SEP Amplitude Research Articles

Comparison of intraoperative neurophysiological monitoring between propofol and remimazolam during total intravenous anesthesia in the cervical spine surgery: a prospective, double-blind, randomized controlled trial.

Although total intravenous anesthesia (TIVA) with propofol and remifentanil is frequently used to optimize intraoperative neurophysiological monitoring (IONM), the exact effect of remimazolam on IONM remains unknown. Here, we compared the effects of propofol and remimazolam along with remifentanil on IONM during TIVA. In this prospective, double-blind, randomized controlled trial, 64 patients requiring IONM during cervical spine surgery were administered either propofol (Group P) or remimazolam (Group R). The preoperative latencies of the somatosensory-evoked potentials (SEP; N20 for the median nerve and P37 for the tibial nerve) were measured. SEP latencies and amplitudes and motor-evoked potential (MEP) amplitudes were measured 30 min after anesthetic induction (T1), 30 min after surgical incision (T2), after laminectomy or discectomy (T3), immediately after plate insertion or pedicle screw fixation (T4), and before surgical wound closure (T5). The primary outcome was the between-group difference in the N20 latency changes measured at T1 and preoperatively. The change in SEP latencies including N20 and P37 at T1 compared with preoperative time was not significantly different between Groups P and R. Except for the amplitude of the right abductor brevis, there was no significant group-by-time interaction effect for intraoperative MEP amplitudes or SEP latencies and amplitudes. TIVA with remimazolam and remifentanil for cervical spine surgery yielded stable IONM, comparable to those observed with conventional TIVA with propofol and remifentanil. Further clinical trials are needed in other surgical contexts and with more diverse patient populations to determine the effects of remimazolam on IONM.

A diagnostic study on the application of segmental somatosensory evoked potential examination to acquired premature ejaculation.

Premature ejaculation (PE), affecting approximate 5%, has an unclear pathogenesis, limited treatment efficacy, and a lack of effective diagnostic methods. This prospective diagnostic study aimed to compare segmental dorsal penile nerve somatosensory evoked potentials (DNSEP) differences among patients with acquired premature ejaculation (APE), primary premature ejaculation (PPE), and healthy controls. This prospective diagnostic study examined patients suffering from PE who visited the outpatient clinic of the Department of Urology of the Second Affiliated Hospital of Zhejiang University School of Medicine from January 1, 2022 to February 28, 2023. According to the definition of PE by the ISSM, 16 cases comprised the healthy control group, 31 in the APE group, and 28 in the PPE group. Each group was examined based on the segmental DNSEP with electrodes recording at multiple locations (the selected location was at the Cz and the C7). The latency time of the evoked potential obtained at Cz was P40, and that obtained at C7 was P30. The P30/P40 ratios of P40, P30, and DNSEP wave amplitudes at C7 and Cz were compared among the 3 groups of patients. No group differences were found in P40 latency at Cz. However, PPE showed higher DNSEP amplitude at Cz, while APE showed lower amplitudes compared with controls. Both APE and PPE had significantly shorter P30 latency at C7 than controls. SEP amplitude at C7 was similar in APE and PPE but lower than in controls. The P30/P40 ratio was lower in APE compared with PPE and controls. Segmental SEP may offer more assistance in localizing neurological lesions, potentially guiding clinical treatment. In this study, the innovative use of the P30/P40 ratio was proposed, maintaining consistency in emotional states and measurement conditions for the same patient. However, limitations include a restricted number of patient cases and challenges in obtaining a diverse control group, potentially introducing bias. In addition, not considering subclinical premature ejaculation and the comorbidity of PE + ED (LCEE) in patient stratification is another limitation of this study. Results suggest a correlation between secondary PE and underlying conditions, such as obesity and lumbar spine injuries. The study validates multi-site somatosensory-evoked potential examination for locating neural lesions but acknowledges the need for future invasive needle electrode AEP testing to analyze neuropathological changes more comprehensively. In conclusion, segmental DNSEP examination aids in localizing neuropathy in APE patients, and the P30/P40 ratio proves more accurate in diagnosing APE than P40 alone.

