Transcranial electrical motor evoked potential (TcMEP) is a modality utilized in intraoperative neurophysiological monitoring to assess the integrity of the corticospinal tract. Traditionally, TcMEPs are obtained by anodal stimulation of the scalp over the motor cortex of the selected hemisphere and referenced to the contralateral hemisphere. Subsequent compound motor action potential responses (CMAPs) are recorded at various muscles. The muscle responses of interest are usually those recorded in the limb contralateral to the hemisphere of stimulation. However, TcMEPs may elicit simultaneous muscle responses in the limbs ipsilateral to the hemisphere of stimulation, otherwise defined as "crossover" responses. Crossover TcMEPs are thought to be generated when electrical stimulation reaches the corticospinal tract at intracranial structures as deep as the medullary pyramids. If electrical stimulation penetrates deeper than the at-risk structures, false negatives motor evoked potential monitoring may occur. Therefore, in surgeries where cerebral cortical structures may be at risk, TcMEPs may be elicited so that contralateral CMAP responses are present without crossover responses. We present three cases using an alternative TcMEP montage in which anodal stimulation of the target hemisphere is referenced to Fpz at midline. Compared with the C3-C4/C4-C3 montage, crossover responses are minimized with the modified montage. C3-Fpz/C4-Fpz TcMEP stimulation may be a potential tool to implement in certain intraoperative neuromonitoring cases.
Keywords: Crossover; TcMEP montage; evoked potentials; intraoperative neurophysiological monitoring; ipsilateral motor evoked responses; transcranial electrical motor evoked potentials.