Facilitation of motor evoked potentials after tetanic peripheral nerve stimulation.

OBJECTIVE: Tetanic stimulation of a peripheral nerve prior to transcranial electrical stimulation (TES) may enhance motor evoked potential (MEP) amplitudes. The purpose of this study was to investigate the post-tetanic MEP (p-MEP) technique in improving MEP amplitudes. METHODS: Conventional TES MEPs (c-MEP) and p-MEPs with left upper limb stimulation (p-MEPUL) or left lower limb stimulation (p-MEPLL) were performed in 26 patients. Bilateral hand and foot MEP amplitudes obtained with each protocol were compared. Subgroup comparisons were performed for myelopathy and peripheral neuropathy patients. Within-subject amplitude differences between c-MEP and each p-MEP technique were compared using a Wilcoxon test. RESULTS: The mean age of the patients was 52.7 years (range, 12-79 years). Overall, p-MEPUL resulted in MEP improvement in 25 of 26 (96%) patients, and p-MEPLL improved MEPs in 19 of 26 (73%) patients. The increase in MEP amplitudes were statistically significant in all muscle groups except left foot. Similar improvements were seen in the myelopathy group; in the neuropathy group, p-MEPUL produced similar results, but p-MEPLL did not. CONCLUSIONS: The p-MEP technique can improve MEP amplitudes, including in patients with myelopathy. In patients with peripheral neuropathy, the results were mixed. SIGNIFICANCE: Tetanic stimulation can enhance intraoperative MEP amplitudes.

Intraoperative Electromyography in Awake Minimally Invasive Transforaminal Lumbar Interbody Fusion: A Case Study on Nerve Activation Under the Effects of Local Anesthesia.

SUMMARY: With the versatility of lumbar spine surgery continually expanding, intraoperative electromyography (EMG) has become a common adjunct used to reduce risk of nerve injury and postoperative neurologic deficit. EMG monitoring has been deemed particularly useful in the minimally invasive transforaminal lumbar interbody fusion. A more recent evolution of the minimally invasive transforaminal lumbar interbody fusion entails complete percutaneous access to the disc through Kambin's triangle, followed by a percutaneous lumbar interbody fusion. Given the lack of direct visualization of nervous structures with percutaneous entrance into the disc, there is risk of injury to surrounding nervous structures with this approach. In effort to reduce risk of nerve injury, application of triggered EMG to gauge proximity of nervous tissue has been evaluated. Recently, patients presenting with contraindications or concerns for general anesthesia have been offered the alternative to undergo their procedure with spinal anesthesia, allowing them to remain awake. Spinal anesthesia entails intrathecal administration of local anesthetic, which mechanistically acts to reduce overall excitability of surrounding neural structures. However, nerve activation under conditions of local anesthetic is relatively unknown, and the ability of triggered EMG monitoring to reliably detect nerve proximity becomes questionable. This case report demonstrates nerve activation at thresholds comparable with those seen under general anesthesia. Although this has sparked interest in the possibility that local anesthetic may not remarkably affect nerve excitability as measured by triggered EMG activation, further investigation is recommended to reliably apply triggered EMG monitoring in awake spine surgery.