the brain to deliver high frequency stimulation. Behavior, motor and sensory functions were tested in three groups: SCI rats that received high-frequency (20 Hz) rTMS within 10 min post-injury (acute-TMS; n = 7); SCI rats that received TMS starting 2 weeks post-injury (chronic-TMS; n = 5), and SCI rats that received sham TMS (no-TMS, n = 5). Locomotion was evaluated by the Basso, Beattie, and Bresnahan (BBB) and gridwalk tests. Motor evoked potentials (MEP) were recorded from the forepaw across all groups to measure integrity of motor pathways. Functional MRI (fMRI) responses to contralateral tactile hindlimb stimulation were measured in an 11.7T horizontal bore small-animal scanner.Results: The acute-TMS group demonstrated the fastest improvements in locomotor performance in both the BBB and gridwalk tests compared to chronic and no-TMS groups. MEP responses from forepaw showed significantly greater difference in the inter-peak latency between acute-TMS and no-TMS groups, suggesting increases in motor function. Finally, the acute-TMS group showed increased fMRI-evoked responses to hindlimb stimulation over the right and left hindlimb (LHL) primary somatosensory representations (S1), respectively; the chronic-TMS group showed moderate sensory responses in comparison, and the no-TMS group exhibited the lowest sensory responses to both hindlimbs.Conclusion: The results suggest that rTMS therapy beginning in the acute phase after SCI promotes neuroplasticity and is an effective rehabilitative approach in a rat model of SCI.

Motometrics: A Toolbox for Annotation and Efficient Analysis of Motor Evoked Potentials.

Stimulating the nervous system and measuring muscle response offers a unique opportunity to interrogate motor system function. Often, this is performed by stimulating motor cortex and recording muscle activity with electromyography; the evoked response is called the motor evoked potential (MEP). To understand system dynamics, MEPs are typically recorded through a range of motor cortex stimulation intensities. The MEPs increase with increasing stimulation intensities, and these typically produce a sigmoidal response curve. Analysis of MEPs is often complex and analysis of response curves is time-consuming. We created an MEP analysis software, called Motometrics, to facilitate analysis of MEPs and response curves. The goal is to combine robust signal processing algorithms with a simple user interface. Motometrics first enables the user to annotate data files acquired from the recording system so that the responses can be extracted and labeled with the correct subject and experimental condition. The software enables quick visual representations of entire datasets, to ensure uniform quality of the signal. It then enables the user to choose a variety of response curve analyses and to perform near real time quantification of the MEPs for quick feedback during experimental procedures. This is a modular open source tool that is compatible with several popular electrophysiological systems. Initial use indicates that Motometrics enables rapid, robust, and intuitive analysis of MEP response curves by neuroscientists without programming or signal processing expertise.

Effect of Paired Associative Stimulation on Corticomotor Excitability in Chronic Smokers.

Chronic smoking has been shown to have deleterious effects on brain function and is an important risk factor for ischemic stroke. Reduced cortical excitability has been shown among chronic smokers compared with non-smokers to have a long-term effect and so far no study has assessed the effect of smoking on short-term motor learning. Paired associative stimulation (PAS) is a commonly used method for inducing changes in excitability of the motor cortex (M1) in a way that simulates short-term motor learning. This study employed PAS to investigate the effect of chronic cigarette smoking on plasticity of M1. Stimulator output required to elicit a motor-evoked potential (MEP) of approximately 1 mV was similar between the groups prior to PAS. MEP response to single pulse stimuli increased in the control group and remained above baseline level for at least 30 min after the intervention, but not in the smokers who showed no significant increase in MEP size. The silent period was similar between groups at all time points of the experiment. This study suggests that chronic smoking may have a negative effect on the response to PAS and infers that chronic smoking may have a deleterious effect on the adaptability of M1.

Safety and effects on motor cortex excitability of five anodal transcranial direct current stimulation sessions in 24 hours

