Origin of the early components of somatosensory evoked potentials by the posterior tibial nerve stimulation--a comparative study of median nerve stimulation in clinical cases

In order to determine the generation sites of short latency somatosensory evoked potentials to the posterior tibial nerve stimulation, scalp topography was performed on 10 normal subjects in the two different band-pass recordings, i.e., wide band-pass filter (5-3000 Hz) and narrow band-pass filter (100-1000 Hz). Furthermore, comparative study of the changes of evoked potentials between posterior tibial nerve stimulation and median nerve stimulation was carried out in 22 cases with well localized lesion of the central nervous system in the same wide band-pass filter setting. The early components of somatosensory evoked potentials elicited by the posterior tibial nerve stimulation were obtained as P 30, N 34, and P 38 in the wide band-pass filter, and P 29, N 32, P 36 in the narrow band-pass filter. Components P 30, N 34 and components P 29, N 32 were widely distributed on the scalp, but were disappeared on the scalp-scalp recording. These results suggested all those components were generated from the deep subcortical structures. In the case with high cervical lesion, component P 30 at the posterior tibial nerve stimulation was remarkably prolonged in latency, and component P 13 at the median nerve stimulation was disappeared. P 30-N 34 interpeak latency at the posterior tibial nerve stimulation was prolonged in the case with pontine lesion, while P 13-N 16 interpeak latency at the median nerve stimulation was also prolonged. In the cases with thalamic and internal capsular lesion, P 30 and N 34 at the posterior tibial nerve stimulation and P 13 and N 16 at the median nerve stimulation were all preserved in normal range. These results revealed that components P 30 and N 34 were almost identical to components P 13 and N 16, respectively. On the other hand, component P 38 at the posterior tibial nerve stimulation was suppressed or disappeared in the cases with well localized lesion at the midcentro parietal region, that includes the primary foot sensory area.(ABSTRACT TRUNCATED AT 400 WORDS)     

The significance of delayed long-loop responses to ankle displacement for the diagnosis of multiple sclerosis

The present study compares the results of sensory evoked potentials after stimulation of the tibial nerve with measurements of short (M1), medium (M2) and long-loop latency (M3) responses of leg muscles in 42 patients suffering from multiple sclerosis. EMG responses were elicited by a movable platform which was tilted in pitch toe-up around the subject's ankle joint. The short latency response of the triceps surae muscle was nearly always normal in latency. The M2 response was lacking in about 30% of the patients and 10% of normals. The long latency response (M3) in the antagonistic anterior tibial muscle was significantly delayed (beyond 162 msec = AM + 2.5 S) in 69% of the patients. The delayed M3 response was found to indicate demyelination as reliably as the delayed somatosensory evoked potential (66% of the patients). The frequency of coincident results in the entire group was 86%. Additional information is gained in cases where SEPs are normal, but M3 is delayed (7%). Furthermore, patients lacking P40 of the SEP still exhibit M3 responses, the latency of which may be measured (24%). The results favour the assumption that M3 is mediated by a supraspinal pathway.     

Middle-latency somatosensory evoked potentials following median and posterior tibial nerve stimulation in Down's syndrome.

Middle-latency somatosensory evoked potentials (SEPs) following median and posterior tibial nerve stimulation were studied in 40 patients with Down's syndrome and in age- and gender-matched healthy controls as well as in middle-aged and aged healthy subjects. In median nerve SEPs, latencies of the initial cortical potentials, N18 and P18, showed no significant difference, but the following potentials N22, P25, N32, P41 and P46 were relatively or significantly shorter in latency in Down's patients than in the controls. Amplitudes of all components in Down's patients were significantly larger than those of age- and gender-matched controls as well as of those of middle-aged healthy subjects, but there was only a small difference in their amplitudes from aged healthy subjects. Results of posterior tibial nerve SEPs were generally consistent with those of median nerve SEPs. Therefore, 'short latency with large amplitude' is the main characteristic of middle-latency SEPs in Down's syndrome, possibly related to accelerated physiological aging of the central nervous system.     

