Pitfalls in cortical localization by intraoperative recording of cortical somatosensory evoked potentials]

Cortical somatosensory evoked potential (SEP) recordings were made in 11 patients who had lesions located in or near the somatosensory or motor gyri to localize the central sulcus and sensorimotor cortex during neurosurgical operations. Cortical localization was successful in 7 of the 11 patients by recording phase reversal waveforms of N20 and P20 at electrode sites in the hand area on opposite sides of the central sulcus. There were 4 cases in which the cortical localization failed. Locations of craniotomy were far distant from the central sulcus retrospectively in 2 of the 4 patients. Cortical SEPs couldn't be recorded despite probable exposure of the hand area and apparently adequate stimulation and recording conditions in 2 patients who had showed no or low amplitude scalp SEP preoperatively. In one of these 2 patients only low amplitude negative waves were recorded at the cortex which was thought far field potentials originated from subcortical structures. In 2 patients cortical SEP was monitored during the removal of the tumors and was useful to estimate the effects of the operative procedures on the sensorimotor cortex. It is concluded that the localization of cortical functions using cortical SEP is useful for reducing risk associated with intracranial surgery. However, we must be aware that there are some pitfalls in this method.     

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.     

Intra-operative localization of sensorimotor cortex by cortical somatosensory evoked potentials: From analysis of waveforms to dipole source modeling

Summary Intra-operative localization of sensorimotor cortex is of increasing importance as neurosurgical techniques allow safe and accurate removal of lesions around the central sulcus. Although direct cortical recordings of somatosensory evoked potentials (SEPs) are known to be helpful for cortical localization, source localization models can provide more precise estimates than subjective visual analysis. In addition to intra-operative analysis of waveforms and amplitudes of SEPs to median nerve stimulation in 20 neurosurgical patients, we used a spatiotemporal dipole model to determine the location of the equivalent dipoles consistent with the cortical distribution of the SEPs. The early cortical SEPs were modeled by 2 equivalent dipoles located in the postcentral gyrus. The first dipole was primarily tangentially oriented and explained N20 and P20 peaks. The second dipole was primarily radially oriented and explained P25 activity. We found consistent localization of the first dipole in the postcentral gyrus, which was always located within 8 mm of the central sulcus, with an average distance of 3 mm. This finding provides an objective basis for using the SEP phase reversal method for cortical localization. We conclude that dipole source modeling of the cortical SEPs can be considered as an objective way of localizing the cortical hand sensory area.     

Spinal cord monitoring during surgery by direct recording of somatosensory evoked potentials. Technical note

A simple method of spinal cord monitoring that can be readily used during surgery for spinal disorders in children or adults is described. A spinal subdural recording electrode is placed rostral to the site of surgery and the peroneal nerve is stimulated in the popliteal fossa; in this way, large-amplitude polyphasic spinal somatosensory evoked potentials (SEP's) can be directly recorded. The large amplitude of the spinal SEP's recorded intrathecally facilitates spinal cord monitoring by allowing: 1) rapid acquisition of the evoked response, which provides continuous monitoring during surgery; 2) relatively easy interpretation of the signal, there being no significant ultrashort- or long-latency components to the waveform; and 3) signal acquisition in an electrical environment that would be unacceptable using standard methods of spinal and cortical SEP recording.     

Pudendal cortical somatosensory evoked potentials

To determine normal reference latencies pudendal cortical somatosensory evoked potentials were evaluated in 20 healthy volunteers. The dorsal nerve of the penis or clitoris was stimulated electrically and the cortical response was recorded from the CZ-2 point (2 cm. behind the central vertex). Reproducible responses were obtained and reference values were established.     

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.     