Application parameters of transcranial direct current stimulation (tDCS) for therapeutic purposes are relatively restricted. The aim of this study was to assess safety and effects on motor cortex excitability of an intensive anodal-tDCS protocol. In 26 healthy subjects, five 15-minute anodal-tDCS sessions were delivered, at increasing time intervals, over 24hours. Safety was defined as absence of serious adverse events including brain tissue alterations on magnetic resonance imaging. Effect on motor cortex excitability was evaluated by motor evoked potential (MEP) amplitude, measured eight times. No serious adverse events occurred. Mild adverse events, such as reversible scalp erythema or transient metallic taste, were observed in 27% of subjects. MEP amplitudes did not change in any of the recording periods. When inter-individual variability was taken into account and threshold values defined, 50% of subjects were classified as responders, 15% were inverse responders, and 35% non-responders. In the responders, normalized MEP was increased by 57% 1hour after the first anodal-tDCS and increased by 50% three hours after two stimulations delivered 1hour apart. Intra-individual, inter-sessional consistency of MEP response over four measurements was 61-77%. Five anodal-tDCS delivered in 24hours are safe and well tolerated, expanding the safety standard of tDCS. However, only half of subjects respond to anodal-tDCS with a robust and durable MEP augmentation. On the other hand, the response to a single anodal-tDCS predicts fairly well the response to other sessions in the same subject. These findings should be considered in clinical trials utilizing repeated anodal-tDCS.

Clinical Usefulness of Intraoperative Motor-Evoked Potential Monitoring during Temporal Lobe Epilepsy Surgery.

Background and PurposeWe aimed to determine the effectiveness of intraoperative neurophysiological monitoring focused on the transcranial motor-evoked potential (MEP) in patients with medically refractory temporal lobe epilepsy (TLE).MethodsWe compared postoperative neurological deficits in patients who underwent TLE surgery with or without transcranial MEPs combined with somatosensory evoked potential (SSEP) monitoring between January 1995 and June 2018. Transcranial motor stimulation was performed using subdermal electrodes, and MEP responses were recorded in the four extremity muscles. A decrease of more than 50% in the MEP or the SSEP amplitudes compared with baseline was used as a warning criterion.ResultsIn the TLE surgery group without MEP monitoring, postoperative permanent motor deficits newly developed in 7 of 613 patients. In contrast, no permanent motor deficit occurred in 279 patients who received transcranial MEP and SSEP monitoring. Ten patients who exhibited decreases of more than 50% in the MEP amplitude recovered completely, although two cases showed transient motor deficits that recovered within 3 months postoperatively.ConclusionsIntraoperative transcranial MEP monitoring during TLE surgery allowed the prompt detection and appropriate correction of injuries to the motor nervous system or ischemic stroke. Intraoperative transcranial MEP monitoring is a reliable modality for minimizing motor deficits in TLE surgery.

Characterization of Intraoperative Motor Evoked Potential Monitoring for Surgery of the Pediatric Population with Brain Tumors

We investigated the relationship between the reliability of the transcranial or transcortical motor evoked potential (MEP) response and age in pediatric patients aged ≤15 years with brain tumor. We retrospectively analyzed the data from 60 consecutive patients aged ≤15 years who had undergone brain tumor surgery that involved intraoperative MEP monitoring from October 2009 to May2016. A total of 41 patients with reliable signals (MEP response group) and 19 patients without reliable signals (MEP nonresponse group) were included in the present study. The mean age at surgery, body height, and body weight were significantly greater in the MEP response group than in the MEP nonresponse group. When the MEP success rates during surgery of the pediatric population with brain tumors were analyzed in relation to patient age, the transcortical MEP success rate in the 0-5-year age group (10.0%) was significantly lower than that in the 6-10-year age group (71.4%; P= 0.009) and that in the 11-15-year age group (75.0%; P= 0.015). The transcortical MEP response was monitored less successfully during brain tumor surgery in patients aged ≤5 years than in patients aged 6-15 years. Although MEP monitoring techniques can be applied during surgery of pediatric populations with brain tumors similar to that used for adult patients, the limitations of the low transcortical MEP response rate in young patients should be considered.

Intra-operative cortical motor mapping using subdural grid electrodes in children undergoing epilepsy surgery evaluation and comparison with the conventional extra-operative motor mapping

ObjectivesThe objective of this study was to review our experience with intra-operative “train of five” stimulation using subdural grid for motor mapping in children undergoing epilepsy surgery evaluation. MethodsTwenty consecutive children below 18-years of age with drug-refractory epilepsy who underwent invasive-EEG monitoring using subdural-grid placement and intra-operative motor mapping using direct cortical stimulation by sub-dural grid electrodes (IODCS-SDG) at our institution between January-2016 and June-2017 were reviewed. Stimulation was delivered through the subdural-grid electrodes using a train-of-five pulses and muscle responses were recorded by motor-evoked-potentials (MEPs). Intra-operative direct cortical stimulation delivered through a ball-tipped probe (IODCS-probe) and extra-operative motor-mapping (EODCS-SDG) were also performed. ResultsIODCS-SDG was completed in 20 patients and subsequent EODCS-SDG was done in 17/20 patients. MEP responses were more commonly obtained in the deltoid (19/20), extensor-digitorum-communis (20/20) and first-dorsal-interosseus (19/20). The median thresholds varied between 40 V and 60 V for the six muscle groups. The respective IODCS-probe thresholds tended to be similar. No stimulation-provoked seizures or anaesthesia-related complications were noted during IODCS-SDG. EODCS-SDG could not be completed in 4/17 children and mapping data obtained was frequently inadequate. Nine patients demonstrated 100% concordance between IODCS-SDG and EODCS-SDG for the common mapped body regions. Stimulation-provoked seizures during EODCS-SDG were seen in 6/17 (35.3%) and after-discharges in 7/17 (41.2%) children. ConclusionsIODCS-SDG could be performed safely in children with drug refractory epilepsy undergoing invasive EEG monitoring. SignificanceIODCS-SDG may be a useful adjunct to EODCS-SDG in motor mapping for children.

