Spinal and far-field components of human somatosensory evoked potentials to posterior tibial nerve stimulation analysed with oesophageal derivations and non-cephalic reference recording

Utility of the tibial nerve somatosensory evoked potentials in differentiating between neuromyelitis optica spectrum disorders and multiple sclerosis

BACKGROUND

Somatosensory evoked potentials (SEPs) are widely used for the diagnosis and evaluation of neuromyelitis optica spectrum disorder (NMOSD) and multiple sclerosis (MS). However, whether the parameters of tibial nerve SEPs can help to distinguish NMOSD from MS remains unclear. Thus, the aim of this study was to investigate the utility of tibial nerve SEP parameters in differentiating patients with NMOSD and MS.

METHODS

The clinical data of patients with NMOSD or MS treated in our institution between 2005 and 2021 were retrospectively extracted from our electronic database. Additional inclusion criteria were presentation with sensory symptoms in the lower extremities with corresponding lesions in the magnetic resonance images as well as available data on anti-aquaporin-4 antibodies and tibial nerve SEPs. The Z-scores of the N21-P38 interval (central sensory conduction time), P38 latency, and P38 amplitude were compared between the patients with NMOSD and MS. The relationship of disease severity with the parameters of the tibial nerve SEPs was also evaluated.

RESULTS

Twenty patients with NMOSD and 13 patients with MS were enrolled. The Z-scores of the N21-P38 interval and P38 latency were significantly higher in the MS group than in the NMOSD group (p < 0.05 and p < 0.01, respectively), whereas there was no difference in the Z-scores of the P38 amplitude between the two groups. In the MS group, only the N21-P38 interval and P38 latency were significantly correlated with disease severity (p < 0.05 and p < 0.01, respectively). In contrast, none of the tibial nerve SEP parameters were significantly correlated with disease severity in the NMOSD group.

CONCLUSION

Evaluation of the N21-P38 interval and P38 latency in tibial nerve SEPs potentially helps in differentiating between NMOSD and MS.

Assessment of thoracic spinal cord electrophysiological activity through magnetoneurography

OBJECTIVE

Noninvasive and detailed visualization of electrophysiological activity in the thoracic spinal cord through magnetoneurography.

METHODS

In five healthy volunteers, magnetic fields around current flowing in the thoracic spinal cord after alternating unilateral and synchronized bilateral sciatic nerve stimulation were measured using a magnetoneurograph system with superconductive quantum interference device biomagnetometers. The current distribution was obtained from the magnetic data by spatial filtering and visualized by superimposing it on the X-ray image. Conduction velocity was calculated using the peak latency of the current waveforms.

RESULTS

A sufficiently high magnetic signal intensity and signal-to-noise ratio were obtained in all participants after synchronized bilateral sciatic nerve stimulation. Leading and trailing components along the spinal canal and inward components flowing into the depolarization site ascended to the upper thoracic spine. Conduction velocity of the inward current in the whole thoracic spine was 42.4 m/s.

CONCLUSIONS

Visualization of electrophysiological activity in the thoracic spinal cord was achieved through magnetoneurography and a new method for synchronized bilateral sciatic nerve stimulation. Magnetoneurography is expected to be a useful modality in functional assessment of thoracic myelopathy.

SIGNIFICANCE

This is the first report to use magnetoneurography to noninvasively visualize electrophysiological activity in the thoracic spinal cord in detail.

Inhibitory effects of heterotopic noxious counter-stimulation on perception and brain activity related to Aβ-fibre activation

Heterotopic noxious counter-stimulation (HNCS) inhibits pain and pain processes through cerebral and cerebrospinal mechanisms. However, it is unclear whether HNCS inhibits non-nociceptive processes, which needs to be clarified for a better understanding of HNCS analgesia. The aim of this study was to examine the effects of HNCS on perception and scalp somatosensory evoked potentials (SEPs). Seventeen healthy volunteers participated in two counter-balanced sessions, including non-nociceptive (selective Aβ-fibre activation) or nociceptive electrical stimulation, combined with HNCS. HNCS was produced by a 20-min cold pressor test (left hand) adjusted individually to produce moderate pain (mean ± SEM: 42.5 ± 5.3 on a 0-100 scale, where 0 is no pain and 100 the worst pain imaginable). Non-nociceptive electrical stimulation was adjusted individually at 80% of pain threshold and produced a tactile sensation in every subject. Nociceptive electrical stimulation was adjusted individually at 120% of RIII-reflex threshold and produced moderate pain (45.3 ± 4.5). Shock sensation was significantly decreased by HNCS compared with baseline for non-nociceptive (P < 0.001) and nociceptive (P < 0.001) stimulation. SEP peak-to-peak amplitude at Cz was significantly decreased by HNCS for non-nociceptive (P < 0.01) and nociceptive (P < 0.05) stimulation. These results indicate that perception and brain activity related to Aβ-fibre activation are inhibited by HNCS. The mechanisms of this effect remain to be investigated to clarify whether it involves inhibition of spinal wide-dynamic-range neurons by diffuse noxious inhibitory controls, supraspinal processes or both.

Subcortical somatosensory evoked potentials after posterior tibial nerve stimulation in children

We report our normative data of somatosensory evoked potentials (SEP) after posterior tibial nerve (PTN) stimulation from a group of 89 children and 18 adults, 0.4-29.2 years of age. We recorded near-field potentials from the peripheral nerve, the cauda equina, the lumbar spinal cord and the somatosensory cortex. Far-field potentials were recorded from the scalp electrodes with a reference at the ipsilateral ear. N8 (peripheral nerve) and P40 (cortex) were present in all children but one. N20 (cauda equina) and N22 (lumbar spinal cord) were recorded in 94 and 106 subjects, respectively. P30 and N33 (both waveforms probably generated in the brainstem) were recorded in 103 and 101 subjects, respectively. Latencies increased with age, while central conduction times including the cortical component, decreased with age (up to about age 10 years). The amplitudes of all components were very variable in each age group. We report our normative data of the interpeak latencies N8-N22 (peripheral conduction time), N22-P30 (spinal conduction time), N22-P40 (central conduction time) and P30-P40 (intracranial conduction time). These interpeak latencies should be useful to assess particular parts of the pathway. The subcortical PTN-SEPs might be of particular interest in young or retarded children and during intraoperative monitoring, when the cortical peaks are influenced by sedation and sleep, or by anesthesia.

Recommendations for the clinical use of somatosensory-evoked potentials

The International Federation of Clinical Neurophysiology (IFCN) is in the process of updating its Recommendations for clinical practice published in 1999. These new recommendations dedicated to somatosensory-evoked potentials (SEPs) update the methodological aspects and general clinical applications of standard SEPs, and introduce new sections dedicated to the anatomical-functional organization of the somatosensory system and to special clinical applications, such as intraoperative monitoring, recordings in the intensive care unit, pain-related evoked potentials, and trigeminal and pudendal SEPs. Standard SEPs have gained an established role in the health system, and the special clinical applications we describe here are drawing increasing interest. However, to prove clinically useful each of them requires a dedicated knowledge, both technical and pathophysiological. In this article we give technical advice, report normative values, and discuss clinical applications.

