Transcranial MEP threshold voltages and current densities simulated with finite element modelling.

OBJECTIVE: The aim of this study was to compare stimulation thresholds and current densities in the brain for transcranial motor evoked potentials (tcMEPs) from the hands and feet with linked quadripolar (LQP), M3-M4 and C1-C2 electrode montages. METHODS: Twenty-five patients underwent cerebral vascular surgery with tcMEP monitoring. tcMEP voltage thresholds were compared between LQP (C1, M3, C2, M4), C1-C2, and M3-M4 montages. In a finite element model (FEM), hand, arm, and leg regions of interest (ROIs) on the cortical motor homunculus were segmented. Current densities in these ROIs at tcMEP thresholds were compared across tcMEP electrode montages. RESULTS: LQP tcMEP thresholds were 61.5 volts for hands and 95.2 volts for feet. Thresholds were higher for M3-M4 (hands, 89.4 V; feet, 141.3 V) and C1-C2 (hands: 137.3 V; feet: 194.7 V). Total current at threshold voltage was greater for LQP (hands, 210.9 mA; feet, 311.3 mA) compared to M3-M4 (hands, 166.8 mA; feet, 256.6 mA), but similar to C1-C2 (hands, 246.7 mA; feet, 341.1 mA). In FEM simulations, current density and local current density topography in the hand ROI at threshold were very similar for LQP, M3-M4 and C1-C2. CONCLUSIONS: TcMEP voltage thresholds were least for LQP, and lesser for M3-M4 compared to C1-C2. In FEM simulations, resistance to current to hand ROI was ordered the same (LQP < M3-M4 < C1-C2). The local distribution of current density in motor cortex with tcMEP was mainly determined by cortical geometry. SIGNIFICANCE: Current densities and resistance to current simulated with FEM may explain threshold requirements for tcMEP electrode montages.

Analyzing the value of IONM as a complex intervention: The gap between published evidence and clinical practice.

OBJECTIVE: Intraoperative neurophysiological monitoring (IONM) was investigated as a complex intervention (CI) as defined by the United Kingdom Medical Research Council (MRC) in published studies to identify challenges and solutions in estimating IONM's effects on postoperative outcomes. METHODS: A scoping review to April 2022 of the influence of setting on what was implemented as IONM and how it influenced postoperative outcomes was performed for studies that compared IONM to no IONM cohorts. IONM complexity was assessed with the iCAT_SR tool. Causal graphs were used to represent this complexity. RESULTS: IONM implementation depended on the surgical procedure, institution and/or surgeon. "How" IONM influenced neurologic outcomes was attributed to surgeon or institutional experience with the surgical procedure, surgeon or institutional experience with IONM, co-interventions in addition to IONM, models of IONM service delivery and individual characteristics of the IONM provider. Indirect effects of IONM mediated by extent of tumor resection, surgical approach, changes in operative procedure, shorter operative time, and duration of aneurysm clipping were also described. There were no quantitative estimates of the relative contribution of these indirect effects to total IONM effects on outcomes. CONCLUSIONS: IONM is a complex intervention whose evaluation is more challenging than that of a simple intervention. Its implementation and largely indirect effects depend on specific settings that are usefully represented in causal graphs. SIGNIFICANCE: IONM evaluation as a complex intervention aided by causal graphs and multivariable analysis could provide a valuable framework for future study design and assessments of IONM effectiveness in different settings.

Intraoperative neuromonitoring for pediatric Chiari decompression: when is it useful?

