Monitoring of somatosensory evoked potentials during surgery for middle cerebral artery aneurysms

Somatosensory evoked potentials (SEPs) were monitored during 53 procedures for aneurysms of the middle cerebral artery (MCA). "Significant" changes were reported to the surgeon, who took corrective action when possible. Changes in the SEPs were categorized as follows: Type I, no change; Type II, significant change with complete return to baseline; Type III, significant change with incomplete return to baseline; Type IV, complete loss with no return; and Type V, no response at baseline. Only 1 of 37 patients with a Type I SEP had a new neurological deficit, and this was a patient who could not be examined for several days after surgery because he was in a pentobarbital coma. All 4 patients with Type III and IV changes had new postoperative neurological deficits. Perhaps of greater importance, 4 of 5 patients with Type II changes had no new deficit. These patients all had changes in SEPs that were completely reversible by clip adjustment (2), prompt removal of temporary clips (1), and inducing hypertension after aneurysm trapping (1). These cases may, therefore, represent instances in which SEP monitoring allowed the clinicians to prevent a neurological deficit. The MCA supplies the area of the somatosensory cortex that controls the hand. Median nerve SEPs are, therefore, a theoretically ideal monitor during surgery for MCA aneurysms. This study suggests that the results of MCA aneurysm surgery may be accurately predicted and improved with SEP monitoring.     

Variability of somatosensory cortical evoked potential monitoring during spinal surgery

The intraoperative variability of somatosensory cortical evoked potentials (SCEPs) has been measured for 320 consecutive spinal surgeries and found to be a function of patient diagnosis, neuromuscular status, age, and procedural factors. In many cases, it is likely that this variability severely limits the reliability and usefulness of spinal cord monitoring in detecting early cord compromise. Patients with idiopathic scoliosis, spondylolisthesis, and pseudarthrosis have the smallest spontaneous variability and strongest amplitudes, while those with congenital, paralytic scoliosis, stenosis, or tumor have very variable, weak SCEPs. Patients with neurologic disorders, particularly cerebral palsy, myelomeningocele, Friedreich's ataxia, and peripheral deficits, also have high variability and weak amplitudes. A monitoring quality scoring system is proposed that may be useful during surgery in judging how well the SCEPs can discern surgically related changes in cord function from background variations.     

Monitoring of somatosensory evoked potentials during surgery for aneurysms of the sylvian artery]

Per-operative use of S.E.P.s for vascular disease is based on: 1. The relationship between electrical cortical responses and cerebral blood flow. 2. The existence of a reversible threshold of cerebral ischemia. Intra-operative monitoring S.E.P.s were used during 30 procedures for aneurysms of the middle cerebral artery (M.C.A.). In 18 cases, a temporary occlusion of M.C.A. was necessary. Occlusion times ranged from 1 to 30 minutes. The central conduction time delay (C.C.T.), i.e. the delay N14-N20 and the cortical peak (N20) amplitude, elicited by median nerve stimulation was bilaterally monitored. A prolongation of the C.C.T. by 1 millisecond (compared with the baseline induction value) and a progressive decrease or a disappearance of the N20 peak were considered as "significant" changes. In response to these changes, immediate corrective actions (interruption of temporary M.C.A. occlusion (T.O.), repositioning of brain retractors, reapplication of aneurysm clips ...) were implemented. The reversibility of the S.E.P.s alterations during surgery was correlated with the post-operative outcome. Significant changes were found in 20 cases (including 13 T.O.). They were totally reversible in 11 cases: 5 of them developed a new but transient immediate post-operative deficit, none had a definitive deficit, and 6 patients had no new deficit. In 9 cases, the per-operative S.E.P.s alterations were not reversible: 3 cases (including 1 T.O.) had a transient deficit, 4 a permanent deficit, and 1 died (aneurysm rupture during craniotomy). An irreversible N20 peak disappearance predicted a permanent post-operative deficit in 4 of 4 patients (100%), whereas an isolated irreversible C.C.T. increase was only followed by a transitory deficit. Only 1 of 9 patients with no change in S.E.P.s (during a 15 min. T.O.), had a transient hemiparesis: this "false-negative" case will be discussed. This study confirms that S.E.P.s monitoring provides useful warning during aneurysm surgery. Median nerve S.E.P.s reflect the functional integrity of cortical M.C.A. territory; it is the pathway a risk during M.C.A. aneurysm surgery. S.E.P.s changes are not real-time information (an average of 500 responses need about 2 min), but these delays allow the surgeon to reverse the situation by immediate intra-operative adjustment, especially during temporary M.C.A. occlusion.     

