Monitoring of subcortical and cortical somatosensory evoked potentials during carotid endarterectomy: comparison with stump pressure levels

Monitoring of multichannel somatosensory evoked potentials (SEPs) has been performed in 40 cases of carotid endarterectomy (CEA). SEPs were obtained after median nerve stimulation at wrist, recording from 2nd cervical and from the scalp parietal (ipsi- and contralateral) and central (contralateral) positions. The reduction of CBF due to clamping of the carotid artery provoked SEP abnormalities in 10 of the 40 cases. None of the 30 patients with unmodified SEPs developed post-surgical neurological sequelae.SEP alterations were characterized exclusively by amplitude decrements and latency increases of the cortical components, the subcortical ones being unaffected. In 5 of these patients, SEPs returned to normal values before the end of the intervention and no neurological deficit was observed on awakening. In the remaining 5 cases SEPs retained their abnormalities and patients developed post-surgery neurological sequelae (4 immediately, 1 the day after).SEP alterations affected parietal and central components to a similar extent; however, in a few cases cerebral blood flow deficits provoked by carotid clamping modified differently the central P22 and the parietal N20–P25 waves.Comparisons with stump (back) pressure in the carotid artery revealed a higher sensitivity of the SEP technique in detecting vascularization problems due to carotid clamping.The time course of the appearance of SEP abnormalities seems to discriminate alterations secondary to collateral revascularization from those determined by embolization.     

Origin of frontal N15 component of somatosensory evoked potential in man

Origin of the frontal somatosensory evoked potential (SEP) by median nerve stimulation was investigated in normal volunteers and in patients with localized cerebrovascular diseases, and the following results were obtained.

• 1.(1) In normal subjects, SEPs recorded at F3 (or F4) contralateral to the stimulating median nerve were composed of P12, N15, P18.5 and N26. Similar components were recognized in SEP recorded at Fz.
• 2.(2) In patients in whom putaminal or thalamic hemorrhages had destroyed the posterior limbs of the internal capsules, frontal N15 and parietal N18 (N20) disappeared. These components were also absent in patients with cortical (parietal) infarctions. Among these patients, the thalamus was not affected in cases with putaminal hemorrhages and cortical infarctions.

These facts indicate that the generator of the frontal N15 does not exist in the thalamus but that it originates from the neural structure central to the internal capsule, which suggests a similarity to the generator of the parietal N18.Because N15 was recorded in the midline of the frontal region with shorter latency than parietal N18, the frontal N15 might represent a response to the sensory input of the frontal lobe via the non-specific sensory system.     

Single-trial detection for intraoperative somatosensory evoked potentials monitoring

Abnormalities of somatosensory evoked potentials (SEPs) provide effective evidence for impairment of the somatosensory system, so that SEPs have been widely used in both clinical diagnosis and intraoperative neurophysiological monitoring. However, due to their low signal-to-noise ratio (SNR), SEPs are generally measured using ensemble averaging across hundreds of trials, thus unavoidably producing a tardiness of SEPs to the potential damages caused by surgical maneuvers and a loss of dynamical information of cortical processing related to somatosensory inputs. Here, we aimed to enhance the SNR of single-trial SEPs using Kalman filtering and time–frequency multiple linear regression (TF-MLR) and measure their single-trial parameters, both in the time domain and in the time–frequency domain. We first showed that, Kalman filtering and TF-MLR can effectively capture the single-trial SEP responses and provide accurate estimates of single-trial SEP parameters in the time domain and time–frequency domain, respectively. Furthermore, we identified significant correlations between the stimulus intensity and a set of indicative single-trial SEP parameters, including the correlation coefficient (between each single-trial SEPs and their average), P37 amplitude, N45 amplitude, P37-N45 amplitude, and phase value (at the zero-crossing points between P37 and N45). Finally, based on each indicative single-trial SEP parameter, we investigated the minimum number of trials required on a single-trial basis to suggest the existence of SEP responses, thus providing important information for fast SEP extraction in intraoperative monitoring.     