Noninvasive Detection of Motor-Evoked Potentials in Response to Brain Stimulation Below the Noise Floor-How Weak Can a Stimulus Be and Still Stimulate.

Motor-evoked potentials (MEP) are one of the most important responses to brain stimulation, such as supra-threshold transcranial magnetic stimulation (TMS) and electrical stimulation. The understanding of the neurophysiology and the determination of the lowest stimulation strength that evokes responses requires the detection of even smallest responses, e.g., from single motor units, but available detection and quantization methods are rather simple and suffer from a large noise floor. The paper introduces a more sophisticated matched-filter detection method that increases the detection sensitivity and shows that activation occurs well below the conventional detection level. In consequence, also conventional threshold definitions, e.g., as 50 μV median response amplitude, turn out to be substantially higher than the point at which first detectable responses occur. The presented method uses a matched-filter approach for improved sensitivity and generates the filter through iterative learning from the presented data. In contrast to conventional peak-to-peak measures, the presented method has a higher signal-to-noise ratio (≥14 dB). For responses that are reliably detected by conventional detection, the new approach is fully compatible and provides the same results but extends the dynamic range below the conventional noise floor. The underlying method is applicable to a wide range of well-timed biosignals and evoked potentials, such as in electroencephalography.

T22. Intra-operative motor mapping using subdural grid electrodes in children undergoing invasive EEG monitoring

Introduction Extra-operative motor mapping (EOMM) in the pediatric population can be challenging, especially in young and/or developmentally delayed children, due to lack of cooperation, and higher motor stimulation thresholds. This study examined the feasibility and safety of intra-operative motor mapping (IOMM) using subdural grid electrodes in children undergoing epilepsy surgery evaluation. Methods Twenty consecutive children under18 years of age with drug refractory epilepsy who underwent invasive EEG monitoring using subdural grid placement and intra-operative motor mapping at our institution between December 2015 and June 2017 were enrolled. Stimulation was delivered to the subdural grid electrode using a train of five pulses, with an interstimulus interval of 1.1 ms and a pulse duration of 50 microseconds. Stimulation intensity was commenced at 25 Volts (V) and increased gradually until acceptable motor evoked potential (MEP) waveforms were generated. MEPs were recorded from the contralateral six muscle groups (orbicularis oris, deltoid, extensor digitorum communis, first dorsal interossei, tibialis anterior and abductor halluces). Extra-operative motor mapping (previously published procedure) was also done 3 days after the subdural grid placement. Results IOMM using sub-dural grid electrodes was completed in 20 patients, and simultaneous EOMM was done in 17 patients. The mean age at the time of mapping was 11.7 years (SD 4.2). 13/20 patients had malformations of cortical development. MEP responses were more commonly obtained in the deltoid (19/20), extensor digitorum communis (20/20) and first dorsal inerossei (19/20). The mean threshold varied from 44.5 V to 55.8 V for the six muscle groups. With increasing age, there was a decline in the motor thresholds with a possible exponential relationship. No seizures or anaesthesia related complications were noted. EOMM could not be completed in 4/17 children (3-clinical seizures, 1-limited patient cooperation). The mapping was incomplete in the majority of cases with 1 being the median number of areas mapped (IQR, 1–2). Seizures were seen in 6/17 (35.3%) and after-discharges in 7/17 (41.2%) children. Nine patients demonstrated 100% concordance between the common mapped areas, 4 patients showed 50% concordance and 3 patients showed no concordance between IOMM and EOMM. The median duration of follow up was 10.2 months (IQR, 3.6 to 14.4 months). ILAE seizure outcome 1 was seen in 15 patients (75%) at the last follow up. Post-operative neurological deficits were seen in 11 patients: contralateral motor deficits (6; 2 were transient), contralateral sensory deficits (2), homonymous hemianopia (1) and seizures (3). Conclusion IOMM was found to be feasible and safe procedure in children with drug refractory epilepsy undergoing invasive EEG monitoring using the subdural grid.

