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

Comparison of muscle MEPs from transcranial magnetic and electrical stimulation and appearance of reflexes in horses

Introduction Transcranial electrical (TES) and magnetic stimulation (TMS) are both used for assessment of the motor function of the spinal cord in horses. Muscular motor evoked potentials (mMEP) were compared intra-individually for both techniques in five healthy horses. mMEPs were measured twice at increasing stimulation intensity steps over the extensor carpi radialis (ECR), tibialis cranialis (TC), and caninus muscles. Significance was set atp< 0.05. To support the hypothesis that both techniques induce extracranially elicited mMEPs, literature was also reviewed. Results Both techniques show the presence of late mMEPs below the transcranial threshold appearing as extracranially elicited startle responses. The occurrence of these late mMEPs is especially important for interpretation of TMS tracings when coil misalignment can have an additional influence. Mean transcranial motor latency times (MLT; synaptic delays included) and conduction velocities (CV) of the ECR and TC were significantly different between both techniques: respectively, 4.2 and 5.5 ms (MLTTMS--MLTTES), and -7.7 and -9.9 m/s (CVTMS-CVTES). TMS and TES show intensity-dependent latency decreases of, respectively, -2.6 (ECR) and -2.7 ms (TC)/30% magnetic intensity and -2.6 (ECR) and -3.2 (TC) ms/30V. When compared to TMS, TES shows the lowest coefficients of variation and highest reproducibility and accuracy for MLTs. This is ascribed to the fact that TES activates a lower number of cascaded interneurons, allows for multipulse stimulation, has an absence of coil repositioning errors, and has less sensitivity for varying degrees of background muscle tonus. Real axonal conduction times and conduction velocities are most closely approximated by TES. Conclusion Both intracranial and extracranial mMEPs inevitably carry characteristics of brainstem reflexes. To avoid false interpretations, transcranial mMEPs can be identified by a stepwise latency shortening of 15-20 ms when exceeding the transcranial motor threshold at increasing stimulation intensities. A ring block around the vertex is advised to reduce interference by extracranial mMEPs. mMEPs reflect the functional integrity of the route along the brainstem nuclei, extrapyramidal motor tracts, propriospinal neurons, and motoneurons. The corticospinal tract appears subordinate in horses. TMS and TES are interchangeable for assessing the functional integrity of motor functions of the spinal cord. However, TES reveals significantly shorter MLTs, higher conduction velocities, and better reproducibility

Multipulse transcranial electrical stimulation (TES):Normative data for motor evoked potentials in healthy horses

Background: There are indications that transcranial electrical stimulation (TES) assesses the motor function of the spinal cord in horses in a more sensitive and reproducible fashion than transcranial magnetic stimulation (TMS). However, no normative data of TES evoked motor potentials (MEP) is available. In this prospective study normative data of TES induced MEP wave characteristics (motor latency times (MLT); amplitude and waveform) was obtained from the extensor carpi radialis (ECR) and tibial cranialis (TC) muscles in a group of healthy horses to create a reference frame for functional diagnostic purposes. For the 12 horses involved in the study 95% confidence intervals for MLTs were 16.1-22.6 ms and 31.9-41.1 ms for ECR and TC muscles respectively. Intraindividual coefficients of variation (CV) and mean of MLTs were: ECR: 2.2-8,2% and 4.5% and TC: 1.4-6.3% and 3.5% respectively. Inter-individual CVs for MLTs were higher, though below 10% on all occasions. The mean +/- sd of MEP amplitudes was respectively 3.61 +/- 2.55 mV (ECR muscle left) and 4.53 +/- 3.1 mV (right) and 2.66 +/- 2.22 mV (TC muscle left) and 2.55 +/- 1.85 mV (right). MLTs showed no significant left versus right differences. All MLTs showed significant (p < 0.05) voltage dependent decreases with slope coefficients of linear regression for ECR: 0.049; - 0.061 ms/V and TC: - 0.082; - 0.089 ms/V (left; right). There was a positive correlation found between height at withers and MLTs in all 4 muscle groups. Finally, reliable assessment of MEP characteristics was for all muscle groups restricted to a transcranial time window of approximately 15-19 ms. Conclusions: TES is a novel and sensitive technique to assess spinal motor function in horses. It is easy applicable and highly reproducible. This study provides normative data in healthy horses on TES induced MEPs in the extensor carpi radialis and tibialis cranialis muscles bilaterally. No significant differences between MLTs of the left and right side could be demonstrated. A significant effect of stimulation voltage on MLTs was found. No significant effect of height at the withers could be found based upon the results of the current study. A study in which both TMS and TES are applied on the same group of horses is needed