A critical event frequent lead to reversible spinal cord injury during vertebral column resection surgery.

To report a "critical phase" (between osteotomy completion and correction beginning) that will frequently lead to the reversible intraoperative neurophysiological monitoring (IOM) change during posterior vertebral column resection (PVCR) surgery. The study sample consisted of 120 patients with severe spine deformity who underwent PVCR and deformity correction surgeries. Those patients were recruited consecutively from 2010 to 2018 January in our spine center. The detailed IOM data (the amplitude of MEP & SEP) and its corresponding surgical points were collected prospectively. Early and long-term postoperative neurologic outcomes were assessed for the following functions: motor, sensory, and pain at immediate postoperative and 1-year post-operation in this cases series. A total of 105 (105/120) patients presented varying degrees of IOM reduction in the critical phase; the mean IOM amplitude retention vs rescue rate was 27% ± 11.2 versus 58% ± 16.9, P < 0.01 (MEP) & 34% ± 8.3 versus 66% ± 12.4 P < 0.01 (SEP). Patients with postoperative spinal deficits often had a significantly longer IOM-alerting duration than the patients without (p < 0.01, Mann-Whitney U-test), and IOM-alerting duration greater than 39.5min was identified as an independent predictor of the risk of postoperative spinal deficits. The reversible IOM events probably often appear in the critical phase during PVCR surgery. The new postoperative spinal deficits are possible for patients without satisfied IOM recovery or alerting duration greater than 39.5min. Timely and suitable surgical interventions are useful for rescuing the IOM alerts.

Balancing the Senses: Electrophysiological Responses Reveal the Interplay between Somatosensory and Visual Processing During Body-Related Multisensory Conflict.

In the study of bodily awareness, the predictive coding theory has revealed that our brain continuously modulates sensory experiences to integrate them into a unitary body representation. Indeed, during multisensory illusions (e.g., the rubber hand illusion, RHI), the synchronous stroking of the participant's concealed hand and a fake visible one creates a visuotactile conflict, generating a prediction error. Within the predictive coding framework, through sensory processing modulation, prediction errors are solved, inducing participants to feel as if touches originated from the fake hand, thus ascribing the fake hand to their own body. Here, we aimed to address sensory processing modulation under multisensory conflict, by disentangling somatosensory and visual stimuli processing that are intrinsically associated during the illusion induction. To this aim, we designed two EEG experiments, in which somatosensory- (SEPs; Experiment 1; N = 18; F = 10) and visual-evoked potentials (VEPs; Experiment 2; N = 18; F = 9) were recorded in human males and females following the RHI. Our results show that, in both experiments, ERP amplitude is significantly modulated in the illusion as compared with both control and baseline conditions, with a modality-dependent diametrical pattern showing decreased SEP amplitude and increased VEP amplitude. Importantly, both somatosensory and visual modulations occur in long-latency time windows previously associated with tactile and visual awareness, thus explaining the illusion of perceiving touch at the sight location. In conclusion, we describe a diametrical modulation of somatosensory and visual processing as the neural mechanism that allows maintaining a stable body representation, by restoring visuotactile congruency under the occurrence of multisensory conflicts.

Modulation of the spinal N13 SEP component by high- and low-frequency electrical stimulation. Experimental pain models matter