How response inhibition modulates nociceptive and non-nociceptive somatosensory brain-evoked potentials

OBJECTIVE

To examine and compare the modulation of nociceptive somatosensory laser-evoked potentials (LEPs) and non-nociceptive somatosensory electrically-evoked potentials (SEPs) by brain processes related to response inhibition.

METHODS

A warning auditory tone was followed by either an electrical or a laser stimulus. Subjects performed a Go/Nogo task in which they were instructed to respond to the laser stimulus and refrain from responding to the electrical stimulus in half of the runs. In the other half, they performed the opposite. The paradigm allowed a direct, within-subject comparison of the electrophysiological correlates of brain processes related to the Go/Nogo task in both somatosensory submodalities.

RESULTS

In the Nogo-condition, SEPs displayed an enhanced N120 (early Nogo-response), a reduced vertex P240 and enhanced frontal P3 (late Nogo-responses). In contrast, LEPs only displayed late Nogo-related responses (reduced vertex P350 and enhanced frontal P3).

CONCLUSIONS

The early Nogo-related enhancement of SEPs may reflect brain processes specific to the processing of non-nociceptive somatosensory stimuli. Later components of the Nogo-response may reflect cortical activity common to the processing of both nociceptive and non-nociceptive somatosensory stimuli.

SIGNIFICANCE

Response inhibition significantly modulates both LEPs and SEPs. Part of these activities may be specific of the eliciting stimulus modality.

Magnetoneurographic registration of propagating magnetic fields in the lumbar spine after stimulation of the posterior tibial nerve

Stimulation of the posterior tibial nerve has been associated with different somatosensory evoked potentials (SEP) recorded along the spine and thorax. The aim of this study was to register and describe the magnetic fields corresponding to different components of spinal SEP after stimulation of tibial nerves. In nine healthy subjects, right and left posterior tibial nerves were transcutaneously electrostimulated at the ankles. Neuromagnetic fields were registered over a circular 800 cm(2) area of the lumbosacral spine using a 61-channel biomagnetometer. Magnetic field maps were constructed and examined visually for dipolar patterns. Equivalent current dipoles (ECD) were calculated for each somatosensory evoked field (SEF) using a least-squares fit in a spherical model. In seven subjects dipolar SEF were detected over the lower back at two separate latencies and locations and propagating ECD could be localized. Both the first and second components found agreed anatomically and functionally with respect to propagation in the underlying nerve fibers. It was possible to record and identify SEF which correspond to the SEP described in the literature. Dipole localization based on an equivalent current dipole model allowed a basic evaluation of the plausibility of the measurements with respect to the processes being examined.

Fundamental principles of somatosensory evoked potentials

Studies of SSEP provide unique opportunities for investigating physioanatomic substrates of sensory pathway and cognitive functions of the sensory system. Progress of clinical investigation and application of SSEP have been stalled in more recent years, although SSEP still remain a useful tool for diagnosis of various neurologic disorders and for the monitoring of spinal cord function during surgery. Reflecting complex sensory system in human, scalp-recorded SSEP consists of multiple waves, having different distribution, amplitude, and latencies among different electrodes. The physioanatomic significance of these multiple waves, especially the late components, is largely unknown. These should be explored further, especially in relation to cognitive function.

Establishment of standard values for the latency, interval and amplitude parameters of tibial nerve somatosensory evoked potentials (SEPs)

OBJECTIVE

To establish standard values for tibial nerve somatosensory evoked potentials (SEPs).

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Evaluation of descending spinal cord tracts in patients with thoracic cord lesions using motor evoked potentials recorded from the paravertebral and lower limb muscles

We evaluated the function of the descending spinal cord motor tracts in patients, with and without spinal cord lesion, using motor evoked potentials. We studied 50 normal volunteers and 15 patients with thoracic lesions. The onset latency of the negative waves of motor evoked potentials for the thoracic spines was obtained, and the descending spinal cord conduction time was measured for the thoracic segments. In normal subjects, motor evoked potentials of paravertebral muscles recorded from T1-T2 to T5-T6 initially appeared as negative waveform with transcutaneous electrical stimulation over occipitocervical junction, although those from T6-T7 to T8-T9 were initially positive and those from more caudal sites were flatter. The motor evoked potential waveforms of tibialis anterior muscles evoked by electrical stimulation over the occipitocervical junction were markedly similar to those over the L1-L2. In patients with upper thoracic lesions, descending spinal cord conduction time from T2-T3 to T5-T6 was prolonged (p < 0.01). The descending spinal cord conduction time from T5-T6 to T11-T12 was also prolonged (p < 0.01) in patients with lower thoracic lesion. The descending spinal cord conduction time from T2-T3 to T11-T12 in patients with smaller motor function scores (<2) was significantly prolonged (p < 0.01) compared with normal subjects and patients with larger function scores. The methods of recording motor evoked potentials from paravertebral muscles with transcutaneous electrical stimulation over occipitocervical junction were useful for evaluating the level and motor function of thoracic cord lesions.

Cerebral responses following stimulation of unmyelinated C-fibers in humans: electro- and magneto-encephalographic study

There are two kinds of pain, a sharp pain ascending through Adelta fibers (first pain) and a second burning pain ascending though C fibers (second pain). By using a novel method, the application of a low intensity CO(2) laser beam to a tiny area of skin using a very thin aluminum plate with numerous tiny holes as a spatial filter, we succeeded in selectively stimulating unmyelinated C fibers of the skin in humans, and could record consistent and clear brain responses using electroencephalography (EEG) and magnetoencephalography (MEG). The conduction velocity (CV) of the C fibers of the peripheral nerve and spinal cord, probably spinothalamic tract (STT), is approximately 1-4 m/s, which is significantly slower than that of Adelta (approximately 10-15 m/s) and Abeta fibers (approximately 50-70 m/s). This method should be very useful for clinical application. Following C fiber stimulation, primary and secondary somatosensory cortices (SI and SII) are simultaneously activated in the cerebral hemisphere contralateral to the stimulation, and then, SII in the hemisphere ipsilateral to the stimulation is activated. These early responses are easily detected by MEG. Then, probably limbic systems such as insula and cingulate cortex are activated, and those activities reflected in EEG components. Investigations of the cortical processing in pain perception including both first and second pain should provide a better understanding of pain perception and, therefore, contribute to pain relief in clinical medicine.

