Nerve Somatosensory Evoked Potentials Research Articles

Somatosensory evoked potentials recorded from DBS electrodes: the origin of subcortical N18.

The different peaks of somatosensory-evoked potentials (SEP) originate from a variety of anatomical sites in the central nervous system. The origin of the median nerve subcortical N18 SEP has been studied under various conditions, but the exact site of its generation is still unclear. While it has been claimed to be located in the thalamic region, other studies indicated its possible origin below the pontomedullary junction. Here, we scrutinized and compared SEP recordings from median nerve stimulation through deep brain stimulation (DBS) electrodes implanted in various subcortical targets. We studied 24 patients with dystonia, Parkinson's disease, and chronic pain who underwent quadripolar electrode implantation for chronic DBS and recorded median nerve SEPs from globus pallidus internus (GPi), subthalamic nucleus (STN), thalamic ventral intermediate nucleus (Vim), and ventral posterolateral nucleus (VPL) and the centromedian-parafascicular complex (CM-Pf). The largest amplitude of the triphasic potential of the N18 complex was recorded in Vim. Bipolar recordings confirmed the origin to be close to Vim electrodes (and VPL/CM-Pf) and less close to STN electrodes. GPi recorded only far-field potentials in unipolar derivation. Recordings from DBS electrodes located in different subcortical areas allow determining the origin of certain subcortical SEP waves more precisely. The subcortical N18 of the median nerve SEP-to its largest extent-is generated ventral to the Vim in the region of the prelemniscal radiation/ zona incerta.

Median Nerve Somatosensory Evoked Potentials in Patients with Chiari Malformation

Median Nerve Somatosensory Evoked Potentials in Patients with Chiari Malformation

Somatosensory pathway dysfunction in patients with amyotrophic lateral sclerosis in a completely locked-in state

ObjectiveTo investigate somatosensory pathway function in patients with amyotrophic lateral sclerosis (ALS) dependent on invasive ventilation and in a completely locked-in state (CLIS). MethodsWe examined median nerve somatosensory evoked potentials (SEPs) in 17 ALS patients in a CLIS, including 11 patients with sporadic ALS, one with familial ALS with genes not examined, four with a Cu/Zn superoxide-dismutase-1 (SOD1) gene variant (Val118Leu, Gly93Ser, Cys146Arg), and one with a fused-in-sarcoma gene variant (P525L). We evaluated N9, N13, N20 and P25, and central conduction time (CCT); the data were compared with those of 73 healthy controls. ResultsN20 and N13 were abolished in 12 and 10 patients, and their latencies was prolonged in four and three patients, respectively. The CCT was prolonged in five patients with measurable N13 and N20. Two patients with SOD1 gene mutations had absent or slightly visible N9. Compared to the CCT and latencies and amplitudes of N13 and N20 in the controls, those in the patient cohort were significantly abnormal. ConclusionsThe central somatosensory pathway is severely involved in patients with ALS in a CLIS. SignificanceOur findings suggest that median nerve SEP cannot be utilized for communication in patients with ALS in a CLIS.

Sensory Gating during Voluntary Finger Movement in Amyotrophic Lateral Sclerosis with Sensory Cortex Hyperexcitability.

Cortical responses in somatosensory evoked potentials (SEP) are enhanced in patients with amyotrophic lateral sclerosis (ALS). This study investigated whether sensory gating is involved in the pathophysiology of sensory cortical hyperactivity in ALS patients. The median nerve SEP was recorded at rest and during voluntary finger movements in 14 ALS patients and 13 healthy control subjects. The parietal N20, P25, and frontal N30 were analyzed, and sensory gating was assessed by measuring the amplitude of each component during finger movement. The amplitudes of the N20 onset-peak, N20 peak-P25 peak, and N30 onset-peak were higher in ALS patients than in controls. Nonetheless, there were no significant differences in the amplitude reduction ratio of SEPs between patients and controls. There was a significant correlation between the baseline amplitudes of the N20 onset-peak or N20 peak-P25 peak and their gating ratios in patients with ALS. Our findings indicate that the excitability of the primary sensory cortex and secondary motor cortex is enhanced in ALS, while sensory gating is preserved in the early stages of ALS. This result suggests that enhanced SEP is caused by the hyperexcitability of the primary sensory and secondary motor cortices but not by the dysfunction of inhibitory mechanisms during voluntary movements.