OBJECTIVE: Surgical treatment for symptomatic Chiari I malformation involves surgical decompression of the craniovertebral junction. Given the proximity of critical brainstem structures, intraoperative neuromonitoring (IONM) is employed for safe decompression in some institutions. However, IONM adds time and cost to the operation, and the benefit to the patient has not been defined. Given the diversity in surgical practices, there is no evidence-based standard of care regarding when to use IONM and which modalities are most helpful. The purpose of this study was to review a single-surgeon experience with IONM in order to determine the sensitivity, specificity, and predictive values of various IONM modalities routinely used in pediatric Chiari I decompression; to examine the associations between patient, clinical, and radiographic characteristics and IONM alerts; and to obtain data regarding the usefulness of these modalities during the surgical process to improve patient outcomes. METHODS: A retrospective review was performed for 300 consecutive pediatric patients who underwent suboccipital craniectomy and C1 laminectomy for Chiari decompression performed by a single surgeon over a 15-year period. Clinical, radiographic, and IONM data were collected. Radiographic measurements of the skull base morphological abnormalities, including clival angle, Chamberlain's line, and Grabb-Oakes line, were compared between patients with and without true IONM signal changes. RESULTS: A total of 291 cases were included, with an age range of 6 months to 19 years. Among 291 cases, somatosensory evoked potentials (SSEPs) were monitored in 291, motor evoked potentials (MEPs) in 209, cranial nerve spontaneous electromyography (sEMG) in 290, and brainstem auditory evoked potentials (BAEPs) in 110. Sensitivity, specificity, positive predictive value, and negative predictive value, respectively, were as follows: 1.00, 1.00, 1.00, and 1.00 for SSEPs; 1.00, 0.99, 0.67, and 1.00 for MEPs; 0.00, 0.88, 0.00, and 1.00 for sEMG; and not appliable, 1.00, not applicable, and 1.00 for BAEPs. Six patients had true IONM signal changes. These patients had radiographic evidence of more severe concomitant craniocervical instability and basilar invagination, with steeper clival angles (124° vs 146°, p = 0.02) and larger Grabb-Oakes lines (10.1 mm vs 6.7 mm, p = 0.02), when compared with the patients without any true IONM changes. CONCLUSIONS: Intraoperative neuromonitoring may be best utilized for patients who show radiographic features of abnormal skull base morphology, defined as a clival angle < 135° or Grabb-Oakes line > 9 mm. When IONM is employed, SSEP and MEP monitoring are the most useful modalities.

Monitoring spinal surgery for extramedullary tumors and fractures.

Meningiomas are the most common intradural extramedullary tumors, followed by nerve sheath tumors that can also grow extradurally. Metastases are the most frequent extradural tumors and most commonly affect the thoracic vertebrae. Spinal fractures with column dislocation and/or instability require surgical fixation. Spine surgery for an extramedullary tumor or fracture usually involves decompression of neural elements and instrumentation for stabilization. These procedures risk spinal cord and nerve root injury. The incidence of nerve root deficits after resection of nerve sheath tumors is particularly high since the tumor grows from the rootlets. Intraoperative neurophysiologic monitoring and mapping techniques have been introduced to prevent iatrogenic neurologic deficits. These include motor and sensory evoked potentials, electromyography, compound muscle action potentials, and the bulbocavernosus reflex. The combination of techniques chosen for a particular procedure depends on the surgical level and the character of the lesion.

Intraoperative Neurophysiology and Transcranial Doppler for Detection of Cerebral Ischemia and Hyperperfusion During Carotid Endarterectomy.

OBJECTIVE: To evaluate and compare efficacy of intraoperative neurophysiological monitoring (IONM) and intraoperative transcranial Doppler (TCD) techniques for identification of hypoperfusion during carotid artery clamp and hyperperfusion after release of occlusion during carotid endarterectomy. METHODS: This was a retrospective, consecutive case series of 152 patients undergoing carotid endarterectomy between June 2018 and March 2020. Somatosensory evoked potentials, motor evoked potentials, electroencephalogram, and TCD were obtained. RESULTS: Three patient cohorts were observed after clamping the carotid artery: A, in 132 of 152 patients (87%), TCD blood flow velocity decreased by <50% and there were no changes in IONM; B, in 5 of 152 (3%) patients, TCD blood flow rate was reduced 50%-100% with no changes in IONM; C, in 15 patients (10%), blood flow velocity was reduced by 50%-100% and all IONM modalities met warning criteria. With increased blood pressure, IONM and blood flow velocities improved to less than warning criteria in 8 of 15 patients. In 6 of the 7 remaining patients, IONM modalities recovered to baseline immediately after clamps were removed from the carotid artery. The 1 patient with persistent motor evoked potential deterioration experienced postoperative proximal muscle weakness, which recovered 48 hours later. In 22 patients, TCD detected hyperperfusion at the moment of clamp release. CONCLUSIONS: TCD blood flow velocity is correlated with motor evoked potential and somatosensory evoked potential amplitude changes after clamping. After declamping, TCD can detect hyperperfusion and help regulate blood pressure to prevent hyperperfusion.