ObjectiveThe N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of cervical dorsal horn neurons. Neurophysiological studies in healthy participants showed that capsaicin-induced central sensitization causes an increase of the N13 SEP amplitude. Consequently, in human research, this spinal component may serve as a valuable readout of central sensitization.In this study, we wanted to verify if the sensitivity of the N13 SEP for detecting central sensitization is consistent across different experimental pain models inducing central sensitization and secondary hyperalgesia, namely high and low-frequency electrical stimulation (HFS and LFS). MethodsIn 18 healthy participants, we recorded SEP after bilateral ulnar nerve stimulation before and after secondary hyperalgesia was induced through HFS and LFS applied on the ulnar nerve territory of the hand of one side. The area of secondary hyperalgesia was mapped with a calibrated 128-mN pinprick probe, and the mechanical pain sensitivity with three calibrated 16–64-256-mN pinprick probes. ResultsAlthough both HFS and LFS successfully induced secondary hyperalgesia only LFS increased the amplitude of the N13 SEP. ConclusionsThese findings suggest that the sensitivity of the N13 SEP for detecting dorsal horn excitability changes may critically depend on the different experimental pain models. SignificanceOur results indicate that LFS and HFS could trigger central sensitization at the dorsal horn level through distinct mechanisms, however this still needs confirmation by replication studies.

Somatosensory evoked potentials of the tibial nerve during the surgical decompression of thoracolumbar intervertebral disk herniation in dogs.

This study aimed to identify the impact on spinal cord integrity and determine the electrophysiological safety level during surgery for thoracolumbar intervertebral disk herniation in dogs. A total of 52 dogs diagnosed with thoracolumbar intervertebral disk herniation were enrolled. The tibial nerve somatosensory evoked potential elicited on the scalp by stimulation of the tibial nerve was recorded before and during hemilaminectomy. Both the amplitude and latency of the somatosensory evoked potential were periodically registered, and the percentage changes from the pre-operative control values (amplitude rate and latency rate) were calculated. When the multifidus muscles were retracted after removal from the spinous processes and vertebrae, the somatosensory evoked potential amplitude rate decreased in all dogs, while the latency rate increased in 33 dogs examined. The amplitude rate remained unchanged during the halting procedure, loosening retraction, and hemilaminectomy. After removing the disc material from the spinal canal, the amplitude rate was increased. The somatosensory evoked potential latency increased when the multifidus muscles were retracted and shortened after multifidus muscles closure in four cases. The outcome of all cases showed improvement in clinical signs 7 days after operation. Spinal cord conduction is impaired by retraction of the multifidus muscles and improved by removal of disk materials. Maintaining intraoperative SEP amplitudes above 50% of control may help avoid additional spinal cord injury during surgery. Since we have no case that worsened after the surgery, however, further studies are necessary to confirm this proposal.

Partial Unweighting in Obese Persons Enhances Tactile Transmission From the Periphery to Cortical Areas: Impact on Postural Adjustments.

Tactile plantar information is known to play an important role in balance maintenance and to contribute to the setting of anticipatory postural adjustments (APAs) prior to stepping. Previous studies have suggested that somatosensory processes do not function optimally for obese individuals due to the increased pressure of the plantar sole resulting in balance issues. Here, we investigated whether decreasing the compression of the mechanoreceptors by unweighting the plantar sole would enhance tactile sensory processes leading to an increased stability and an accurate setting of the APAs in obese individuals. More specifically, we tested the hypothesis that the somatosensory cortex response to electric stimulation (SEP) of the plantar sole in standing obese persons will be greater with reduced body weight than with their effective weight. The level of unweighting was calculated for each participant to correspond to a healthy body mass index. We showed an increase SEP amplitude in the unweighted condition compared to the effective body weight for all participants. This increase can be explained by the reduction of weight itself but also by the modified distribution of the pressure exerted onto the foot sole. Indeed, in the unweighted condition, the vertical ground reaction forces are evenly distributed over the surface of the foot. This suggests that decreasing and equalizing the pressure applied on the plantar mechanoreceptors results in an increase in somatosensory transmission and sensory processes for obese persons when unweighted. These sensory processes are crucial prior to step initiation and for setting the anticipatory postural adjustments (i.e., thrust). These cortical changes could have contributed to the observed changes in the spatiotemporal characteristics of the thrust prior to step initiation.

Overpressure on fingertips prevents state estimation of the pen grip force and movement accuracy.