A rare case of a somatosensory evoked potentials recording during cardiopulmonary resuscitation

Background: Neuronal damage is a possible complication of cardiac surgery. To reduce the potential risk of postoperative neurological deficit, the functional state of affectedcentral nervous system pathways is monitored intraoperatively by recording evoked potentials (EPs). Apart from animal research, there is little clinical evidence of EPs recordingduring cardiac arrest and cardio-pulmonary resuscitation (CPR).
Methods: Both scalp (SCEPs) and spinal (SSEPs) short-latency somatosensory EPs were recorded as the response to theelectrical stimulation of the right median nerve during mitral valve replacement surgery. Evoked potentials were recorded before, during, and after sudden ventricular fibrillationfollowed by CPR.
Results: Preoperative control recordings of both SCEPs and SSEPs were in the normal ranges. During the first 4 min of cardiac arrest and resuscitation, all SCEPs waves disappeared, while the spinal component of the SSEPs was still recognizable. After CPR, all waves of both EPs recordings recovered completely. The patient woke from anesthesia without neurological deficits.
Conclusions: As expected, scalp-recorded EPs are more sensitive to the cardiac arrest than spinal EPs. Rapid and almost complete recovery of postoperative EPs, namely SCEPs, correlated well with normal neurological recovery.     

右美托咪定对特发性脊柱侧弯矫形术中体感诱发电位和运动诱发电位的影响

目的:研究右美托咪定（dexmedetomidine, Dex）对特发性脊柱侧弯矫形术中体感诱发电位（somatosensory evoked potentials, SEPs）和经颅电刺激运动诱发电位（transcranial electric motor evoked potentials, TCeMEPs）的影响...     

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.     

Benefits and effectiveness of recording somatosensory evoked potentials in surgery on the carotid artery

BACKGROUND: Operative treatment of high-grade carotid stenosis is an established procedure. The question whether a temporary - either selective or routine - shunt is needed, is a matter of controversy, and the decision is based on a number of available monitoring procedures. Within the framework of quality assurance based on the regular collection of our own patient data, carotid thromboendarterectomy (TEA) with recording of somatosensory evoked potentials (SEP) was analysed for its effectiveness. PATIENTS AND METHODS: Two non-randomised groups of patients were analysed retrospectively: Group I: 99 carotid TEAs with no recording of SEP (1.1.99-31.12.99); Group II: 139 carotid TEAs with SEP recording (1.1.01-31.12.01). These two groups were unselected in terms of procedure, as also with regard to age, sex, stage or degree of carotid stenosis or surgeon, and were homogeneously distributed. A comparison was made of anaesthesia and operating times, shunt rate and the outcome of the two groups. Additionally, the two subgroups surgery with no shunt, and surgery with shunt, and the subgroups thromboendarterectomy with patchplasty (TEA) and eversion endarterectomy (EEA), were analysed. RESULTS: A temporary shunt was employed in 41 (41.4 %) of the patients in Group I (no SEP recording) and in 16 (11.5 %) of those in Group II (SEP recording). The average operating time in Group II was 11.4 min shorter (p < 0.001) than in Group I. The average carotid clamping time in Group II was significantly reduced (by 4.2 min; p < 0.001), while the duration of anaesthesia prior to skin incision was increased by an average of 18.3 min (p < 0.001), and the overall duration of anaesthesia by an average of 15.8 min (p < 0.001). A comparison of the subgroups surgery with no shunt and surgery with shunt revealed - both in Group I and Group II - a significant prolongation of the anaesthesia time and operating time (p < 0.001). In both Groups I and II, the subgroup TEA revealed a significant prolongation of both the anaesthesia and operating times vis-à-vis EEA. The major stroke rate was 2.0 % in Group I and 1.4 % in Group II, and the minor stroke rate 3.0 in Group I and 3.6 % in Group II; no deaths were observed. CONCLUSIONS: A reduction in the shunt rate to 11 % (by means of SEP) significantly decreased the average operating time (incision - suture) and the clamping time, with identical outcome in Groups I and II. Despite a reduction in the average incision-suture time in Group II (with SEP recording), the average overall operating time (anaesthesia time) was significantly increased due to the greater technical effort required. Carotid TEA with a selective shunt as determined by SEP is a high-cost procedure with no demonstrable benefit. At a stroke rate < 5 % and a need for stratification into several groups in accordance with the AHA classification, it is not possible to achieve adequate patient recruitment for a randomised analysis of outcome of the individual monitoring procedures. Alternative procedures are the routine use of a shunt and operation under regional anaesthesia.     

