No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Individually Optimizing Posterior Tibial Somatosensory Evoked Potential P37 Scalp Derivations for Intraoperative Monitoring
Abstract
This investigation sought the optimal (highest amplitude) derivation for monitoring the posterior tibial P37 for each side in each individual, and determined whether this may change intraoperatively. Fifty monitored patients were studied using a partial P37 map consisting of FPz, Fz, Cz, Cz', Pz, POz, C4', and C3' to a noncephalic reference. From this, the highest amplitude scalp derivation was determined for each side. Of 100 tibial nerves, the initial optimal input 1 was Cz' in 52%, Pz in 28%, and Cz or iC' in 10%, and optimal input 2 was cC' in 69% and FPz in 31%. The optimal derivation was the same for each side in 34% of patients and different in 66%. Of 31 patients with at least one subsequent trial later during surgery, P37 topography changed in 14 and affected optimal inputs in 12. This occurred regularly during sitting-position posterior fossa surgery because of intracranial air, but sometimes occurred during other surgeries as well. The most common change consisted of FPz replacing cC' as optimal input 2. Input 1 changes were predominantly in an anterior or posterior sagittal direction. The results demonstrate great inter- and intraindividual P37 variability, and document intraoperative topographic changes. Both phenomena can be addressed by a practical method to refine intraoperative monitoring by individually optimizing scalp derivations and identifying topographic P37 changes during surgery.
... The P37 most often projects maximally to the centroparietal midline and its field paradoxically spreads over the scalp ipsilateral to the stimulated nerve because of its mesial source (Rossini et al., 1981;Cruse et al., 1982;Lesser et al., 1987). However, it may be maximal at the vertex, parietal midline, or ipsilateral scalp (MacDonald, 2001;Miura et al., 2003;MacDonald et al., 2004bMacDonald et al., , 2005). An N37 pole usually projects to the contralateral scalp but may be unapparent or at the parietal midline when the P37 is maximal at the vertex. ...
... Table 3 summarizes optimal derivations. They are based on SEP optimization that was developed through a series of investigations including prospective study (MacDonald, 2001;MacDonald and Janusz, 2002;MacDonald et al., 2003MacDonald et al., , 2004aMacDonald et al., , 2004bMacDonald et al., , 2005MacDonald et al., , 2007MacDonald et al., , 2009. Optimization minimizes surgical feedback time by selecting highest-SNR derivations while omitting low-SNR channels to gain 1-200 sweep medium-high reproducibility, and includes decussation assessment. ...
... Occasionally a gradual unilateral change of an optimal derivation causes asymmetric amplitude reduction, but rarely enough to risk a false positive (MacDonald, 2001). One could monitor additional derivations to guard against this, but they might not include the newly optimal one. ...
Intraoperative somatosensory evoked potentials (SEPs) provide dorsal somatosensory system functional and localizing information, and complement motor evoked potentials. Correct application and interpretation require in-depth knowledge of relevant anatomy, electrophysiology, and techniques. It is advisable to facilitate cortical SEPs with total intravenous propofol–opioid or similarly favorable anesthesia. Moreover, SEP optimization is recommended to enhance surgical feedback speed and accuracy by maximizing signal-to-noise ratio (SNR); it consists of selecting highest-SNR peripheral and cortical derivations while omitting low-SNR channels. Confounding factors causing non-surgical SEP reduction should be excluded before issuing a warning. It is advisable to facilitate their identification with peripheral SEP controls and cortical SEP systemic controls whenever possible. Warning criteria should adjust for baseline drift and reproducibility. The recommended adaptive warning criterion is visually obvious amplitude reduction from recent pre-change values and clearly exceeding trial-to-trial variability, particularly when abrupt and focal. Acquisition and interpretation should be done by qualified technical and professional level personnel. Indications for SEP monitoring include intracranial, posterior fossa, and spinal neurosurgery, as well as orthopedic spine, cerebrovascular, and descending aortic surgery. Indications for SEP mapping include sensorimotor cortex and dorsal column midline identification. Future advances could modify current recommendations.
... We reviewed 206 consecutively monitored thoracolumbar spine surgeries involving 173 patients (124 females and 49 males, age 2-43 years, median 14 years) of whom 143 had one surgery, 27 had two and 3 had three. The etiologies in 190 scoliosis surgeries were idiopathic (109), congenital (37), neurofibromatosis (15), neuromuscular (13), horizontal gaze palsy and progressive scoliosis (HGPPS) (6) and other causes (10). The indications of 16 other surgeries were vertebral or pedicle tumor (7), spondylolisthesis (6), spinal fracture (2) or previous laminectomy requiring stabilization (1). ...
... First, tibial cortical SEP derivations were optimized to highest SNR as previously described [13,18,19]. The left tibial nerve procedure follows: (Table 1). ...
... Marked topographic variability between individuals and sides makes tibial cortical SEP optimization more complex [13,18,19]. Input 1 should be the P37 maximum that is commonly at CPz, but may be at Cz, Pz, iCPi or CPi (iCPi being the intermediate CP1 or CP2 site ipsilateral to the stimulated nerve). ...
The objective of this study was to improve upon leg somatosensory-evoked potential (SEP) monitoring that halves paraplegia risk but can be slow, miss or falsely imply motor injury and omits arm and decussation assessment. We applied four-limb transcranial muscle motor-evoked potential (MEP) and optimized peripheral/cortical SEP monitoring with decussation assessment in 206 thoracolumbar spine surgeries under propofol/opioid anesthesia. SEPs were optimized to minimal averaging time that determined feedback intervals between MEP/SEP sets. Generalized changes defined systemic alterations. Focal decrements (MEP disappearance and/or clear SEP reduction) defined neural compromise and prompted intervention. They were transient (quickly resolved) or protracted (>40 min). Arm and leg MEP/SEP monitorability was 100% and 98/97% (due to neurological pathology). Decussation assessment disclosed sensorimotor non-decussation requiring ipsilateral monitoring in six scoliosis surgeries (2.9%). Feedback intervals were 1-3 min. Systemic changes never produced injury regardless of degree. They were gradual, commonly included MEP/SEP fade and sometimes required large stimulus increments to maintain MEPs or produced >50% SEP reductions. Focal decrements were abrupt; their positive predictive value for injury was 100% when protracted and 13% when transient. Six transient arm decrements predicted one temporary radial nerve injury; five suggested arm neural injury prevention (2.4%). There were 15 leg decrements: six MEP-only, four MEP before SEP, three simultaneous and two SEP-only. Five were protracted, predicting four temporary cord injuries (three motor, one Brown-Sequard) and one temporary radiculopathy. Ten were transient, predicting one temporary sensory cord injury; nine suggested cord injury prevention (4.4%). Two radiculopathies and one temporary delayed paraparesis were unpredicted. The methods are reliable, provide technical/systemic control, adapt to non-decussation and improve spinal cord and arm neural protection. SEP optimization speeds feedback and MEPs should further reduce paraplegia risk. Radiculopathy and delayed paraparesis can evade prediction.
... [11,13,[19][20][21][22][23]. [6,11,13,[19][20][21][22][23][24]. 일반적으로 상지 SSEP 기록전극의 위치는 말초부위는 'EPi- EPc' 혹은 'Epi-Fz', 척수 부위는 'C5-EPc'나 'C5-Fz', 피질 하 부위는 'CPi-EPc', 피질 부위는 'CPc-Fz', 'CPc-Fpz' 혹 은 'CPc-CPi' 등을 사용해 왔다. ...
... . [6,11,13,[19][20][21][22][23][24]. ...
... Possible causes include interference with the OR surrounding and anesthetic effects that lead to "wandering topography" of the optimal recording site. MacDonald suggested this to be caused by changes in the scalp topography due to intraoperative brain shifts of the cerebral volume and location (23). This suggestion does not explain the changes seen in our population, as they did not contain any craniotomies. ...
... CzCc is still better in most qualities than CiCc. A small body of evidence is starting to build naming CzCc a better single channel choice than CzFz (21)(22)(23)(24)(25). It might be sensible to omit the subcortical trace, when not satisfying or monitorable, and create space for an additional cortical montage recording. ...
Intraoperative monitoring is performed to provide real time assessment of the neural structures that can be at risk during spinal surgery. Somatosensory evoked potentials (SEPs) are the most commonly used modality for intraoperative monitoring. SEP stability can be affected by many factors during the surgery. This study is a prospective review of SEP recordings obtained during intraoperative monitoring of instrumented spinal surgeries that were performed for chronic underlying neurologic and neuromuscular conditions, such as scoliosis, myelopathy and spinal stenosis. We analyzed multiple montages at the baseline, and then followed their development throughout the procedure. Our intention was to examine the stability of the SEP recordings throughout the surgical procedure on multiple montages of cortical SEP recordings, with the goal of identifying the appropriate combination of the least number of montages that gives the highest yield of monitorable surgeries. Our study shows that it is necessary to have multiple montages for SEP recordings, as it reduces the number of non-monitorable cases, improves IOM reliability, and therefore could reduce false positives warnings to the surgeons. Out of all the typical montages available for use, our study has shown that the recording montage Cz-C4/Cz-C3 (Cz-Cc) is the most reliable and stable throughout the procedure and should be the preferred montage followed throughout the surgery.
... In April 2010 this system was replaced with a 32-channel dedicated NIM Eclipse monitoring machine. From July 2007, optimization techniques for cortical SSEPs were introduced, as described by Macdonald [13]; this introduction further enhanced our service, by improving the stability of cortical SSEPs. Individual monitoring strategies were used depending on the case, for example lumbar nerve root monitoring was frequently used for degenerative lumbar scoliosis though results of the nerve root monitoring are not reported here. ...
... The study does not set out to compare multimodal monitoring with our previous practice of using SSEP monitoring alone. This would not be comparing like with like, due to noteworthy improvements in SSEP equipment and technology since the introduction of multimodal neuromonitoring at our hospital [13]; since the adoption of multimodal monitoring other multifactorial improvements in the practice of scoliosis surgery have also been effected. ...
