Three-dimensional human somatosensory evoked potentials

Median nerve somatosensory evoked potentials were recorded from 30 normal adults using conventional scalp derivations and an orthogonal bipolar surface electrode montage. This allowed the determination of the spatial orientation of the hypothetical centrally located equivalent dipole derived from the evoked response recorded in 3-dimensional voltage space. The 3-dimensional voltage trajectory describing changes in equivalent dipole orientation and magnitude revealed 4 major apices between 5 and 25 msec, 3 of which corresponded to the traditional P14, N20 and P25 peaks. A fourth apex at 17 msec was not as evident in the conventional recordings and signaled a transition from a vertical P14-N18 generator process to a horizontal N20 generator process. The normal within- and between-subject variability of trajectory apices, segments and planes are described, along with the theoretical and practical implications of this recording technique.     

Effect of movement on human spinal and subcortical somatosensory evoked potentials

Sensory transmission in dorsal column nuclei is inhibited during voluntary movement in experimental animals. We have studied the human response by recording spine and scalp somatosensory evoked potentials. Finger movement attenuated the amplitude and duration of the cervical N13 and the scalp N18 and N20 waves. Foot movement did not alter the lumbar N22 after foot stimulation, but the scalp P38 was attenuated. N22 results solely from activation of interneurons in the dorsal gray of the cord at the root entry zone, but N13 may receive contributions from the nucleus cuneatus. Therefore, the movement-induced attenuation of N13 is attributed to decreased contribution from the nucleus cuneatus.     

Monitoring somatosensory evoked potentials in spinal cord ischemia-reperfusion injury

It remains unclear whether spinal cord ischemia-reperfusion injury caused by ischemia and other non-mechanical factors can be monitored by somatosensory evoked potentials. Therefore, we monitored spinal cord ischemia-reperfusion injury in rabbits using somatosensory evoked potential detection technology. The results showed that the somatosensory evoked potential latency was significantly prolonged and the amplitude significantly reduced until it disappeared during the period of spinal cord ischemia. After reperfusion for 30-180 minutes, the amplitude and latency began to gradually recover; at 360 minutes of reperfusion, the latency showed no significant difference compared with the pre-ischemic value, while the somatosensory evoked potential amplitude in- creased, and severe hindlimb motor dysfunctions were detected. Experimental findings suggest that changes in somatosensory evoked potentia~ ~atency can reflect the degree of spinat cord ischemic injury, while the amplitude variations are indicators of the late spinal cord reperfusion injury, which provide evidence for the assessment of limb motor function and avoid iatrogenic spinal cord injury.     

Characterization and processing of surface recorded spinal somatosensory evoked potentials

Somatosensory evoked potentials were recorded from the skin surface overlying the spinal cord, from the lower lumbar to the lower cervical regions. The recorded responses did not vary over time in any one individual and at each level a consistent wave shape was obtained across all individuals tested. Since the initial signal-to-noise ratio (SNR) for the evoked response recorded at any cord level is very low, the signal data must be processed. In this study, bandpass filtering or matched filtering was used together with ensemble averaging to obtain a usable signal in a reasonable processing time. SNR was improved approximately 1.5 X with bandpass filtering and 2 X with matched filtering. Although the output of the matched filter is a distorted version of the input signal, detection of information is enhanced and processing time using the matched filter and ensemble averaging can be reduced to 1/4 that required for ensemble averaging alone.     

Long-latency somatosensory evoked potentials

Theoretically, long-latency somatosensory evoked potentials (SEPs) provide information on the function of somatosensory associative cortical structures. Their potential role in clinical studies and research has been hampered by the lack of standardized methodology in the use of these SEPs. Other factors, such as drugs, simultaneous stimuli, and state of consciousness, also have far-reaching influences on the various parameters of long-latency SEPs. The knowledge of the origin of most SEP components is at best fragmentary; studies on clinical-electrophysiological correlations seem to be hopeful in this respect. As yet, clinical applications of long-latency SEPs are limited; for future research, studies of disturbances of SEPs are most promising, mainly with regard to diseases of the gray matter, the influence of drugs on the cerebral function, and psychopathology.     

Cortical somatosensory evoked potentials in spinal cord injury patients.

