Transcranial electric stimulation for intraoperative motor evoked potential monitoring: Stimulation parameters and electrode montages

Timing-dependent synergies between motor cortex and posterior spinal stimulation in humans

Volitional movement requires descending input from the motor cortex and sensory feedback through the spinal cord. We previously developed a paired brain and spinal electrical stimulation approach in rats that relies on convergence of the descending motor and spinal sensory stimuli in the cervical cord. This approach strengthened sensorimotor circuits and improved volitional movement through associative plasticity. In humans, it is not known whether posterior epidural spinal cord stimulation targeted at the sensorimotor interface or anterior epidural spinal cord stimulation targeted within the motor system is effective at facilitating brain evoked responses. In 59 individuals undergoing elective cervical spine decompression surgery, the motor cortex was stimulated with scalp electrodes and the spinal cord was stimulated with epidural electrodes, with muscle responses being recorded in arm and leg muscles. Spinal electrodes were placed either posteriorly or anteriorly, and the interval between cortex and spinal cord stimulation was varied. Pairing stimulation between the motor cortex and spinal sensory (posterior) but not spinal motor (anterior) stimulation produced motor evoked potentials that were over five times larger than brain stimulation alone. This strong augmentation occurred only when descending motor and spinal afferent stimuli were timed to converge in the spinal cord. Paired stimulation also increased the selectivity of muscle responses relative to unpaired brain or spinal cord stimulation. Finally, clinical signs suggest that facilitation was observed in both injured and uninjured segments of the spinal cord. The large effect size of this paired stimulation makes it a promising candidate for therapeutic neuromodulation. KEY POINTS: Pairs of stimuli designed to alter nervous system function typically target the motor system, or one targets the sensory system and the other targets the motor system for convergence in cortex. In humans undergoing clinically indicated surgery, we tested paired brain and spinal cord stimulation that we developed in rats aiming to target sensorimotor convergence in the cervical cord. Arm and hand muscle responses to paired sensorimotor stimulation were more than five times larger than brain or spinal cord stimulation alone when applied to the posterior but not anterior spinal cord. Arm and hand muscle responses to paired stimulation were more selective for targeted muscles than the brain- or spinal-only conditions, especially at latencies that produced the strongest effects of paired stimulation. Measures of clinical evidence of compression were only weakly related to the paired stimulation effect, suggesting that it could be applied as therapy in people affected by disorders of the central nervous system.

Motor-Evoked Potential Monitoring With Multi-train Electrical Stimulation During Thoracoabdominal Aortic Aneurysm Surgery: A Case Report

Intraoperative motor-evoked potentials (MEPs) are measured for assessing motor function during surgery. MEP monitoring is often performed in thoracoabdominal aortic aneurysm (TAAA) surgery, but false positives are common and amplification methods are needed to obtain waveforms under severe conditions to assess proper spinal cord function. One method of amplitude amplification in transcranial-stimulated MEP monitoring is multitrain stimulation. There are few reports on multitrain-stimulated MEP monitoring for this surgery. A 57-year-old woman underwent open repair of the thoracoabdominal aorta due to a dissecting aortic aneurysm. After opening the chest, the aneurysm was incised proximally, and anastomosis with an artificial vessel was initiated. The lumbar artery leading to the Adam-Kiewicz artery was reconstructed at a body temperature of 25 °C. However, the single-train stimulation did not produce MEPs. When the measurement was switched to multitrain stimulation, MEPs were elicited in the lower extremity muscle groups and the waveforms were maintained until the end of the measurement. This case illustrates that MEP monitoring using multitrain stimulation during descending thoracic aortic aneurysm surgery can effectively elicit MEPs under challenging conditions, in which conventional single-train stimulation may be insufficient.

Timing dependent synergies between motor cortex and posterior spinal stimulation in humans

Volitional movement requires descending input from motor cortex and sensory feedback through the spinal cord. We previously developed a paired brain and spinal electrical stimulation approach in rats that relies on convergence of the descending motor and spinal sensory stimuli in the cervical cord. This approach strengthened sensorimotor circuits and improved volitional movement through associative plasticity. In humans it is not known whether dorsal epidural SCS targeted at the sensorimotor interface or anterior epidural SCS targeted within the motor system is effective at facilitating brain evoked responses. In 59 individuals undergoing elective cervical spine decompression surgery, the motor cortex was stimulated with scalp electrodes and the spinal cord with epidural electrodes while muscle responses were recorded in arm and leg muscles. Spinal electrodes were placed either posteriorly or anteriorly, and the interval between cortex and spinal cord stimulation was varied. Pairing stimulation between the motor cortex and spinal sensory (posterior) but not spinal motor (anterior) stimulation produced motor evoked potentials that were over five times larger than brain stimulation alone. This strong augmentation occurred only when descending motor and spinal afferent stimuli were timed to converge in the spinal cord. Paired stimulation also increased the selectivity of muscle responses relative to unpaired brain or spinal cord stimulation. Finally, paired stimulation effects were present regardless of the severity of myelopathy as measured by clinical signs or spinal cord imaging. The large effect size of this paired stimulation makes it a promising candidate for therapeutic neuromodulation.

Comparison Between Cz-C3/C4 and C3-C4 Montages to Protect Against Peripheral Stimulation in Transcranial Facial Motor-Evoked Potential Monitoring

INTRODUCTION

In facial motor-evoked potential monitoring, efforts to reduce peripheral stimulation are necessary because it can cause false-negatives. The effects of peripheral stimulation on Cz-C3/C4 and C3-C4 montages were compared.

METHODS

Facial motor-evoked potentials were recorded from bilateral orbicularis oculi (Oculi) and oris (Oris) muscles. The double-train approach combining single-pulse and five-train pulse stimulation was used to determine the effect of peripheral stimulation. If the five-train pulse produced a significant waveform, it was defined as "total success." In total success cases, "true success" was defined as a case in which no waveform appeared after the single pulse at the threshold level of the five-train pulse. The total and true success rates and the threshold value of Oculi and Oris were compared between Cz-C3/C4 and C3-C4 montages.

RESULTS

Thirty-six muscles each of Oculi and Oris of 18 patients were used for the analysis. True success was more likely to be obtained by the Cz-C3/C4 montage than the C3-C4 montage in Oculi (42% vs. 22%, p = 0.039). Both Oculi and Oris had higher thresholds to elicit facial motor-evoked potentials with the Cz-C3/C4 montage (Oculi: 101.7 vs. 71.4 mA, p = 0.038; Oris: 94.8 vs. 73.1 mA, p = 0.016).

CONCLUSIONS

Cz-C3/4 montage is more effective at reducing peripheral stimulation compared with the C3-4 montage. This effect was primarily seen in the orbicularis oculi muscle. It should be noted that the Cz-C3/C4 montage has a higher threshold than the C3-C4 montage in facial muscles. In facial motor-evoked potential monitoring, the Cz-C3/C4 montage may be more suitable to eliminate peripheral stimulation.