The early negative potential evoked by stimulation of the tibial nerve in man

The scalp response to stimulation of the tibial nerve at the level of the medial malleolus was systematically analysed. It was recorded 2 cm posterior to the vertex and at the sites corresponding to cortical representation of the hand. The existence of an early negative wave with a peak latency of 37.2 ± 2.29 ms and amplitude of ?0.69 ± 0.40 μV was established (being half the amplitude of the first positive wave (P40) over the vertex). This wave was named N37 in respect of the peak latency and polarity. N37 was the first event recorded after stimulation of the tibial nerve at this level as the onset latency was 32.2 ± 1.75 ms and that of P40 over the vertex 33.8 ± 2.28 ms. It was recorded with the highest amplitude over the hand primary somatosensory area after stimulation of the opposite foot.N37 evoked by stimulation of the tibial nerve at the ankle and N20 evoked by stimulation of the arm nerve are both the primary negativities of the evoked potential. However, N37 is not recorded with maximum amplitude over the leg primary somatosensory area and it is rounded and longer lasting than N20. In spite of these differences the two initial negative electrical phenomena are not necessarily generated by different functional structures. The possible generators of N37 are discussed.     

椎旁定位腰骶体感诱发电位对腰椎间盘突出症的诊断意义

目的探讨椎旁定位腰骶体感诱发电位（LSSEP）在腰椎间盘突出症中的变化及其诊断意义。方法对84例有L4、L5和S1神经根损害表现的腰椎间盘突出症的患者进行胫后神经体感诱发电位（SEP）、皮节体感诱发电位（DSEP）和椎旁定位LSSEP检查，测定其N40的潜伏期，并对多水平突出症患者的LSSEP与腰椎MRI比较。结果胫后神经SEP检测异常率为42．86％；L5、S1 DSEP检测的异常率为90．48％；椎旁定位LSSEP异常率为95．24％，比胫后神经SEP检测异常率明显增高（P〈0．005）。在L3。椎间盘突出中，以L4的LSSEP异常为主；在L4-5椎间盘突出中以L5的LSSEP为主。结论椎旁定位LSSEP检测可能是诊断腰椎间盘突出症快捷、可靠、敏感的检测方法，其结果与神经根受累水平相一致。     

Ultrastructural findings in Lafora disease

The examination of somatosensory evoked potentials following trigeminal nerve stimulation in 20 subjects with idiopathic trigeminal neuralgia revealed a pathological increase in latency of the first positive component on the involved siEN in 7 (41%) of the 17 patients without anteceENnt surgical treatment. The mean difference in latency between the two siENs was 1.3 msec (2p ≥ 0.005). These results are indicative of microtraumatization of the trigeminal nerve in the posterior fossa by blood vessels, for example, leading to local ENmyelinization. The method seems to be of diagnostic value in selecting patients for nerve ENcompression by posterior fossa operations.     

Serial activation of distinct cytoarchitectonic areas of the human S1 cortex after posterior tibial nerve stimulation.

MEG recordings visualized non-invasively a dynamic anterior-posterior activation in the pyramidal cell population of the human primary somatosensory cortex (S1) after posterior tibial nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the foot area in response to right posterior tibial nerve stimulation at the ankle in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources in the mesial wall of the left hemisphere around the primary P37m response typically separated by 1.3 cm. The first source was located in area 3b and oriented toward the contralateral hemisphere. The second source was assumed to be in an area near the marginal sulcus and the source orientation was directed posteriorly. The first source began to be active during the initial slope of the P37m. The second source was active after the P37m peak and the signal intensities of the first and second sources were equal at a mean latency of 2.6 ms after the peak of P37m. Then the first source became inactive and the second source was dominant after about 5 ms post-P37m peak. These findings suggest that a single peaked posterior tibial nerve P37m consists of partially overlapping two subcomponents generated in area 3b and an area near the marginal sulcus.     

Dermatomal/segmental somatosensory evoked potential evaluation of L5/S1 unilateral/unilevel radiculopathies

Dermatomal and segmental somatosensory evoked potentials (SEPs) have been reported to be of diagnostic utility in unilateral/unilevel L5 and S1 radiculopathies. This investigation employs history, physical examination, imaging studies, and electrodiagnostic medicine evaluations to clearly define unilateral/unilevel L5 or S1 nerve root compromise. Inclusion criteria require all of the preceding diagnostic methods to corroborate a specific nerve root lesion. Regression equation analysis for cortical P1 latencies evaluating age and height based on comparable patient and control reference populations reveals segmental and dermatomal sensitivities for L5 radiculopathies to be 70% and 50%, respectively, at 90% confidence intervals. Similar sensitivities are obtained for 2 standard deviation mean cortical P1 latencies. Side-to-side cortical P1 latency difference data reveal segmental and dermatomal sensitivities for L5 radiculopathies to be 40% at 2 standard deviations. Regression equation analysis for age and height regarding segmental and dermatomal studies for S1 radiculopathies reveal sensitivities of 30% and 20%, respectively, at 90% confidence intervals. Similar data are delineated for 2 standard deviation mean cortical P1 latencies. Side-to-side cortical P1 latency difference data reveal segmental and dermatomal sensitivities for S1 radiculopathies to be 50% and 10%, respectively, at two standard deviations. The clinical utility of both segmental and dermatomal SEPs are questionable in patients with known unilateral/unilevel L5 and S1 nerve root compromise. © 1996 John Wiley & Sons, Inc.     