Evidence of a specific spinal pathway for the sense of warmth in humans

While research on human sensory processing shows that warm input is conveyed from the periphery by specific, unmyelinated primary sensory neurons, its pathways in the central nervous system (CNS) remain unclear. To gain physiological information on the spinal pathways that convey warmth or nociceptive sensations, in 15 healthy subjects, we studied the cerebral evoked responses and reaction times in response to laser stimuli selectively exciting Adelta nociceptors or C warmth receptors at different levels along the spine. To minimize the conduction distance along the primary sensory neuron, we directed CO(2)-laser pulses to the skin overlying the vertebral spinous processes. Using brain source analysis of the evoked responses with high-resolution electroencephalography and a realistic model of the head based on individual magnetic resonance imaging scans, we also studied the cortical areas involved in the cerebral processing of warm and nociceptive inputs. The activation of C warmth receptors evoked cerebral potentials with a main positive component peaking at 470-540 ms, i.e., a latency clearly longer than that of the corresponding wave yielded by Adelta nociceptive input (290-320 ms). Spinal neurons activated by the warm input had a slower conduction velocity (2.5 m/s) than the nociceptive spinal neurons (11.9 m/s). Brain source analysis of the cerebral responses evoked by the Adelta input yielded a very strong fit for one single generator in the mid portion of the cingulate gyrus; the warmth-related responses were best explained by three generators, one within the cingulate and two in the right and left opercular-insular cortices. Our results support the existence of slow-conducting second-order neurons specific for the sense of warmth.

Functional plasticity in the human primary somatosensory cortex following acute lesion of the anterior lateral spinal cord: neurophysiological evidence of short-term cross-modal plasticity

The primary somatosensory cortex (S1) in adult animals and humans is capable of rapid modification after deafferentation. These plastic changes may account for a loss of tonic control by nociceptive inputs over inhibitory mechanisms within structures of the dorsal column-medial lemniscal system. Most studies, however, have been performed under conditions where deafferentation of C and A delta fibres coexists with large-diameter fibres deafferentation. In this study the effect of the acute lesion of one ascending anterior lateral column on neuronal activity within the dorsal column-medial lemniscal system was assessed by recording somatosensory evoked potentials (SEPs) in seven patients who underwent unilateral percutaneous cervical cordotomy (PCC) as treatment for drug-resistant malignant pain.Spinal, brainstem and cortical SEPs were recorded 2h before and 3h after PCC by stimulating the posterior tibial nerve at both ankles. Amplitudes of cortical potentials obtained by stimulation of the leg contralateral to PCC were significantly increased after PCC. No significant changes in spinal or brainstem potentials were observed. PCC did not affect SEP components obtained by stimulation of the leg ipsilateral to PCC. Our results suggest that nociceptive deafferentation may induce a rapid modulation of cortical neuronal activity along the lemniscal pathway, thus providing the first evidence in humans of short-term cortical plasticity across the spinothalamic and lemniscal systems.

Somatosensory and brainstem auditory evoked potentials in the Gunn rat model of acute bilirubin neurotoxicity

Brainstem auditory evoked potentials (BAEPs) are a sensitive indicator of bilirubin neurotoxicity. Somatosensory evoked potentials (SEPs) have been proposed as another measure of toxicity, though the lemniscal pathways that generate the SEP are not involved in kernicterus. In 16 to 17-d-old jaundiced (jj) Gunn rats, serial BAEPs and SEPs were obtained up to 8 h after acute bilirubin toxicity. jjs were injected with 150 mg/kg sulfadimethoxine to displace bilirubin into brain tissue (n = 8); littermate controls were jjs given saline (n = 4) and nonjaundiced given sulfadimethoxine or saline (n = 7). After anesthesia, BAEP and SEP recordings were obtained at baseline and serially after injection. SEPs to median nerve stimulation were recorded from surface electrodes over the brachial plexus (Erb's) and contralateral parietal cortex, and subtracted to obtain central conduction time (CCT). There were no statistically significant different baseline values between groups. Baseline BAEP I, I-II, and I-III were 1.22 +/- 0.13, 1.11 +/- 0.12, and 2.10 +/- 0.15 ms, and SEP Erb's and CCT were 1.48 +/- 0.20 and 5.59 +/- 0.50 ms, respectively (n = 19). At 6.8 +/- 1.5 h after injection BAEP I, I-II, and I-III increased 0.10 +/- 0.08, 0.18 +/- 0.17, and 0.56 +/- 0.33 ms over baseline, respectively (p = 0.005, 0.01, and 0.001, respectively, paired, 1-tailed t-tests), in experimental but not control groups. SEP Erb's decreased slightly, -0.06 +/- 0.04 ms in experimental and -0.08 +/- 0.08 ms in control groups, while CCT did not change significantly. BAEPs were completely abolished in two jjs with no SEP changes. When injection of sulfonamide induced significant peripheral and central BAEP abnormalities in jaundiced rats, no SEP abnormalities occurred. SEPs assess proprioception but not other somatosensory function or sensory integration. The results demonstrate the selectivity of acute bilirubin toxicity for the auditory nervous system.

Examination of nasopharyngeal and tracheal somatosensory evoked potential recordings in dogs

OBJECTIVE

To investigate the value of nasopharyngeal and tracheal recordings of somatosensory evoked potentials (SSEP) in anesthetized dogs.

ANIMALS

10 healthy mixed-breed dogs (5 males and 5 females).

PROCEDURE

Square-ware electrical stimuli (50 microseconds duration, 4Hz) were delivered through bipolar surface electrodes to the median nerve of the right forelimb with 7 to 12 mA constant current. The SSEP were recorded with soft electrodes placed on the epipharynx and dorsal wall of the trachea, respectively. Traditional scalp and neck recordings of SSEP were also performed, using SC-inserted needle electrodes. The potentials recorded dorsally and ventrally from the neuraxis were compared to assess the application of these signals for intraoperative neurophysiologic monitoring.

RESULTS

Electrical stimulation of the median nerve resulted in multiphasic potentials recorded from all 4 recording sites. Latency and phase shifts were observed between the far-field potentials placed ventrally and dorsally from the neuraxis.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentials recorded with nasopharyngeal and tracheal electrodes are regarded suitable for intraoperative neurophysiologic monitoring in anesthized dogs.