The role of intraoperative extensor digitorum brevis muscle MEPs in spinal surgery

PURPOSE

Intraoperative muscle motor evoked potentials (m-MEPs) are widely used in spinal surgery with the aim of identifying a damage to spinal cord at a reversible stage. Generally, lower limb m-MEPs are recorded from abductor hallucis [AH] and the tibialis anterior [TA]. The purpose of this work is to study an unselected population by recording the m-MEPs from TA, AH and extensor digitorum brevis (EDB), with the aim of identifying the most adjustable and stable muscles responses intraoperatively.

METHODS

Transcranially electrically induced m-MEPs were intraoperative recorded in a total of 107 surgical procedures. m-MEPs were recorded by a needle electrode placed in the muscle from TA, AH and EDB muscles in the lower extremities.

RESULTS

Overall monitorability (i.e., at least 1 Lower Limb m-MEP recordable) was 100/107 (93.5%). In the remaining 100 surgeries in 3 cases, the only muscle that could be recorded at baseline was one AH, and in other 2 the EDB. Persistence (i.e., the recordability of m-MEP from baseline to the end of surgery) was 88.7% for TA, 89.8% for AH and 93.8% for EDB.

CONCLUSION

In our series, EDB m-MEPs have demonstrated a recordability superior to TA and a stability similar to AH. The explanations may be different and range from changes in the excitability of the cortical motor neuron to the different sensitivity to ischemia of the spinal motor neuron. EDB can be used alternatively or can be added to TA and AH as a target muscle of the lower limb in spinal surgery.

Fast or Slow? A Comparison Between Two Transcranial Electrical Stimulation Techniques for Eliciting Motor-Evoked Potentials During Supratentorial Surgery

PURPOSE

During intraoperative neurophysiological monitoring of motor pathways, two types of transcranial electrical stimulation are available, i.e., constant-current and constant-voltage stimulation. Few previous studies, performed only during spinal surgery, analyzed and compared them during intraoperative neurophysiological monitoring. The aim of our study was to compare these two stimulation techniques for eliciting motor-evoked potentials during intraoperative neurophysiological monitoring in a group of patients affected by supratentorial lesions.

METHODS

Supratentorial lesions from 16 patients were retrospectively collected and analyzed. Motor-evoked potentials were performed only from transcranial electrical stimulation because the inability to place the subdural strip electrodes correctly did not permit to perform direct cortical stimulation. At the beginning of surgery, in each patient, motor-evoked potentials were monitored by using both "fast-charge" constant-voltage and "slow-charge" constant-current stimulation. Several neurophysiological parameters were collected and compared between the two stimulation techniques by means of statistical analysis.

RESULTS

"Fast-charge" constant-voltage stimulation allowed statistically higher efficiency rates for eliciting motor-evoked potentials compared with "slow-charge" constant-current stimulation, both for upper and lower limbs. We also found that threshold and maximal charge as well as charge density were significantly lower during constant-voltage stimulation, thus lowering the potential tissue damage.

CONCLUSIONS

"Fast-charge" constant-voltage transcranial electrical stimulation is feasible and safe during intraoperative neurophysiological monitoring for supratentorial surgery and may be preferable to "slow-charge" constant-current stimulation.

Transcranial electrical stimulation technique for induction of unilateral motor evoked potentials

OBJECTIVE

Transcranial electrical stimulation motor evoked potentials (TES-MEP) are widely used to monitor motor function; however, broad current spread and induced body movement are limitations of this technique. We herein report a localized stimulation technique for TES-MEP that induces unilateral MEP responses.

METHODS

The stimulation of C1(+)-C4(-) or C2(+)-C3(-) was performed to induce right- or left-sided muscle contraction, respectively, in 70 patients. Electromyography was recorded by placing electrodes on the bilateral abductor pollicis brevis (APB) and abductor hallucis (AH) muscles. Stimulation conditions were regulated in the range to induce unilateral muscle contractions contralateral to the anodal stimulation. The thresholds and amplitudes of TES-MEP were retrospectively analyzed.

RESULTS

The thresholds of APB were lower than those of AH in 47 patients, AH thresholds were lower than those of APB in 6 patients, and both APB and AH started to respond at the same intensity in 15 patients. This technical stimulation induced contralateral limb contractions with a suprathreshold stimulation of 129.4 ± 35.6 mA (mean ± standard deviation) in 68 patients (97%). Amplitudes in the suprathreshold stimulation of APB and AH responses were 727.5 ± 695.7 and 403.3 ± 325.7 μV, respectively.

CONCLUSIONS

The C1(+)-C4/C2(+)-C3(-) stimulation in TES-MEP enables a localized stimulation to induce unilateral MEP responses.

SIGNIFICANCE

Our stimulation technique enables the stable and safe monitoring of unilateral limbs, and contributes to the reliable monitoring of motor function in neurosurgery.

Stimulation parameters for motor evoked potentials during intraoperative spinal cord monitoring. A systematic review

OBJECTIVE

The aim of this systematic review was to find the optimal stimulation parameters for muscle recorded transcranial electrical stimulation motor evoked potential (mTc-MEP) and D-wave monitoring during spinal cord monitoring.

METHODS

A PRISMA systematic search in Medline and EMBASE and a QUADAS-2 quality evaluation was performed to identify studies that compared stimulation parameters consisting of stimulation location, number of pulses, pulse duration, interstimulus interval, double train (DTS) or recurrent train stimulation (RTS) and intertrain interval (ITI) for performing mTc-MEP and D-wave monitoring. Only studies that used total intravenous anaesthesia (TIVA) were included.

RESULTS

Ten studies that compared stimulation parameters for performing mTc-MEP monitoring (stimulation location n = 4, number of pulses n = 2, pulse duration n = 1, interstimulus interval n = 4, DTS n = 1, RTS n = 2, ITI n = 2) were included. No studies compared stimulation parameters (stimulation location and pulse duration) for performing D-wave monitoring.

CONCLUSIONS

Few studies examined the optimal stimulation parameters for monitoring mTc-MEPs and no studies were included for D-wave monitoring. There is a need for prospective research to investigate the optimal stimulation parameters for mTc-MEP with the use of TIVA and D-wave monitoring.

SIGNIFICANCE

For mTc-MEP monitoring, a table is provided in which the recommended stimulation parameters are stated.

Updates on Intraoperative Neurophysiology During Surgery for Spinal Dysraphism

Spinal dysraphism is a group of disorders resulting from an embryologic failure of spinal cord development which can lead to a radicular-medullary mechanical stretch that generates vascular compromise and hypoxic-ischemic damage to the nervous structures of the conus-cauda region.Thus, the clinical relevance of the different types of spinal dysraphism is related to the possible neurologic deficits resulting from spinal cord tethering. The clinical presentation is heterogenous: from asymptomatic to very compromised patients. The indications and the time of a detethering surgery are still subject of debate, although there is an agreement on the high standards of treatment that have to be offered by the surgery. Intraoperative neurophysiology (ION) contributes to the safety of tethered cord surgery in reducing the risks of iatrogenic neurological damages.

