Spinal pathways mediating somatosensory evoked potentials from cutaneous and muscle nerves in the cat

Summary The Authors give evidence on the function of a pathway mediating somatosensory evoked potentials (SEPs) from muscle nerves, other than the dorsal columns: physiological and anatomical data prove its location to be in the spinothalamic tract. Previous contrasting results on the topic are discussed.     

Spinal cord evoked potential

Spinal cord evoked potentials (SCEPs) used as a technique for intraoperative neurophysiological monitoring can be divided into several groups depending on the stimulation method. SCEP evoked by direct motor cortex stimulation enables the recording of the descending volley passing through the corticospinal tract. Such potentials are termed D (direct)- and I (indirect)-waves. D-wave reflects the function of the corticospinal tract directly and is regarded as an appropriate parameter for the intraoperative monitoring of motor function. On the other hand, the corticospinal tract may be activated by transcranial stimulation and a potential similar to D-wave can be recorded from the spinal epidural space. Previous studies, however, indicated that the site of transcranial stimulation is around the lower level of the brainstem and the response includes the potentials coming from other neural tracts. In addition, the responses of SCEPs evoked by spinal cord and peripheral nerve stimulations are generally considered to reflect mainly the function of the deep sensory pathway. Therefore, SCEPs evoked by transcranial, spinal cord and peripheral nerve stimulations cannot be used as a parameter to monitor motor function directly. It is particularly important to know the characteristics of each SCEP and apply each potential to the most appropriate situation.     

Facial nerve evoked potentials in the cat

Electrophysiologic facial nerve testing usually involves stimulation of the peripheral nerve in order to make some indirect inference about nerve integrity at a more proximal site of lesion. In an attempt to develop a test of facial nerve function by use of across-the-lesion testing, the cat facial nerve was stimulated percutaneously at the stylomastoid foramen while retrograde activity through the temporal bone and cranium was monitored with scalp electrodes. A biphasic evoked potential could be identified within 3 milliseconds of stimulation with the use of a signal-averaging computer. This potential remained when the animal was paralyzed and disappeared when the facial nerve was cut proximal to the stimulation site. A potential of similar latency and duration but larger amplitude was recorded from the subarachnoid space. Mapping studies indicated its origin to be a dipole located between ipsilateral mastoid and parietal recording sites that corresponded to the region of the intracranial facial nerve. Optimal stimulation and recording techniques for subsequent studies of human beings are discussed.     

Spinal evoked potentials from the cervical spine

Spinal cord evoked potentials were recorded from spinous processes of the lumbar, thoracic, and cervical levels of the spinal column of normal cats in response to direct stimulation of the sciatic nerve and subcutaneous stimulation of the posterior tibial nerve. The typical primary response was altered by changes in location, magnitude, and frequency of the stimulation. Further changes in the primary response could be associated with spinal level of observation. Reduction in amplitude and decrease in signal complexity with increasing frequency of stimulus indicated the loss of the slower components of the primary response of average evoked potentials. Sharp reduction of signal amplitude and increase in latency was evident as the recording moved from L1 to C3 and C2.     

Spinal and muscle motor evoked potentials following magnetic stimulation in cats.

Motor evoked potentials (MEP) from transcranial magnetic stimulation were successfully recorded from the spinal cord and gastrocnemius muscle in cats. The spinal MEP consisted of four negative peaks, N1 through N4, and subsequent small, multiphasic waves. The conduction velocity calculated from N1 corresponded to that of the nonpyramidal cerebellar evoked potential. After ablation of the pyramidal tract at various levels, N1 through N3 persisted, whereas N4 disappeared. The muscle MEP showed the same latency as that of MEP obtained by direct cortical electrical stimulation and disappeared after similar ablation. These findings suggest that the first three peaks of the spinal MEP are responses of the nonpyramidal cerebellospinal pathways and that the fourth peak of the spinal MEP and the muscle MEP are pyramidal tract responses. Spinal and muscle MEP from magnetic stimulation may be valuable in the qualitative evaluation of the function of the pyramidal and nonpyramidal pathways.     

