运动诱发电位研究现状
运动诱发电位(motor evoked potentials'MEP)是继体感诱发电位(somatosensory evoked potentials，SEP)后，为检查运动神经系统功能而设计的一项神经电生理学检查方法。作为一种无创伤性的检测手段，MEP已广泛应用于运动神经 系统疾病的诊断、术中监护和预后估计，尤其是近年来，随着电生理学和叠加平均技术的完善，MEP的适用范围日益拓广。现就其基本原理、特征以及临床应用等 研究现状简介如下。
1 MEP的基本原理
MEP是指应用电或磁刺激皮层运动区产生的兴奋通过下行传导径路，使脊髓前角细胞或周围神经运动纤维去极化，在相应肌肉或神经表面记录到的电位。早在1954年，Patton和Amassion等用重复电刺激经颅兴奋猴的皮质运动区，在颈髓部用球状电极记录到MEP。但由于刺激局部剧痛， 病人难以忍受，故临床应用受限。八十年代初，Merton和Morton使用高压脉冲电流(750V，5μs，1200mA）作为刺激源，局部疼痛明显减 轻。1985年，Barker等首先应用经颅磁刺激人运动皮层技术诱发 MEP，由于磁性刺激在头皮上产生的诱导电流很弱，不足以兴奋痛觉感受器，因此受检者无任何不适，使MEP真正得以在临床上越来越广泛应用。
MEP的传导途径，各作者尚有不同看法。多数学者认为MEP是沿皮质脊髓束、红核脊髓束等位于脊髓前索和前外侧索的运动束传导。Levy等在动 物实验中发现，手术显微镜下单独切断皮质脊髓束，MEP的大部分波形消失，进一步论证了皮质脊髓束是MEP的主要传导途径。但也有作者提出 MEP的传导途径中，同样包含了可逆行传导的感觉束，其依据为保留后束的脊髓切除术并不能完全消除MEP。
2 MEP的基本特征及影响因素
2．1 基本特征
MEP是由一组不同极性的波组成，其潜伏期和波幅各不相同。通常第一个波叫D波或直接波，呈单个的正相波，它的潜伏期较短，是皮层运动区第V层 锥体细胞的轴突始段兴奋的结果，其传导不经过突触传递，受麻醉药物的影响最小。D波之后的一系列波称为I波或间接波，表现为5个左右的正相／负相波，是联 络纤维间接兴奋锥体细胞所致，潜伏期长，易受外界因素影响。所以，临床上多用D波的潜伏期和波幅作为监护指标。
2．2 影响因素
2.2．1 麻醉药物 麻醉药物对MEP的波幅与潜伏期影响较大。1993年，Glassman报道了多种麻醉药物对MEP的影响，认为在诱导 麻醉期，硫喷妥钠对MEP的影响较大，而甲苄咪酯的影响较小；在维持麻醉期，氟烷对MEP的影响较大，而芬太尼和氯胺酮的影响较小。Yamada也 证实了麻醉剂对肌肉MEP的波幅与潜伏期有显著影响，而对脊髓MEP的影响甚小。
2．2．2 刺激强度 脊髓前角细胞包括小运动神经元和大运动神经元。小运动神经元兴奋阈值低，发出慢神经纤维；大运动神经元兴奋阈值高，发出 快神经纤维。当刺激电压较低时，只能兴奋小运动神经元，产生长潜伏期、低波幅的MEP；当刺激电压逐步升高后，可同时兴奋大小运动神经元，产生短潜伏期、 高波幅的MEP。鉴于刺激强度对MEP有如此的影响，检测时恒定的刺激参数对检测结果的正确解释至关重要。
2．2．3 肌肉收缩 1992年，Hayes等发现在脊髓损伤病人中，经颅磁刺激前若先辅以经皮电刺激可使局部肌肉的MEP更易被引出。这点对于脊髓损伤程度的判断极为重要。对于那些脊髓损伤后MEP消失的病人，若在经皮电刺激的基础上经颅磁刺激能诱发出MEP，则提示脊髓为不 全损伤；若在经皮电刺激的基础上仍未引出MEP，则说明脊髓为完全损伤。
3 MEP的应用
3．1 脊髓疾病诊断
神经系统疾病的诊断过去多依赖于临床的问诊，查体与CT、MRI等形态学检查相结合，缺乏直接的运动神经系统或感觉神经系统功能检查。因此，对于某些早期病变或亚临床病变，漏诊误诊率较高。MEP直接反映了运动系统功能的完整性，为神经系统疾病的诊断开辟了新的途径。
Maerten'Dvorak等人的研究指出，在脊椎病变和椎间盘突出症中，MEP的敏感性为84％，较SEP的36％明显增高，并且MEP对 颈椎管狭窄的敏感性略高于腰椎管狭窄(72％VS65%）。推测与颈椎管体积小，狭窄后更易压迫脊髓所致〔13、14〕。Machida则报导了MEP对 外伤性脊髓损伤病人的敏感性为85％。
1994年，Linden和Berlit等人对脊髓病中MEP的改变进行了较多细致的研究，结果显示MEP诊断脊髓运动损伤的特异性明显优于 SEP。在肿瘤性脊髓疾病中，MEP的改变通常表现为波幅的下降或波形的消失，而炎症性疾病中，潜伏期的延长更多见，推测原因为肿瘤性疾病的病理改变以神 经元和轴突的破坏为主，故波幅下降；炎症性疾病以脱髓鞘为主，故潜伏期延长。此外，有些学者观察到急性病变较慢性病变更易引起MEP的改变，可能 与慢性病变中，代偿机制发挥作用保护脊髓功能有关。
1988年，Caramia对79名有感觉、运动功能障碍的患者进行了MEP检测，结果显示49名多发性硬化病人中，MEP有改变者占54％，多表现为潜伏期的延长；其中有临床症状的病人，MEP检测阳性率为100％，而亚临床症状的病人，阳性率为40％。9名肌萎缩性侧索硬化病人中，MEP有 改变者占67％，多表现为波幅的下降或消失。至于Parkinson′s病人和Hungtington′s病人，MEP的波幅与潜伏期未见异常'这可能与 疾病未直接影响锥体系功能有关。
由于MEP是一项客观的功能检测，因此，也有作者将其应用于鉴别心因性瘫痪和器质性瘫痪，虽属个案报导，但值得借鉴。除了观察波幅与潜伏期的变化，MEP后抑制期和神经传导时间的测定也对脊髓损伤有诊断意义。
3.2 预后的判断
在脊髓疾病或损伤中，MEP的表现是由脊髓破坏的程度决定的：白质纤维脱髓鞘越重，前角运动细胞损伤数目越多，则MEP的潜伏期延长和波幅降低越显著。因此，通过观察MEP的潜伏期和波幅改变，可以对脊髓运动功能的损伤程度以及预后情况作出判断。