Facial Nerve Branching Patterns Vary With Vascular Anomalies.

OBJECTIVES: At our institution, in vivo facial nerve mapping (FNM) is used during vascular anomaly (VAN) surgeries involving the facial nerve (FN) to create an FN map and prevent injury. During mapping, FN anatomy seemed to vary with VAN type. This study aimed to characterize FN branching patterns compared to published FN anatomy and VAN type. STUDY DESIGN: Retrospective study of surgically relevant facial nerve anatomy. METHODS: VAN patients (n = 67) with FN mapping between 2005 and 2018 were identified. Results included VAN type, FN relationship to VAN, FNM image with branch pattern, and surgical approach. A Fisher exact test compared FN relationships and surgical approach between VAN pathology, and FN branching types to published anatomical studies. MATLAB quantified FN branching with Euclidean distances and angles. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) analyzed quantitative FN patterns amongst VAN types. RESULTS: VANs included were hemangioma, venous malformation, lymphatic malformation, and arteriovenous malformation (n = 17, 13, 25, and 3, respectively). VAN FN patterns differed from described FN anatomy (P < .001). PCA and HCA in MATLAB-quantified FN branching demonstrated no patterns associated with VAN pathology (P = .80 and P = .91, one-way analysis of variance for principle component 1 (PC1) and priniciple component 2 (PC2), respectively). FN branches were usually adherent to hemangioma or venous malformation as compared to coursing through lymphatic malformation (both P = .01, Fisher exact). CONCLUSIONS: FN branching patterns identified through electrical stimulation differ from cadaveric dissection determined FN anatomy. This reflects the high sensitivity of neurophysiologic testing in detecting small distal FN branches. Elongated FN branches traveling through lymphatic malformation may be related to abnormal nerve patterning in these malformations. LEVEL OF EVIDENCE: NA Laryngoscope, 130:2708-2713, 2020.

Correction to: Is the new ASNM intraoperative neuromonitoring supervision "guideline" a trustworthy guideline? A commentary.

The article Is the new ASNM intraoperative neuromonitoring supervision "guideline" a trustworthy guideline? A commentary, written by Stanley A. Skinner, Elif Ilgaz Aydinlar, Lawrence F. Borges, Bob S. Carter, Bradford L. Currier, Vedran Deletis, Charles Dong, John Paul Dormans, Gea Drost, Isabel Fernandez­Conejero, E. Matthew Hoffman, Robert N. Holdefer, Paulo Andre Teixeira Kimaid, Antoun Koht, Karl F. Kothbauer, David B. MacDonald, John J. McAuliffe III, David E. Morledge, Susan H. Morris, Jonathan Norton, Klaus Novak, Kyung Seok Park, Joseph H. Perra, Julian Prell, David M. Rippe, Francesco Sala, Daniel M. Schwartz, Martín J. Segura, Kathleen Seidel, Christoph Seubert, Mirela V. Simon, Francisco Soto, Jeffrey A. Strommen, Andrea Szelenyi, Armando Tello, Sedat Ulkatan, Javier Urriza and Marshall Wilkinson, was originally published electronically on the publisher's internet portal (currently SpringerLink) on 05 January 2019 without open access. With the author(s)' decision to opt for Open Choice the copyright of the article changed on 30 January 2019 to © The Author(s) 2019 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The original article has been corrected.

