Bipolar Stimulation Research Articles

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.

Role of Microelectrode Recording in Deep Brain Stimulation of the Pedunculopontine Nucleus: A Physiological Study of Two Cases

BackgroundDeep brain stimulation (DBS) of the pedunculopontine nucleus (PPN) has been reported to improve gait disturbances in Parkinson's disease (PD); however, there are controversies on the radiological and electrophysiological techniques for intraoperative and postoperative confirmation of the target and determination of optimal stimulation parameters. ObjectivesWe investigated the correlation between the location of the estimated PPN (ePPN) and neuronal activity collected during intraoperative electrophysiological mapping to evaluate the role of microelectrode recording (MER) in identifying the effective stimulation site in two PD patients. Materials and MethodsBilateral PPN DBS was performed in two patients who had suffered from levodopa refractory gait disturbance. They had been implanted previously with DBS in the internal globus pallidus and the subthalamic nucleus, respectively. The PPN was determined on MRI and identified by intraoperative MER. Neuronal activity recorded was analyzed for mean discharge rate, bursting, and oscillatory activity. The effects were assessed by clinical ratings for motor signs before and after surgery. ResultsThe PPN location was detected by MER. Groups of neurons characterized by tonic discharges were found 9–10 mm below the thalamus. The mean discharge rate in the ePPN was 19.1 ± 15.1 Hz, and 33% of the neurons of the ePPN responded with increased discharge rate during passive manipulation of the limbs and orofacial structures. PPN DBS with bipolar stimulation at a frequency range 10–30 Hz improved gait disturbances in both patients. Although PPN DBS provided therapeutic effects post-surgery in both cases, the effects waned after a year in case 1 and three years in case 2. ConclusionsEstimation of stimulation site within the PPN is possible by combining physiological guidance using MER and MRI findings. The PPN is a potential target for gait disturbances, although the efficacy of PPN DBS may depend on the location of the electrode and the stimulation parameters.

Deep brain stimulation in the bed nucleus of the stria terminalis: A symptom provocation study in patients with obsessive-compulsive disorder

Deep brain stimulation (DBS) is an emerging therapy for treatment-resistant obsessive-compulsive disorder (OCD), and several targets for electrode implantation and contact selection have been proposed, including the bed nucleus of the stria terminalis (BST). Selecting the active electrode contacts (patients typically have four to choose from in each hemisphere), and thus the main locus of stimulation, can be a taxing process. Here, we investigated whether contact selection based purely on their neuroanatomical position in the BST is a worthwhile approach. For the first time, we also compared the effects of uni- versus bilateral BST stimulation. Nine OCD patients currently receiving DBS participated in a double-blind, randomized symptom provocation study to compare no versus BST stimulation. Primary outcomes were anxiety and mood ratings in response to disorder-relevant trigger images, as well as ratings of obsessions, compulsions, tendency to avoid and overall wellbeing. Furthermore, we asked whether patients preferred the electrode contacts in the BST over their regular stimulation contacts as a new treatment setting after the end of the task. We found no statistically significant group differences between the four conditions (no, left, right and bilateral BST stimulation). Exploratory analyses, as well as follow-up data, did indicate that (bilateral) bipolar stimulation in the BST was beneficial for some patients, particularly for those who had achieved unsatisfactory effects through the typical contact selection procedure. Despite its limitations, this study suggests that selection of stimulation contacts in the BST is a viable option for DBS in treatment-resistant OCD patients.

Comparison of Thresholds between Bipolar and Monopolar Electrical Cortical Stimulation.

Electrical cortical stimulation is widely performed and is the gold standard for functional mapping in intractable epilepsy patients; however, a standard protocol has not yet been established. With respect to stimulation methods, two techniques can be applied: monopolar and bipolar stimulation. We compared the threshold to induce clinical symptoms between these two stimulation techniques. Twenty patients with intractable epilepsy who underwent electrical cortical stimulation for functional mapping were retrospectively investigated. We evaluated the stimulation intensity thresholds required to induce motor, sensory, and language symptoms. A total of 114 electrodes in 20 patients were used to investigate motor, sensory, and language symptoms. The thresholds required to induce motor (median value, bipolar: 4 mA, monopolar: 5 mA, p < 0.05) and language symptoms (bipolar: 8 mA, monopolar: 10 mA, p < 0.0005) were significantly higher for monopolar stimulation than those for bipolar stimulation. However, for sensory symptoms, no significant differences were found in the required thresholds between monopolar and bipolar stimulation (bipolar: 4 mA, monopolar: 4 mA, p = 0.474). Bipolar cortical stimulation required lower intensities to produce clinical motor and language symptoms and thus would be safe and suitable for screening of the eloquent area in functional mapping.