Conduction velocity of the spinothalamic tract following CO2 laser stimulation of C-fibers in humans

Pain-related somatosensory-evoked potential following CO(2) laser stimulation (laser-evoked potential (LEP)) is now used not only for research objectives, but also for clinical applications. Estimating the conduction velocity (CV) of the spinothalamic tract (STT) by analyzing LEP following activation of Adelta-fibers (Adelta-CVSTT) by CO(2) laser stimulation has been performed previously, but estimating the CV of STT following activation of C-fibers (C-CVSTT) has not. This is the first report to estimate the C-CVSTT in humans; by using the novel method of CO(2) laser stimulation applied to tiny skin areas. The calculation method was based on that of Kakigi and Shibasaki (Electroenceph clin Neurophysiol 80 (1991) 39) who measured Adelta-CVSTT by conventional CO(2) laser stimulation. The C-CVSTT ranged between 1.4 and 4.0 m/s, and its mean+/-SD was 2.9+/-0.8 m/s. This C-CVSTT was significantly slower than the Adelta-CVSTT, which ranged approximately from 10 to 21 m/s. The nociceptive signal of the C-fibers in STT is probably conveyed by unmyelinated axons of projection neurons to reach the thalamus. Our findings provide the first physiological evidence of the signals ascending through unmyelinated axons in the spinal cord in humans. In addition, estimating C-CVSTT and Adelta-CVSTT combined with conventional methods to measure the CV of the posterior column using electrical stimulation should be useful and have important clinical applications, particularly in patients with spinal cord lesions showing various kinds of sensory disturbances.

Visualization of signal propagation from sciatic nerve to spinal cord in canine

The neuroelectric activity that ascends the sciatic nerve and moves to the spinal cord was visualized by measuring the magnetic compound action fields (CAFs) with a superconducting quantum interference device gradiometer. The sciatic nerve of a dog was stimulated electrically, and propagating evoked CAFs were measured non-invasively. Isomagnetic field maps were made on the basis of this data, and the signal propagation was visualized. The evoked magnetic fields presented a quadrupole consisting of two elements: depolarization and repolarization. Measuring the magnetic CAFs of the sciatic nerve on the body surface enabled us to visualize the non-invasively the signal movement continuously from the sciatic nerve to the spinal cord.

Source generators of the early somatosensory evoked potentials to tibial nerve stimulation: an intracerebral and scalp recording study

OBJECTIVE

To investigate the location of the cerebral generators of the early scalp somatosensory evoked potentials (SEPs) after tibial nerve stimulation.

METHODS

Tibial nerve SEPs were recorded in 15 patients, suffering from Parkinson's disease, who underwent implantation of intracerebral (IC) electrodes in the subthalamic nucleus, in the globus pallidum or in the thalamic ventralis intermediate nucleus. SEPs were recorded both from the scalp surface and from the IC leads.

RESULTS

The lemniscal P30 response was recorded by all the electrodes. The IC waveforms included a negative N40IC response, followed by a positive (P50IC) and a negative (N60IC) potential. The N40IC, the P50IC and the N60IC potentials did not differ in latency from the P40, the N50 and the P60 responses recorded by the Cz electrode. In 6 patients, in which SEPs were recorded also during the voluntary movement of the stimulated foot (active gating), an amplitude reduction of the SEP components following the P30 potential was observed during movement at the vertex and in the IC traces. Instead, in the contralateral temporal traces the SEP components (N40temp and P50temp) were not modified by active gating, and in the ipsilateral parietal traces only the positive potentials at about 60ms of latency was decreased.

CONCLUSIONS

Two differently oriented generators are active in the contralateral hemisphere at both 40 and 50ms of latency after tibial nerve stimulation. One source is oriented perpendicularly to the mesial hemispheric surface and generates the potentials recorded by the contralateral temporal and the ipsilateral parietal leads; the other dipolar source is radial to the hemispheric convexity, and generates the potentials at the vertex and those recorded by the IC electrodes.

Responses of the supra-sylvian (SII) cortex in humans to painful and innocuous stimuli. A study using intra-cerebral recordings

In this study we compare the intrinsic characteristics and localization of nociceptive CO(2) laser evoked potential (LEP) and non-nociceptive electrical EP (SEP) sources recorded by deep electrodes (one to two electrodes per patient, 10-15 contacts per electrode) directly implanted in the supra-sylvian cortex of 15 epileptic patients. Early CO(2) laser (N140-P170) and electrical (N60-P90) evoked potentials were recorded by all of the electrodes implanted in the supra-sylvian cortex contralateral to stimulation. SEPs and LEPs had similar waveforms and inter-peak latencies. The LEPs appeared 84+/-15 ms later and were, on average, 14.2+/-22.2 microV smaller than the SEPs. These differences may be accounted for by the characteristics and the sizes of the different peripheral fibers (Adelta vs. Abeta) activated by the two types of stimuli. The stereotactic Talairach coordinates of the SEP and LEP sources covered the pre- and post-rolandic upper bank of the sylvian fissure, and were not significantly different for noxious and non-noxious stimuli. The spatial distribution of these contralateral responses fits with that of the modeled sources of scalp CO(2) LEPs, magneto-encephalographic studies, and PET data from pain and vibrotactile activation studies. These results permit us to define the SII cortex as a cortical integration area of non-nociceptive and nociceptive inputs. This is supported by: (i) anatomical data reporting that the SII area receives inputs from both posterior columns and spino-thalamic pathways conveying the non-noxious and noxious information, respectively, and (ii) single cell recordings in monkeys, demonstrating that the SII area contains both nociceptive-specific neurons and wide-dynamic-range neurons receiving convergent input from nociceptive and non-nociceptive somatosensory afferents.

Tracing of proximal lumbosacral nerve conduction--a comparison of simultaneous magneto- and electroneurography

OBJECTIVE

The reconstruction of nerve impulse conduction along proximal lumbosacral plexus and nerve roots is compared using simultaneous magneto- and electroneurography.

METHODS

In 3 healthy subjects the left tibial nerve was electrostimulated at the ankle. Evoked magnetic fields and electric surface potentials were measured simultaneously over the lumbosacral spine using a multichannel SQUID-detector with a planar measuring area and 25 surface electrodes covering a comparable area centered around L4. Based on either magnetic field or electric potential maps the depolarization front of the evoked compound action currents (CAC) was spatio-temporally reconstructed using a simple equivalent current dipole model in a half-space volume conductor.

RESULTS

The mean signal-to-noise ratio in the magnetic (electric) recordings was around 4 (8). Yet, the localization quality for the propagating CAC was lower for electric than magnetic recordings. The local nerve conduction velocity was around 47 m/s (calculated from magnetic data), but fluctuated unphysiologically for electric data.

CONCLUSION

In comparison to electroneurography, an anatomically reasonable localization of evoked compound action currents propagating in lumbosacral roots can be obtained by magnetoneurography.

Magnetoneurography of evoked compound action currents in human cervical nerve roots

OBJECTIVE

A measurement protocol for magnetoneurography (MNG) is established which allows the non-invasive localization and tracing of evoked compound action currents propagating along cervical nerve roots in man.

METHODS

Inside a magnetically shielded room either both median or both ulnar nerves of healthy subjects were conventionally electrostimulated in alternation. Evoked magnetic responses were recorded using a multichannel SQUID-detector with a planar measuring area centered over the neck. Simultaneously, electric surface potentials were recorded using cervical bipolar electrode montages.