Preoperative Facial Nerve Mapping to Plan and Guide Pediatric Facial Vascular Anomaly Resection.

Importance: Facial vascular anomalies are surgical challenges due to their vascularity and facial nerve distortion. To assist facial vascular anomaly surgical treatment, presurgical percutaneous facial nerve stimulation and recording of compound motor action potentials can be used to map the facial nerve branches. During surgery, the nerve map and continuous intraoperative motor end plate potential monitoring can be used to reduce nerve injury. Objective: To investigate if preoperative facial nerve mapping (FNM) is associated with intraoperative facial nerve injury risk and safe surgical approach options compared with standard nerve integrity monitoring (NIM). Design, Setting, and Participants: This investigation was a historically controlled study at a tertiary vascular anomaly center in Seattle, Washington. Participants were 92 pediatric patients with facial vascular anomalies undergoing definitive anomaly surgery (from January 1, 1999, through January 1, 2015), with 2 years' follow-up. In retrospective review, a consecutive FNM patient cohort after 2005 (FNM group) was compared with a consecutive historical cohort (1999-2005) (NIM group). Main Outcomes and Measures: Postoperative facial nerve function and selected surgical approach. For NIM and FNM comparisons, statistical analysis calculated odds ratios of nerve injury and operative approach, and time-to-event methods analyzed operative time. Results: The NIM group had 31 patients (median age, 3.3 years [interquartile range, 2.2-11.4 years]; 20 [65%] male), and the FNM group had 61 patients (median age, 4.4 years [interquartile range, 1.5-11.0 years]; 26 [43%] male). In both groups, lymphatic malformation resection was most common (19 of 31 [61%] in the NIM group and 32 of 61 [52%] in the FNM group), and the median anomaly volumes were similar (52.4 mL; interquartile range, 12.8-183.3 mL in the NIM group and 65.4 mL; interquartile range, 18.8-180.2 mL in the FNM group). Weakness in the facial nerve branches at 2 years after surgery was more common in the NIM group (6 of 31 [19%]) compared with the FNM group (1 of 61 [2%]) (percentage difference, 17%; 95% CI, 3%-32%). Anterograde facial nerve dissection was used more in the NIM group (27 of 31 [87%]) compared with the FNM group (28 of 61 [46%]) (percentage difference, 41%; 95% CI, 24%-58%). Treatment with retrograde dissection without identification of the main trunk of the facial nerve was performed in 21 of 61 (34%) in the FNM group compared with 0 of 31 (0%) in the NIM group. Operative time was significantly shorter in the FNM group, and patients in the FNM group were more likely to complete surgery sooner (adjusted hazard ratio, 5.36; 95% CI, 2.00-14.36). Conclusions and Relevance: Facial nerve mapping before facial vascular anomaly surgery was associated with less intraoperative facial nerve injury and shorter operative time. Mapping enabled direct identification of individual intralesional and perilesional nerve branches, reducing the need for traditional anterograde facial nerve dissection, and allowed for safe removal of some lesions after partial nerve dissection through transoral or direct excision.

A comparison of the effects of desflurane versus propofol on transcranial motor-evoked potentials in pediatric patients.

