Surgical Neurophysiology Program, Department of Neurology, University of Southern California, Los Angeles, California, U.S.A.
J Clin Neurophysiol. 2019 May;36(3):236-241. doi: 10.1097/WNP.0000000000000570.
Transcranial motor evoked potentials (TcMEPs) are used to assess the corticospinal tract during surgery. Transcranial motor evoked potentials are elicited by preferentially activating the anode over the target cortex. Crossover occurs when stimulation also induces activation of ipsilateral motor evoked responses. These responses are believed to be generated by activation of corticospinal tract on more caudal neural structures. The presence of cross activation poses a problem in craniotomy surgeries because activation of neural structures occurs distal to the area of interest leading to false negatives. Eliminating crossover may lead to activation of the motor pathway proximal to the surgical site, thus potentially reducing false-negative responses. There are no data on how often crossover signals occur or the conditions in which they take place. This study examines the frequency of crossover, the surgical procedures in which they occur, and their stimulation parameters.
We reviewed all the TcMEP data files for intracranial procedures performed in 2016 at Keck Hospital of USC. We recorded demographic information about the surgical side, lobe, diagnosis, age, and sex. Only baseline TcMEPs were analyzed. Crossover responses were deemed present if recorded amplitudes were greater than 25μv on the ipsilateral side. We evaluated the rate of crossover presence, the lowest voltage associated with crossover, the highest voltage without crossover, if crossover resolved, and the last muscles to remain present when crossover is eliminated. Transcranial motor evoked potentials were divided into four groups. Group A: crossover present and was not resolved, group B1: crossover present but resolved with desired signals, group B2: no crossover seen with desired signals in both limbs, and group C: crossover resolved with loss of signals in either limb. The Difference between lowest amplitude with crossover and highest amplitude without crossover was obtained for each patient, and the mean of this difference was calculated using paired t-test.
We analyzed 186 TcMEPs. Forty-four TcMEPs were in group A, 52 in B1, 68 in B2, and 22 TcMEPs were in group C. Of total crossovers (118), 63% resolved at baseline, whereas 37% did not resolve. The mean difference between minimum value with crossover and maximum value without crossover was 50 V (P < 0.0001). In five TcMEPs, this difference was 0 and the median was 250 V. There was no significant difference between surgical site, stimulation side, pathology, or sex between crossover (A) and noncrossover (B + C) groups. There was a significant association found between age group ≤50 years versus >50 years and being in crossover versus noncrossover groups (P = 0.01). For 95% of the cases in group C, the last muscles to stay were hand muscles.
Transcranial motor evoked potential crossover may pose a problem during surgeries leading to false-negative results. Crossover is a frequent phenomenon that should not be overlooked. Stimulation intensity is the main factor contributing to the reduction of crossover. Crossover can be reduced in most TcMEPs performed (63%) leading to adequate monitoring in 76% of TcMEPs. Despite best efforts, there are still one quarter (24%) of TcMEPs where crossover cannot be eliminated. Newer strategies should be sought to reduce crossover. Teams should focus their efforts on reducing crossover of TcMEPs to make monitoring of intracranial surgeries more reliable.
经颅运动诱发电位（TcMEP）用于在手术期间评估皮质脊髓束。经颅运动诱发电位通过优先激活目标皮层上的阳极来引出。当刺激也诱导同侧运动诱发电位的激活时，会发生交叉激活。据信这些反应是通过激活更尾端的神经结构中的皮质脊髓束产生的。交叉激活的存在在开颅手术中是一个问题，因为神经结构的激活发生在感兴趣区域的远端，导致假阴性。消除交叉激活可能导致手术部位近端的运动通路被激活，从而潜在地减少假阴性反应。关于交叉信号发生的频率或发生的条件，尚无数据。本研究检查了交叉激活的频率、发生交叉激活的手术程序以及它们的刺激参数。
我们回顾了 2016 年在南加州大学凯克医院进行的所有颅内手术的 TcMEP 数据文件。我们记录了有关手术侧、叶、诊断、年龄和性别的信息。仅分析了基线 TcMEP。如果同侧记录的振幅大于 25μv，则认为存在交叉激活反应。我们评估了交叉激活存在的比率、与交叉激活相关的最低电压、无交叉激活时的最高电压、交叉激活是否解决以及当消除交叉激活时最后保留的肌肉。经颅运动诱发电位分为四组。组 A：交叉激活存在且未解决，组 B1：交叉激活存在但与所需信号一起解决，组 B2：双侧均未看到所需信号的交叉激活，组 C：交叉激活解决，双侧信号丢失。对于每个患者，获得了交叉激活时的最低振幅与无交叉激活时的最高振幅之间的差异，并用配对 t 检验计算了该差异的平均值。
我们分析了 186 个 TcMEP。44 个 TcMEP 位于组 A，52 个位于 B1，68 个位于 B2，22 个 TcMEP 位于组 C。在总共 118 个交叉激活中，63%在基线时得到解决，而 37%没有得到解决。交叉激活时最小幅度与无交叉激活时最大幅度之间的平均差异为 50 V（P < 0.0001）。在 5 个 TcMEP 中，这个差异为 0，中位数为 250 V。交叉（A）和非交叉（B+C）组之间，手术部位、刺激侧、病理学或性别没有显著差异。≤50 岁与>50 岁的年龄组与交叉与非交叉组之间存在显著关联（P = 0.01）。对于组 C 的 95%的病例，最后保留的肌肉是手部肌肉。
经颅运动诱发电位交叉激活可能在手术中导致假阴性结果。交叉激活是一种常见现象，不应忽视。刺激强度是减少交叉激活的主要因素。在大多数（63%）进行的 TcMEP 中可以减少交叉激活，从而使 76%的 TcMEP 得到充分监测。尽管尽了最大努力，仍有四分之一（24%）的 TcMEP 无法消除交叉激活。应寻求新的策略来减少交叉激活。团队应将重点放在减少 TcMEP 的交叉激活上，以提高颅内手术监测的可靠性。
