Systemic lidocaine and human somatosensoryevoked potentials during sufentanil-isoflurane anaesthesia

The effect of systemically administered lidocaine on somatosensory evoked potentials (SSEPs) during general anaesthesia has not been widely reported. Knowledge of the influence of anaesthetic agents on evoked potentials assists in interpreting evoked potential waveforms. Accordingly, we studied the behaviour of cortical and subcortical (recorded at the second cervical vertebra) SSEPs after administration of intravenous lidocaine (3 mg.kg-1 bolus followed by infusion at 4 mg.kg-1.hr-1) during a sufentanil-based anaesthetic regimen in 16 patients undergoing abdominal or orthopaedic surgery. When compared to awake baseline recordings, the sufentanil-nitrous oxide, low-dose isoflurane anaesthetic depressed N1 amplitude by approximately 40% and prolonged latency by 10%. Fifteen minutes after establishment of this anaesthetic, the amplitude and latency of N1 were 1.13 +/- 0.56 microV and 19.81 +/- 1.63 msec, respectively. Within five minutes of adding lidocaine, amplitude decreased further to 0.84 +/- 0.39 microV (P = 0.001), while latency was extended to 20.44 +/- 1.48 msec (P = 0.01). Lidocaine did not affect cervical amplitude and prolonged latency only minimally. Despite the observed effects on amplitude and latency, SSEP waveforms were preserved and interpretable. Plasma lidocaine levels obtained at 5, 20, and 40 minutes after lidocaine were 5.17 +/- 1.33, 3.76 +/- 1.14, and 3.66 +/- 0.9 micrograms.dl-1, respectively. Our results indicate that systemically administered lidocaine at therapeutic plasma levels acts synergistically with a sufentanil-based anaesthetic to depress the amplitude and prolong the latency of SSEPs.     

Sensitivity of scalp EEG,cortical EGG,and somatosensory evoked responses during surgery for intracranial aneurysms

BACKGROUND: We estimated the relative sensitivity and reliability of scalp EEG, cortical EEG and somatosensory evoked potentials (SSEPs) to detect significant changes during aneurysm surgery. METHODS: Two observers independently reviewed data from 18 patients who were monitored with scalp EEG, cortical EEG, and SSEPs to determine which if any modality demonstrated significant changes during 25 different episodes of temporary intracranial vascular occlusion. RESULTS: Kappa scores indicating the degree of agreement between the two observers were highest for the cortical strip EEG (kappa = 0.92) and the SSEPs (kappa = 0.82) and much greater than for the scalp EEG data (kappa = -0.07). The cortical strip recordings showed changes more often than either the scalp EEG or SSEP during temporary vascular occlusion. In no case did the scalp EEG change when neither the strip nor SSEP changed. In only 4% of events did the observers feel that the SSEP changed when the strip did not, but in 16% of cases, the strip changed without changes in any of the other modalities. CONCLUSIONS: Recording of EEG from strip electrodes placed on the cortical surface detects changes more frequently than either scalp EEG or SSEPs during vascular occlusion. Different observers were more likely to agree on whether the cortical strip EEG changed during vascular occlusion than the other modalities. This argues for the possible advantages of recording from strip electrodes during aneurysm surgery.     

Effects of adenosine triphosphate (ATP) on somatosensory evoked potentials in humans anesthetized with isoflurane and nitrous oxide

In order to examine the usefulness of adenosine triphosphate (ATP) as an adjuvant to anesthesia for surgery requiring intraoperative somatosensory evoked potential (SSEP) monitoring, we have studied the effects of ATP on SSEPs in patients anesthetized with isoflurane and nitrous oxide (N₂O). A control recording of SSEP was performed while anesthesia was maintained with 0.5% end-tidal concentration of isoflurane in 60% N₂O. The recordings were repeated after an ATP infusion had been added to this basal anesthesia at the rates of 100 μgkg bw^-1 min^-1 and 200 μg kg bw^-1 min^-1. SSEP was also studied when end-tidal isoflurane concentration was increased to 1.5% after cessation of ATP infusion. An infusion of ATP combined with 0.5% isoflurane and 60% N₂O effectively inhibited an increase in blood pressure during surgery. The amplitude of the cortical component of SSEP was lowered by 1.5% isoflurane, which also increased both cortical and spinal latencies as well as central conduction time (CCT). In contrast ATP infusions at both rates induced no significant changes in latencies, amplitude and CCT. The results indicate that ATP infusion combined with 0.5% isoflurane in 60% N₂O can be a useful anesthetic technique for intraoperative SSEP monitoring because adequate anesthetic depth can be maintained by a low concentration of anesthetics without further suppression of SSEPs.     