To evaluate the effectiveness of multimodal intraoperative neuromonitoring in the early detection of impending spinal cord injury during surgery for spinal deformity.
A retrospective review of prospectively collected data in 354 consecutive spinal deformity operations from June 2003 to October 2013. Patients were sub-grouped according to demographics, diagnosis and operative features. Post-operative neurological deficit was defined as either spinal cord, nerve root or transient deficit.
Combined monitoring with SSEPs and MEPs was possible in 315 cases. The overall incidence of significant alerts was 7.1 % and overall permanent neurological deficit was 1.6 %. When results were collated, the overall combined sensitivity of multimodal monitoring was 100 % with a specificity of 99.3 %.
Multimodal monitoring allows early detection of impending neurological deficit that is superior to a single monitoring modality. To achieve optimal use of monitoring, continuous communication between surgical, anaesthetic and neurophysiology teams are required. As a result of our experience we have incorporated in our consent procedure the discussion of monitoring and the possibility of needing to abandon the procedure, and completing in a staged fashion at a later date. We believe multimodal monitoring is the current gold standard for complex spinal deformity surgery.
... D. B. MACDONALD considered safe in awake, unpredisposed subjects (Wassermann, 1998). Table 2 lists 68 publications involving 2,915 anesthetized patients without seizures during low-frequency single-pulse (n 1,201) or very brief high-frequency pulse train (n 1,714) MEP monitoring, including direct cortical stimulation in predisposed subjects (n 289) ( Aglio et al., 2002;Andersson and Ohlin, 1999;Bartley et al., 2002;Boyd et al., 1986;Burke et al., 1992Burke et al., , 2000Calancie et al.,1998Calancie et al., , 2001Cedzich et al., 1996Cedzich et al., , 1998Deletis et al., 2000a, b;de Noordhout et al., 1996;Firsching et al., 1991;Glassman et al., 1995;Gokaslan et al., 1997;Herdmann et al., 1993;Hicks et al., 1991;Horikoshi et al., 2000;Jacobs et al., 1999Jacobs et al., , 2000Jellinek et al., 1991a, b;Jones et al., 1996;Kakimoto et al., 2000;Kalkman et al., 1991Kalkman et al., , 1992Katayama et al., 1988;Kawaguchi et al., 1996Kawaguchi et al., , 2000Kitagawa et al., 1995;Kombos et al., 2000aKombos et al., , b, 2001Kothbauer et al., 1997Kothbauer et al., , 1998Krombach et al., 1998;Lang et al., 1996;Lee et al., 1995;Levy, 1987;MacDonald, 2001;MacDonald and Janusz, 2002;Meylaerts et al., 1999;Morota et al., 1997;Pechstein, 1996Pelosi et al., 2001;Rodi et al., 1996;Sihle-Wissel et al., 2000;Stephen et al., 1996;Tabaraud et al., 1993;Taniguchi et al., 1993a, b;Thompson et al., 1991;Ubags et al., 1996Ubags et al., , 1997Ubags et al., , 1998Ubags et al., , 1999van Donegen, 1999avan Donegen,-d, 2000Yang et al., 1994;Zentner 1989Zentner , 1991Zentner et al., 1989;Zhou and Zhu, 2000). ...
... I am not aware of any evidence that awareness is more likely than with inhalational anesthesia. Similar considerations apply to cortical sensory evoked potential monitoring, which also benefits from intravenous anesthesia (MacDonald, 2001). ...
This article reviews intraoperative transcranial electrical stimulation (TES) motor evoked potential (MEP) monitoring safety based on comparison with other clinical and experimental brain stimulation methods and clinical experience in more than 15000 cases. Comparative analysis indicates that brain damage and kindling are highly unlikely. There have been remarkably few adverse events. Pulse train TES-induced or coincidental seizures (n = 5) are rare, probably because of very brief (<0.03 second) stimuli, anesthesia, and the general absence of predisposing cerebral conditions. Soft bite blocks may prevent tongue or lip laceration (n = 29) or mandibular fracture (n = 1). Rare cardiac arrhythmia (n = 5) and intraoperative awareness (n = 1) may be coincidental. Minor scalp burns (n = 2) are rare. Although possible, no spinal epidural recording electrode complications or injuries resulting from TES-induced movement were found. There have been no recognized adverse neuropsychological effects, headaches, or endocrine disturbances. Comprehensive relative contraindications include epilepsy, cortical lesions, convexity skull defects, raised intracranial pressure, cardiac disease, proconvulsant medications or anesthetics, intracranial electrodes, vascular clips or shunts, and cardiac pacemakers or other implanted biomedical devices. Otherwise unexplained intraoperative seizures and possibly arrhythmias are indications to abort TES. With appropriate precautions in expert hands, the well-established benefits of TES MEP monitoring decidedly outweigh the associated risks.
... Cz' for the exploring electrode and Cc' for the reference electrode in the largest number of cased [9]. However, other studies have never explicitly recommended the Cz' e Cc lead as a routine derivation for the PTN-SEPs. ...
Background
A reference interval exists for posterior tibial nerve somatosensory evoked potentials (PTN-SEPs) in awake. However, the reference interval for intraoperative- PTN-SEPs (I-PTN-SEPs) remains unclear. As a substitute for PTN-SEPs in awake, we considered I-PTN-SEPs can provide functional information about the dorsal somatosensory system. No report evaluated the physiologic and analytical issues in the measurement of I-PTN-SEPs. We investigated the sources of variation and reference intervals for I-PTN-SEPs.
Methods
We studied 143 patients with unilateral radiculopathy and without neurologic deficit who underwent surgery. Stimulation was delivered to the PTN at the ankle. The scalp recording electrode was placed at the Cz with a reference electrode located on the forehead at the Fz. SEPs were recorded from patients during electrical stimulation of the I-PTN.
Results
P1 and N1 latencies showed significant positive linear correlations with age (P1 latency = 36.52 + 0.0814 × age, P = 0.00003; N1 latency = 46.21 + 0.081 × age, P = 0.00022), and body height (P1 latency = 16.94 + 14.91 × body height, P = 0.00000; N1 latency = 25.42 + 15.64 × body height, P = 0.00002). In contrast, I-PTN-SEPs amplitude showed no correlation with age or body height. The 95% confidence interval for I-PTN-SEPs amplitude, or the reference interval, was determined as 0.31–5.91 μV.
Conclusions
The lower normal limit value was 0.31 μV, and this reference interval may be useful to evaluate function of the posterior funiculus, such that as during surgery for patients with intramedullary tumor.
... Our method using the Cz'-Cc lead may well be criticized. However, this method has a definite advantage in that the P38 potential is clearly observable for all subjects in this single lead, and is best suited for routine examinations (MacDonald, 2001;Miura et al., 2003). We can probably introduce some measure to avoid the right-left error, such as confirming that the Cc and contralateral iliac crest ICc electrodes are placed on the same side. ...
OBJECTIVE
At our laboratory, we routinely record tibial nerve somatosensory evoked potentials (SEPs) using 5 channels including the second cervical vertebra (C2S)-contralateral central area (Cc) and Cz’ (2 cm posterior to Cz)-Cc derivations. In a man with lumbar spondylotic myelopathy, symptoms improved after surgery, although the N21-P38 interval was markedly prolonged in comparison with that before surgery. We presumed that the Cc electrode was actually placed on the ipsilateral central area (Ci) at the second examination. Inspired by this episode, we investigated the influence of the right-left error in the placement of the Cc electrode.
METHODS
Subjects were 20 healthy volunteers. Tibial nerve SEPs were recorded with 8 leads including Cz’-Cc, Cz’-Ci, C2S-Cc and C2S-Ci.
RESULTS
For the Cz’-Ci lead, the P38 potential diminished in amplitude, was absent or became negative. For the C2S-Ci lead, a large negative potential corresponding to the phase reversal of P38 was frequently observed.
CONCLUSIONS
Tibial nerve SEPs using the Cz’-Cc or C2S-Cc lead are distorted if the Cc electrode is placed on the opposite side.
SIGNIFICANCE
When a strange result is obtained in tibial nerve SEPs, we should check for a right-left error in the Cc electrode placement.
... During surgery, the Sap-SEP waveform probably changes, reflecting a shift in the dipole orientation secondary to the anesthesia effect. As previously advocated, the use of two derivations throughout the surgery helps avoid a wrong interpretation of SEP decrements [8,10,19]. We wish to stress that distal stimulation of the saphenous nerve to obtain Sap-SEP poses no additional technical difficulty compared to the TN-SEP, although Sap-SEP conveys pure cutaneous information. ...
The demand for intraoperative monitoring (IOM) of lumbar spine surgeries has escalated to accommodate more challenging surgical approaches to prevent perioperative neurologic deficits. Identifying impending injury of individual lumbar roots can be done by assessing free-running EMG and by monitoring the integrity of sensory and motor fibers within the roots by eliciting somatosensory (SEP), and motor evoked potentials. However, the common nerves for eliciting lower limb SEP do not monitor the entire lumbar plexus, excluding fibers from L1 to L4 roots. We aimed to technically optimize the methodology for saphenous nerve SEP (Sap-SEP) proposed for monitoring upper lumbar roots in the operating room. In the first group, the saphenous nerve was consecutively stimulated in two different locations: proximal in the thigh and distal close to the tibia. In the second group, three different recording derivations (10–20 International system) to distal saphenous stimulation were tested. Distal stimulation yielded a higher Sap-SEP amplitude (mean ± SD) than proximal: 1.36 ± 0.9 µV versus 0.62 ± 0.6 µV, (p < 0.0001). Distal stimulation evoked either higher (73%) or similar (12%) Sap-SEP amplitude compared to proximal in most of the nerves. The recording derivation CPz–cCP showed the highest amplitude in 65% of the nerves, followed by CPz–Fz (24%). Distal stimulation for Sap-SEP has advantages over proximal stimulation, including simplicity, lack of movement and higher amplitude responses. The use of two derivations (CPz–cCP, CPz–Fz) optimizes Sap-SEP recording.