The value of intraoperative neurophysiological monitoring in tethered cord surgery

The value of intraoperative neurophysiological monitoring (IONM) with surgical detethering in dysraphic patients has been questioned. A retrospective analysis of our series of 65 patients is presented with special focus on technical set-up and outcome. All patients were diagnosed with a tethered cord (TC) due to spinal dysraphism. A high-risk group (HRG) was determined consisting of 40 patients with a lipomyelomeningocele and/or a split cord malformation sometimes in combination with a tight filum terminale. The surgical procedure was a detethering operation in all cases performed by a single surgeon during a 9-year period (1999-2008). A standard set-up of IONM was used in all patients consisting of motor-evoked potentials (MEP) evoked by transcranial electrical stimulation (TES) and electrical nerve root stimulation. In young patients, conditioning stimulation was applied in order to improve absent or weak MEPs. IONM responses could be obtained in all patients. Postoperative deterioration of symptoms was found in two patients of whom one patient belonged to the HRG. Mean maximal follow-up of all 65 patients was 4.6 years (median 4.1 years). Long-term deterioration of symptoms was found in 6 of 65 patients with a mean follow-up of 5 years (median 5.3 years). The use of IONM is feasible in all TC patients. The identification of functional nervous structures and continuous guarding of the integrity of sacral motor roots by IONM may contribute to the safety of surgical detethering

The Percentage of Amplitude Decrease Warning Criteria for Transcranial MEP Monitoring

Muscle motor evoked potentials (MEPs) from transcranial electrical stimulation (TES) became a standard technique for monitoring the motor functions of the brain and spinal cord at risk during spinal and brain surgery. However, a wide range of criteria based on the percentage of amplitude decrease is used in practice. A survey of the current literature on clinical outcome parameters reveals a variety of percentages in a range of 30% to 100% (50% to 100% spinal procedures) with no consensus. The interpretation of muscle MEPs is hampered by their sensitivity to many interfering factors. Trial-to-trial MEP variations may partly be reduced by controllable parameters of which TES parameters are in the hands of the neuromonitorist. We propose an operational model based on basic neurophysiologic knowledge to interpret the characteristics of MEP-TES voltage curves and predict the influences of the location on the sigmoid voltage curve on spontaneous MEP-variations and influences of factors affecting the voltage curve. The model predicts a correlation between the slope, expressed by a gain, and variations of muscle MEP amplitudes. This complies with two case examples. The limited specificity/sensitivity of warning criteria based on the percentage of amplitude reduction can possibly be improved by developing standards for set-up procedures of TES paradigms. These procedures include strategies for desensitizing MEPs for variations of controllable parameters. The TES voltage or current is a feasible controlling parameter and should be related to the motor threshold and the onset of the supramaximal level being landmarks of MEP-voltage functions. These parameters may offer a valuable addition to multicenter outcome studies

Detection of tremor bursts by a running second order moment function and analysis using interburst histograms