Spinal cord stimulation evoked potentials during thoracoabdominal aortic aneurysm surgery.

Although monitoring of somatosensory evoked potentials elicited from stimulation of lower extremity peripheral nerves has been suggested as a method for assessing neural function during thoracoabdominal aortic aneurysm surgery, this technique has been reported to yield a large number of false positives. It was believed that direct stimulation of the spinal cord would eliminate some of the problems associated with peripheral evoked potentials. The present study compared in 18 patients the use of scalp recorded evoked potential following stimulation of either the posterior tibial nerve via percutaneous needles or the spinal cord via an epidural electrode previously placed fluoroscopically. In 10 patients in whom distal bypass or shunt was not used, peripheral evoked potentials totally disappeared within 5-30 min of aortic clamping. Spinal cord stimulation evoked potentials disappeared permanently in 2 patients shortly after aortic cross-clamping; 1 died shortly after the procedure, and the other awoke densely paraplegic and died the next day. When distal perfusion was maintained by shunt or bypass, the disappearance of both peripheral and spinal evoked potentials accurately predicted the neurologic outcome of 1 paralyzed patient. Loss of spinal cord stimulation evoked potentials was found to be correlated with adverse neurologic outcome. Over the period of aortic clamping a gradual decrease in mean amplitude (50% at 45 min [P less than 0.05]) and a 20% increase in mean latency time were observed. Maintenance of adequate distal perfusion may permit the use of peripheral evoked potentials in the assessment of spinal cord ischemia during aortic cross-clamping.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spinal cord evoked potentials in cervical and thoracic myelopathy

Summary 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.

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Résumé Nous avons étudié 70 patients ayant eu une décompression chirurgicale avec ou sans fusion pour une myélopathie cervicale et/ou dorsale afin d'évaluer l'utilité des potentiels évoqués de la moelle épinière (PEME). Quarante-deux patients avaient une myélopathie spondylosique cervicale, 10 une ossification du ligament longitudinal postérieur cervical et 18 une ossification ligamentaire dorsale. Des PEME anormaux ont été observés chez 88% des malades présentant une myélopathie cervicale. Le rétablissement rapide des PEME per et post-opératoires a été suivi des résultats les meilleurs. Dans la myélopathie dorsale une corrélation significative a été mise en évidence entre la gravité de la myélopathie et les PEME anormaux. Les PEME pourraient refléter la gravité de la maladie et une récupération per-opératoire rapide des PEME pourrait également faire prévoir une amélioration neurologique.

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Reprint requests to: H. Baba     

Spinal cord monitoring in patients with nonidiopathic spinal deformities using somatosensory evoked potentials.

Seventy-nine somatosensory evoked potentials were intraoperatively recorded in 52 patients undergoing spinal surgery for nonidiopathic spinal deformities. There were 37 true-negative, 28 true-positive (a significant change in the somatosensory evoked potential related to the surgical process), and 14 false-positive (a significant change in the somatosensory evoked potential not related to a surgical event) readings. There were, however, no postoperative neurologic deficits with any of the true-positive readings and no false negatives. Spinal and subcortical somatosensory evoked potentials gave few false-positive readings. True-positive somatosensory evoked potentials occurred in 44% of the patients with neuromuscular deformities, 17% with congenital deformities, 45% with Luque instrumentation, 22% with Harrington instrumentation, and none with fusion in situ. Fifty percent of the true positives occurred while the sublaminar wires were tightened. The predictive accuracy of intraoperative spinal cord monitoring in this patient population is not high, but the sensitivity to potentially harmful surgical events is high.     