We tested the hypothesis that the inability to move a pen accurately in a graphic task is partly due to a decrease of afferent somatosensory information resulting from overpressure on the tactile receptors of the fingers holding the pen. To disentangle the depressed somatosensory origin from an altered motor command, we compared a condition in which the participant actively produces pressure on the pen (active grip) with a condition in which pressure is passively applied (passive grip, no grip-related motor command). We expected that the response of the somatosensory cortex to electric stimulation of the wrist's tactile nerve (i.e., SEP) would be greater in the natural pen grip (baseline condition) than in the two overpressure conditions (actively or passively induced). Fifteen adults were required to trace a geometrical shape in the three grip conditions. The SEP amplitude was not significantly different between the baseline and both overpressure conditions. However, behavioral results showed that drawing accuracy is impaired when the pressure on the pen is increased (passively or actively). Cortical source analyses revealed that the activity of the superior parietal areas (SPL) increased in both overpressure conditions. Our findings suggest that the SPL is critical for sensorimotor integration, by maintaining an internal representation of pen holding. These cortical changes might witness the impaired updating of the finger-pen interaction force for such drawing actions under visual guidance.

Modulation of the N13 component of the somatosensory evoked potentials in an experimental model of central sensitization in humans

The N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of dorsal horn neurons. In this neurophysiological study, we aimed to verify whether N13 SEP might reflect excitability changes of dorsal horn neurons during central sensitization. In 22 healthy participants, we investigated how central sensitization induced by application of topical capsaicin to the ulnar nerve territory of the hand dorsum modulated N13 SEP elicited by ulnar nerve stimulation. Using a double-blind placebo-controlled crossover design, we also tested whether pregabalin, an analgesic drug with proven efficacy on the dorsal horn, influenced capsaicin-induced N13 SEP modulation. Topical application of capsaicin produced an area of secondary mechanical hyperalgesia, a sign of central sensitization, and increased the N13 SEP amplitude but not the peripheral N9 nor the cortical N20-P25 amplitude. This increase in N13 SEP amplitude paralleled the mechanical hyperalgesia and persisted for 120 min. Pregabalin prevented the N13 SEP modulation associated with capsaicin-induced central sensitization, whereas capsaicin application still increased N13 SEP amplitude in the placebo treatment session. Our neurophysiological study showed that capsaicin application specifically modulates N13 SEP and that this modulation is prevented by pregabalin, thus suggesting that N13 SEP may reflect changes in dorsal horn excitability and represent a useful biomarker of central sensitization in human studies.

Application of Magnetic Resonance Diffusion Tensor Imaging in the Clinical Diagnosis of Disc Herniation after Lumbar Spine Injury.

Magnetic resonance diffusion-weighted imaging (DTI) provides a unique perspective on the pathophysiological and microstructural changes during spinal cord injury, with high spatial specificity; meanwhile, NM reflects the conduction and integrity of neuroelectrical signals in spinal cord fiber tracts, with time-specific and dynamic evaluation effects. The fractional anisotropy (FA) value, SEP amplitude, and neurological function score or improvement rate are correlated. The combination of DTI and NM can more reliably quantify the spinal cord function, evaluate the effectiveness of treatment, and determine the patient's prognosis, which can provide reference for clinical decision making and future research for SCI patients. That is, the lower the preoperative FA value and the lower the SEP amplitude, the worse the preoperative and postoperative neurological function, the lower the improvement rate, and the worse the prognosis of patients. Therefore, we believe that spinal cord function can be graded according to JOA scores to find the corresponding FA and SEP amplitude ranges and that, by measuring FA and SEP amplitude in the future, we can reverse the assessment of spinal cord function, expected postoperative improvement, and long-term prognosis. At the same time, FA values can also help determine the nature of the lesion to some extent.

The influence of an acute bout of moderate-intensity cycling exercise on sensorimotor integration.