Effects of halothane on intraoperative scalp-recorded somatosensory evoked potentials to posterior tibial nerve stimulation in man

Somatosensory evoked potentials (SEPs) were monitored in 116 patients receiving halothane anesthesia during spinal fusion surgery. Whereas it has been generally assumed that the use of halogenated inhalational anesthetics should be avoided with SEP monitoring because of their purported deleterious effects on scalp-recorded sensory responses, we found that reproducible SEPs were obtained throughout the surgical procedure in 91% of the cases we monitored while using halothane at concentrations of 0.25–2.0%. In those cases in which halothane was delivered continuously at 0.5%, reproducible evoked responses were recorded in 96% (75 of 78) of the patients. Our data demonstrated 3 major effects of halothane on the SEP: (a) a small but significant decrease in the average amplitude of the first two components (N₂₅ and P₃₀), (b) a significant increase in the average latency of the late positive component (P₅₃) of the wave form, and (c) occasional obliteration of components N₂₅, N₄₀, P₅₃, and N₇₁,but never of P₃₀. These effects did not, in most cases, interfere with our ability to obtain clinically useful recordings. Our results suggest that in many instances the use of halothane anesthesia can be combined successfully with the recording of intraoperative SEPs.     

Right or left ear reference changes the voltage of frontal and parietal somatosensory evoked potentials

Short-latency cortical somatosensory evoked potentials (SEPs) to left median nerve stimulation were recorded with either the left or right earlobe as reference. With a right earlobe reference the voltage of the parietal N20 and P27 was reduced while the voltage of the frontal P20 and N30 was enhanced. The effects were consistent, but their size varied with the SEP component considered and also among the subjects. Analysis of SEPs at different scalp sites and at either earlobe suggested that the ear contralateral to the side stimulated picked up transient potential differences, depending a.o. on side asymmetry and geometry of the neural generators as disclosed in topographic mapping. For example, the right ear potential can be shifted negatively by the right N20 field evoked by left median nerve stimulation. The changes involve the absolute potential values, but not the time features of the gradients of potential fields. Scalp current density (SCD) maps are not affected. The results are pertinent for current discussions about which reference to use and document the practical recommendation of recording short-latency cortical SEPs with a reference at the ear ipsilateral (not contralateral) to the side of stimulation.     

Abnormalities of parietal and prerolandic somatosensory evoked potentials in Huntington's disease

Cervical, parietal and prerolandic somatosensory evoked potentials (SEPs) to median nerve stimulation at the wrist were recorded with an earlobe reference in 24 patients with Huntington's disease (HD) and in 24 age-matched normal controls. Cortical responses of abnormal wave form and reduced amplitude were constantly observed in HD patients. SEP changes affected more severely the prerolandic (P22/N30) pattern, which could not be recognized in two-thirds of patients, than the parietal (N20/P27) pattern, which could be identified in all cases. The N20 latency and the central conduction time (N13–N20 interval) were significantly increased. The occurrence of abnormalities of central conduction and of a predominant involvement of the prerolandic SEP pattern suggests an impairment of impulse transmission along the somatosensory lemniscal pathway at subcortical, possibly thalamic, level in HD.     

High-rate sequential sampling of auditory brain-stem and somatosensory evoked responses in hypoxia

We developed a high-rate sequential recording technique that allowed simultaneous measurements of both auditory brain-stem response (ABR) and somatosensory evoked potential (SEP) every 10 sec. Using this method, a transient increase in amplitude of all the ABR and SEP components in response to hypoxia in dogs could be detected. The increase in amplitude preceded the prolongation of latency. Our study showed that there were succesive changes of evoked potentials in response to hypoxia. A transient increase in amplitude is the first to occur, followed by a latency prolongation and an amplitude decrease for both ABRs and SEPs.     

Reorganization of the pattern of the brain somatosensory evoked potentials after spinal cord injury

Recordings of somatosensory evoked potentials (SEPs) and SEP brain maps of 23 patients with vertebral injury were analyzed. All patients were operated at the Clinic of Military Traumatology and Orthopedics, Military Medical Academy. SEPs (19 sites) were recorded in real time and mapped with "Brain surveyor" neuromapper. Brain lesions were localized before surgery by of MR imaging and CT standard techniques. The results demonstrate that electrical stimulation of peripheral nerves produces a substantially variable SEP pattern. It was shown that SEP formation in more than 60% of recordings suggests a favorable prognosis for rehabilitation, whereas if more than 50% of recording sites are unresponsive, the rehabilitation prognosis is unfavorable. The amplitude of early components (below 60 ms) increases, and that of the late waves reduces.     

