Spinal cord monitoring

Tactile Location Perception Encoded by Gamma-Band Power

BACKGROUND

The perception of tactile-stimulation locations is an important function of the human somatosensory system during body movements and its interactions with the surroundings. Previous psychophysical and neurophysiological studies have focused on spatial location perception of the upper body. In this study, we recorded single-trial electroencephalography (EEG) responses evoked by four vibrotactile stimulators placed on the buttocks and thighs while the human subject was sitting in a chair with a cushion.

METHODS

Briefly, 14 human subjects were instructed to sit in a chair for a duration of 1 h or 1 h and 45 min. Two types of cushions were tested with each subject: a foam cushion and an air-cell-based cushion dedicated for wheelchair users to alleviate tissue stress. Vibrotactile stimulations were applied to the sitting interface at the beginning and end of the sitting period. Somatosensory-evoked potentials were obtained using a 32-channel EEG. An artificial neural net was used to predict the tactile locations based on the evoked EEG power.

RESULTS

We found that single-trial beta (13-30 Hz) and gamma (30-50 Hz) waves can best predict the tactor locations with an accuracy of up to 65%. Female subjects showed the highest performances, while males' sensitivity tended to degrade after the sitting period. A three-way ANOVA analysis indicated that the air-cell cushion maintained location sensitivity better than the foam cushion.

CONCLUSION

Our finding shows that tactile location information is encoded in EEG responses and provides insights on the fundamental mechanisms of the tactile system, as well as applications in brain-computer interfaces that rely on tactile stimulation.

Somatosensory evoked potentials of the tibial nerve during the surgical decompression of thoracolumbar intervertebral disk herniation in dogs

This study aimed to identify the impact on spinal cord integrity and determine the electrophysiological safety level during surgery for thoracolumbar intervertebral disk herniation in dogs. A total of 52 dogs diagnosed with thoracolumbar intervertebral disk herniation were enrolled. The tibial nerve somatosensory evoked potential elicited on the scalp by stimulation of the tibial nerve was recorded before and during hemilaminectomy. Both the amplitude and latency of the somatosensory evoked potential were periodically registered, and the percentage changes from the pre-operative control values (amplitude rate and latency rate) were calculated. When the multifidus muscles were retracted after removal from the spinous processes and vertebrae, the somatosensory evoked potential amplitude rate decreased in all dogs, while the latency rate increased in 33 dogs examined. The amplitude rate remained unchanged during the halting procedure, loosening retraction, and hemilaminectomy. After removing the disc material from the spinal canal, the amplitude rate was increased. The somatosensory evoked potential latency increased when the multifidus muscles were retracted and shortened after multifidus muscles closure in four cases. The outcome of all cases showed improvement in clinical signs 7 days after operation. Spinal cord conduction is impaired by retraction of the multifidus muscles and improved by removal of disk materials. Maintaining intraoperative SEP amplitudes above 50% of control may help avoid additional spinal cord injury during surgery. Since we have no case that worsened after the surgery, however, further studies are necessary to confirm this proposal.

Neuromonitoring

Neuromonitoring is used during surgery to assess the functional integrity of the brain, brain stem, spinal cord, or peripheral nerves. The aim of monitoring is to prevent permanent damage by early intervention when changes are detected in the monitor. Neuromonitoring is also used to map areas of the nervous system in order to guide management in some cases. The best neuromonitor remains the awake patient. In the conscious state, the function of individual parts of the nervous system and the complex interactions of its different parts can be assessed more accurately. However, most surgical procedures involving the nervous system require general anaesthesia. Procedures that require neuromonitoring can have changes in their monitored parameters corrected by modifying the surgical approach or by having the anaesthesiologist manipulate the parameters under their control. An ideal neuromonitor would be one that is specific for the parameter of interest, and gives reliable, reproducible, or continuous results.

The Virtual-Spine Platform-Acquiring, visualizing, and analyzing individual sitting behavior

Back pain is a serious medical problem especially for those people sitting over long periods during their daily work. Here we present a system to help users monitoring and examining their sitting behavior. The Virtual-Spine Platform (VSP) is an integrated system consisting of a real-time body position monitoring module and a data visualization module to provide individualized, immediate, and accurate sitting behavior support. It provides a comprehensive spine movement analysis as well as accumulated data visualization to demonstrate behavior patterns within a certain period. The two modules are discussed in detail focusing on the design of the VSP system with adequate capacity for continuous monitoring and a web-based interactive data analysis method to visualize and compare the sitting behavior of different persons. The data was collected in an experiment with a small group of subjects. Using this method, the behavior of five subjects was evaluated over a working day, enabling inferences and suggestions for sitting improvements. The results from the accumulated data module were used to elucidate the basic function of body position recognition of the VSP. Finally, an expert user study was conducted to evaluate VSP and support future developments.

Predicting intraoperative feasibility of combined TES-mMEP and cSSEP monitoring during scoliosis surgery based on preoperative neurophysiological assessment

BACKGROUND CONTEXT

Combined monitoring of muscle motor evoked potentials elicited by transcranial electric stimulation (TES-mMEP) and cortical somatosensory evoked potentials (cSSEPs) is safe and effective for spinal cord monitoring during scoliosis surgery. However, TES-mMEP/cSSEP is not always feasible. Predictors of feasibility would help to plan the monitoring strategy.