The influence of interfering input from the peroneal nerve on tibial-nerve somatosensory evoked potential.

Using a conditioning-test paradigm, we studied the recovery function of tibial nerve somatosensory evoked potentials (SEPs) conditioned by preceding peroneal nerve stimulation. The inter-stimulus intervals (ISIs) ranged from 0 to 400 msec, where 0 msec indicated simultaneous arrival of tibial and peroneal nerve volleys at the L1 spine. The recovery curve was W-shaped, showing two peaks of SEP suppression, maximum at 6 msec ISI (1st phase) and 50-75 ISI msec (2nd phase). In the 1st phase suppression, we found distinct differences in wave forms between 0-2 msec ISI and 4-6 msec ISI. At 0-2 msec ISI, P40-N50-P60 amplitude decreased and latencies shortened, while P31 and N35 were unchanged. At 4-6 msec ISI, all peaks, possibly excluding P31, were markedly depressed. We attribute the former change to an "occlusive effect" and the latter to an "inhibitory effect," each mediated via a central synaptic network between the two nerves. The attenuation of the 2nd but not the 1st phase suppression by peroneal nerve block distal to the stimulating electrodes provided evidence that the 2nd phase suppression resulted primarily from interfering afferent signals generated by peroneal nerve peripheral receptors, activated by foot movement.     

Vibration-induced inhibition of the early components of the tibial nerve somatosensory evoked potential is mediated at a spinal synapse.

OBJECTIVES: To investigate whether afferent-induced suppression of cortical somatosensory evoked potentials (SEPs) occurs at a spinal site along the transmission route of afferent signals from the tibial nerve to the primary somatosensory cortex. METHODS: Evoked potentials were recorded at 4 points (sciatic nerve, L5, C1, and cortex) along the path of transmission following electrical stimulation of the tibial nerve in halothane-anesthetized cats. The amplitudes of evoked potentials sampled during vibration of quadriceps were compared to evoked potentials sampled without the vibration. RESULTS: The spinal SEP recorded at C1 and the cortical SEP were both substantially reduced by patellar tendon vibration. The L5 spinal SEP and the sciatic nerve potential were unaffected. Vibration of quadriceps did not influence the latency of the evoked potentials. CONCLUSIONS: These results indicate that afferent-induced suppression of the initial complex of the SEP can be mediated at a spinal synapse.     

Reliability of dermatomal somatosensory-evoked potential in the evaluation of lumbosacral nerve root injury*☆ A concurrent case-control study

BACKGROUND： It has been reported that dermatomal somatosensory evoked potential （DSEP） can be used for diagnosing nerve root injury in patients with lumbar disc herniation （LDH）, and that 83% 95% of patients suffer from the disease. Body height correction is not performed prior to determinations of latency and latency difference between the healthy and affected sides. However, latency noticeably correlates to body height. OBJECTIVE： This study aims to determine the lumbosacral nerve root injury in patients with LDH by DSEP, and to evaluate the sensitivity of the DSEP difference between the healthy and affected sides using a diagnostic index following body height correction. DESIGN： A case-control observation. SETTING： Department of Orthopedic Surgery, Hainan Provincial People＇s Hospital. PARTICIPANTS： Ninety-six patients, comprised of 67 males and 29 females, with an average age of 43 years and a mean body height of 1.65 m （range 1.48-1.81 m）, were recruited for this study. These patients suffered from unilateral lower limb radiation pain and received treatment at the Department of Orthopedic Surgery, Hainan Provincial People＇s Hospital between January 2004 and December 2006. All patients were confirmed to suffer from LDH at the L3-4, L-5, and/or Ls-SI by CT and/or MRI examinations. Central nervous system diseases were excluded. In order to obtain a normal reference value, DSEP was determined for a group of 50 subjects, who concurrently received health examinations in the same department. The subjects had no previous history of back leg pain or nervous system disease. The group of healthy controls included 26 males and 24 females, with an average age of 37 years and a mean body height of 1.63 m （range 1.50-1.80 m）. Written informed consent was obtained from all subjects for laboratory samples. The protocol was approved by the Hospital＇s Ethics Committees. DSEP was determined with myoelectricity-evoked potential equipment （Keypoint, Batch No. 9020A0042591, Dantec Company, Denmark?     