Directional recordings of somatosensory evoked potentials from the sensory thalamus in chronic poststroke pain patients

ObjectiveThe aim of this feasibility study was to investigate the properties of median nerve somatosensory evoked potential (SEPs) recorded from segmented Deep Brain Stimulation (DBS) leads in the sensory thalamus (VP) and how they relate to clinical and anatomical findings. MethodsWe analyzed four patients with central post-stroke pain and DBS electrodes placed in the VP. Median nerve SEPs were recorded with referential and bipolar montages. Electrode positions were correlated with thalamus anatomy and tractography-based medial lemniscus. Early postoperative clinical paresthesia mapping was performed by an independent pain nurse. Finally, we performed frequency and time–frequency analyses of the signals. ResultsWe observed differences of SEP amplitudes recorded along different directions in the VP. SEP amplitudes did not clearly correlate to both atlas-based anatomical position and fiber-tracking results of the medial lemniscus. However, the contacts of highest SEP amplitude correlated with the contacts of lowest effect-threshold to induce paraesthesia. ConclusionsSEP recordings from directional DBS leads offer additional information about the neurophysiological (re)organization of the sensory thalamus. SignificanceDirectional recordings of thalamic SEPs bear the potential to assist clinical decision-making in DBS for pain.

Optimization of signal-to-noise ratio in short-duration SEP recordings by variation of stimulation rate

ObjectiveThe intraoperative averaging of the somatosensory evoked potential (SEP) requires reliable recordings within the shortest possible duration. We here systematically optimized the repetition rate of stimulus presentation. MethodsWe recorded medianus and tibial nerve SEP during 22 surgeries and varied the rate of stimulus presentation between 2.7 Hz and 28.7 Hz. We randomly sampled a number of sweeps corresponding to recording durations up to 20 s and calculated the signal-to-noise ratio (SNR). ResultsFor the medianus nerve at 5 s recording duration, SEP stimulation rate at 12.7 Hz obtained the highest median SNR = 22.9 for the N20, which was higher than for rate 4.7 Hz (p = 1.5e-4). When increasing the stimulation rate, latency increased and amplitude decayed for cortical but not for peripheral recording sites. For the tibial nerve, the rate 4.7 Hz achieved the highest SNR for all durations. ConclusionsWe determined the time-dependence of SNR for N20 and elucidated the underlying physiology. For short recordings, rapid reduction of noise through averaging at high stimulation rate outweighs the disadvantage of smaller amplitude. SignificanceFor a short duration of medianus nerve SEP recording only, it may be advantageous to stimulate with a repetition rate of 12.7 Hz.

Utility of the tibial nerve somatosensory evoked potentials in differentiating between neuromyelitis optica spectrum disorders and multiple sclerosis.

Somatosensory evoked potentials (SEPs) are widely used for the diagnosis and evaluation of neuromyelitis optica spectrum disorder (NMOSD) and multiple sclerosis (MS). However, whether the parameters of tibial nerve SEPs can help to distinguish NMOSD from MS remains unclear. Thus, the aim of this study was to investigate the utility of tibial nerve SEP parameters in differentiating patients with NMOSD and MS. The clinical data of patients with NMOSD or MS treated in our institution between 2005 and 2021 were retrospectively extracted from our electronic database. Additional inclusion criteria were presentation with sensory symptoms in the lower extremities with corresponding lesions in the magnetic resonance images as well as available data on anti-aquaporin-4 antibodies and tibial nerve SEPs. The Z-scores of the N21-P38 interval (central sensory conduction time), P38 latency, and P38 amplitude were compared between the patients with NMOSD and MS. The relationship of disease severity with the parameters of the tibial nerve SEPs was also evaluated. Twenty patients with NMOSD and 13 patients with MS were enrolled. The Z-scores of the N21-P38 interval and P38 latency were significantly higher in the MS group than in the NMOSD group (p < 0.05 and p < 0.01, respectively), whereas there was no difference in the Z-scores of the P38 amplitude between the two groups. In the MS group, only the N21-P38 interval and P38 latency were significantly correlated with disease severity (p < 0.05 and p < 0.01, respectively). In contrast, none of the tibial nerve SEP parameters were significantly correlated with disease severity in the NMOSD group. Evaluation of the N21-P38 interval and P38 latency in tibial nerve SEPs potentially helps in differentiating between NMOSD and MS.