Localization of stereotactic targets by microrecordings of thalamic somatosensory evoked potentials

To improve the localization of stereotactic targets, somatosensory evoked potentials (SEPs) were recorded from the thalamus and subthalamic area using a specially designed semimicroelectrode in 61 patients and a conventional "macroelectrode" in 17 patients. By means of the semimicroelectrode, median nerve stimulation evoked two distinct SEPs, consisting of a diphasic wave with a huge positivity restricted to the nucleus ventrocaudalis (Vc) and a triphasic wave of lower amplitude with a major negativity in the ventral part of the nucleus ventrointermedius (Vim) and nucleus ventrooralis posterior (Vop) as well as the subthalamic lemniscal pathway. The Vim-Vc junction could thus be clearly delineated by an abrupt transition of SEPs from one type to the other with a precision of 1 mm. The parvicellular part of the Vc (Vcpc), situated in its basal region, was distinguishable from the Vc proper by a significant reduction of the positivity elicited by stimulation of the median nerve and by a rapid growth of a diphasic SEPs to stimulation of the posterior tibial nerve. In the other thalamic nuclei, stimulation of the median nerve elicited triphasic SEPs of a very small amplitude, suggesting a volume conduction current from the lemniscal pathway. With the macroelectrode, the positivity in the Vc was sensitive to electrode manipulation and the thalamic nuclei could not be distinctly outlined. SEP monitoring using the semimicroelectrode significantly improved the precision of target localization, which allowed minimizing of the volume of the therapeutic lesion without losing surgical effectiveness, while avoiding complications associated with increased penetration of the coagulating electrode. It is suggested that recording serial thalamic SEPs with the semimicroelectrode is a practical method to refine stereotactic targets in the thalamus.     

Effects of temperature on somatosensory evoked potentials during open heart surgery

Somatosensory evoked potentials (SEPs) have been found to be useful for early detection of brain ischemia during hypothermic cardiopulmonary bypass in cardiac surgery. However, the relationship between temperature and latency period remains unclear. We prospectively analyzed SEPs obtained during hypothermic cardiopulmonary bypass in 20 patients who had valvular replacement.
We concluded that i) a linear correlation was found between temperature and latency period during cooling and rewarming, ii) no hysteresis effect existed in cooling and rewarming, iii) there was a greater hypothermic effect on the synaptic transmission than on the conduction velocity, and, iv) age had also more profound effect on relationship between temperature and latency of SEPs.     

Investigation of brachial plexus injuries by intraoperative cortical somatosensory evoked potentials

Summary With the intention of estimating the extent and site of damage in brachial plexus injuries which involve close to the root outlet and also the distal portion, cortical somatosensory evoked potentials (SEP) were recorded in 21 patients by directly stimulating the exposed brachial plexus. In 38 avulsed nerve roots which showed positive sensory action potentials (SNAP)/nerve action potentials (NAP) in their peripheral part, 15 roots (nine patients) were apparently in continuity and confirmed as root avulsion injury by the absence of cortical SEP. However, 11 roots (ten patients) which showed neither SNAP nor SEP would suggest either extensive lesions involving root and more distal segment or a combination of root avulsion and postganglionic injury. Fourteen patients who showed a positive Tinel's sign had at least one root with a postganglionic type lesion, but cortical SEP evoked by stimulation of the most proximal root zone sometimes revealed a reduced amplitude and prolonged latency. This would indicate the retrograde extension of damage. To confirm the extent and degree of the nerve lesions in brachial plexus injuries, an intraoperative SEP and NAP recording is very useful, practical, and also indispensable.