Spinal and cortical somatosensory evoked potential monitoring during corrective spinal surgery with 108 patients

To reduce the incidence of neurologic complications following spinal surgery, somatosensory evoked potentials (SEPs) were monitored in 108 patients. An electrode with four in-line contacts was used to record spinal SEPs in the epidural space in 33 patients at locations both rostral and caudal to the surgical site. Cortical SEPs were successfully monitored in 107/108 patients and spinal SEPs in all 33 attempted epidurally. Spinal conduction velocities were found to range from 43.9 to 110.5 m/s depending on vertebral level and the time location of the measured peak in the response waveform. Frequency power spectra of the SEP waveforms were found to be a reliable adjunct to peak latency amplitude analysis in the time domain. Use of caudal and rostral epidural, subcortical, and cortical electrode sites were found to be the most reliable technique for the maximum patient safety.     

Effects of propofol, propofol-nitrous oxide and midazolam on cortical somatosensory evoked potentials during sufentanil anaesthesia for major spinal surgery

Recording of cortical somatosensory evoked potentials (CSEP) enables monitoring of spinal cord function. We studied the effects of propofol, propofol-nitrous oxide or midazolam during sufentanil anaesthesia on CSEP monitoring during major spinal surgery. Thirty patients with normal preoperative CSEP were allocated randomly to one of the following anaesthesia regimens: propofol (2.5 mg kg-1 followed by 10-6 mg kg-1 h-1) with or without nitrous oxide, or midazolam (0.3 mg kg-1 followed by 0.15 mg kg-1 h-1) combined with sufentanil 0.5 microgram kg- 1 h-1 in the propofol and midazolam groups, or 0.25 microgram kg-1 h-1 in the propofol-nitrous oxide group. CSEP were elicited by alternate right and left tibial posterior nerve stimulation and recorded before and after induction (15 min, 1, 2 and 3 h), and during skin closure. CSEP latencies were not significantly modified in the three groups. CSEP amplitude decreased significantly in the propofol-nitrous oxide group (from mean 2.0 (SEM 0.3) to 0.6 (0.1) microV; P < 0.05) but not in the propofol (from 1.8 (0.6) to 2.2 (0.3) microV) or midazolam (1.7 (0.5) to 1.6 (0.5) microV) groups. The time to the first postoperative voluntary motor response (recovery) delay was significantly greater in the midazolam group (115 (19) min) compared with the propofol and propofol-nitrous oxide groups (43 (8) and 41 (3) min, respectively). Consequently, the use of propofol without nitrous oxide can be recommended during spinal surgery when CSEP monitoring is required.      

Localization of human sensorimotor cortex during surgery by cortical surface recording of somatosensory evoked potentials

The traditional means of localizing sensorimotor cortex during surgery is Penfield's procedure of mapping sensory and motor responses elicited by electrical stimulation of the cortical surface. This procedure can accurately localize sensorimotor cortex but is time-consuming and best carried out in awake, cooperative patients. An alternative localization procedure is presented that involves cortical surface recordings of somatosensory evoked potentials (SEP's), providing accurate and rapid localization in patients under either local or general anesthesia. The morphology and amplitude of median nerve SEP's recorded from the cortical surface varied systematically as a function of spatial location relative to the sensorimotor hand representation area. These results were validated in 18 patients operated on under local anesthesia in whom the sensorimotor cortex was independently localized by electrical stimulation mapping; the two procedures were in agreement in all cases. Similar SEP results were demonstrated in an additional 27 patients operated on under general anesthesia without electrical stimulation mapping. The following three spatial relationships between SEP's and the anatomy of the sensorimotor cortex permit rapid and accurate localization of the sensorimotor hand area: 1) SEP's with approximately mirror-image waveforms are recorded at electrode sites in the hand area on opposite sides of the central sulcus (P20-N30 precentrally and N20-P30 postcentrally); 2) the P25-N35 is recorded from the postcentral gyrus as well as a small region of the precentral gyrus in the immediate vicinity of the central sulcus: this waveform is largest on the postcentral gyrus about 1 cm medial to the focus of the 20- and 30-msec potentials; and 3) regardless of component identification, maximum SEP amplitudes are recorded from the hand representation area on the precentral and postcentral gyri.     