Intraoperative decrease in amplitude of somatosensory-evoked potentials of the lower extremities with interbody fusion cage placement during lumbar fusion surgery

STUDY DESIGN.: A retrospective analysis was performed. OBJECTIVE.: To characterize neurophysiological data of patients who had a decrease in amplitude of somatosensory-evoked potentials (SSEP) of the lower extremities secondary to interbody fusion cage placement during lumbar fusion surgery with no alert of the electromyography (EMG). SUMMARY OF BACKGROUND DATA.: The most consistently used and studied modalities of neurophysiological monitoring during spine surgery are SSEPs, motor-evoked potentials (MEPs), and EMG. In general, it is accepted that MEPs along with SSEPs are used to detect spinal cord injury and EMGs are used to detect nerve root injury. METHODS.: The medical records of a consecutive series of 115 patients who had undergone a transforaminal lumbar interbody fusion (TLIF) procedure in which SSEPs, MEPs, and EMGs were utilized for neurophysiological monitoring were retrospectively reviewed. RESULTS.: One hundred fifteen cases of TLIF procedures were reviewed. The follow-up was 2 years after the last procedure. A total of 5 cases that demonstrated intraoperative SSEP changes were found. The age range for these cases was from 39 to 81 years (mean age, 61 yr). All 5 patients developed SSEP changes that were secondary to interbody fusion cage placement. All 5 cases demonstrated reversal of the SSEP changes to baseline after removal of the interbody cage. Three of these cases had no new postoperative neurological findings. However, given that these 3 cases of SSEP change were associated with a surgical event that improved secondary to an intervention (in this case removal of the interbody cage), those cases were classified as presumed positive. Two of the 5 cases were in fact associated with a new postoperative neurological deficit. CONCLUSION.: To our knowledge this study demonstrates the first reported SSEP alerts that were associated with a posterior lumbar interbody cage placement without a corresponding EMG alert.     

Prevention of positional brachial plexopathy during surgical correction of scoliosis

Continuous intraoperative monitoring of spinal cord function using somatosensory evoked potentials (SSEP) has gained nearly universal acceptance as a reliable and sensitive method for detecting and possibly preventing neurologic injury during surgical correction of spinal deformities. In several reports, spinal cord injury was identified successfully based on changes in SSEP response characteristics, specifically amplitude and latency. Less well documented and used, however, is monitoring of peripheral nerve function with SSEPs to identify and prevent the neurologic sequelae of prolonged prone positioning on a spinal frame. The authors describe a patient who underwent surgical removal of spinal instrumentation but was not monitored. A brachial plexopathy developed in this patient from pressure on the axilla exerted by a Relton-Hall positioning frame during spinal surgery. In addition, data are presented from 15 of 500 consecutive pediatric patients who underwent surgical correction of scoliosis between 1993 and 1997 with whom intermittent monitoring of ulnar nerve SSEPs was used successfully to identify impending brachial plexopathy, a complication of prone positioning. A statistically significant reduction in ulnar nerve SSEP amplitude was observed in 18 limbs of the 500 patients (3.6%) reviewed. Repositioning the arm(s) or shoulders resulted in nearly immediate improvement of SSEP amplitude, and all awoke without signs of brachial plexopathy. This complication can be avoided by monitoring SSEPs to ulnar nerve stimulation for patients placed in the prone position during spinal surgery.     

Comparison of transcranial motor evoked potentials and somatosensory evoked potentials during thoracoabdominal aortic aneurysm repair