... Prepositional SEP (Ulnar nerve) baselines showed prolonged latencies. Unilateral stimulation of tibial nerves did not elicit any cortical SEP response, requiring a modification involving simultaneous bilateral stimulation of both tibial nerves at the ankle with a different montage recording (M1: Fpz-Cpz,M2: Cpz-Cp3, and M3:Cpz-Cp4) (International 10-20 System) [5], which elicited a barely discernible tibial SEP at M1 (Fig. 2). The Halo was removed, and a Mayfield head clamp was applied. ...
Spinal cord compression from severe C1–C2 subluxation is a complex and challenging neurosurgical problem. A combination of surgical approaches may be required to decompress the spinal cord, including anterior transoral odontoidectomy, posterior cervical laminectomy, or indirect decompression by subluxation reduction. After decompression, an instrumented posterior fusion is then required to stabilize the spine. We report a case of a 13-year-old-girl who presented with spastic quadriparesis secondary to severe atlantoaxial subluxation, where intraoperative neurophysiologic monitoring (IOM) guided surgical decision making. The patient had congenital osseous abnormalities, including incomplete segmentation of C2 on C3, predisposing her to atlantoaxial dislocation. She underwent open surgical reduction of the C1–C2 subluxation with multi-modal intraoperative neurophysiologic guidance. Translaminar C2 screws and C1 lateral mass screws were placed bilaterally. Using a previously described distraction technique, we performed ventral translation of C2 relative to C1, thereby reducing the subluxation. Intraoperative radiographic images and neurophysiologic monitoring revealed significant improvements, confirming that adequate indirect decompression had been achieved. After subluxation reduction with intraoperative monitoring (IOM) improvements, we decided to proceed with a C1–C2 posterior fusion and avoid a cervical laminectomy and more extensive occipital cervical fusion procedure. After one year, the girl made a complete neurologic recovery and remains symptom-free 3 years post-surgery. Fusion to the occiput would have potentially caused greater morbidity in this patient. The neurophysiologic monitoring confirmed indirect decompression and served as a critical tool for the prediction of favorable neurologic outcome.
... It is important to extract SEP signals as fast as possible so that any injury can be reversed. After the general anesthesia had been administered and before the spinal surgery, six needle electrodes, as shown in Fig. 8a, were applied to the scalp of the subject at positions Cz 0 , C3 0 , C4 0 , Pz, P7, and P8 (international 10-20 system) [34]. An additional pair of surface electrodes was applied to the skin over the cheek area to serve as ground. ...
When prior knowledge is available, it is beneficial to use constrained blind source separation (BSS) algorithms that can utilize more information to distinguish the desired source from artifacts and noise. This paper proposes a one-unit second-order blind identification with reference (SOBI-R) algorithm for short transient signal extraction, which reformulates the conventional second-order blind identification (SOBI) algorithm in an iterative manner to achieve joint diagonalization and the reference information incorporated. The proposed algorithm was applied to single trial extraction of somatosensory evoked potential (SEP). The experimental results demonstrated its effectiveness. Compared with other algorithms including the autoregressive model with exogenous input (ARX), artificial neural networks (ANN) and one-unit the independent component analysis with reference (ICA-R), the proposed SOBI-R algorithm shows high robustness under conditions with low signal-to-noise ratios and less sensitivity to the reference signal.
... Recording derivations and nomenclature followed guidelines of the American Electroencephalographic Society (1994a). An exception to this was the tibial P37 cortical response, which was often recorded using an individually optimized scalp derivation for each side after partially mapping its scalp distribution bilaterally after induction (MacDonald, 2001). SSEP surface recording electrodes were applied with collodion at measured sites by a registered EEG technologist experienced in intraoperative monitoring. ...
Thoracoabdominal aneurysm surgery carries an approximate 10% risk of intraoperative paraplegia. Abrupt cord ischemia and the confounding effects of systemic alterations and limb or cerebral ischemia challenges neurophysiologic spinal cord monitoring. This investigation sought a rapid differential monitoring approach to predict or help prevent paraplegia. Thirty-one patients were monitored with motor evoked potentials (MEPs) and median and tibial somatosensory evoked potentials (SSEPs). MEPs involved single-pulse transcranial electrical stimulation with D wave recording (n = 16), arm and leg muscle MEPs following multiple-pulse transcranial electrical stimulation (n = 12), or both (n = 3). D wave recordings required averaging, invasive epidural electrode insertion, and produced both false positives and false negatives. Muscle MEPs were instantaneous and reliably sensitive and specific for cord ischemia. Cortical and peripheral nerve SSEPs provided rapid detection of systemic alterations and cerebral or limb ischemia. Cord and subcortical SSEPs required excessive averaging time. In conclusion, bilateral arm and leg muscle MEPs with median and tibial peripheral nerve and cortical SSEPs provide sufficiently rapid detection and differentiation of cord ischemia from confounding factors. There were two predicted intraoperative spinal cord infarctions (6.5%) and nine circumstantial examples of possible contributions to deficit prevention.
... 9). Significant advantages could be achieved more simply by using the most commonly optimal CPz–CPc derivation that will have lower noise and frequently higher amplitude than CPz–FPz (MacDonald, 2001; MacDonald et al., 2004b; Miura et al., 2003). However, even this derivation is bilaterally optimal in only 17% of patients (MacDonald et al., 2004b), so many additional SNR advantages will be missed (e.g.Fig. ...
To compare the intraoperative signal-to-noise ratio (SNR), reproducibility and rapidity of popliteal fossa (PF), optimized P37, standard P37 and P31 potentials.
Raw sweeps and 11 averages doubling sweep number from 2 to 2048 were compared in 37 patients undergoing scoliosis surgery. Optimized (highest amplitude or SNR) P37 derivations were Cz-CPc (22), CPz-CPc (27), Pz-CPc (7), iCPi-CPc (8), CPi-CPc (1), Cz-Pz (2) or Pz-FPz (3), and in two patients with non-decussation, Cz-CPi (1) or CPz-CPi (3). Standard P37 and P31 derivations were CPz-FPz and FPz-C5S. Signal amplitude was measured in 2048-sweep averages; peak noise was measured in raw sweeps and +/- averages; SNR was amplitude/noise. Visual superimposability and < 20-30% amplitude variation determined reproducibility. Sweeps to reproducibility determined rapidity.
The SNR order was PF > optimized P37 > standard P37 > P31. Mean optimized P37 SNR advantages over the standard P37 and P31 were 2.1:1 and 4.9:1. SNR had powerful non-linear correlations to reproducibility and rapidity. Median sweeps to reproducibility were PF: 2, optimized P37: 128, standard P37: 512 and P31: 1024. EEG noise was greatest in FPz derivations. Burst-suppression increased scalp potential SNR and rapidity.
Optimized P37 and PF recordings are most rapidly reproducible due to superior SNRs and are recommended. FPz should be avoided. Burst-suppression may be desirable.
CPz-FPz and FPz-C5S should no longer be standard.
This chapter reviews the history, methodology, and criteria for intraoperative somatosensory evoked potentials (SEPs). It emphasizes optimal monitoring methods that enhance efficacy by maximizing the speed and accuracy of surgical feedback. Specifically, total intravenous anesthesia with peripheral and cortical SEP derivations optimize to highest signal-to-noise ratio for fastest reproducibility, while omitting slower subcortical and other traditional channels. It also recommends adjusting for baseline drift and reproducibility with an adaptive warning criterion defined as visually obvious amplitude reduction from recent prechange values and clearly exceeding trial-to-trial variability, particularly when abrupt and focal. It then reviews cortical SEP mapping, and finally, presents some possible future advances.
Purpose:
Monitoring of somatosensory evoked potentials (SSEPs) serves as an early warning system to detect spinal cord injury and is correlated with postoperative sensory findings. It is an indirect indicator of motor function. This study aimed to evaluate the usefulness of intraoperative SSEPs monitoring as a stand-alone tool during spinal surgeries when motor evoked potentials are not available, to prevent and predict new postoperative neurologic deficits. Motor evoked potentials were not used as the equipment needed to record them was not available at the time of this study.
Methods:
This study included 50 patients, aged 14 to 67 years, undergoing extramedullary manipulations, decompression of an epidural abscess or neoplasm, removal of intramedullary tumor, or arteriovenous malformation or spine correction procedures. Somatosensory evoked potentials were analyzed for latency and peak-to-peak amplitude. Critical SSEP changes were defined as a 50% decrease in amplitude or a 10% increase in latency.
Results:
Somatosensory evoked potentials had an overall sensitivity of 81.8%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 91.3%.
Conclusions:
Intraoperative SSEPs have proved to be highly sensitive and specific for iatrogenic injury, mechanical stress caused by cord traction/compression, dural traction, lowered systemic blood pressure, and cord hypothermia. The reversibility of intraoperative SSEP changes showed a highly significant relation to the number of cases with new postoperative deficits as well as type and site of pathologic study (P = 0.00, P = 0.01, and P = 0.00, respectively) but not with the level of pathologic study (P = 0.49).
The cortical response of the somatosensory potential of the posterior tibial nerve is recorded in the most of laboratories using only one derivation and requiring even more the design of the new protocols involving the record with the most cortical areas; the present research tries to design and implement the Somatosensory Evoked Potential with 19 derivations from the International System 10/20 in the 4-MEDICID-equipment, allowing a mapping of the cortical response in P40 of the posterior tibial nerve. Initially, we look for the lower cutting value in the high frequencies which allowed the record with any change in the characteristics. For this purpose the potential in 10 healthy subjects is recorded using the 4- NEURONICA- electromyography, with the application of different filters in high frequencies. Later, a protocol with 19 derivations is designed in the 4-MEDICI electromyography using a band weigh between 0.5 and 300 Hz. It was necessary to modify the Trackwalker software for allowing a 1 kHz sample frequency and for obtaining that the synchronization traces of the external stimulator were recognized by EP Workstation software. Findings yielded that cuts at high frequencies less than 300 Hz modified significatively the latency of component P40.With this cut ,responses with no significant differences were obtained ( p>0.05) according to latency, weigh and frequency in comparison to those obtained with the cut at high frequencies which is more recommended in literature. It is concluded that with this protocol is possible to record the cortical response of the somatosensory evoked potential of the posterior tibial nerve in more than a derivation, contributing to the characterization of abnormal pattern in some pathologic conditions.