The usefulness of the spinal and subcortical components of the posterior tibial nerve SEPs for spinal cord monitoring during aortic coarctation repair

This study examines how the recording of the lumbar and subcortical components of the posterior tibial nerve (PTN) SEPs may usefully replace that of cortical components in situations in which these components cannot be reliably obtained (infants, high concentrations of halogenated gasses). Lumbar, brain-stem, and cortical PTN SEPs were intraoperatively monitored in 7 patients undergoing repair of aortic coarctation under variable isoflurane concentration (up to 1.2%). Four patients were less than 1 year old. Two distinct activities were evidenced at the lumbar level in all of the patients: the dorsal root component (DRC) and the dorsal horn negativity (DHN). The equivalent of the adult P30 (lemniscal positivity; LP) was also present in all of the patients, whatever their age or the concentration of isoflurane. By contrast, the parietal activities were absent intraoperatively in the youngest patients. Spinal-cord ischemia consecutive to aortic cross-clamping gave rise to early DHN changes and later alterations of the LP in the two patients in which it occurred, while the DRC and the peripheral nerve activities remained unchanged. This elective sensitivity of the DHN is likely due to it being dependent on the gray matter of the spinal cord, the basal metabolism of which is greater than that of the white matter and to the situation of the DHN generator in a watershed zone of the spinal cord. This study emphasizes the interest of PTN SEPs for spinal-cord monitoring in vascular surgery and the importance of combining the recording of parietal activities with that of the lumbar spinal components.     

Gating of the early components of the frontal and parietal somatosensory evoked potentials in different sensory-motor interference modalities

Three different interfering conditions were studied during the recording of pre- and postcentral somatosensory evoked potentials (SEPs) following median nerve stimulation at the wrist in 16 normal subjects: active finger movement (MVT), light superficial massage (LSM) and deep muscular massage (DMM) of the hand. Special attention was focused on selective effects on individual SEP components. The frontal N₃₀ component showed the most significant amplitude reduction during the three interfering conditions (76.4% of reduction in MVT, 36.4% in DMM and 32.9% in LSM). In contrast the frontal N₂₃ was not significantly changed and the preceding P₂₂ component was only reduced in the MVT condition.Postcentral N₂₀ was unchanged by the three conditions while P₂₇ was clearly gated by movement but not significantly by LSM and DMM. The three interfering conditions enhanced the parietal N₃₂ and had no significant effect on the parietal P₄₅.An important point was the interindividual variability of these effects and it appeared that group average wave forms would therefore be confusing.The peak latency of some SEP components was changed during the interfering conditions. The most important effect was an increase of postcentral P45 latency which was found to be related to the amplitude enhancement of N₃₂.     

Evaluation of reference sites for scalp potentials evoked by painful and non-painful sural nerve stimulation

The objective of this study was to evaluate reference sites for recording the middle- and long-latency scalp potentials elicited by painful and non-painful sural nerve stimulation. Somatosensory evoked potentials (SEPs) were recorded from the scalp, the mastoid, the earlobe, the neck, and the wrist. Each site was referenced to the sterno-vertebral (SV) electrode, which is a balanced non-cephalic reference with essentially no ECG contamination.There was little or no activity recorded between the wrist and SV, and the SV was located within a region extending from the rostral neck to the wrist where the potentials were stable over space. Hence, the SV reference is indifferent for the middle- and long-latency potentials evoked by painful and non-painful sural nerve stimulation. There was, however, significant activity recorded from the earlobe and mastoid, sites which are frequently used as references for the SEP. It is important that investigators using these cephalic references to study the middle- and long-latency peaks of the SEP be aware of this activity as it will distort SEPs recorded from single sites and the SEP scalp topography, distortions which could unnecessarily complicate their interpretation.     

Unmasking of cortical SEP components by changes in stimulus rate: a topographic study

We performed topographic mapping of somatosensory responses to median nerve stimulation delivered at 2, 5 and 10 Hz. Parietal N20 was significantly attenuated in 10 Hz somatosensory evoked potentials (SEPs), while central P22 diminished between 2 and 5 Hz, remaining stable thereafter. The single component most affected by increasing stimulus rate was N30, which abated by more than 50% in 10 Hz SEPs, as compared with basal responses. N30 attenuation disclosed the existence of an earlier negative component, N24, which appeared as a notch on the N30 ascending slope in 2 Hz SEPs, but became a well-defined peak at higher stimulus rates. The N24 negativity was not significantly modified by stimulus rate; it had a parietal counterpart (P24) with the same peak latency and identical behavior during the experimental procedure. Both P24 and N24 could be differentiated from central P22 on the basis of topographical distribution and response to stimulus frequency. P22 topography could be the result of a radially oriented generator, while P24/N24 appeared as the two poles of a neural source tangential to the scalp. P27 was seen in 40% of the subjects only; it is suggested that P27 is itself a composite potential to which the generator of N30 could contribute in part. We conclude that there is no single “optimal” stimulation rate for SEP recording. On the contrary, combination of different frequencies of stimulation should enhance the diagnostic utility of this technique by allowing a more selective assessment of overlapping activities.     