Cerebral magnetic responses to stimulation of tibial and sural nerves

We report somatosensory evoked magnetic fields (SEFs) to stimulation of the mixed posterior tibial nerve (PTN) and the sensory sural nerve (SN) in 6 healthy subjects. The first peak of the responses occurred at 39-50 ms (P40m), with a 2-3 ms longer latency for SN than PTN stimulation. Within 200 ms several other deflections followed, with interindividually varying waveforms and latencies. Magnetic field mappings indicated that the source of P40m for PTN can be modelled by a single equivalent current dipole at the primary sensorimotor cortex; for the smaller responses to SN stimulation the single dipole model was less applicable. Field patterns for later deflections differed from those of P40m, indicating that several current sources within or near the primary foot projection area are sequentially activated after stimulation of both a mixed and a sensory lower limb nerve. These late deflections could not always be satisfactorily explained by single current dipoles, suggesting more complex geometries for the underlying neural activity.     

Selectivity of attenuation (i.e., gating) of somatosensory potentials during voluntary movement in humans

Attenuation of somatosensory evoked potentials (SEPS) recorded from the scalp during voluntary movement occurs for specific combinations of the finger moved and the peripheral nerve stimulated. The cerebral potential component occurring at a latency of 27 msec (P27) evoked either by stimulation of median nerve at the wrist or by stimulation of 1st and 2nd digit nerves in the fingers were selectively attenuated during movement of 1st digit but were not altered during movement of 5th digit. By contrast, the cerebral P27 component evoked by stimulation of ulnar nerve at the wrist or by stimulation of 5th digital nerve were attenuated during movement of that digit but were not altered during movement of 1st digit. Gating of somatosensory activity is a selective phenomenon occurring when movement involves the areas being stimulated.     

Evoked potentials in HIV infection]

The study of the literature data on the multimodal evoked potentials in HIV infected patients shows many abnormalities as well in asymptomatic subjects without AIDS as in AIDS subjects with or without neurological signs. Visual evoked potentials (VEPs) reveal prolonged P100 wave latency in 22% of HIV asymptomatic subjects and in 26% of HIV symptomatic subjects; brainstem auditory evoked potentials (BAEPs) reveal an increase of the interpeak latency I-V in 16% of asymptomatic subjects and in 32% of symptomatic subjects; somatosensory evoked potentials (SEPs) by median nerve stimulation reveal prolonged central conduction time in 6% of asymptomatic subjects and in 11% of symptomatic subjects; somatosensory evoked potentials (SEPs) by tibial nerve stimulation reveal prolonged central conduction time in 4% of asymptomatic subjects and in 45% of symptomatic subjects; motor evoked potentials (MEPs) by magnetic stimulation reveal prolonged central motor conduction time in 46% of asymptomatic subjects.     

High-frequency magnetic oscillations evoked by posterior tibial nerve stimulation

Magnetocephalographic recordings of the primary somatosensory response (P37m) and high-frequency oscillations (HFOs) evoked by posterior tibial nerve stimulation were obtained in normal subjects. Electrical stimuli were delivered to the posterior tibial nerve and magnetic recordings were taken over the superior aspect of the left hemisphere with a 37-channel biomagnetometer. In order to separate the high-frequency oscillations from the underlying P37m, the wide-band (0.1-1200 Hz) recorded responses were digitally filtered with a 500-800 Hz band-pass filter. The localization of the HFOs were estimated to be in somatosensory area 3b, very close to the P37m source. Our data suggest that the HFOs are somatotopically arranged in the primary somatosensory cortex, and are a ubiquitous phenomenon of the primary somatosensory cortex.     

Effects of age and body height on somatosensory evoked potentials]

The influence of age and height on somatosensory evoked potentials (SEP) following median and tibial nerve stimulation was studied. Age correlated with increase of latencies and decrease of amplitudes; exceptionally the amplitude of cortical N20 component increased with age. The central conduction time P31-P40 (tibial nerve stimulation) was longer in elderly subjects, whereas the time N13-N20 (median nerve stimulation) was independent of age. Height showed a positive correlation with latencies and peripheral conduction times; central conduction times (N13-N20 and P31-P40) were independent on height. The correlations of SEP parameters with age and height were expressed quantitatively by regression equations. The presented equations should be treated as a valuable complement to normative data in interpretation of SEP testing results.     