Intraoperative recordings of somatosensory evoked potentials (SSEP) were recorded among the earliest used electrophysiological methods for monitoring function of the spinal cord, and for that matter, of any neurological system. Orthopedics was the first specialty of surgery where this method was used, beginning in the 1970s in operations for scoliosis (1–3).
In the early days of intraoperative monitoring, either custom-made equipment or equipment taken from the clinical testing laboratory of the neurophysiological animal laboratories was used in the operating room. Now, there is specialized equipment commercially available for nearly all needs of intraoperative monitoring. This means that the persons who perform monitoring and intraoperative neurophysiology do not need to know as much about recording and stimulating equipment as they once did. However, knowledge about the basic function of the equipment that is used for intraoperative monitoring enables optimal use of the equipment and is important for troubleshooting. The equipment now commonly utilized for intraoperative neurophysiology is capable of appropriate signal processing, has several ways of filtering the recorded responses, and has many options for displaying the potentials recorded. The user must have sufficient knowledge, however, about the basis for filtering and signal averaging to use these methods in optimal ways.
Intraoperative neurophysiological monitoring is used to guide surgery and predict the postoperative neurological function of patients. Surgery for basal neurosurgical tumors is fraught with difficulties due to the important neurovascular anatomy at risk. During these often long surgical cases, the function of the nerves and long tracts at risk can be monitored, giving the surgeon near real time feedback on how the patient is doing and what functions if any are being affected. This information is useful in trying to reduce and prevent injuries for quality control and improve patient outcomes. With the advent and effectiveness of alternative treatment strategies for many of these tumors, such as focused stereotactic radiation as primary treatment or surgery plus radiation (for planned subtotal resections), surgical outcomes need to be looked at critically; it becomes necessary to be able to perform these operations with minimal morbidity. Intraoperative monitoring is an adjunct to surgery that aids the surgeon in procedures for patients with these difficult tumors.
This article reviews the anatomical generators and physiologic basis of motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs), emphasizing intraoperative monitoring (IOM). Fast conducting thick myelinated corticospinal axons and alpha motor neurons selectively generate spinal D-wave and muscle MEPs due to the combination of approximately focused brain stimulation and motor system anatomy and physiology. Similarly, fast conducting thick myelinated axons of the dorsal column-medial lemniscus system selectively generate SEPs due to the combination of peripheral nerve stimulation and somatosensory anatomy and physiology. For IOM, one should use optimized SEP derivations based on individual anatomy and physiology to maximize signal to noise ratio (SNR) and thereby, accelerate surgical feedback; low SNR SEPs should be omitted. The generators and physiologic basis of MEPs and SEPs must be understood for their most effective use and correct interpretation.
The third edition of this classic text again provides practical, comprehensive coverage of the anatomical and physiological basis for intraoperative neurophysiological monitoring. Written by a leading authority in the field, Dr. Aage Moller has updated this important title to again offer all the leading-edge knowledge needed to perform electrophysiological recordings in the operating room, to interpret the results, and to present the results to the surgeon. The field known as "intraoperative monitoring" has expanded rapidly to cover other uses of neurophysiology and electrophysiologic recordings during surgical operations that affect the brain, spinal cord, and other parts of the nervous system. These new areas are covered in this new edition. To better represent the content of the book and the field as it now stands, many of the chapters have been revised and new material has been added. While the general organization of the book is maintained, chapters such as monitoring of motor systems have been revised and extended with new material, including more detailed description of the anatomy and physiology of motor systems and new information about intraoperative monitoring. © Springer Science+Business Media, LLC 2011. All rights reserved.
No existe un consenso en la descripción de la distribución topográfica de la respuesta cortical P40 del potencial evocado somatosensorial por la estimulación del nervio tibial. Se han reportado diferencias de distribución con máxima respuesta en línea media. Otros autores, sin embargo, la refieren en la región centro-parietal ipsilateral al estímulo, mientras que en menor escala también ha sido referida en dicha región, pero contralateral al estímulo. Considerando lo antes expuesto se decidió realizar el presente trabajo para evaluar la distribución topográfica de la respuesta cortical P40 del potencial evocado somatosensorial del nervio tibial. Se estudiaron 18 sujetos sanos con estimulación eléctrica del nervio tibial y se registró P40 sobre
toda la cabeza a partir de las 19 derivaciones del Sistema Internacional 10/20. Se utilizó un ancho de banda entre 0,5 y 300 Hz . En todos los sujetos se obtuvo la respuesta cortical P40, la cual mostró una distribución con máxima amplitud en línea media centro-parietal, así como una representación paradójica hacia la misma región del lado estimulado. Estos hallazgos muestran que el P40 no se registra solamente con un máximo en línea media, como lo describe la generalidad de los textos sobre potenciales evocados, sino que simultáneamente se obtiene en la propia región centroparietal ipsilateral al estímulo. Estos resultados sugieren la posibilidad de registrar la respuesta cortical del potencial evocado somatosensorial de nervio tibial desde más de una derivación, como habitualmente se realiza, lo que permitirá contribuir a
caracterizar el patrón de anormalidad en ciertos estados patológicos.
This work aims at investigating the somatosensory evoked potential topographical distribution by applying the Magnitude Squared
Coherence (MSC), an Objective Response Detection technique in the frequency domain. The EEG was collected from eight volunteers
at derivations according to the 10–20 International System during stimulation of the right posterior tibial nerve. The stimuli
were applied at the rate of 5 Hz and with intensity slightly above the motor threshold. Detection was identified based on
the null hypothesis of response absence rejection (significance level α = 0.05 and M = 500). The best percentages of detection were achieved in the parietal and central regions ipsilateral to the stimulation
limb. C4, P4, Cz and Pz were considered the best derivations for SEP monitoring when monopolar derivations are used.
The purpose of this study was to examine the utility and feasibility of using alternative anterior reference leads when measuring left posterior tibial nerve somatosensory evoked potentials (SEPs).
With IRB approval, 12 patients were monitored using both traditional (FPz and C4') and alternative anterior (F3 and F4) reference leads during routine spine surgery with SEP monitoring. Recordings from the routine and novel electrode pairs were collected and analyzed.
All of the SEP amplitudes measured were of similar magnitude except for that of F3-F4, which was significantly lower (P < 0.001) than all of the other five lead combinations which were assessed (Cz'-FPz, C3'-C4', C3'-F4, Cz'-F3, and Cz'-F4). The latencies of the novel lead combinations (C3'-F4, Cz'-F3, Cz'-F4, and F3-F4) were similar to those of the "gold standards" (Cz'-FPz and C3'-C4') (pooled median, 45.6 ms with 25-75th percentiles, 44.0-47.8 ms, P = 0.308). The coefficients of variation (CV %) of the amplitudes were not statistically significantly different (P = 0.341).
The use of alternative frontal reference leads (F3 and F4) for left posterior tibial nerve SEP monitoring yields signals of equal quality and reproducibility compared to signals with standard (FPz and C4') referencing. These alternative leads may substitute for traditional referencing when placement of FPz or C4' is precluded by the location of surgery.
The objective of the present study was to determine the adequate cortical regions based on the signal-to-noise ratio (SNR) for somatosensory evoked potential (SEP) recording. This investigation was carried out using magnitude-squared coherence (MSC), a frequency domain objective response detection technique. Electroencephalographic signals were collected (International 10-20 System) from 38 volunteers, without history of neurological pathology, during somatosensory stimulation. Stimuli were applied to the right posterior tibial nerve at the rate of 5 Hz and intensity slightly above the motor threshold. Response detection was based on rejecting the null hypothesis of response absence (significance level alpha= 0.05 and M = 500 epochs). The best detection rates (maximum percentage of volunteers for whom the response was detected for the frequencies between 4.8 and 72 Hz) were obtained for the parietal and central leads mid-sagittal and ipsilateral to the stimulated leg: C4 (87%), P4 (82%), Cz (89%), and Pz (89%). The P37-N45 time-components of the SEP can also be observed in these leads. The other leads, including the central and parietal contralateral and the frontal and fronto-polar leads, presented low detection capacity. If only contralateral leads were considered, the centro-parietal region (C3 and P3) was among the best regions for response detection, presenting a correspondent well-defined N37; however, this was not observed in some volunteers. The results of the present study showed that the central and parietal regions, especially sagittal and ipsilateral to the stimuli, presented the best SNR in the gamma range. Furthermore, these findings suggest that the MSC can be a useful tool for monitoring purposes.
To form median somatosensory evoked potential (SEP) monitoring recommendations based on signal-to-noise ratio (SNR).
Two 1024-sweep right median SEP trials were recorded in 35 patients undergoing spine surgery. The SNR (signal power/noise power) and sweeps to reproducibility (<30% and <20% signal variation) were compared between the following derivations: cubital fossa (CF), Erb's point (EPi-EPc, EPi-M, EPi-Fz), cervical (C5S-EPc, C5S-AC, C5S-M, C5S-Fz), subcortical (CPi-EPc, CPi-M), and cortical (CPc-EPc, CPc-M, CPc-FPz, CPc-Fz, CPc-CPi, CPc-CPz), where M was the mastoid.
Higher SNR produced markedly faster reproducibility. The CF derivation had very high SNR and single-sweep reproducibility. Of cortical derivations, CPc-CPz had highest mean SNR and fastest overall reproducibility (median 50 and 120 sweeps to <30% and <20% signal variation); occasionally CPc-Fz or CPc-CPi was better. Of Erb's point and cervical derivations, EPi-M and C5S-M had highest mean SNR and fastest reproducibility. Subcortical derivations had very low mean SNR and slow or non-reproducibility. High voltage EEG degraded cortical and subcortical derivation SNR and reproducibility in young children.
