Direct Nerve Root Stimulation Research Articles

Wednesday, September 26, 2018 1:00 PM – 2:00 PM Navigation and Intraoperative Monitoring: 43. Direct nerve root stimulation: a novel approach to neuromonitoring for reduction of high grade spondylolisthesis

BACKGROUND CONTEXT Traditional methods of neuromonitoring for spinal deformity cases lack accuracy and reproducibility in monitoring for high grade spondylolisthesis. Free run electromyography (EMG) has not been shown to be accurate in detecting radicular injuries in lumbar degenerative surgeries. PURPOSE The purpose of this project is to introduce direct nerve stimulation as a novel alternative to nerve root monitoring in the reduction of high grade spondylolisthesis. METHODS Prospectively collected intraoperative neuromonitoring data including MEP, SSEP and free run EMG, was collected on pediatric patients undergoing posterior reductions for high grade L5 or S1 spondylolisthesis. A fourth modality using direct nerve root stimulation (DNS) of the L5 and S1 nerve roots was recorded by placing a stimulator directly on the exposed nerve root and recording threshold stimulus. Along with the other modalities, DNS was carried out prior to reduction, following all reduction maneuvers and prior to closure. RESULTS Five patients with high grade spondylolisthesis were analyzed, four with Grade IV and one with Grade V. All patients had positive bilateral straight leg raises. Four of 5 patients had intact preoperative lower extremity motor examinations, with one patient having left-sided 4/5 EHL weakness. Three patients did not have any neuromonitoring alerts during their surgical procedure. Their average change in threshold value from baseline to final stimulation in these cases for the L5 nerve root was 1.3 mA. Two patients had intraoperative alerts of MEPs, SSEPs and EMGs which resulted in greater nerve root stimulation threshold values at closing compared to baseline. One patient showed a three-fold increase in DNS threshold while a second patient had a 25-30-fold increase. In both cases, further nerve root exploration and decompression was performed. Both cases resulted in immediate postoperative neurologic deficits in bilateral ankle dorsiflexion. CONCLUSIONS After establishing a prereduction threshold of direct nerve stimulation, increases in DNS threshold were associated with postoperative neurologic deficits in cases of posterior reduction of high grade spondylolisthesis. DNS provided an accurate measure of nerve function in this series. Recognizing increases in DNS thresholds intraoperatively can alert the surgeon to potential real time nerve injuries. A larger multicenter series can help determine the utility of this technique. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs. Traditional methods of neuromonitoring for spinal deformity cases lack accuracy and reproducibility in monitoring for high grade spondylolisthesis. Free run electromyography (EMG) has not been shown to be accurate in detecting radicular injuries in lumbar degenerative surgeries. The purpose of this project is to introduce direct nerve stimulation as a novel alternative to nerve root monitoring in the reduction of high grade spondylolisthesis. Prospectively collected intraoperative neuromonitoring data including MEP, SSEP and free run EMG, was collected on pediatric patients undergoing posterior reductions for high grade L5 or S1 spondylolisthesis. A fourth modality using direct nerve root stimulation (DNS) of the L5 and S1 nerve roots was recorded by placing a stimulator directly on the exposed nerve root and recording threshold stimulus. Along with the other modalities, DNS was carried out prior to reduction, following all reduction maneuvers and prior to closure. Five patients with high grade spondylolisthesis were analyzed, four with Grade IV and one with Grade V. All patients had positive bilateral straight leg raises. Four of 5 patients had intact preoperative lower extremity motor examinations, with one patient having left-sided 4/5 EHL weakness. Three patients did not have any neuromonitoring alerts during their surgical procedure. Their average change in threshold value from baseline to final stimulation in these cases for the L5 nerve root was 1.3 mA. Two patients had intraoperative alerts of MEPs, SSEPs and EMGs which resulted in greater nerve root stimulation threshold values at closing compared to baseline. One patient showed a three-fold increase in DNS threshold while a second patient had a 25-30-fold increase. In both cases, further nerve root exploration and decompression was performed. Both cases resulted in immediate postoperative neurologic deficits in bilateral ankle dorsiflexion. After establishing a prereduction threshold of direct nerve stimulation, increases in DNS threshold were associated with postoperative neurologic deficits in cases of posterior reduction of high grade spondylolisthesis. DNS provided an accurate measure of nerve function in this series. Recognizing increases in DNS thresholds intraoperatively can alert the surgeon to potential real time nerve injuries. A larger multicenter series can help determine the utility of this technique.