---

Zusammenfassung Zur Schätzung des Grades und der Lokalisation der Armplexusverletzungen, die entweder in der Nähe des Austrittspunkts der Nervenwurzel oder weiter distal stattfand, waren die mit einer somatosensorischen Reizung erregten Hirnrindenaktions-potentiale abgeleitet und aufgezeichnet wurden. Anhand der 21 Patienten wurden die Armgeflechte ausgesetzt und direkt elektrisch stimuliert. Unter 38 ausgerissenen Nervenwurzeln, die positives sensorisches Aktionspotential in ihren peripheren Teilen zeigten, waren 15 Wurzeln (neun Patienten) scheinbar in Kontinuität, wurden jedoch wegen der Negativität des sensorisch erregten Rindenpotentials als Wurzelausriß-typverletzung bestimmt. Im Gegenteil, 11 Nervenwurzeln (zehn Patienten), die weder somatosensorisches Rindenaktionspotential noch sensorisches Nervenaktionspotential zeigten, würden entweder ausgedehnte Schädigungen an der Wurzel und mehr distalen Teilen oder eine Kombination von Wurzelausriß und postganglionischer Läsion vermuten lassen. Vierzehn Patienten, die ein positives Tinelsches Zeichen zeigten, hatten mindestens eine Wurzel mit einer postganglionischen Verletzung, oder ihr durch die Reizung der extrem proximalen Wurzel erregtes Rindenaktionspotential zeigte öfters eine reduzierte Schwingungsweite und eine Verzögerung der Latenzzeit. Dies würde auf eine retrograde Extension der Läsion hinweisen. Die intraoperative somatosensorische Rindenpotentialsleitung ist ein sehr wertvolles, praktisches und auch unentbehrliches Verfahren, um den Grad und die Lokalisation der Armplexusverletzungen zu bestimmen.

     

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.     

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.     

Effects of abdominal surgery on somatosensory evoked potentials during nitrous oxide-enflurane anesthesia

The effect of abdominal surgery on median nerve somatosensory evoked potentials (SEPs) was studied in 8 enflurane and nitorous oxide anesthesia (GOE) patients. We further compared the effect of epidural anesthesia. The first recording was done immediately prior to induction. Anesthesia was then induced with 5mg·kg^–1 i.v. of thiopental and maintained with 1.0% enflurane, 66% N₂O and 33% O₂. Before skin incision for abdominal surgery, the second recording was performed under GOE anesthesia and the third recording during surgery. Then 2% lidocaine was injected into the epidural space through a preinserted catheter, and after 15min the fourth recording was obtained. The latencies of peaks N1, P2 and N2 and the amplitudes of N1-P2 and P2-N2 were measured. The latencies of N1, P2 and N2 increased and the amplitudes of N1-P2 and P2-N2 deceased significantly after the induction of anesthesia compared with the control values. During abdominal surgery the latencies of N1 and P2 decreased and the amplitudes of N1-P2 and P2-N2 increased. After epidural anesthesia, however, the latencies of N1 and P2 increased and the amplitudes of N1-P2 and P2-N2 decreased significantly and returned almost to the values recorded under preoperative GOE anesthesia. These phenomena indicated that the excitations produced by surgical stimulation in nerve ending might have been transmitted to the central nervous system via spinal nerves and blocked by epidural anesthesia.(Kasaba T, Kosaka Y: Effects of abdominal surgery on somatosensory evoked potentials during oxide-enflurane anesthesia. J Anesth 5: 281–286, 1991)     

Continuous monitoring of short-latency somatosensory evoked potentials during cardiac and aortic surgery

The effectiveness of monitoring somatosensory evoked potentials (SEPs) intraoperatively to detect brain damage early remains controversial. To assess the diagnostic accuracy of this modality, a study was conducted between 1991 and 1994, recording SEPs in 287 consecutive patients undergoing cardiac and aortic surgery using cardiopulmonary bypass (CPB) with moderate hypothermia or deep hypothermic circulatory arrest. From P1 to N2 of the SEPs occurring within 50 ms latency in response to electrical stimulation of the median nerve were recorded over the contralateral postcentral cortex at 5-min intervals using a Neuropack-2 (Nihon Koden, Tokyo, Japan). Normal SEPs were recovered in 247 patients postoperatively; however, 2 of these patients had suffered a cerebral infarction and 1, a transient stroke intraoperatively, demonstrating a false-negative incidence of 1.2%. On the other hand, three different types of abnormal SEPs were recorded postoperatively. P1 and N1 absence, probably caused by a subcortical lesion, was observed in 4 patients; P2 and N2 absence, probably caused by a cortical lesion, was observed in 8 patients; and a flat SEP, representing diffuse damage, was observed in 2 patients. Among these 14 patients with abnormal SEPs, 7 showed no neurologic disturbance at all, demonstrating a false-positive incidence of 50%. Thus, we concluded that when normal SEPs are recovered during weaning from CPB, the incidence of brain damage could be predicted at below 5%. Conversely, when abnormal SEPs are demonstrated, the incidence of brain dysfunction impeding a return to active life is estimated to be about 70%.     

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.