S114. Comparison of Transcranial Electrical Motor Evoked Potentials (TcMEPs) of foot recording montages

Abstract

TcMEPs are widely used for monitoring of corticospinal motor pathways in a variety of surgical procedures. However, there is no published standard recording technique indicating the optimal electrode placement for recording myogenic responses from the foot. The purpose of our survey was to asses the different foot recording montages used by neurodiagnostic technologists in the surgical setting and which is the preferred recording site. Fifty neurodiagnostic technologists in the USA were sent diagrams of the foot asking them to indicate where they place the recording electrodes on the foot of their patients undergoing a variety of thoracic or lumbar spinal column surgeries. The diagrams sent to the technologists pictured the sole of the foot, the top of the foot, and the medial aspect of the foot. The technologists were instructed to place an X of where they place the recording electrodes for a foot motor evoked potential recording. Twenty technologists submitted their diagrams. The bulk of the data was broken down to show the top three muscles used by the technologists who returned their submissions. All technologists indicated that they used subdermal needle electrodes to record the TcMEPs from the foot. The survery showed that TcMEPs were obtained from four different muscles: the Abductor Hallicus Brevis (AHB); Flexor Hallicus Brevis (FHB); Flexor Digitorium Brevis (FDB); and the Abductor Digiti Minimi (ADM). The most commonly used recording montage was AHB-FHB at 65%; followed by AHB-FDB at 20%; and lastly AHB-ADM at 15%. Our survey results indicate that the majority of technologists prefer using the AHB-FHB montage to record the TcMEP from the foot. Unfortunately, technologists did not provide reasons for why a particular recording montage was used and the survey did not specifically ask that reasons be provided. These results confirm our suspicion that multiple montages are used to record the foot TcMEP. We postulate that the reason for this is that there is no consensus or guideline indicating which muscle(s) should be used to record the TcMEP. We were also surprised that the majority of the recordings were obtained from two different muscles and not a single muscle. We recognize that the small number of respondents limits our study but we feel that these results provide useful insight and understanding on how technologists in the US record foot TcMEPs. This is an important step in trying to standardize how these recordings should be performed. Further studies are needed to determine the best muscles and montages to record the foot TcMEPs. We are doing a prospective study using the results from this survey to analyze TcMEPs and help determine the optimal recording montage.