Intraoperative Neuromonitoring for Pediatric Pelvic Tumors

Background

Tumors of the pre-sacral and sacral spaces are a rare occurrence in children. Total tumor excision is required due to the significant risk of relapse in the event of partial surgery, but the surgical procedure may lead to postoperative problems such as urinary, sexual, and anorectal dysfunctions. Intraoperative neuromonitoring (IONM) has gained popularity in recent years as a strategy for preventing the onset of neurologic impairments by combining several neurophysiological techniques. The aim of our study is to describe the experience of Bambino Gesù Children's Hospital in the use of IONM in pediatric pelvic surgery.

Materials and Methods

The data of patients treated for pelvic malignancies at Bambino Gesù Children's Hospital from 2015 to 2019 were retrospectively collected. All patients were assessed from a neurologic and neuro-urologic point of view at different time-points (before and immediately after surgery, after 6 months, and 1-year follow-up). They were all monitored during a surgical procedure using multimodal IONM including transcranial motor evoked potentials (TcMEP), triggered-EMG (t-EMG), pudendal somatosensory evoked potentials (PSSEP), and bulbocavernosus reflex (BCR).

Results

During the study period, ten children underwent pelvic tumor removal at our Institution. In all cases, intraoperative neurophysiological recordings were stable and feasible. The preservation of neurophysiological response at the same intensity during surgical procedures correlated with no new deficits for all neurophysiological techniques.

Discussion

Although the impact of the IONM on surgical strategies and clinical follow-up is unknown, this preliminary experience suggests that the appropriate use of several neurophysiological techniques can influence both the radicality of pelvic tumor removal and the neurological and urological outcome at clinical follow-up. Finally, because of the highly complex anatomy and inter-individual variances, this is especially useful in this type of surgery.

Transcranial Magnetic Stimulation and Transcranial Electrical Stimulation in Horses

Depending on the localization of the lesion, spinal cord ataxia is the most common type of ataxia in horses. Most prevalent diagnoses include cervical vertebral stenotic myelopathy (CVSM), equine protozoal myeloencephalitis (EPM), trauma and equine degenerative myeloencephalopathy (EDM). Other causes of ataxia and weakness are associated with infectious causes, trauma and neoplasia. A neurologic examination is indispensable to identify the type of ataxia. In addition, clinical neurophysiology offers tools to locate functional abnormalities in the central and peripheral nervous system. Clinical EMG assessment looks at the lower motoneuron function (LMN) and is used to differentiate between neuropathy in peripheral nerves, which belong to LMNs and myopathy. As LMNs reside in the spinal cord, it is possible to grossly localize lesions in the myelum by muscle examination. Transcranial (tc) stimulation techniques are gaining importance in all areas of medicine to assess the motor function of the spinal cord along the motor tracts to the LMNs. Applications in diagnostics, intraoperative neurophysiological monitoring (IONM), and evaluation of effects of treatment are still evolving in human medicine and offer new challenges in equine medicine. Tc stimulation techniques comprise transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES). TMS was first applied in horses in 1996 by Mayhew and colleagues and followed by TES. The methods are exchangeable for clinical diagnostic assessment but show a few differences. An outline is given on the principles, current clinical diagnostic applications and challenging possibilities of muscle evoked potentials (MEP) from transcranial stimulation in horses.

Monophasic-Quadripulse Theta Burst Magnetic Stimulation for Motor Palsy Functional Evaluation After Intracerebral Hemorrhage

Transcranial magnetic stimulation (TMS) is commonly employed for diagnostic and therapeutic purposes to enhance recovery following brain injury, such as stroke or intracerebral hemorrhage (ICH). Single-pulse TMS, most commonly used for diagnostic purposes and with motor evoked potential (MEP) recordings, is not suitable for clinical use in patients with severe motor paresis. To overcome this problem, we developed a quadripulse theta burst transcranial magnetic stimulation (QTS) device that combines the output from 16 stimulators to deliver a train of 16 monophasic magnetic pulses through a single coil. High-frequency theta rhythm magnetic bursts (bursts of four monophasic pulses, at 500 Hz, i.e., with a 2-ms interpulse interval, repeated at 5 Hz) were generated via a set of 16 separate magnetic stimulators connected to a specially designed combination module. No adverse effects or electroencephalogram (EEGs) abnormalities were identified during or after the recordings. MEP amplification in the QTS during four-burst theta rhythm stimulations produced four independent MEPs 20 ms after each burst onset maximizing the final third or fourth burst, which exhibited significantly greater amplitude than those resulting from a single burst or pulse. Motor functional palsy grades after ICH and QTS-MEP parameters and resting motor threshold (RMT) and amplitudes were significantly correlated (r = −0.83/−0.81 and 0.89/0.87; R2 = 0.69/0.66 and 0.79/0.76, p < 0.001; anterior/posterior-stimulus polarity, respectively). In conclusion, QTS-MEPs enabled a linear functional evaluation in patients with various degrees of motor paresis. However, the benefits, safety, and limitations of this device should be further explored in future studies.

Intraoperative mapping and monitoring during brain tumor surgeries

Many different methodologies and paradigms are available to guide surgery of supratentorial tumors with the aim to preserve quality of life of the patients and to increase the extent of tumor resection. Neurophysiologic monitoring techniques (such as different evoked potentials) may help to continuously assess functional integrity of the observed systems and warn about vascular injury. For neurophysiologic mapping methods, the focus is not only to preserve cortical sites, but also to prevent injury to subcortical pathways. Therefore, cortical mapping is not enough but should be combined with subcortical mapping to identify tracts. This may be done by alternating resection and stimulation, or by continuous mapping via an electrified surgical tool such as a stimulating suction tip. Increasingly refined techniques are evolving to improve mapping of complex motor networks as well as language and higher cortical functions. Finally, in deciding between an awake vs asleep intraoperative setting, various factors need to be considered, such as the surgical goal, patient expectation and cooperation, treating team expertise, and neurooncologic aspects including histopathology. Therefore, the choice of protocol depends on the clinical context and the experience of the interdisciplinary team treating the patients.

Safety issues during surgical monitoring

While intra-operative neuro-physiologic assessment and monitoring improve the safety of patients, its use may also introduce new risks of injuries. This chapter looks at the electric safety of equipment and the potential hazards during the set-up of the monitoring. The physical and functional physiologic effects of electric shocks and stimulation currents, standards for safety limits, and conditions for tissue damage are described from basic physical principles. Considered are the electrode-tissue interface in relation to electrode dimensions and stimulation parameters as applied in various modalities of evoked sensory and motor potentials as to-date used in intra-operative monitoring, mapping of neuro-physiologic functions. A background is given on circumstances for electric tissue heating and heat drainage, thermal toxicity, protection against thermal injuries and side effects of unintended activation of neural and cardiac tissues, adverse effects of physiologic amplifiers from transcranial stimulation (TES) and excitotoxicity of direct cortical stimulation. Addressed are safety issues of TES and measures for prevention. Safety issues include bite and movement-induced injuries, seizures, and after discharges, interaction with implanted devices as cardiac pacemaker and deep brain stimulators. Further discussed are safety issues of equipment leakage currents, protection against electric shocks, and maintenance.