PURPOSE

To identify predictors of the feasibility of TES-mMEP/cSSEP during scoliosis surgery.

STUDY DESIGN/SETTING

Prospective cohort study in a clinical neurophysiology unit and pediatric orthopedic department of a French university hospital.

PATIENT SAMPLE

A total of 103 children aged 2 to 19 years scheduled for scoliosis surgery.

OUTCOME MEASURES

Feasibility rate of intraoperative TES-mMEP/cSSEP monitoring.

METHODS

All patients underwent a preoperative neurological evaluation and preoperative mMEP and cSSEP recordings at both legs. For each factor associated with feasibility, we computed sensitivity, specificity, positive predictive value (PPV), and negative predictive value. A decision tree was designed.

RESULTS

Presence of any of the following factors was associated with 100% feasibility, 100% specificity, and 100% PPV: idiopathic scoliosis, normal preoperative neurological findings, and normal preoperative mMEP and cSSEP recordings. Feasibility was 0% in the eight patients with no recordable mMEPs or cSSEPs during preoperative testing. A decision tree involving three screening steps can be used to identify patients in whom intraoperative TES-mMEP/cSSEP is feasible.

CONCLUSIONS

Preoperative neurological and neurophysiological assessments are helpful for identifying patients who can be successfully monitored by TES-mMEP/cSSEP during scoliosis surgery.

S100B, intraoperative neuromonitoring findings and their relation to clinical outcome in surgically treated intradural spinal lesions

BACKGROUND

Neurophysiological monitoring (IOM) consisting of somatosensory (SEPs), muscle (MEPs) and spinal motor evoked (D-wave; spinal MEPs) potentials is used to indicate injury related to surgical treatment of intradural and intramedullary lesions. Combining spinal and muscle MEPs reliably predicts long-term motor deficit. If spinal MEPs recording is not possible, additional markers-e.g. S100B, a serum marker for glial injury-may be a helpful adjunct. Thus, serial serum S100B measurements were related to both the intraoperative IOM recordings and the long-term neurological outcome in patients surgically treated for cervical and thoracic intradural lesions.

METHODS

In 33 patients (9 men, 24 women, 54 ± 17 years) during intramedullary (8) or intradural (25) cervical or thoracic spinal surgeries significant intraoperative SEP-amplitude decrement >50 % or MEP loss and serial S100B serum concentration (perioperative days 0, 1-3, 5) were related to outcome (>1 year after discharge, grouped into improved and unchanged/altered neurological symptoms).

RESULTS

Differences in S100B levels between patients with improved and unchanged/altered neurological outcome were significantly on postoperative days 2 (0.085 ± 0.08 μg/l vs 0.206 ± 0.07 μg/l, p = 0.005) and 3 (0.076 ± 0.03 μg/l vs 0.12 ± 0.05 μg/l, p = 0.007). All patients with permanent altered neurological symptoms developed S100B levels >0.08 μg/l (0.09-0.35 μg/l). Eighty-one percent of patients with improved neurological symptoms presented with S100B levels ≤0.08 μg/l (0.02-0.08 μg/l). Nine out of ten patients (90 %) without changes in EP and S100B had an improved long-term outcome, whereas 9/13 patients (69 %) with changes in EP and S100B had altered neurological symptoms in long-term outcome.

CONCLUSION

Intraoperative stable EPs and S100B ≤0.08 μg/l may be used as a marker to predict long-term neurological improvement, whereas EP-changes and elevated S100B levels on the 3rd postoperative day may be useful as a marker to predict long-term neurological alteration. In summary, the combined use of S100B and EPs might be helpful in the prediction of the severity of adverse spinal cord affection following surgery and guidance of patients.

Predictive Value of Somatosensory Evoked Potential Monitoring during Resection of Intraparenchymal and Intraventricular Tumors Using an Endoscopic Port

BACKGROUND AND PURPOSE

Intraoperative neurophysiological monitoring (IONM) using upper and lower somatosensory evoked potentials (SSEPs) is an established technique used to predict and prevent neurologic injury during intracranial tumor resections. Endoscopic port surgery (EPS) is a minimally-invasive approach to deep intraparenchymal and intraventricular brain tumors. The authors intended to evaluate the predictive value of SSEP monitoring during resection of intracranial brain tumors using a parallel endoscopic technique.

METHODS

A retrospective review was conducted of patients operated on from 2007-2010 utilizing IONM in whom endoscopic ports were used to remove either intraparenchymal or intraventricular tumors. Cases were eligible for review if an endoscopic port was used to resect an intracranial tumor and the electronic chart included all intraoperative monitoring data as well as pre- and post-operative neurologic exams.

RESULTS

139 EPS cases met criteria for inclusion. Eighty five patients (61%) had intraparenchymal and fifty four (39%) had intraventricular tumors or colloid cysts. SSEP changes were seen in eleven cases (7.9%), being irreversible in three (2.2%) and reversible in eight cases (5.8%). Seven patients (5.0%) with intraparenchymal tumors had SSEP changes which met our criterea for significant changes while there were four (2.9%) with intraventricular (p-value=0.25). Five patients suffered post operative deficits, two reversible and two irreversible SSEP changes. Only one case exhibited post operative hemiparesis with no SSEP changes. The positive predictive value of SSEP was 45.4% and the negative predictive value was 99.2%.

