Evoked spinal electrograms recorded from epidural space in man

A multichannel electrophysiological approach to noninvasively and precisely record human spinal cord activity

The spinal cord is of fundamental importance for integrative processing in brain-body communication, yet routine noninvasive recordings in humans are hindered by vast methodological challenges. Here, we overcome these challenges by developing an easy-to-use electrophysiological approach based on high-density multichannel spinal recordings combined with multivariate spatial-filtering analyses. These advances enable a spatiotemporal characterization of spinal cord responses and demonstrate a sensitivity that permits assessing even single-trial responses. To furthermore enable the study of integrative processing along the neural processing hierarchy in somatosensation, we expand this approach by simultaneous peripheral, spinal, and cortical recordings and provide direct evidence that bottom-up integrative processing occurs already within the spinal cord and thus after the first synaptic relay in the central nervous system. Finally, we demonstrate the versatility of this approach by providing noninvasive recordings of nociceptive spinal cord responses during heat-pain stimulation. Beyond establishing a new window on human spinal cord function at millisecond timescale, this work provides the foundation to study brain-body communication in its entirety in health and disease.

First Report on Real-World Outcomes with Evoked Compound Action Potential (ECAP)-Controlled Closed-Loop Spinal Cord Stimulation for Treatment of Chronic Pain

INTRODUCTION

A novel closed-loop spinal cord stimulation (SCS) system has recently been approved for use which records evoked compound action potentials (ECAPs) from the spinal cord and utilizes these recordings to automatically adjust the stimulation strength in real time. It automatically compensates for fluctuations in distance between the epidural leads and the spinal cord by maintaining the neural response (ECAP) at a determined target level. This data collection was principally designed to evaluate the performance of this first closed-loop SCS system in a 'real-world' setting under normal conditions of use in a single European center.

METHODS

In this prospective, single-center observational data collection, 22 patients were recruited at the outpatient pain clinic of the St. Antonius Hospital. All candidates were suffering from chronic pain in the trunk and/or limbs due to PSPS type 2 (persistent spinal pain syndrome). As standard of care, follow-up visits were completed at 3 months, 6 months, and 12 months post-device activation. Patient-reported outcome data (pain intensity, patient satisfaction) and electrophysiological and device data (ECAP amplitude, conduction velocity, current output, pulse width, frequency, usage), and patient interaction with their controller were collected at baseline and during standard of care follow-up visits.

RESULTS

Significant decreases in pain intensity for overall back or leg pain scores (verbal numerical rating score = VNRS) were observed between baseline [mean ± SEM (standard error of the mean); n = 22; 8.4 ± 0.2)], 3 months (n = 12; 1.9 ± 0.5), 6 months (n = 16; 2.6 ± 0.5), and 12 months (n = 20; 2.0 ± 0.5), with 85.0% of the patients being satisfied at 12 months. Additionally, no significant differences in average pain relief at 3 months and 12 months between the real-world data (77.2%; 76.8%) and the AVALON (71.2%; 73.6%) and EVOKE (78.1%; 76.7%) studies were observed.

CONCLUSIONS

These initial 'real-world' data on ECAP-controlled, closed-loop SCS in a real-world clinical setting appear to be promising, as they provide novel insights of the beneficial effect of ECAP-controlled, closed-loop SCS in a real-world setting. The presented results demonstrate a noteworthy maintenance of pain relief over 12 months and corroborate the outcomes observed in the AVALON prospective, multicenter, single-arm study and the EVOKE double-blind, multicenter, randomized controlled trial.

TRIAL REGISTRATION

The data collection is registered on the International Clinical Trials Registry Platform (Trial NL7889).

Measuring Spinal Cord Potentials and Cortico-Spinal Interactions After Wrist Movements Induced by Neuromuscular Electrical Stimulation

Electroencephalographic (EEG) correlates of movement have been studied extensively over many years. In the present work, we focus on investigating neural correlates that originate from the spine and study their connectivity to corresponding signals from the sensorimotor cortex using multivariate autoregressive (MVAR) models. To study cortico-spinal interactions, we simultaneously measured spinal cord potentials (SCPs) and somatosensory evoked potentials (SEPs) of wrist movements elicited by neuromuscular electrical stimulation. We identified directional connections between spine and cortex during both the extension and flexion of the wrist using only non-invasive recording techniques. Our connectivity estimation results are in alignment with various studies investigating correlates of movement, i.e., we found the contralateral side of the sensorimotor cortex to be the main sink of information as well as the spine to be the main source of it. Both types of movement could also be clearly identified in the time-domain signals.

Electrically Evoked Compound Action Potentials in Spinal Cord Stimulation: Implications for Preclinical Research Models

OBJECTIVES

The study aimed to assess the feasibility of recording electrically evoked compound action potentials (ECAPs) from the rat spinal cord. To achieve this, we characterized electrophysiological responses of dorsal column (DC) axons from electrical stimulation and quantified the relationship between ECAP and motor thresholds (ECAPTs and MTs).

MATERIAL AND METHODS

Naïve, anesthetized and freely behaving rats were implanted with a custom-made epidural spinal cord stimulation (SCS) lead. Epidural stimulation and recordings were performed on the same lead using specifically designed equipment.

RESULTS

The ECAPs recorded from the rat spinal cord demonstrated the expected triphasic morphology. Using 20 μsec pulse duration and 2 Hz frequency rate, the current required in anesthetized rats to generate ECAPs was 0.13 ± 0.02 mA, while the average current required to observe MT was 1.49 ± 0.14 mA. In unanesthetized rats, the average current required to generate ECAPs was 0.09 ± 0.02 mA, while the average current required to observe MT was 0.27 ± 0.04 mA. Thus, there was a significant difference between the ECAPT and MT in both anesthetized and unanesthetized rats (MT was 13.39 ± 2.40 and 2.84 ± 0.33 times higher than ECAPT, respectively). Signal analysis revealed average conduction velocities (CVs) suggesting that predominantly large, myelinated fibers were activated. In addition, a morphometric evaluation of spinal cord slices indicated that the custom-made lead may preferentially activate DC axons.

CONCLUSIONS

This is the first evidence demonstrating the feasibility of recording ECAPs from the rat spinal cord, which may be more useful in determining parameters of SCS in preclinical SCS models than MTs. Thus, this approach may allow for the development of a novel model of SCS in rats with chronic pain that will translate better between animals and humans.