Spinal somatosensory evoked potentials after epidural isoproterenol in awake sheep

Purpose

The use of 10–15 μg epinephrine as an epidural test-dose is controversial. Isoproterenol would be a better alternative. However before 5μg isoproterenol can be incorporated in an epidural test-dose, neurotoxicological studies have to be performed. The present study was designed to assess spinal somatosensory evoked potentials (spinal SSEP) before and after epidural isoproteronol.

Methods

Spinal SSEPs were recorded before, 30 min after, and 72 hr after 50 μg isoproterenol were given epidurally (L_3–4) to six chronically instrumented awake sheep. The spinal SSEPs after epidural (L_3–4 administration of 15 ml lidocaine 2% were used to evaluate the model. The SSEPs were generated by transcutaneous stimulation of the sciatic nerve in the thigh. Spinal SSEPs were recorded directly from the spinal cord at vertebra T₁₂ using a monopolar epidural electrode referenced to a subcutaneous needle electrode in the adjacent paraspinal area.

Results

Thirty minutes and 72 hr after epidural injection of 50 μg isoproterenol the latency and the amplitude of the SSEP waves were similar to baseline values. After lidocaine, no SSEPs could be generated in three sheep while in three sheep the latency of wave 2 (W2) was prolonged and the amplitude diminished.

Conclusion

Administration of epidural isoproterenol did not affect spinal SSEPs in this study indicating an absence of neurotoxic side effects.     

Diagnosis of the level and severity of cord lesion in cervical spondylotic myelopathy. Spinal evoked potentials

Experimental and clinical studies were performed to measure the segmental spinal evoked potential (SEP) of the cervical cord after stimulation of the median nerve, to determine the location, severity, and outcome of cord lesion in patients who had cervical spondylotic myelopathy. The SEP in control subjects consisted of two waves, the primary reactive R-wave and the secondary reactive N-wave. The R-wave related to both the dorsal root potential and the funiculus posterior potential; the N-wave related to the post-synaptic potential in the spinal cord. In the early stages of cervical spondylotic myelopathy, the N-wave weakens or disappears at the level of lesion. Then, as damage progresses, the R-wave also weakens or disappears.     

Spinal sensory and motor tract activation after epidural electrical stimulation in the cat

Somatosensory evoked potentials (SEPs) after peripheral nerve stimulation and motor evoked potentials (MEPs) after transcranial stimulation have been routinely used as monitors of the viability of pathways in the posterior and anterior spinal cord, respectively, in patients undergoing spinal cord surgery. To assess total spinal cord function, both of these procedures must be performed simultaneously, which is both cumbersome and technically difficult. The objectives of this study were to demonstrate both sensory and motor spinal tract activation after epidural electrical stimulation of the cat spinal cord. Thirty-seven adult mongrel cats were anesthetized with ketamine, intubated, and maintained with Ethrane and nitrous oxide. Stimulating electrodes were placed over the right dorsolateral spinal cord epidurally at T7 after a laminectomy. Recording electrodes were placed over the right L3 spinal cord epidurally, on the right L7 dorsal and ventral nerve roots, on the right and left sciatic nerves in the popliteal fossa, and in the right gastrocnemius muscle. After epidural stimulation of the spinal cord at T11, distinct reproducible potentials were recorded at each site. Activity in the L7 dorsal root implicated activation of spinal sensory tracts. Activity in the L7 ventral root and in the gastrocnemius implicated activation of spinal motoneurons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spinal cord monitoring as a clinical utilization of the spinal evoked potential

For the purpose of monitoring spinal cord function, evoked spinal cord potential was observed by recording electrodes in the subarachnoid space at the level of the conus medullaris and stimuli delivered to the rostral spinal cord from the epidural space. A specially designed, flexible tube-type electrode was used for both recording and stimulating. The spinal cord evoked potential consisted of an initial spike followed by polyphasic waves. The peak amplitude change was considered to be the main indicator of events that would potentially be hazardous to spinal cord function. Two hundred and twenty-nine operations were monitored by the recording electrode. Six of the patients experienced postoperative neurologic complications. During operation, the peak amplitudes of the evoked potentials diminished to less than 50% of the control values. Simultaneous monitoring of evoked potential revealed that this method was more sensitive than the wake-up test. No complications resulted from the application of this method. Thus, monitoring is a sufficiently sensitive, safe, and reliable method to be useful during aggressive surgery in the spine and spinal cord.     