Sensorimotor integration and motor learning during a novel force-matching task in young adults with attention-deficit/hyperactivity disorder.

Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder that exhibits unique neurological and behavioral characteristics. Those with ADHD often have noted impairments in motor performance and coordination, including during tasks that require force modulation. The present study provides insight into the role of altered neural processing and SMI in response to a motor learning paradigm requiring force modulation and proprioception, that previous literature has suggested to be altered in those with ADHD, which can also inform our understanding of the neurophysiology underlying sensorimotor integration (SMI) in the general population. Adults with ADHD (n = 15) and neurotypical controls (n = 15) performed a novel force-matching task, where participants used their right-thumb to match a trace template that varied from 2-12% of their Abductor Pollicis Brevis maximum voluntary contraction. This motor task was completed in pre, acquisition, and post blocks. Participants also completed a retention test 24 h later. Median nerve somatosensory-evoked potentials (SEPs) were collected pre and post motor acquisition. SEPs were stimulated at two frequencies, 2.47 Hz and 4.98 Hz, and 1,000 sweeps were recorded using 64-electrode electroencephalography (EEG) at 2,048 Hz. SEP amplitude changes were normalized to each participant's baseline values for that peak. Both groups improved at post measures (ADHD: 0.85 ± 0.09; Controls: 0.85 ± 0.10), with improvements maintained at retention (ADHD: 0.82 ± 0.11; Controls: 0.82 ± 0.11). The ADHD group had a decreased N18 post-acquisition (0.87 ± 0.48), while the control N18 increased (1.91 ± 1.43). The N30 increased in both groups, with a small increase in the ADHD group (1.03 ± 0.21) and a more pronounced increase in controls (1.15 ± 0.27). Unique neural differences between groups were found after the acquisition of a novel force-matching motor paradigm, particularly relating to the N18 peak. The N18 differences suggest that those with ADHD have reduced olivary-cerebellar-M1 inhibition when learning a novel motor task dependent on force-modulation, potentially due to difficulties integrating the afferent feedback necessary to perform the task. The results of this work provide evidence that young adults with ADHD have altered proprioceptive processing when learning a novel motor task when compared to neurotypical controls.

Tibial nerve SEPs in diagnosing lumbar spinal stenosis: The utility of segmental evaluation using P15 and N21

ObjectiveTo establish the utility of the additional evaluation of the P15 potential generated at the greater sciatic foramen in the tibial nerve somatosensory evoked potentials (SEPs) in diagnosing lumbar spinal stenosis (LSS). MethodsWe retrospectively reviewed tibial nerve SEP findings in patients having MRI-confirmed LSS at the cauda equina or conus/epiconus region. P15 and N21 potentials were recorded and the following findings were defined as localizing abnormalities: 1) normal P15 latency either with prolonged P15-N21 interval or with absent N21; 2) decreased ratio of the N21 amplitude to P15 amplitude. As non-localizing abnormalities, N21 and P38 latencies were also evaluated. Tibial nerve F-wave findings were also investigated. ResultsAccording to the entry criteria, 18 patients were included, 15 with cauda equina lesions and 3 with conus/epiconus lesions. Localizing abnormalities in SEPs were found in 67% of patients, achieving significantly higher sensitivity than delayed P38 latency (28%), and higher sensitivity than N21 abnormalities (39%), though this was not significant. Localizing abnormalities were observed even in 6 out of 11 patients lacking both sensory symptoms and signs. Tibial nerve F-wave was abnormal in 36% of 14 patients with F-wave examinations, whereas the localizing abnormalities in SEPs were found in 64% of the same patient population. P15 amplitude was depressed in 4 patients (22%), which may indicate the involvement of the dorsal root ganglion in LSS, although its latency was normal even for these patients. ConclusionsTibial nerve SEPs with the recording of P15 and N21 potentials achieved sufficiently high sensitivity in diagnosing LSS. They have the advantage over F-wave in that they can localize the lesion at the cauda equina or conus/epiconus level. SignificanceTibial nerve SEPs are promising in evaluating LSS, especially in documenting sensory tract involvement in cases lacking sensory symptoms/signs.

