Anatomic and Physiologic Bases of Posterior Tibial Nerve Somatosensory Evoked Potentials

Identifying Intraoperative Spinal Cord Injury Location from Somatosensory Evoked Potentials' Time-Frequency Components

Excessive distraction in corrective spine surgery can lead to iatrogenic distraction spinal cord injury. Diagnosis of the location of the spinal cord injury helps in early removal of the injury source. The time-frequency components of the somatosensory evoked potential have been reported to provide information on the location of spinal cord injury, but most studies have focused on contusion injuries of the cervical spine. In this study, we established 19 rat models of distraction spinal cord injury at different levels and collected the somatosensory evoked potentials of the hindlimb and extracted their time-frequency components. Subsequently, we used k-medoid clustering and naive Bayes to classify spinal cord injury at the C5 and C6 level, as well as spinal cord injury at the cervical, thoracic, and lumbar spine, respectively. The results showed that there was a significant delay in the latency of the time-frequency components distributed between 15 and 30 ms and 50 and 150 Hz in all spinal cord injury groups. The overall classification accuracy was 88.28% and 84.87%. The results demonstrate that the k-medoid clustering and naive Bayes methods are capable of extracting the time-frequency component information depending on the spinal cord injury location and suggest that the somatosensory evoked potential has the potential to diagnose the location of a spinal cord injury.

Intraoperative Neurophysiological Monitoring for Endoscopic Endonasal Approaches to the Skull Base: A Technical Guide

Intraoperative neurophysiological monitoring during endoscopic, endonasal approaches to the skull base is both feasible and safe. Numerous reports have recently emerged from the literature evaluating the efficacy of different neuromonitoring tests during endonasal procedures, making them relatively well-studied. The authors report on a comprehensive, multimodality approach to monitoring the functional integrity of at risk nervous system structures, including the cerebral cortex, brainstem, cranial nerves, corticospinal tract, corticobulbar tract, and the thalamocortical somatosensory system during endonasal surgery of the skull base. The modalities employed include electroencephalography, somatosensory evoked potentials, free-running and electrically triggered electromyography, transcranial electric motor evoked potentials, and auditory evoked potentials. Methodological considerations as well as benefits and limitations are discussed. The authors argue that, while individual modalities have their limitations, multimodality neuromonitoring provides a real-time, comprehensive assessment of nervous system function and allows for safer, more aggressive management of skull base tumors via the endonasal route.

Rare true-positive isolated SSEP loss with preservation of MEPs response during scoliosis correction

STUDY DESIGN

Case report.

OBJECTIVE

To report a case of a true-positive isolated somatosensory evoked potential (SSEP) loss with preservation of motor evoked potential (MEP) response during scoliosis correction.

SUMMARY OF BACKGROUND DATA

Combined intraoperative monitoring uses SSEPs and MEPs to decrease the probability of observing false-negative events. In combination, SSEPs and MEPs have become a standard of care for spinal deformity surgery. However, literature review reveals several cases of false-negative response with combined SSEPs and MEPs, raising the contention that intraoperative monitoring does not reliably identify all isolated selective spinal cord dysfunction.

METHODS

A 15-year-old female patient with a 65° right thoracic adolescent idiopathic scoliosis underwent correction and posterior spinal fusion with segmental pedicle screw instrumentation. After capture and derotation of the left concave rod, left-sided irreversible SSEP loss occurred whereas MEPs remained unchanged. After excluding systemic factors, anesthetic causes, or technical fault, deformity correction was released and instrumentation removed. No cortical breach was reported during pedicle screw removal.

RESULTS

Postoperatively, no clinical sensory or motor deficit was present; computed tomography demonstrated a burst left pedicle at T10 with the medial pedicle wall fragment in direct contact with the dorsal spinal cord. Magnetic resonance imaging excluded cord edema or other evidence of injury. Three days after surgery, intraoperative monitoring showed delayed latencies and amplitudes of the left SSEP. An uneventful reinsertion of instrumentation and correction excluding the left T10 pedicle screw was performed 7 days after the initial surgery.

CONCLUSION

This case report provides evidence of selective posterior spinal cord dysfunction with sparing of the anterior columns immediately after a correction maneuver and emphasizes the importance of simultaneous SSEP and MEP monitoring. To the authors' knowledge, there is no previous report of a true-positive isolated SSEP loss with preservation of MEP response during scoliosis correction.