The highest SNR derivations should be used to speed surgical feedback; slower low-SNR derivations should be omitted. Consequently, the CF is the best technical control and CPc-CPz should be the standard cortical derivation, with CPc-Fz and CPc-CPi as alternates. EPi-M and C5S-M are the best Erb's point and cervical derivations, but are optional. Subcortical derivations should be omitted. A presence/absence criterion or SEP omission may be indicated for some young children.
The results should influence future guidelines.
A retrospective case review was performed.
To assess the value, rapidity, and safety of combined multiple-pulse transcranial electric stimulation motor-evoked potential and somatosensory-evoked potential monitoring during scoliosis surgery.
Leg somatosensory-evoked potentials can miss motor deficits, and a 50% amplitude warning criterion can produce false alarms.
For this study, 33 scoliosis surgeries in neurologically normal patients under propofol/fentanyl anesthesia omitting neuromuscular blockade were monitored with four-extremity multiple-pulse transcranial electric stimulation muscle motor-evoked potentials and cortical somatosensory-evoked potentials. Instead of amplitude criteria, parallel (same-direction) change was used to identify systemic alteration and nonparallel (one- or two-limb) deterioration to identify focal neurologic compromise. Clinical observation and intraoperative electroencephalography were used to assess adverse effects.
Instantaneous motor-evoked potentials and rapidly reproducible cortical somatosensory-evoked potentials provided comprehensive feedback every 0.8 to 6.7 minutes (median, 2.4 minutes) without adverse effects. Parallel (systemic) changes without alarm or deficit included motor-evoked potential fading or temporary loss and leg somatosensory-evoked potential amplitudes below 50% of initial, maximum, or median intraoperative values in 10% to 37% of the cases. Three nonparallel changes occurred: 1) abrupt bilateral leg somatosensory-evoked potential 20% to 30% reduction without motor-evoked potential change during instrumentation resolving spontaneously over 30 minutes, with transient postoperative sensory symptoms; 2) right-arm somatosensory-evoked potential and motor-evoked potential reduction during hyperabduction restored after repositioning, without deficit; 3) abrupt bilateral leg motor-evoked potential loss preceding 30% to 60% somatosensory-evoked potential reduction during derotation rapidly restored after instrumentation release, without deficit.
In neurologically normal patients, the combined methods are safe and rapid, and could improve the sensitivity and specificity of scoliosis monitoring. Arm controls facilitate differentiation between systemic alterations and focal neurologic compromise.
To report the intraoperative neurophysiologic discovery of clinically unsuspected non-decussation of the somatosensory and motor pathways.
We performed somatosensory evoked potential (SEP) and transcranial electric stimulation (TES) muscle motor evoked potential (MEP) monitoring during scoliosis surgery for a 16 year old patient with familial horizontal gaze palsy and progressive scoliosis. Our routine procedures included optimizing tibial cortical SEP monitoring derivations through saggital and coronal (C4', C2', Cz', C1', C3'-mastoid) P37 mapping, which surprisingly indicated non-decussation. Consequently, we also obtained coronal median nerve SEPs and simultaneous bilateral muscle recordings to lateralized TES (C3-Cz, C4-Cz) intraoperatively and focal hand area transcranial magnetic stimulation (TMS) postoperatively.
For each nerve, tibial P37/N37 distribution was contralateral/ipsilateral and median N20 ipsilateral. For each hemisphere, ipsilateral TES MEPs had lower thresholds and TMS MEPs were exclusively ipsilateral. Accurate monitoring required reversed montages. Reevaluation of an MRI (previously reported normal) disclosed a ventral midline cleft of the medulla.
The results indicate uncrossed dorsal column-medial lemniscal and corticospinal pathways due to brain-stem malformation with absent internal arcuate and pyramidal decussations.
Simultaneous bilateral recording to unilateral stimulation demonstrates SEP/MEP hemispheric origin and is important for accurate interpretation and monitoring because decussation anomalies exist.
To compare P37 derivation optimization to Cz'-FPz.
After induction in 120 patients, monitoring derivations optimized by mapping FPz, Cz, Cz', Pz, C4', C2', C1' and C3'-mastoid to determine the P37 and N37 maximums for use as inputs 1 and 2 were compared to Cz'-FPz. This was repeated later in 35 surgeries.
Eleven optimal derivations occurred and usually differed between sides. Input 1 was Cz', Pz, Cz, iCi', or Ci' and input 2 was Cc', FPz, Ci' or Pz. Even the most frequent Cz'-Cc' derivation was optimal for both sides of an individual in only 17% and this was true for Cz'-FPz in only 4%. Optimization produced higher amplitudes than Cz'-FPz (P<0.001). The ratio was [squareroot of 2] : 1 in 61% of patients and > or =2:1 in 28%, approximately halving or quartering averaging times. Optimization assessed decussation, disclosing non-decussation in one patient while Cz'-FPz did not. Alterations of P37 topography that reduced initially optimal derivation amplitude and made a different derivation optimal were demonstrated by repeat optimization in 13 of 35 patients, preventing misinterpretation in one. While also affected, Cz'-FPz neither detected nor adjusted for potentially misleading topographic changes.
Higher amplitudes, decussation assessment and topographic adjustment make P37 derivation optimization superior to Cz'-FPz for monitoring this highly variable potential.
To establish standard values for tibial nerve somatosensory evoked potentials (SEPs).
We examined SEPs following left tibial nerve stimulation in 65 normal subjects of various ages, and performed multiple regression analysis using height, age, (age-20)(2) and gender as predictor variables. We objectively selected the latency or interval parameters with less intersubject variability as the standard parameters for evaluation.
Among 3 cortical bipolar derivations investigated, the Cz'-Cc lead gave a more constant and stable P38 component than the Cz'-Fz or Ci-Cc lead. The latencies of the N8o (N8 onset) of the popliteal potential, P15 (P15 peak) in the contralateral iliac crest-ipsilateral greater trochanter lead, N21, N30 and P38o/P38 in the Cz'-Cc lead, as well as the intervals between these components were selected as standard parameters. P15 was easily identified in all of the subjects and is expected to be a new parameter to evaluate the proximal segment of the tibial nerve. The amplitudes of P15 and the other components were also evaluated. We present nomograms for the normal limit values of each parameter.
We present a thorough set of standard values for tibial SEPs where the subject factors were fully considered, and which is easily applicable to clinical practice.
The effect of isoflurane on the subcortical P14 component of the median nerve somatosensory evoked potential (SEP) is poorly known. We studied whether the P14 wave from the upper brainstem, recorded with a nasopharyngeal electrode, was attenuated at the isoflurane-induced EEG burst-suppression level. We also compared the effect of isoflurane on the P14, cervical N13 and cortical N20, N35 and N6, components.
Seventeen elective patients were anaesthetized with isoflurane. Somatosensory evoked potentials were recorded prior to anaesthesia, at 0.5 MAC and 1 MAC end-tidal isoflurane as well as at the level when EEG was in burst-suppression (mean 1.9 vol% end-tidal isoflurane).
Isoflurane had varying effects on the subcortical components of median SEP. The amplitude of nasopharyngeal P14 was stable, but the mean latency increased from 14.4 +/- 1.2 msec at 0.5 MAC to 15.2 +/- 1.1 msec at burst-suppression level (P < 0.05). In contrast, the N13 neck response amplitude was attenuated from 3.3 +/- 0.6 microV to 2.6 +/- 0.5 microV (P < 0.005) without latency changes. The latency of the cortical N20 wave was increased from 19.7 +/- 1.1 msec at awake to 24.4 +/- 1.6 msec at burst-suppression level (P < 0.0001) and amplitude was reduced from 3.3 +/- 1.1 microV to 1.3 +/- 0.6 microV (P < 0.0001). The later cortical components were attenuated even during 0.5 MAC isoflurane and were not recordable during EEG burst-suppression.
We conclude that P14 can reliably be recorded with nasopharyngeal electrodes during isoflurane anaesthesia, even during EEG burst-suppression, when the N20 wave is attenuated. In contrast, the middle-latency SEP components are sensitive to isoflurane anaesthesia.
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.
Background: Electroencephalogram (EEG) and somatosensory evoked potentials (SEPs) are altered by inhalation anaesthesia. Nitrous oxide is commonly used in combination with volatile anaesthetics. We have studied the effects of nitrous oxide on both EEG and SEPs simultaneously during isoflurane burst‐suppression anaesthesia.
Methods: Twelve ASA I‐II patients undergoing abdominal or orthopaedic surgery were anaesthetized with isoflurane by mask. After intubation and relaxation the isoflurane concentration was increased to a level at which an EEG burst‐suppression pattern occurred (mean isoflurane end‐tidal concentration 1.9 (SD 0.2) %. With a stable isoflurane concentration, the patients received isoflurane‐air‐oxygen and isoflurane‐nitrous oxide‐oxygen (FiO 2 0.4) in a randomized cross‐over manner. EEG and SEPs were simultaneously recorded before, and after wash‐out or wash‐in periods for nitrous oxide. The proportion of EEG suppressions as well as SEP amplitudes for cortical N 20 were calculated.
Results: The proportion of EEG suppressions decreased from 53.5% to 34% ( P < 0.05) when air was replaced by nitrous oxide. At the same time, the cortical N 20 amplitude was reduced by 69% ( P < 0.01).
Conclusion: The results suggest that during isoflurane anaesthesia, nitrous oxide has a different effect on EEG and cortical SEP at the same time. The effects of nitrous oxide may be mediated by cortical and subcortical generators.
The scalp topography of the short latency somatosensory evoked potentials (SEPs) to unilateral posterior tibial nerve stimulation at the ankle was studied by using a non-cephalic reference in 22 normal young adults. At least 3 components (P28, N31 and N32) were identified preceding the major positive peak (P36). The first 2 components had similar peak latency at all scalp electrodes, and were considered to be generated in deep structures. However, N32 was localized to the hemisphere contralateral to the side of stimulation. P36 was maximal at the midline foot sensory area, or at the contralateral parasagittal area, and its amplitude decreased more steeply anteriorly than posteriorly. The peak latency of P36 progressively increased from ipsilateral to the side of stimulation in the coronal plane. P36 occurred earlier in the somatosensory area, and increased in peak latency anteriorly. Generator source of scalp-recorded far-field potentials (P28 and N31) remains to be elucidated. N32 might reflect activities of the thalamo-cortical pathway or an initial cortical response. P36 appeared to be generated in the somatosensory foot area.