Somatosensory evoked potential spinal cord monitoring reduces neurologic deficits after scoliosis surgery: results of a large multicenter survey

Neurologic deficits were compared to somatosensory evoked potential (SEP) spinal cord monitoring in a survey of spinal orthopedic surgeons. Experienced SEP spinal cord monitoring teams had fewer than one-half as many neurologic deficits per 100 cases compared to teams with relatively little monitoring experience. Experienced SEP monitoring teams also had fewer neurologic deficits than were seen in previous surveys of this group. Definite neurologic deficits, despite stable SEPs (false negative monitoring), occurred during surgery in only 0.063% of patients. Factors independently associated with fewer neurologic deficits also included the surgeon's years of experience in orthopedic surgery and the use of the wake-up test. Other technical survey results are also presented here. These results confirm the clinical efficacy of experienced SEP spinal cord monitoring for prevention of neurologic deficits during spinal surgery such as for scoliosis.     

Somatosensory evoked potential, neurological examination and magnetic resonance imaging for assessment of cervical spinal cord decompression

The present study was designed to determine the relationship between neurological testing, anatomical imaging, and electrophysiological monitoring for assessing outcome of cervical spinal cord decompression. We prospectively studied 28 consecutive patients (age 39-76 yr) who were subjected to presurgical-(1-3 wk) and postsurgical (3-4 mo) neurological examination and recording of the median nerve somatosensory evoked potential (SEP). In 13 patients, magnetic resonance imaging (MRI) was also performed. Changes in neurological function, SEP and MRI were evaluated and graded as (1) improvement,(2) no change or (3) deterioration. Neurological outcome (NO) was based on changes in motor grade strength, sensory, reflexes and gait. The SEP outcome was based on changes in latency and disappearance of SEP waveform components whereas MRI evaluation was based on changes in spinal cord and canal diameters. Significance of association between NO, SEP and MRI was determined by Pearson's Chi-Square statistic (P<.05). The SEP improved in 71% (20/28) and deteriorated in 28% (8/28) of the subjects. An association between SEP changes and NO was found in 82% (23/28) of the subjects (P = .0038). Decompression increased the spinal canal diameter in 92% (12/13), and the spinal cord diameter in 38% (5/13) of the subjects. An association between NO, or SEP and MRI was not detected. Changes in median nerve SEP latency appear to be predictive of the neurological status of patients subjected to cervical spinal cord decompression. Postoperative increments in SEP latency or disappearance of the SEP waves were indicative of poor outcome after surgical decompression of the cervical spinal cord.     

颈动脉狭窄的显微外科治疗

目的:评价颈动脉内膜剥脱术治疗颅外颈动脉狭窄的疗效,以及经颅多普勒超声(TCD)、微血管多普勒超声(MVD)应用的价值。方法:回顾性分析2012年5月至2013年5月采用颈动脉内膜剥脱术治疗的19例颅外颈动脉狭窄的资料。均伴有不同程度的脑缺血症状。颈动脉狭窄程度2例中度狭窄,17例重度狭窄。同时行颈动脉内膜剥脱术均在显微镜下操作。1例采用补片成形。18例术中采用TCD及MVD监测下完成,1例未采用超声监测。结果:手术成功率为100%,无死亡率。术前脑缺血症状术后患者均有不同程度的恢复。均未发现过度灌注的并发症。结论:颈动脉内膜剥脱术治疗颅外颈动脉狭窄是一种安全、有效的措施;MVD及TCD监测对于显微手术有着重要意义。     