Scalp-recorded short and middle latency peroneal somatosensory evoked potentials in normals: comparison with peroneal and median nerve SEPs in patients with unilateral hemispheric lesions

Peroneal somatosensory evoked potentials (SEPs) were performed on 23 normal subjects and 9 selected patients with unilateral hemispheric lesions involving somatosensory pathways. Recording obtained from right and left peroneal nerve (PN) stimulations were compared in all subjects, using open and restricted frequency bandpass filters. Restricted filter (100-3000 Hz) and linked ear reference (A1-A2) enhanced the detection of short latency potentials (P1, P2, N1 with mean peak latency of 17.72, 21.07, 24.09) recorded from scalp electrodes over primary sensory cortex regions. Patients with lesions in the parietal cortex and adjacent subcortical areas demonstrated low amplitude and poorly formed short latency peroneal potentials, and absence of components beyond P3 peak with mean latency of 28.06 msec. In these patients, recordings to right and left median nerve (MN) stimulation showed absence or distorted components subsequent to N1 (N18) potential. These observations suggest that components subsequent to P3 potential in response to PN stimulation, and subsequent to N18 potential in response to MN stimulation, are generated in the parietal cortical regions.     

Somatosensory evoked potentials: correlations with height

Somatosensory evoked potentials (SEPs) to median and posterior tibial nerve stimulation were studied in 160 subjects aged 20-90 years. Height was highly correlated with latencies of spinal and cortical SEPs (N13, N20, N22, and P40). Although tibial central conduction (N22-P40) was also highly correlated with height, median conduction (N13-N22) was not correlated with the latter. Multiple correlation and regression analysis showed that except for the median N13-N20 latency, height provided the best prediction of the remaining SEP latencies. Age alone was not correlated with SEP latencies, but its significance was observed when age and height were considered together as the predictors. Effects of age and height on SEP latencies were independent of gender. The present data indicate that except for the N13-N20 conduction, height is the most important parameter for SEP latencies and can be used for construction of normograms.     

Spinal somatosensory evoked potentials following segmental sensory stimulation. A direct measure of dorsal root function

Dorsal root function cannot presently be measured directly. The H-reflex is an indirect measure of dorsal root function but only for the S1 root. Spinal somatosensory evoked potentials (SEPs) following dermatomal stimulation of the legs have the potential of providing direct data reflecting dorsal root function but have not been reliably recorded in normal subjects. We have developed a reliable technique for recording SEPs at the lumbar root entry zone following segmental sensory stimulation of the legs. The saphenous, superficial peroneal, and sural nerves were stimulated representing the L3/L4, L5 and S1 roots respectively. Reproducible responses (N-wave) were recorded over the lumbar spine in all 60 normal limbs examined. The N-wave peak latency was significantly correlated with lower limb length. The conduction velocities from the stimulation sites to the lumbar spine were similar to published values for peripheral conduction velocities in these nerves. The mean inter-limb latency differences for the N-wave peak were: L3/L4 0.61 msec; L5 0.35 msec; and S1 0.57 msec. The mean N-wave amplitudes were: L3/L4 0.11 microV; L5 0.28 microV; and S1 0.23 microV. This technique is a direct measure of dorsal root integrity. Unlike scalp recorded SEPs, the lumbar N-wave is not state-dependent and is unaffected by lesions within the brain and rostral cord.     

Lumbar cord potentials evoked by stimulation of the nerves of the lower limb in man

Lumbar cord potentials evoked by electrical stimulation of the posterior tibial and sural nerves at the ankle were recorded with monopolar epidural electrodes, at T11-T12 level in 20 subjects and were compared with surface recorded potentials. Two quadriplegic patients with spinal section were included in this group. Curare was given in two cases. Xylocaine block of peripheral nerve was carried out in 4 cases. Double shock study was done in 5 cases. The lumbar cord evoked potentials show two successive components: a 'primary' negative-positive spike response with a latency of 19-35 msec, and the 'secondary' waves with latencies up to 200 msec. The 'primary' response is mainly produced by the afferent volley in the fibres of the dorsal roots and of their intramedullary prolongations. There is no evidence which suggests that it is correlated with presynaptic inhibition. The secondary components may be divided into the early and the late waves. The early waves (40-90 msec) are related to the polysynaptic activities from the afferent fibres of small diameters. The late waves are under the influence of supraspinal mechanism and may be related to long-loop reflexes. The clinical implications of these evoked potentials are discussed.