RESULTS

Upon median (ulnar) nerve stimulation somatosensory evoked magnetic fields up to 20 fT (10 fT) amplitude were detected propagating over the cervical transforaminal root entry zone, with corresponding electrical surface potentials of 1.5 microV (0.5 microV). Furthermore, the signal-to-noise ratio of the spatiotemporal magnetic field mappings in median nerve stimulation experiments allowed dipolar source reconstructions and tracing of the propagation of the compound action currents along nerve root fibers.

CONCLUSION

Magnetoneurography allows tracing of the propagation of evoked compound action currents along cervical roots in healthy subjects with millisecond temporal and high spatial resolution. Thus, MNG offers a sensitivity appropriate to serve as a clinical diagnostic tool for localizing focal neuropathies of cervical nerve roots.

A simple method for estimating conduction velocity of the spinothalamic tract in healthy humans

OBJECTIVES

The object of this study was to establish a method for estimating the conduction velocity (CV) of the spinothalamic tract (STT) in relation to clinical application.

METHODS

The CV of the STT was estimated by an indirect method based on that reported by Kakigi and Shibasaki in 1991 (Kakigi R, Shibasaki H. Electroenceph clin Neurophysiol 80 (1991) 39). Laser-evoked potentials (LEP) were measured in 8 subjects following hand (LEPH) and foot (LEPF) laser stimulation. The conduction times recorded at the scalp (P340, P400 and N150 potentials) were considered as the summation of peripheral and central components. The peripheral conduction times were calculated by measuring the latency of the electrical cutaneous silent period (from the same stimulus site of LEPs), corrected for F- and M-wave latency values.

RESULTS

The CV of the STT ranged between 8.3 and 11.01 m/s and its mean value was found to be approximately 9.87+/-1.24 m/s. The CV of the STT obtained by the N150 latencies overlapped that obtained by the P340/P400 latencies.

CONCLUSIONS

Our data suggest that our method appears appropriate and useful for practical clinical purposes, furnishing an additional tool for investigating the physiological function of small-fiber pathways.

Scalp distribution of the earliest cortical somatosensory evoked potential to tibial nerve stimulation: proposal of a new recording montage

OBJECTIVE

To investigate the most reliable method to record the earliest cortical somatosensory evoked potential (SEP) after tibial nerve stimulation. The 'gating' phenomenon was used to dissociate the overlapping cortical SEP components.

METHODS

In 11 subjects we recorded the scalp SEPs at rest, during the voluntary (active gating) and passive (passive gating) foot movement and during the isometric calf muscle contraction (isometric gating).

RESULTS

At the vertex the P40 amplitude was reduced in all the gating conditions. Instead, both the P40 response recorded in the parietal region ipsilateral to the stimulation (indicated as P40par) and the fronto-temporal N37 potential were reduced in amplitude only during the passive foot movement.

CONCLUSIONS

The same behaviour of the N37 and P40par potentials suggests that they can represent the opposite counterparts of the same dipolar generator. Instead, the real P40 amplitude, which is affected in all the gating conditions, is recorded at the vertex and might be generated by a different source. We conclude that the montage obtained by referring a temporal electrode contralateral to the stimulation to an ipsilateral parietal lead can reliably record the earliest cortical component (N37/P40par) after tibial nerve stimulation.

Somatosensory evoked potentials after multisegmental lower limb stimulation in focal lesions of the lumbosacral spinal cord

OBJECTIVES

Recording techniques permit the separate analysis of the response from cauda equina roots and the spinal potential that is probably generated by the activation of dorsal horn cells. To improve the functional assessment of focal lesions of the lumbosacral cord, lower limb somatosensory evoked potentials (SEPs) were measured by multisegmental stimulation.

METHODS

Common peroneal and tibial nerves SEPs were recorded in 14 patients in whom MRI demonstrated compressive cord damage ranging from T9 to L1 levels. SEPs were recorded in each patient at the lumbar level (cauda equina response), lower thoracic level (spinal response), and from the scalp (cortical response).

RESULTS

Abnormalities in spinal response occurred in 50% and 70% of tibial and common peroneal nerve SEPs respectively; these findings were well explained by the radiological compression level, involving in most of the patients lumbar rather than sacral myelomeres. The SEPs were often more effective than the clinical examination in showing the actual extension of damage.

CONCLUSIONS

The recording of spinal SEPs after multisegmental lower limb stimulation proved useful in assessing cord dysfunction and determining the cord levels mainly involved by the compression.

Neuroanatomic substrates of lower extremity somatosensory evoked potentials

After stimulation of the lower extremity nerve (tibial nerve), N21 and N23 are recorded from L4 and T12 spine respectively. The far-field potentials of P31 and N35 are registered from Fpz-C5s (fifth cervical spine) or CPi (ipsilateral with respect to the side of stimulation)-ear derivation. Additional far-field potentials of P17 and P24 may be recorded from the scalp when a noncephalic (knee) reference is used. The major positive peak, P40, is registered at the vertex and the CPi. Preceding P40, there is a small negative peak, N37, recorded at the contralateral (CPc) hemisphere. Neuroanatomic substrates of these somatosensory evoked potential (SSEP) components are less well clarified compared with those of upper extremity (median nerve) SSEPs, primarily because clinical application of lower extremity SSEPs is more difficult, and all of the aforementioned potentials but one (P40) are not obligatory components. The concept of "paradoxical lateralization" complicates the issue further. Accumulating evidence, however, suggests that the far-field potentials of P17 and P31 arise from the distal portion of the sacral plexus and brainstem respectively. These correspond to P9 and P14 of the median nerve SSEPs respectively. The spinal potential of N23 is equivalent to the N13 cervical potential of the median nerve SSEP. N35 recorded from the ipsilateral hemisphere is analogous to N18 of the median nerve. Paradoxically lateralized P40 has been thought to represent the positive end of a dipole field, reflected by the negativity at the mesial surface of the contralateral hemisphere, and has commonly been considered to be equivalent to the first cortical potentials (N20) of the median nerve SSEP. However, more recent evidence suggests that the primary positivity is at the mesial cortical surface, and it more likely corresponds to P26 of the median nerve SSEP. Thus the first cortical potential corresponding to N20 is probably a small and inconsistent N37 recorded on the contralateral hemisphere. These assumptions need to be verified further by more extensive clinical studies applied to various neurologic disorders.