Utility of evoked potentials during anterior cerebral artery and anterior communicating artery aneurysm clipping

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For anterior cerebral artery aneurysm clipping dual SSEPs and tcMEPs enhance detection of lower extremity deficits.

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Evoked potentials have limited utility in predicting upper extremity deficits related to subcortical ischemia.

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Four-extremity dual-modality monitoring can also detect pathogenetic mechanisms that are remote from the surgical site.

Objective

To investigate the optimal combination of somatosensory- and transcranial motor-evoked potential (SSEP/tcMEP) modalities and monitored extremities during clip reconstruction of aneurysms of the anterior cerebral artery (ACA) and its branches.

Methods

A retrospective review of 104 cases of surgical clipping of ruptured and unruptured aneurysms was performed. SSEP/tcMEP changes and postoperative motor deficits (PMDs) were assessed from upper and lower extremities (UE/LE) to determine the diagnostic accuracy of each modality separately and in combination.

Results

PMDs were reported in 9 of 104 patients; 7 LE and 8 UE (3.6% of 415 extremities). Evoked potential (EP) monitoring failed to predict a PMD in 8 extremities (1.9%). Seven of 8 false negatives had subarachnoid hemorrhage. Sensitivity and specificity in LE were 50% and 97% for tcMEP, 71% and 98% for SSEP, and 83% and 98% for dual-monitoring of both tcMEP/SSEP. Sensitivity and specificity in UE were 38% and 99% for tcMEP, and 50% and 97% for tcMEP/SSEP, respectively.

Conclusions

Combined tcMEP/SSEP is more accurate than single-modality monitoring for LE but is relatively insensitive for UE PMDs.

Significance

During ACA aneurysm clipping, multiple factors may confound the ability of EP monitoring to predict PMDs, especially brachiofacial hemiparesis caused by perforator insufficiency.

Neurophysiology during epilepsy surgery

Intraoperative neuromonitoring (IONM) complements modern presurgical investigations by providing information about the epileptic focus as well as real-time identification of critical functional tissue and assessment of ongoing neural integrity during resective epilepsy surgery. This chapter summarizes current IONM methods for mapping the epileptic focus and for mapping and monitoring functionally important structures with direct brain stimulation and evoked potentials. These techniques include electrocorticography, computerized high-frequency oscillation mapping, single-pulse electric stimulation, cortical and subcortical motor evoked potentials, somatosensory evoked potentials, visual evoked potentials, and cortico-cortical evoked potentials. They may help to maximize epileptic tissue resection while avoiding permanent postoperative neurologic deficits.

Surgery and intraoperative neurophysiologic monitoring for aneurysm clipping

This chapter describes the feasibility, utilization, and value of intraoperative neurophysiologic monitoring (IONM) in cerebrovascular cases. Practical advice on the integration of these adjunct methods into the modern neurosurgical operating room is based on our own neurophysiologic and neurosurgical experience. Most IONM is done for anterior circulation aneurysms. Somatosensory and motor evoked potentials are the modalities of choice covering vascular territories of the internal, anterior, and middle cerebral arteries. While monitoring both hemispheres with the unoperated side as control, monitoring focus is laid upon those territories at risk and bearing the aneurysm. The specificity of IONM is close to 1, and sensitivity ranges from 0.2 to 1, depending on the categorization of transient changes. The overall likelihood of worsened neurologic outcome after any intraoperative signal deterioration (transient or permanent) is 0.4.

Intraoperative evoked potential techniques

There are many recent advances in intraoperative evoked potential techniques for mapping and monitoring neural function during surgery. In particular, somatosensory evoked potential optimization speeds surgical feedback, motor evoked potentials provide selective motor system information, and new visual evoked potential methods promise reliable visual system monitoring. This chapter reviews these advances and provides a comprehensive background for understanding their context and importance.

"Quadripolar" Transcranial Electrical Stimulation for Motor Evoked Potentials

PURPOSE

To determine if transcranial electrical stimulation (TES)-induced motor evoked potentials (MEPs) are of higher amplitude when using two electrodes as anodes and two as cathodes, known as "quadripolar stimulation."

METHODS

Patients who underwent TES MEP monitoring in which control, bipolar stimulation and four variations of quadripolar stimulation were used were evaluated. The bipolar stimulation montage was C3-C4 (C3 was used as anode for stimulation first, then the polarity was switched to stimulate the contralateral side). Four quadripolar montages were used: C3/C1-C4/C2 (step 1), M3/M1-M4/M2 (step 2), C3/M1-C4/M2 (step 3), and M3/C1-M4/C2 (step 4). The area under the curve for the right foot TES MEP was compared for the various montages using descriptive statistics and Fisher exact test for proportions.

RESULTS

Sixteen patients were retrospectively evaluated. The mean age as 51.6 years, range 4 to 80 years; 11 were female. The transcranial electrical stimulation MEP area under the curve for the right foot MEP was highest in the bipolar montage in 1 of 16 patients (6.3%). Meanwhile, it was highest in step 4 (M3/C1-M4/C2) in 9 of 16 patients (56.3%; P = 0.027). The highest right foot MEP area under the curve with one of the quadripolar montages was seen in 15 of 16 patients (93.8%; P = 0.0001).

CONCLUSIONS

Quadripolar stimulation resulted in higher area under the curve for right foot MEP compared with conventional bipolar stimulation.

Transcranial Electrical Motor Evoked Potential in Predicting Positive Functional Outcome of Patients after Decompressive Spine Surgery: Review on Challenges and Recommendations towards Objective Interpretation

Spine surgeries impose risk to the spine's surrounding anatomical and physiological structures especially the spinal cord and the nerve roots. Intraoperative neuromonitoring (IONM) is a technology developed to monitor the integrity of the spinal cord and the nerve roots via the surgery. Transcranial motor evoked potential (TcMEP) (one of the IONM modalities) is adopted to monitor the integrity of the motor pathway of the spinal cord and the motor nerve roots. Recent research suggested that the IONM is conducive as a prognostic tool towards the patient's functional outcome. This paper summarizes the researches of IONM being adopted as a prognostic tool. In addition, this paper highlights the problems associated with the signal parameters as the improvement criteria in the previous researches. Lastly, we review the challenges of TcMEP to achieve a prognostic tool focusing on the factors that could interfere with the generation of a stable TcMEP response. The final section will discuss recommendations for IONM technology to achieve an objective prognostic tool.

