The effect of changing stimulus intensities on median nerve somatosensory-evoked potentials

We investigated median nerve somatosensory-evoked potentials (SEP) in 31 healthy volunteers to test the hypothesis that 1) increasing stimulus intensity influences SEP components in both amplitude and latency 2) SEP components respond differently to changing intensities. Cluster analysis and analysis of variance were used for statistical testing. Three groups of components could be found according to latency changes in response to increasing stimulus intensities: N13, and P15, the primary cortical response (N19, P22) and the components over 30 ms. In general, SEP components below 30 ms significantly shortened in latency and increased in amplitude with subsequent saturation. In contrast, in components over 30 ms latencies decreased linearly and amplitudes changed inhomogeneously. The clear effect of stimulus intensity on most median nerve SEP components makes it necessary to maintain comparable stimulus intensities when comparing intra- and interindividual registrations.     

High frequency vibration induced gating of subcortical and cortical median nerve somatosensory evoked potentials: different effects on the cervical N13 and on the P13 and P14 far-field SEP components.

Subcortical and cortical somatosensory evoked potentials (SEP) to median nerve stimulation were recorded before, during and after high frequency (270 Hz) vibration of the fingers 1-3 in 8 healthy subjects. A marked decrease of the amplitude of all potentials was observed. The attenuation of the sensory nerve action potential (SNAP) of the median nerve and the attenuation of SEP components N9, N11 and N13 showed no differences, while the attenuation of the subcortical P14 component was significantly higher. This is in accordance with a generator of the cervical N13 in the interneurons beside the lemniscal pathway. The cortical N20 (post-rolandic) was significantly more decreased in amplitude than P14 while P22 (pre-rolandic) remained reduced in amplitude like P14. An increased latency of the far-field subcortical P14 was observed, while P13 recorded in the same montage remained unchanged in latency. These findings suggest different generators of these peaks. A generator of P14 above the nucleus cuneatus is confirmed. A presynaptic generator of P13 is suspected.     

65例颈椎型脊髓病体感诱发电位分析

目的研究颈椎型脊髓病皮质体感诱发电位（ＳＥＰ）变化。方法对６５例颈椎型脊髓病患者和２６例正常人进行正中神经和胫后神经刺激的ＳＥＰ对照研究，并对１０例患者作治疗前后对照观察。结果本组异常率为４５％，主要表现为各波替伏期和波间期（Ｎ２０—Ｐ２５，Ｐ２５—Ｎ３５，Ｐ４０—Ｎ４５）延长，且下肢的延长更加明显，部分患者出现波形分化不良。经保守治疗后６例正常，２例好转，且ＳＥＰ的好转先于临床的改善。结论ＳＥＰ对评判颈椎型脊髓病的脊髓传导功能具有重要的意义，且有助于临床预后的评价。     

肝硬化失代偿期病人体感诱发电位观察

目的 :探讨肝硬化失代偿期病人的神经功能状态。方法 :对 30例肝硬化失代偿期病人进行体感诱发电位 (SEP)检查 ,刺激部位均为右腕正中神经。结果 :肝硬化失代偿期病人的SEP有着不同程度的改变 ,其中P15、N2 0 、P2 5、N3 5的潜伏期延长和N13 -P15的波间期延长分别占本组异常数的 85 %、40 %、35 %、2 5 %和 2 0 %。这些值与正常人组相比较 ,其两组间差异均有极显著性意义 (P <0 0 1)。结论 :SEP可为研究亚临床肝性脑病提供电生理学的客观依据。     

Median sep and blink reflex in thyroid diseases

Pathological disturbances of thyroid hormones is associated with central and peripheral nervous system disturbances. The aim of this study is to evaluate median nerve stimulated somatosensory evoked potential (SEP) and blink reflex of thyroid patients (hypo and hyperthyroidism). Median SEP was performed in 40 patients (21 with hyperthyroidism and 19 with hypothyroidism). We evaluated the latencies of N9, N11, N13, P9, P11, P14, N20 and P25 waves and the N9-N20, N9-N13, N13-N20 and P14-N20 interpeak latencies. We compared the results of patients with the control group (26 persons). We found that the N20 latency was longer in patients with hyperthyroidism than in the control group and the difference was statistically significant. There was not any statistically significant difference regarding the N9, N11, N13, P9, P11, P14, N20 and P25 latencies and the N9-N20, N9-N13, N13-N20 and P14-N20 interpeak latencies between hypothyroid patients and controls. We performed the blink reflex study in 28 of 40 patients (14 patients with hyperthyroidism and 14 patients with hypothyroidism). Comparing the R1, R2, CR2 (contralateral R2) latencies and durations of the patients and controls, we found that R2 and CR2 duration was shorter in patients with hyperthyroidism. This difference was statistically significant.     