Effects of hydrocephalus and increased intracranial pressure on auditory and somatosensory evoked responses

Recent studies in human and animal subjects have suggested a relationship between intracranial pressure (ICP) and ventricular dilatation and multimodality evoked responses which, if substantiated, would be of value to clinical practice as a noninvasive way of assessing the need for shunting in selected patients in whom computed tomography (CT) is not definitive. In an attempt to better define these changes, auditory evoked response (BAER) and somatosensory evoked response (SER) were performed on 16 cats as a base line, after which they were made hydrocephalic by the cisternal injection of kaolin. Nine cats survived, and CT or magnetic resonance scans were performed on them 4 to 6 weeks later. In those animals in which ventricular dilatation was noted, repeat evoked responses were recorded. In the 6 hydrocephalic cats, the ventricle was punctured to measure ICP, which in all cases was less then 5 mm Hg. The lumbar spinal dural sac was then ligated, which resulted in periodic plateau waves up to 75 to 100 mm Hg after 4 to 6 hours, lasting up to 10 minutes. In neither group of cats was any change in either BAER or SER observed until preterminally, when ICP was in the range of 75 to 100 mm HG and cerebral perfusion pressure was compromised. This suggests that the BAER and SER are not sensitive to either ventricular dilatation or intracranial hypertension.     

皮层体感诱发电位监护在胸椎管狭窄症手术中的应用

目的：探讨皮层体感诱发电位（CSEP）监护在胸椎管狭窄症手术中的应用价值。方法：自2000年4月～2003年11月共有32例胸椎管狭窄症患者接受术中体感诱发电位监护，男21例，女11例，年龄38～75岁；其中单纯胸椎黄韧带骨化症13例，胸椎间盘突出症合并胸椎黄韧带骨化症10例，胸椎间盘突出症合并胸椎孤立后纵韧带骨化9例。单纯胸椎后路全椎板切除术4例，单纯后路全椎板截骨原位再植、椎管扩大减压术9例，全椎板截骨原位再植、环脊髓减压、椎管扩大减压成形术19例。术中均应用丹迪Key—Poim脊髓监护系统进行皮层体感诱发电位监护。结果：23例术中监护无异常，术后未出现神经系统并发症。其中12例患者术中即可见波形改善。4例术中出现波形异常，其中2例术后发生神经功能障碍。4例术中监护未见异常。术后症状加重，假阴性率12．5％。1例术中波形异常，但术后无脊髓损伤表现，假阳性率3.1％。结论：皮层体感诱发电位（CSEP）监护可及时发现术中危及脊髓的因素，但存在一定的假阳性或假阴性率。与其它监测方法合用可提高手术安全性。     

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.     

Somatosensory evoked potential monitoring during intracranial surgery

In the neurosurgical approach to intracranial aneurysms which are often accompanied by arterial spasm and cortical ischaemia, monitoring procedures aim to obtain useful information on cerebral function. SEPs evoked by stimulation of the median nerve at the wrist and of the tibial nerve at the medial malleolus were registered in 45 patients with intracranial aneurysms during neurosurgical procedures. Our results show SEP abnormalities during different stages of neurosurgical procedures in 36 patients out of the monitored 45. Significant abnormalities of SEPs with respect to the control group were decrease of the amplitude of N 20-P 25 complex, lengthening of the absolute latency of the waves N 20- and P 25 and lengthening of the central conduction time (CCT) (N 13-N 20). The greatest SEP abnormalities were registered during the neurosurgical approach to aneurysm and during the clipping procedure. However, the changes were reversible in the majority of the patients. The aim of this paper was to focus on early detection of some cerebral function disturbances during the neurosurgical procedure as well as the prevention of possible brain damage.     