Cortical control of spinal pathways mediating group II excitation to human thigh motoneurones

1. The possibility was investigated that cortical excitation to human thigh motoneurones is relayed via lumbar premotoneurones. 2. Test responses were evoked by transcranial magnetic stimulation (TMS) in voluntarily contracting quadriceps (Q) and semitendinosus (ST) muscles: either a motor evoked potential (MEP) in surface recordings or a peak of cortical excitation in the post-stimulus time histogram (PSTH) of single motor units was used. These test responses were conditioned by stimuli to the common peroneal (CP) or gastrocnemius medialis (GM) nerves. 3. CP stimulation evoked a large biphasic facilitation of the Q MEP, with early, short-lasting, low-threshold (0.6-0.8 x motor threshold (MT)) and late, longer lasting and higher threshold (1.2-1.5 x MT) peaks separated by a period of depression. GM nerve stimulation evoked a similar early depression and late facilitation in the ST MEP. 4. CP-induced effects in the Q H reflex were different (smaller late facilitation not preceded by any depression), suggesting that CP and cortical volleys interact at a premotoneuronal level to modify the Q MEP. 5. Peaks of cortical excitation evoked by TMS in single motor unit PSTHs were modulated by the conditioning volley like the MEPs with, in Q motor units, early and late CP-induced facilitations separated by a depression, and in ST motor units a late GM-induced facilitation. Facilitations on combined stimulation (i) were greater than the sum of effects by separate stimuli and (ii) never affected the initial part of the cortical peak. 6. It is concluded that the features of the reported facilitatory interactions between cortical and peripheral volleys are consistent with interactions in a population of lumbar excitatory premotoneurones co-activated by group I and group II afferents. The potency of the effects suggests that a significant part of the cortical excitation to motoneurones of thigh muscles is relayed via these interneurones. 7. It is argued that the early depression in ST motoneurones and the separation of the two peaks of facilitation in Q motoneurones reflect a cortical facilitation of spinal inhibitory interneurones projecting on excitatory premotoneurones.

Clinical and neurophysiological abnormalities before and after reconstruction of the anterior cruciate ligament of the knee

OBJECTIVES

We aimed to study knee proprioception and somatosensory evoked potentials (SEPs) to stimulation of the common peroneal nerve (CPN) in 7 patients with lesion of the anterior cruciate ligament (ACL) before and after ACL reconstruction.

MATERIALS AND METHODS

We recorded the spinal N14 and scalp P27 potentials in 5 patients, while in the remaining 2 patients we calculated scalp SEP maps by 20 electrodes. The knee proprioception was tested by comparing the sensitivity to movement of both the knees.

RESULTS

Before surgery, all patients showed decreased knee position sense and lack of the cortical P27 potential on the side of the ACL lesion. Arthroscopic reconstruction of the ligament improved neither the knee proprioception nor the somatosensory central conduction.

CONCLUSION

We suggest that the loss of the knee mechanoreceptors can be followed by modifications of the central nervous system, which are not compensated by other nervous structures.

Generators of short latency human somatosensory-evoked potentials recorded over the spine and scalp

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.

Dissociation induced by voluntary movement between two different components of the centro-parietal P40 SEP to tibial nerve stimulation

Whether the two earliest cortical somatosensory evoked potentials (SEPs) to tibial nerve stimulation (N37 and P40) are generated by the same dipolar source or, instead, originate from different neuronal populations is still a debated problem. We recorded the early scalp SEPs to tibial nerve stimulation in 10 healthy subjects at rest and during voluntary movement of the stimulated foot. We found that the P40, which reached its highest amplitude on the vertex at rest, changed its topography during movement, since its amplitude was reduced much more in the central than in the parietal traces. These findings suggest that two different components contribute to the centro-parietal positivity at rest: (1) the P37 response, which is parietally distributed and is not modified by movement, and (2) the 'real' P40 SEP, which is focused on the vertex and is reduced in amplitude during voluntary movement. Since, also, the N37 response did not vary its amplitude under interference condition, it is possible that the N37 and P37 potentials are generated by the same dipolar source. Other later components, namely P50 and N50 were significantly reduced in amplitude during foot movement. Lastly, the subcortical P30 far-field remained unchanged and this suggests that the phenomenon of amplitude reduction during movement (i.e. gating) occurs above the cervico-medullary junction.

Somatosensory evoked potentials after posterior tibial nerve stimulation--normative data in children

We report normative data of somatosensory evoked potentials to posterior tibial nerve stimulation from 47 children 4-15 years of age. We recorded near-field potentials from the peripheral nerve, the cauda equina, the lumbar spinal cord and the somatosensory cortex. Far-field potentials were recorded from the scalp electrodes with a reference at Erb's point and on the earlobe. The near-field potentials N8 (peripheral nerve) and P40 (cortex) were present in all children. N20 (near-field from the cauda equina) was recorded in 38 subjects. N22 (near-field from the lumbar spinal cord), P30 and N37 ( both far-field waveforms probably generated in the brainstem) were recorded in 46 subjects each. The latencies and the peripheral conduction time (N8-N22) increased with age, while the central conduction time (N22-P40) and the intracranial conduction time (P30-P40) both decreased with age (up to about 10 years of age). The spinal conduction time (N22-P30) was relatively independent of age. The interpeak latencies allow the assessment of specific portions of this pathway. The subcortical posterior tibial nerve-somatosensory evoked potentials are of particular interest in children when the cortical peaks are influenced by sedation and sleep, or by anaesthesia.

Neuromonitoring in the operating room: why, when, and how to monitor?

This review considers the main principles and indications of EEG and evoked potential (EP) neuromonitoring in the operating room. Neuromonitoring has a threefold purpose: to warn the surgeon that he has to adjust his strategy, to confirm his decision, and to help him improve subsequent procedures. The pathophysiology of intraoperative events liable to alter the EEG or the EPs is first considered. The usefulness of neuromonitoring in preventing neurological complication relies on its ability to detect neurological dysfunction at a reversible stage. This applies especially to ischemia and compressive damage. The anesthetic influences on EEG and EPs are then considered. Knowledge of them is essential to disentangle these neurophysiological alterations due to intraoperative events from those merely due to anesthesia and to use neurophysiological parameters to evaluate the depth of anesthesia. Third, the main indications and limitations of neuromonitoring are considered: prevention of ischemic brain or spinal cord damage, prevention of mechanical injuries of the brain, spinal cord or peripheral nerve, and localization of the motor cortex in cortical neurosurgery or of cranial nerves in posterior fossa surgery. Finally, the 3 levels of neuromonitoring (neurophysiological feature extraction, neurophysiological pattern recognition, clinical integration of the neurophysiological patterns) are discussed together with the rules that should guide the dialogue between the surgeon, the anesthesiologist, and the neurophysiologist.