Intraoperative Spinal Cord Monitoring: Focusing on the Basic Knowledge of Orthopedic Spine Surgeon and Neurosurgeon as Members of a Team Performing Spine Surgery under Neuromonitoring

An intraoperative functional spinal cord monitoring system is a technology used by spine and spinal cord surgeons to perform a safe surgery and to gain further surgical proficiency. However, no existing clinical neurophysiological method used in the operating room can monitor all complex spinal cord functions. Therefore, by observing the activities of certain neural action potentials transferred via limited neural tissues, surgeons need to deductively estimate the function of the whole spinal cord. Thus, as the number of spinal cord functions that need to be observed increases, spinal cord monitoring can be more reliable. However, in some situations, critical decision-making is affected by the limited capability of these methods. Nevertheless, good teamwork enables sharing of seamless information within the team composed of a surgeon, anesthesiologist, monitoring technician and nurses greatly contributes to making quick and accurate decisions. The surgeon, who is the person in charge of the team, should communicate with multidisciplinary team members using common technical terms. For this reason, spine and spinal cord surgeons must have appropriate knowledge of the methods currently used, especially of their utility and limitations. To date, at least six electrophysiological methods are available for clinical utilization: three are used to monitor sensory-related tracts, and three are used to monitor motor-related spinal cord functions. If surgeons perform electrode setting, utilizing their expertise, then the range of available methods is broadened, and more meticulous intraoperative functional spinal cord monitoring can be carried out. Furthermore, if the team members share information effectively by utilizing a clinically feasible judicious checklist or tools, then spinal cord monitoring will be more reliable.

Convergence of flexor reflex and corticospinal inputs on tibialis anterior network in humans

OBJECTIVE

Integration between descending and ascending inputs at supraspinal and spinal levels is a key characteristic of neural control of movement. In this study, we characterized convergence of the flexor reflex and corticospinal inputs on the tibialis anterior (TA) network in healthy human subjects. Specifically, we characterized the modulation profiles of the spinal TA flexor reflex following subthreshold and suprathreshold transcranial magnetic stimulation (TMS). We also characterized the modulation profiles of the TA motor evoked potentials (MEPs) following medial arch foot stimulation at sensory and above reflex threshold.

METHODS

TA flexor reflexes were evoked following stimulation of the medial arch of the foot with a 30 ms pulse train at innocuous intensities. TA MEPs were evoked following TMS of the leg motor cortex area.

RESULTS

TMS at 0.7 and at 1.2 MEP resting threshold increased the TA flexor reflex when TMS was delivered 40-100 ms after foot stimulation, and decreased the TA flexor reflex when TMS was delivered 25-110 ms before foot stimulation. Foot stimulation at sensory and above flexor reflex threshold induced a similar time-dependent modulation in resting TA MEPs, that were facilitated when foot stimulation was delivered 40-100 ms before TMS. The flexor reflex and MEPs recorded from the medial hamstring muscle were modulated in a similar manner to that observed for the TA flexor reflex and MEP.

CONCLUSION

Cutaneomuscular afferents from the distal foot can increase the output of the leg motor cortex area. Descending motor volleys that directly or indirectly depolarize flexor motoneurons increase the output of the spinal FRA interneuronal network. The parallel facilitation of flexor MEPs and flexor reflexes is likely cortical in origin.

SIGNIFICANCE

Afferent mediated facilitation of corticospinal excitability can be utilized to strengthen motor cortex output in neurological disorders.

Thiamylal antagonizes the inhibitory effects of dorsal column stimulation on dorsal horn activities in humans

In humans, peripheral somatosensory information converges upon dorsal horn neurons in the spinal cord, which can be recorded from the dorsal epidural space as spinal cord potentials (SCPs) following segmental dorsal root stimulation (SS) employing epidural catheter electrodes. Antidromic action potentials and descending inhibition from the dorsolateral funiculus may contribute to SCPs following dorsal column stimulation (DCS). Effects of thiamylal (2.5-7.5 mg/kg, i.v.) on SCPs evoked by independent DCS or SS were compared with those evoked by simultaneous DCS and SS (DCS/SS). DCS- and SS-evoked SCPs recorded from the lumbar enlargement consisted of a sharp negative (N) followed by a slow positive (P) potential. Thiamylal induced dose-dependent increases in amplitude and duration of both N and P potentials evoked by DCS and SS, whether the responses were summed or evoked simultaneously. In awake subjects, N and P potentials produced by simultaneous DCS/SS were significantly smaller than the sum of independent responses. Thiamylal anesthesia antagonized this inhibition; responses to simultaneous DCS/SS were larger than the sum of independent responses. These results suggest that in wakefulness DCS inhibits dorsal horn neuron activity in the lumbar spinal cord, while thiamylal antagonizes DCS-induced inhibition in dose-dependent fashion.

Evaluation of segmental spinal cord evoked magnetic fields after sciatic nerve stimulation

OBJECTIVE

We have previously reported that the measurement of spinal cord evoked magnetic fields (SCEFs) could be a helpful method for evaluating spinal cord function or detecting conduction blocks in the spinal cord. However, there have been no reports about segmental-SCEFs as a complex of axonal and synaptic activities in the spinal cord. The purpose of this study is to record and evaluate segmental-SCEFs.

METHODS

The segmental-SCEFs were measured over the lumbar dural tubes of adult rabbits using our SQUID system following sciatic nerve stimulation; spinal cord evoked potentials (SCEPs) were also measured to compare the results.

RESULTS

SCEPs showed conductive sharp waves following gentle waves, suggesting action potentials and synaptic potentials, respectively. The isomagnetic field maps of SCEFs showed a quadrupolar pattern propagating from the caudal to the cranial region within a short latency time, and after the conductive magnetic fields passed, stationary dipolar fields appeared and were sustained at some vertebral levels.

CONCLUSIONS

The quadrupolar magnetic fields were estimated to be generated from conducting action potentials, and the dipolar fields were thought to be caused by synaptic activities.

SIGNIFICANCE

Through the measurement of segmental-SCEFs, the conductive neural and synaptic activities in the spinal cord can be visualized and distinguished. This is the first report to record and visualize the sequence of events ranging from the axonal activities of peripheral nerves and the spinal tract to the synaptic activities in the spinal cord.

Effects of systemic deep hypothermia and subarachnoid block on the longitudinally conducting evoked spinal cord potentials in man

The present study reports the effects of systemic deep hypothermia (SDH) and subarachnoid block (SAB) on the longitudinally conducting evoked spinal cord potential (conducting ESCP) in man. Before induction of anesthesia, a pair of bipolar catheter electrodes was introduced to the epidural space: one at the level of the cervical enlargement and the other at the lumbosacral enlargement. The conducting ESCP was produced by electrical stimulation through the upper electrode and recorded through the lower electrode, and vice versa. SDH Study: Subjects were 6 patients who underwent replacement surgery of an aortic aneurysm with deep hypothermia anesthesia. The peak latency of the ESCP was gradually prolonged and the duration was widened with cooling via extracorporeal circulation. The amplitude of ESCP showed a biphasic change over the course of cooling with a turning point of around 30 degrees C in esophageal temperature. The ESCP was well observed until blood temperatures as low as near 10 degrees C. The result shows that ESCP is available as an intra-operative monitoring parameter of the spinal function even under SDH. SAB Study: Subjects were 7 patients, 6 of whom had SAB and the remaining 1 intravenous application of a local anesthetic. The conducting ESCP was markedly depressed or disappeared completely even after SAB with clinical doses of various local anesthetics, while it was hardly affected by the intravenous application. The result implies that SAB causes, at least partially, the conduction block within the spinal cord.