Acute cycling exercise can modulate motor cortical circuitry in the non-exercised upper-limb. Within the primary motor cortex, measures of intracortical inhibition are reduced and intracortical facilitation is enhanced following acute exercise. Further, acute cycling exercise decreases interhemispheric inhibition between the motor cortices and lowers cerebellar-to-motor cortex inhibition. Yet, investigations into the effects of acute exercise on sensorimotor integration, referring to the transfer of incoming afferent information from the primary somatosensory cortex to motor cortex, are lacking. The current work addresses this gap in knowledge with two experimental sessions. In the first session, we tested the exercise-induced changes in somatosensory and motor excitability by assessing somatosensory (SEP) and motor evoked potentials (MEPs). In the second session, we explored the effects of acute cycling exercise on short- (SAI) and long-latency afferent inhibition (LAI), and afferent facilitation. In both experimental sessions, neurophysiological measures were obtained from the non-exercised upper-limb muscle, tested at two time points pre-exercise separated by a 25-min period of rest. Next, a 25-min bout of moderate-intensity lower-limb cycling was performed with measures assessed at two time points post-exercise. Acute lower-limb cycling increased LAI, without modulation of SAI or afferent facilitation. Further, there were no exercise-induced changes to SEP or MEP amplitudes. Together, these results suggest that acute exercise has unique effects on sensorimotor integration, which are not accompanied by concurrent changes in somatosensory or motor cortical excitability.

Somatosensory cortical facilitation during step preparation restored by an improved body representation in obese patients

BackgroundThe anticipatory postural adjustments (APA) associated with step initiation are impaired in obese patients (e.g. longer duration, greater lateral center of pressure excursion). This could arise from the known altered internal representation of the body in obese individuals as this representation is crucial for enhancing the processing of foot cutaneous inputs prior to step initiation and for setting the APA. Research questionThe purpose of the study was to examine if the processing of foot cutaneous inputs and the preparation of the APA when planning a step are impaired in obese patients due to their damaged body internal representation (BIR). We also investigated whether these sensorimotor processes will be restored after a 15-day intervention program composed of motor and cognitive activities engaging the BIR without aiming weight loss. MethodsWe compared, prior to (D1) and after (D15) the program, the amplitude of the cortical response evoked by foot cutaneous stimulation (SEP) occurring either during quiet standing or during the planning of a step in 18 obese patients (mean body mass index, BMI: 35). The APA were analyzed by measuring the amplitude and latency of the lateral force exerted on the ground. Results and significanceThe SEP amplitude was not significantly different between the standing and stepping tasks at D1, but increased in the stepping task at D15. This enhanced sensory processing was associated with an increased activation of the posterior parietal cortex, suggesting a stronger involvement of the body representation during the planning of the stepping movement after the program. These cortical changes could have contributed to the changes in the temporal dimension of the APA observed at D15. These results suggest that programs targeting different dimensions of the BIR could be beneficial in improving the dynamic balance in obesity.

High-frequency component in flash visual evoked potentials in type 3 Gaucher disease

ObjectiveTo characterize the visual evoked potentials (VEP) in patients with type 3 Gaucher disease (GD) with or without progressive myoclonus epilepsy. MethodsThree young adults with progressive myoclonus epilepsy (type 3a GD) and two children without progressive myoclonus epilepsy (type 3b GD) were enrolled. Flash visual and somatosensory evoked potentials (F-VEP and SEP, respectively) were retrospectively reviewed in all patients under enzyme replacement therapy. Pattern reversal visual evoked potentials (PR-VEP) were recorded in the type 3a group. ResultHigh-frequency components were provoked at early latencies on averaged F-VEP in all patients with type 3a and one patient with type 3b GD. Conversely, no activities were recorded in PR-VEP. The onset latency of the components began at 15.3–19.8 ms after flash stimulation. Four-five of the F-VEP examination the activities’ duration overrode the N75 waveforms and were prolonged toward the P100 peak latency. The F-VEP amplitude did not differ between the type 3a and type 3b groups, while the SEP amplitude was higher in the type 3a than in the type 3b group. SignificanceHigh-frequency components on F-VEP using bandpass filter at 10–200 Hz may help assess augmented excitability in the visual cortex of type 3 GD patients.

