Contralateral Response in Triggered Electromyography: Midline Touching Pedicle Screws

Introduction: The use of pedicle screws is the treatment of choice in thoracic spinal deformity procedures but insertion and confirmation of well-placement of the screws is challenging due to the significant change of normal anatomy. Therefore, intraoperative neurophysiological monitoring with triggered-electromyography (t-EMG) is highly recommended in which if any pedicle breach is present an ipsilateral response is expected. The aim is to expose a cause of contralateral response in t-EMG. Clinical case: Case of 14-year-old boy who underwent correction of a high-grade double thoracic curve with pedicle screw instrumentation from T2 to L4. After thoracic pedicle screw placement at T12 level, assessment by t-EMG was performed evincing a left-sided muscle response elicited stimulating the right-sided pedicle screw at 10 mA. Pedicle palpation was normal, fluoroscopy did not alert of any pedicle breach at both sides, somatosensory and motor evoked potentials did not change from baseline, and any mistake of montage was ruled out. Replacement of right-sided pedicle screw was carried out in a more lateral position and the evoked response disappeared using the same cut-off threshold. Postoperative computed tomography shown the first pedicle track of the right side having a midline touching pedicle screws with a 2 mm medial pedicle breach on the left side. The patient did not develop any neurological worsening or symptoms due to this mispositioning. Conclusion: A contralateral response on t-EMG could have different causes but a less considered is due to midline touching pedicle screws consequent to contralateral medial pedicle breach explaining properly this phenomenon.