Median somatosensory evoked potential intraoperative monitoring: Recommendations based on signal-to-noise ratio analysis

Intraoperative somatosensory evoked potential (SEP) monitoring: an updated position statement by the American Society of Neurophysiological Monitoring

Somatosensory evoked potentials (SEPs) are used to assess the functional status of somatosensory pathways during surgical procedures and can help protect patients' neurological integrity intraoperatively. This is a position statement on intraoperative SEP monitoring from the American Society of Neurophysiological Monitoring (ASNM) and updates prior ASNM position statements on SEPs from the years 2005 and 2010. This position statement is endorsed by ASNM and serves as an educational service to the neurophysiological community on the recommended use of SEPs as a neurophysiological monitoring tool. It presents the rationale for SEP utilization and its clinical applications. It also covers the relevant anatomy, technical methodology for setup and signal acquisition, signal interpretation, anesthesia and physiological considerations, and documentation and credentialing requirements to optimize SEP monitoring to aid in protecting the nervous system during surgery.

Intraoperative Monitoring of Scoliosis Surgery in Young Patients

SUMMARY

Intraoperative neurophysiologic monitoring has added substantially to the safety of spinal deformity surgery correction since its introduction over four decades ago. Monitoring routinely includes both somatosensory evoked potentials and motor evoked potentials. Either modality alone will detect almost all instances of spinal cord injury during deformity correction. The combined use of the two modalities provides complementary information, can permit more rapidly identification of problems, and enhances safety though parallel redundancy should one modality fail. Both techniques are well established and continue to be refined. Although there is room for provider preference, proper monitoring requires attention to technical detail, understanding of the underlying physiology, and familiarity with effects of commonly used anesthetic agents.

Intraoperative evoked potential techniques

There are many recent advances in intraoperative evoked potential techniques for mapping and monitoring neural function during surgery. In particular, somatosensory evoked potential optimization speeds surgical feedback, motor evoked potentials provide selective motor system information, and new visual evoked potential methods promise reliable visual system monitoring. This chapter reviews these advances and provides a comprehensive background for understanding their context and importance.

Multi-modal Neuroelectrophysiological Monitoring in the Treatment of Thoracic Tuberculosis with Debridement and Bone Grafting and Posterior Pedicle Screw Fixation via Costal Transverse Process Approach

Objective

To explore the value of multi‐mode neuroelectrophysiological monitoring (MIOM) in evaluating spinal cord and nerve root function in the treatment of thoracic tuberculosis via costal transverse process approach.

Methods

From December 2017 to September 2019, a retrospective study of thoracic tuberculosis patients in our hospital was conducted. This study included 25 patients (14 men and 11 women). The average age of patients at the time of surgery was 63.3 years (range, 20–83 years). All patients (three cases with the destruction of a single vertebral body, 13 cases with the destruction of two vertebral bodies, and nine cases with the destruction of three or more vertebral bodies) underwent costal transverse process approach with debridement and bone grafting and internal fixation combined with intraoperative multimodal neuroelectrophysiological monitoring. During the operation, somatosensory evoked potential (SEP), transcranial electrical stimulation motor evoked potential (TES‐MEP), and spontaneous electromyography (EMG) were used to monitor progress. ESR, visual analogue scale (VAS), Cobb angle, and Oswestry disability index (ODI) were statistically analyzed to evaluate the treatment effects and patient satisfaction.

Results

All 25 patients were successfully monitored. The follow‐up time ranged from 12 to 21 months, with an average of 15.3 months. SEP waveform abnormalities occurred in five patients during the operation, the incidence rate was 28%. Of these five patients, three patients changed their instruments and postures, and adjusted the flushing water flow in time; one patient received pressure therapy in time; the operation was suspended for 10 min for one patient. There were seven cases with abnormal TES‐MEP waveform, the incidence rate was 28%. Among these seven cases, five cases adjusted the nail path during the operation and adjusted the nail position in time. One case adjusted the inclination angle of the operating table in time; one case completed the contralateral nail stick correction in time; five of them had abnormal TES‐MEP waveforms, and EMG burst potential was also detected, the incidence rate was 20%. After prompt treatment, the abnormal waveforms of all patients returned to normal; no abnormal waveforms, recurrence of tuberculosis, loosening of internal fixation, nerve and spinal cord dysfunction, etc. The VAS score, erythrocyte sedimentation rate (ESR), Cobb angle, and ODI scores of the patients 1 year after operation were significantly improved compared with 1 week after operation (P < 0.05).