OBJECTIVE: To compare transcranial motor evoked potentials (tc-MEPs) and somatosensory evoked potentials (SSEPs) as indicators of spinal cord function during thoracoabdominal aortic aneurysm repair. SUMMARY BACKGROUND DATA: Somatosensory evoked potentials reflect conduction in dorsal columns. tc-MEPs represent anterior horn motor neuron function. This is the first study to compare the techniques directly during thoracoabdominal aortic aneurysm repair. METHODS: In 38 patients, thoracoabdominal aortic aneurysm repair (type I, n = 10, type II, n = 14, type III, n = 6, type IV, n = 8) was performed using left heart bypass and segmental artery reimplantation. tc-MEP amplitudes <25% and SSEP amplitudes <50% and/or latencies >110% were considered indicators of cord ischemia. The authors compared the response of both methods to interventions and correlated the responses at the end of surgery to neurologic outcomes. RESULTS: Ischemic tc-MEP changes occurred in 18/38 patients and could be restored by segmental artery reperfusion (n = 12) or by increasing blood pressure (n = 6). Significant SSEP changes accompanied these tc-MEP events in only 5/18 patients, with a delay of 2 to 34 minutes. SSEPs recovered in only two patients. In another 11 patients, SSEP amplitudes fell progressively to <50% of control without parallel tc-MEP changes or association with cross-clamp events or pressure decreases. At the end of the procedure, tc-MEP amplitudes were 84 +/- 46% of control. In contrast, SSEP amplitudes were <50% of control in 15 patients (39%). No paraplegia occurred. CONCLUSION: In all patients, tc-MEP events could be corrected by applying protective strategies. No patient awoke paraplegic. SSEPs showed delayed ischemia detection and a high rate of false-positive results.     

Somatosensory-evoked potentials during aortic coarctation repair.

OBJECTIVE: To determine the incidence of somatosensory-evoked potential (SSEP) changes and the interventions based on these changes during aortic coarctation repair. DESIGN: Retrospective review. SETTING: Single-institution, university hospital. PARTICIPANTS: Eighty-four children who had undergone surgical repair of aortic coarctation from January 1984 to May 1996. INTERVENTIONS: SSEPs were monitored in all patients throughout the procedure. A persistent decrease in amplitude greater than 50% from baseline was considered significant. Duration of SSEP changes in relation to the time course of surgical repair and whether a surgical or anesthetic intervention resulted from a change in SSEPs were documented. MEASUREMENTS AND MAIN RESULTS: Eighty-four patients underwent 87 surgical procedures. SSEP changes occurred in 40% of the procedures: 38.5% with repair and 15% with test clamp, with 9% occurring during both test clamp and repair. Interventions, which included repositioning the aortic cross-clamp, elevating blood pressure, and aborting surgery, occurred in 26.4% of all procedures based on SSEP changes. No patient sustained a neurologic deficit. CONCLUSION: This is the largest series to date describing the use of SSEPs in aortic coarctation repair. These SSEP changes were often immediately amenable to changes in surgical and anesthetic management. SSEP changes and interventions based on these changes occurred with a considerable frequency.     

Somatosensory-evoked potential monitoring during instrumented scoliosis corrective procedures: validity revisited

BackgroundIntraoperative monitoring (IOM) using somatosensory-evoked potentials (SSEPs) plays an important role in reducing iatrogenic neurologic deficits during corrective pediatric idiopathic procedures for scoliosis. However, for unknown reasons, recent reports have cited that the sensitivity of SSEPs to detect neurologic deficits has decreased, in some to be less than 50%. This current trend, which is coincident with the addition of transcranial motor-evoked potentials, is surprising given that SSEPs are robust, reproducible responses that were previously shown to have sensitivity and specificity of >90%.PurposeOur primary aim was to assess whether SSEPs alone can detect impending neurologic deficits with similar sensitivity and specificity as originally reported. Our secondary aim was to estimate the potential predictive value of adding transcranial motor-evoked potentials to SSEP monitoring in idiopathic scoliosis procedures.DesignThis was a retrospective review to analyze the efficacy of SSEP monitoring in the group of pediatric instrumented scoliosis fusion cases.Patient sampleWe retrospectively reviewed all consecutive cases of patients who underwent idiopathic scoliosis surgery between 1999 and 2009 at Children’s Hospital of Pittsburgh. We identified 477 patients who had the surgery with SSEP monitoring alone. Exclusion criteria included any patients with neuromuscular disorders or unreliable SSEP monitoring. Patients who had incomplete neurophysiology data or incomplete postoperative records were also excluded.Outcome measuresMajor outcomes measured were clinically significant postoperative sensory or motor deficits, as well as significant intraoperative SSEP changes.MethodsContinuous interleaved upper- and lower-extremity SSEPs were obtained throughout the duration of all procedures. We considered a persistent 50% reduction in primary somatosensory cortical amplitude or a prolongation of response latency by >10% from baseline to be significant. Persistent changes represent significant deviation in SSEP amplitude or latency in more than two consecutive averaged trials. Patients were classified into one of four categories with respect to SSEP monitoring: true positive, false positive, true negative, and false negative. The sensitivity, specificity, positive predictive value, and negative predictive value were then calculated accordingly.ResultsOur review of 477 idiopathic scoliosis surgeries monitored using SSEPs alone revealed a new deficit rate of 0.63% with no cases of permanent injury. Sensitivity = 95.0%, specificity = 99.8%, positive predictive value = 95%, negative predictive value = 99.8%. Using evidence-based epidemiologic measures, we calculated that the number needed to treat was 1,587 patients for one intervention to be performed that would have been missed by SSEP monitoring alone. In addition, the number needed to harm, which represents the increase in false positives with the addition of transcranial electrical motor-evoked potentials, was 200.ConclusionSSEP monitoring alone during idiopathic scoliosis continues to be a highly reliable method for the detection and prevention of iatrogenic injury. Our results confirm the high sensitivity and specificity of SSEP monitoring alone published in earlier literature. As such, we suggest the continued use of SSEP alone in idiopathic scoliosis surgeries. At this time we do not believe there are sufficient data to support the addition of MEP monitoring, although more studies and revised criteria for the use of MEP may provide added value for its use in the future.     