Misconceptions in IONM Part II: Does Anodal Blocking Occur and Is Bipolar Stimulation Necessary with Intraoperative Somatosensory Evoked Potentials?

ABSTRACT Current intraoperative somatosensory evoked potential (SSEP) guidelines recommend bipolar stimulation with the anode at or near the crease of the wrist and the cathode 2–4 cm proximal to the anode for median nerve SSEPs. The rationale for this cathode proximal bipolar configuration appears to be the avoidance of anodal blocking; however, there is a paucity of experimental support for the existence of anodal blocking. Evidence that bipolar stimulation preferentially drives stimulation from the cathode better than monopolar cathodal or monopolar anodal in peripheral nerves in human neurophysiology is also lacking. This study compared anode proximal to anode distal bipolar stimulation of median nerve SSEPs and the efficacy of monopolar cathode to monopolar anode stimulation in generating median, ulnar, and tibial nerve SSEPs. No difference in median nerve cortical SSEP amplitude was observed between anode proximal and anode distal bipolar stimulation at supramaximal stimulation suggesting cathode proximal bipolar is equal to anode proximal bipolar stimulation at supramaximal intensity. This data suggests that anodal blocking does not occur inintraoperative SSEPs. Furthermore, no differences were observed in ulnar, median, and tibial nerve SSEP cortical or subcortical amplitudes and latencies between monopolar cathodal or monopolar anodal stimulation suggesting monopolar cathode and anode stimulation are equally effective at evincing intraoperative SSEPs at supramaximal intensity.

Abstract WMP93: Somatosensory Evoked Potentials Monitoring In Acute Ischemic Stroke (Promise) Study: A Biomarker Of Functional Recovery Prior To Mechanical Thrombectomy (clinicaltrials.gov: NCT04099615)

Hypothesis: Somatosensory evoked potentials (SEPs), as surrogate of cerebral blood flow, may give substantial additional predictive value to that provided by clinical and imaging factors and optimize the selection of patients benefiting from mechanical thrombectomy (MT) Methods: Bilateral median nerve SEPs were recorded before and continuously during MT in consecutive patients with AIS and anterior LVO. N20 response ipsilateral to the ischemic hemisphere was measured. The adjusted predictive value of the N20 on functional Independence (modified Rankin Scale score ≤2) after MT was analyzed by binary logistic regression and its predictive value on the full range of disability by ordinal logistic regression. We constructed different regression models with other clinical and imaging predictors at the pre- and in- hospital setting to determine the independent predictive power of N20 for a potential treatment decision-making Results: A total of 223 patients were recruited. A blinded reading of SEPs recordings identified presence of N20 in 110 (49.3%), absence in 58 (26%) and not assessable in 55 patients due to radiofrequency interferences. Prior to MT, N20 had positive predictive value (PPV) of 93% (95%CI, 0.8 to 0.98) and increased 10 fold the likelihood of good functional outcome at 7 days (adjusted OR, 9.9; 95%CI, 3.1-44.6). Receiver operating curves showed that N20 had a higher capacity to predict good functional outcome (AUC 0.82) than models constructed with pre-hospital (age, sex, serum glucose, median blood pressure, NIHSS; AUC 0.75) or in-hospital (NIHSS, ASPECTS; AUC 0.72) factors. During MT, N20 had PPV of 100% (95% CI, 0.85-1) and was the unique independent factor associated with functional independence (adjusted OR, 2.74; 95%CI, 1.90-4.29). Perfusion CT or MR was available in a subgroup of 116 patients. Baseline N20 showed a higher capacity to predict good functional outcome (n=168, AUC 0.71) compared with ischemic core (n=127, AUC 0.66), ischemic penumbra (Tmax&gt;6) (n=116, AUC 0.54) and collateral status (n=191, AUC 0.61) Conclusions: SEPs monitoring is a fast and bedside technique that could complement current clinical factors in terms of increasing the eligibility of AIS patients for MT and improving prognosis.

