D-wave Monitoring Research Articles

Intraoperative neurophysiologic monitoring for intramedullary spinal-cord tumor surgery

During resection of intramedullary spinal-cord tumors intraoperative neurophysiological monitoring has become a true surgical technology. Motor evoked potentials are the most important modality for this purpose. Its use requires neurophysiological expertise from the surgeon, and a monitoring team in place able to handle the necessary equipment. Motor potentials are evoked by transcranial electrical motor cortex stimulation. A "single stimulus technique" evokes D-waves recorded from the spinal cord. The "multipulse (or train) stimulation technique" evokes electromyographic responses in peripheral muscles. These are optimally recorded from the thenar, hypothenar, tibialis anterior, and flexor hallucis brevis muscles, which are known to have strong pyramidal innervation. D-wave monitoring looks primarily at the peak-to-peak amplitude. When monitoring muscle MEPs, the presence or absence of the response irrespective of stimulation intensity is the important parameter. Preparations for neurophysiological monitoring fit quite well into a neurosurgical operating room environment. Recording and interpretation of MEPs is fast and straightforward. Pre- and postoperative clinical motor findings correlate with intraoperative MEP results. Thus correct prediction of the clinical status at a given time during surgery is possible with a very high certainty. The sensitivity of muscle MEPs for postoperative motor deficits is nearly 100%, its specificity is about 90%. Thus MEP data indeed reflect the clinical "reality". Present and stable recordings document intact motor pathways and allow the surgeon to confidently proceed with a tumor resection. Loss of muscle MEPs and/or decrease of the D-wave amplitude constitutes a "window of warning". It reflects a pattern of MEP change indicating a reversible injury to the essential motor pathways. Using this information, the surgical strategy can be adapted before irreversible neurological damage is caused by the surgical manipulation. Such adaptation comprises simply waiting for the recordings to spontaneously improve again, irrigating with warm saline solution to wash out blocking potassium. Other measures include the elevation of mean arterial pressure to improve local perfusion. Even staged resection can be considered if intraoperative measures do not sufficiently improve the recordings.

Surgery for intramedullary spinal cord tumors: the role of intraoperative (neurophysiological) monitoring

In spite of advancements in neuro-imaging and microsurgical techniques, surgery for intramedullary spinal cord tumors (ISCT) remains a challenging task. The rationale for using intraoperative neurophysiological monitoring (IOM) is in keeping with the goal of maximizing tumor resection and minimizing neurological morbidity. For many years, before the advent of motor evoked potentials (MEPs), only somatosensory evoked potentials (SEPs) were monitored. However, SEPs are not aimed to reflect the functional integrity of motor pathways and, nowadays, the combined used of SEPs and MEPs in ISCT surgery is almost mandatory because of the possibility to selectively injury either the somatosensory or the motor pathways. This paper is aimed to review our perspective in the field of IOM during ISCT surgery and to discuss it in the light of other intraoperative neurophysiologic strategies that have recently appeared in the literature with regards to ISCT surgery. Besides standard cortical SEP monitoring after peripheral stimulation, both muscle (mMEPs) and epidural MEPs (D-wave) are monitored after transcranial electrical stimulation (TES). Given the dorsal approach to the spinal cord, SEPs must be monitored continuously during the incision of the dorsal midline. When the surgeon starts to work on the cleavage plane between tumor and spinal cord, attention must be paid to MEPs. During tumor removal, we alternatively monitor D-wave and mMEPs, sustaining the stimulation during the most critical steps of the procedure. D-waves, obtained through a single pulse TES technique, allow a semi-quantitative assessment of the functional integrity of the cortico-spinal tracts and represent the strongest predictor of motor outcome. Whenever evoked potentials deteriorate, temporarily stop surgery, warm saline irrigation and improved blood perfusion have proved useful for promoting recovery, Most of intraoperative neurophysiological derangements are reversible and therefore IOM is able to prevent more than merely predict neurological injury. In our opinion combining mMEPs and D-wave monitoring, when available, is the gold standard for ISCT surgery because it supports a more aggressive surgery in the attempt to achieve a complete tumor removal. If quantitative (threshold or waveform dependent) mMEPs criteria only are used to stop surgery, this likely impacts unfavorably on the rate of tumor removal.