Experimental investigation on the spinal cord evoked injury potential

Neurophysiological assessment of spinal cord injuries in dogs using somatosensory and motor evoked potentials

Somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) are non-invasive neurophysiological tests that reflect the functional integrity of sensory and motor pathways. Despite their extensive use and description in human medicine, reports in veterinary medicine are scarce. SSEPs are obtained via peripheral stimulation of sensory or mixed nerves; stimulation induces spinal and cortical responses, which are recorded when sensory pathways integrity is preserved. MEPs can be obtained via transcranial electrical or magnetic stimulation; in this case, thoracic and pelvic limb muscle responses are captured if motor pathways are preserved. This review describes principles, methodology and clinical applicability of SSEPs and MEPs in companion animal medicine. Potential interferences of anesthesia with SSEP and MEP recording are also discussed.

Somatosensory and spinal evoked potentials in patients with upper cervical neurinoma

Scalp somatosensory evoked potentials (SEPs) and spinal evoked potentials (SpEP) were simultaneously recorded from the exposed surface of the upper cervical cord after median nerve stimulation in five patients undergoing surgery for upper cervical neurinomas. Two of the neurinomas were localized at C1 nerve root, two at C2, and one at C3. All patients showed good postsurgical recovery, suggesting that the tumors had not progressed to the stage where most of the nerve fibers were irreparably damaged. In patients with unaffected superficial and deep skin sensation, both SEP and SpEP were normal. In patients with more advanced tumor, the superficial sensation was abnormal but the deep skin sensation was intact. In these patients, the action potential propagation slowed down but continued partially through the tumor site on the relatively less affected side contralateral to the tumor; however, it stopped at the site of the tumor on the ipsilateral side. It is possible that full functional recovery becomes more difficult during the next stage of tumor development when the propagation of action potentials ceases bilaterally. The intraoperative monitoring of both SEP and SpEP thus appears useful for inferring details of functional integrity and prognosis of the spinal cord near a space-occupying tumor during the critical first two stages of neoplasm in which the spinal function is normal, or a sufficiently large fraction of ascending and descending nerve fibers are functionally suppressed, but are capable of recovery after a surgical intervention.

Objective assessment of cervical spinal cord injury levels by transcranial magnetic motor-evoked potentials

BACKGROUND

The neurologic examination serves as the optimal method to record the level of spinal cord injury (SCI). However, this test is subject to interexaminer variability. To address this shortcoming, we describe a technique that uses transcranial magnetic motor-evoked potentials (tcMMEPs) and dermatomal somatosensory-evoked potentials (d-SSEPs) to more accurately measure the precise level of SCI.

METHODS

Two groups of subjects were studied: (1) complete cervical SCI (n = 10) and (2) neurologically intact volunteers (n = 10). Two additional patients were evaluated: one with a cervical central spinal cord syndrome and another with a head injury with a suspected cervical SCI. Each subject underwent upper extremity tcMMEPs and d-SSEPs.

RESULTS

Transcranial magnetic motor-evoked potentials were elicited from all upper limb myotomes (C4-T1, bilaterally) in neurologically intact volunteers (20 sides). The level of injury was determined using tcMMEPs by observing the lowest level of measurable response. The level of injury obtained using tcMMEPs was the same as that determined by neurologic examination in 13 (65%) of the 20 sides. In 7 sides, tcMMEP responses were obtained 1 level lower than that assessed by physical examination. Dermatomal somatosensory-evoked potentials were obtained from all dermatomes of volunteers tested in the laboratory compared with only 5 of the 9 patients with SCI who underwent d-SSEP testing.

CONCLUSION

Testing using tcMMEPs provides an objective supplement to the neurologic examination after acute cervical SCI. Dermatomal somatosensory-evoked potentials were of limited value in determining the level of cervical SCI.

TIBIAL NERVE SOMATOSENSORY EVOKED POTENTIALS IN DOGS WITH DEGENERATIVE LUMBOSACRAL STENOSIS

OBJECTIVE

To determine somatosensory evoked potentials (SEPs) in dogs with degenerative lumbosacral stenosis (DLS) and in healthy dogs.