Reconstructing subcortical and cortical somatosensory activity via the RAMUS inverse source analysis technique using median nerve SEP data

This study concerns reconstructing brain activity at various depths based on non-invasive EEG (electroencephalography) scalp measurements. We aimed at demonstrating the potential of the RAMUS (randomized multiresolution scanning) technique in localizing weakly distinguishable far-field sources in combination with coinciding cortical activity. As we have shown earlier theoretically and through simulations, RAMUS is a novel mathematical method that by employing the multigrid concept, allows marginalizing noise and depth bias effects and thus enables the recovery of both cortical and subcortical brain activity. To show this capability with experimental data, we examined the 14–30 ms post-stimulus somatosensory evoked potential (SEP) responses of human median nerve stimulation in three healthy adult subjects. We aim at reconstructing the different response components by evaluating a RAMUS-based estimate for the primary current density in the nervous tissue. We present source reconstructions obtained with RAMUS and compare them with the literature knowledge of the SEP components and the outcome of the unit-noise gain beamformer (UGNB) and standardized low-resolution brain electromagnetic tomography (sLORETA). We also analyzed the effect of the iterative alternating sequential technique, the optimization technique of RAMUS, compared to the classical minimum norm estimation (MNE) technique. Matching with our previous numerical studies, the current results suggest that RAMUS could have the potential to enhance the detection of simultaneous deep and cortical components and the distinction between the evoked sulcal and gyral activity.

Contralateral and Ipsilateral Interactions in the Somatosensory Pathway in Healthy Humans.

Hyper-adaptability, the ability to adapt to changes in the internal environment caused by neurological disorders, is necessary to recover from various disabilities, such as motor paralysis and sensory impairment. In the recovery from motor paralysis, the pre-existing neural pathway of the ipsilateral descending pathway, which is normally suppressed and preserved in the course of development, is activated to contribute to the motor control of the paretic limb. Conversely, in sensory pathways, it remains unclear whether there are compensatory pathways which are beneficial for the recovery of sensory impairment due to damaged unilateral somatosensory pathways, such as thalamic hemorrhage. Here, we investigated the interaction between the left and right somatosensory pathways in healthy humans using paired median nerve somatosensory evoked potentials (SEPs). Paired median nerve SEPs were recorded at CP3 and CP4 with a reference of Fz in the International 10–20 System. The paired median nerve stimulation with different interstimulus intervals (ISIs; 1, 2, 3, 5, 10, 20, 40, 60, and 100 ms) was performed to test the influence of the first stimulus (to the right median nerve) on the P14, P14/N20, and N20/P25 components induced by the second stimulus (left side). Results showed that the first stimulation had no effect on SEP amplitudes (P14, P14/N20, and N20/P25) evoked by the second stimulation in all ISI conditions, suggesting that there might not be a neural connectivity formed by a small number of synapses in the left–right interaction of the somatosensory pathway. Additionally, the somatosensory pathway may be less diverse in healthy participants.