Levy曾在造成脊髓慢性不全性损伤的动物模型中，做连续1个月的MEP跟踪检查发现，动物在恢复瘫痪肢体的活动功能之前有MEP出现，且出现率达100％。国内孙天胜、胥少汀等也通过动物实验证实早期出现MEP是脊髓损伤预后良好的指征，MEP的恢复常先于动物的运动功能改善。
临床应用方面，1993年，De-Mattei对12名脊髓受压的患者进行了术前、术后2周、术后2月的MEP对比。结果提示，11名中枢神经 传导时间在术后增快的患者，术后运动功能恢复良好；而神经传导时间无明显变化者，术后症状缓解较差。Clarke对外伤性脊髓损伤的长期MEP跟 踪检查结果也证实，凡MEP在术后6个月有恢复的患者，瘫痪症状明显减轻，而术后6个月内持续无MEP恢复的患者，瘫痪症状无改变。Kai等则将 MEP的波形分为快波和慢波两个组成成分，指出快波成分与运动功能完整性的相关性较高。凡快波恢复者，术后运动功能正常；快波消失，仅慢波恢复者，术后运 动功能轻度障碍；快慢波均未恢复者，术后运动功能严重障碍。
但是，也有些学者提出：MEP对脊髓损伤的发生敏感性很高，但对于损伤后运动功能的恢复'则无明显的相关一致性。
3．3 术中监护
随着外科技术的进步，脊柱手术的种类大为扩展。但术后并发脊髓损伤的患者也较过去明显增多了。为了减少或避免此项严重并发症，临床采用的方法有 以下两种：①术中唤醒试验；②术中诱发电位监护。唤醒试验法(wake up test)是Stagnara于1973年首次报道的，试验结果可靠。但由于存在反应不可逆和唤醒后可能导致肺栓塞、内固定器械脱落等问题，近年来已逐渐 被术中诱发电位监护所取代。诱发电位包括了MEP和SEP两大类，分别监测运动传导功能和感觉传导功能，二者互为补充。其中有关SEP的术中监护研究较 多，而MEP的研究则有待进一步深入。
1988年，Owen报导了111例病人的术中MEP监护，是一次较早期的大宗病例调查，结果有90％的患者在手术过程中监测到了稳定的 MEP，其余未监测到MEP的患者后被证实是由于检测手段的错误所致，并及时得到了纠正。术后功能检测，凡在术中监护期间有稳定MEP出现者，无一人并发 脊髓损伤。从而论证了MEP用于术中监护的可行性。
但具体的监护标准该如何界定，各国的学者意见尚不统一。Machida在动物实验中发现，用类似Harrington的器械牵拉猫的脊 柱后，所有猫均出现了波幅不同程度的降低，当降低幅度超过50%、持续时间大于7min时，术后截瘫发生率为100％。Lee的试验也证实：MEP用于术 中监护时，波幅的改变比潜伏期的改变更有意义，因为潜伏期的变异性较大。至于波幅下降的程度，应不超过2/3，否则就难免会导致术后神经功能受损。而 Glassman通过试验提出MEP术中的改变(包括波幅，潜伏期)与术后神经功能的完整性密切相关。潜伏期延长小于10％组，无脊髓损伤发生；波幅瞬时 消失组'脊髓损伤发生率为50％;波形消失时间大于10min组，脊髓损伤发生率为100％。因此建议把潜伏期延长10％作为MEP的监护标准。
综上所述，MEP是一种极为有效的神经电生理学检测方法，对于监护运动神经系统的完整性具有良好的敏感性与特异性。临床上通过测定其波形的潜伏 期和波幅，能够对神经系统疾病起到诊断、估计预后的作用，并且MEP与SEP联合用于术中监护可克服假阴性的出现，从而提高手术的安全性。因此，多数学者 目前主张MEP和SEP联合用于术中监护，但仍有许多问题尚未解决，例如联合监护的标准是什么，如何提高波形的稳定性等问题。可以预见，经过不断深入的研 究，在不远的将来，MEP将会是一种敏感性高、安全可靠的脊髓疾病检查方法和术中监护手段。
Current status of exercise evoked potentials
Article Source: Medical Network Published: 2008-07-24 10:01:27
Key words: potential research
Motor evoked potentials (MEP) is a neuroelectrophysiological examination method designed to examine motor neurological function after somatosensory evoked potentials (SEP). As a noninvasive means of detection, MEP has been widely used in the diagnosis of motor neurological diseases, intraoperative monitoring and prognosis estimation, especially in recent years, with the improvement of electrophysiology and superposition of the average technology, MEP applicable to the increasingly Broaden. Now on its basic principles, characteristics and clinical application of the status quo as follows.
1 Basic principles of MEP