Dipole-tracing of 'awareness' attenuating the cortical components of somatosensory evoked potentials

Using the dipole-tracing method, the source generators of N18, P22 and P40 of the somatosensory evoked potential (SEP) were estimated as the equivalent dipole. After voluntary action of the thumb flexion, no changes were observed in N18 or P40, but the amplitude of P22 was suppressed. The after-effects of intention accompanied by a voluntary action or the subject's awareness that electrical stimulation will be given after the voluntary action were treated as 'awareness'. By subtracting the pure SEP from SEP during 'awareness', it was found that the equivalent dipole of 'awareness' of P22 was located at the same region of pure P22, but the vector was of opposite orientation. 'Awareness' attenuated the perceptive potential of SEP like P22 generated in the cortex.     

Human brain cortical correlates of short-latency afferent inhibition: a combined EEG-TMS study

When linking in time electrical stimulation of the peripheral nerve with transcranial magnetic stimulation (TMS), the excitability of the motor cortex can be modulated to evoke clear inhibition, as reflected by the amplitude decrement in the motor-evoked potentials (MEPs). This specific property, designated short-latency afferent inhibition (SAI), occurs when the nerve-TMS interstimulus interval (ISI) is approximately 25 ms and is considered to be a corticothalamic phenomenon. The aim of the present study was to use the electroencephalographic (EEG) responses to navigated-TMS coregistration to better characterize the neuronal circuits underlying SAI. The present experimental set included magnetic resonance imaging (MRI)-navigated TMS and 60-channel TMS-compatible EEG devices. TMS-evoked EEG responses and MEPs were analyzed in eight healthy volunteers; ISIs between median nerve and cortical stimulation were determined relative to the latency of the individual N20 component of the somatosensory-evoked potential (SEP) obtained after stimulation of the median nerve. ISIs from the latency of the N20 plus 3 ms and N20 plus 10 ms were investigated. In all experimental conditions, TMS-evoked EEG responses were characterized by a sequence of negative deflections peaking at approximately 7, 44, and 100 ms alternating with positive peaks at approximately 30, 60, and 180 ms post-TMS. Moreover, ISI N20+3 ms modulated both EEG-evoked activity and MEPs. In particular, it inhibited MEP amplitudes, attenuated cortical P60 and N100 responses, and induced motor cortex beta rhythm selective decrement of phase locking. The findings of the present experiment suggest the cortical origin of SAI that could result from the cortico-cortical activation of GABAergic-mediated inhibition onto the corticospinal neurons modulated by cholinergic activation able to reducing intralaminar inhibition and promoting intracolumnar inhibition.     

Multichannel derived median nerve SEP compared to EEG in patients with vascular cerebral lesions.

In order to compare the sensitivity of multichannel derived median nerve SEP with EEG in vascular cerebral lesions we examined 22 normals and 23 patients. SEP components within the first 50 ms could be divided into main waveform patterns: (1) a W-shaped parietal pattern consisting of N20, P25, N35 and P45 in most cases. (2) a frontal pattern with P20 and N30 as well as possibly detectible N24, P28, P33, N40 and P50. (3) a central P22. Two younger normals showed a V-shaped parietal pattern with N20 and P35, a frontal pattern with P20 and N36, and central P22 with a remarkably long latency. All components could be analysed sufficiently by means of three representative electrode positions (stimulation right/left): P3/P4, C3/C4, and F3/F4, which reduces the expense of recording and analysing considerably. 21 patients (91.3%) showed abnormal results in SEP, whereas 14 patients (60.9%) in EEG. A three channel electrode array can increase the usefulness of SEP and detect cerebral dysfunctions in cerebral lesions in spite of normal EEG under routine examination conditions. Analysis of multichannel derived SEPs during treatment and recovery after stroke and search for the prognostic value in the acute stage of the disease should be done in future.     

Altered cortical integration of dual somatosensory input following the cessation of a 20 min period of repetitive muscle activity

