Motor Evoked Potential (MEP) Recording from Vocal Cords
The main concern involved in any spinal or spinal cord surgeries is the injury of any nerve that supplies to the spinal cord. To monitor and reduce the frequency of such injuries, Neurophysiological Intra-operative Monitoring (NIOM) proceduresare developed to record electrical signals produced by nerves in the hope that prompt intervention prevents permanent deficits (Dominici et al., 2014). NIOM encompasses various techniques such as Electromyography (EMG), Somatosensory Evoked Potential (SEP), and Motor Evoked Potential (MEP). Past studies conducted by Li et al. (2012) and MacDonald et al. (2013) state that SEP was based on the assumption that any lesions that occur on the motor pathway has to affect the sensory pathway and vice versa, while electromyography is a system that employs the use of electrodes on endotracheal tube. While SEP showed good results (MacDonald et al., 2013), it was found that this held true only for larger lesions (Li et al., 2012), wherein the sensitiveness was recorded only in 37.5% of patients as opposed to 62.5% for patients where MEP was used. In view of this, MEP was developed that tested the motor pathway directly. Literature in conjunction with the American Clinical Neurophysiology Society (ACNS) which has included MEP as an important technique for IOM of spinal cord surgeries, recommends that the use of MEP should gain more importance and prevalence (MacDonald et al., 2013). The focus of MEP lies in the reduction/ elimination of neurological deficits as opposed to the change in methodology. In a study by Ito et al. (2013), the authors enrolled 15 patients with brainstem tumours to evaluate the reliability of MEP on the Vagus nerve. Transcranial electrical stimulation at C3 and C4 and recording from an electrode on the endotracheal tube, revealed that MEP was a safe and effective method to prevent intraoperational injury to the vagus nerve as only five patients of the ten recorded neural damage that led to dysphagia but restored in four weeks. Similarly a study that was conducted on skull base surgery for cerebral metastases showed a 93% success rate in reduction of the neurological damage in spine surgery (Krieg et al., 2013).
In the case of anterior cervical discectomy and fusion (ACDF), two authors Davis et al. (2013) and Epstein (2013), discussed that quadriplegia is the most commonly occurring nerve injury along with damage to the vagus nerve. Davis et al. (2013)conducted the study on non-myelopathic patients undergoing ACDF, which revealed that nerve injury was averted in the majority of the cases. The author also recommends the increased use of monitoring by MEP technique especially in the cases of ACDF as ACDF is on the rise and hence also makes a viable option. The study by Epstein (2013) on the other hand compared the techniques and suggested that there is need for the use of MEP as it is a better technique. The review substantiated that MEP if used in conjunction to SEP or EMG can minimise post or intraoperative neurological injury in cervical spine surgeries. The author also enunciates that current literature supports the sensitivity, specificity as well as reliability of MEP. Further literature by Dominici et al. (2014) explored the accuracy of MEP by comparing two warning criteria which are amplitude of MEP or SEP of less than 50% (that indicative of warning to neuro-motor damage/reduced activity of the nerve) against the complete loss of amplitude (that indicates permanent central neurological damage). The study findings discuss that the first warning criteria that measures MEP amplitude of <50% is a more accurate method to determine the loss of central neural function at a reversible stage. This helps to correct it momentarily rather than post operatively, by which point the damage maybe irreversible. Currently assessing the risk of irritation to the recurrent laryngeal nerve only EMG is in use (Gremillion et al., 2012; Birkholz et al., 2011). The proposed study however aims to prove the use of MEP to detect the same for which much literature is currently minimal.
Birkholz, T., Saalfrank-Schardt, C., Irouschek, A., Klein, P., Albrecht, S. & Schmidt, J. (2011) Comparison of Two Electromyographical Endotracheal Tube Systems for Intraoperative Recurrent Laryngeal Nerve Monitoring: Reliability and Side Effects. Langenbeck’s Archives of Surgery [Online], 396 (8) December, pp. 1173–1179.
Davis, S. F., Corenman, D., Strauch, E. & Connor, D. (2013) Intraoperative Monitoring May Prevent Neurologic Injury in Non-Myelopathic Patients Undergoing ACDF. Neurodiagnostic Journal [Online], 53 (2), pp. 114–120.
Dominici, M., Chaloult, E., Ventura, F. D. & Comerlato, E. A. (2014) Accuracy of Two Different Criteria for Neurophysiological Intraoperative Monitoring (NIOM) in Spine/spinal Cord Surgeries. Arq Bras Neurocirurgia, 33 (1), pp. 1–5.
Epstein, N. (2013) The Need to Add Motor Evoked Potential Monitoring to Somatosensory and Electromyographic Monitoring in Cervical Spine Surgery. Surgical Neurology International [Online], 4 (6), p. 383.
Gremillion, G., Fatakia, A., Dornelles, A. & Amedee, R. G. (2012) Intraoperative Recurrent Laryngeal Nerve Monitoring in Thyroid Surgery: Is It Worth the Cost? The Ochsner Journal [Online], 12 (4), pp. 363–366.
Ito, E., Ichikawa, M., Itakura, T., Ando, H., Matsumoto, Y., Oda, K., Sato, T., Watanabe, T., Sakuma, J. & Saito, K. (2013) Motor Evoked Potential Monitoring of the Vagus Nerve with Transcranial Electrical Stimulation during Skull Base Surgeries. Journal of Neurosurgery [Online], pp. 1–7.
Krieg, S. M., Schäffner, M., Shiban, E., Droese, D., Obermüller, T., Gempt, J., Meyer, B. & Ringel, F. (2013) Reliability of Intraoperative Neurophysiological Monitoring Using Motor Evoked Potentials during Resection of Metastases in Motor-Eloquent Brain Regions: Clinical Article. Journal of neurosurgery [Online], 118 (6), pp. 1269–1278.
Li, F., Gorji, R., Allott, G., Modes, K., Lunn, R. & Yang, Z.-J. (2012) The Usefulness of Intraoperative Neurophysiological Monitoring in Cervical Spine Surgery. Journal of Neurosurgical Anesthesiology [Online], 24 (3) July, pp. 185–190.
MacDonald, D. B., Skinner, S., Shils, J. & Yingling, C. (2013) Intraoperative Motor Evoked Potential Monitoring – A Position Statement by the American Society of Neurophysiological Monitoring. Clinical Neurophysiology [Online], 124 (12) December, pp. 2291–2316.
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