The effects of isoflurane on conditioned inhibition by dorsal column stimulation

Spinal dorsal column stimulation (DCS) modulates sensory transmission, including pain, at the dorsal horn of the cord. However, the mechanisms of DCS modulatory actions and the effects of anesthetics on these mechanisms remain to be investigated. We studied the effects of isoflurane (1.0% and 2.0%) on conditioned inhibition, the amplitude decrease of the spinal cord potentials (SCPs) after a conditioning volley (DCS), in the ketamine-anesthetized rat by recording the sharp negative (N) and slow positive (P) waves of the SCPs evoked by conditioning dorsal column (DC) and testing segmental stimulations. The N wave is believed to be the synchronized activity of the dorsal horn neurons, and the P wave, primary afferent depolarization (PAD), reflecting presynaptic inhibition. The P potentials evoked by either DC or segmental stimulation were depressed by isoflurane, whereas the N waves remained unchanged, indicating that the pharmacological characteristics of these N and P waves are similar between DC-evoked and segmentally evoked SCPs. The conditioned inhibition of segmental N and P waves by DC stimulation was almost completely suppressed by 2.0% isoflurane. The conditioned inhibition of the segmental N wave was not changed by spinal cord transection, whereas the conditioned inhibition of the segmental P wave was decreased. The results indicate that isoflurane depresses presynaptic inhibition without affecting the synchronized activity of dorsal horn neurons and, most profoundly, depresses the conditioned inhibition by DC stimulation of the dorsal horn neurons and PAD. Further, the results indicate that conditioned inhibition by DC stimulation of PAD receives a facilitatory influence from the supraspinal structures, whereas that of the synchronized activity of the dorsal horn neurons does not.

IMPLICATIONS

To investigate how anesthetics affect supraspinal modulation of sensory transmission in the spinal cord, the spinal cord potential (SCP) evoked by dorsal cord stimulation (DCS) and segmentally evoked SCP conditioned by DCS were recorded in intact and spinal cord-transected rats during isoflurane anesthesia.

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.

Spinal somatosensory evoked potential evaluation of acute nerve-root injury associated with pedicle-screw placement procedures: an experimental study

Pedicle screws for spinal fixation risk neural damage because of the proximity between screw and nerve root. We assessed whether spinal somatosensory evoked potential (SSEP) could selectively detect pedicle-screw-related acute isolated nerve injury. Because pedicle screws are too large for a rat's spine, we inserted a K-wire close to the pedicle in 32 rats, intending not to injure the nerve root in eight (controls), and to injure the L4 or L5 root in 24. We used sciatic-nerve-elicited SSEP pre- and postinsertion. Radiologic, histologic, and postmortem observations confirmed the level and degree of root injury. Sciatic (SFI), tibial (TFI), and peroneal function indices (PFI) were calculated and correlated with changes in potential. Although not specific for injuries to different roots, amplitude reduction immediately postinsertion was significant in the experimental groups. Animals with the offending wire left in place for one hour showed a further non-significant deterioration of amplitude. Electrophysiologic changes correlated with SFI and histologic findings in all groups. SSEP monitoring provided reliable, useful diagnostic and intraoperative information about the functional integrity of single nerve-root injury. These findings are clinically relevant to acute nerve-root injury and pedicle-screw insertion. If a nerve-root irritant remains in place, a considerable neurologic deficit will occur.

The placement of the epidural catheter at the predicted site by electrical stimulation test

More accurate segmental and sagittal positioning of the epidural catheter tip is required for the success of continuous epidural analgesia, spinal cord monitoring, and percutaneous epidural spinal cord stimulation. We examined the usefulness of an electrical stimulation test for verifying the proper placement of the epidural catheter tip at the predicted site in the posterior epidural space by using a locally developed epidural catheter with electrodes at its tip. The test included the observation of segmental bilateral muscle twitches and the patient's report of feeling in the region stimulated by moving the epidural catheter electrode back and forth and changing the direction of the bevel of the Tuohy needle. The success rate of midline placement at the required spinal segment was significantly more frequent (99%; P < 0.001) in the group (n = 289) receiving the electrical stimulation test compared with the group (n = 277) not receiving the test (success rate 57%). The results indicate the usefulness of this method. We concluded that the electrical stimulation test is effective for verifying the proper placement of the catheter electrode tip.

IMPLICATIONS

Ideally the epidural catheter tip should be positioned in the posterior epidural space near the midline. We concluded that the electrical stimulation test is effective for verifying the proper placement of the catheter electrode tip.

Direct Tramadol Application on Sciatic Nerve Inhibits Spinal Somatosensory Evoked Potentials in Rats

We sought to determine the possible neural conduction blockade of tramadol and whether there is evidence of localized neural toxicity with spinal somatosensory evoked potential (SSEP) measurements. Male Wistar rats were used. SSEP, elicited by supramaximally stimulating the hind paw and recorded from the thoracolumbar and the first and second lumbar interspinous ligaments, was monitored. SSEPs were obtained before drug application as the pretreatment baseline and measured every 15 min after treatment for 2 h and at 60-min intervals thereafter until SSEP returned to baseline or for another 4 h. Two small strips of Gelfoam (0.6 x 1.0 cm(2)) soaked with the drug were placed under and over the left sciatic nerve for a 30-min period. Gelfoam was prepared with tramadol hydrochloride (Tramal; the US trade name is Ultram) 5, 2.5, and 1.25 mg, diluted if needed with saline to a total volume of 100 microL (5%, 2.5%, and 1.25%, respectively). The control data were obtained from the right side limb with normal saline by following the same method. Spinal SSEPs were measured after 48 h to detect the late neural damage. The results showed that direct tramadol application on sciatic nerves dose-dependently reduced both the amplitude and conduction velocity of SSEPs when compared with the pretreatment baseline. All SSEPs returned to pretreatment baseline, and no significant changes of SSEP between bilateral limbs were noted at the 48-h measurements. No evidence of irreversible conduction blockade indicative of local neural toxicity was seen. Pretreatment with naloxone 1 mg/kg failed to block the changes of SSEP produced by 2.5% tramadol 100 microL. We conclude that tramadol exerts a local anesthetic-type effect on peripheral nerves.