STUDY DESIGN

Clinical and experimental study.

ANIMALS

Dogs with DLS (n = 21) and 11 clinically normal dogs, age, and weight matched.

METHODS

Under anesthesia, the tibial nerve was stimulated at the caudolateral aspect of the stifle, and lumbar SEP (LSEP) were recorded percutaneously from S1 to T13 at each interspinous space. Cortical SEP (CSEP) were recorded from the scalp.

RESULTS

LSEP were identified as the N1-P1 (latency 3-6 ms) and N2-P2 (latency 7-13 ms) wave complexes in the recordings of dogs with DLS and control dogs. Latency of N1-P1 increased and that of N2-P2 decreased as the active recording electrode was moved cranially from S1 to T13. Compared with controls, latencies were significantly delayed in DLS dogs: .8 ms for N1-P1 and 1.7 ms for the N2-P2 complex. CSEP were not different between groups.

CONCLUSIONS

Surface needle recording of tibial nerve SEP can be used to monitor somatosensory nerve function of pelvic limbs in dogs. In dogs with DLS, the latency of LSEP, but not of CSEP, is prolonged compared with normal dogs.

CLINICAL RELEVANCE

In dogs with lumbosacral pain from DLS, the cauda equina compression is sufficient to affect LSEP at the lumbar level.

Multivariate analysis of the neurological outcome of surgery for cervical compressive myelopathy

BACKGROUND

The neurological outcome of decompressive surgery for cervical myelopathy is influenced by several factors. Although each factor may have an independent effect, it is more likely that the outcome is influenced by more than one factor. We examined the results of multivariate analysis and multiple regression analysis of the neurological outcome of patients treated by cervical cord decompression.

METHODS

A total of 77 patients with cervical spondylotic myelopathy (43 men, 34 women) and 58 with ossification of the posterior longitudinal ligament (OPLL) (39 men, 19 women) were studied with an average follow-up interval of 8.3 years. The clinical data, neurological and radiological findings, and results of spinal cord evoked potentials (SCEPs) were retrieved from the medical records and included in the analysis.

RESULTS

Multivariate analysis indicated that the outcome for patients with spondylosis was positively influenced, in order of importance, by increased transverse area of the cord >or=60%, presence of single-level anterior fusion, a high preoperative neurological score, normal epidural SCEPs, and clinical features of brachialgia and cord type. In patients with OPLL, multivariate analysis showed that the long-term outcome was positively influenced, in order of importance, by the presence of mixed or localized OPLL, normal epidural SCEPs, high preoperative neurological score, a single-vertebra spondylectomy with anterior fusion, laminoplasty, widening of the transverse area of the cord >or=40%, and an expansion rate of the spinal canal after laminoplasty >or=40%.

CONCLUSIONS

We suggest that multivariate analysis is useful for assessing the neurosurgical outcome in patients with cervical compressive myelopathy.

Dorsal root entry zone (DREZ) localization using direct spinal cord stimulation can improve results of the DREZ thermocoagulation procedure for intractable pain relief

The dorsal root entry zone (DREZ) thermocoagulation for intractable pain after brachial plexus avulsion was performed in 21 patients. Good results in pain relief (relief of more than 75% of preoperative pain) were achieved in 62% of patients, whereby fair results (relief of 25-75% of preoperative pain) in 38% of patients. There was no patient with poor result (relief of less than 25% of preoperative pain). Complication rate was 14%. The whole patient population was subdivided into two groups (Group 1 and Group 2). Direct spinal cord bipolar stimulation and registration with the goal to localize DREZ was performed in the Group 2 consisting of 12 patients (n=12). The point on the spinal cord surface where no response after stimulus of low intensity was obtained was the site (the posterolateral sulcus) we identified as the most suitable point for the placement of radiofrequency thermocoagulation electrode. Comparing with the Group 1 consisting of nine patients (n=9), where the localization of DREZ by evoked potentials was not performed, significantly better effect of pain relief was recorded (P<0.05, odds ratio 10). There was no statistically significant difference (P>0.7) in complication rate in Group 1 and Group 2. Described electrophysiological technique is very helpful in identifying of DREZ and, in combination with microsurgical technique, can create DREZ thermocoagulation more effective.