The amplitude and latency of somatosensory evoked potentials (SEPs) in healthy subjects depend on intensity of stimulation. The effect of this parameter on SEPs in patients with neurologic disorders has not been systematically studied, although it could have a profound impact if SEPs are to be used for prognostication. We have compared the latency and amplitude of SEPs in healthy subjects and patients with spinal cord injury (SCI). Stimulation intensity was standardized at two different biologically calibrated levels. Cortical SEPs in patients with SCI showed greater decrease in latency and increase in amplitude with increased intensity of stimulation in comparison to healthy subjects. These phenomena were observed in the majority of patients with incomplete SCI who subsequently showed improvement in cortical SEPs. We observed situations in which the SEP was absent with the usual intensity of stimulation and present only with the stronger stimulation intensity. Furthermore, SEP latencies often changed dramatically with different intensities of stimulation, potentially making any calculation of central conduction velocity meaningless without precise standardization of stimulation. These findings demonstrate a necessity for a biological calibration of stimulation intensity to improve the repeatability of SEPs. We suggest the use of two different standardized intensities of stimulation for SEP studies in SCI patients, one of which should be stronger than the intensity presently recommended.     

Monitoring spinal cord motor and somatosensory evoked potentials in anesthetized primates

Abstract

Monitoring Motor Evoked Potential (MEP) to Transcranial Stimulation (TMS) monitoring (MEP) is a growing technique to assess motor function under anesthesia. The following primate study was conducted to analyze the non-myogenic spinal motor and sensory volleys and to examine their reproducibility under nitrous oxide-methohexidone anesthesia. The traveling periodic spinal descending MEP to TMS and ascending somatosensory (SEP) to posterior tibial nerve stimulation across the thoracic cord were recorded in 12 cynomolgus monkeys. Through a small Tn~Tu laminotomy, an insulated stainless steel electrode was inserted into the epidural thoracic space. The potentials were analyzed under 50 vol% NO in 02 with methohexital (0.1-0.2 mg kg~1 min~1). A well-defined periodic TMS-MEPs and PTN-SEPs were recorded with high reproducibility and consistency in repeated trials under N20-methohexital anesthesia. MEP tracing consisted of an initial peak (direct (D) wave), occurring at 2.43 (±0.28) msec followed by subsequent five positive (indirect (I) waves). Spinal SEPs-MEPs were clearly defined, morphologically stable, and consistent over time under N20-methohexitone anesthesia. The present primate study may set a model to monitor both modalities in anesthetized neurosurgical patients.[Neurol Res 1999; 21: 359-367]     

运动诱发电位联合体感诱发电位监测在脊髓髓内肿瘤手术中的应用

目的 探讨全静脉麻醉下运动诱发电位(MEP)联合体感诱发电位(SEP)术中监测应用于脊髓髓内肿瘤手术的优越性、可靠性及临床应用价值.方法 对72例脊髓髓内肿瘤患者术中行SEP和MEP联合监测,参照McCormick评分标准对术前、术后脊髓功能的改变和诱发电位变化之间的关系进行统计分析.结果 14例脊髓神经功能改善,18例术后脊髓神经功能下降者与诱发电位监测结果具有一致性(P＜0.05).结论 对脊髓髓内肿瘤手术进行SEP与MEP监测有利于避免"假阴性/假阳性"结果及术后神经功能障碍的发生.     

Early somatosensory evoked potentials.

The early somatosensory evoked potential secondary to median nerve stimulation in the human had an onset latency of 9--12 msec when recorded from scalp electrodes at vertex-to-mastoid, vertex-to-inion or at the base of the skull. Similar latencies were observed from responses recorded over the cervical dorsal columns during neurologic surgery. A latency difference of 1.5 msec was observed between the early response and the responses recorded from the junction of medial lemniscus and nucleus ventralis posterior lateralis of the thalamus during human stereotaxic surgery. Cervical cord transections and transection at the midpontine levels of the monkey showed that the evoked potential was due to generators between these levels. Depth recording of the monkey indicate that the early evoked potential originates in the region of dorsal column nuclei, while the later components are secondary to generators in cerebral cortex.     

Segmentally specific somatosensory evoked potentials

The recording of segmentally specific somatosensory evoked potentials (SEPs) is time-consuming and the findings have generally been clinically unhelpful. This article critically evaluates the role of these SEPs in patients with disorders of the spinal cord or nerve roots.     