MEP refers to the use of electrical or magnetic stimulation of the cortical motor area generated by the excitement through the downstream conduction path, the spinal cord anterior horn cells or peripheral nerve motion fibers depolarize the potential in the corresponding muscle or nerve surface recorded. As early as 1954, Patton and Amassion, etc. with repeated electrical stimulation of the craniotomy monkey cortical movement area, in the cervical pulp with spherical electrodes recorded to the MEP. But because of local pain, the patient is unbearable, so the clinical application is limited. In the early eighties, Merton and Morton used high-voltage pulsed current (750V, 5μs, 1200mA) as the source of stimulation, local pain was significantly reduced. In 1985, Barker et al first used transcranial magnetic stimulation of human motor cortex induced MEP, due to magnetic stimulation in the scalp on the induced current is very weak, not enough to stimulate the pain sensory, so the subjects without any discomfort, so that MEP really Clinically more and more widely used.
MEP conduction path, the authors have different views. Most scholars believe that MEP is along the corticospinal tract, red nucleus spinal cord and other spinal cord in the anterior scoliosis and anterolateral lobe of the motor bundle conduction. Levy and so on in animal experiments found that surgery under the microscope alone cut off the corticospinal tract, MEP most of the waveform disappeared, further demonstrated the corticospinal tract is the main transmission pathway of MEP. However, the authors also suggest that the MEP pathway also includes a reversible conduction sensory bundle, which is based on a spinal cord resection that retains the posterior bundle and does not completely eliminate the MEP.
2 Basic characteristics and influencing factors of MEP
2.1 Basic features
MEP is composed of a group of different polar waves, the latency and amplitude vary. Usually the first wave called the D wave or direct wave, was a single positive phase wave, its latency is shorter, is the cortical movement area V-layer pyramidal cells of the axis of the initial segment of the results of excitement, its conduction without synapse Transmission, the least affected by narcotic drugs. D wave after a series of waves known as the I-wave or indirect wave, the performance of about 5 of the positive phase / negative phase wave, is the contact fiber indirect excitement caused by pyramidal cells, long incubation period, vulnerable to external factors. Therefore, the clinical use of D-wave latency and amplitude as a monitoring index.