Use of somatosensory evoked potentials to detect peripheral ischemia and potential injury resulting from positioning of the surgical patient: case reports and discussion.

BACKGROUND: Somatosensory evoked potentials (SSEPs) have long been recognized as an excellent tool for detecting neural and vascular compromise during vascular, neurosurgical and orthopedic procedures. SSEPs have the ability to localize, central versus peripheral, the area of compromise. Many surgeons use only lower-limb SSEP monitoring when performing lumbar spinal surgery. The upper extremities are usually not monitored during such procedures, and monitoring oxygen saturation does not detect neural compromise. PURPOSE: To report that the expanded use of SSEP monitoring during surgery can be beneficial in detecting peripheral ischemia or neural compromise resulting from positioning. STUDY DESIGN: Three case reviews of orthopedic spine surgeries where SSEP monitoring provided early warnings of vascular and neural compression. METHODS: The cases review three different lumbar procedures in which evidence of peripheral ischemia and nerve compression were detected by SSEP monitoring. RESULTS: By the use of upper- and lower-extremity monitoring during lumbar procedures, early detection of ischemia and nerve compression were noted intraoperatively. These changes prompted examination of the patient and repositioning to correct the ischemia or compression. The repositioning in these cases corrected the problem, and no lasting effects were found. CONCLUSIONS: Including SSEP monitoring of the bilateral upper extremities should be considered during lumbar spinal procedures. Such monitoring can be offered for a slightly increased expense and only minimal time delay to place the additional required electrodes by the technician. As a direct result of the early warning of the SSEP monitoring, we were able to avoid potential ischemic injuries and improve patient outcomes.     

Somatosensory evoked potential

Summary Somatosensory evoked potentials (SSEPs) have been used to help minimize neurologic morbidity during spinal surgery. While this is a sensory test it has been used as an inference of motor function. The failure to always achieve the latter goal has resulted in some pessimism regarding the value of this test. In this series of 161 operations in 150 patients, it was demonstrated that SSEPs were recordable under anesthesia in 87% of patients. Of these patients, 12% had their spinal surgery interrupted due to significant neurophysiologic changes; of these patients, 18% had new neurologic deficits postoperatively. There were no cases with new neurologic deficits who had no changes in their SSEPs. It was concluded that SSEP monitoring may be helpful in identifying potentially neurologically threatening surgical maneuvers in a significant number of patients.     