P118. Establishing baseline motor and somatosensory evoked potentials (MEPs and SSEPs) prior to cervical spine surgery: Association with diagnosis, demographics, comorbidities, and patient-reported measures

<h3>BACKGROUND CONTEXT</h3> During cervical spine procedures, motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs) are the main modalities used for intraoperative neuromonitoring (IONM). Although MEPs and SSEPs monitoring have been recommended in cervical procedures, there is little research regarding risk factors for unmonitorable signals. <h3>PURPOSE</h3> To determine diagnosis, health history, demographic, and patient-reported symptom variables associated with attainability of monitorable baseline MEPs and SSEPs. <h3>STUDY DESIGN/SETTING</h3> Retrospective cohort review. <h3>PATIENT SAMPLE</h3> Records of adult patients who underwent elective surgery for cervical radiculopathy or myelopathy with IONM between January 2017 and December 2019 at a high-volume, single center were retrospectively reviewed. Patients with tumor or trauma were excluded. <h3>OUTCOME MEASURES</h3> Short Form-12 Physical (PCS-12), modified Japanese Orthopaedic Association (mJOA) survey, MEP and SSEP monitorability. <h3>METHODS</h3> Demographic data included age at time of procedure, sex, body mass index (BMI) and race. Health history data included 18 health conditions of the circulatory system, joints and other chronic disorders. Patient reported outcome measures (PROM) were derived from the SF-12 survey and the modified Japanese Orthopaedic Association (JOA) survey. All surveys and data were collected preoperatively. The baseline response of each muscle MEP and nerve SSEP was reported as monitorable or unmonitorable and documented in the electronic record during the procedure. Patients were separated into two cohorts based on the primary diagnosis of radiculopathy or myelopathy. Odds ratios were used to compare baseline monitorability of SSEPs and MEPs between cohorts, and within cohort comparisons utilized T-tests, Mann-Whitney U, and chi-squared tests, as appropriate, and logistic regression models. <h3>RESULTS</h3> Baseline MEPs were established in all myotomes in 93.5% of radiculopathic patients but just 87.6% of myelopathic patients, and for just lower extremity myotomes, baselines were established in 97.9% of radiculopathic patients but just 89.2% of myelopathic patients. Relative to radiculopathic patients, the odds ratios for unmonitorable baseline MEPs were significantly increased in myelopathic patients for all myotomes and lower extremity myotomes in particular (2.04 [1.52;2.78], and 2.68 [1.89, 3.82], p<0.001, respectively). For the myelopathic group, unmonitorable MEP signals were associated with age (p < 0.001), being male (p = 0.002) having high blood pressure (p < 0.001), peripheral vascular disease (p < 0.001), rheumatoid arthritis (p = 0.018), and type II diabetes (p < 0.001). For the radiculopathic group, unmonitorable MEP signals were only associated with gout (p < 0.001). For both groups, the majority of patients with unmonitorable lower extremity MEPs reported modest to moderate to major ambulatory dysfunction. Regarding SSEPs, the odds ratios for unmonitorable baselines were also increased in the myelopathic group relative the radiculopathic group (2.72 [1.86, 4.09], P<0.001), and for both groups, unmonitorable SSEPs were significantly associated with age, male sex, BMI, and type II diabetes (p values <0.005). <h3>CONCLUSIONS</h3> Baseline IONM signals are less likely to be established in myelopathic patients than radiculopathic patients. Age, male gender, and elevated BMI, and a history of a chronic circulatory or metabolic disorder such as high blood pressure, peripheral vascular disease, thyroid disorder, or type II diabetes were predictive of unmonitorable signals. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.

Long-term effects of concussion on relevancy-based modulation of somatosensory-evoked potentials

ObjectiveThe purpose of this investigation was to better understand the effects of concussions on the ability to selectively up or down-regulate incoming somatosensory information based on relevance. MethodsMedian nerve somatosensory-evoked potentials (SEPs) were elicited from electrical stimulation and recorded from scalp electrodes while participants completed tasks that altered the relevance of specific somatosensory information being conveyed along the stimulated nerve. ResultsWithin the control group, SEP amplitudes for task-relevant somatosensory information were significantly greater than for non-relevant somatosensory information at the earliest cortical processing potentials (N20-P27). Alternatively, the concussion history group showed similar SEP amplitudes for all conditions at early processing potentials, however a pattern similar to controls emerged later in the processing stream (P100) where both movement-related gating and facilitation of task-relevant information were present. ConclusionsPreviously concussed participants demonstrated impairments in the ability to up-regulate relevant somatosensory information at early processing stages. These effects appear to be chronic, as this pattern was observed on average several years after participants’ most recent concussion. SignificanceGiven the role of the prefrontal cortex in relevancy-based facilitation during movement-related gating, these findings lend support to the notion that this brain area may be particularly vulnerable to concussive forces.

