Crossover Phenomena in Motor Evoked Potentials During Intraoperative Neurophysiological Monitoring of Cranial Surgeries

Abstract

Purpose: 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. Methods: 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. Results: 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. Conclusions: 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.

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

“…In our study, UE tcMEP monitoring predicted only two cases of seven new UE deficits (including one reversible change that resolved with intervention) and therefore did not prove to be clinically reliable in predicting UE neurological deficits. In three cases there was evidence of intermittent crossover activation of the ipsilateral side during tcMEP stimulation indicating inappropriately deep penetration of current ( Gonzalez et al, 2019 ). Increased activation threshold in the setting of SAH is a likely culprit.…”

“…Penetration of current deeper into the brain parenchyma potentially also leads to a failure to detect impending brachiofacial paresis from subcortical ischemia by RAH compromise. Crossover activation of the ipsilateral muscles may help to determine if overstimulation is occurring ( Gonzalez et al, 2019 ). Even in the absence of crossover activation, current penetration may still be unacceptably deep and may cause false-negative monitoring.…”

Anodal Stimulation Latency Response Differences Between Ipsilateral and Contralateral Motor Evoked Potentials in Hand Muscles

“…In that scenario, we would suggest that if you can get rid of crossover, then anodal side will likely be more superficial. 7,15 If crossover cannot be eliminated, because it occurs with 24% of cases, then the cathodal side is likely more superficial and could also be the most useful. For example, in the setting of a left side brain tumor surgery, it is useful to have right side MEPs without crossover or use the crossover responses when obtaining the left side MEP.…”

“…In some instances, activation can occur on the ipsilateral side leading to the phenomena of crossover. 7…”

Deterioration of non-lesional side evoked potential during unilateral single unruptured intracranial aneurysm clipping: incidental finding or true warning signal?

“…However, we can infer these mechanisms from the results of previous studies.The non-lesional side motor response has been explained as a cause of the crossover phenomenon of the motor pathway or non-decussated corticofugal fibers[16,17]. Gonzalez et al[18] reported the crossover phenomenon of transcranial MEP on IONM during open cranial surgery. This study suggests that the crossover phenomenon leads to false-negative results and may be the basis for explaining who two of our patients first showed non-lesional side MEP amplitude decrease and then lesional side MEP amplitude decrease; one showed PND, and one presented cerebral vasospasm on postoperative brain CT.…”