PURPOSE: The aim was to compare the effects of propofol and desflurane anesthesia on transcranial motor evoked potentials (MEPs) from pediatric patients undergoing surgery for spinal deformities. METHODS: Desflurane and propofol cohorts (25 patients each) were obtained retrospectively and matched for patient characteristics and surgical approach. MEPs from the thenar eminence and abductor hallucis were compared during maintenance anesthesia on desflurane (0.6-0.8 MAC) or propofol infusion (150-300 µg/kg/min). MEP amplitudes and durations were obtained for successive 30-min intervals for 150 min, beginning 60 min after maintenance anesthesia. RESULTS: Mean peak to peak amplitudes of MEPs under desflurane anesthesia from the thenar eminence (419 µV) and abductor hallucis (386 µv) were not significantly different from those under propofol (608 µV, 343 µV, thenar, and abductor hallucis, respectively). Stimulation was greater by 42 V and 136 mA, and trains were slightly longer in the desflurane compared to the propofol group (p < 0.05). Most MEP amplitudes for the desflurane and propofol cohorts remained the same or increased (71 % of cases) when those after 150 min were compared to those in the first 30-min interval. CONCLUSIONS: MEPs with good amplitudes were obtained under desflurane only anesthesia that were comparable to propofol only anesthesia in pediatric patients during surgery for spinal deformities. There was no evidence for anesthetic fade over the time period examined. When used by itself, desflurane can be considered a viable alternative to propofol anesthesia.

Intraoperative neuromonitoring alerts that reverse with intervention: treatment paradox and what to do about it.

Intervention-mediated recovery from adversely changed evoked potential recordings may provide evidence for improved outcomes during neurophysiological intraoperative monitoring. However, these reversible signal changes (RSCs) are ambiguous because the patient's neurologic status cannot be known either at signal decline or after intervention. This article describes methods to reduce this ambiguity. Randomized control trials are not always possible or ethical. Recent thought on grading evidence has acknowledged that guidelines first described by Sir Austin Bradford Hill may support evidence for causation. Causality guidelines identified RSCs most likely to be truly positive in three reported studies. Diagnostic statistics were revised accordingly. A range of revised positive predictive values and likelihood ratios was calculated in the three studies, using causality guidelines. The revised data were similar to those reported for other diagnostic tests used in medicine. The RSCs may be assessed using causality guidelines for more accurate reporting of diagnostic statistics while preserving information related to surgical intervention and recovery that is lost with end of surgery diagnostics or when RSCs are ignored. A method is described for including RSCs in diagnostic statistics. This approach will more readily permit assessment of the value of neurophysiological intraoperative monitoring in prediction and prevention of neurologic deficits.

Alternative sites for intraoperative monitoring of cranial nerves X and XII during intracranial surgeries.

During intracranial surgeries, cranial nerve (CN) X is most commonly monitored with electromyographic endotracheal tubes. Electrodes on these endotracheal tubes may be displaced from the vocal folds during positioning, and there is a learning curve for their correct placement. Cranial nerve XII is most commonly monitored with electrodes in the dorsum of the tongue, which are also prone to displacement because of their proximity to the endotracheal tube. A retrospective review was conducted of a consecutive series of 83 skull base surgeries using alternative sites for monitoring CN X and XII. On-going (spontaneous) and evoked electromyography (EMG) were obtained from the cricothyroid muscle for CN X and submental genioglossus for CN XII. Stimulation of CN X or XII evoked specific compound motor action potentials from these muscles, and well-defined on-going EMG was observed during tumor resection in the vicinity of CN X and XII. Volume-conducted responses from the adjacent platysma muscle during CN VII stimulation were identified by concomitant responses from the orbicularis oris and oculi. In conclusion, during skull base surgeries, CN X may be monitored with electrodes in the cricothyroid muscle and CN XII with electrodes in the submental genioglossus. These alternative sites are less prone to displacement of electrodes compared with the more commonly used EMG endotracheal tube and electrodes in the dorsum of the tongue. The cricothyroid muscle should not be used when the recurrent laryngeal nerve is at risk.

Utility of evoked EMG monitoring to improve bone screw placements in the cervical spine.