Cortical somatosensory processing after botulinum toxin therapy in post-stroke spasticity.

In dystonic and spastic movement disorders, abnormalities of motor control and somatosensory processing as well as cortical modulations associated with clinical improvement after botulinum toxin A (BoNT-A) treatment have been reported, but electrophysiological evidence remains controversial. In the present observational study, we aimed to uncover central correlates of post-stroke spasticity (PSS) and BoNT-A-related changes in the sensorimotor cortex by investigating the cortical components of somatosensory evoked potentials (SEPs). Thirty-one chronic stroke patients with PSS of the upper limb were treated with BoNT-A application into the affected muscles and physiotherapy. Clinical and electrophysiological evaluations were performed just before BoNT-A application (W0), then 4 weeks (W4) and 11 weeks (W11) later. PSS was evaluated with the modified Ashworth scale (MAS). Median nerve SEPs were examined in both upper limbs with subsequent statistical analysis of the peak-to-peak amplitudes of precentral P22/N30 and postcentral N20/P23 components. At baseline (W0), postcentral SEPs were significantly lower over the affected cortex. At follow up, cortical SEPs did not show any significant changes attributable to BoNT-A and/or physiotherapy, despite clear clinical improvement. Our results imply that conventional SEPs are of limited value in evaluating cortical changes after BoNT-A treatment and further studies are needed to elucidate its central actions.

Enlarged high frequency oscillations of the median nerve somatosensory evoked potential and survival in amyotrophic lateral sclerosis

ObjectiveA large N20 and P25 of the median nerve somatosensory evoked potential (SEP) predicts short survival in amyotrophic lateral sclerosis (ALS). We investigated whether high frequency oscillations (HFOs) over N20 are enlarged and associated with survival in ALS. MethodsA total of 145 patients with ALS and 57 healthy subjects were studied. We recorded the median nerve SEP and measured the onset-to-peak amplitude of N20 (N20o-p), and peak-to-peak amplitude between N20 and P25 (N20p-P25p). We obtained early and late HFO potentials by filtering SEP between 500 and 1 kHz, and measured the peak-to-peak amplitude. We followed up patients until endpoints (death or tracheostomy) and analyzed the relationship between SEP or HFO amplitudes and survival using a Cox analysis. ResultsPatients showed larger N20o-p, N20p-P25p, and early and late HFO amplitudes than the control values. N20p-P25p was associated with survival periods (p = 0.0004), while early and late HFO amplitudes showed no significant association with survival (p = 0.4307, and p = 0.6858, respectively). ConclusionsThe HFO amplitude in ALS is increased, but does not predict survival. SignificanceThe enlarged HFOs in ALS might be a compensatory phenomenon to the hyperexcitability of the sensory cortex pyramidal neurons.

Distal Stimulation Site at the Medial Tibia for Saphenous Nerve Somatosensory Evoked Potentials (DSn-SSEPs) in Lateral Lumbar Spine Procedures