Hand or foot train-of-four tests and surgical site muscle relaxation assessed with multiple motor evoked potentials: A prospective observational study

BACKGROUND

Intra-operative muscle relaxation is often required in orthopaedic surgery and the hand train-of-four (TOF) test is usually used for its quantification. However, even though full muscle relaxation is claimed by anaesthesiologists based on a TOF count of zero, surgeons observe residual muscle activity.

OBJECTIVE

The aim of the study was to assess if hand or foot TOF adequately represents intra-operative muscle relaxation compared with multiple motor evoked potentials.

DESIGN

Prospective observational study.

SETTING

A single-centre study performed between February 2016 and December 2018 at the Balgrist University Hospital, Zurich, Switzerland.

PATIENTS

Twenty patients scheduled for elective lumbar spinal fusion were prospectively enrolled in this study after giving written informed consent.

INTERVENTIONS

To assess neuromuscular blockade (NMB) with the intermediate duration nondepolarising neuromuscular blocking agent rocuronium, hand TOF (adductor pollicis) and foot TOF (flexor hallucis brevis) monitoring, and muscle motor evoked potentials (MMEPs) from the upper and lower extremities were assessed prior to surgery under general anaesthesia. Following baseline measurements, muscle relaxation was performed with rocuronium until the spinal surgeon observed sufficient relaxation for surgical intervention. At this timepoint, NMB was assessed by TOF and MMEP.

MAIN OUTCOME MEASURES

The primary outcome was to determine the different effect of rocuronium on muscle relaxation comparing hand and foot TOF with the paraspinal musculature assessed by MMEP.

RESULTS

Hand TOF was more resistant to NMB and had a shorter recovery time than foot TOF. When comparing MMEPs, muscle relaxation occurred first in the hip abductors, and the paraspinal and deltoid muscles. The most resistant muscle to NMB was the abductor digiti minimi. Direct comparison showed that repetitive MMEPs simultaneously recorded from various muscles at the upper and lower extremities and from paraspinal muscles reflect muscle relaxation similar to TOF testing.

CONCLUSION

Hand TOF is superior to foot TOF in assessing muscle relaxation during spinal surgery. Hand TOF adequately represents the degree of muscle relaxation not only for the paraspinal muscles but also for all orthopaedic surgical sites where NMB is crucial for good surgical conditions.

TRIAL REGISTRATION

ClinicalTrials.gov (NCT03318718).

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.

Preoperative nTMS and Intraoperative Neurophysiology - A Comparative Analysis in Patients With Motor-Eloquent Glioma

Background

The resection of a motor-eloquent glioma should be guided by intraoperative neurophysiological monitoring (IOM) but its interpretation is often difficult and may (unnecessarily) lead to subtotal resection. Navigated transcranial magnetic stimulation (nTMS) combined with diffusion-tensor-imaging (DTI) is able to stratify patients with motor-eloquent lesion preoperatively into high- and low-risk cases with respect to a new motor deficit.

Objective

To analyze to what extent preoperative nTMS motor risk stratification can improve the interpretation of IOM phenomena.

Methods

In this monocentric observational study, nTMS motor mapping with DTI fiber tracking of the corticospinal tract was performed before IOM-guided surgery for motor-eloquent gliomas in a prospectively collected cohort from January 2017 to October 2020. Descriptive analyses were performed considering nTMS data (motor cortex infiltration, resting motor threshold (RMT), motor evoked potential (MEP) amplitude, latency) and IOM data (transcranial MEP monitoring, intensity of monopolar subcortical stimulation (SCS), somatosensory evoked potentials) to examine the association with the postoperative motor outcome (assessed at day of discharge and at 3 months).

Results

Thirty-seven (56.1%) of 66 patients (27 female) with a median age of 48 years had tumors located in the right hemisphere, with glioblastoma being the most common diagnosis with 39 cases (59.1%). Three patients (4.9%) had a new motor deficit that recovered partially within 3 months and 6 patients had a persistent deterioration (9.8%). The more risk factors of the nTMS risk stratification model (motor cortex infiltration, tumor-tract distance (TTD) ≤8mm, RMT_ratio <90%/>110%) were detected, the higher was the risk for developing a new postoperative motor deficit, whereas no patient with a TTD >8mm deteriorated. Irreversible MEP amplitude decrease >50% was associated with worse motor outcome in all patients, while a MEP amplitude decrease ≤50% or lower SCS intensities ≤4mA were particularly correlated with a postoperative worsened motor status in nTMS-stratified high-risk cases. No patient had postoperative deterioration of motor function (except one with partial recovery) when intraoperative MEPs remained stable or showed only reversible alterations.

Conclusions

The preoperative nTMS-based risk assessment can help to interpret ambiguous IOM phenomena (such as irreversible MEP amplitude decrease ≤50%) and adjustment of SCS stimulation intensity.

Transcranial Motor Evoked Potential Monitoring in a Patient With a Deep Brain Stimulator: A Case Report

SUMMARY

A major complication of surgical scoliosis correction is permanent injury of the spinal cord. Intraoperative neuromonitoring continually evaluates spinal cord function through monitoring sensory and corticospinal motor tracts. There is no literature or manufacturer recommendation on whether transcranial motor evoked potential (tcMEP) monitoring can be performed safely in the presence of a deep brain stimulator (DBS) system. A 17-year-old adolescent boy with severe neuromuscular scoliosis presented for a posterior spinal fusion. The patient suffered from generalized dystonia treated with a DBS terminating in the left and right globus pallidus internus. The competing goals of monitoring motor function during the spinal fusion and preserving the integrity of the DBS system were discussed preoperatively. The DBS system was deactivated for the duration of surgery, and tcMEPs were used sparingly at the lowest suitable stimulation voltage. Intraoperative management focused on facilitating neurophysiologic monitoring through a total intravenous anesthetic of propofol, methadone, and remifentanil. The tcMEPS remained unchanged throughout the operation and the patient emerged able to move his lower extremities to command. Postoperatively, the DBS system was turned back on and showed retained settings, normal functioning, and unchanged impedance of the DBS leads. Neither the patient nor his parents reported any subjective changes in the symptoms of dystonia. The authors conclude that monitoring tcMEPs in the presence of a DBS implant may be done safely, when the clinical circumstances suggest that the added information gained from tcMEPs outweighs the theoretical risk to the DBS system and the course of the medical condition treated by the DBS.

Success Rate of Obtaining Baseline Somatosensory and Motor Evoked Potentials in 695 Consecutive Cranial and Spine Surgeries

PURPOSE

Intraoperative neurophysiological monitoring has been well documented as an adjunctive technique that significantly decreases the risk of developing inadvertent sensory and motor deficits during cranial and spine surgeries. The ability to detect neurologic problems intraoperatively depends largely on accurately identifying changes that occur in somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) during each procedure. Therefore, obtaining accurate and reproducible SSEP and MEP data during the initial setup is paramount for intraoperative monitoring. In 2007, Chen et al. found the overall success rate for establishing reliable MEP responses to be 94.8% in the upper extremities and 66.6% in the lower extremities. Since then, the success rate of obtaining baseline sensory and motor evoked potential responses has not been specifically reevaluated. The main goal of this study was to evaluate the current success rates of obtaining adequate SSEP and MEP baseline data in the current era, as well as take a closer look into some of the factors that can reduce the success rates.