Electrophysiologic intraoperative monitoring for spine procedures

The advent of equipment capable of performing SEPs, MEPs, and EMG in a multiplexed manner and in a timely fashion brings a new level of monitoring that far exceeds the previous basic monitoring done with SEPs only. Whether this more comprehensive monitoring will result in greater protection of the nervous system awaits future analysis. In any event, monitoring of the spinal cord with SEPs is an accepted standard of care for cases that place the spinal cord at risk. Likewise, nerve root monitoring with EMG is a widely practiced form of monitoring and shows great benefit. MEPs and reflex monitoring, which address the descending pathways and the interneuronal connections, is efficacious in detecting abnormalities that may be missed by SEPs.

Motor Evoked Potential Monitoring for the Surgery of Brain Tumours and Vascular Malformations

Brain surgery incurs a significant risk of a new motor deficit in lesions within or adjacent to the motor areas and pathways which, for the patient, presents one of the most disabling complications of such operations. It is a major concern of intracranial procedures to delineate and monitor motor regions in order to preserve their structural and functional integrity, while still achieving maximal cytoreduction. The technique of motor evoked potential recording has had to be adapted to intraoperative recording conditions under general anaesthesia, but has been available for clinical use now for almost ten years. This contribution summarizes the current technique and related methods, as well as our clinical experience in some 400 cases of MEP monitoring in supratentorial tumors, lesions in and around the brainstem, and aneurysm surgery. Intraoperative MEP recordings have been shown to reliably reflect an impending new motor deficit. Irreversible MEP deterioration heralds new paresis, and unaltered recordings predict preserved motor function. This is also true in aneurysm surgery where conventional SEP monitoring may yield false-negative results with regard to development of a new motor deficit. Moreover, if MEP deterioration can be reversed, or halted by early surgical intervention, the presence of only a transient motor deficit, or even the lack of a new postoperative deficit, indicates the success of the MEP monitoring method in the prevention of a significant motor impairment. Certain complicated lesions can only be operated on at all because MEP monitoring is available. In conclusion, intraoperative MEP monitoring is a useful aid in brain surgery with which to avoid a new motor deficit without compromise to the surgical result. Controlled prospective studies will be required to verify the clinical value of the method.

Exposure to pulsed magnetic fields enhances motor recovery in cats after spinal cord injury

STUDY DESIGN

Animal model study of eight healthy commercial cats was conducted.

OBJECTIVE

To determine whether pulsed electromagnetic field (PMF) stimulation results in improvement of function after contusive spinal cord injury in cats.

SUMMARY OF BACKGROUND DATA

PMF stimulation has been shown to enhance nerve growth, regeneration, and functional recovery of peripheral nerves. Little research has been performed examining the effects of PMF stimulation on the central nervous system and no studies of PMF effects on in vivo spinal cord injury (SCI) models have been reported.

MATERIALS AND METHODS

PMF stimulation was noninvasively applied for up to 12 weeks to the midthoracic spine of cats with acute contusive spinal cord injury. The injury was produced using a weight-drop apparatus. Motor functions were evaluated with the modified Tarlov assessment scale. Morphologic analyses of the injury sites and somatosensory-evoked potential measurements were conducted to compare results between PMF-stimulated and control groups.

RESULTS

There was a significant difference in locomotor recovery between the PMF-stimulated and control groups. Although not statistically significant, PMF-stimulated spinal cords demonstrated greater sparing of peripheral white matter and smaller lesion volumes compared to controls. Somatosensory-evoked potential measurements indicated that the PMF-stimulated group had better recovery of preinjury waveforms than the control group; however, this observation also was not statistically significant because of the small sample size.

CONCLUSIONS

This preliminary study indicates that pulsed magnetic fields may have beneficial effects on motor function recovery and lesion volume size after acute spinal cord injury.