CONCLUSIONS

Based on the high negative and low positive predictive values, the utility of SSEP monitoring for cylindrical port resections may be limited. However, the use of SSEP monitoring can be helpful in reducing the impact of endoscopic port manipulation when the tumor is closer to the somatosensory pathway.

Time-frequency analysis of somatosensory evoked potentials for intraoperative spinal cord monitoring

PURPOSE

To evaluate the potential use of time-frequency analysis and its reliability in intraoperative somatosensory evoked potential (SEP) monitoring.

METHODS

One hundred ninety-one patients undergoing thoracic and/or lumbar spinal surgery were studied retrospectively. The SEP signals were recorded during different stages of surgery. Averaged SEP was analyzed by short-time Fourier transform. The main peak in the time-frequency interpretation of SEP was measured in peak power, peak time, and peak frequency. The variability of these parameters was compared with that of amplitude and latency during different stages of surgery. The reliability of these parameters was also compared in true-positive and false-positive cases.

RESULTS

During different surgical stages for the posterior tibial nerve SEP, the intrasubject variability of peak power was found to be more stable than that of amplitude, while the intrasubject variability of peak time did not show any difference compared with that of latency. The peak frequency presented stable during surgery. Moreover, the true-positive SEP case showed that peak power may detect the potential injury earlier than amplitude does. The false-positive outcomes could be reduced by the proposed method.

CONCLUSIONS

The SEP peak component was found stable and reliable during the different stages of surgery. For clinical application purpose, time-frequency analysis was suggested to be an additional monitoring method besides the conventional amplitude/latency measurement since it provided a more reproducible and prompt response to the potential injury in intraoperative SEP monitoring.

Clinical neurophysiology in the prognosis and monitoring of traumatic spinal cord injury

Preclinical studies for the repair of spinal cord injury (SCI) and potential therapies for accessing the inherent plasticity of the central nervous system (CNS) to promote recovery of function are currently moving into the translational stage. These emerging clinical trials of therapeutic interventions for the repair of SCI require improved assessment techniques and quantitative outcome measures to supplement the American Spinal Injuries Association (ASIA) Impairment Scales. This chapter attempts to identify those electrophysiological techniques that show the most promise for provision of objective and quantitative measures of sensory, motor, and autonomic function in SCI. Reviewed are: (1) somatosensory evoked potentials, including dermatomal somatosensory evoked potentials, and the electrical perceptual threshold as tests of the dorsal (posterior) column pathway; (2) laser evoked potentials and contact heat evoked potentials as tests of the anterior spinothalamic tract; (3) motor evoked potentials in limb muscles, in response to transcranial magnetic stimulation of the motor cortex as tests of the corticospinal tract, and the application of the technique to assessment of trunk and sphincter muscles; and (4) the sympathetic skin response as a test of spinal cord access to the sympathetic chain.

Histogram based quantification of spinal cord injury level using somatosensory evoked potentials

This paper uses an entropy based metric to study the somatosensory evoked potential (SEP) in rodents afflicted with focal demyelination spinal cord injury (SCI). It has been shown that amplitude characteristics of the SEP signal are a strong indicator of the integrity of the spinal cord sensory pathways. Compared to conventional correlation based metrics, the metric used in this paper exploits the amplitude histogram of SEP signals to provide a robust assessment of the different degrees of demyelination in the spinal cord. Results are presented using actual SEP signals collected on rodents with various levels of SCI.

Intraoperative multimodality monitoring in adult spinal deformity: analysis of a prospective series of one hundred two cases with independent evaluation

STUDY DESIGN

A retrospective analysis of prospectively collected data of 102 consecutive adult patients who underwent intraoperative neurophysiological monitoring (IOM) during spinal deformity corrective surgery.

OBJECTIVE

To report the sensitivity and specificity of combined IOM in this study population using the postoperative neurologic examination as the "gold standard."

SUMMARY OF BACKGROUND DATA

IOM is recommended during corrective spinal surgery and has been widely used in the pediatric deformity population. However, there are limited data describing the application of IOM in adults undergoing spinal deformity corrective surgery.

METHODS

The study group consisted of 102 patients undergoing spinal deformity corrective surgery between 2001 and 2004. Patients were monitored using at least 2 or more electrophysiological methods including somatosensory-evoked potentials (SSEP), motor-evoked potentials (MEP), and electromyography (EMG).