Using topographic maps, we studied the scalp field distribution of somatosensory evoked potentials (SEPs) in response to the stimulation of the tibial (TN), sural (SN) and lateral femoral cutaneous (LFCN) nerves in 24 normal volunteers. Cortical peaks, i.e., N35, P40, N50 and P60 were generally dominant in the contralateral hemisphere for the LFCN-SEP, whereas all peaks except N35 had dominance in the ipsilateral hemisphere for TN- and SN-SEPs. The findings imply that ipsilateral or contralateral peak dominance for the lower extremity SEP is determined by where the cortical leg representation occurs. As a result, mesial hemisphere representation results in peak dominance projected to the hemisphere ipsilateral to stimulation. Representations at the superior lip of the interhemispheric fissure or lateral convexity lead to midline or contralateral peak dominance. These findings also suggest that the paradoxically lateralized P40 is not the result of a positive field dipole shadow generated by the primary negative wave in the mesial hemisphere, but is the primary positive wave, analogous to P26 of the median nerve SEP. Accordingly, contralaterally dominant N35 is likely equivalent to the first cortical potential of N20 in the median nerve SEP. The difference in vector directions of potential fields between N35 and P40 may account for the opposite hemispheric dominance for these peaks in TN- and SN-SEPs.
Two anesthetic regimens for monitoring somatosensory evoked potentials (SEPs) during intracranial aneurysm surgery were compared. Eighty-four sequential cases of intracranial aneurysms were operated on employing SEP monitoring. The first group of 22 cases was anesthetized with "balanced anesthesia" and the second group of 62 cases received total intravenous anesthesia (TIVA) consisting of propofol and alfentanil. In the TIVA group, the amplitude of early cortical SEP responses (N20-P25, or P40-N50) was significantly higher than that of responses in the balanced anesthesia group. In median nerve SEPs, the averaged amplitude of N20-P25 was 3.22 microV with TIVA and 1.69 microV with balanced anesthesia (P = 0.006). Similarly, posterior tibial nerve SEPs showed a P40-N50 response of 1.85 microV and 1.00 microV, respectively (P = 0.017). The superior signal-to-noise ratio obtained with TIVA allowed more frequent and reliable intraoperative SEP recordings than was possible with balanced anesthesia, resulting in rapid and reliable feedback for the surgeon. In 19% of median nerve SEPs recorded with TIVA, the cortical responses were over 5 microV in amplitude, so that reproducible N20-P25 responses were obtainable by averaging only 10 to 50 serial responses, that is, two to three recordings per minute. The higher amplitude of posterior tibial nerve SEPs recorded with TIVA made monitoring during surgery for anterior communicating artery aneurysms possible in all cases. This was not always the case with balanced anesthesia. The late deflection of median nerve SEPs (N30) was more frequently observed with TIVA.(ABSTRACT TRUNCATED AT 250 WORDS)
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.
The effect of ketamine alone and in combination with N2O (70% inspired) on median nerve somatosensory evoked potentials (SSEPs) was investigated in 16 neurologically normal patients undergoing elective abdominopelvic procedures. The anesthetic regimen consisted of ketamine (2 mg/kg iv bolus followed by continuous infusion at a rate of 30 micrograms.kg-1.min-1) [corrected], neuromuscular blockade (atracurium), and mechanical ventilation with 100% oxygen. SSEP recordings were obtained immediately preinduction and at 2, 5, 10, 15, 20, and 30 min postinduction. Thereafter, N2O was added with surgical incision and maintained for 15 min. At 5-min intervals, SSEP recordings were again taken during and after N2O. With minor exceptions, mean cortical and noncortical latencies as well as noncortical-evoked potential amplitude were unaffected by either ketamine or N2O. Ketamine induction increased cortical amplitude significantly with maximal increases occurring within 2-10 min. For example, at 5-min postinduction, mean N1-P1 amplitude increased from 2.58 +/- 1.05 (baseline) to 2.98 +/- 1.20 microV and P1-N2 amplitude increased from 2.12 +/- 1.50 (baseline) to 3.99 +/- 1.76 microV. Throughout the 30-min period after ketamine induction, mean P1-N2 amplitude increased generally by more (57-88%) than did mean N1-P1 amplitude (6-16%). N2O added to the background ketamine anesthetic produced a rapid and consistent reduction in both N1-P1 and P1-N2 amplitude. Thus, at 1 min after N2O, mean N1-P1 amplitude decreased from 2.74 +/- 1.11 to 1.64 +/- 0.63 microV, while P1-N2 amplitude decreased from 3.32 +/- 1.52 to 1.84 +/- 0.87 microV.(ABSTRACT TRUNCATED AT 250 WORDS)
Somatosensory evoked potentials (SEP) after median nerve stimulation were recorded in 40 patients during infusion of either 15 mg/kg bw thiopentone or 1 mg/kg bw etomidate (n = 10) within 15 min and after 0.3 mg/kg bw etomidate (n = 20). Marked alterations of SEP waveforms and changes in latencies were observed in all patients. Central conduction time (CCT) was significantly correlated to plasma thiopentone concentration. Infusion of high doses of thiopentone and etomidate was followed by a complete loss of middle and long latency components. Amplitude of the primary cortical SEP N20 was found to be unchanged after thiopentone and to be increased after etomidate, indicating the synchronizing properties of this drug. A pronounced increase in SEP latencies and CCT and waveform alterations have to be considered during hypnotic drug administration in intensive care medicine and intraoperatively.
The effects of halothane, enflurane, and isoflurane were studied at 0.5, 0.75, and 1 MAC in 60% N2O on subcortical sensory evoked potentials recorded at the popliteal fossa (PF), the spine (L-3, C-6) and on cortical potentials recorded at the scalp (SC) following bilateral posterior tibial nerve stimulations at the ankle in 28 patients undergoing scoliosis surgery. Latencies and amplitudes of the resulting potentials at each level were compared with postinduction control values. With increasing MAC, latency and amplitude changes seen at C6 (subcortical) were also compared with those at SC (cortical). Increasing the concentrations of each agent resulted in a graded increase in latency and a graded decrease in amplitude, at all levels. At SC each increase in MAC with each agent resulted in an increase in latency (P less than 0.05) and a decrease in amplitude (P less than 0.005), respectively. The increases in SC latency at 0.75, 1 MAC were larger than the increase in latency at C-6 (P less than 0.005) and the decreases in SC amplitudes at 0.5, 0.75 and 1 MAC were greater than the decrease in amplitude at C-6 (P less than 0.01). Halothane, enflurane, and isoflurane in 60% N2O altered subcortical potentials less than cortical potentials. Enflurane and isoflurane at 0.5, 0.75, and 1 MAC, and halothane at 0.5, 0.75 MAC maintained subcortical and cortical potentials that were adequate for evaluation. However, 1 MAC of halothane suppressed cortical potentials but maintained subcortical potentials. Subcortical C-6 potential may serve as an additional monitor.
Median nerve somatosensory evoked potentials (SSEPs) were recorded in 21 healthy subjects anesthetized with halothane, isoflurane, or enflurane (with and without nitrous oxide) for abdominal or pelvic surgery. Recordings were made prior to induction, then at 0.5 MAC increments of each volatile agent with 60% N2O up to 1.5 MAC, and, finally, at 1.5 MAC without N2O. All three volatile anesthetics produced dose-related reductions in the amplitude and increases in the latency of the cortical component of the SSEP. These changes were most pronounced with enflurane and least with halothane. At 1.5 MAC of each volatile agent, cortical latency decreased and amplitude increased when nitrous oxide was discontinued. The results suggest that in neurologically intact patients, end-tidal concentrations of 1.0 MAC halothane and 0.5 MAC enflurane or isoflurane (each in 60% N2O) can be compatible with effective SSEP monitoring. Volatile anesthetic concentrations consistent with satisfactory somatosensory-evoked potential recording may be greater if N2O is not employed.
As with the following case, the intraoperative evoked reponses may be too poor to be reliably monitored. An anesthetic that would increase the amplitude of evoked responses might make the response usable for monitoring. Etomidate may be such an agent. As shown in several studies, etomidate increases the amplitude of cortically derived median nerve SSEPs. We now report a case in which the introduction of etomidate during a narcotic anesthetic improved SSEPs responses from posterior tibial nerve stimulation and allowed the patient's neural function to be monitored when it would otherwise have been considered unreliable.
In 30 patients undergoing spinal disc operations, the effects of bolus injections followed by intravenous infusions of thiopental, etomidate, and midazolam on median nerve somatosensory-evoked potentials (SSEPs) were studied. Possible additive effects of fentanyl and nitrous oxide were also evaluated. Serial SSEP measurements were made before and for 25 minutes after the start of anesthesia. After induction with one of the three intravenous agents, fentanyl (10 micrograms/kg) was administered and SSEPs were again measured 1 and 5 minutes after administration. Sixty-five% nitrous oxide in 35% oxygen was administered after tracheal intubation and was followed by final SSEP measurements. The three intravenous agents affected SSEP signals differently. Etomidate increased both amplitude and latency. Thiopental decreased amplitude and increased latency. Midazolam had no effect on amplitude but increased latency. The addition of fentanyl and nitrous oxide had different effects in response to the three intravenous induction agents. This study emphasizes the differences in SSEP responses not only to different intravenous induction agents but also to the addition of fentanyl and nitrous oxide.