Utility of Direct Stimulation of Roots in Spinal Surgery

Prevention of postoperative neurological deficits is a major concern of spinal surgeons and has led to the introduction and current development of intraoperative neurophysiological monitoring. We have used motor evoked potentials and somatosensory evoked potentials as routine monitoring techniques and, in some cases, added optional methods such as direct stimulation of nerve roots and spinal evoked potentials. We report our experience of direct nerve root stimulation as an optional monitoring method during spinal surgeries in 7 patients with lesions affecting the proximal nerve roots aged from 1 day to 78 years (mean 23.5 years). Four patients had anomalous lesions, two had spinal nerve root schwannomas, and one had a far-lateral lumbar disc herniation. Direct stimulation was used for detection of motor nerve roots in the anomalous lesions and schwannomas, and to distinguish the nerve root from the paraspinal soft tissues in the case of a far-lateral herniated disc at the L5-S1 level. Although some patients had slight transient neurological symptoms such as motor weakness and sensory disturbance, none developed severe permanent neurological impairment. Direct stimulation allows detection of the motor nerve during spinal surgery in real time. Our limited experience suggests that the direct stimulation technique could reduce the risk of motor or vesicorectal disturbance after surgery of lesions affecting or involving the spinal nerve roots.

Intraoperative neurophysiological monitoring during surgery for tethered cord syndrome

The tethered cord syndrome refers to a variety of lesions that can cause the conus medullaris to be low-lying or incapable of movement within the spinal canal. Permanent or temporary neurological complications were reported following surgical release. In this report, peri- and postoperative results in cases with tethered cord syndrome that were followed by multimodal intraoperative neurophysiological monitoring (MIONM) methods are presented. An IONM system (Nicolet CR Endeavor) was used for monitoring during tethered cord surgery. Somatosensory evoked potentials (SEPs), motor evoked potentials (MEPs), direct nerve root/rootlet stimulation, free-run electromyography (EMG) and F-waves were used during tethered cord surgery of 10 cases to prevent possible nerve injuries. MEP and SEP recordings did not change in any of the cases during surgery. The nervous tissue was identified and differentiated from connective tissue in three cases when motor responses were elicited with direct stimulation of nerve roots. None of the cases had neurological deficits following the operation. Direct nerve root/rootlet stimulation should be one of the components of MIONM during surgery for tethered cord syndrome to prevent postoperative neurological deficits.

Recording Diffusion Responses From Contralateral Intercostal Muscles After Stimulus-Triggered Electromyography

A new stimulus-triggered electromyography (EMG) test for detecting stimulus diffusion to contralateral intercostal muscles during thoracic pedicle screw placement was assessed in a porcine model. To determine if electromyographic thresholds in the intercostal muscles of both sides of the thorax could discriminate thoracic pedicle screw malpositions with and without neural contact at different aspects of the spinal cord and nerve roots. There is controversy about the value of triggered EMG stimulation for aiding precise insertion of thoracic pedicle screws. A universally validated threshold that confirms screw malposition has not been established. Diffusion of EMG responses to the contralateral intercostal muscles has not previously been investigated. Nine domestic pigs weighing 60 to 75 kg had 108 pedicle screws placed bilaterally in the thoracic spine from T8-T13. Before spine instrumentation, neural structures were stimulated in 4 animals under direct vision at different anatomic locations from T9-T12. Recording electrodes were placed over the right and left intercostal muscles. Increasing stimulus intensity was applied until muscle response was detected at the contralateral side (EMG diffusion phenomenon). After this first experiment, the thoracic spine was instrumented in all 9 animals. Screws were placed in the pedicle in different positions, the anatomic intrapedicular location and within the spinal canal, with and without contact with the neural elements. Response thresholds to direct nerve root stimulation were significantly lower than those obtained by stimulation of the dorsal aspect of the spinal cord (0.44 +/- 0.22 mA vs. 1.38 +/- 0.71 mA, P < 0.01). However, a 14-fold stimulation intensity (6.50 +/- 0.29 mA) was necessary to obtain diffusion of the EMG response to the opposite (left) side if the right nerve root was stimulated. A 2-fold increment (3.17 +/- 0.93 mA) elicited diffusion of the EMG responses to the contralateral side when stimulation was applied to the dorsal aspect of the spinal cord. EMG recordings of the 108 stimulated screws showed a significant decrease in the EMG response when the screw was in contact with the spinal cord (2.72 +/- 1.48 mA; P < 0.01) compared with that found when the pedicle track was intact (mean: 5.01 +/- 1.89 mA). Screws violating the medial wall of the pedicle, but not touching neural tissues, responded to slightly lower intensities than well-positioned screws, but this was not statistically significant (3.91 +/- 1.39 mA vs. 4.89 +/- 1.30 mA, P > 0.05). Stimulus-triggered EMG can identify screws that violate the medial pedicle wall if they are in contact with neural tissues. EMG thresholds could not discriminate screws that violated the medial pedicle wall without neural contact from screws with accurate intraosseous placement. However, recording EMG potentials at the contralateral intercostal muscles (stimulus diffusion phenomenon) proved to be a reliable method for identifying the neural structures at risk.