The effect of acute hypocapnia on human median nerve somatosensory evoked responses

This study was undertaken to define the effect of acute hypocapnia on intraoperatively recorded somatosensory evoked potentials (SSEPs). Median nerve SSEPs were studied in ten anesthetized neurologically normal adult patients undergoing elective nonneurologic surgery. End-tidal carbon dioxide tension (ETCO2) was allowed to stabilize for 15 min before SSEP recordings were obtained during normocapnia (N) (ETCO2 = 39.9 +/- 1.45 mm Hg), hypocapnia (H) (ETCO2 = 20.6 +/- 1.07 mm Hg), and after return to normocapnia (NR). Although a trend toward a reduction in the latencies of all SSEP components was evident, only the cervical (CII), and cortical (N1 and P1) latencies decreased significantly with H when compared to N. Mean latencies for CII, N1, and P1 (+/- SD) were, respectively, 14.80 +/- 1.14, 20.93 +/- 1.50, and 25.17 +/- 2.88 msec during N, and 14.50 +/- 1.13, 20.25 +/- 1.49, and 24.23 +/- 2.52 msec during H. On return to normocapnia, latencies were unchanged from N. Cortical latencies were affected to a greater extent than subcortical ones. Aside from a small but statistically significant increase in CII amplitude at H (2.05 +/- 0.55 microV vs 1.83 +/- 0.49 microV at N), SSEP amplitudes were unaffected by hypocapnia. The authors conclude that acute hypocapnia in neurologically normal patients results in a small reduction of SSEP latencies. The magnitude of the change is such that it is unlikely to interfere with recording or interpretation of intraoperative SSEPs.     

Rate of change of somatosensory evoked potentials during isoflurane anesthesia in newborn piglets

Most studies of the effects of inhalation anesthetics on somatosensory evoked potentials (SSEPs) have examined SSEP at single times after initiation of an anesthetic. This study describes SSEP changes as functions of time of exposure to isoflurane. Both transient and sustained SSEP changes were observed. Nonlinear regression was used to fit exponential terms to the trend curves for end-tidal anesthetic concentration and SSEP peak latency. End-tidal concentrations could be well described by two exponential terms, one with a long and one with a short time constant. Isoflurane at 1% inspired concentration produced a sustained SSEP latency change that could be fit by a single exponential term; the time constant was essentially identical to the long time constant of end-tidal concentration. The long time constants ranged from 7 to 33 min. At 0.5% isoflurane, SSEP changes were often small or not sustained, and the changes could not always be well described by an exponential curve. These data suggest that the time-course of anesthetic effects on SSEPs may be prolonged and complex, and the possibility of changes over time should be considered both in experimental studies and during intra-operative monitoring.     

Monitoring of motor evoked potentials compared with somatosensory evoked potentials and microvascular Doppler ultrasonography in cerebral aneurysm surgery

OBJECT: The aims of this study were to compare the efficiency of motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), and microvascular Doppler ultrasonography (MDU) in the detection of impending motor impairment from subcortical ischemia in aneurysm surgery; to determine their sensitivity for specific intraoperative events; and to compare their impact on the surgical strategy used. METHODS: Motor evoked potentials, SSEPs, and MDU were monitored during 100 operations for 129 aneurysms in 95 patients. Intraoperative events, monitoring results, and clinical outcome were correlated in a prospective observational design. Motor evoked potentials indicated inadequate temporary clipping, inadvertent occlusion, inadequate retraction, vasospasm, or compromise to perforating vessels in 21 of 33 instances and deteriorated despite stable SSEPs in 18 cases. Microvascular Doppler ultrasonography revealed inadvertent vessel occlusion in eight of 10 cases and insufficient clipping in four of four cases. Stable evoked potentials (EPs) allowed safe, permanent vessel occlusion or narrowing despite reduced flow on MDU in five cases. Two patients sustained permanent and 10 showed transient new weakness, which had been detected by SSEPs in two of 12 patients and MEPs in 10 of 11 monitored cases. The surgical strategy was directly altered in 33 instances: by MEPs in 16, SSEPs in four, and MDU in 13. CONCLUSIONS: Monitoring of MEPs is superior to SSEP monitoring and MDU in detecting motor impairment, particularly that from subcortical ischemia. Microvascular Doppler ultrasonography is superior to EP monitoring in detecting inadvertent vessel occlusion, but cannot assess remote collateral flow. Motor evoked potentials are most sensitive to all other intraoperative conditions and have a direct influence on the course of surgery in the majority of events. A controlled study design is required to confirm the positive effect of monitoring on clinical outcome in aneurysm surgery.     