Acute respiratory and metabolic acidosis induced by excessive muscle contraction during spinal evoked stimulation

Spinal somatosensory evoked potentials (SSEPs) have been used to monitor spinal cord function during corrective scoliosis surgery. We report three cases in which direct epidural stimulation for measurement of SSEPs produced paraspinal muscle contraction, resulting in respiratory and metabolic acidosis. In two of the cases, SSEP-induced acidosis was observed even when only the first twitch of the train-of-four response was detectable after a second dose of muscle relaxant. In one of these two cases, the acidosis was abolished after a sufficient dose of vecuronium to ablate the twitch response. To prevent SSEP-induced respiratory and metabolic acidosis, we recommend that SSEPs should be measured only when profound neuromuscular blockade has been obtained.

Neuromonitoring of an experimental model of clip compression on the spinal nerve root to characterize acute nerve root injury

OBJECTIVES

To evaluate the sensitivity of an electro-monitoring method in acute nerve root injury, and to determine a proposed criterion for irreversible electrophysiologic degradation.

STUDY DESIGN

Acute nerve root injury was induced by a clip compression model in rabbits, mimicking nerve root injury by a transpedicular screw. A common neuromonitoring technique, spinal somatosensory-evoked potential, was used to study the electrophysiologic change during the procedure.

SUMMARY OF BACKGROUND DATA

With the advent of the transpedicular screw system, increased risk of injury to the spinal root because of the passage of screws is not unexpected. Although both an experimental model and a clinical application in intraoperative neuromonitoring of spinal cord function have been established, the value of neuromonitoring of an acute spinal root injury remains obscure. Several neurophysiologic surveillance techniques have been used successfully to monitor the potential injury to the spinal cord during orthopedic procedures around the spinal cord and spinal column. Spinal somatosensory-evoked potential, which has the advantages of high amplitude and quick recording time, is used to detect nerve root impairment during the insertion of transpedicular screws.

METHODS

Experimental acute nerve root injury was induced in rabbits by direct hemostatic clip compression on the nerve root (S1) during different time intervals. Spinal somatosensory-evoked potential elicited by stimulating the sciatic nerve and recorded from a needle electrode at the L6-L7 interspinous ligament was monitored immediately before and after compression.

RESULTS

Spinal somatosensory-evoked potential is sensitive enough to detect the compromise of a single nerve root and that a decrease in the amplitude is the most reliable and sensitive sign. With this model, there was a statistically significant correlation between the compression time and reduction of amplitude and delay of latency. The criterion for irreversible electrophysiologic change was an amplitude loss of more than 20% and a delay in latency immediately after nerve root compression.

CONCLUSIONS

It was concluded that spinal somatosensory-evoked potential can provide immediate feedback of nerve root injury and should be considered for use during the dynamic phase of transpedicular screw insertion.

Lumbar spinal canal stenosis examined electrophysiologically in a rat model of chronic cauda equina compression

STUDY DESIGN

A model of chronic cauda equina compression with conductive stress was studied electrophysiologically.

OBJECTIVE

To analyze the pathophysiology arising from chronic compression electrophysiologically.

SUMMARY OF BACKGROUND DATA

This rat model of cauda equina compression that is chronic, not acute, has been reported elsewhere.

METHODS

A stainless steel wire and plate were fastened to the spine at L5 of 8 rats 3 weeks old. One year later, the ascending and descending nerve action potentials were recorded and the conduction velocities (CVs) were measured. Electrophysiologic changes after high-frequency stimulation (HFS) were observed.

RESULTS

The waveform of the ascending cauda equina action potential at the cauda equina had three peaks, and that at the conus medullaris had a peak followed by a broad wave. The waveform of the descending nerve action potential had two peaks. The mean ascending and descending CVs of the treated rats were slower (P < 0.001) than those of the control rats. In the control rats, the mean CV and mean amplitude after HFS decreased slightly and returned to normal within 30 seconds, and the waveform was unchanged. In treated rats, the mean CV decreased after HFS but returned to normal within 10 minutes. The mean amplitude decreased after HFS and did not return to normal within 10 minutes. The waveform was unchanged.

CONCLUSIONS

Because the differences between treated and control rats in amplitude (and CVs) were greater before HFS than after HFS, we concluded that treated rats had disturbance of the blood flow in vessels around the nerves of the cauda equina with histologic damage. In human patients, such disturbance may be one cause of intermittent claudication.

Somatosensory evoked potentials recorded from the posterior pharynx to stimulation of the median nerve and cauda equina

Somatosensory evoked potentials (ppSEPs) in response to stimulation of the median nerve at the wrist and the cauda equina at the epidural space (the L4 level) were recorded from the posterior wall of the pharynx in 15 patients who underwent spinal surgery under general anesthesia, using disc electrodes attached to the endotracheal tube, and compared with segmental spinal cord potentials (seg-SCPs) that were recorded simultaneously from the posterior epidural space (PES). ppSEPs consisted of the initially positive spike (P9) followed by slow positive (P13) and negative (N22) waves. The P13 and N22 of ppSEPs had phase reversal relationship with the P2 and N2 recorded from the PES, respectively. The peak latencies of P9 (9.40 +/- 0.7 ms) (mean +/- SD), P13 (13.1 +/- 0.9 ms), and N22 (22.0 +/- 2.1 ms) of ppSEPs coincided with those of P1, N1 and P2 of seg-SCPs, respectively, ppSEPs were recorded more clearly with a reference electrode on the dorsal surface of the neck than with the reference electrode at the earlobe or back of the hand. The threshold and maximal stimulus intensities were also similar between the ppSEPs and seg-SCPs. Thus, the P9, P13, and N22 components of ppSEPs were thought to have the same origin as the P1, N1 and P2 of seg-SCPs, respectively. Therefore, the P9, P13 and N22 of ppSEPs may reflect incoming volleys through the root, synchronized activities of the interneurons and primary afferent depolarizations (PAD), respectively. ppSEPs in response to cauda equina stimulation showed that the latencies of the two initial components (4.6 +/- 0.4 and 6.4 +/- 0.6 ms) corresponded to those of the SCPs recorded from the PES (4.6 +/- 0.3 and 6.3 +/- 0.5 ms), suggesting that these potentials reflect impulses conducting through the spinal cord, similar to epidurally recorded SCPs.

Spinal cord evoked potential monitoring after spinal cord stimulation during surgery of spinal cord tumors

Spinal cord evoked potentials (SCEPs) after spinal cord stimulation were used as a method of spinal cord monitoring during surgery of 6 extramedullary and 14 intramedullary spinal cord tumors. SCEPs were recorded from an epidural electrode placed rostral to the level of the tumor. Electrical stimulation was applied on the dorsal spinal cord from a caudally placed epidural electrode. The wave forms of SCEPs consisted of a sharp negative peak (N1) in 15 cases and two negative peaks (N1 and N2) in 5 cases. The N2 wave was markedly attenuated by posterior midline myelotomy, whereas the N1 activity showed less-remarkable changes by myelotomy. An increase in N1 amplitude was observed after the removal of the tumor in four extramedullary and three intramedullary cases. Of six patients that showed decreased N1 amplitude after the removal of the tumor, five patients developed postoperative motor deficits. However, there were four false-negative cases and one false-positive case in regard to changes of N1 amplitude and postoperative motor deficits. Four false results occurred in intramedullary cases. In two of them, postoperative symptoms indicated intraoperative unilateral damage to the spinal cord. The position of the stimulating electrode, the difference in thresholds of the axons for electrical stimulation between the right and left side of the spinal cord, or the change of the distance between the electrode and the spinal cord surface may account for these false results. Thus, our analysis of the changes of SCEP wave forms and early postoperative symptoms indicates that the sensitivity of this monitoring method to detect intraoperative insults to the spinal cord is unsatisfactory in spite of the reproducible wave forms. We conclude that SCEP monitoring can be used as an alternative method or in combination with other types of evoked potentials in patients with severe spinal cord lesions who show abnormal somatosensory evoked potentials preoperatively.