RESULTS

The mean age of patients was 41.5 years (+/-17). The majority of the operative procedures involved instrumented fusion from thoracic to lumbar/sacral spine (n = 55), thoracic-pelvis fusion (n = 26), and a combined total of 32 osteotomies (including 25 pedicle subtraction osteotomies and 7 Smith-Peterson osteotomies). SSEPs were recorded successfully in 101 (99%), EMGs in 89 of 102 (87%), and MEPs in 12 of 16 (75%). Five cases were true positives (4.95%), and these were all detected by combined monitoring (2-SSEP, 2-EMG, 1-MEP). There were no false positives with SSEPs, but EMG resulted in 30 of 89 (34%) false positives. There were 4 false negatives with SSEPs, which reduced its sensitivity to 33%. There was 1 false negative with EMG, and 0 with MEPs. When these results were collated, the overall sensitivity of combined multimodality IOM in this adult deformity series was 100%, specificity 84.3%, PPV 13.9%, and NPV 97%. The combined sensitivity in the osteotomy group (n = 32) was 67%, specificity 98%, PPV 80%, and NPV 96%. In comparison, there were no IOM abnormalities in those patients who had in situ/minor corrective procedures (n = 18; largely adult degenerative scoliosis).

CONCLUSION

Multimodality IOM of spinal cord sensory and motor function during surgical correction of adult spinal deformity is feasible and provides useful neurophysiological data with an overall sensitivity of 100% and a specificity of 84.3% (67% and 98%, respectively in patients undergoing major deformity correction).

Foramen magnum meningiomas--experience with the posterior suboccipital approach

The aim of this study is to analyse short- and long-term results after surgical treatment of foramen magnum meningiomas and to identify the possible advantages of the posterior suboccipital approach over lateral and anterior approaches. Between 1992 and 2006, 16 patients with foramen magnum meningiomas were operated on in our institution, and in all cases a posterior suboccipital approach was utilised with lateral extension of the bone opening according to the position of the tumour. In 14 patients, intraoperative monitoring of the lower cranial nerves was performed. Localisation of the tumours was ventral (3), ventrolateral (10), dorsal (1) and dorsolateral (2). Mean age of the patients was 61 years (ranging from 40 to 85 years). Preoperative and postoperative function was classified according to the McCormick scale. We found in eight patients a postoperative upgrading of at least one grade, in five patients an unchanged status and a deterioration in only two patients. Complete removal of the tumour was possible in 14 cases (Simpson 1-2). The follow-up period varied from 24 to 119 months (mean 43.5 months), during this time there were no recurrences. Removal of foramen magnum meningiomas can be performed safely today with the use of microsurgical techniques and intraoperative monitoring. In our experience, the posterior suboccipital approach is suitable for the majority of these tumours.

Evoked potential versus behavior to detect minor insult to the spinal cord in a rat model

Reliable outcome measurement is needed for spinal cord injury research to critically evaluate the severity of injury and recovery thereafter. However, such measurements can sometimes be affected by minor, injury to the spinal cord during surgical procedures, including laminectomy. The open-field Basso, Beattie and Bresnahan (BBB) behavior motor scores are subjective and prone to human error. We investigated somatosensory evoked potential (SEP) as an electrophysiological measure to assess the integrity of the spinal cord after injury. In our experiment, control rats with a minor unintentional spinal cord insult during laminectomy showed a decrease in SEP amplitude by 16% to 18%, which recovered in around 7 days. However, there was no change in the BBB scores for the same animals over the same period. This highlights the sensitivity of SEP to minor insult as compared to BBB. These differences may be beneficial in accurate evaluation of the severity and progression of spinal cord injury, and subsequent recovery.

Current approach on spinal cord monitoring: the point of view of the neurologist, the anesthesiologist and the spine surgeon

Optimal outcome in spine surgery is dependent of the coordination of efforts by the surgeon, anesthesiologist, and neurophysiologist. This is perhaps best illustrated by the rising use of intraoperative spinal cord monitoring for complex spine surgery. The challenges presented by neurophysiologic monitoring, in particular the use of somatosensory and motor evoked potentials, requires an understanding by each member for the team of the proposed operative procedure as well as an ability to help differentiate clinically important signal changes from false positive changes. Surgical, anesthetic, and monitoring issues need to be addressed when relying on this form of monitoring to reduce the potential of negative outcomes in spine surgery. This article provides a practical overview from the perspective of the neurophysiologist, the anesthesiologist, and the surgeon on the requirements which must be understood by these participants in order to successfully contribute to a positive outcome when a patient is undergoing complex spine surgery.

Multimodality intraoperative neurophysiologic monitoring findings during surgery for adult tethered cord syndrome: analysis of a series of 44 patients with long-term follow-up

STUDY DESIGN

Prospective analysis of a consecutive series in which multimodality intraoperative neurophysiologic monitoring was used as an adjunct to microneurosurgery for adult tethered cord syndrome. The results of multimodality intraoperative neurophysiologic monitoring were compared with the "gold standard" (neurologic outcomes).

OBJECTIVE

To assess the sensitivity, specificity, and positive and negative predictive values of multimodality intraoperative neurophysiologic monitoring in surgery for adult tethered cord syndrome.

SUMMARY OF BACKGROUND DATA

Although intraoperative electrophysiologic techniques may help to minimize neural injury during spinal microneurosurgery, to our knowledge, no study has quantitatively evaluated the value of multimodality intraoperative neurophysiologic monitoring in the management of adult tethered cord syndrome.

METHODS

Multimodality intraoperative neurophysiologic monitoring included posterior tibial nerve somatosensory evoked potentials (SSEPs), continuous electromyographic (EMG) monitoring of the L2 to S4 myotomes, and evoked EMG. Follow-up neurologic evaluations were performed for at least 1 year.