Abnormalities of somatosensory and motor evoked potentials in adrenomyeloneuropathy: comparison with magnetic resonance imaging and clinical findings

We studied 6 patients with adrenomyeloneuropathy (AMN) showing mild signs of central nervous system involvement. All patients underwent brain and spinal magnetic resonance imaging (MRI) and somatosensory (SEP) and motor (MEP) evoked potential study. Whereas SEPs and MEPs were abnormal in all patients, only 1 patient showed brain MRI abnormalities; spinal MRI showed hypotrophy without focal abnormalities in 4 of 6 patients. Median nerve SEPs, which were recorded with noncephalic reference montage, revealed delayed or absent scalp P14 far-field potential in all patients and abnormal spinal N13 in 2. Moreover, tibial nerve SEPs revealed abnormalities of the subcortical P30 response in all 4 patients in whom scalp-to-ear recording was employed. These findings strongly suggest that in the early stages of disease neurological dysfunction is localized in the spinal cord, where it is difficult to assess using MRI. However, SEPs and MEPs, which show a typical pattern of abnormality in these patients, could be useful in disclosing signs of long tract involvement and in monitoring treatment.

Pudendal nerve somatosensory evoked potentials in paediatrics: maturation aspects

Pudendal nerve somatosensory evoked potentials (PN-SSEPs) were recorded in 21 healthy children (age range: 3.3-13.3 years). The dorsal nerve of the penis/clitoris was stimulated and SSEPs were recorded at spinal L1-D12 and at cortical Cz'-Fz. Morphology, latency and amplitude of the cortical SSEPs were evaluated. A cortical response was obtained in all but two subjects. Cortical SSEPs were broader and less defined in shape in the youngest subjects. There was a progressive shortening of the latency of the P and N components during growth. Spinal responses were obtained only in 6 cases. Nine subjects also underwent tibial nerve stimulation. Pudendal and tibial SSEPs differed in their degree of maturation.

Somatosensory evoked potentials elicited by stimulation of lower-limb nerves in amyotrophic lateral sclerosis

To determine lower limb somatosensory modifications in amyotrophic lateral sclerosis (ALS), we studied somatosensory evoked potentials (SEPs) elicited by stimulation of tibial posterior nerves (TP), sural nerves (SN), saphenous internous nerves (SA), and medial plantar nerves (PL) of both limbs in 24 ALS patients, and compared the results with those from 17 normal subjects. Responses were recorded according to the international 10-20 system. Normal sensory conduction velocities of SN, SA and PL and H reflexes in soleus muscles were prerequisites for patient inclusion in this study. The results showed marked alterations in SEPs cortical components of all lower limb nerves, which could be related to abnormal sensory transmission (after spinal N22), or cortical abnormalities. We put forward the hypothesis of impairment of pyramidal control of the sensory system and Clark's column involvement to explain such anomalies. It was concluded that SEPs abnormalities in the lower limbs are a common feature in ALS.

Mapping of tibial nerve evoked magnetic fields over the lower spine

Using a low-noise 49-channel dc-SQUID system spinal somatosensory evoked fields (SEF) were recorded which were generated by compound action currents evoked upon posterior tibial nerve stimulation. The SEF mapping showed the action current propagation along the sciatic nerve, lumbosacral plexus and cauda equina in parallel to simultaneously recorded electrical potentials (SEP). For a reliable intraindividual side-to-side comparison of spinal SEFs the right and left tibial nerves were stimulated in alternating order; this procedure minimizes artifactual inter-nerve SEF map differences due to eventual patient-to-sensor displacements which might occur in serial measurements. These large-area lumbar SEF mappings open up several clinical perspectives for magnetoneurography, in particular with respect to the 3D-localization of proximal conduction blocks.

Selective gating of lower limb cortical somatosensory evoked potentials (SEPs) during passive and active foot movements

We evaluated subcortical and cortical somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation in 4 experimental conditions of foot movement and compared them with the baseline condition of full relaxation. The experimental conditions were: (a) active flexion-extension of the stimulated foot; (b) active flexion-extension of the non-stimulated foot; (c) passive flexion-extension of the stimulated foot in complete relaxation; (d) tonic active flexion of the stimulated foot. We analyzed latencies and amplitudes of the subcortical P30 potential, of the contralateral pre-rolandic N37 and P50 responses and of the P37, N50 and P60 potentials recorded over the vertex. Latencies did not vary in any of the paradigms. The amplitude of subcortical P30 potential did not change during any of the paradigms. Among the cortical waves, P37, N50 and P60 amplitudes were significantly attenuated in all conditions except active movement of the non-stimulated foot (b). This attenuation was less during passive (c) than during active movements of the stimulated foot (a and d). The contralateral pre-rolandic waves N37 and P50 showed no significant decrease during any of the paradigms. These results suggest that gating occurs rostrally to the cervico-medullary junction, probably at cortical level. The different behavior of N37, P50 and P37, N50 cortical responses during movement of the stimulated foot provides evidence suggestive of a highly localized gating process occurring at cortical level. These potentials could reflect activation of separate, functionally distinct generators.

Functional involvement of central nervous system in mitochondrial disorders

Thirty-nine patients with mitochondrial diseases were studied with somatosensory and motor evoked potentials. Sixteen patients (41%) had clinical and 12 (31%) had neuroradiological evidence of central nervous system involvement. The overall incidence of electrophysiological abnormalities was 64%. Abnormal evoked potentials were also found in a significant percentage (33%) of patients with pure myopathic forms of mitochondrial diseases and in an asymptomatic carrier of MERRF mutation. Of the individual tests, somatosensory evoked potentials were abnormal in 49% of the patients and motor evoked potentials were abnormal in 46% of the patients. The outcome is that electrophysiological evidence of central nervous system involvement is present in a high percentage of patients with mitochondrial disorders, and that the threshold for central nervous system electrophysiological abnormalities is well below that for clinical and/or radiological manifestations.

Amplitude changes of tibial nerve cortical somatosensory evoked potentials when the ipsilateral or contralateral ear is used as reference

We performed topographical mapping of somatosensory evoked potentials (SEPs) to the posterior tibial nerve using earlobe references both ipsilateral and contralateral to the stimulation side. The voltage of the frontal contralateral N37 and P50 components was enhanced, while the voltage of the parietal ipsilateral P37 and N50 components was reduced when the contralateral earlobe was substituted by the ipsilateral earlobe reference. Maps of the same data documented concomitant changes in negative and positive potential fields, showing an expansion of the pre-Rolandic N37 toward the centrotemporal contralateral regions, and a tendency of the parietal ipsilateral P37, N50 components to be more focally distributed at the vertex. SEPs recorded at each earlobe (Cv6 reference) provided an explanation of these results: The contralateral earlobe detected a negative potential corresponding to the N37 potential recorded over the scalp, followed by a P50 potential that attenuated the contralateral responses and enhanced the ipsilateral ones. The ipsilateral earlobe had no significant effects on scalp SEPs, since it detected only a large N33 negativity. Current source density (CSD) maps were, of course, not influenced by the ear used as reference. Our results suggest that the ipsilateral ear reference is better than the contralateral one for recording "genuine" cortical SEPs. Therefore, it can be recommended in the clinical domain for mapping studies of lower-limb cortical SEPs.