Response variability of somatosensory evoked potentials during scoliosis surgery

Somatosensory evoked potentials (SSEP) were recorded from the scalp for intraoperative monitoring of patients undergoing surgical correction of spine deformities or spine fractures. Alterations in the SSEP with distraction, spine manipulation, anesthesia, hypotension, and other intraoperative variables are described. When loss of the SSEP occurred and a waiting period was undertaken until it returned, all patients with an SSEP present upon closing, which was within +/- 2 SD of their anesthetized control values, had no neurologic complications. Alterations in SSEP consisting of increases in latency of 15% and decreases in amplitude of 50% were not associated with any postoperative neurologic deficits.     

Variability of somatosensory cortical evoked potentials during spinal surgery. Effects of anesthetic technique and high-pass digital filtering

The effects of anesthetic technique (nitrous oxide or propofol) and high-pass digital filtering on within-patient variability of posterior tibial nerve somatosensory cortical evoked potentials (PTN-SCEP) were compared prospectively in two groups of 20 patients undergoing spinal surgery. Average P1N1 amplitude was significantly higher and P1N1 amplitude variability lower during propofol/alfentanil anesthesia than during nitrous oxide/alfentanil anesthesia. Off-line 30-Hz high-pass digital filtering significantly reduced P1N1 amplitude variability without decreasing P1N1 amplitude. In 93 patients studied retrospectively, a significant negative logarithmic correlation (r = -0.77) was observed between P1N1 amplitude and P1N1 amplitude variability. This study shows the importance of maintaining the highest possible PTN-SCEP amplitudes during spinal surgery. Propofol/opioid anesthesia may be an alternative anesthetic technique to nitrous oxide/opioid anesthesia during spinal cord function monitoring.     

Intra-operative recording of cortical somatosensory evoked potentials as a method of spinal cord monitoring during spinal surgery

The experience with cortical somatosensory evoked potential (SEP) recording during 13 cases of spinal neurological surgery is described. Good quality cortical SEPs were obtained in eight patients with a variety of intradural and extradural spinal disorders. The short latency components of these waveforms were stable during anaesthesia with nitrous oxide, 0.5% halothane and fentanyl. Fluctuations in signal amplitude were, however, common. In the one patient in whom the cortical SEP waveform was distorted intra-operatively, there was an increased neurological deficit. In one normal and four patients with impairment of dorsal column function, no intra-operative cortical SEP was recorded. In these five patients spinal SEPs were recorded rostral to the level of spinal disease. Monitoring spinal cord function using cortical SEPs can provide useful neurophysiological information, however, there are limitations to its utility. These relate to difficulties in signal acquisition, the low signal amplitude, attenuation of the signal during intramedullary surgery and uncertainties in signal interpretation. All these problems are exacerbated if the patient has a pre-operative clinical somatosensory deficit. Although most of these problems can be overcome using spinal SEP monitoring, intra-operative SEP recordings are not an infallible guide to spinal cord integrity since they reflect the functional status of only the dorsal column-medial lemniscus pathway.     