Evaluation of time-dependent spread of tissue damage in experimental spinal cord injury by killed-end evoked potential: effect of high-dose methylprednisolone

OBJECT

Histopathological studies on spinal cord injury (SCI) have demonstrated time-dependent spread of tissue damage during the initial several hours postinjury. When the long tract within the spinal cord is stimulated, a large monophasic positivity occurs at the injury site. This type of potential, termed the killed-end evoked potential (KEEP), indicates that a nerve impulse approaches but does not pass beyond the injury site. The authors tested the hypothesis that the damage spread can be evaluated as a progressive shift of the KEEP on a real-time basis. The effect of high-dose methylprednisolone sodium succinate (MPSS) on the spread of tissue damage was also examined by this methodology.

METHODS

The KEEP was recorded using an electrode array placed on the spinal cord at the T-10 level in cats. This electrode array consisted of multiple 0.2-mm-diameter electrodes, each separated by 0.5 mm. Spinal cord injury was induced using a vascular clip (65 g pinching pressure for 30 seconds). The midline posterior surface of the spinal cord was stimulated bipolarly at the C-7 level by applying a single pulse at supramaximal intensity. During the initial period of 6 hours postinjury, the localization of the largest KEEP shifted progressively up to 2.5 mm rostral from the injury site. The amplitude of the KEEP recorded at the injury site decreased to 55 to 70% and became slightly shortened in latency as the localization of the largest KEEP shifted rostrally. These findings imply that the injury site KEEP represents the volume-conducted potential of the largest KEEP at the site of the conduction block. It moved away from the injury site in association with the damage spread, and this was confirmed histopathologically. A decrease in amplitude of KEEP at the injury site appeared to be the most sensitive measure of the damage spread, because the amplitude of the volume-conducted KEEP is inversely proportional to the square of the distance between the recording site and site of conduction block. Administered immediately after SCI, MPSS clearly inhibited these events, especially within 30 minutes postinjury.

CONCLUSIONS

The KEEP enables sequential evaluation to be made of the time-dependent spread of tissue damage in SCI in the same animal. It is, therefore, useful for detecting the effect of therapeutic interventions and for determining the therapeutic time window. The efficiency of MPSS to inhibit the spread of damaged tissue appeared to be maximized when it was administered within the initial 30-minute period postinjury.

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.

Three-dimensional topographic analysis of spinal accessory motoneurons under chronic mechanical compression: an experimental study in the mouse

We investigated the effect of chronic mechanical compression of the cervical spinal cord on the number of spinal accessory motoneurons in 25 tiptoe-walking Yoshimura mice. The animals had calcified deposits in the atlantoaxial membrane at the C1-C2 vertebral level, compressing the spinal cord posterolaterally. Motoneurons of the spinal accessory nerve between C1 and C5 segments were labelled using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected into the sternocleidomastoid muscles. The counted cells were processed into a three-dimensional computer display to analyse the cytoarchitectonic changes caused by external cord compression. The number of WGA-HRP-labelled spinal accessory motoneurons was significantly reduced on the affected side. The number of motoneurons in compromised C2 and C3 cord segments correlated linearly with the extent of mechanical compression, but no such relationship was present on the contralateral side. There was an increase in the number of WGA-HRP-labelled spinal accessory motoneurons in the medial cell pools of the anterior grey horn at a level most rostral to the compression, and in the ventrolateral cell pools at levels immediately rostral to the compression. Our findings suggest that the spinal accessory motoneurons translocate rostral to the area of external compression in order to avoid mechanical injury.

Waveform changes due to conduction block and their underlying mechanism in spinal somatosensory evoked potential: a computer simulation. Technical note