ABSTRACT Lateral lumbar interbody fusion procedures are performed with multimodality neuromonitoring of the femoral nerve to prevent lumbosacral plexus and peripheral nerve injury from positioning, dilation, retraction, and hardware implantation. The integrity of the femoral nerve can be continuously assessed during these procedures by Somatosensory Evoked Potentials of the Saphenous nerve (Sn-SSEPs). Sn-SSEPs are technically challenging to acquire and necessitate advanced troubleshooting skills with a more rigid anesthetic regimen and physiological parameters. We performed a retrospective analysis of Sn-SSEP data for 100 consecutive lateral lumbar surgeries where the stimulation electrodes were placed distally below the knee and medial to the tibia bone (i.e., DSn-SSEPs). Monitorable baseline responses were present in 87% of patients after the exclusion of fourteen cases where the tibial nerve SSEP was absent, quadriceps transcranial electrical motor evoked potentials (TCeMEPs) were absent or not utilized. Sex, age, body mass index (BMI), diagnosis, mean arterial pressure (MAP), inhalational anesthetic levels, reliability of ulnar and posterior tibial nerve SSEPs, and the reliability of femoral nerve innervated quadriceps TCeMEPs were evaluated but were not of statistically significant consequence between cases where the DSn-SSEP was present or absent in this study. We found the utilization of DSn-SSEPs to be a valuable adjunct to femoral nerve monitoring. Stimulation electrode placement is easy to palpate with clear anatomical borders. Significant muscle artifact and patient movement from stimulation do not affect waveform morphology, allowing for continuous and reliable monitoring. We recommend including DSn-SSEPs to optimize recordings during lateral lumbar procedures.

The Vagus Nerve Somatosensory-evoked Potential in Neural Disorders: Systematic Review and Illustrative Vignettes.

Objective. To review the scientific publications reporting vagal nerve somatosensory-evoked potential (VSEP) findings from individuals with brain disorders, and present novel physiological explanations on the VSEP origin. Methods. We did a systematic review on the papers reporting VSEP findings from individuals with brain disorders and their controls. We evaluated papers published from 2003 to date indexed in PubMed, Web of Science, and Scielo databases. We extracted the following information: number of patients and controls, type of neural disorder, age, gender, stimulating/recording and grounding electrodes as well as stimulus side, intensity, duration, frequency, and polarity. Information about physiological parameters, neurobiological variables, and correlation studies was also reviewed. Representative vignettes were included to add support to our conclusions. Results. The VSEP was studied in 297 patients with neural disorders such as Parkinson's disease (PD), Alzheimer's disease, vascular dementia, mild cognitive impairment, subjective memory impairment, major depression, and multiple sclerosis. Scalp responses marked as the VSEP showed high variability, low validity, and poor reproducibility. VSEP latencies and amplitudes did not correlate with disease duration, unified PD rating scale score, or heart function in PD patients nor with cerebrospinal fluid β amyloid, phosphor-τ, and cognitive tests from patients with mental disorders. Vignettes demonstrated that the VSEP was volume conduction propagating from muscles surrounding the scalp recording electrodes. Conclusion. The VSEP is not a brain-evoked potential of neural origin but muscle activity induced by electrical stimulation of the tragus region of the ear. This review and illustrative vignettes argue against assessing the parasympathetic system using the so-called VSEP.

The influence of right-left error in the placement of the Cc electrode in tibial nerve somatosensory evoked potentials (SEPs)

ObjectiveAt our laboratory, we routinely record tibial nerve somatosensory evoked potentials (SEPs) using 5 channels including the second cervical vertebra (C2S)-contralateral central area (Cc) and Cz’ (2 cm posterior to Cz)–Cc derivations. In a man with lumbar spondylotic myelopathy, symptoms improved after surgery, although the N21-P38 interval was markedly prolonged in comparison with that before surgery. We presumed that the Cc electrode was actually placed on the ipsilateral central area (Ci) at the second examination. Inspired by this episode, we investigated the influence of the right-left error in the placement of the Cc electrode. MethodsSubjects were 20 healthy volunteers. Tibial nerve SEPs were recorded with 8 leads including Cz’–Cc, Cz’-Ci, C2S-Cc and C2S-Ci. ResultsFor the Cz’-Ci lead, the P38 potential diminished in amplitude, was absent or became negative. For the C2S-Ci lead, a large negative potential corresponding to the phase reversal of P38 was frequently observed. ConclusionsTibial nerve SEPs using the Cz’–Cc or C2S-Cc lead are distorted if the Cc electrode is placed on the opposite side. SignificanceWhen a strange result is obtained in tibial nerve SEPs, we should check for a right-left error in the Cc electrode placement.