皮层体感诱发电位信号改善对脊髓型颈椎病手术预后的评价

[目的]研究皮层体感诱发电位(cortical somatosensory evoked potential,CSEP)在脊髓型颈椎病(cervicalspondylotic myelopathy,CSM)手术后出现信号改善(潜伏期缩短,或/和波幅增高)与临床症状恢复的相关性。[方法]对2008年7月～2010年5月本院收治的58例CSM患者,行术前、术中及术后CSEP监护并记录CSEP值,根据脊髓监护手术前后CSEP是否改善分为两组:波形改善组(A组),表现为波幅升高(>50%),或(和)潜伏期缩短(<5%);波形无改善组(B组)。于颈椎手术术前、术后1周和6个月分别行JOA评分(Japanese Orthopaedic As-sociation scoring system)评价神经功能,观察CSEP变化与神经功能恢复之间的关系。[结果]58例患者中36例(62.1%)CSEP出现改善;A组JOA评分术前、术后1周及术后6个月分别为8.42±1.06,14.71±1.31,15.43±1.26;B组分别为8.61±1.13,11.92±1.15,15.21±1.23。术后1周A组恢复高于B组(P<0.05...     

Intraoperative cortical somatosensory evoked potentials for detection of sciatic neuropathy during total hip arthroplasty

Approximately 3% of patients undergoing hip arthroplasty develop postoperative sciatic neuropathy. The factors associated with changes in somatosensory evoked potentials (SSEP) and sciatic neuropathy were examined in patients undergoing hip arthroplasty, to evaluate whether the use of intraoperative SSEP could help reduce the incidence of postoperative sciatic neuropathy. Eighty-eight patients were assigned to either monitored or unmonitored groups. SSEP were recorded following peroneal nerve stimulation, using contralateral stimulation to detect systemic influences on SSEP. Amplitude reduction of less than 50% of control and/or latency increase of greater than 10% of control was considered significant, and surgical intervention was attempted to restore SSEP. Previous surgery and a lateral incision approach tended to be associated with sciatic neuropathy (p less than 0.053). The incidence of sciatic neuropathy in the monitored group (4.3%) was not different from the unmonitored group (2.4%). Isolated reduction in amplitude or prolongation in latency of the SSEP was not predictive of postoperative neurologic function of the sciatic nerve. Six patients, two of whom developed sciatic neuropathy, demonstrated complete flattening of the SSEP. Both of these patients had flattened SSEP for two or more surgical events (p less than 0.01) and flattened SSEP were present at the end of the surgical procedure. There were no false-negative SSEP changes. Simultaneous amplitude and latency changes appear to be predictive of sciatic nerve function following hip arthroplasty.     

Intra-operative spinal cord monitoring during surgery for scoliosis using somatosensory evoked potentials

The method of intra-operative monitoring of spinal cord function by spinal somatosensory evoked potentials (SEP), as used at the Royal Alexandra Hospital for Children during surgery for scoliosis is described. Using a non-polarisable platinum spinal epidural electrode, SEP elicited by tibial and peroneal nerve stimulation in the popliteal fossa are recorded proximal to the level of spinal correction. The large amplitude and discrete waveform of the SEP enable rapid signal acquisition and easy interpretation. The spinal SEP is stable under both therapeutic hypotension and general anaesthesia. Electrophysiological monitoring has now superceded the 'wake-up' test as an index of spinal cord function during corrective surgery for scoliosis.     

Effects of temporary arterial occlusion on somatosensory evoked responses in aneurysm surgery

Changes in the central conduction time (CCT) during the application of temporary clips were studied in 40 patients who had undergone operations for intracranial aneurysms in relation to postoperative neurological outcome. Ten of these 40 patients (25%) showed postoperative morbidity, although promptly recoverable in 5. None of the patients whose CCTs did not change following temporary occlusion of major vessels showed any postoperative morbidity, except in one case of anterior cerebral artery aneurysm. In 6 patients, temporary vascular occlusion caused a considerable transient prolongation in CCT of up to 10 msec. Two of these 6 patients were associated with postoperative neurological deficit (which was recoverable in 1). The cortical response became flat in 15 patients. Seven of these 15 patients showed hemispheric deficits postoperatively, although recoverable in 4. There was a correlation between the change in the somatosensory evoked response and postoperative outcome. Disappearance of the N20 potential following occlusion is regarded as a danger signal, but postoperative, irrecoverable neurological deficit seems to be unlikely if its disappearance takes more than 3-4 minutes. Even if the cortical response disappears, the clinical outcome is expected to be good if the N20 potential recovers within 20 minutes after recirculation.