METHODS

Somatosensory evoked potential and MEP monitoring was attempted in a total of 695 consecutive brain and spine surgeries performed by neurosurgeons and orthopedic surgeons between January 2010 and July 2011. Somatosensory evoked potential and MEP baseline data were obtained after initiation of general anesthesia and before skin incision. The primary measure is the ability to obtain adequate SSEP and MEP baseline in each extremity. A secondary measure was to stratify the success rate based on preoperative diagnosis.

RESULTS

Six hundred ninety-five consecutive cranial and spinal cases that required intraoperative monitoring were reviewed. Baseline upper extremity SSEPs were successfully obtained in 679 cases (98.1%), and baseline lower extremity SSEPs were successfully obtained in 626 cases (90.1%). However, if the preoperative diagnosis was in the category spine trauma or spine infection, the success rate of obtaining adequate baseline in the lower extremities dropped to around 60% for both SSEPs and MEPs.

CONCLUSIONS

The success rates of obtaining adequate baseline SSEP and MEP data are overall higher than previously reported. Preoperative diagnosis like spinal infection or trauma may predict lower success rates for acquiring adequate baseline SSEPs and MEPs.

The Effects of Transcranial Electrical Stimulation on Human Motor Functions: A Comprehensive Review of Functional Neuroimaging Studies

Transcranial electrical stimulation (tES) is a promising tool to enhance human motor skills. However, the underlying physiological mechanisms are not fully understood. On the other hand, neuroimaging modalities provide powerful tools to map some of the neurophysiological biomarkers associated with tES. Here, a comprehensive review was undertaken to summarize the neuroimaging evidence of how tES affects human motor skills. A literature search has been done on the PubMed database, and 46 relative articles were selected. After reviewing these articles, we conclude that neuroimaging techniques are feasible to be coupled with tES and offer valuable information of cortical excitability, connectivity, and oscillations regarding the effects of tES on human motor behavior. The biomarkers derived from neuroimaging could also indicate the motor performance under tES conditions. This approach could advance the understanding of tES effects on motor skill and shed light on a new generation of adaptive stimulation models.

Extramuscular Recording of Spontaneous EMG Activity and Transcranial Electrical Elicited Motor Potentials in Horses: Characteristics of Different Subcutaneous and Surface Electrode Types and Practical Guidelines

Introduction

Adhesive surface electrodes are worthwhile to explore in detail as alternative to subcutaneous needle electrodes to assess myogenic evoked potentials (MEP) in human and horses. Extramuscular characteristics of both electrode types and different brands are compared in simultaneous recordings by also considering electrode impedances and background noise under not mechanically secured (not taped) and taped conditions.

Methods

In five ataxic and one non-ataxic horses, transcranial electrical MEPs, myographic activity, and noise were simultaneously recorded from subcutaneous needle (three brands) together with pre-gelled surface electrodes (five brands) on four extremities. In three horses, the impedances of four adjacent-placed surface-electrode pairs of different brands were measured and compared. The similarity between needle and surface EMGs was assessed by cross-correlation functions, pairwise comparison of motor latency times (MLT), and amplitudes. The influence of electrode noise and impedance on the signal quality was assessed by a failure rate (FR) function. Geometric means and impedance ranges under not taped and taped conditions were derived for each brand.

Results

High coherencies between EMGs of needle-surface pairs degraded to 0.7 at moderate and disappeared at strong noise. MLTs showed sub-millisecond simultaneous differences while sequential variations were several milliseconds. Subcutaneous MEP amplitudes were somewhat lower than epidermal. The impedances of subcutaneous needle electrodes were below 900 Ω and FR = 0. For four brands, the FR for surface electrodes was between 0 and 80% and declined to below 25% after taping. A remaining brand (27G DSN2260 Medtronic) revealed impedances over 100 kΩ and FR = 100% under not taped and taped conditions.

Conclusion

Subcutaneous needle and surface electrodes yield highly coherent EMGs and TES-MEP signals. When taped and allowing sufficient settling time, adhesive surface-electrode signals may approach the signal quality of subcutaneous needle electrodes but still depend on unpredictable conditions of the skin. The study provides a new valuable practical guidance for selection of extramuscular EMG electrodes. This study on horses shares common principles for the choice of adhesive surface or sc needle electrodes in human applications such as in intraoperative neurophysiological monitoring of motor functions of the brain and spinal cord.

A practical guide for anesthetic management during intraoperative motor evoked potential monitoring

Postoperative motor dysfunction can develop after spinal surgery, neurosurgery and aortic surgery, in which there is a risk of injury of motor pathway. In order to prevent such devastating complication, intraoperative monitoring of motor evoked potentials (MEP) has been conducted. However, to prevent postoperative motor dysfunction, proper understanding of MEP monitoring and proper anesthetic managements are required. Especially, a variety of anesthetics and neuromuscular blocking agent are known to attenuate MEP responses. In addition to the selection of anesthetic regime to record the baseline and control MEP, the measures to keep the level of hypnosis and muscular relaxation at constant are crucial to detect the changes of MEP responses after the surgical manipulation. Once the changes of MEP are observed based on the institutional alarm criteria, multidisciplinary team members should share the results of MEP monitoring and respond to check the status of monitoring and recover the possible motor nerve injury. Prevention of MEP-related adverse effects is also important to be considered. The Working Group of Japanese Society of Anesthesiologists (JSA) developed this practical guide aimed to help ensure safe and successful surgery through appropriate anesthetic management during intraoperative MEP monitoring.

Effects of electrodes length and insulation for transcranial electric stimulation

Background:

The aim of this study is to investigate the effects of length and insulation of the corkscrew electrodes for transcranial motor evoked potential (tMEP) monitoring.

Methods:

We used the finite element method to visualize the electric field in the brain, which was generated by electrodes of different lengths (4, 7, and 12 mm). Two types of head models were generated: A model that included a subcutaneous fat layer and another without a fat layer. Two insulated needle types of conductive tip (5 and 2 mm) were studied. The stimulation threshold levels of hand tMEP were measured in a clinical setting to compare normal corkscrew and insulated 7-mm depth corkscrew.

Results:

The electric field in the brain depended on the electrode depths in the no fat layer model. The deeper the electrodes reached, the stronger the electric fields generated. Electrode insulation made a difference in the fat layer models. The threshold level recordings of tMEP revealed that the 7-mm insulated electrodes showed a lower threshold than the normal electrodes by one-side replacement in each patient: 33.6 ± 9.6 mA and 36.3 ± 11.0 mA (n =16, P < 0.001), respectively. The 7-mm insulated electrodes also showed a lower threshold than the normal electrodes when both sides, electrodes were replaced: 34.4 ± 8.6 mA and 37.5 ± 9.2 mA (n =10, P = 0.003), respectively.