T140. Identifying the best stimulation configuration to use for linked-quadripolar motor evoked potentials

Neurophysiologic intraoperative monitoring (NIOM) utilizes motor evoked potentials (MEP) to assess the corticospinal tract during surgery. In most patients, 200 V or higher stimulation intensity is needed to elicit reliable MEP in the foot muscles. High stimulation intensity may result in more patient movement and adverse events such as tongue lacerations. This study is a continuation of the linked-quadripolar study that couples multiple electrodes over the scalp to assess which combination results in the most robust MEP response. MEP were obtained with different combination of anode and cathode placements. Linked double anode and cathode combinations were used. Electrodes were placed at C1, C2, C3, C4, M1, M2, M3, M4. A four-step process was used to determine the best montage. Demographic and clinical data were noted. Amplitude of both the right and left foot MEP were assessed at threshold intensity levels in order to see which electrode combination resulted in the most robust MEP. This was a retrospective study of 16 patients who underwent the 4-step linked-quadripolar study looking for the most robust foot MEP responses. On average, patients showed the highest response using a diagonal linked electrode configuration (68.75%). Seven patients (43.75%) had the highest amplitude at threshold using C1-M3/C2-M4 linked as anode/cathode. Four patients (25%) showed the highest amplitude using the C3-M1/C4-M2 linked as anode/cathode. Stimulation was lower on average from the standard bipolar montage to the linked quadripolar method (193.25–140.5 V). The linked quadripolar stimulation configuration using C1-M3/C2-M4 showed the most robust MEP response in the right foot using the lowest stimulation.

Intra- and Inter-Regional Priming of Ipsilateral Human Primary Motor Cortex With Continuous Theta Burst Stimulation Does Not Induce Consistent Neuroplastic Effects.

Human responses to non-invasive brain stimulation (NIBS) techniques can be highly variable. Recently, priming protocols involving a conditioning round of NIBS applied to a target brain region prior to the application of a test protocol have shown promise in inducing more reliable effects. We investigated whether intra- or inter-regional priming of the left primary motor cortex (M1) using continuous theta burst stimulation (cTBS) can induce consistent, and reliable modulation of corticospinal excitability. Twenty healthy adults (six males) underwent four cTBS protocols. For intra-regional priming, cTBS was applied twice to the left M1 (M1-M1). For inter-regional M1 priming, cTBS was applied to the ipsilateral (left) dorsal premotor cortex (dPMC-M1), and ipsilateral (left) dorsolateral prefrontal cortex (DLPFC-M1). In the control condition, sham stimulation was applied to left M1, followed by active cTBS also applied to the left M1 (sham-M1). Each round of cTBS was separated by 10 min. Neuroplastic responses were indexed using motor evoked potentials (MEPs) elicited from the left M1 hand region, and measured from the contralateral first dorsal interosseous (right hand). MEP measurements were taken before the first round of cTBS priming, then immediately, 10, 20 and 30 min after the second test round of cTBS. The primary two-way repeated measures ANOVA revealed no significant differences in MEP responses across each condition (no main effects or interaction). Intra- and inter-regional priming of the left M1 using cTBS does not induce consistent neuroplastic effects. Further work is required to identify factors which contribute to such variability in human responses to NIBS.

The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor.

Motor evoked potentials (MEPs) are commonly used during surgery for spinal cord tumor resection. However, it can be difficult to record reliable MEPs from the muscles of the lower extremities during surgery in patients with preoperative weakness due to spinal cord compression. In this study, motor function of patients' lower extremities and their association with intraoperative MEP recording were compared. Patients undergoing thoracic spinal cord tumor resection were studied. Patients' motor function was checked immediately before the surgical procedure. MEP responses were recorded from the tibialis anterior and foot muscles, and the hand muscles were used as control. Electrical current with train of eight pulses, 200 to 500 V was delivered through 2 corkscrews placed at C3' and C4' sites. Anesthesia was maintained by total intravenous anesthesia using a combination of propofol and remifentanil after induction with intravenous propofol, remifentanil, and rocuronium. Rocuronium was not repeated. Bispectral Index was maintained between 40 to 50. From 178 lower limbs of 89 patients, myogenic MEPs could be recorded from 100% (105/105) of the patients with 5 of 5 motor strength in lower extremity; 90% (36/40) from the patients with 4/5 motor strength; only 25% (5/20) with 3/5; and 12.5% (1/8) with 2/5 motor strength; none (0/5) were able to be recorded if the motor strength was 1/5. The ability to record myogenic MEPs is closely associated with the patient's motor function. They are difficult to obtain if motor function is 3/5 motor strength in the lower extremity. They are almost impossible to record if motor function is worse than 3/5.