Based on a square-wave solid-angle analysis, a simplified mathematical model was produced for computing a sequence of potential change in a volume conductor generated by an impulse traveling along a nerve fiber. A conduction block was simulated as a phenomenon in which a depolarization wavefront stops traveling when it reaches a certain point, although the following repolarization wavefront continues to travel until it reaches the same point. The spinal somatosensory evoked potential (SSEP) was produced as an algebraic sum of simulated nerve fiber action potentials (NFAPs). With a conduction block, an NFAP that was normally triphasic showed a positive-negative diphasic wave with reduced negativity at the point of the block, diphasic waves with enhanced negativity at points immediately preceding the block, and initial-positive waves alone or abolition of any wave at points beyond the block. The absence of their terminal-positive phases paradoxically enhanced the negative peak of the spinal SSEPs in a partial block that involved only the constituent fastest fibers, because phase cancellation of the phases between the terminal-positive phases of the fastest fibers and the negative phases of the slower fibers, which normally happens, failed to occur. At the points immediately preceding the block, the identical mechanism sustained the spinal SSEP enhancement even when every fiber was included in the block. The computer model predicted that localization of the precise site of conduction block can be achieved by demonstrating an abrupt reduction in the amplitude of the spinal SSEP, which is accompanied by an increased negative wave caudally and an enhanced monophasic positive wave rostrally.

Radiofrequency as a lesioning model in experimental spinal cord injury

Many models have been developed to study spinal cord injury (SCI), such as cryogenic lesioning, hot water injury, scalpel lesioning, compressive trauma using clips, electromechanical devices, extradural cuffs, and weight-drop techniques. In this study, the radiofrequency (RF) lesion was used for inducing an experimental SCI in cats. The neuropathology was correlated with the MRI. In this model, 4 cats were injured at the thoracic spinal cord (T11-T12) with a lesion of 65 degrees C for 1 min using a micromanipulated penetrating RF electrode. The MRI of the lesions after 2, 3, 5, and 6 weeks post-injury as well as the correlative histological changes were obtained. The RF-induced lesion was discrete with little spreading across the spinal cord. There was a good correlation between the histopathology findings and the MRI. We conclude that experimental RF lesioning of the spinal cord can produce a consistent lesion with predictable histopathological changes in experimental animals. A 65 degree C injury for 1 min induced a clinical picture of an incomplete SCI. The RF lesioning should be considered as a new model to study SCI, particularly those with a penetrating component.

Spinal cord evoked potential monitoring for cervical and thoracic compressive myelopathy

Spinal cord evoked potentials (SCEPs) were recorded epidurally in 95 surgical cases of cervical and/or thoracic compressive myelopathy. Abnormal SCEPs occurred in 91% of the patients with cervical myelopathy and in all with thoracic involvement at the levels suspected to be responsible for neurological damage. Abnormal SCEPs correlated significantly with the severity of spinal cord compromise and symptoms, such as myelopathy. It was not possible to predict postoperative neurological improvement on the basis of the preoperative SCEP findings alone, but better neurological improvement after surgery was closely associated with early recovery of intra- and postoperative SCEPs. Epidural SCEPs are therefore useful in making a level-specific diagnosis, especially in the patient with multilevel vertebral involvement, and in assessing the severity of neurological compromise and surgical outcome to some extent.

Spinal cord evoked potentials in cervical and thoracic myelopathy

We have studied 70 patients who underwent surgical decompression with or without fusion for cervical and/or thoracic myelopathy to assess the value of spinal cord evoked potentials (SCEP). The 70 patients included 42 with cervical spondylotic myelopathy, 10 with ossification of the posterior longitudinal ligament of the cervical spine and 18 with thoracic ligamentous ossification. Abnormal SCEP occurred in 88% of the patients with cervical myelopathy. Early recovery in intra- and postoperative SCEP was associated with a better result. The severity of myelopathy correlated significantly with an abnormal SCEP in thoracic myelopathy. SCEP may reflect the severity of disease, and early recovery of intraoperative SCEP may also predict neurological improvement.

Motor versus somatosensory evoked potential changes after acute experimental spinal cord injury in rats

In this study, averaged cortical somatosensory evoked potentials (SEP) after sciatic nerve stimulation, and lower extremity muscle responses after motor cortex stimulation (MEP) were compared in rats. 10 animals served as light (25 g-cm) and 10 animals as severe (80 g-cm) acute spinal cord injury group after weight dropping trauma. After the initial loss of components, both SEP and MEP recovered in most cases in the light injury group. In the severe injury group, however, no recovery was observed in cortical SEPs, while the muscle MEP recovered in some animals. Light spinal cord injury had little effect on muscle MEPs and caused a paradoxical amplitude increase in some MEP recordings. Latency values of muscle MEPs did not show great changes after either kind of injury, while cortical SEP latency was considerably delayed. In this model cortical SEPs were more sensitive to light spinal cord injury than muscle MEPs after single electrical cortical stimuli. Severe spinal cord injury caused amplitude changes or loss of waves from both SEP and MEP.