The adult human central nervous system (CNS) retains its ability to reorganize itself in response to altered afferent input. Intracortical inhibition is thought to play an important role in central motor reorganization. However, the mechanisms responsible for altered cortical sensory maps remain more elusive. The aim of the current study was to investigate changes in the intrinsic inhibitory interactions within the somatosensory system subsequent to a period of repetitive contractions. To achieve this, the dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was utilized in 14 subjects. SEPs were recorded following median and ulnar nerve stimulation at the wrist (1 ms square wave pulse, 2.47 Hz, 1× motor threshold). SEP ratios were calculated for the N9, N11, N13, P14–18, N20–P25 and P22–N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar (MU) stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median (M) and ulnar (U) nerves. There was a significant increase in the MU/M + U ratio for both cortical SEP components following the 20 min repetitive contraction task, i.e. the N20–P25 complex, and the P22–N30 SEP complex. These cortical ratio changes appear to be due to a reduced ability to suppress the dual input, as there was also a significant increase in the amplitude of the MU recordings for the same two cortical SEP peaks (N20–P25 and P22–N30) following the typing task. No changes were observed following a control intervention. The N20 (S1) changes may reflect the mechanism responsible for altering the boundaries of cortical sensory maps, changing the way the CNS perceives and processes information from adjacent body parts. The N30 changes may be related to the intracortical inhibitory changes shown previously with both single and paired pulse TMS. These findings may have implications for understanding the role of the cortex in the initiation of overuse injuries.     

Changes in excitability of the cortical projections to the human tibialis anterior after paired associative stimulation

Paired associative stimulation (PAS) based on Hebb's law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of this study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 min with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions (n = 5). Subsequently, PAS was applied (n = 14, ISI = 55 ms) with the TA relaxed or dorsi-flexing. Finally, an optimized ISI based on the subject somatosensory evoked potential (SEP) latency plus a central processing delay (6 ms) was used (n = 13). Motor-evoked potentials (MEPs) were elicited in the TA before and after the intervention, and the size of the TA MEP was extracted. ISIs of 45, 50, and 55 ms increased and 40 ms decreased TA MEP significantly (P = 0.01). PAS during dorsi-flexion increased TA MEP size by 92% (P = 0.001). PAS delivered at rest resulted in a nonsignificant increase; however, when the ISI was optimized from SEP latency recordings, all subjects showed significant increases (P = 0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased after PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.     

Sensory nerve somatosensory evoked potentials (SEP) in the evaluation of patients with sciatica: false P1 latency prolongation may be due to admixture of dermatomal SEP.

Patient-reported stimulus-related radiating sensory symptoms within the territory of the stimulated nerve have been used to verify stimulation in sensory nerve scalp recorded somatosensory evoked potentials (SEP). The main aim of the present study of false positive P1 latency prolongation in lumbosacral sensory nerve SEP was to investigate whether elicitation of such symptoms secures adequate sensory nerve stimulation. Nerve roots were studied on the asymptomatic side in 64 patients with unilateral sciatica. Saphenous (L4), superficial peroneal (L5), and sural (S1) nerve SEP were registered in all patients. Pretibial dermatomal SEP were registered in ten of them. Stimulation was equidistant from the registration electrode in all SEP registrations. The false positive rate was lower in saphenous nerve SEP with than without verified supramaximal stimulation (1/30 vs. 6/22, P = 0.03) in spite of radiating stimulus-related sensory symptoms in both groups. This difference was not caused by subclinical myelographic nerve root compression or general peripheral nerve dysfunction. The P1 latency was longer in the pretibial dermatomal SEP than in the saphenous and superficial peroneal nerve SEP with the same conduction distance (mean difference 4.7 (95% CI = 3.8 to 5.6) and 4.4 ms (95% CI = 3.4 to 5.4), respectively). It is concluded that dermatomal SEP have longer P1 latency than sensory nerve SEP. Verified supramaximal nerve simulation is recommended to avoid false results due to admixture of dermatomal to sensory nerve SEP.     

一种新型慢性压迫性颈脊髓症大鼠模型的建立

目的建立一种新型慢性压迫性颈脊髓症动物模型。方法 20只SD大鼠经颈后路手术、于C5～6水平椎板下植入吸水性聚氨酯胶片,植入体内逐渐吸水胀大,形成对脊髓的慢性持续压迫。术后6个月,进行体感诱发电位(SEP)、微焦点CT(Micro-CT)、组织学(H-E染色)和组织化学(LuxolFastBlue,LFB染色)等检测。结果 20只造模大鼠脊髓侧后方出现明显压迫性形态学改变,Micro-CT显示脊髓灰质和白质扭曲变形,14只出现SEP异常,其中波幅减低5例、潜伏期延长5例、二者均异常4例。SEP反应异常者与正常者组织学、组织化学比较,脊髓后索髓鞘染色显著减少(102±13vs.138±7;P0.05),脊髓后索的对比剂密度有显著性差异(98±5vs.88±6;P0.05),后角内神经元也明显较少(23±8/mm2vs.27±6/mm2;P0.05);造模大鼠脊髓出现基质海绵样变和静脉扩张、血管增生。结论该方法提供了一种操作方便、稳定性好、成功率高、评价指标完整的慢性压迫性脊髓症动物模型。     

Non-Painful and Painful Surface and Intramuscular Electrical Stimulation at the Thenar and Hypothenar Sites: Differential Cerebral Dynamics of Early to Late Latency SEPs