Spinal cord monitoring during surgery by direct recording of somatosensory evoked potentials. Technical note

A simple method of spinal cord monitoring that can be readily used during surgery for spinal disorders in children or adults is described. A spinal subdural recording electrode is placed rostral to the site of surgery and the peroneal nerve is stimulated in the popliteal fossa; in this way, large-amplitude polyphasic spinal somatosensory evoked potentials (SEP's) can be directly recorded. The large amplitude of the spinal SEP's recorded intrathecally facilitates spinal cord monitoring by allowing: 1) rapid acquisition of the evoked response, which provides continuous monitoring during surgery; 2) relatively easy interpretation of the signal, there being no significant ultrashort- or long-latency components to the waveform; and 3) signal acquisition in an electrical environment that would be unacceptable using standard methods of spinal and cortical SEP recording.     

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.     

Spinal function monitoring by evoked spinal cord potentials in aortic aneurysm surgery

Evoked spinal cord potentials (ESCPs) were monitored in 12 patients who underwent repair of thoracoabdominal aortic aneurysm with a high risk of spinal ischemia. A pair of bipolar catheter electrodes were introduced into the epidural space, one at the level of the C5-T2 vertebrae and the other at the level of T11-L2. Conductive mixed ESCP in seven patients, conductive sensory ESCP in one patient, and segmental descending ESCP in three patients were observed by applying a rectangular electric current to one of each pair of epidural electrodes and recording through the other. Segmental ESCP in response to posterior tibial nerve stimulation was observed in one patient. Following aortic cross-clamping, the I wave of conductive mixed ESCPs gradually decreased in amplitude with latency prolongation in five of the seven patients and disappeared in one of these five; transient augmentation of amplitude was observed before eventual decline in four of these five patients. The N wave of segmental descending ESCP subsequently flattened in two of the three patients and the N₁ wave of segmental ESCP in the one patient. Three of the four patients in whom the ESCPs disappeared during aorta clamping recovered the ESCPs after declamping and showed no neurological disorders postoperatively. Intraoperative ESCP monitoring appears to be useful to detect spinal cord ischemia in the early stage and to alert surgeons and anesthesiologists so that timely resuscitative steps can be taken.     

Transient suppression of event-related evoked potentials produced by mild head injury in the cat

Studies in humans have shown that sensory stimuli, presented in the context of certain tasks, can elicit a late positive component (LPC), namely P300, in the scalp-recorded evoked potential believed to reflect neural activity related to attentional processes. A similar LPC has been reported in cats and monkeys. In this study, the LPC of the auditory evoked potential (AEP) in the cat was used to detect impairment in attention to a relevant stimulus after low levels of cerebral concussion produced by a fluid percussion device. A hollow screw (for fluid percussion) and stainless steel screws (for AEP recording) were surgically placed in the skull. After recovery from surgery, animals were trained in the paradigm to obtain an LPC. Pupillary dilation was conditioned to tones. A random sequence of two discriminable tones was presented. The lower tone had a probability of 0.1 and was followed by a tail shock (tone-shock). After 400 to 1000 tone-shock presentations, animals attended to the lower tone stimulus as inferred by selective pupillary dilation. In the AEP an early positive component at 50 to 120 msec related to an alerting response was enhanced, and an LPC at 250 to 450 msec appeared in response to the paired tone-shock. Animals were then subjected to cerebral concussion. Complete recovery of normal reflexes, motor coordination, and orienting response was seen within 2 hours after injury. The LPC was suppressed for a period of at least 3 days, suggesting that low magnitudes of brain injury can disrupt higher-order neural activities. This disruption can persist despite recovery of other neurological functions.