Origin and distribution of brain-stem somatosensory evoked potentials in humans

The distribution of somatosensory evoked potentials (SEPs) recorded from the brain-stem surface was studied to investigate their generator sources in 14 patients during surgical exploration of the posterior fossa. Two distinct SEPs of different morphologies and electrical orientation were obtained by median nerve stimulation. A small positive-large negative-late prolonged positive wave was recorded from the cuneate nucleus and its vicinity. There was a phase-reversal between the cuneate nucleus and the ventral surface of the medulla, depicting a dipole for dorso-ventral organization. From the pons and midbrain, triphasic waves with predominant negativity were obtained. This type of SEP had identical wave forms between the dorsal, lateral and ventral surface of the pons and midbrain. It showed an increase in negative peak latency as the recording sites moved rostrally, suggesting an ascending axial orientation. In a patient with pontine hemorrhage, the killed end potential, a large monophasic positive potential was obtained from the lesion. This potential occurs when an impulse approaches but never passes beyond the recording electrode. Therefore, the triphasic SEP from the pons and midbrain reflects an axonal potential generated in the medial lemniscal pathway.

Vestibulospinal evoked potential versus motor evoked potential monitoring in experimental spinal cord injuries of cats

Changes in vestibulospinal evoked potentials (VsEP) and motor evoked potentials (MEP) were examined in 10 cats before and after two different weight-dropping spinal cord injuries. In six animals somatosensory evoked potentials (SEP) were also monitored. The recordings were done from epidural spinal cord electrodes. Before and after severe and light weight-dropping spinal cord injuries all 3 modalities were recorded at the same time intervals till the end of 4th hour postinjury. According to a scoring system, evoked potential changes below and above the level of injury were monitored, and compared with each other. This study showed that the different motor stimulation methods use different descending spinal tracts, and both can be useful as a monitoring tool. Both descending tracts carrying VsEP and MEP had similarly remarkable changes after severe spinal cord injury. These consisted of major deformation, development of an evoked injury potential and complete potential loss. During the 4 hour monitoring period, no case showed EP recovery in the severe injury group. Light spinal cord injury caused somewhat more deterioration in MEPs than VsEP. The higher numbers of severe potential alterations in the lightly injured animals suggest that MEP is a more sensitive method for spinal cord monitoring compared to VsEP and also to SEP. On the other hand, this sensitivity might be a disadvantage during intraoperative monitoring, if MEP alone were used.

Subpial spinal evoked potentials in patients undergoing junctional dorsal root entry zone coagulation for pain relief

Seven patients with complete avulsion of the brachial plexus underwent junctional coagulation lesions of the dorsal root entry zone (DREZ) for relief of intractable pain in the paralyzed arm. Intra-operative monitoring by recording spinal cord somatosensory evoked potentials (SEP) resulting from tibial nerve stimulation was done using subpial recording electrodes situated dorsal to the posterior median sulcus at the C4 and T2 segment. SEP on the normal side showed an initial positive wave and two negative waves followed by a group of high frequency waves of relatively high amplitude which continued into high frequency, low amplitude potentials. The conduction velocity of the fastest spinal evoked potential components were, on average, 86 m/s. Recordings from the side of avulsion revealed a steep positive potential of high amplitude which appeared in five patients prior to the creation of the DREZ lesion. This effect was assumed to be secondary to spinal cord damage caused by avulsion. During the DREZ coagulation the SEP from the unaffected side did not change. On the side of DREZ coagulation the velocity of the fastest fibres decreased. Four patients reported sensory deficits after the operation, which were transient in three. In one of these patients, the first two negative potentials disappeared. In the fourth patient, who had permanent sensory deficits, the positive steep potential appeared after generation of the lesion. Our results point to the usefulness of the subpial SEPs monitoring during microneuro-surgical procedures on the spinal cord to provide further insight into evoked electrical activity of the normal and injured spinal cord, and to minimize post-operative neurological morbidity.