Little is known about somatosensory evoked potentials (SEPs) from muscle stimulation compared to that from skin stimulation. The current study examined this issue in the full SEP spectrum (0 - 440 ms). The aims of the study were to (1) establish the dynamics of early to late latency SEPs from intramuscular stimulation in contrast to surface stimulation, (2) compare the effect of non-painful and painful stimuli on SEP latencies and amplitudes of the two methods, and (3) investigate to which extent these results can be shared between the median nerve innervated thenar site and ulnar nerve innervated hypothenar site. Stimuli were delivered (2 Hz) at a non-painful and a painful intensity above or within the thenar and hypothenar muscles of the hand. Maximas of the SEPs were extracted by a combination of global field power and visual inspection of the topographies. Amplitudes and latencies of the maximas were analysed by a two-way ANOVA with repeated measures. In the early phase (0 - 50 ms) the topographic patterns showed different dynamics between surface and intramuscular stimulation and in the late phase (100- 440 ms) prolonged latencies were found for intramuscular stimulation. Apart from this, similar topographic patterns and time sequences were obtained. Significant higher SEP amplitudes for most of the isolated components (C4'/P25, Fz/N35, C4'/P45, Fc2/N65, P4/P90, T4/N137, F3/P150, Cz/P240-P270) were found with surface stimulation compared to intramuscular stimulation. In contrast to surface stimulation, intramuscular stimulation at a stimulation frequency of 2 Hz did not result in a differentiation in amplitude for any of the isolated components. These results indicate differences in the early and late processing of sensory input from skin and muscle.     

脑瘫病人选择性后根切断术的体感诱发电位、神经传导速度检测

报告２０例脑瘫病人选择性后根切断术（ＳＰＲ）治疗前后体感诱发电位（ＳＥＰ）、运动神经传导速度（ＭＣＶ）检测结果。ＳＥＰ较术前潜伏期缩短者在ＥＢ点为６３％、ＦＯ点４１％、Ｎ１点４１％、Ｐ１点４１％、Ｎ２点５０％，神经传导速度亦较术前明显缩短。对ＳＰＲ的机制进行了讨论。     

体感诱发电位磁共振对脊髓炎的定位诊断价值对比研究

目的：比较体感诱发电位(SEP)和脊髓磁共振(MRI)检查对视神经脊髓炎(NMO)和急性横贯性脊髓炎(ATM)病人进行定位诊断的价值。方法：对21例临床确诊为NMO和36例ATM病人的SEP和MRI检查结果进行回顾性分析，比较其定位符合率。结果：①ATM组病人MRI以胸髓损害为主，NMO组病人以颈胸段脊髓联合损害为主；②ATM组病人SEP主要表现为N20正常、P40异常，而NMO组病人则表现为N20和P40均异常；当病变位于颈髓时，NMO组SEP与MRI的定位符合率为94．1％，ATM组为78．6％；当病变位于胸髓时，ATM组为100％，NMO组为66．7％。结论：在NMO和ATM病人中，SEP提示的神经电生理异常部位基本与MRI所证实的病变部位相吻合。     

The effect of stimulus intensity on visual evoked potential estimates of interhemispheric transmission time

Summary Visual evoked potentials (VEPs) to bright or dim lateralised light flashes were recorded from homologous occipital and central sites. In a GO/NOGO reaction time task (Experiment 1) the latency of the N160 component of the VEP was found to be shorter from the contralateral hemisphere by approximately 16 ms at occipital sites, but only 3 ms centrally. In addition, there was a trend for the occipital contralateral latency advantage to increase with decreasing stimulus brightness. In Experiment 2 a wider intensity range and a simple visual reaction time task were employed. Contralateral N160 latency advantages were again found to be larger occipitally (approx 13 ms) than centrally (3 ms). Furthermore the occipital contralateral latency advantage was significantly increased at the lower stimulus intensity, while that from central sites remained constant. These data suggest that two types of interhemispheric relay can be distinguished — a sensory one recorded occipitally and a non-sensory one recorded from central sites.     

椎基底动脉供血不足家兔模型体感诱发电位分析

目的：研究椎基底动脉供血不足(VBI)时体感诱发电位(SEP)的动态变化及其诊断意义。方法：选择成年家兔10只，通过结扎家兔一侧椎动脉人为造成VBI模型，以N27、P40峰潜期及峰间期为指数，动态检测其右侧胫后神经SEP的变化，研究其变化规律。结果：VBI后SEPN27，p40的峰潜伏期明显延长，峰间期无明显变化。结论：SEP对VBI发作期感觉功能的变化有诊断价值，并可评价发作的严重程度。