Comparison of transcranial electrical stimulation regimens for effects on inhibitory circuit activity in primary somatosensory cortex and tactile spatial discrimination performance

Transcranial electrical stimulation (tES) can be used to modulate inhibitory circuits in primary somatosensory cortex, resulting in improved somatosensory function. However, efficacy may depend on the specific stimulus modality and patterns. For instance, transcranial alternating current stimulation (tACS), transcranial random noise stimulation (tRNS), and transcranial pulsed current stimulation (tPCS) were found to stably and effectively modulate neuronal excitability, while anodal transcranial direct current stimulation (tDCS) appeared less effective overall but with substantial response heterogeneity among subjects. Therefore, we compared the effects of tES applied to primary somatosensory cortex on somatosensory evoked potential paired-pulse depression (SEP-PPD) and tactile discrimination performance in 17 neurologically healthy subjects. In Experiment 1, somatosensory evoked potential N20/P25_SEP-PPD, N20_SEP-PPD, and P25_SEP-PPD responses were assessed before and immediately after anodal tDCS, tACS (stimulation frequency, 140 Hz), tRNS (stimulation frequency, 0.1–640 Hz), anodal tPCS (pulse width, 50 ms; inter-pulse interval, 5 ms), and sham stimulation applied to primary somatosensory cortex. In Experiment 2, a grating orientation task (GOT) was performed before and immediately after the same anodal tDCS, tRNS, anodal tPCS, and sham stimulation regimens. Anodal tDCS and anodal tPCS decreased N20_SEP-PPD, and tRNS increased the first N20 SEP amplitude. Furthermore, tRNS and anodal tPCS decreased GOT discrimination threshold (improved performance). These results suggest that tRNS and anodal tPCS can improve sensory perception by modulating neuronal activity in primary somatosensory cortex.

Progressive myoclonus epilepsy caused by a gain-of-function KCNA2 mutation

Progressive myoclonus epilepsy caused by a gain-of-function KCNA2 mutation

T94. Stable “Crossover” TcMEP with post-op weakness in CEA

Introduction Carotid endarterectomies can be performed in patients with carotid artery stenosis in order to prevent future strokes. Electroencephalogram and somatosensory evoked potentials are intraoperative neuromonitoring modalities used to provide the surgeon with real-time feedback in detecting decreases in cerebral blood flow during carotid artery clamping. This may help the surgeon decide whether to shunt the carotid artery, which has potential complications. Transcranial motor evoked potentials have been used as an adjunct to EEG and SSEPs to monitor carotid endarterectomies, with Malcharek et al. reporting 0.4% false-negative TcMEPs in a review of 264 patients. We report a case where TcMEPs remain unchanged with unilateral SSEP changes that was associated with new post-operative deficits. Methods This is a case report of a 54 year old female with multiple vascular risk factors and a prior stroke three months prior with trace right upper (RUE) and lower (RLE) weakness. She presented with acute worsening of weakness and numbness, due to severe narrowing of the proximal left internal carotid artery. She underwent left carotid endarterectomy for symptomatic carotid stenosis. Three-channel EEG, SSEP with stimulation at the median nerve and posterior tibial nerve, and TcMEPs of the upper and lower extremities were setup for intraoperative neuromonitoring. The patient was anesthetized with propofol and fentanyl. Results Baseline SSEPs in all four extremities were adequate. TcMEPs were reproducible in all four extremities, except the left foot had variable responses. Of note, there were TcMEP responses ipsilateral to the hemisphere of stimulation. Seventy minutes post-incision, EEG showed burst suppression and the surgeons clamped the left carotid artery. Two minutes post-clamping, the right sided SSEP began to decrease in amplitude. The RUE and RLE SSEPs became absent at 10 min and 19 min post-clamping, respectively. TcMEPs run after the SSEPs changes were unchanged from baseline and the surgeons moved forward without shunting. At 42 min post-clamping, surgeons completed the CEA and removed the clamp. The RLE SSEP returned 3 min after unclamping, and the RUE SSEP 29 min after unclamping. The RLE SEP improved to baseline, but the RUE SEP amplitude remained significantly diminished. Left sided SSEPs were stable and bilateral TcMEPs remained unchanged throughout the entire surgery. EEG demonstrated burst suppression without any notable asymmetry. Pre-operative exam showed RUE drift and 4+/5 strength on RLE. Post-operative exam revealed flaccid RUE. Her RLE motor strength was anti-gravity, but she did not follow commands for individual muscle groups. Conclusion TcMEPs may be an unreliable measure of the nervous system when crossover responses are present in the limbs ipsilateral to the hemisphere of stimulation. In this left CEA, right-sided SSEP changes correctly heralded potential injury immediately after clamping, while right-sided TcMEPs remained unchanged.