低体温对患者体感诱发电位的影响

目的 评价低体温对患者体感诱发电位(SSEP)的影响.方法 择期心脏手术患者13例,性别不限,年龄23～51岁,体重45～82 kg,ASA分级Ⅱ或Ⅲ级.麻醉诱导后测定双侧正中神经体感诱发电位:于Erb点和第2颈椎棘突分别放置臂丛和颈部记录电极,随后放置头皮电极,分别记录臂丛电位(N9)、颈髓电位(N13)和皮层电位(N20)的峰潜伏期和波幅.记录降温阶段和复温阶段达到目标体温(36、35、34、33 ℃)时的MAP、峰潜伏期和波幅.术后记录显性神经功能缺陷的发生情况.结果 降温阶段随体温降低,峰潜伏期延长,MAP降低(P＜0.05),波幅差异无统计学意义(P＞0.05);复温阶段随体温升高,峰潜伏期缩短(P＜0.05),MAP和波幅差异无统计学意义(P＞0.05);降温阶段峰潜伏期与体温呈负相关,相关系数分别为-0.673(N9)、-0.702(N13)、-0.702(N20)(P＜0.05),以体温为自变量X,峰潜伏期为因变量Y,得到直线回归方程分别为:Y9=-0.558X+28.994,YN13=-1.121X+53.242,YN20=-1.458X+72.036;复温阶段峰潜伏期与体温呈负相关,相关系数分别为-0.634(N9)、-0.619(N13)、-0.600(N20)(P＜0.05),直线回归方程分别为:YN9=-0.505X+27.313,YN13=-0.905X+46.249,YN20=-1.142X+61.668.所有患者术后均未发生神经系统功能缺陷.结论 低体温可延长SSEP峰潜伏期,对波辐无明显影响.

Abstract：

Objective To evaluate the effect of hypothermia on somatosensory evoked potentials (SSEPs). Methods Thirteen ASA Ⅱ or Ⅲ patients aged 23-51 yr weighing 45-82 kg scheduled for cardiac surgery were enrolled in this study. Bilateral median nerve SSEPs (N9, N13, N20) were recorded after induction.The MAP, peak latency and amplitude of N9, N13 and N20 were recorded when the target temperature (36, 35,34, 33 ℃ ) was reached during the cooling and rewarming periods. The neurological dysfunction was recorded after operation. Results The peak latency was prolonged and MAP was decreased with the decrease in the body temperature during the cooling period, the peak latency was shortened with the increase in the body temperature during the rewarming period ( P ＜ 0.05), but no significant change in the amplitudes was found ( P ＞ 0.05). The regression equation of the interaction between the peak latency and body temperature was YN9= -0.558X + 28.994(r=-0.673), YN13 =-1.121X+53.242 (r= -0.702) , YN20 = -1.458X+72.036(r= -0.702) during the cooling period (P ＜ 0.05), and YN9 = - 0.505X + 27.313 ( r = - 0.634), YN13 = - 0.905X + 46.249(r= -0.619), YN20 = - 1.142X + 61.668 (r= -0.600) during the rewarming period (P ＜0.05). No neurological dysfunction was found in all the patients. Conclusion Hypothermia can prolong the peak latency of SSEP and does not alter the SSEP amplitude.     

Somatosensory evoked potential monitoring of lumbar pedicle screw placement for in situ posterior spinal fusion.

BACKGROUND CONTEXT: Somatosensory evoked potentials (SSEP) are commonly used to monitor the spinal cord and nerve roots during operative procedures that put those structures at risk. The utility of SSEPs to evaluate cauda equina and nerve root function during posterior spinal arthrodesis with pedicular fixation for degenerative lumbar disease has been reported anecdotally and remains controversial. PURPOSE: An institution-wide review of the ability of SSEP readings to monitor nerve function during posterior lumbar spinal arthrodeses with transpedicular fixation for degenerative lumbar spinal disorders was undertaken. STUDY DESIGN/SETTING: A retrospective review was undertaken. Patient history, preoperative physical examination, intraoperative anesthesia, SSEP records and the postoperative course were reviewed. METHODS: A total of 186 consecutive arthrodeses as described above were reviewed. Patients who had anterior procedures, spondyloreduction or scoliosis correction were excluded from the study. There were 76 male and 110 female patients. Five fellowship-trained spine surgeons placed a total of 888 pedicle screws. Sixty-five percent of the patients had a principal preoperative diagnosis of spinal stenosis with degenerative spondylolisthesis. Other common diagnoses were isthmic spondylolisthesis and degenerative scoliosis. Ninety-three percent of the cases involved decompressive laminectomy. Eight percent had posterior interbody fusions. All pedicle screws were placed without the assistance of fluoroscopy or stereotactic computer-assisted guidance. Screw position was evaluated intraoperatively with standard posteroanterior and lateral radiographs.Anesthetic agents compatible with SSEP monitoring were used in all patients. SSEP baseline readings were obtained in all patients in the operating room soon after induction of general anesthesia. An acute and sustained loss of 50% of the SSEP amplitude and/or increase by 10% of latency from baseline was considered to be pathologic. RESULTS: None of the 186 patients had significant SSEP changes. There were, however, 5 patients with postoperative radiculopathies distinct from their preoperative presentations. Early postoperative plain radiographs and computed assisted tomography (CAT) scans revealed malpositioned pedicle screws. Consequently, eight pedicle screws were either revised or removed. All patients had partial or full recovery of their new deficits after revision surgery. CONCLUSION: We conclude that the use of SSEPs in evaluating pedicle screw placement during lumbar arthrodesis is limited. In this setting, if monitoring is required, alternative methods with greater sensitivity and efficacy should be explored.     