Measurement of motor-evoked potential resting threshold and amplitude of proximal and distal arm muscles in healthy adults. A reliability study.

PurposeReliability of motor-evoked potential threshold and amplitude measurement of upper limb muscles is important when detecting changes in cortical excitability. The objective of this study was to investigate intra-rater, test–retest reliability and minimal detectable change of resting motor threshold and amplitude of a proximal and distal upper limb muscles, anterior deltoid and distal extensor digitorum communis in healthy adults.MethodTo measure motor-evoked potential responses, transcranial magnetic stimulation was interfaced with electromyography and neuronavigation equipment. Two measurements were conducted on day 1 and a third measurement three days later. Reliability was analysed using intraclass correlation coefficients.ResultsTwenty participants completed the study. Excellent intra-rater (intraclass correlation coefficient = 0.91 (extensor digitorum), 0.94 (anterior deltoid)) and good to excellent test–retest reliability (intraclass correlation coefficient = 0.69 (anterior deltoid), 0.84 (extensor digitorum)) was found for resting motor threshold. Minimal detectable change for resting motor threshold was found at 10.95% (extensor digitorum) and 16.35% (anterior deltoid) between first and third measurements. Motor-evoked potential amplitude of extensor digitorum communis had fair to good intra-rater (intraclass correlation coefficient = 0.50) and test–retest reliability (intraclass correlation coefficient = 0.65).ConclusionsOur results suggest that resting motor threshold is a reliable neurophysiological measure even for proximal shoulder muscles. Future research should further explore the reliability of motor-evoked potential amplitude before integration into neurological rehabilitation.

The effect of shoulder position on motor evoked and maximal muscle compound action potentials of the biceps brachii.

The purpose of the study was to assess the effect of shoulder position, 0° versus 90° shoulder flexion, on stimulation intensity and maximal muscle compound action potentials (Mmax) and motor evoked potentials (MEP) of the biceps brachii during both rest and 10% maximum voluntary contraction (MVC). Nine participants completed two experimental sessions with four conditions. During each condition, transcranial magnetic (TMS) and Erb's point stimulation were used to elicit MEPs and Mmax, respectively. During rest, the TMS intensity to elicit a MEP response (p<0.001), was significantly lower by 28.6±6.8%, in the 90° compared to the 0° position, but the stimulation intensity to elicit a Mmax was not different. MEP (p<0.001) and Mmax (p<0.001) amplitudes were significantly higher by 212.4±43.3% and 86.5±38.0%, respectively in the 90° compared to the 0° position. During 10% MVC the Mmax stimulation intensity (p=0.022), but not TMS intensity, was significantly lower by 7.4±3.8% in the 90° compared to the 0° position. Mmax (p<0.001) amplitudes were significantly higher by 92.2±20.2% in the 90° compared to the 0° position whereas MEP (p<0.001) amplitudes were significantly lower by 24.5±6.0% in the 90° compared to the 0° position. In conclusion, TMS intensity and Mmax intensity were both shoulder-position and state-dependent, whereas MEP and Mmax amplitudes were only shoulder position-dependent.

43. Intraoperative neurophysiological monitoring in spine surgery: A significant tool for neuronal protection and functional restoration

Although there is recent evidence for the role of intraoperative neurophysiological monitoring (IONM) in spine surgery, there are no uniform opinions on the optimal combination of the different tools. We analyzed the impact of the multimodal IONM (mIONM) on the preservation of neuronal structures and on functional restoration in a prospective series of our patients who underwent spine surgery. The mIONM approach in spine surgery involves the evaluation of somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) with electrical transcranial stimulation, including the use of a multipulse technique with multiple myomeric registration of responses from limbs, and a single-pulse technique with D-wave registration through epi- and intradural recording, and free running and evoked electromyography. We observed an improvement of neurological status in 50% of the patients. The D-wave registration was the most useful intraoperative tool, especially when MEP and SEP responses were absent or poorly recordable. Our preliminary data confirm that mIONM plays a fundamental role in the identification and functional preservation of the spinal cord and nerve roots. It is highly sensitive and specific for detecting and avoiding neurological injury during spine surgery and represents a helpful tool for achieving optimal postoperative functional outcome.