Effects of dorsal root entry zone lesion on spinal cord potentials evoked by segmental, ascending and descending volleys

The spinal cord potentials (SCPs) were recorded from the dorsal root entry zone (DREZ) and posterior epidural space in patients before and after dorsal root entry zone lesion (DREZL) during general anaesthesia. The SCPs from the DREZ activated by segmental, ascending and descending volleys were basically the same in fundamental waveform as those recorded from the posterior epidural space. Segmentally activated slow negative (N1) wave, reflecting synchronized activities of dorsal horn neurones, and positive (P2) wave, thought to indicate primary afferent depolarization, were affected by DREZL in all 4 subjects tested, even by contralateral stimulation, suggesting that these components of the segmental SCPs in man partly reflect the activities of the contralateral dorsal horn. The spike-like potentials activated by ascending volleys were not affected by DREZL, while the subsequent slow components were decreased in the lesioned level. This may indicate that ascending spinal cord tracts are not affected by the operation, and suggests that the origin of the slow components by ascending volleys lies at least in part in the segmental dorsal horn. The slow negative and positive components, recorded at a remote segment from DREZL, in response to the descending volleys, were augmented after DREZL, suggesting that activation of ascending or descending inhibition through a feedback loop via the supraspinal structures might occur at least transiently following DREZL. All components of the SCPs activated by descending volleys were decreased or disappeared in recording from the lesioned level, as expected. Thus, intra-operative recording of the SCPs during DREZL might be beneficial for monitoring and studying human spinal cord function.

Evaluation of the central nervous function in resuscitated comatose patients by multilevel evoked potentials

Multilevel evoked potentials were examined in 17 patients who became comatose after cardiac arrest and resuscitation. In 4 patients, the P1 through N3 components of the somatosensory evoked cerebral potential (SECP) were present altogether within 100 ms after the ischemic insults. They all subsequently regained consciousness, though three of them developed intelligence and motor disturbances to some extent. In 11 patients who regained consciousness, or remained in a vegetative state, the evoked potentials which reflect brainstem functions, such as the auditory evoked brainstem potential, the R1 wave of the orbicularis oculi reflex and the slow positive wave of the somatosensory evoked brainstem potential, were recognized. The somatosensory evoked spinal potential and spinal monosynaptic reflex showed normal appearances in the state of vegetation and even after the determination of brain death. The measures of SECP could be useful in predicting restoration of consciousness.

Intraoperative recordings of spinal somatosensory evoked potentials to tibial nerve and sural nerve stimulation

Somatosensory evoked potentials (SSEPs) to stimulation of the tibial nerve at the knee (TN-K) and ankle (TN-A), and the sural nerve at the ankle (SN-A), were recorded from 3 or 4 spinal levels during surgery for scoliosis in 11 neurologically normal subjects. With stimulation of all 3 nerves, the propagation velocity along the spine was nonlinear: it was faster over cauda equina and midthoracic cord than over caudal spinal cord. Over the mid-thoracic cord, TN-K SSEP propagation was faster than that of TN-A and SN-A SSEPs, whereas over the caudal spinal cord these values were similar on stimulation of all 3 nerves. These data suggest that fast conducting second order afferent fiber systems contribute to spinal cord SSEPs evoked by stimulating both mixed and cutaneous peripheral nerves.

Distribution of lumbar spinal evoked potentials and their correlation with stimulation-induced paresthesiae

In 7 awake patients with neuropathic lower extremity pain, spinal somatosensory evoked potentials (SEP) were elicited from the non-painful leg by electrical stimulation of the peroneal nerve and mechanical stimulation of the hallux ball. Recording was made epidurally in the thoraco-lumbar region by means of an electrode temporarily inserted for trial of pain-suppressing stimulation. In response to peroneal nerve stimulation, two major SEP complexes were found. The first complex consisted, as has been described earlier, of an initial positivity (P12), a spike-like negativity (N14), a slow negativity (N16) and a slow positivity (P23). The second complex consisted of a slow biphasic wave, conceivably mediated by a supraspinal loop. Both complexes had a similar longitudinal distribution with amplitude maxima at the T12 vertebral body. The SEP evoked by mechanical hallux ball stimulation had a relatively small amplitude, and there was no significant second complex. The relationship between stimulus intensity and SEP amplitude was negatively accelerating. The longitudinal distribution of spinal SEP was compared with the somatotopic distribution of paresthesiae induced by stimulation through the epidural electrode. It was found that stimulation applied at the level of maximal SEP generally induced paresthesiae in the corresponding peripheral region. Therefore, spinal SEP may be used as a guide for optimal positioning of a spinal electrode for therapeutic stimulation when implanted under general anesthesia. An attempt was made to record the antidromic potential in the peroneal nerve elicited from the dorsal columns by epidural stimulation. The antidromic response was, however, very sensitive to minimal changes of stimulus strength and body position of the patient, and was also contaminated by simultaneously evoked muscular reflex potentials. Thus, peripheral responses evoked by epidural stimulation appeared too unreliable to be useful for the permanent implantation of a spinal electrode for therapeutic stimulation.

Spinal cord responses to feline transcranial brain stimulation: evidence for involvement of cerebellar pathways

The physiological characteristics of spinal cord responses recorded from the spinal epidural space of the cat to transcranial brain stimulation were studied, in comparison with the spinal cord responses to direct stimulation of the motor cortex or cerebellum. The conduction velocity of the initial wave of the responses to transcranial brain stimulation (122.3 +/- 16.3 m/sec mean +/- SD, n = 5) was much faster than the conduction velocity of the initial wave of the responses to motor cortex stimulation (68.3 +/- 14.7, n = 5) and similar to the conduction velocity of the initial wave of the responses to cerebellar stimulation (120.2 +/- 16.2, n = 5). Furthermore, the conduction velocity of any component in the subsequent polyphasic waves at any intensity was not similar to the conduction velocity of the initial wave of the responses to motor cortex stimulation. All components of the responses to motor cortex stimulation disappeared after intercollicular transection. In contrast, the initial wave of the responses to cerebellar stimulation and transcranial brain stimulation remained unaffected by intercollicular transection. The initial wave caused by anodal transcranial brain stimulation was eliminated by ablation of the cerebellum. However, cathodal transcranial brain stimulation sometimes can produce an initial wave that can be eliminated only by transection at the medullospinal junction. The initial wave of the responses to cerebellar stimulation was largest in amplitude when the vicinity of the dentate nucleus was stimulated. These results suggest that responses to activation of the cerebellum, rather than corticospinal neurons arising from the motor cortex, represent a major component of the spinal cord responses to transcranial brain stimulation in cats. The data obtained indicate that it is difficult to activate the motor cortex selectively by transcranial brain stimulation in cats.