Conclusion

Multi‐mode intraoperative electrophysiological detection combined with costal transverse process approach for the treatment of thoracic tuberculosis could avoid intraoperative nerve and blood vessel damage, reduce surgical risk, improve surgical efficiency, and ensure curative effect.

Neurophysiological Changes in the First Year After Cell Transplantation in Sub-acute Complete Paraplegia

Neurophysiological testing can provide quantitative information about motor, sensory, and autonomic system connectivity following spinal cord injury (SCI). The clinical examination may be insufficiently sensitive and specific to reveal evolving changes in neural circuits after severe injury. Neurophysiologic data may provide otherwise imperceptible circuit information that has rarely been acquired in biologics clinical trials in SCI. We reported a Phase 1 study of autologous purified Schwann cell suspension transplantation into the injury epicenter of participants with complete subacute thoracic SCI, observing no clinical improvements. Here, we report longitudinal electrophysiological assessments conducted during the trial. Six participants underwent neurophysiology screening pre-transplantation with three post-transplantation neurophysiological assessments, focused on the thoracoabdominal region and lower limbs, including MEPs, SSEPs, voluntarily triggered EMG, and changes in GSR. We found several notable signals not detectable by clinical exam. In all six participants, thoracoabdominal motor connectivity was detected below the clinically assigned neurological level defined by sensory preservation. Additionally, small voluntary activations of leg and foot muscles or positive lower extremity MEPs were detected in all participants. Voluntary EMG was most sensitive to detect leg motor function. The recorded MEP amplitudes and latencies indicated a more caudal thoracic level above which amplitude recovery over time was observed. In contrast, further below, amplitudes showed less improvement, and latencies were increased. Intercostal spasms observed with EMG may also indicate this thoracic “motor level.” Galvanic skin testing revealed autonomic dysfunction in the hands above the injury levels. As an open-label study, we can establish no clear link between these observations and cell transplantation. This neurophysiological characterization may be of value to detect therapeutic effects in future controlled studies.

Feasibility of Full Neuromuscular Blockade During Transcranial Motor Evoked Potential Monitoring of Neurosurgical Procedures

BACKGROUND

Transcranial motor evoked potential (TcMEP) monitoring is conventionally performed during surgical procedures without or with minimal neuromuscular blockade (NMB) because of its potential interference with signal interpretation. However, full blockade offers increased anesthetic management options and facilitates surgery. Here, the feasibility of TcMEP interpretation was assessed during full NMB in adult neurosurgical patients.

METHODS

Patients undergoing cervical or lumbar decompression received a rocuronium bolus producing 95% or greater blockade by qualitative train-of-four at the ulnar nerve. TcMEPs were recorded in bilateral thenar-hypothenar and abductor hallucis muscles. Adequacy of response for reliable signal interpretation was determined on the basis of repeatability and clarity, assessed by coefficient of variation and signal-to-noise ratio, respectively.

RESULTS

All patients had at least 3 of 4 measurable TcMEP limb responses present during full NMB, and 70.8% of patients had measurable responses in all 4 limbs. In total, 82.2% of thenar-hypothenar responses and 62.8% of abductor hallucis responses were robust enough for reliable signal interpretation on the basis of clarity. In addition, 97.8% of thenar-hypothenar responses and 79.1% of abductor hallucis responses met the criteria for reliable signal interpretation on the basis of consistency. Patient demographics, medical comorbidities, and preoperative weakness were not predictive of absent responses during full NMB.

CONCLUSIONS

TcMEP interpretation may be feasible under greater levels of NMB than previously considered, allowing for monitoring with greater degrees of muscle relaxation. Consideration for monitoring TcMEP during full NMB should be made on a case-by-case basis, and baseline responses without blockade may predict which patients will have adequate responses for interpretation.