LEVEL OF EVIDENCE

N/A.

False-negative transcranial motor-evoked potentials during scoliosis surgery causing paralysis: a case report with literature review

STUDY DESIGN

Case report.

OBJECTIVE

To report a case of false-negative intraoperative motor-evoked potentials (MEP) that developed paraplegia after surgery.

SUMMARY OF BACKGROUND DATA

Although several false-negative results have been reported with somatosensory-evoked potentials, there is no report noted with MEP. Therefore, several authors have preferred using MEPs as a gold standard in neuromonitoring.

METHODS

We report a case of false-negative MEP during the scoliosis surgery which is the first report showing false-negative MEPs during operation.

RESULTS

A 15-year-old girl with severe kyphoscoliosis (Cobb angle, 140 degrees) in neurofibromatosis was operated for correction and posterior spinal fusion surgery, using pedicle screw instrumentation. Intraoperative neuromonitoring did not show any change in MEPs throughout the procedure, however, she woke-up with paraplegia. Immediate implant release could not recover her neurology functionally at last follow-up. Positive event during the operation was massive blood loss which could not show drop in MEPs as an ischemic cord injury (probable cause). Postoperative CT scan in both patients did not show any injury with pedicle screw as implants were well placed within the pedicles. Reviewing the literature, we could not find out any prospective study in animals identifying false-negative results with MEPs.

CONCLUSION

From our experience of false-negative MEPs, we conclude that unwanted events with use of MEP in scoliosis or other spinal surgeries. We propose further prospective research on animals to solve this issue.

Current practice of motor evoked potential monitoring: results of a survey

Centers responding to a survey of MEP monitoring practices predominantly used transcranial electrical brain stimulation (TCES) with brief pulse trains and/or spinal cord stimulation (SCS) to elicit MEPs; transcranial magnetic stimulation and single-pulse TCES were not techniques of choice. Most centers using TCES had patient exclusion criteria (e.g., cochlear implants, cardiac pacemakers, prior craniotomy or skull fracture, history of seizures). Adverse effects included rare tongue injuries or seizures from TCES, and minor bleeding from needle electrodes in muscle. Spinal cord, peripheral nerve, and muscle recording sites were all employed. TCES with recording of muscle responses was the preferred MEP monitoring technique at the plurality of the centers. MEPs suitable for monitoring were obtained in about 91.6% of patients overall. Most of the failures were attributed to technical factors; preexisting neurologic dysfunction precluded MEP monitoring in approximately 1.7% of patients. Almost all centers monitored SEPs concurrently with MEPs. Overall, both measures remained stable during about 90.2% of cases. Adverse MEP changes occurred in about 8.3%; a little over half of these were accompanied by SEP changes. Adverse SEP changes without MEP changes occurred in about 1.5% of cases. SEPs and MEPs should be used together to optimally monitor the spinal cord.

Findings of somatosensory evoked potential to stimulation of the sciatic nerve in two different rat strains

No comparative study about somatosensory evoked potentials (SEP) on different rat strains has been done yet. It is evident that comparative SEP studies are important since different rat strains have different physiological properties. We aimed to compare early latency SEP values from stimulation of sciatic nerve in Wistar (Wr) and Sprague-Dawley (SDr) rats which are frequently used rat strains in experimental studies. In Wr group, the mean of first far field potential (Ff1) latency was shorter and the mean Ff1 amplitude was lower than that of Sprague-Dawley rat group. Mean cortical potential latency in Wr group was longer than that of SDr group while amplitude was not different. Central conduction time (CCT) in Wistar rat group was found to be longer than that of SDr group. Shorter Ff1 latency in Wr group implies that afferent volley reaches cervical posterior fasciculus from sciatic nerve earlier than SDr group while longer CP latency implies that afferent volley reaches cortex later than SDr group. Similarity between the latencies of lumbar potentials implies that peripheral conduction velocity has no effect on the difference of Ff1 latencies.