Pain-Related somatosensory evoked potentials.

The authors reviewed basic and clinical reports of pain-related somatosensory evoked potentials (SSEP) after high-intensity electrical stimulation [pain SSEP(E)] and painful laser stimulation [pain SSEP(L)]. The conduction velocity of peripheral nerves for both pain SSEP(E) and pain SSEP(L) is approximately 10 to 15 m/second, in a range of Adelta fibers. The generator sources are considered to be the secondary somatosensory cortex and insula, and the limbic system, including the cingulate cortex, amygdala, or hippocampus of the bilateral hemispheres. The latencies and amplitudes are clearly affected by vigilance, attention-distraction, and various kinds of stimulation applied simultaneously with pain. Abnormalities of pain SSEP(L) reflect an impairment of pain-temperature sensation, probably relating to dysfunction of A5 fibers of the peripheral nerve and spinothalamic tract. In contrast, conventional SSEP after nonpainful electrical stimulation reflects an impairment of tactile, vibratory, and deep sensation, probably relating to dysfunction of Aalpha or Abeta fibers of the peripheral nerve and dorsal column. Therefore, combining the study of pain SSEP(L) and conventional SSEP is useful to detect physiologic abnormalities, and sometimes subclinical abnormalities, of patients with peripheral and central nervous system lesions.     

脊椎旁刺激法体感诱发电位在脊髓病变中的临床应用

目的 探讨脊椎旁刺激法体感诱发电位（SEPs）在脊髓病变患者中进行脊髓功能判断的临床应用价值，并将诱发电位结果与脊髓MRI结果进行比较，初步探讨该方法在脊髓病变中的生理定位价值。方法 应用脊椎旁刺激、头皮记录的直接SEPs法对脊髓病变患者进行SEPs测定及脊髓传导速度（SCCV）测定。结果 共有96例脊髓病变患者行脊椎旁刺激法SEPs及SCCV测定，男53例，女43例，年龄15-75岁，平均46．78岁。其中脊髓脱髓鞘病36例，脊髓亚急性联合变性26例，脊髓病19例，急性脊髓炎10例，脊髓压迫性疾病5例。96例脊髓病变患者均做脊椎旁刺激法SEPs检查，脊髓诱发电位及SCCV的异常率在各脊髓病变中阳性率均较高，总异常率为81．25％（78／96）。其中脊髓脱髓鞘病异常率75．00％（27／36），脊髓亚急性联合变性异常率88．46％（23／26），脊髓病19例中有15例异常，脊髓炎异常率10例中有8例异常，脊髓压迫症5例均异常。共有68例患者做脊髓MRI检查，其中42例MRI异常，26例MRI正常。42例MRI异常患者中35例诱发电位结果异常，26例MRI正常患者中21例诱发电位结果异常。结论 脊椎旁刺激法SEPs及SCCV的测定客观地记录了脊髓病变的异常电生理改变，其异常电生理改变可能会出现在脊髓结构改变及影像学改变之前，可为脊髓病变患者早期脊髓功能性改变提供比较可靠的客观诊断依据，尤其在脊髓MRI少有阳性所见的营养代谢性脊髓病中意义更大。本方法简单、无创、具有可重复性，可在临床中应用。     

Topographical displays of somatosensory evoked potentials

The distribution of somatosensory evoked potentials (SEPs) after stimulation of the median nerve at the wrist was examined in 10 normal subjects using isopotential maps. The latencies of continuous negative and positive peaks were measured in each lead. The differences of the potentials at these latencies were measured in all the leads and the isopotential maps were constructed. The distribution of P0-NI was all similar. The latencies of P0 were almost the same in all the leads at about 13 msec. The distribution of NI-PI-NII was divided into three types--N16-P20-N28 localized in the frontal region, N17-P22-N30 localized in the central region and N19-P25-N33 distributed in the parieto-occipito-temporal regions. The distributions of NII-PII and PII-NIII were all similar, with high amplitudes in the central region. The latencies of PII and NIII were almost the same in all the leads at about 45 msec and 68 msec.     