Posterior tibial somatosensory evoked responses (SSERs) were recorded during administration of isoflurane and nitrous oxide. Responses arising from cortical and subcortical neural generators were examined to compare their relative resistance to anesthetic-related degradation. Recordings were performed in ten adults during anesthesia with 0.5 MAC isoflurane/60% N2O, 1.0 MAC isoflurane/60% N2O, and 1.5 MAC isoflurane/60% N2O. Thereafter, N2O was omitted and recordings were repeated during anesthesia with 1.5 and 1.0 MAC isoflurane/O2. Isoflurane resulted in a significant (P less than 0.001) dose-related decrease in the amplitude of cortical waveforms. The amplitude loss was substantial; e.g., for the first cortical waveform, amplitude decreased from 1.21 +/- 0.67 microV during 0.5 MAC isoflurane/N2O to 0.28 +/- 0.29 microV during 1.5 MAC/N2O. Elimination of N2O resulted in an increase in amplitude of approximately 100% (P less than 0.04). By contrast, the amplitude of the subcortical response as recorded in vertex to linked mastoid and vertex to upper cervical spine derivations was not significantly altered by changing concentrations of isoflurane or N2O. The results suggest that subcortical SSERs may be preferable to those of cortical origin for spinal cord monitoring in situations where isoflurane and nitrous oxide, especially in varying concentrations, are the primary anesthetic agents.
Somatosensory potentials evoked in response to median nerve stimulation were studied in 10 patients during surgery under general
anaesthesia with halothane (five patients) or enflurane (five patients). Bipolar scalp responses (near field) and scalp to
non-cephalic reference (far field) potentials were recorded before the induction of anaesthesia, and after 15 min stabilization
at each of 0.5, 1.0, 1.5 and 2.0% end-tidal concentrations of anaesthetic agent. Both agents produced similar effects. The
latency of the bipolar responses was increased and the amplitude decreased. The amplitude of the far field, subcortically
generated, potentials measured from the scalp to non-cephalic electrodes did not decrease as much as the near field potential
with increasing concentrations of volatile anaesthetic, although the latency of the potential recorded at the frontal electrode
increased. At higher anaesthetic concentrations the virtual elimination of the near field potential caused the frontal and
rolandic potentials to appear to be identical. Since far field somatosensory evoked potentials are preserved during deep anaesthesia,
they should be considered for use when measurement of evoked responses is required sfor monitoring purposes.
Scalp topographies and distributions of initial cortical positive-negative components of somatosensory evoked potentials (SEPs) to posterior tibial nerve stimulation were studied in 12 normal controls in order to investigate the generator sources of the cortical SEPs. There are three variations in topographies and distributions of early cortical SEPs in normal subjects. In 5 of 12 cases initial cortical positive-negative components were distributed over the centro-parietal areas ipsilateral to the stimulation site or midline parasagittal areas. The origins of these components are speculated to be vertical dipolar generators located at the contralateral interhemispheric fissure. In 5 of 12 cases P37/N36, and N45/P43 showed a phase reversal between the left and right hemispheres and may be generated from horizontal dipoles located at the contralateral interhemispheric fissure. In 2 of 12 cases N36 and P43 were distributed predominantly over the contralateral hemisphere. Oblique dipolar generators located at the contralateral interhemispheric fissure may be oriented prominently to the contralateral hemisphere. Normal variations of initial cortical positive-negative components of SEPs to posterior tibial nerve stimulation should be considered in their clinical applications.
We report the potentials elicited by posterior tibial nerve stimulation and recorded simultaneously from the scalp and from electrodes within the interhemispheric fissure. The primary cortical potential was recorded from cortex contralateral but from scalp ipsilateral to the stimulated nerve. The scalp recordings thus demonstrated "paradoxical lateralization" as reported previously, and the similar morphology of the scalp and contralateral cortical recordings confirm that this "paradoxical lateralization" is most likely the result of a horizontal dipole located within the interhemispheric fissure.
The effects of nitrous oxide, enflurane, and isoflurane on cortical somatosensory evoked potentials (SEPs) were studied in 29 patients undergoing intracranial or spinal operations. Anesthesia was induced with fentanyl (25 micrograms/kg, iv) plus thiopental (0.5-1.0 mg/kg, iv). In one group of patients (n = 12), nitrous oxide (50%) was compared with enflurane (0.25-1.0%), and in another group (n = 12) nitrous oxide (50%) was compared with isoflurane (0.25-1.0%). In a third group of patients (n = 5) with preexisting neurologic deficits, nitrous oxide (50%) was compared with enflurane (0.25-1.0%). In all three groups, one gas was administered for 30 min, and then the alternate gas was administered for 30 min; then the cycle was repeated for a total of two administrations of each of the two anesthetics. SEPs were determined before and after induction of anesthesia and at the end of each 30-min study period. The latencies and amplitudes of the early cortical components of the upper- and lower-extremity SEP were examined. Induction of anesthesia resulted in increases of latency in both upper- and lower-extremity SEPs without any alteration of amplitude. Nitrous oxide, enflurane, and isoflurane each decreased the amplitude of the upper-extremity SEPs compared with the postinduction value. The amplitude of the upper-extremity SEPs was less during nitrous oxide than with either enflurane or isoflurane. Nitrous oxide decreased the amplitude of lower-extremity SEPs below postinduction value, while enflurane and isoflurane had no effect. Isoflurane and enflurane increased the latency of both upper- and lower-extremity SEPs slightly, while nitrous oxide had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)
The effects of the inhalation of50% nitrous oxide on somatosensory evoked potentials during a fentanyf-oxygen anaesthetic
technique for central nervous system surgery were evaluated. The latency and amplitude of the first cortical wave were obtained
using conventional somatosensory techniques with median or posterior tibialnerve stimulation. Data were collected before and
after the inhalation of 50% nitrous oxide in oxygen introduced at the conclusion of the surgical procedure. The addition of
nitrous oxide was associated with consistent decreases in the amplitude of somatosensory evoked potentials, but with no significant
changes in latency. Since no electrical, physiological, or surgical event was associated with these changes, the results suggest
that they were attributable to nitrous oxide per se.
Somatosensory evoked potentials (SEPs) were elicited by stimulation of the posterior tibial nerve (PTN) in 12 normal adults. Recording using both cephalic and non-cephalic references were obtained from multiple electrodes placed over the spine and scalp. Following PTN stimulation, the fastest recorded potentials of the afferent sensory volley proceeds up the spinal cord at constant velocity. After arrival of the volley at cervical cord levels, 3 widely distributed waves, P28, P31 and N34, are recorded from scalp electrodes. These 'far-field' potentials are followed by a localized positivity (P38) which has a peak voltage either at the vertex or just laterally toward the side of stimulation. A contralateral negativity (N38) was present in most individuals. We propose that P28 arises from medial lemniscus; that P31 is generated by ventrobasal thalamus; and that N34 is probably the result of further activity in thalamus and/or thalamocortical radiations. The P38/N38 complex represents the primary cortical response to PTN stimulation. Its most consistent characteristic is a positivity at the vertex or immediately adjacent scalp areas ipsilateral to the stimulated leg. The topography of the P38/N38 potential varies slightly from individual to individual in a manner consistent with a functional dipole situated in the leg and foot area on the mesial aspect of the postcentral gyrus, whose exact location and orientation changes in accordance with known variations in the location of the leg area.
The scalp distribution of the response to stimulation of the tibial nerve at the medial malleolus was systematically analysed. The somatosensory evoked potential (SEP) was recorded with electrodes placed in a transversal line over the ipsilateral and contralateral postcentral gyri and in a sagittal line over the longitudinal brain fissure. The SEPs recorded over the ipsilateral hemisphere and along the sagittal line were similar to the F response (the response over the foot primary somatosensory region). Over the contralateral hemisphere the waveform of the responses changed obviously from point F to the point C (contralateral hand primary somatosensory region). The C response started with N37, P40 had a longer latency, N50 was not present and the subsequent waves were also considerably different. Mathematical simulation of the responses recorded from the electrodes between points F and C has shown that they represent an electrical algebraic summation of the activity over points F and C. Although the F and C responses may be 2 potentials arising from the opposite sides of a single dipole generator which is located in the medial fissure, it is more probable that the somatosensory evoked potential on tibial nerve stimulation reflects the activity of 2 separate generators.
To study the distribution of the early (first 80 ms) human cortical potentials evoked by stimulation of the posterior tibial nerve at the ankle, scalp electrodes were placed within a 12-cm radius from the vertex and were separated by approximately 3 cm. With unilateral stimulation the response at the hemisphere ipsilateral to the stimulus was consistently of substantially higher amplitude and at times opposite in polarity to the contralateral response. An explanation of this paradoxical lateralization is that the cortical generators of the evoked potentials to posterior tibial nerve stimulation are located in the mesial surface of the cortex, adjacent to the the interhemispheric tissue, and therefore project transversely or parallel (not perpendicular) to the scalp surface. A similar paradoxical lateralization with similar paradoxical lateralization with similar cause has been reported concerning occipital evoked potentials in response to half-field pattern stimulation.
We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained unchanged for the 3 stimulus rates tested. The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral. N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators.
Most techniques used to monitor spinal cord tracts are sensitive to the effects of anesthesia, particularly to volatile anesthetic agents. The aim of this prospective study was to show that evoked potentials recorded from the peripheral nerves after spinal cord stimulation, so-called neurogenic motor evoked potentials, are resistant to clinical concentrations of isoflurane or desflurane, compared with somatosensory-evoked potentials.
Twenty-three patients were studied during surgery to correct scoliosis. The background anesthetic consisted of a continuous infusion of propofol. Isoflurane (n = 12) or desflurane (n = 11) were then introduced to achieve 0.5 and 1.0 end-tidal minimum alveolar concentrations (MAC), both in 50% oxygen-nitrous oxide and in 100% oxygen. Somatosensory-evoked potentials were elicited and recorded using a standard method, defining cortical P40 and subcortical P29. Neurogenic motor-evoked potentials were elicited by electric stimulation of the spinal cord via needle electrodes placed by the surgeon in the rostral part of the surgical field. Responses were recorded from needle electrodes inserted in the right and left popliteal spaces close to the sciatic nerve. Stimulus intensity was adjusted to produce a supramaximal response; that is, an unchanged response in amplitude with subsequent increases in stimulus intensity. Measurements were obtained before introducing volatile agents and 20 min after obtaining a stable level of each concentration.