Epidurally recorded cervical spinal activity evoked by electrical and mechanical stimulation in pain patients

Spinal SEPs to electrical and mechanical stimulation of the upper limb of the non-painful side in 7 pain patients were recorded from the cervical epidural space. In response to electrical stimulation of the median nerve, the longitudinal distribution of the spinal postsynaptic negativity (N13) along the cord had a distinct level of maximal amplitude at the C5 vertebral body. When recorded at increasing distances cranial or caudal to this level, the latency of N13 was successively prolonged, in agreement with a spread-out near-field generator in the dorsal horn. Similar patterns of distribution and levels of maximal amplitude were demonstrated for the N13 wave evoked by electrical stimulation of the ulnar and thumb nerves as well as by mechanical stimulation of the thumb ball. The amplitude ratios of the N13 waves evoked by electrical stimulation of the median nerve and the thumb nerves, and by mechanical stimulation of the thumb ball were 3.9 to 1.4 to 1. The slow positive wave (P18), which has been assumed to represent recurrent presynaptic activity, had a somewhat different distribution, with a lower maximal amplitude and a less marked falling off in amplitude along the cord, as compared to the N13 component. The initial presynaptic positivity (P10) appeared with an almost constant amplitude along the cord. Tactile stimuli produced responses with considerably longer latency and duration than those obtained with electrical stimulation. There seemed to be a non-linear relationship between the amplitude of the response and the depth of skin indentation. The presented data contribute a more detailed picture of epidurally recorded spinal SEPs than previous studies. They will serve as a reference for further analysis of SEPs evoked by stimulation of the affected side in pain patients, to explore whether the painful state is associated with altered SEPs before or after therapeutic intervention.

The origins of lumbosacral spinal evoked potentials in humans using a surface electrode recording technique

Somatosensory evoked potentials were recorded over the lumbar spine and scalp in 12 normal subjects after stimulating the posterior tibial nerve at the knee and ankle and the sural nerve at the ankle. The H-reflex from the soleus muscle was recorded at the same time. The effects of stimulus intensity, frequency of stimulation and vibration were assessed. It was concluded that when the posterior tibial nerve was stimulated in the popliteal fossa, three negative peaks were recorded over the lumbosacral area. They arose from activity in the dorsal roots, the dorsal horn of the spinal cord (SD) and the ventral roots. In contrast when the posterior tibial nerve and the sural nerve were stimulated at the ankle only two negative peaks were recorded, a dorsal root potential and a spinal cord dorsum potential. In addition the data suggested that the peripheral nerve fibres that are involved with generating the surface recorded spinal potential with mixed nerve stimulation are primarily muscle afferents.

Paraspinal stimulation to elicit somatosensory evoked potentials: an approach to physiological localization of spinal lesions

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the paraspinal region. Simultaneous bilateral stimulation, 2 cm lateral to the midline, sufficient to induce a visible muscle twitch, was applied opposite vertebral levels L3, T12, T6 and T1 and intervening segments in some subjects. The potentials were recorded over the scalp (Cz-Fz). The stimulus excludes most of the peripheral nervous system; the volley being initiated in the cutaneous branches of the primary dorsal root rami with some contribution from paraspinal muscle Ia afferents. In normal subjects, paraspinal evoked SEPs are easily elicitable and measurable. Mean spinal cord conduction velocity between T12 and T1 measured 64.1 m/sec (N = 25). The upper thoracic cord propagated faster than the lower thoracic cord which conducted faster than the lumbar segment. The technique was used to confirm the approximate level of radiologically visible spinal lesions that were surgically treated and to identify diffuse, focal or multisegmental spinal conduction slowing in patients devoid of radiologically visible lesions. The method has potential for intraoperative spinal cord monitoring.

Conduction characteristics of somatosensory evoked potentials to peroneal, tibial and sural nerve stimulation in man

Lumbar spine and scalp short latency somatosensory evoked potentials (SSEPs) to stimulation of the posterior tibial, peroneal and sural nerves at the ankle (PTN-A, PN-A, SN-A) and common peroneal nerve at the knee (CPN-K) were obtained in 8 normal subjects. Peripheral nerve conduction velocities and lumbar spine to cerebral cortex propagation velocities were determined and compared. These values were similar with stimulation of the 3 nerves at the ankle but were significantly greater with CPN-K stimulation. CPN-K and PTN-A SSEPs were recorded from the L3, T12, T6 and C7 spines and the scalp in 6 normal subjects. Conduction velocities were determined over peripheral nerve-cauda equina (stimulus-L3), caudal spinal cord (T12-T6) and rostral spinal cord (T6-C7). Propagation velocities were determined from each spinal level to the cerebral cortex. With both CPN-K and PTN-A stimulation the speed of conduction over peripheral nerve and spinal cord was non-linear. It was greater over peripheral nerve-cauda equina and rostral spinal cord than over caudal cord segments. The CPN-K response was conducted significantly faster than the PTN-A response over peripheral nerve-cauda equina and rostral spinal cord but these values were similar over caudal cord. Spine to cerebral cortex propagation velocities were significantly greater from all spine levels with CPN-K stimulation. These data show that the conduction characteristics of SSEPs over peripheral nerve, spinal cord and from spine to cerebral cortex are dependent on the peripheral nerve stimulated.

Intraoperative evoked potentials recorded in man directly from dorsal roots and spinal cord

Direct spinal cord surface recordings of evoked spinal cord potentials have been made in 26 patients during neurosurgical procedures for intractable pain. Monopolar recordings at the dorsal root entry zone after peripheral nerve stimulation have been made at multiple levels for segmental localization and to monitor the state of the afferent path and dorsal horn. Dorsal root and dorsal column conduction has been tested on diseased and intact sides. Normal afferent conduction velocity was found to have an overall mean of 61.33 m/sec for cervicothoracic and lumbosacral peripheral nerves, and 50 m/sec for the dorsal columns. The normal mean amplitude for the slow negative wave (N1) recorded at the root entry was 52.54 muV, while that for the dorsal column conducted response recorded within 4 cm of the stimulus point on the dorsal columns was 347.5 muV. Several different placements of stimulating and recording electrodes are described, as well as their application. An interpretation of the resulting data is proposed.