The importance of early postoperative radiation in spinal myxopapillary ependymomas

We read with interest the report by Aykurek et al. [1] on the success rates of combined surgical and radiation treatment of myxopapillary ependymomas. Myxopapillary ependymomas are commonly found within the cranio-spinal axis and are typically found in the caudal spinal canal, conus and cauda equina [2–4]. Total surgical extirpation has been found to a highly effective treatment in these low grade ependymomas [5, 6]. Subtotal resection of these tumors requires postoperative radiation, which has been also highly effective in preventing recurrence [5, 7]. The time frame between surgical resection and radiation therapy is yet to be fully elucidated. We recently operated on a 47 year-old male who presented for neurosurgical evaluation of an enhancing lesion within the conus. The patient was originally seen by an orthopedic surgeon for an L5-S1 disc herniation and an enhancing lesion was discovered on the preoperative MRI. The patients’ past medical history was insignificant. On neurological examination the patient had classic right S1 radiculopathy. The MRI revealed mild foraminal stenosis with a 4 mm disc bulge at L5-S1 while also demonstrating a 7 mm homogeneous contrast enhancing lesion within the conus (Fig. 1). The craniospinal axis was subsequently scanned via MRI and demonstrated no other lesions. After informed consent the patient was taken to the operating room for a microscopic L5-S1 discectomy and an L1-L2 laminectomy, durotomy for resection of conus lesion. The patient had continuous EMG and SSEP monitoring throughout the surgery. The lesion was gray in color and had several roots traversing the core of the tumor. Microdissection allowed for approximately 80% resection of the mass. Direct nerve root stimulation demonstrated several sacral roots in the core of the tumor. Postoperatively the patient was neurologically intact with full resolution of the S1 radiculopathy. The MRI performed approximately six weeks after surgery demonstrated three additional lesions cephalad to the initial surgery (Fig. 2). Further imaging of the entire craniospinal axis demonstrated no other lesions. The patient subsequently underwent stereotactic radiation therapy at a dose of 50 gy on the four lesions without complications. The metastatic potential of myxopapillary ependymomas is well documented, although not routinely recognized [8–10]. The reports of myxopapillary dissemination are reported as long as forty years ago with reports of both intracranial seeding and extraneural metastasis [3, 8–12]. Graf et al [8] report on a case of pulmonary metastasis of a myxopapillary ependymoma, although this occurred decades after the initial surgery and no focal radiation was performed after surgery. We report the first case of metastasis of myxopapillary spinal ependymoma within six weeks of initial surgery. Our case exemplifies the classic pattern of metastasis, through cerebrospinal fluid, cephalad into the ascending spinal cord. Davis et al. [3] reported two decades ago on three cases of aggressive behavior of a supposed ‘benign’ myxopapillary spinal ependymoma, speculating that a few of these tumors while R. D. Dickerman (&) A. S. Reynolds E. Gilbert B. Morgan Neurosurgery Research Foundation of Texas — Brain and Spinal Cord Tumor Institute, North Texas Neurosurgical Associates, Plano Presbyterian and Medical Center of Plano Hospitals, 6200 West Parker Road, Suite 1-503, Plano, TX 75093, USA e-mail: drrdd@yahoo.com

Higher electrical stimulus intensities are required to activate chronically compressed nerve roots. Implications for intraoperative electromyographic pedicle screw testing.

A comparison of the electrical thresholds required to evoke myogenic responses from direct stimulation of normal and chronically compressed nerve roots. To determine whether intraoperative electromyographic testing to confirm the integrity of instrumented pedicles should be performed at higher stimulus intensities in cases where there is preoperative lumbosacral radiculopathy. Postoperative neurologic deficits may occur as a result of pedicle screw misplacement during spinal instrumentation. The failure to evoke myogenic responses from stimulation of pedicle holes and screws at intensities of 6-8 mA is commonly used to exclude bony pedicular wall perforation. Direct nerve root stimulation was used to compare the stimulus thresholds of normal and compressed nerve roots in six patients with limb weakness from chronic lumbosacral radiculopathy. The stimulus thresholds of chronically compressed nerve roots significantly exceeded those of normal nerve roots, indicating partial axonal loss (axonotmesis). In most cases, the direct stimulus thresholds of compressed nerve roots exceeded 10 mA. When instrumentation is placed at spinal levels where there is preexisting chronic lumbosacral radiculopathy, holes and screws may need to be stimulated at higher intensities to exclude pedicular perforation and prevent further iatrogenic nerve root injury.