Conclusions:

The electrodes depth reached enough to skull is considered to be efficient. Insulation of the electrodes with a conductive tip is efficient when there is subcutaneous fat layer.

State-of-the-Art Diagnostic Methods to Diagnose Equine Spinal Disorders, With Special Reference to Transcranial Magnetic Stimulation and Transcranial Electrical Stimulation

Spinal cord disorders are a common problem in equine medicine. However, finding the site of the lesion is challenging for veterinarians because of a lack of sensitive diagnostic methods that can assess neuronal functional integrity in horses. Although medical imaging is frequently applied to help diagnose corticospinal disorders, this approach does not reveal functional information. For the latter, transcranial magnetic stimulation (TMS) and more recently transcranial electrical stimulation (TES) can be useful. These are brain stimulation techniques that create either magnetic or electrical fields passing through the motor cortex, inducing muscular responses, which can be recorded either intramuscularly or extramuscularly by needle or surface electrodes. This permits the evaluation of the functional integrity of the spinal motor tracts and the nerve conduction pathways. The interest in TES in human medicine emerged these last years because unlike TMS, TES tends to bypass the motor cortex of the brain and predominantly relies on direct activation of corticospinal and extrapyramidal axons. Results from human medicine have indicated that TMS and TES recordings are mildly if not at all affected by sedation. Therefore, this technique can be reliably used in human patients under either sedation or full anesthesia to assess functional integrity of the corticospinal and adjunct motor tracts. This opens important new avenues in equine medicine.

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.

Influence of the Montage of Stimulation Electrodes for Intraoperative Neuromonitoring During Orthopedic Spine Surgery

PURPOSE

In transcranial electrical stimulation, induced motor evoked potentials (MEPs) are influenced by the montage of stimulation electrodes. Differences are to be examined between coronal and sagittal stimulation.

METHODS

Forty-five patients with idiopathic scoliosis were included. Coronal and sagittal montages were obtained by electrode placement at C3C4 and Cz'F using large contact electrodes. Corkscrew and short needle electrodes were additionally placed at C3C4 in five patients. Voltage motor thresholds (MTvoltage) and MEP amplitudes at 2 times MTvoltage (MEP2MTvoltage) were obtained of upper and lower extremity muscles. Differences of MTvoltage and MEP2MTvoltage at Cz'F and C3C4 and between electrodes were analyzed.

RESULTS

MEP2MTvoltage benefits from coronal positioning. Correlations between MTvoltage and impedance were not significant for large electrodes at Cz'F, very low for C3C4, and high for short needles or corkscrew electrodes. MTvoltage of short needles and corkscrews was up to 200% higher compared with MTvoltage of long needles. MTcurrent is increased by 20% to 30% and 2% to 10% for the arm and leg muscles, respectively.

CONCLUSIONS

Biphasic stimulation at C3C4 is advised when constant voltage stimulation is used to monitor the spinal cord during orthopedic spine surgery. MTvoltage of corkscrew and small needle electrodes are highly sensitive to electrode impedances.

An Alternative Transcranial Motor Evoked Potential Montage to Minimize Ipsilateral "Crossover" Motor Responses

Transcranial electrical motor evoked potential (TcMEP) is a modality utilized in intraoperative neurophysiological monitoring to assess the integrity of the corticospinal tract. Traditionally, TcMEPs are obtained by anodal stimulation of the scalp over the motor cortex of the selected hemisphere and referenced to the contralateral hemisphere. Subsequent compound motor action potential responses (CMAPs) are recorded at various muscles. The muscle responses of interest are usually those recorded in the limb contralateral to the hemisphere of stimulation. However, TcMEPs may elicit simultaneous muscle responses in the limbs ipsilateral to the hemisphere of stimulation, otherwise defined as "crossover" responses. Crossover TcMEPs are thought to be generated when electrical stimulation reaches the corticospinal tract at intracranial structures as deep as the medullary pyramids. If electrical stimulation penetrates deeper than the at-risk structures, false negatives motor evoked potential monitoring may occur. Therefore, in surgeries where cerebral cortical structures may be at risk, TcMEPs may be elicited so that contralateral CMAP responses are present without crossover responses. We present three cases using an alternative TcMEP montage in which anodal stimulation of the target hemisphere is referenced to Fpz at midline. Compared with the C3-C4/C4-C3 montage, crossover responses are minimized with the modified montage. C3-Fpz/C4-Fpz TcMEP stimulation may be a potential tool to implement in certain intraoperative neuromonitoring cases.

Tractography Verified by Intraoperative Magnetic Resonance Imaging and Subcortical Stimulation During Tumor Resection Near the Corticospinal Tract

BACKGROUND

Tractography is a popular tool for visualizing the corticospinal tract (CST). However, results may be influenced by numerous variables, eg, the selection of seeding regions of interests (ROIs) or the chosen tracking algorithm.

OBJECTIVE

To compare different variable sets by correlating tractography results with intraoperative subcortical stimulation of the CST, correcting intraoperative brain shift by the use of intraoperative MRI.

METHODS

Seeding ROIs were created by means of motor cortex segmentation, functional MRI (fMRI), and navigated transcranial magnetic stimulation (nTMS). Based on these ROIs, tractography was run for each patient using a deterministic and a probabilistic algorithm. Tractographies were processed on pre- and postoperatively acquired data.

RESULTS

Using a linear mixed effects statistical model, best correlation between subcortical stimulation intensity and the distance between tractography and stimulation sites was achieved by using the segmented motor cortex as seeding ROI and applying the probabilistic algorithm on preoperatively acquired imaging sequences. Tractographies based on fMRI or nTMS results differed very little, but with enlargement of positive nTMS sites the stimulation-distance correlation of nTMS-based tractography improved.

CONCLUSION

Our results underline that the use of tractography demands for careful interpretation of its virtual results by considering all influencing variables.

[Technique and advantages of multimodal intraoperative neuromonitoring for complex spinal interventions in older patients]

BACKGROUND

Complex spinal surgery in elderly patients mostly treats degenerative spine alterations. The use of multimodal intraoperative neuromonitoring (IONM) has proven to be a useful tool to recognize neural deterioration during such operations. Elderly patients often have preexisting neural impairment, which leads to difficulties in deriving some potentials or can even lead to not obtaining any potentials at all.

PRACTICE AND PROSPECTS

For reliable benefits from IONM a combined use of monitoring and mapping methods as well as the right choice of methods according to the spine level to be treated and a definition of the neural structures in danger is needed. This article intends to explain IONM methods in procedures treating degenerative spine alterations in a comprehensive way and to show our point of view on pedicle stimulation. Readers should be motivated to deepen their knowledge in these methods and to gain confidence and experience to increase the safety of these operations for the benefit of our patients.