Spinal cord evoked injury potentials in patients with acute spinal cord injury

Six patients were examined in the acute stage of spinal cord injury, between 11 h and 12 days posttrauma. Quadripolar epidural electrodes were positioned either percutaneously using a Tuohy needle or directly into the epidural space during surgical intervention. These electrodes were combined with a common reference to obtain monopolar recordings of spinal cord evoked potentials resulting from either median nerve stimulation at the wrist or tibial nerve stimulation at the popliteal fossa. Spinal cord evoked injury potentials (SCEIPs), stationary potentials with positive polarity on the distal aspect of the lesion and negative polarity on the proximal aspect, were recorded in all cases. The average amplitude (n = 3) of the SCEIP resulting from tibial nerve stimulation as measured across the lesion was 13.5 microV with an average duration of 12.7 msec. For median nerve stimulation, the average amplitude (n = 3) of the SCEIP was 16.3 microV with an average duration of 6.7 msec. There was a change in polarity in all cases over a distance of less than 6 mm, the distance between the electrode contacts on the epidural electrode. In one case, recordings were performed initially at 11 h and repeated at 21 days posttrauma. In the latter recording, the SCEIP was still present but was five times smaller in amplitude. Coincidentally, the patient also showed clinical signs of improvement in sensory and motor spinal cord function. This study demonstrates the feasibility of recording the SCEIP in patients with acute spinal cord injury, describes the features of these SCEIPs, discusses their origins, and explores the utility of recording the SCEIP as an aid in determining the severity of the injury as well as a means of monitoring changes in spinal cord function.

Scalp recorded somatosensory evoked potentials in response to cauda equina stimulation in neurosurgical operations on the spinal cord

Somatosensory evoked potentials were recorded from the scalp in response to cauda equina stimulation in a total of 30 patients who were treated neurosurgically for spinal space-occupying lesions. Reproducible potentials could be obtained intraoperatively in 26 patients (86.7%). Preoperatively all of the four remaining patients had severe neurological deficits and an incomplete para- or tetraparesis. On the basis of an acceptable amplitude reduction of up to 50% at the end of the operation, it was possible to make an accurate statement as to the expected postoperative neurological state in all of the 26 patients in whom potentials could be obtained intraoperatively. There were no false positive or false negative findings. Our results confirm the reliability and usefulness of this invasive stimulation and non-invasive recording technique when applied intraoperatively.

Preoperative determination of the level of spinal cord lesions from the killed end potential

To determine preoperatively the level of lesions in acute cervical cord injury, the killed end potential of the spinal cord was recorded with a pair of electrodes placed in the spinal epidural space, one initially being placed rostrally to the lesions for obtaining recordings and the other placed caudally to the lesions for stimulation. The level associated with the largest killed end potential was clearly determined without much difficulty, with sequential recordings on stepwise withdrawal of the recording electrode, in four of five cases investigated. In two cases subjected to surgery, the recording electrode left in place at the level associated with the largest killed end potential was found to be located at, or a few millimeters below, the center of the lesions. This demonstrates the preoperative localizing value of the killed end potential for determining the level of lesions responsible for myelopathy.

Spinal cord contusion in the rat: somatosensory evoked potentials as a function of graded injury

A weight-drop technique was used to produce mild, moderate, or severe spinal cord contusive injury in rats. At 4 weeks after injury, somatosensory evoked potentials (SEPs) were recorded with silver ball electrodes placed over the somatosensory cortex of anesthetized rats to measure the response to sciatic nerve stimulation. Both SEP area and amplitude were measured and were highly correlated with each other. Both indices of the SEP correlated inversely with the height of the weight drop and directly with the degree of residual function assessed at 4 weeks after injury. Measures of residual function consisted of a motor score, inclined plane test, and a combined behavioral score based on several neurologic functions. No correlation between latency of the SEP with degrees of contusive injury was observed. The data indicate that the SEP can be used as one criterion in the assessment of the severity of a lesion in a rat model of a graded spinal cord injury.