Isoflurane and desflurane in both 50% oxygen-nitrous oxide and 100% oxygen were associated with a significant decrease in the amplitude and an increase in the latency of the cortical P40, whereas subcortical P29 latency did not vary significantly. Typical neurogenic motor-evoked potentials were obtained in all patients without volatile anesthetic agents, consisting of a biphasic wave, occurring 15 to 18 ms after stimulation, with an amplitude ranging from 1.3 to 4.1 microV. Latency or peak-to-peak amplitude of this wave was not significantly altered with isoflurane and desflurane, either in the presence or in the absence of nitrous oxide.
Compared with cortical somatosensory-evoked potentials, neurogenic motor-evoked potential signals are well preserved in patients undergoing surgery to correct scoliosis under general anesthesia supplemented with isoflurane or desflurane in concentrations as great as 1 MAC.
The suppressive effect of the halogenated inhalation anesthesia on cortical somatosensory evoked potentials (cSSEPs) has been well documented. Less studied and appreciated is the effect of nitrous oxide often with a narcotic as an alternative to a potent agent for spinal cord monitoring. This study sought to define more clearly the influence of nitrous oxide on cSSEPs elicited to posterior tibial nerve stimulation. A secondary purpose was to demonstrate the advantage of a total intravenous propofol anesthesia in facilitating uncompromised large-amplitude cSSEPs. Fifty adult patients undergoing anterior cervical discectomy served as the study sample. Brainstem and cortical posterior tibial nerve SSEPs were recorded under two independent anesthesia conditions, namely, nitrous oxide and propofol. Results demonstrated a significant amplitude reduction and latency prolongation with the nitrous oxide versus propofol protocol. cSSEP amplitude with propofol was, on the average, approximately two times larger than that with nitrous oxide. Based on these findings, the use of nitrous-oxide anesthesia is not recommended when limited to monitoring cSSEPs that are already amplitude compromised secondary to existing spinal cord disease.
Electroencephalogram (EEG) and somatosensory evoked potentials (SEPs) are altered by inhalation anaesthesia. Nitrous oxide is commonly used in combination with volatile anaesthetics. We have studied the effects of nitrous oxide on both EEG and SEPs simultaneously during isoflurane burst-suppression anaesthesia.
Twelve ASA I-II patients undergoing abdominal or orthopaedic surgery were anaesthetized with isoflurane by mask. After intubation and relaxation the isoflurane concentration was increased to a level at which an EEG burst-suppression pattern occurred (mean isoflurane end-tidal concentration 1.9 (SD 0.2) %. With a stable isoflurane concentration, the patients received isoflurane-air-oxygen and isoflurane-nitrous oxide-oxygen (FiO2 0.4) in a randomized cross-over manner. EEG and SEPs were simultaneously recorded before, and after wash-out or wash-in periods for nitrous oxide. The proportion of EEG suppressions as well as SEP amplitudes for cortical N20 were calculated.
The proportion of EEG suppressions decreased from 53.5% to 34% (P < 0.05) when air was replaced by nitrous oxide. At the same time, the cortical N20 amplitude was reduced by 69% (P < 0.01).
The results suggest that during isoflurane anaesthesia, nitrous oxide has a different effect on EEG and cortical SEP at the same time. The effects of nitrous oxide may be mediated by cortical and subcortical generators.
Cortical somatosensory evoked potentials (CSEP) allow monitoring of spinal cord function during surgery. Ketamine has been shown to enhance CSEP amplitude, but there is no previous study comparing its effects with those of other anaesthetic regimens. Therefore, we have compared the effects of ketamine with those of fentanyl, both combined with midazolam, on CSEP monitoring during major spine surgery. Twenty patients with normal preoperative CSEP were allocated randomly to a ketamine or fentanyl group. Anaesthesia was induced with ketamine 3 mg kg-1 or fentanyl 6 μg kg-1 i.v., and midazolam 0.3 mg kg-1 i.v in both groups, and maintained with continuous i.v infusion of ketamine 2 mg kg-1 h-1 or fentanyl 3 μg kg-1 h-1, combined in both groups with midazolam 0.15 mg kg-1 h-1 and 60% nitrous oxide in oxygen. CSEP were elicited by tibial posterior nerve stimulation and measured P1 and N1 latencies, and P1-N1 amplitude. CSEP were recorded before and after induction, at 15 min, 1 and 2 h after induction, during skin closure and after removal of nitrous oxide. Both groups were comparable in characteristics, duration of surgery, mean arterial pressure and temperature. CSEP latencies were not significantly affected in either group. CSEP amplitude decreased significantly over time in the fentanyl group (from mean 2.02 (SEM 0.41) to 0.95 (0.17) μV, P < 0.05), but not in the ketamine group (from 1.33 (0.36) to 1.05 (0.31) μV, ns). Nevertheless, we did not observe any significant differences in amplitudes or latencies between the two groups. The delay in obtaining the first voluntary postoperative motor response was significantly greater in the ketamine group (170 (54) vs 55 (17) min, P < 0.01). Both ketamine and fentanyl allowed us to obtain reliable CSEP during major spine surgery, and there were no significant difference between these two anaesthetic regimens for CSEP monitoring, but a longer delay for voluntary postoperative motor assessment was observed in the ketamine group.
Somatosensory evoked potentials (SEPs) elicited by right posterior tibial nerve stimulation were simultaneously recorded from 21-27 epidural electrodes in three monkeys. N23-P40 was recorded anterior to the left central sulcus, and P23-N40 was recorded on the parietal midline and the middle portion of the right hemisphere. These potentials were thought to be the primary cortical responses elicited by posterior tibial nerve stimulation in the monkey, since a topographical map made of them corresponded to the paradoxical lateralization of the primary cortical components in human posterior tibial nerve SEPs. Current source generators (dipoles) of these potentials were 3-dimensionally identified dipoles located in the left side of the mesial wall of the anterior parietal cortex, and oriented obliquely toward the right hemisphere by a dipole tracing (DT) method in which the 3-dimensional localization of dipoles in the brain were estimated and superimposed on magnetic resonance imaging (MRI) images.
Somatosensory evoked potentials (SEPs) are most commonly obtained after stimulation of the median nerve and the posterior tibial nerve. SEPs reflect conduction of the afferent volley along the peripheral nerve, dorsal columns, and medial lemniscal pathways to the primary somatosensory cortex. Short-latency SEPs are recorded over the spine and scalp. After posterior tibial nerve stimulation, the following waveforms are recorded: N22, W3, the dorsal column volley, N29, P31, N34, and P37. After median nerve stimulation, the brachial plexus volley, dorsal column volley (N11), N13, P14, N18, N20, and P22 potentials are recorded. We discuss the current state of knowledge about the generators of these SEPs. Such information is crucial for proper interpretation of SEP abnormalities.
Continuous measurement of somatosensory evoked potentials (SEP) by means of characteristic changes in the signal pattern makes it possible to identify cerebral or spinal cord ischemia during critical phases of the operative procedure. A correct interpretation of the measurements is only possible, however, if the influence of drugs acting on the central nervous system is known. The authors were able to show that inhaled anesthetics have an impact on latencies and response amplitudes. This study examined the influence of various concentrations of desflurane on the conduction of SEP of the Median nerve. In addition, the authors determined how the supplementation of nitrous oxide (N2O) influences the stimulus response of the medianus nerve's SEP. Desflurane has been shown to produce dose-dependent increases in SEP latency (data in part for latency N2O: 0.5 minimum alveolar concentration [MAC] = 20.8 +/- 0.9; 1.5 MAC = 22.2 +/- 1.5; 1.5 MAC/N2O= 23.8 +/- 1.5) and decreases in amplitude, whereas cervically recorded subcortical SEP components are minimally influenced by desflurane. When nitrous oxide is added, there were marked reductions in amplitude (p<0.01) of the cortical stimulus response (1.5 MAC = 2.4 +/- 0.9; 1.5 MAC/N2O = 1.1 +/- 1). It can therefore be recommended that supplementation with N2O should be avoided in the presence of low initial amplitudes. Based on the study's results, the use of desflurane (up to 1.0 MAC) seems to be compatible with intraoperative monitoring of median somatosensory evoked potentials.
Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34-46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5-500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45 +/- 0.14 microV) than contralaterally (-0.49 +/- 0.16 microV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2 +/- 4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4 +/- 1.6 mm) with a more tangential orientation (delta theta = 17.5 +/- 2.3 degrees) in the left hemisphere.
Studies of the effects on lower-limb cortical somatosensory evoked potentials (CSEP) during total intravenous anesthesia are sparse for propofol and are lacking for midazolam. This study was designed to compare the effects of propofol and midazolam on CSEP under total intravenous anesthesia during intraoperative monitoring for surgical treatment of scoliosis.
CSEPs were recorded in two groups of 15 patients during posterior instrumentation for treatment of idiopathic scoliosis. The anesthesia used the combination of atracurium, alfentanil, and an hypnotic agent (propofol for Group I or midazolam for Group II). The main characteristics of the CSEPs (P40 latency and N34-P40 and P40-N50 amplitudes) were recorded using ankle posterior tibial nerve stimulation. The CSEPs were recorded before induction, 10, 70, 100, 130, and 160 minutes after induction, and before the wake-up test. The statistical analysis involved analysis of variance for repeated measures. Both groups were homogeneous before induction.
Neither CSEP deterioration during risk-associated surgical procedures nor postoperative clinical abnormalities were observed. Both propofol and midazolam induced increases in P40 latencies, with the increases being greater and more regular for the propofol-treated group. The amplitude values changed with time for both groups, decreasing mainly after induction; in the midazolam-treated group, the amplitudes were smaller but more stable. Propofol modified the morphological characteristics of the response by decreasing the late P60 component amplitude; the W-shaped CSEP morphological pattern was maintained with midazolam.
This study demonstrates the appropriate use of either propofol or midazolam in scoliosis monitoring. Preoperative small-amplitude CSEPs might favor the use of propofol anesthesia.
- Tinazzi
- Taniguchi
- Wolfe
- Langeron