F172. Comparison of inhalational anesthetic effects on cortical SEPs to upper limb and lower limb stimulation

During surgery on the thoracic and lumbar spine, SEPs to lower limb (LL) stimulation are used to monitor the dorsal column somatosensory pathways from the legs. SEPs to upper limb (UL) stimulation are often also recorded, to monitor for brachial plexus or peripheral nerve dysfunction related to the positioning of the patient’s arms and to serve as a ”control” for the effects of anesthesia on the LL SEPs. The latter use presumes that cortical SEPs to UL and LL stimulation are affected equally by anesthesia. We examined this assumption by measuring amplitude changes in UL and LL SEPs associated with the transition from TIVA (total intravenous anesthesia) with propofol and a narcotic to a halogenated inhalational anesthetic (HIA) during closing in a neurologically normal patient population. We retrospectively identified surgeries for idiopathic scoliosis in adolescents without neuromuscular disease during which there were no adverse SEP or MEP changes and at least 2 runs of both UL and LL SEPs were recorded after the transition to inhalational anesthesia. The average SEP amplitudes were calculated for those runs and for several SEP waveforms immediately preceding the anesthetic change. Percentage amplitude changes were calculated for SEPs for each limb, the results for the left and right sides were averaged together, and the amplitude ratios (amplitudes under HIA divided by amplitudes under TIVA) for UL SEPs were compared to those for LL SEPs. In all cases, the SEPs were elicited by ulnar nerve and posterior tibial nerve stimulation. We identified 17 surgeries meeting inclusion criteria; desflurane was used in 9 and sevoflurane in 8. The patients included 12 girls and 5 boys ages 12–17 years. In one patient the UL SEPs were larger after the anesthetic change (amplitude ratio = 118%); in the other patients the amplitude ratio for UL SEPs ranged from 29% to 91%. Overall, the mean amplitude ratio for UL SEPs was 76%. For LL SEPs, the amplitude ratios ranged from 50% to 97% with a mean of 74%. In 10 patients the relative difference between the amplitude ratios for UL SEPs and LL SEPs was ⩽15% and in 6 patients it was between 16% and 40%. In the patient with the largest relative difference, the amplitude ratio was 29% for UL SEPs and 55% for LL SEPs. Cortical SEPs were almost always smaller under HIA than under TIVA. On average, the anesthetic effects in the UL SEPs were very similar to those in the LL SEPs (mean amplitude ratio 76% vs. 74%). The effects on UL SEPs versus LL SEPs were also similar in most individual patients, but there were some patients in whom the quantitative effects on UL SEPs and LL SEPs differed by more than 15%. In conclusion, in this patient population UL SEPs provide a good anesthetic control for LL SEPs overall, but it should not be assumed that they can accurately predict quantitative amplitude changes in LL SEPs in individual patients.