Cortical evoked potentials and somatosensory perception in chronic spinal cord injury patients

The correlation between somatosensory evoked potentials (SEPs) and sensory perception was studied in 110 patients with traumatic chronic spinal cord lesions. Perception thresholds over the legs for light touch, vibratory sensibility, temperature and thermal pain were tested together with recordings of tibial and peroneal SEPs. Tibial nerve SEPs correlated better with sensory perception than peroneal nerve SEPs. Normal tibial nerve SEPs were not present with absent or trace vibratory sensibility and vice versa. However, we found many exceptions to the correlation between temperature and pain perception and SEPs. Light touch, vibratory sensibility, and SEPs were highly correlated between each other, while temperature and pain perception correlated poorly to these other modalities. This represents an evident segregation of touch perception, vibratory sensibility and SEPs, which are thought to share dorsal columns as a common ascending pathway, and temperature and pain perception known to be related to the spinothalamic system.     

Monitoring spinal cord motor and somatosensory evoked potentials in anesthetized primates.

Monitoring Motor Evoked Potential (MEP) to Transcranial Stimulation (TMS) monitoring (MEP) is a growing technique to assess motor function under anesthesia. The following primate study was conducted to analyze the non-myogenic spinal motor and sensory volleys and to examine their reproducibility under nitrous oxide-methohexidone anesthesia. The traveling periodic spinal descending MEP to TMS and ascending somatosensory (SEP) to posterior tibial nerve stimulation across the thoracic cord were recorded in 12 cynomolgus monkeys. Through a small T11-T12 laminotomy, an insulated stainless steel electrode was inserted into the epidural thoracic space. The potentials were analyzed under 50 vol% NO in O2 with methohexital (0.1-0.2 mg kg-1 min-1). A well-defined periodic TMS-MEPs and PTN-SEPs were recorded with high reproducibility and consistency in repeated trials under N2O-methohexital anesthesia. MEP tracing consisted of an initial peak (direct (D) wave), occurring at 2.43 (+/- 0.28) msec followed by subsequent five positive (indirect (I) waves). Spinal SEPs-MEPs were clearly defined, morphologically stable, and consistent over time under N2O-methohexitone anesthesia. The present primate study may set a model to monitor both modalities in anesthetized neurosurgical patients.     

Maturation of cerebral somatosensory evoked potentials

Cerebral somatosensory evoked potentials (SEPs) were elicited by stimulation of the median nerve and/or posterior tibial nerve in 117 children of 1 day to 16 years old. A major negative wave (N) was consistently recorded from the parietal region of the scalp when the arm was stimulated. The peak latency, the onset latency, the rising time and the duration of H wave are closely correlated with age and body length. The latencies are shortest in the subjects of 1-3 years old. SEPs to lower extremity stimulation were inconstant in the infants before the age of one. The major positive wave (P) has a variable topographic distribution along the middle line, over the scalp. The latencies are also very variable in the different subjects of the same age as well as in the same subject with different locations of active electrode. Among the parameters studied as for N wave, only the rising time of P wave is significantly correlated with age. The latencies of P wave have the shortest value in the subjects of 1-3 years old. The comparison of SEPs to upper and to lower limb stimulations shows that there is no relationship between them in respect to their morphology and amplitude. The minimum value of the latencies of N and P waves was observed at the same age but the difference between the peak latencies of P and N waves in the same subject increases considerably after 2 years of age and reaches the adult value after 5 years of age. These resultats indicate that the maturation of the peripheral somatosensory pathways proceeds at a higher rate than that of the central somatosensory pathways, that the maturation of the somatosensory pathways of the upper limb precedes that of the lower limb, and that the rising time of N or P waves is a good index of cortical maturation. The clinical utility of these SEPs in pediatrics is discussed.     

Intersession stability of somatosensory evoked potentials

Upper and lower limb nerve somatosensory evoked potentials (SEPs) were recorded from a group of normal adults in order to assess their intersession stability. Median, peroneal and sural nerve SEPs were recorded from each subject on 3 successive occasions at weekly intervals. For median nerve SEPs, the intrinsic variability was more pronounced for interpeak latencies (N9-N20, N13-N20) than for the latencies of individual components. Their inherent fluctuation was not, however, notably greater for the primary cortical response (N20) than for the more caudally generated components (N9 and N13). These relationships were not so apparent for lower limb SEPs where intersession variability was markedly greater overall than for upper limb SEPs.