RESULTS

A total of 44 consecutive patients, including 19 males and 25 females (aged 43 +/- 15 years), who underwent microsurgery for adult tethered cord syndrome were evaluated. After surgery, new neurologic deficits, including 1 transient and 1 permanent, developed in 2 patients. There was 1 patient who had persistent posterior tibial nerve SSEP amplitude reduction following microsurgical manipulation. In 1 patient, a transient posterior tibial nerve SSEP amplitude reduction prompted a change in microneurosurgical strategy. This patient awoke with no new postoperative neurologic deficits. For SSEPs, the sensitivity was 50% and specificity 100%. EMG bursts were recorded in 36 patients (82%). The 2 patients with postoperative neurologic worsening had EMG activity in the myotomes, where their new deficits presented. Continuous EMG had a sensitivity of 100% and a specificity of 19%.

CONCLUSIONS

To our knowledge, this is the largest series to date reporting the use of multimodality intraoperative neurophysiologic monitoring in the surgical management of adult tethered cord syndrome. Posterior tibial nerve SSEPs have high specificity, but low sensitivity, for predicting new neurologic deficits. In contrast, continuous EMG showed high sensitivity and low specificity. Evoked EMG accurately identified functional neural tissue. The combined recording of SSEPs in concert with continuous and evoked EMGs may provide a useful adjunct to complex microsurgery for adult tethered cord syndrome.

Continuous EEG–SEP monitoring of severely brain injured patients in NICU: methods and feasibility

AIMS

To evaluate the feasibility of a continuous neurophysiologic monitoring (electroencephalography (EEG)-somatosensory evoked potentials (SEPs)) in the neuro-intensive care unit (NICU), taking into account both the technical and medical aspects that are specific of this environment.

METHODS

We used an extension of the recording software that is routinely used in our unit of clinical neurophysiology. It performs cycles of alternate EEG and SEP recordings. Raw traces and trends are simultaneously displayed. Patient head and stimulator box are placed behind the bed and linked to the ICU monitoring terminal through optic fibers. The NICU staff has been trained to note directly clinical events, main artefacts and therapeutic changes. The hospital local area network (LAN) enables remote monitoring survey.

RESULTS

Continuous EEG (CEEG)-SEP monitoring was performed in 44 patients. Problems of needle detachment were seldomly encountered, thanks to the use of a sterile plastic dressing, which covers needles. We never had infection or skin lesions due to needles or the electrical stimulator. The frequent administration of sedative at high doses prevented us from having a clinically valuable EEG in several cases but SEPs were always monitorable, independently of the level of EEG suppression. The diagnosis of seizures and non-epileptic status was based on raw EEG, while quantitative EEG (QEEG) was used to quantify ictal activity as a guide to treatment.

CONCLUSIONS

EEG and EP waveforms collected in NICU were of comparable quality to routine clinical measurements and contained the same clinical information. A continuous SEP monitoring in a comatose and sedated patient in NICU is not technically more difficult and potentially less useful than in operating room. This monitoring appears to be feasible provided the observance of some requirement regarding setting, electrodes, montages, personnel integration, consulting and software.

Multi-modality neurophysiological monitoring during surgery for adult tethered cord syndrome

During complex microneurosurgery performed in patients with tethered cord syndrome, the conus medullaris and the roots that innervate the lower limbs, bladder and bowel are potentially exposed to damage. The aim of multimodality intraoperative monitoring (IOM) is to reduce the risk of inadvertent injury of neural tissue. We simultaneously record tibial nerve somatosensory evoked potentials (SSEPs) from the scalp and free run electromyography (EMG) of limb muscles supplied by L2 to S2 roots, anal and urethral sphincters. We also identify critical neural structures in the operative field, including the conus and exiting nerve roots, with a nerve stimulator to evoke EMG. SSEPs assess the sensory pathways mainly mediated by the S1 roots. Continuous EMG provides the surgeon with immediate auditory feedback resulting from irritative discharges triggered by manipulation of nerve fibres. Microstimulation can distinguish the filum terminale, scar tissue and invasive tumors from functional neural tissue, thus minimizing the risk of iatrogenic injury. Overall multimodality IOM proves a valuable adjunct to microneurosurgery of the lumbosacral spine.

Palsies of the fifth cervical nerve root after cervical decompression: prevention using continuous intraoperative electromyography monitoring

OBJECT

A desire to prevent complications resulting from spinal surgery led to the development of intraoperative monitoring. Intraoperative electromyography (EMG) provides useful diagnostic information regarding nerve root function during spinal and peripheral nerve surgeries. The C-5 nerve root is considered particularly vulnerable to injury during cervical surgery. Despite advances in techniques, the incidence of postoperative C-5 palsy has not changed.

METHODS

The authors reviewed prospectively collected data obtained in 161 patients who underwent 171 cervical procedures. In 116 procedures, operative monitoring was modified to include continuous C-5 EMG from the deltoid muscle. In cases in which spontaneous C-5 activity occurred, an appropriate change in operative manipulation was made. A historical control group consisted of a retrospective review of 55 procedures that were monitored using conventional techniques. In the retrospective cohort, four (7.3%) of 55 patients presented after undergoing surgery for C-5 nerve root palsy. In each patient conventional monitoring revealed unremarkable findings. In the prospective cohort, intraoperative spontaneous EMG activity necessitated a change in either positioning or operative technique in three cases. Only one patient (0.9%) experienced postoperative C-5 palsy. Postoperative C-5 palsy occurred in no patient in whom there was no intra-operative evidence of root irritation (p < 0.03, chi-square test).