Origin and distribution of brain-stem somatosensory evoked potentials in humans

The distribution of somatosensory evoked potentials (SEPs) recorded from the brain-stem surface was studied to investigate their generator sources in 14 patients during surgical exploration of the posterior fossa. Two distinct SEPs of different morphologies and electrical orientation were obtained by median nerve stimulation. A small positive-large negative-late prolonged positive wave was recorded from the cuneate nucleus and its vicinity. There was a phase-reversal between the cuneate nucleus and the ventral surface of the medulla, depicting a dipole for dorso-ventral organization. From the pons and midbrain, triphasic waves with predominant negativity were obtained. This type of SEP had identical wave forms between dorsal, lateral and ventral surface of the pons and midbrain. It showed an increase in negative peak latency as the recording sites moved rostrally, suggesting an ascending axial orientation. In a patient with pontine hemorrhage, the killed end potential, a large monophasic positive potential was obtained from the lesion. This potential occurs when an impulse approaches but never passes beyond the recording electrode. Therefore, the triphasic SEP from the pons and midbrain reflects an axonal potential generated in the medial lemniscal pathway.     

Detection of neurological injury using time-frequency analysis of the somatosensory evoked potential

Somatosensory evoked potentials (SEPs) can be monitored during critical surgery to help detect or possibly prevent post-operative injury to the brain. This paper presents the application of time-frequency analysis to detect both temporal and spectral changes in the SEP waveform that may occur due to injury. Time-frequency distributions, which provide a measure of signal energy at both a specific time and frequency, were computed for averaged SEPs acquired from anesthetized cats during various stages of hypoxic injury and then recovery. Wigner distributions of SEP waveforms were found to contain a peak of signal energy at a specific time and frequency, a peak that is altered during injury. Four characteristics of the distribution peak that demonstrate changes due to injury were computed: peak time, peak frequency, peak power, and peak sharpness. Peak time was found to increase while peak frequency, peak power, and peak sharpness were found to decrease during injury. Furthermore, the total signal power in a time-frequency space around the normal peak location was monitored by developing a time-frequency window filter (TFWF) method. For all cases, onset of hypoxia was detected an average of 2.75 min earlier by the TFWF method than by the conventional amplitude measurement method. Time-frequency analysis of EP signals may therefore be useful as a monitoring tool for early detection of brain injury.     

Somatosensory evoked potentials recovery (SEP-R) in myoclonic patients

Recovery of somatosensory evoked potentials (SEPs) was studied by paired stimulation of the median nerve in patients with various kinds of myoclonus. This technique revealed the hyperexcitability of the central nervous system (CNS) which could not be detected by the conventional SEP technique using a single stimulus. This technique would be useful for studying the excitability of the CNS.     

Laser-evoked brain potentials in patients with dissociated loss of pain and temperature sensibility

Brief heat stimuli, elicited by a CO₂ laser (10.6 μm wave length), activate the most superficial cutaneous nerve terminals of the thin myelinated Aδ and unmyelinated C fibres which mediate heat and pain sensations. This paper investigates late cerebral potentials (SEPs) in response to laser pulses in comparison with those to conventional electrical stimulation in 18 patients with a dissociated sensory deficit (intact mechanosensibility and disturbed temperature and pain sensation). Patients were stimulated in the most disturbed limb (affected area) and in a corresponding control area.In all 18 patients the SEPs elicited by laser stimuli were able to identify the body site with heaviest disturbances in pain and thermosensibility: the SEPs from the affected area were reduced or delayed, compared to the control area. In contrast, no alterations in SEPs could be observed after conventional electrical nerve stimulation, in agreement with the normal mechanosensibility. However, the degree of SEP modulation in response to cutaneous heat stimuli did not correspond to the severity of the subjectively reported sensory deficit. Highest correlations between sensory deficits and abnormal SEPs were found in all those patients in whom computer tomography or MR imaging documented a localized destructive process in the CNS. All patients with the smallest SEP modulations despite a considerable sensory deficit had an inflammatory aetiology. Preliminary criteria to define a laser-evoked SEP as pathological are discussed.     

The value of ulnar somatosensory evoked potentials (SEPs) in cervical myelopathy

In 57 patients with clinical signs and surgical documentation of compressive myelopathy, ulnar nerve somatosensory evoked potentials (SEPs) were more sensitive (with 74% abnormal) than either median or tibial nerve SEPs. The most frequent abnormalities were reduced or absent neck evoked responses and prolonged central conduction time. All subjects who had an SEP abnormality were identified by combined tibial and ulnar SEPs. Median nerve SEP added no additional information. Normal ulnar and tibial nerve SEPs were also able to exclude major cord damage in patients with cervical radiculopathy but little evidence of myelopathy.