Warning criteria for intraoperative neurophysiologic monitoring

Intraoperative changes in somatosensory (SEP) and motor evoked potentials (MEPs) may indicate potential injury to the spinal cord and will require timely intervention to prevent permanent damage. This review focuses on the validity of currently recommended warning criteria for intraoperative evoked potential monitoring. Current guideline recommends a decrease in SEP amplitude by 50% and MEP amplitude by 50-100% as warning signals for injury to the ascending sensory and descending motor pathway, respectively. On the basis of cohort studies, the diagnostic accuracy of SEP and MEP to predict postoperative neurologic deficits was variable. Importantly, 0.1-4.1% of monitored patients suffered postoperative neurologic deficit despite apparently normal SEP and MEP recordings (i.e. false negative events). These data suggested that the true warning criteria may be lower than previously acknowledged. A systematic review of studies that reported changes in SEP or MEP monitoring and postoperative neurological outcome showed an association between changes in monitoring signals and postoperative neurological deficits. However, the confidence intervals were wide and it is not possible to determine a threshold value in SEP or MEP amplitude beyond which may indicate neurologic deficit. Current recommendations for warning criteria during intraoperative evoked potential monitoring are empirically derived. Until a threshold that predicts spinal cord injury can be accurately determined, it remains difficult to define the clinical utility of intraoperative neurophysiologic monitoring.

P328 Not only the absence but also amplitude reduction of cortical SEPs predict ominous outcome in comatose patients after cardiac arrest

In comatose patients after cardiac arrest (CA) bilateral absence of cortical SEPs is a high reliable predictor of poor outcome but it is present in no more than 40% (1). Low amplitude pathological SEPs were found in 28.9% (2) of subjects but only few papers analysed the prognostic significance of this pattern (3–4). To evaluated prognostic value of N20-P25 amplitude of SEPs, recorded at 6–12 h after CA. Retrospectively, we analysed comatose patients after CA admitted to our hospital. SEPs were recorded at 6–12 h after CA. We determined the lowest SEP amplitude in a subject that recovery consciousness We evaluated the accuracy of SEP amplitude in predicting the “no consciousness recovery” by ROC curves. Outcome was evaluated at 6 months by GOS. One-hundred and eighteen patients were analysed. None of subjects with N20-P25 amplitude less than 1.2 mV recovered consciousness. The area under the ROC curves was 0.96 for “no consciousness recovery”. SEP amplitude threshold determined by ROC analysis was 1.21 mV. For this SEP amplitude threshold, we found a sensibility: 85.9% and specificity: 100.0%. Not only the absence but also the amplitude reduction (less than 1.2 mV) of N20-P25 is associated with no consciousness recovery with very high predictive value.

Natural variation in sensory-motor white matter organization influences manifestations of Huntington's disease.

While the HTT CAG-repeat expansion mutation causing Huntington's disease (HD) is highly correlated with the rate of pathogenesis leading to disease onset, considerable variance in age-at-onset remains unexplained. Therefore, other factors must influence the pathogenic process. We asked whether these factors were related to natural biological variation in the sensory-motor system. In 243 participants (96 premanifest and 35 manifest HD; 112 controls), sensory-motor structural MRI, tractography, resting-state fMRI, electrophysiology (including SEP amplitudes), motor score ratings, and grip force as sensory-motor performance were measured. Following individual modality analyses, we used principal component analysis (PCA) to identify patterns associated with sensory-motor performance, and manifest versus premanifest HD discrimination. We did not detect longitudinal differences over 12 months. PCA showed a pattern of loss of caudate, grey and white matter volume, cortical thickness in premotor and sensory cortex, and disturbed diffusivity in sensory-motor white matter tracts that was connected to CAG repeat length. Two further major principal components appeared in controls and HD individuals indicating that they represent natural biological variation unconnected to the HD mutation. One of these components did not influence HD while the other non-CAG-driven component of axial versus radial diffusivity contrast in white matter tracts were associated with sensory-motor performance and manifest HD. The first component reflects the expected CAG expansion effects on HD pathogenesis. One non-CAG-driven component reveals an independent influence on pathogenesis of biological variation in white matter tracts and merits further investigation to delineate the underlying mechanism and the potential it offers for disease modification. Hum Brain Mapp 37:4615-4628, 2016. © 2016 Wiley Periodicals, Inc.