Intraoperative Erb's Point-Vertex recording increases brainstem auditory evoked potential wave V amplitude

OBJECTIVE

Recording derivations for intraoperative brainstem auditory evoked potential (BAEP) monitoring consist of a preauricular electrode referenced to Cz'. These derivations are prone to unfavorable signal amplitude. This study analyses whether an alternative noncephalic electrode positioned over ipsilateral Erb's point, thereby generating a new Erb's point-vertex recording derivation, improves BAEP recordings.

METHODS

Electrodes were placed preauricularly (A1/A2) and at left and right Erb's point (EP1/EP2). They were referenced to Cz'. Click sound stimulation (80-95 dB above hearing level) was applied. At intraoperative baseline conditions, latencies and amplitudes of waves I-V of all derivations were analyzed.

RESULTS

Data of 30 patients (54 ± 15 years/17 females) with normal hearing or mild symmetrical presbycusis undergoing infratentorial surgeries (15 microvascular decompressions) were analyzed. Using EP1-Cz'/EP2-Cz' derivations compared to A1-Cz'/A2-Cz', amplitudes for wave IV (left +65%, p < 0.001; right +43%, p = 0.002) and wave V (left +54%, p < 0.001; right +48%, p < 0.001) were significantly increased. Only in the left (EP1) derivation, there was a tendency towards less reproducibility of wave I, resulting in a decrease of amplitude (-35%, p = 0.005).

CONCLUSIONS

Adding an Erb's point electrode derivation resulted in larger amplitudes of waves IV to V. whereas conventional preauricular or mastoid derivation is preferential for wave I assessment.

SIGNIFICANCE

Increased wave amplitudes facilitate detection of pathologically reduced wave forms (wave V in particular) which represents a significant advancement.

Clinical and Neurophysiological Changes after Targeted Intrathecal Injections of Bone Marrow Stem Cells in a C3 Tetraplegic Subject

High-level quadriplegia is a devastating condition with limited treatment options. Bone marrow derived stem cells (BMSCs) are reported to have immunomodulatory and neurotrophic effects in spinal cord injury (SCI). We report a subject with complete C2 SCI who received three anatomically targeted intrathecal infusions of BMSCs under a single-patient expanded access investigational new drug (IND). She underwent intensive physical therapy and was followed for >2 years. At end-point, her American Spinal Injury Association Impairment Scale (AIS) grade improved from A to B, and she recovered focal pressure touch sensation over several body areas. We conducted serial neurophysiological testing to monitor changes in residual connectivity. Motor, sensory, and autonomic system testing included motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), electromyography (EMG) recordings, F waves, galvanic skin responses, and tilt-table responses. The quality and magnitude of voluntary EMG activations increased over time, but remained below the threshold of clinically obvious movement. Unexpectedly, at 14 months post-injury, deep inspiratory maneuvers triggered respiratory-like EMG bursting in the biceps and several other muscles. This finding means that connections between respiratory neurons and motor neurons were newly established, or unmasked. We also report serial analysis of MRI, International Standards for Neurological Classification of SCI (ISNCSCI), pulmonary function, pain scores, cerebrospinal fluid (CSF) cytokines, and bladder assessment. As a single case, the linkage of the clinical and neurophysiological changes to either natural history or to the BMSC infusions cannot be resolved. Nevertheless, such detailed neurophysiological assessment of high cervical SCI patients is rarely performed. Our findings indicate that electrophysiology studies are sensitive to define both residual connectivity and new plasticity.

Motor and somatosensory evoked potential spinal cord monitoring during intubation and neck extension for thyroidectomy in a Down syndrome boy with atlantoaxial instability

Intubation or neck extension can compress the spinal cord in patients with craniocervical instability. Protective motor evoked potential (MEP) and somatosensory evoked potential (SEP) monitoring of these maneuvers is an obvious consideration when these patients undergo already-monitored spinal surgery, but might be overlooked when they undergo other normally unmonitored procedures. Here we report monitoring intubation and neck extension for the unusual indication of thyroidectomy in a Down syndrome boy with atlantoaxial instability. Transcranial electric stimulation thenar MEPs and optimized median nerve SEPs were acquired about every minute throughout intubation and neck extension under propofol and remifentanil anesthesia without neuromuscular blockade. Potentials were stable and there was no neurologic deficit. This approach could protect craniocervical instability patients against cord compression when they undergo intubation and neck extension for surgical procedures that would not otherwise indicate spinal cord monitoring.