Recovery functions of spinal cord and subcortical somatosensory evoked potentials to posterior tibial nerve stimulation: intrasurgical recordings

The recovery function of evoked potentials to posterior tibial nerve stimulation was studied. Intrasurgical recordings were made from interspinous ligaments at lumbar levels and from high thoracic-low cervical level. In addition, surface recordings were obtained from neck-scalp derivations. The recovery function of the potentials recorded from lumbar and from high thoracic-low cervical spinal cord were very similar, showing an early period of supernormality (5-20 ms) followed by a period of subnormality which reached its lowest point at 40-60 ms. Assuming that the potentials recorded at the lumbar level reflect activity in the cauda equina, we conclude that the results support the hypothesis that the potentials recorded from the thoraco-cervical level reflect activity in the dorsal columns. The recovery curve of the amplitude between the far field potentials P27 (which most probably reflects activity of the afferent volley at the level of foramen magnum) and N30 (which, by latency criteria, would reflect lemniscal or thalamic activity) showed a similar shape but with a shorter duration of the periods of super- and subnormality. It is likely that this modification was due to the synapse at the gracilis nucleus. The first cortical component (P32) recorded in the neck-scalp derivation was totally abolished within the recovery period studied (50 ms interval).

Pathways of ascending evoked spinal cord potentials of dogs

Pathways of the ascending evoked potentials (AEPs) of the spinal cord of the dog were investigated by stimulating rear leg nerves and recording at thoracic levels before and after making selective partial or complete transections of topographic regions of the spinal cord in the L-1 segment: hemisection; ventral quadrant (VQ); dorsal quadrant (DQ); dorsolateral fasciculus (DLF); dorsal columns (DCs). The AEPs were found to be propagated in all topographic areas of the spinal cord. The contribution by the DQ ipsilateral to the stimulated nerve appeared to be the largest. Within the DQ, both the DLF and the DC participated in the AEP but the DLF contribution tended to predominate in the earliest parts while the DC contribution tended to lag somewhat behind that of DLF. The predominance of DLF activity in early phases of the AEP probably reflected the influence of high conduction velocities in muscle afferents and in postsynaptic DLF axons having connections with muscle afferents or with cutaneous afferents. The VQ contribution to the AEP appeared to arise from both crossed and uncrossed pathways but was not otherwise defined. Based on the results and on data in the literature, important considerations in interpretation of AEPs are: the relative numbers of muscle or cutaneous afferent fibers in the stimulated nerves; temporal dispersion of action potentials associated with differences in conduction velocities; conduction distances in the stimulated primary afferent axons and in postsynaptic spinal cord axons.

A method of calculating spinal cord transit time from potentials evoked by tibial nerve stimulation in normal subjects and in patients with spinal cord disease

Somatosensory potentials were evoked by stimulation of the tibial nerve at the ankle and recorded over the spine and scalp in 16 normal subjects and 26 patients with known or suspected spinal cord disease, with the aim of developing a method of measuring spinal sensory conduction velocity using a tolerable number of stimuli, applied unilaterally to alert subjects. In normal subjects N21 was consistently recorded over L1 vertebra and in most subjects a complex, N27/N29/P33, was recorded over the cervical spine referred to the vertex. Constant latencies at different spinal levels and, in one subject, comparison with the latency of the ascending volley indicate that the complex was not derived from the spinal cord but from more rostral structures, and therefore only transit time, rather than velocity, could be measured. In patients with clinically definite multiple sclerosis, even with minimal clinical signs, the N27/N29/P33 complex was always abnormal. Abnormalities in this and other forms of spinal cord disease were commonly absence or distortion of the complex, prolonged transit time being rare. The clinical value of the method is limited by the very low amplitude of the responses.

The cervical somatosensory evoked potential in man: far-field, conducted and segmental components

Somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded from neck and scalp electrodes in 23 normal adults using cephalic and non-cephalic (knee) references simultaneously. With a cephalic reference, the neck SEP consisted of several 'negative' potentials that had the same latency at all recording locations. Simultaneous recordings from neck-scalp, neck-knee and scalp-knee derivations demonstrated that scalp far-field potentials significantly contributed to neck-to-scalp recordings and obscured the cervical SEPs. With a non-cephalic reference, the neck SEP consisted of a prominent positive wave (P9) followed by a large negative component (N13). A small positive potential, P10, seen in the lower neck, gradually increased in latency and amplitude from lower to upper neck and appeared as a P11 potential at upper cervical levels. In lower neck recordings, a negative wave, N11, was also present and in some subjects exhibited a latency shift from lower to upper neck. P9, P11 and N11 had a short refractory period suggesting a presynaptic origin whereas N13 had a longer refractory period indicating a postsynaptic generator. The consensus that P9 originates in the peripheral nervous system is consistent with its rapid recovery cycle. The bipolar characteristics of N11 and P11 as well as their latency shift and their short recovery cycle suggest that they reflect activity in the cervical dorsal columns. N13, that displayed no latency shift and had a longer recovery cycle, may originate in spinal cord dorsal horn interneurons.

[Spinal and subcortical somatosensory evoked potentials after stimulation of the tibial nerve]

Evoked potentials in response to unilateral stimulation of the posterior tibial nerve at the ankle were recorded above the spinous processes L5, L1, C2, and at Cz' in 30 normal subjects. The "cauda-potential" recorded above L5 consists of two small negative peaks with a mean latency of 18 and 22.5 ms respectively, whereas the "cord-potential" recorded above L1 exhibited a peak latency of 21.2 ms and on average a three-times larger amplitude than the first of the two "cauda-potentials" (Fig. 1). Leads from the spinous process C2 revealed a sharp negative peak with a mean peak latency of 28.8 ms (N 30). Scalp recordings with a midfrontal (Fz-) reference inconsistently showed 1-2 small waves (P31, N33) prior to the primary cortical response (P40). Recordings with an ear- or non-cephalic reference consistently showed a large positive deflection (P30) which corresponded in latency with the simultaneously recorded cervical response (N30) and was followed by a distinct negative potential (N33) (Fig. 2a and b). Average latencies and amplitudes of the different spinal and subcortical evoked potentials (Tables 1 and 2), as well as the diagnostically more important interpeak-intervals, amplitude relations, and side-differences of latencies and amplitudes (Tables 3 and 4) were calculated. The diagnostic significance of these parameters will be shown in selected cases with spinal cord pathology.