Negative difference potential isolates scalp potentials generated by activity in supraspinal nociceptive pathways

A negative difference potential (75-240 ms poststimulus) was computed by subtracting the sural nerve-evoked somatosensory evoked potential (SEP) elicited at the pain threshold level from SEPs elicited at noxious levels. The effects of stimulus intensity and interstimulus interval on the negative difference potential amplitude plus conduction velocity measurements and a dipole source localization analysis all suggest that the negative difference potential reflects the response of neurons in the primary somatosensory cortex to inputs that arise from the nociceptive A delta peripheral afferents. Furthermore, a comparison of these results with our earlier sural nerve-evoked SEP studies suggests that these pain-related inputs to the primary somatosensory cortex are largely inhibitory.

Effects of interstimulus interval on scalp topographies evoked by noxious sural nerve stimulation

The amplitude of the late pain-related negative-positive peak complex, which we have labeled SP3 (134-150 ms) and SP6 (277-331 ms), respectively, increased with increasing interstimulus interval (ISI). This contrasts with the nociceptive spinal withdrawal reflex and subjective pain rating data, which implied that nociceptive somatosensory processes were unaffected by ISI at stimulus levels that were well within the pain range. A scalp topographic analysis strongly suggested that none of the brain areas responsible for SP3 or SP6 are involved exclusively in nociception. We also observed a pain-related positive potential approximately 161-177 ms following sural nerve stimulation that has not been reported by others. A dipole source localization analysis and the effects of ISI and stimulus intensity on this potential suggest that it is generated by the response of primary somatosensory cortex neurons to inputs arising from the innocuous peripheral afferents and that this response is inhibited by noxious inputs.

SEP topographies elicited by innocuous and noxious sural nerve stimulation. III. Dipole source localization analysis

The dipole source localization method was used to determine which of the brain areas known to be involved in somatosensation are the best candidate generators of the somatosensory evoked potential evoked by sural nerve stimulation. The ipsilateral central negativity and contralateral frontal positivity which occurred between 58 and 90 msec post stimulus (stable period 1) were best represented by a single source located in the primary somatosensory cortex (SI). The symmetrical central negativity and bilateral frontal positivity which occurred between 92 and 120 msec post stimulus (stable period 2) was best represented by 3 sources. One of these sources was located in SI and the other 2 were located bilaterally in either the frontal operculum or near the second somatosensory cortex (SII). The widespread negativity whose minimum was located in the contralateral fronto-temporal region and which occurred between 135 and 157 msec post stimulus (stable period 3) was also best represented by 3 sources. Two of these sources may be located bilaterally in the hippocampus. We cannot, however, eliminate the possibility that multiple sources in the cortex overlying the hippocampus (e.g., SII and frontal cortex) are responsible for these potentials. At innocuous stimulus levels the third source for stable period 3 was located near the vertex, possibly involving the supplementary motor cortex, whereas at noxious levels this source appears to be located in the cingulate cortex. We were unable to achieve any convincing source localization for the widespread positivity which occurred between 178 and 339 msec post stimulus (stable periods 4-6). Available evidence suggests that more sources were active during this interval than the three we could reliably test under these conditions.

SEP topographies elicited by innocuous and noxious sural nerve stimulation. I. Identification of stable periods and individual differences

Scalp potential topographies evoked by innocuous and noxious sural nerve stimulation were obtained from 15 human subjects. The SEP scalp topography could be separated into 6 different stable periods (SP), that is, consecutive time points where there were no major changes in the topographic pattern. SP1 (occurring 58-90 msec post stimulus) was characterized by a contralateral frontal positivity and a central negativity oriented ipsilateral to the evoking stimulus; SP2 (92-120 msec) by a bilateral frontal positivity and a symmetrical central negativity; SP3 (135-158 msec) by a widespread negativity with a minimum at the contralateral temporo-frontal region; and SP4 (178-222 msec), SP5 (223-277 msec) and SP6 (282-339 msec) by a widespread positivity with a maximum located along the centro-parietal midline. SP4, SP5, and SP6 could be distinguished by changes in the orientation of the isovoltage contour lines and/or by changes in the location of the maximum. The stable periods had similar onset and offset latencies and the same major features across subjects. However, the topographic patterns were not identical across subjects. These individual differences are likely due to the expected variability in the orientation of the equivalent regional dipole sources generating these potentials.

The use of somatosensory evoked potentials in infants and children

Somatosensory evoked potentials (SSEPs) are a useful, reliable means of assessing function of the somatosensory system. Complex maturational changes of the CNS such as synaptogenesis and myelination, as well as body growth, complicate interpretation of SSEPs. An understanding of these factors enhances clinical interpretation in infants and children.