Continuous somatosensory evoked potential monitoring during brain tumor resection. Report of four cases and review of the literature

The neurosurgical management of intrinsic brain tumors and brain metastases mandates maximum resection with preservation of functional cortex. There have been previous reports on the use of cortical somatosensory evoked potentials (SSEPs) for localization of functional cortex prior to resection. The identification of rolandic cortex with the use of intraoperative SSEP monitoring enables the neurosurgeon to tailor the surgery to achieve a greater extent of resection while minimizing the risk of morbidity. The use of continuous SSEP monitoring during resection to provide an ongoing functional assessment of somatosensory cortex has not been reported. This powerful technique is illustrated using four case examples.     

Effect of sevoflurane/nitrous oxide versus propofol anaesthesia on somatosensory evoked potential monitoring of the spinal cord during surgery to correct scoliosis

Background. Use of intraoperative somatosensory evoked potential(SSEP) monitoring is helpful in spinal corrective surgery butmay be affected by anaesthetic drugs. An anaesthetic techniquethat has less effect on SSEP or allows faster recovery is anadvantage. We compared the effects on SSEP and the clinicalrecovery profiles of sevoflurane/nitrous oxide and propofolanaesthesia during surgery to correct scoliosis. Methods. Twenty adolescent patients were randomized into twogroups of 10. One group received sevoflurane–nitrous oxideanaesthesia and the other received propofol i.v. anaesthesia.An alfentanil infusion was used for analgesia in both groups. Results. Changes in anaesthetic concentration produced littleeffect on the latency of SSEP, but the effect on the variabilityof SSEP amplitude was significant (P<0.05). Sevoflurane produceda faster decrease in SSEP and a faster recovery than propofol(P<0.05). On emergence, patients who received sevofluranetended to have shorter recovery times to eye opening (mean 5.1vs 20.6 min, P=0.09) and toe movement (mean 7.9 vs 15.7 min,P=0.22). Those who had received sevoflurane were significantlymore lucid and cooperative in recovery. Conclusions. Sevoflurane produces a faster decrease and recoveryof SSEP amplitude as well as a better conscious state on emergencethan propofol. Br J Anaesth 2002; 88: 502–7     

Intraoperative spinal cord monitoring during descending thoracic and thoracoabdominal aneurysm surgery

BACKGROUND: Postoperative paraplegia is one of the most dreaded complications after descending thoracic and thoracoabdominal aneurysm surgery. In this study, intraoperative monitoring was applied during resection of descending thoracic and thoracoabdominal aneurysms to detect spinal cord ischemia and help prevent paraplegia. METHODS: Fifty-six patients (descending thoracic, 25; thoracoabdominal, 31) were monitored intraoperatively with both motor- (MEP) and somatosensory- (SSEP) evoked potentials. MEPs were elicited with transcranial electrical stimulation and recorded from the spinal epidural space (D wave) or peripheral muscles (myogenic MEP). SSEPs were obtained with median and tibial nerve stimulation. RESULTS: A total of 16 patients (28.6%) showed MEP evidence of spinal cord ischemia, only 4 of whom had delayed congruent SSEP changes. In 13 patients (23.2%), ischemic changes in MEPs were reversed by reimplanting segmental arteries or increasing blood flow or blood pressure. None of these 13 patients suffered acute paraplegia regardless of the status of SSEP at the end of the procedure, but 1 of them developed delayed postoperative paraplegia after multisystem failure. Three patients (5.4%) who had persistent loss of MEPs despite of recovery of SSEPs awoke paraplegic. CONCLUSIONS: The results demonstrate that compared with SSEP, MEP, especially myogenic MEP, is more sensitive and specific in detection of spinal cord ischemia, and that intraoperative monitoring can indeed help prevent paraplegia.     