2.2 influencing factors
2.2.1 narcotic drugs narcotic drugs on the amplitude and latency of MEP greater impact. In 1993, Glassman reported the effects of various narcotic drugs on MEP, suggesting that thiopental had a greater effect on MEP during the induction of anesthesia, whereas methadiazem had less effect; during maintenance anesthesia, MEP has a greater impact, while fentanyl and ketamine have less impact. Yamada also confirmed that narcotics had a significant effect on the amplitude and latency of muscle MEP, but had little effect on spinal MEP.
2.2.2 Stimulation intensity Spinal anterior horn cells include small motor neurons and large motor neurons. Small motor neurons have low excitatory thresholds and slow nerve fibers; high motor neurons have high excitatory thresholds and fast nerve fibers. When the stimulation voltage is low, only excited small motor neurons, resulting in long latency, low amplitude of the MEP; when the stimulus voltage gradually increased, can also excited the size of motor neurons, resulting in short latency, high amplitude MEP. In view of the fact that the stimulus intensity has such an effect on the MEP, a constant stimulus parameter at the time of detection is critical to the correct interpretation of the test results.
2.2.3 muscle contraction In 1992, Hayes et al found in patients with spinal cord injury, before the transcranial magnetic stimulation before the first supplemented by transcutaneous electrical stimulation can make the local muscle MEP more easily lead. This is extremely important for judging the degree of spinal cord injury. For patients with spinal cord injury after MEP disappeared, if the transcutaneous electrical stimulation on the basis of transcranial magnetic stimulation can induce MEP, then suggest that the spinal cord is incomplete injury; if the transcutaneous electrical stimulation on the basis of still did not lead to MEP, then Indicating that the spinal cord is completely damaged.
3 MEP applications
3.1 Diagnosis of spinal cord disease
Diagnosis of neurological diseases in the past more dependent on clinical inquiry, physical examination and CT, MRI and other morphological examination of the combination of the lack of direct motor nervous system or sensory nervous system function tests. Therefore, for some early lesions or subclinical lesions, missed misdiagnosis rate is higher. MEP directly reflects the integrity of the function of the motor system, for the diagnosis of neurological diseases opened up a new way.
Maerten'Dvorak et al. Noted that the sensitivity of MEP was 84% in vertebral lesions and intervertebral disc herniation, which was significantly higher than that of SEP (36%), and the sensitivity of MEP to cervical spinal stenosis was slightly higher than that of lumbar spinal stenosis 72% VS65%). Presumably with the cervical spinal canal volume is small, more easily after compression caused by spinal cord [13,14]. Machida reported that the sensitivity of MEP to traumatic spinal cord injury was 85%.
In 1994, Linden and Berlit et al. Examined the changes of MEP in myelopathy. The results showed that the specificity of MEP in the diagnosis of spinal cord injury was significantly better than that of SEP. In the case of neoplastic spinal cord disease, changes in MEP are usually manifested as a decrease in amplitude or disappearance of waveforms. In inflammatory diseases, the prolongation of latency is more common, suggesting that the pathological changes of neoplastic diseases are neuronal and axonal Destruction of the main, so volatility decreased; inflammatory diseases to demyelination, so the incubation period extended. In addition, some scholars observed that acute disease is more likely to cause chronic changes in MEP changes, may be associated with chronic diseases, the compensatory mechanism to play a role in protecting spinal cord function.
In 1988, Caramia conducted 79 MEP tests for 79 patients with sensory and motor dysfunction. The results showed that 49% of patients with multiple sclerosis had a MEP change of 54%, and showed more prolonged latency. Among them, there were clinical symptoms Patient, MEP detection rate was 100%, while subclinical symptoms of patients, the positive rate was 40%. 9 patients with amyotrophic lateral sclerosis, MEP changes accounted for 67%, more than the decline or disappearance of volatility. As for Parkinson's patients and Hungtington's patients, there was no abnormality in the amplitude and latency of MEP. "This may be related to the fact that the disease does not directly affect the function of the cone system.
Because MEP is an objective function of detection, therefore, there are also used by the author to identify heart due to paralysis and organic paralysis, although a case report, but worth learning. In addition to observing changes in amplitude and latency, MEP postconditioning and nerve conduction time were also measured for spinal cord injury.