Introduction: Conventional linear signal processing techniques are not always suitable for the detection of tremor bursts in clinical practice due to inevitable noise from electromyographic (EMG) bursts. This study introduces (1) a non-linear analysis technique based on a running second order moment function (SOMF) and (2) auto- and cross-interburst interval histograms (IBIH) showing distributions of interburst interval EMG bursts of pathological tremors illustrating an application of the SOME Materials and methods: EMG recordings from extensors and flexors of two patients with Parkinson's disease with a rest tremor and from a healthy subject during sustained muscular contraction were preliminary analyzed in a pilot study. The SOMF was obtained by repeated second order moment calculations within a window of fixed width W (time scale parameter) plotted as a function of time. Minimum SOMF values indicate local "moments of inertia" of each EMG burst. Bursts were detected and located when minimum SOMF values were below level L (decision parameter). Optimal settings of parameters W and L were calculated empirically for pathological tremor EMGs. Auto- and cross-IBIHs were obtained from minimum SOMF values of detected bursts. Results: Tremor frequency and phase relation between EMG bursts from auto- and cross-IBIHs agreed with those derived from spectral analysis. Burst detection by SOMF has a high sensitivity and selectivity even with noisy background. Conclusion: The SOMF is appropriate for detection of individual EMG bursts of pathological tremors. The technique is sensitive to non-stationary changes of tremor bursts regardless of their amplitude. IBIHs provide a measure of tremor frequency and phase difference between EMG bursts. (c) 2007 IPEM. Published by Elsevier Ltd. All rights reserved

Cervical osteotomy in ankylosing spondylitis:evaluation of new developments

Objectives: Cervical osteotomy can be performed on patients with cervical kyphosis due to ankylosing spondylitis. This study reviews the role of two new developments in cervical osteotomy surgery: internal fixation and transcranial electrical stimulated motor evoked potential monitoring (TES-MEP). Methods: From 1999 to 2004, 16 patients underwent a C7-osteotomy with internal fixation. In 11 patients, cervical osteotomy was performed in a sitting position with halo-cast immobilization (group S), five patients underwent surgery in prone position with Mayfield clamp fixation (group P). In group P, longer fusion towards T4-T6 could be obtained that created a more stable fixation. Therefore, post-operative immobilization protocol of group P was simplified from halo-cast to cervical orthosis. Results: Consolidation was obtained in all patients without loss of correction. Post-operative chin-brow to vertical angle measured 5° (range 0–15). TES-MEP was successfully performed during all surgical procedures. In total, nine neurological events were registered. Additional surgical intervention resulted in recovery of amplitudes in six of nine events. In two patients spontaneous recovery took place. One patient showed no recovery of amplitudes despite surgical intervention and a partial C6 spinal cord lesion occurred. Conclusion: We conclude that C7 osteotomy with internal fixation has been shown to be a reliable and stable technique. When surgery is performed the in prone position, distal fixation can be optimally obtained allowing post-operative treatment by cervical orthosis instead of a halo-cast. TES-MEP monitoring has been shown to be a reliable neuromonitoring technique with high clinical relevancy during cervical osteotomy because it allows timely intervention before occurrence of permanent cord damage in a large proportion of the patients

Recovery of TES-MEPs During Surgical Decompression of the Spine: A Case Series of Eight Patients

Purpose: This study aimed to illustrate the recovery of transcranial electrical stimulation motor evoked potentials during surgical decompression of the spinal cord in patients with impaired motor function preoperatively. Specific attention was paid to the duration of neurologic symptoms before surgery and the postoperative clinical recovery. Methods: A case series of eight patients was selected from a cohort of 74 patients that underwent spine surgery. The selected patients initially had low or absent transcranial electrical stimulation motor evoked potentials followed by a significant increase after surgical decompression of the spinal cord. Results: A significant intraoperative increase in amplitude of motor evoked potentials was detected after decompression of the spinal cord or cauda equina in patients suffering from spinal canal stenosis (n = 2), extradural meningioma (n = 3), or a herniated nucleus polposus (n = 3). This was related to an enhanced neurologic outcome only if patients (n = 6) had a short onset (less than = year) of neurologic impairment before surgery. Conclusions: In patients with a short onset of neurologic impairment because of compression of the spinal cord or caudal fibers, an intraoperative recovery of transcranial electrical stimulation motor evoked potentials can indicate an improvement of motor function postoperatively. Therefore, transcranial electrical stimulation motor evoked potentials can be considered as a useful tool to the surgeon to monitor the quality of decompression of the spinal cord