CONCLUSIONS

The incidence of postoperative C-5 palsies was reduced from 7.3% to 0.9% due to intraoperative continuous EMG monitoring. No patient suffered a postoperative C-5 palsy when intraoperative evidence of root irritation was absent.

Effectiveness of Intraoperative Somatosensory Evoked Potential Monitoring During Cervical Spinal Operations on Animals with Spinal Cord Dysfunction

We conducted somatosensory evoked potential (SEP) monitoring on 3 dogs with cervical spinal cord dysfunction caused by various diseases throughout operative procedures to examine whether the intraoperative SEP monitoring was effective for prediction of spinal cord conductive function. The SEP was recorded on the scalp via stimulation of the ulnar nerve. Stable SEP was recorded in all animals examined. Its amplitude was decreased by surgical manipulations of the regio vertebralis, but the amplitude gradually recovered once the manipulations were halted. The latency showed small variation throughout the operations. This evidence suggests that intraoperative SEP monitoring may provide continuous and instantaneous information regarding the functional integrity of the central nervous system.

Variability of Somatosensory Evoked Potential Monitoring During Scoliosis Surgery

OBJECTIVE

Somatosensory evoked potentials (SEPs) of 65 patients undergoing scoliosis surgery were monitored by stimulation of posterior tibial nerve to observe variations in latencies and amplitudes.

METHODS

Monitoring was divided into five stages: pre incision, spine exposure, instrumentation loading, deformity correction, and wound closure (stages 1-5, respectively).

RESULTS

We found the latency showed significant increase and the amplitude significant reduction from stages 1 to 2. There was no significant variability from stages 2, 3, and 4, but both latency and amplitude recovered significantly from stage 4 to 5. This variability correlated with the changes in mean arterial pressure and end-tidal concentrations of isoflurane and was not dependent on the type of surgical procedure. If either 50% amplitude reduction or 10% latency prolongation of SEP compared with baseline recordings at stage 1 (pre incision) was used as warning criterion, the overall false-positive rate was 23.1%. It was significantly reduced to 7.7% if stage 2 (spine exposure) recordings were used as the baseline (P < 0.05). The false-positive rate decreased to 0% if a combined 50% amplitude reduction and 10% latency prolongation of SEP compared with the stage 2 baseline were used (P < 0.001).

CONCLUSION

Based on these findings, we concluded that the time to obtain SEP baseline data should be adjusted to be post incision instead of pre incision.

The effects of isoflurane and propofol on intraoperative neurophysiological monitoring during spinal surgery

OBJECTIVES

To compare the effects of isoflurane and propofol on intraoperative neurophysiological monitoring (IONM) during spinal surgery.

METHODS

Thirty-five patients were randomly assigned to receive isoflurane (n = 17) or propofol (n = 18) anesthesia. Somatosensory evoked potentials (SEPs) following posterior tibial nerve stimulation were recorded before induction as baselines. Isoflurane concentrations and propofol infusions were adjusted to obtain four pre-determined BIS ranges: 65-55, 55-45, 45-35 and 35-25. For each range, a stable state was maintained for at least 10 min to perform IONM. The SEP latency P40 and amplitude P40-N50, the onset latency and amplitude of transcranial motor evoked potentials (tcMEPs), and threshold intensity of triggered electromyographic activity (EMG) following pedicle screw stimulation were statistically analyzed.

RESULTS

Compared with baseline values, P40 latency increased and P40-N50 amplitude decreased after anesthesia with isoflurane or propofol. Isoflurane caused a dose-dependent depression of SEPs, but propofol did not. TcMEPs were recordable and stable in all patients receiving propofol in each BIS range, but only recordable in 10 (58.8%) receiving isoflurane with BIS >55, and 3 (17.8%) with BIS <55. No difference was noted in triggered EMG.

CONCLUSIONS

Isoflurane inhibited IONM more than propofol. Propofol is recommended for critical spinal surgery, particularly when motor pathway function is monitored.

Electrophysiologic intraoperative monitoring for spine procedures

The advent of equipment capable of performing SEPs, MEPs, and EMG in a multiplexed manner and in a timely fashion brings a new level of monitoring that far exceeds the previous basic monitoring done with SEPs only. Whether this more comprehensive monitoring will result in greater protection of the nervous system awaits future analysis. In any event, monitoring of the spinal cord with SEPs is an accepted standard of care for cases that place the spinal cord at risk. Likewise, nerve root monitoring with EMG is a widely practiced form of monitoring and shows great benefit. MEPs and reflex monitoring, which address the descending pathways and the interneuronal connections, is efficacious in detecting abnormalities that may be missed by SEPs.