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.

Neural generators of tibial nerve P30 somatosensory evoked potential studied in patients with a focal lesion of the cervicomedullary junction

The tibial nerve P30 potential was studied in 6 patients with focal lesions located in the vicinity of the cervicomedullary junction. P30 potential was unaffected while cortical P39 was abnormal in the patients with a supramedullary lesion affecting the somatosensory pathway just above its decussation. Conversely, P30 was abnormal in the presence of a lesion situated caudally to the cervicomedullary junction affecting the lower limb sensory fibers just below their decussation. Median nerve P14 behaved similarly to the P30 potential in these cases. These clinical observations suggest that P30 potential, as P14 of median nerve somatosensory evoked potentials, is generated in the lower brain stem probably before the decussation of the sensory fibers; nucleus gracilis and medial lemniscus fibers in the lower brain stem are probably the anatomical structures generating P30 potential. This suggests that P30 potential may be used to study intraspinal and intracranial conduction times separately in the afferent somatosensory pathways.

Changes in spinal cord excitability in a patient with rhythmic segmental myoclonus

Paired stimulation of the common peroneal and posterior tibial nerve was used to study the recovery cycle of lumbosacral somatosensory evoked potentials in 10 control subjects and in one patient with rhythmic segmental myoclonus of the leg involving the L2-L4 myotomes. In normal subjects the peripheral nerve volley in the cauda equina had recovered at an interstimulus interval of 3 ms whereas the postsynaptic dorsal horn potential was reduced to about 60% of its control size. Similar results were found in the patient after posterior tibial nerve but not common peroneal nerve stimulation. The second, which evokes afferent input to the affected lumbar segments, produced facilitation of the postsynaptic response at 3 ms. This finding suggests that the physiological suppression of dorsal horn interneurons which usually takes place after paired stimulation fails to occur in segmental myoclonus. This may indicate that dorsal horn interneurons are abnormally hyperactive and are involved in the pathophysiology of spinal myoclonus.

Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

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.

Measurement of motor conduction in the thoracolumbar cord. A possible predictor of surgical outcome in cervical spondylotic myelopathy

STUDY DESIGN

A prospective motor-evoked potential study with measurement of spinal cord motor conduction velocity in the thoracolumbar cord was performed before and after decompression surgery in 30 patients with cervical spondylotic myelopathy.

OBJECTIVES

To evaluate the neurofunctional integrity of the spinal motor pathways in cervical spondylotic myelopathy in patients compared with age-matched control subjects; to assess any changes after posterior surgical decompression; and to correlate such changes with functional outcomes so that the predictability of preoperative motor-evoked potentials could be determined.

SUMMARY OF BACKGROUND DATA

Previous studies evaluating neurologic function and predictability of surgical results in cervical spondylotic myelopathy patients always depended on the morphologic changes of the cord and spinal structures. The recently developed motor-evoked potential study and noninvasive measurement of spinal cord motor conduction velocity may provided an objective method to evaluate physiologic motor function in cervical spondylotic myelopathy patients.

METHODS

Spinal cord motor conduction velocity in the thoracolumbar cord was measured using percutaneous magnetic stimulation over the motor cortices and F-wave studies in median and peroneal nerves. Motor function of cervical spondylotic myelopathy patients was graded according to evaluation of signs of cord involvement, ambulation, and degree of dependence in activities of daily living. Evaluation was performed at 6 months, 1 year, and 2 years after decompression surgery.

RESULTS

Motor functional improvement accompanied by increased spinal cord motor conduction velocity occurred in Grade I patients with a mild neurologic dysfunction but not in Grade II or III patients with a moderate-to serve neurologic deficit. Neurologic improvement does not appear to occur until 6 months after surgery.

CONCLUSIONS

Measurement of spinal cord motor conduction velocity may provide an objective and quantitative approach to assessing the motor functional integrity of the spinal cord and serving as a predictor in evaluating surgical outcome in patients with cervical spondylotic myelopathy.

Inter- and intraindividual variability of posterior tibial nerve somatosensory evoked potentials in comatose patients

Thirty-two sequential posterior tibial nerve somatosensory evoked potentials (PTN-SEP) were recorded in 30 neurologically impaired, ventilated, comatose patients. To establish the time invariance of PTN-SEP in this population, Spearman rank correlations of latencies, interpeak latency, amplitudes, and mean absolute amplitude with time were computed. The results revealed no significant time dependency. The mean, standard deviation, and 5th and 95th percentiles for the inter- and intraindividual distribution of PTN-SEP parameters, pairwise PTN-SEP parameter differences, and direct cross-correlation of PTN-SEP were estimated using bootstrap procedures. The standard deviations of the interindividual distribution of PTN-SEP parameters are two to three times higher than the standard deviations of the intraindividual distributions. The coefficients of variation, that is, standard deviations divided by means, for the intraindividual distribution of latencies ranged from 0.012 to 0.042, of amplitudes from 0.146 to 0.230. The mean maximal cross-correlation coefficient of two randomly chosen PTN-SEP across patients equaled 0.65, and within patients 0.91. These data demonstrate the interindividual variability and intraindividual stability of PTN-SEP. The normal limits of intraindividual variability (1.96 x standard deviation) are 3.76, 1.33, 2.92, 6.00, and 3.04 ms for latencies N1, P1, N2, P2, and interpeak latency P1-N2, respectively. The intraindividual differences of amplitudes N1/P1, P1/N2, N2/P2, and the mean absolute amplitude should not exceed 0.67, 0.67, 0.90, and 0.27 microV or, expressed as quotients, 61, 52, 41, and 61%, respectively. The intraindividual maximal cross-correlation coefficient should not be lower than 0.74 with a lag of < 2.00 ms. The results of this study are of use for discrete and continuous PTN-SEP monitoring on intensive care units and during neuroradiological interventions and neurosurgery.

Subcortical P30 potential following tibial nerve stimulation: detection and normative data

Stimulation of the tibial nerve evokes a P30 far-field potential over the scalp which, like the median nerve P14, probably originates in the vicinity of the cervico-medullary junction. Unlike the P14 potential, P30 recording has not been systematically performed in clinical practice, probably because of doubts about the generator of the potential and the possibility of consistently recording it on the scalp after the unilateral stimulation of the tibial nerve. In this study, we tested the reliability of the tibial nerve scalp far-field P30 potential in 34 normal subjects using different montages, of which the Fpz-Cv6 derivation gave the highest signal to noise ratio, making it possible to obtain a P30 potential peaking at 29.2 +/- 1.6 msec in all normal subjects. This suggests that this component should to be included in the routine recording of tibial nerve SEPs in order to evaluate the spinal and intracranial conduction of the somatosensory pathway separately.