Intraoperative Neurophysiologic Monitoring for Degenerative Cervical Myelopathy

Multimodal intraoperative neurophysiologic monitoring is a reliable tool for detecting intraoperative spine injury and is recommended during surgery for degenerative cervical myopathy (DCM). Somatosensory evoked potential (SEP) can be used to monitor spine and peripheral nerve injury during positioning in surgery for DCM. Compensation technique for transcranial evoked muscle action potentials (tcMEPs) should be adopted in intraoperative monitoring during surgery for DCM. Free-running electromyography is a useful real-time monitoring add-on modality in addition to SEP and tcMEP.

Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie

OBJECTIVE

To determine optimal interstimulus interval (ISI) and pulse duration (D) for direct cortical stimulation (DCS) motor evoked potentials (MEPs) based on rheobase and chronaxie derived with two techniques.

METHODS

In 20 patients under propofol/remifentanil anesthesia, 5-pulse DCS thenar MEP rheobase and chronaxie with 2, 3, 4 and 5ms ISI were measured by linear regression of five charge thresholds at 0.05, 0.1, 0.2, 0.5 and 1msD, and estimated from two charge thresholds at 0.1 and 1msD using simple arithmetic. Optimal parameters were defined by minimum threshold energy: the ISI with lowest rheobase²×chronaxie, and D at its chronaxie. Near-optimal was defined as threshold energy <25% above minimum.

RESULTS

The optimal ISI was 3 or 4 (n=7 each), 2 (n=4), or 5ms (n=2), but only 4ms was always either optimal or near-optimal. The optimal D was ∼0.2 (n=12), ∼0.1 (n=7) or ∼0.3ms (n=1). Two-point estimates closely approximated five-point measurements.

CONCLUSIONS

Optimal ISI/D varies, with 4ms/0.2ms being most consistently optimal or near-optimal. Two-point estimation is sufficiently accurate.

SIGNIFICANCE

The results endorse 4ms ISI and 0.2msD for general use. Two-point estimation could enable quick individual optimization.

Neurophysiological assessment of spinal cord injuries in dogs using somatosensory and motor evoked potentials

Somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) are non-invasive neurophysiological tests that reflect the functional integrity of sensory and motor pathways. Despite their extensive use and description in human medicine, reports in veterinary medicine are scarce. SSEPs are obtained via peripheral stimulation of sensory or mixed nerves; stimulation induces spinal and cortical responses, which are recorded when sensory pathways integrity is preserved. MEPs can be obtained via transcranial electrical or magnetic stimulation; in this case, thoracic and pelvic limb muscle responses are captured if motor pathways are preserved. This review describes principles, methodology and clinical applicability of SSEPs and MEPs in companion animal medicine. Potential interferences of anesthesia with SSEP and MEP recording are also discussed.

Corticobulbar motor evoked potentials from tongue muscles used as a control in cervical spinal surgery

Objective

Motor evoked potentials (MEPs) changes might be caused to the non-surgically induced factors during cervical spinal surgery. Therefore, control MEPs recorded cranially to the exit of the C5 root are highly recommendable in cervical spinal surgery. We studied whether corticobulbar MEPs (C-MEPs) from tongue muscle could be used as a control MEPs in cervical spinal surgery.

Methods

Twenty-five consecutive cervical spinal surgeries were analyzed. Stimulation of motor area for tongue was done by subcutaneous electrodes placed at C3/C4 (10-20 EEG System), and recording was done from both sides of tongue.

Results

C-MEPs were recorded successfully 24 out of the 25 (96%) tested patients. Forty-six out of fifty MEPs (92%) from tongue muscles were monitorable from the baseline. In two patients, we could obtain only unilateral C-MEPs. Mean MEPs latencies obtained from the left and right side of the tongue were 11.5 ± 1 ms and 11.5 ± 0.8 ms, respectively.

Conclusions

Monitoring C-MEPs from tongue muscles might be useful control in cervical spinal surgery. They were easily elicited and relatively free from phenomenon of peripheral stimulation of the hypoglossal nerves.

Significance

This is first study to identify the usefulness of C-MEPs as a control of cervical spinal surgery.

Intraoperative Mapping and Monitoring of the Pyramidal Tract Using Endoscopic Depth Electrodes

OBJECTIVE

To evaluate motor function during neuroendoscopic resectioning of deep-seated brain tumors using endoscopically guided depth electrodes.

METHODS

For 12 cases of thalamic tumors, including high-grade gliomas, germinomas, and malignant lymphomas, depth electrodes were inserted using endoscopic guides between the tumor and the pyramidal tract in the thalamus. Motor-evoked potentials (MEPs) were continuously recorded during neuroendoscopic resectioning of the tumors.

RESULTS

Monitoring of MEP responses using depth electrodes in all 12 cases was successful. The minimum stimulus intensity threshold required to induce MEP responses was 3 mA. Gross total or subtotal resections were successful with this technique for all patients with glioma. No additional neurologic impairments were found after surgery in any of the cases.

CONCLUSIONS

Continuous MEP measurement using depth electrodes can serve as a new monitoring technique for endoscopic resectioning of deep-seated brain tumors.

Intraoperative neurophysiological monitoring during resection of infratentorial lesions: the surgeon's view

OBJECTIVE

Methods of choice for neurophysiological intraoperative monitoring (IOM) within the infratentorial compartment mostly include early brainstem auditory evoked potentials, free-running electromyography, and direct cranial nerve (CN) stimulation. Long-tract monitoring with somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) is rarely used. This study investigated the incidence of IOM alterations during posterior fossa surgery stratified for lesion location.

METHODS

Standardized CN and SEP/MEP IOM was performed in 305 patients being treated for various posterior fossa pathologies. The IOM data were correlated with lesion locations and histopathological types as well as other possible confounding factors.

RESULTS

Alterations in IOM were observed in 158 of 305 cases (51.8%) (CN IOM alterations in 130 of 305 [42.6%], SEP/MEP IOM alterations in 43 of 305 [14.0%]). In 15 cases (4.9%), simultaneous changes in long tracts and CNs were observed. The IOM alterations were followed by neurological sequelae in 98 of 305 cases (32.1%); 62% of IOM alterations resulted in neurological deficits. Sensitivity and specificity for detection of CN deficits were 98% and 77%, respectively, and 95% and 85%, respectively, for long-tract deficits. Regarding location, brainstem and petroclival lesions were closely associated with concurrent CN IOM and SEP/MEP alterations.

CONCLUSIONS

The incidence of IOM alterations during surgery in the posterior fossa varied widely between different lesion locations and histopathological types. This analysis provides crucial information on the necessity of IOM in different surgical settings. Because MEP/SEP and CN IOM alterations were commonly observed during posterior fossa surgery, the authors recommend the simultaneous use of both modalities based on lesion location.