Motor Evoked Potential Monitoring for the Surgery of Brain Tumours and Vascular Malformations

Brain surgery incurs a significant risk of a new motor deficit in lesions within or adjacent to the motor areas and pathways which, for the patient, presents one of the most disabling complications of such operations. It is a major concern of intracranial procedures to delineate and monitor motor regions in order to preserve their structural and functional integrity, while still achieving maximal cytoreduction. The technique of motor evoked potential recording has had to be adapted to intraoperative recording conditions under general anaesthesia, but has been available for clinical use now for almost ten years. This contribution summarizes the current technique and related methods, as well as our clinical experience in some 400 cases of MEP monitoring in supratentorial tumors, lesions in and around the brainstem, and aneurysm surgery. Intraoperative MEP recordings have been shown to reliably reflect an impending new motor deficit. Irreversible MEP deterioration heralds new paresis, and unaltered recordings predict preserved motor function. This is also true in aneurysm surgery where conventional SEP monitoring may yield false-negative results with regard to development of a new motor deficit. Moreover, if MEP deterioration can be reversed, or halted by early surgical intervention, the presence of only a transient motor deficit, or even the lack of a new postoperative deficit, indicates the success of the MEP monitoring method in the prevention of a significant motor impairment. Certain complicated lesions can only be operated on at all because MEP monitoring is available. In conclusion, intraoperative MEP monitoring is a useful aid in brain surgery with which to avoid a new motor deficit without compromise to the surgical result. Controlled prospective studies will be required to verify the clinical value of the method.

Propofol Suppresses the Cortical Somatosensory Evoked Potential in Rats

The dose-response curve for the effect of volatile anesthetics on the somatosensory evoked potential (SEP) is well described, but for propofol, the large dose segment of the curve is undefined. We describe the effect of increasing plasma concentrations of propofol on cortical SEPs in 18 rats. After surgical preparation under ketamine anesthesia, a remifentanil infusion was begun at 2.5, 5, or 10 microg x kg(-1) x min(-1). After 20 min, the propofol infusion was initiated at 20 mg x kg(-1) x h(-1) and was increased to 40, 60, and 80 mg x kg(-1) x h(-1) at 20-min intervals. SEP was recorded before remifentanil infusion, before propofol infusion rate changes, and 30 min after discontinuing propofol infusion. In six additional rats, the plasma concentrations of propofol after each 20-min infusion were measured using gas chromatography. Remifentanil did not have a significant effect, but propofol significantly depressed the SEP amplitude and prolonged the latency at infusion rates of 40 mg x kg(-1) x h(-1) and more. Propofol's effect was dose-dependent, but even at 80 mg x kg(-1) x h(-1) with an estimated plasma concentration of 31.6 +/- 3.4 microg/mL (10.8 50% effective concentration), a measurable response was present in 44.5% of rats. These results suggest that even at large doses, propofol and remifentanil provide adequate conditions for SEP monitoring.

IMPLICATIONS

Rats demonstrate dose-dependent somatosensory evoked potential (SEP) suppression with propofol but not with remifentanil. However, SEP suppression by 50% occurred only at large (1.5 EC(50)) concentrations of propofol, and a measurable SEP was present in 8 of 18 rats, even at 10.8 EC(50).

Reliability of perioperative SSEP recordings in spine surgery

OBJECTIVE

The reproducibility and clinical reliability of perioperative somatosensory-evoked potentials (SSEP) were prospectively evaluated in uneventful scoliosis surgery. The influence of anesthesia owing to induction of total intravenous anesthesia (TIVA) upon preoperative SSEP and the variability of intraoperative SSEP were calculated. The potential effect of spine surgery was assessed by comparing pre- to postoperative SSEP.

METHODS

A total of 2,143 pre-, intra- and postoperative tibial and median SSEP recorded in 25 patients undergoing spine surgery owing to idiopathic scoliosis were analyzed. The anesthesia protocol consisted of a computerized target controlled infusion (TCI) device for propofol and intravenous application of an opioid.

RESULTS

Anesthesia induced a significant and comparable prolongation of the tibial SSEP onset, P40 and P60 latencies, while the N50 latency was less changed. Throughout anesthesia, latencies of median (onset, N20, P25 and N35) and tibial (onset, P40, N50 and P60) SSEP showed mean variations of less than 6%. The intraoperative SSEP amplitudes were less stable with a relative standard deviation of 30-40%. In uneventful spine surgery, the postoperative tibial SSEP were not significantly changed in comparison to preoperative recordings.

CONCLUSIONS

By using a standardized anesthesia protocol, the impact of anesthesia on preoperative SSEP can be predicted. Furthermore, the controlled application of sedatives and analgesics allows recording of stable SSEP parameters for intraoperative monitoring purposes. As in uneventful spine surgery pre- to postoperative SSEP are unchanged the latter comparison can be applied as an additional perioperative neuromonitoring procedure to assess the influence of spine surgery or other invasive interventions on spinal cord function.