3.2 judgment of prognosis
In the spinal cord disease or injury, the performance of MEP is determined by the extent of spinal cord destruction: the more the white matter fiber demyelination, the more the number of anterior horn kinetic cell damage, the MEP latency and volatility decreased significantly. Therefore, by observing the latency and amplitude of MEP changes, spinal cord motor function can be the degree of damage and prognosis to make judgments.
Levy had a 1-month MEP follow-up in animal models that caused chronic spinal cord injury, and found that MEP appeared at 100% of the animals before resuming the activity of paralyzed limbs. Domestic Sun Tiansheng, Xu Shaoting also confirmed by animal experiments early MEP is a good prognosis of spinal cord injury indications, MEP recovery often before the animal's motor function improved.
In clinical use, in 1993, De-Mattei had 12 patients with spinal cord compression who underwent preoperative, postoperative 2 weeks, postoperative MEP comparison. The results suggest that 11 patients with central nervous conduction time in patients with postoperative faster postoperative motor function recovery is good; and nerve conduction time no significant changes in postoperative symptoms of poor relief. Clarke's long-term MEP follow-up examination of traumatic spinal cord injury also confirmed that patients with MEP who had recovered 6 months after surgery had significantly reduced paralysis symptoms and who had no MEP recovery within 6 months after surgery had no symptoms of paralysis change. Kai et al. Classify the waveforms of MEP into two components: fast wave and slow wave, which indicates that the fast wave component has a high correlation with the motion function integrity. Where fast recovery, postoperative motor function is normal; fast wave disappeared, only slow wave recovery, postoperative motor function mild obstacle; fast wave were not restored, postoperative motor function serious obstacles.
However, some scholars have suggested that: MEP on the occurrence of spinal cord injury is very sensitive, but for the recovery of motor function after injury 'is no significant correlation.
3.3 Intraoperative monitoring
With the advances in surgical techniques, the types of spine surgery are greatly expanded. But patients with postoperative spinal cord injury than in the past significantly increased. In order to reduce or avoid this serious complications, clinical methods used are the following two: ① intraoperative wake-up test; ② intraoperative evoked potential monitoring. Wake up test is Stagnara reported in 1973 for the first time, the test results are reliable. However, due to the existence of irreversible response and wake up may lead to pulmonary embolism, internal fixation device shedding and other issues, in recent years has been gradually replaced by intraoperative evoked potential monitoring. Evoked potentials include MEP and SEP two categories, respectively, to monitor the motor conduction function and sensory conduction function, the two complement each other. There are more studies on intraoperative care of SEP, and MEP research needs to be further studied.
In 1988, Owen reported 111 patients with intraoperative MEP monitoring, an earlier case study, with 90% of patients monitoring stable MEP during surgery, and the remaining patients without MEP were confirmed Is due to the error of the means of detection, and in a timely manner to be corrected. Postoperative functional testing, where there are stable MEP during intraoperative monitoring, no one complicated with spinal cord injury. Thus demonstrating the feasibility of MEP for intraoperative care.
But the specific monitoring standards how to define the views of scholars in different countries is not uniform. In the animal experiment, Machida found that after cat's spine with a Harrington-like device, all cats were reduced in varying degrees. When the reduction was more than 50% and the duration was greater than 7 min, the incidence of postoperative paralysis was 100 %. Lee's trial also confirms that changes in volatility are more meaningful than changes in latency during MEP for intraoperative care because of the variability of latency. As for the extent of volatility decline should not exceed 2/3, otherwise it will inevitably lead to postoperative neurological impairment. Glassman's experiments suggest that changes in MEP (including amplitude, latency) are closely related to postoperative neurological integrity. The incidence of spinal cord injury was 50%. The incidence of spinal cord injury was 100%. The incidence of spinal cord injury was 50%. The incidence of spinal cord injury was 100%. It is therefore recommended that the latency be extended by 10% as the monitoring standard for MEP.
In summary, MEP is an extremely effective method of neurophysiological detection, which has good sensitivity and specificity for monitoring the integrity of motor system. Clinically, by measuring the latency and amplitude of its waveforms, can diagnose neurological diseases and estimate the prognostic role, and MEP combined with SEP for intraoperative monitoring can overcome the occurrence of false negatives, thereby improving the safety of surgery. Therefore, most scholars now advocate MEP and SEP combined for intraoperative monitoring, but there are still many problems yet to be solved, such as what is the standard of joint monitoring, how to improve the stability of the waveform and other issues. It is foreseeable that, after in-depth research, in the near future, MEP will be a sensitive, safe and reliable spinal cord disease examination methods and intraoperative monitoring means.