The somatosensory evoked potential predicts neurologic deficits and serotonergic pathochemistry after spinal distraction injury in experimental scoliosis

The validity of the somatosensory evoked potential as an intraoperative spinal cord monitor was evaluated in an experimental model of scoliosis in the rat and a Harrington distraction model of injury. Under these conditions, it was found that any change in latency or amplitude of the major negative wave above a certain level was a significant predictor of an adverse neurologic outcome. Changes in latency of 4% or greater and changes in amplitude of 50% or greater were unequivocal indicators of spinal cord injury. Postmortem analyses of the spinal neurotransmitter serotonin revealed that apparent false-positive results of the SEP were, in fact, true-positive results.

Recording of spinal somatosensory evoked potentials for intraoperative spinal cord monitoring

The authors' experience with intradural and epidural recording of spinal somatosensory evoked potentials (SSEP's) during 26 cases of spinal surgery is described. The techniques of monitoring spinal cord function provided good quality SSEP waveforms in patients both with and without neurological deficits. The SSEP configuration and peak latencies remained stable for up to 5 hours during anesthesia with nitrous oxide, halothane, and fentanyl. Patterns of baseline SSEP's were characteristic of different spinal segments. Distortion and asymmetry of these baseline patterns were seen in several patients with spinal neoplasms. Loss of waveform components during surgery occurred with profound hypotension, overdistraction of the vertebral axis, dorsal midline myelotomy, and removal of intramedullary tumors. Persistent loss of waveform components was associated with an acquired neurological deficit. Fluctuations in the amplitude of the SSEP's were common but were not associated with postoperative neurological deficits. Spinal cord monitoring by means of SSEP recording would appear to be useful during extradural spinal surgery, but there are limitations associated with this technique during some types of intradural surgery.

Spinal cord monitoring: current status and new developments

A review of current techniques and results of monitoring spinal cord function by the intraoperative testing of somatosensory evoked potentials is given. The criteria for an ideal monitoring method are defined: (1) potential alterations occur before the lesion is irreversible, (2) monitoring itself does not harm the patient, (3) there are no false-positive or false-negative results, (4) warning criteria are defined by objective and quantifiable parameters. In recording and stimulation, two different approaches are applied: cortical or spinal recording and peripheral or spinal stimulation. Spinal stimulation techniques are considered more invasive, but an averaged potential is obtained quicker and more reliably by spinal methods. Failure rates in establishing useful monitoring procedures vary between 2.85 and 5%. The N2O-analgesic-relaxant-type of anesthesia is recommended. A precise definition of criteria indicating spinal cord damage has been difficult because of the natural variability of intraoperative evoked potentials. Wide ranges of physiologic, anesthesiologic, and technical and surgical factors have been found to influence intraoperative potential monitoring adversely. The so-called warning criteria drawn from evoked potential changes have so far been set arbitrarily: amplitude reductions of 30-50% for several recordings or at least 15 minutes have mostly been used. It has become clear, however, that warning criteria should be different for healthy or impaired spinal cord function and for cortical and spinal recordings. The value of a lesion-specific spinal cord potential for monitoring remains to be clarified. SEPs are sensitive for demonstrating ischemic changes to the spinal cord, but the limited experience with these lesions does not allow firm conclusions regarding the reversibility of clinical and evoked potential changes in spinal cord ischemia in man. The limited experience with multilevel recording, i.e., simultaneously recording at spinal and cortical level, indicates that epidural recordings are less variable and less failure-prone than cortical recording. Simultaneous multilevel recording also gives more information and allows easier recognition of false-positive or false-negative results. Poor preoperative SEP nearly always preclude useful monitoring. The results obtained so far point out areas where further development is necessary in order to increase the efficacy of this method. Major unsolved problems are (1) definition of warning criteria, (2) incidence of false-positive and false-negative findings, and (3) improvement of data acquisition.(ABSTRACT TRUNCATED AT 400 WORDS)