Mechanisms of signal change during intraoperative somatosensory evoked potential monitoring of the spinal cord

In scoliosis surgery, intraoperative somatosensory evoked potential (SSEP) monitoring has reduced the incidence of postoperative neurologic deficits. Many factors affect the amplitude and latency of SSEP waveforms during surgery. Somatosensory evoked potential amplitude decreases with ischemia and anoxia because of temporal dispersion of the afferent volley and conduction block in damaged axons. In conjunction with surgical manipulations, minor drops in blood pressure may result in substantial SSEP changes that reverse when perfusion pressure is increased. Irreversible anoxic injury to central nervous system white matter with loss of SSEP waveforms is dependent on calcium influx into the intracellular space. Somatosensory evoked potential monitoring may be less sensitive for detecting acute insults in the presence of preexisting white matter lesions. Increased extracellular potassium from acute baro-trauma can block axonal conduction transiently even when there is no axonal disruption. Marked temperature-related drops in SSEP amplitude may occur after exposure of the spine but before instrumentation and deformity correction. Hypothermia may increase false-negative outcomes. Short-interval double-pulse stimulation may improve the sensitivity of the SSEP in detecting early ischemic changes. For neurosurgical procedures on the spinal cord the use of SSEP monitoring in improving postoperative outcome is less well established.

Complications associated with the prophylactic use of methylprednisolone during surgical stabilization after spinal cord injury

OBJECT

The authors attempted to determine if there is a significant relationship between the incidence of medical complications and the prophylactic use of methylprednisolone (MP) during spine surgery in patients with acute spinal cord injury (SCI) who had already received MP on hospital admission (typically in the setting of an Emergency Room/Trauma Center).

METHODS

The authors studied 73 patients with acute SCI who were admitted to the hospital for at least 7 days postinjury. All patients 1) received a 24-hour regimen of MP in the acute period of hospitalization; and 2) underwent surgery to stabilize the spine and/or decompress the spinal cord. Patients were separated into two groups on the basis of whether they received additional MP therapy during spine surgery. A chart review was conducted retrospectively to determine the incidence of complications up to 6 weeks postinjury. Muscle strength and American Spinal Injury Association grades were determined prospectively throughout the follow-up period. In patients who received two courses of MP following acute SCI (one at initial hospitalization and one during surgery), a significantly increased probability of complications was demonstrated compared with those who received no MP therapy during surgery. This was particularly evident when the incidences of serious complications were compared.

CONCLUSIONS

Prophylactic use of MP as a neuroprotective agent during spine surgery in patients with acute SCI should be avoided in those in whom MP was administered on admission to the hospital.

Threshold-level repetitive transcranial electrical stimulation for intraoperative monitoring of central motor conduction

OBJECT

The authors conducted a study to evaluate repetitive transcranial electrical stimulation (TES) to assess spinal cord motor tract function in individuals undergoing spine surgery, with emphasis on safety and efficacy.

METHODS

Somatosensory evoked potentials (SSEPs) were elicited using standard technique. Muscle electromyographic values were measured in response to a three- or four-pulse train of stimulation delivered to the motor cortex via subdermal electrodes. They also evaluated whether changes in the minimum stimulus intensity (that is, threshold level) needed to elicit a response from a given muscle predict motor status immediately postoperatively, as well as whether changes in SSEP response amplitude and latency predict sensory status immediately postoperatively. Anesthesia was routinely induced with intravenous propofol and remifentanil, supplemented with inhaled nitrous oxide. Use of neuromuscular block was avoided after intubation. Satisfactory monitoring of muscle response to threshold-level repetitive TES was achieved in all but nine of the 194 patients studied. In contrast, cortical SSEP responses could not be elicited in 42 of 194 individuals. In cases in which responses were present, TES-based evoked responses proved to be extremely accurate for predicting postoperative motor status. Somatosensory evoked potential monitoring was nearly as accurate for predicting postoperative sensory status. There were frequent instances of postoperative motor or sensory deficit that were not predicted by SSEP- and TES-based monitoring, respectively. There were no adverse events attributable to TES-based monitoring, although since this study ended we have had a single adverse event attributable to threshold-level repetitive TES.

CONCLUSIONS

Intraoperative threshold-level repetitive TES-based monitoring of central motor conduction has proven to be a simple, safe, and highly accurate technique for the prevention or minimization of inadvertent motor deficit during surgery involving the spine or spinal cord.

Anterior spinal cord injury with preserved neurogenic 'motor' evoked potentials

OBJECTIVE

To describe two cases in which intraoperative monitoring of neurogenic 'motor' evoked potentials (NMEPs) did not identify a spinal cord injury that resulted in paraplegia.

METHODS

Bilateral tibial nerve somatosensory evoked potential (SEP) and NMEP testing was performed in two patients during spinal deformity corrective surgery using standard stimulation and recording parameters. These potentials were obtained repetitively throughout the primary procedures and were performed again during a subsequent procedure that took place after the discovery of paraplegia.

RESULTS

SEP and NMEP signals were preserved in both patients and no adverse events were identified during the initial procedures. Postoperatively, paraplegia was identified immediately upon recovery from anesthesia and preserved posterior column function was apparent on clinical exam. In the procedures following the discovery of paraplegia, SEP and NMEP signals remained comparable with signals elicited in the initial surgeries.

CONCLUSIONS

Based on these cases and previously published experimental evidence, we conclude that while 'NMEPs' remain a useful second test of spinal cord function, they are not reliable indicators of motor tract function. An alternate term, such as 'spinally-elicited peripheral nerve responses' should be used.