Effects of hypothermia on median nerve somatosensory evoked potentials during spontaneous circulation

Perioperative-induced hypothermia is a common means of reducing ischemic injury in neurosurgical procedures and cardiac surgery, and it may occur accidentally. Somatosensory evoked potentials (SSEPs) are used frequently for neurophysiologic monitoring of these procedures. The effects of hypothermia on SSEPs have been studied widely in humans with cardiopulmonary bypass (CPB) during nonpulsatile flow. However, changes of latency and amplitude of early SSEP components during spontaneous circulation have not yet been studied. Median nerve SSEPs were recorded in 21 patients during rewarming from 32 to 36 degrees C core temperature. Latencies and amplitudes of N9, N13, N20, and central conduction time were registered at 32, 34, and 36 degrees C. Latencies of N9, N13, and N20 were prolonged at 32 degrees C compared with 36 degrees C (N9: 13.4 +/- 1.4 msec versus 11.8 +/- 1.4 msec, P <.05; N13: 17.6 +/- 1.9 msec versus 15.4 +/- 1.4 msec, P <.01; N20: 26.5 +/- 1.8 msec versus 22.4 +/- 1.6 msec, P <.001). Amplitude of N20 was higher at 32 degrees C compared with 36 degrees C (2.86 +/- 1.94 microV versus 2.07 +/- 1.47 microV, P < .05). Central conduction time decreased by 27%, and peripheral latency of N13 decreased by 14%. The increase in SSEP latency (N9, N13, and N20) and central conduction time during moderate hypothermia of 32 degrees C and spontaneous circulation are comparable with those during nonpulsatile flow on CPB. In contrast to nonpulsatile flow, the amplitude of N20 was increased significantly (P < .05) during moderate hypothermia and pulsatile circulation. These results suggest to be cautious about generalizing the effects of hypothermia on SSEP during CPB to spontaneous circulation.     

Transcranial motor evoked potentials during basilar artery aneurysm surgery: technique application for 30 consecutive patients

OBJECTIVE: Microsurgical clipping of basilar artery aneurysms carries a risk of neurological compromise resulting from midbrain or thalamic ischemia. Somatosensory evoked potential (SSEP) monitoring and electroencephalography are the standard techniques for assessing the level of cerebroprotective anesthesia and monitoring ischemia during temporary occlusion or after permanent clipping. Transcranial motor evoked potential (TcMEP) monitoring was added to determine whether this modality improved intraoperative monitoring. METHODS: Combined SSEP/electroencephalographic/TcMEP monitoring was used for 30 consecutive patients with basilar artery apex aneurysms in the past 1.5 years. Voltage thresholds were recorded before, during, and after aneurysm treatment for the last 10 patients. RESULTS: All 30 patients underwent an orbitozygomatic craniotomy for clipping (28 patients), wrapping (1 patient), or superficial temporal artery-superior cerebellar artery bypass (1 patient). Electrophysiological changes occurred for 10 patients (33%), elicited by temporary clipping (6 patients), permanent clipping (3 patients), or retraction (1 patient). Isolated SSEP changes were observed for one patient, isolated TcMEP changes for five patients, and changes in both TcMEPs and SSEPs for four patients. Among patients with simultaneous changes, TcMEP abnormalities were more robust and occurred earlier than SSEP abnormalities. Impaired motor conduction was detected first with an increase in the voltage threshold (from 206 +/- 22 to 410 +/- 49 V, P < 0.05, n = 3) and then with loss of TcMEP responses. SSEP and TcMEP signals returned to baseline values for all patients after corrective measures were taken. CONCLUSION: TcMEP monitoring can be safely and easily added to traditional neurophysiological monitoring during basilar artery aneurysm surgery. These results suggest that TcMEPs may be more sensitive than SSEPs to basilar artery and perforating artery ischemia. This additional intraoperative information might minimize the incidence of ischemic complications attributable to prolonged temporary occlusion or inadvertent perforator occlusion.