Conducted somatosensory evoked potentials during spinal surgery. Part 1: control conduction velocity measurements

Intraoperative recordings of conducted bipolar epidural somatosensory evoked potentials (SEP's) generated by unilateral common peroneal nerve stimulation have been obtained in 27 patients. The SEP's were multiphasic, 0.3 to 1.5 microV in amplitude, and recorded in 100% of patients with normal cords or in patients with spinal lesions, at a site caudal to the lesions. Control spinal conduction velocities (CV's), measured in the midthoracic to lower cervical regions, were in the range of 65 to 85 m/sec. Control lumbar and lower thoracic CV's were in the range of 30 to 45 m/sec. The CV values were obtained periodically throughout the course of surgery and were plotted as a function of time. In control patients with extradural lesions and neuroleptic anesthesia, the CV's remained constant (+/- 3%). The consistency, sensitivity, and safety of SEP recordings obtained by this technique make precise monitoring readily available during spinal operations.

Sensory nerve conduction in the human spinal cord: epidural recordings made during scoliosis surgery

This report describes the waveform and properties of somatosensory evoked potentials recorded from various levels of the human spinal cord, with electrodes inserted into the epidural space and the stimulus delivered to the posterior tibial nerve at the knee. The object was to provide a means of monitoring spinal cord function during surgery for the correction of spinal deformities. The responses could be resolved into at least three components with different activation thresholds and different conduction velocities within the spinal cord (45-80 m/s approximately). The findings are in accord with recent studies, suggesting that the fast activity may be conducted in the dorsal spinocerebellar tract and the slower waves in the posterior columns.

Spinal evoked potentials evaluated with two relevant electrode types

The spinal evoked potentials (SEPs) were recorded at the level of the 7th lumbar segment in five cats by stimulating the popliteal nerve separately on both sides. The recordings were performed both with silver-silver chloride cup electrodes and stainless steel needle electrodes. The shapes and latencies of the responses were highly similar, when comparing the responses recorded with the two electrode types as shown by means of transfer function: attenuation and phase curves for both electrode types are highly similar in the bandpass used in the present study. It is concluded that the properties of stainless steel needle electrodes are highly correspondent with the conventional silver-silver chloride electrodes when somatosensory evoked responses are recorded.

Somatosensory evoked potentials in diagnostics of cervical spondylosis and herniated disc

Somatosensory evoked potentials (SEPs) were recorded in 16 healthy controls and 12 patients suffering symptoms typical of cervical spondylosis and/or herniated disc and showing abnormal findings both in clinical and myelographic examinations. SEP recordings were performed at 3 different levels of the nervous system, at Erb's point, over the 7th cervical spine and at the inion, by stimulating the median nerve separately at both wrists. Nine of the 12 patients were chosen for surgical treatment, while 3 patients received physiotherapy. Six out of the 12 patients showed abnormal findings in SEP recordings, while the 3 patients chosen for the physiotherapy group showed normal SEP recordings. In the present study the amplitudes of the responses may seem to reflect nerve lesions caused by spondylotic deformities more than the latencies of responses. The results suggest that the SEP recordings may have value as a diagnostic aid in cervical spondylosis and spinal stenosis, which finding is in agreement with those of some other authors.

Conducted and segmental components of the somatosensory cervical response

Cervical responses evoked by stimulation of the median nerve have been concurrently recorded from C7--Fz and C7--Sn (suprasternal notch). The existence of two different waveforms (RI and RII) has been confirmed. RI (from C7--Fz) consists of four negative peaks (N9, N11, N13, N14) followed by a large positive deflection (P16). RII (from C7--Sn) is characterised by an early positive--negative spike (P1--N1a) followed by a slow negative--positive wave (N1b--P2). The study of the most relevant parameters (polarity, latency and refractory period) of each component of RI and RII did not indicate whether the generators underlying RI differ from those responsible for RII. However, stimulation of the lower limb, which does not involve segmental events at cervical level, showed a clearcut difference: no response was recorded from C7--Sn, while evoked activity equivalent to RI was obtained from C7--Fz. Therefore it is suggested that RII is entirely generated by segmentally evoked potentials while RI is mainly due to conducted potentials.

Central somatosensory conduction in man: neural generators and interpeak latencies of the far-field components recorded from neck and right or left scalp and earlobes

Early somatosensory evoked potential (SEP) components to median nerve or finger stimulation were recorded with non-cephalic references in normal young adults. Detailed topographic data over scalp and neck were related to anatomical observations on the actual conduction distances in dorsal column, medial lemniscus and thalamo-cortical parts of the somatosensory pathway. The extrapolation of afferent conduction velocity (CV) measured from sensory nerve potentials along the peripheral nerve to the C6-C7 spinal segments identified the spinal entry time with the onset of the neck N11 or scalp P11 (far field 2 or FF2). The first far field (FF1) is generated in the nerve proximal to axilla. The definite latency shift of the spinal negativity along the neck indicates a CV of 58 m/sec. Data about the maximal diameter of lemniscal axons in man were used to calculate a CV of 40.5 m/sec. Consideration of transit times from spinal entry to cortex and of synaptic delays clarified the arrival times of the afferent volley at various relay nuclei, and also suggested a thalamo-cortical CV of about 33 m/sec. Interpeak and onset-to-peak measures on scalp far fields suggest that FF3-FF4 are generated in medial lemniscus rather than above the thalamus. Consistent differences in amplitude, but not in wave form, were recorded at right and left earlobes for FF2 (larger ipsilaterally) and FF3-FF4 (larger contralaterally). The scalp topography of far fields was analysed in detail.

Estimation of the brain and spinal cord conduction time in man by means of the somatosensory evoked potentials and F and H responses

Somatosensory evoked potential (SSEP) recordings from the scalp were performed in 17 healthy subjects. In seven of these SSEP was also recorded at the level of the second lumbar spine. In the other ten F and H responses and the corresponding M responses were studied. By means of the SSEP recordings at the level of the second lumbar spine and the F- and H-responses, the conduction time in the brain and spinal cord, that is central latency, was calculated and the following results were obtained: 16.0 ms with standard deviation (SD) +/- 1.1 ms (by means of SSEPs), 9.5 +/- 2.4 ms (by means of F response) and 13.1 +/- 1.5 ms (by means of H response). Of the three methods used the H response method seems to be the best for clinical purposes: it is easy to perform and statistically it is more stable than the F response recording; moreover the recording can be performed reliably even in persons with thick back muscles and subcutaneous fat, unlike the evoked potential procedure which only with difficulty shows detectable responses at the lumbosacral levels in such persons. Three patients are presented to illustrate the technique; in one of these the recording evoked potentials from the epidural space were recorded.

Brachial plexus and radicular neurography in relation to cortical evoked responses

An application of somatosensory potential recording suitable for clinical neurodiagnostics is described. Evoked responses were recorded with surface electrodes at four levels between wrist and scalp: Erb's point, seventh cervical spine, inion, and the somatosensory area of the scalp. The normal latency and latency difference values based on 16 healthy subjects are presented as well as those of four examples of pathological cases with lesions at various levels in the nervous system. The method presented offers novel possibilities for solving problems of differential diagnosis, especially at the level of the brachial plexus.