STUDY DESIGN: EMG threshold testing as a guide to accurate screw placement was examined during posterior cervical spine instrumentation. The accuracy of screw placements was compared with the surgeon blinded and unblinded to EMG thresholds. OBJECTIVE: To determine the utility of EMG threshold testing in improving screw placements in the lateral mass and pedicles of the cervical/upper thoracic spine. SUMMARY OF BACKGROUND DATA: EMG threshold testing in the lumbar spine is generally thought to improve the accuracy of pedicle screw placements. These results may not generalize to the cervical spine, where smaller pedicles, bicortical pilot holes, and the orientation of lateral mass screws away from midline may result in different alert thresholds. METHODS: Triggered EMG thresholds were obtained from pilot holes in 244 lateral mass and 113 pedicles from 32 patients. Thresholds were compared with the accuracy of screw placements as determined from postoperative computed tomography scans. The percentage of inaccurate and potentially dangerous (IAPD) screws with the surgeon blinded and unblinded to EMG thresholds was determined. RESULTS: EMG threshold testing was more accurate in predicting IAPD screw trajectories in the pedicles (likelihood ratio 5.1) as compared with the lateral mass (likelihood ratio 2.9). In the pedicles, the number of IAPD screws decreased from 4.5% in the blinded controls to 0% in the unblinded group. In the lateral mass, there were no IAPD screw placements in the blinded control group, whereas 2% of the screws in the unblinded group were IAPD. CONCLUSIONS: EMG threshold testing in the cervical spine (C3, T1) is a moderately accurate diagnostic test and more predictive of potentially dangerous screw trajectories in the pedicles (C7, T1) as compared with the lateral mass (C3-C6). EMG threshold testing may decrease potentially dangerous screw placements in the pedicles, but may have less utility in the case of the lateral mass because of less reproducible placement of the stimulating probe.

Intraoperative local field recording for deep brain stimulation in Parkinson's disease and essential tremor.

Oscillations in the beta frequency range (ß-LFP) are widely distributed throughout the motor system, modulated by dopaminergic medications, and locally generated in the subthalamic nucleus (STN) and ventral intermediate nucleus of the thalamus (VIM). We investigated the feasibility of recording intraoperative ß-LFP signals and their descriptive summary statistics during surgeries for deep brain stimulation (DBS). ß-LFP from the microelectrode and stimulating lead were obtained from the STN in Parkinson's patients, and from the stimulating lead in the VIM of patients with Parkinson's disease or essential tremor. ß-LFP power was obtained over 8 second epochs and displayed online as compressed spectral and density arrays and trend plots. In agreement with other studies, ß-LFP power along microelectrode penetrations was greater in the STN as compared to sites dorsal and ventral to the nucleus. Differences in ß-LFP power were also observed across the contacts of stimulating leads in the STN and VIM. The contact with greatest ß-LFP power was either the most effective contact for clinical stimulation or adjacent to it. These results were obtained from conventional power measurements, spectral displays, and trend plots with equipment commonly used for intraoperative neuromonitoring. We conclude that ß-LFP is an accessible and easily recorded signal intraoperatively with potential usefulness for DBS lead localization and clinical programming of the stimulating lead.

Dynamic correspondence between Purkinje cell discharge and forelimb muscle activity during reaching.

There remain conflicting models of the cerebellar control of limb movement, ranging from the suggestion that the inhibitory output from Purkinje cells (PCs) is meant to suppress unwanted muscle activity, to the hypothesis that the cerebellar cortex embodies complex internal models of limb dynamics. To test these ideas, we undertook a quantitative comparison of PC simple spike dynamics to those of muscle activity. We recorded simultaneously from Purkinje cells in the paravermal anterior lobe and from muscles of the hand and arm in the behaving monkey during a simple, sequential button pressing task. The task-related discharge of each neuron was determined from peri-event histograms aligned to the onset of the behavior. Bursts of discharge were more than twice as common as pauses, but there was no difference in their timing relative to movement. From the same recordings, the similarity between discharge and muscle activity was evaluated by calculating the cross correlation between firing rate and rectified EMG. Surprisingly, given the inhibitory projection of PCs, most of the bursts of PC discharge were positively correlated with muscle activity. Although our results do not support a simple correspondence of pauses and bursts with limb acceleration and deceleration respectively, they are consistent with a more complex PC regulation of cerebellar nuclear activity from task-related, corticopontine drive.