335 Low Frequency Oscillations in the Ventral Nucleus Accumbens Region Guides Brain Responsive Neurostimulation for Loss of Control Eating: An Initial Review of the BITES Trial

INTRODUCTION: Loss of control (LOC) eating is pervasive in eating disorders and obesity. The presence of this phenotype appears to predict treatment-resistance, including gastric bypass surgery. Brain-responsive neurostimulation guided by low frequency changes in the nucleus accumbens (NAc-RNS) was previously found to block LOC eating behavior in mice. Following this novel preclinical work, the U.S. Food and Drug Administration approved an Investigational Device Exemption for a first-in-human study. METHODS: Two participants (female, 45 and 55 years old) with binge-eating disorder and morbid obesity (body mass index, 45 and 50 kg/m2) refractory to gastric bypass were surgically implanted with a brain-responsive neurostimulator (RNS System; NeuroPace, Inc.) connected to bilateral NAc depth leads. NAc field potentials were recorded postoperatively in the clinic while the participants completed eating tasks. Outside of the clinic, participants maintained a diary describing their LOC episodes, specifically describing craving and satiety severity (Likert scale), and triggered the neurostimulator to record timestamped NAc field potentials. NAc field potentials and bilateral NAc connectivity were analyzed for the development and implementation of a novel NAc-RNS paradigm. RESULTS: In the clinic, the left ventral NAc region activity showed increased delta (2-4 Hz)- and theta (4-8 Hz)-band power during anticipatory bites of highly caloric, palatable food when compared to standard meal bites. In the ambulatory setting, we observed increased bilateral ventral NAc delta band activity and connectivity that appeared selective for states of food craving. Therefore, the RNS System was programmed to detect brief bursts of low-frequency activity in the bilateral ventral NAc, and delivered bilateral, bipolar stimulation (∼1 μC/cm2 charge density per 10s stimulation bout). This paradigm resulted in decreased LOC episodes (∼25% in both participants) and subjective improvement in decision-making surrounding food choices. CONCLUSION: We present early findings of an electrographic signal associated with LOC eating with promising preliminary evidence of restored control using a NAc-RNS paradigm. Future directions include refining control signals, adaptive parameters, and examining the long-term effects of restoring inhibitory control on obesity.

A Minimally Invasive Method for Observing Wind-Up of Flexion Reflex in Humans: Comparison of Electrical and Magnetic Stimulation.

Wind-up like pain or temporal summation of pain is a phenomenon in which pain sensation is increased in a frequency-dependent manner by applying repeated noxious stimuli of uniform intensity. Temporal summation in humans has been studied by observing the increase in pain or flexion reflex by repetitive electrical or thermal stimulations. Nonetheless, because the measurement is accompanied by severe pain, a minimally invasive method is desirable. Gradual augmentation of flexion reflex and pain induced by repetitive stimulation of the sural nerve was observed using three stimulation methods—namely, bipolar electrical, magnetic, and monopolar electrical stimulation, with 11 healthy male subjects in each group. The effects of frequency, intensity, and number of repetitive stimuli on the increase in the magnitude of flexion reflex and pain rating were compared among the three methods. The reflex was measured using electromyography (EMG) from the short head of the biceps femoris. All three methods produced a frequency- and intensity-dependent progressive increase in reflex and pain; pain scores were significantly lower for magnetic and monopolar stimulations than for bipolar stimulation (P < 0.05). The slope of increase in the reflex was steep during the first 4–6 stimuli but became gentler thereafter. In the initial phase, an increase in the reflex during the time before signals of C-fibers arrived at the spinal cord was observed in experiments using high-frequency stimulation, suggesting that wind-up was caused by inputs of A-fibers without the involvement of C-fibers. Magnetic and monopolar stimulations are minimally invasive and useful methods for observing the wind-up of the flexion reflex in humans. Monopolar stimulation is convenient because it does not require special equipment. There is at least a partial mechanism underlying the wind-up of the flexion reflex that does not require C-fibers.

HP08: Deep Brain Stimulation Evoked Potentials in Children with dystonia

Pallidal Deep Brain Stimulation (DBS) is an established therapy for dystonia. Stimulation parameters are currently determined empirically. Recording the cortical response to the pallidal stimulus, known as Deep Brain Stimulation Evoked Potentials (DBSEPs) could provide a more objective measure of optimal contact selection (Tisch et al, MovDisord 2008;23:265-73; Bhanpuri et al, BrainStim 2014;7:718-26). Methods: Fifteen children with dystonia participated (age 7-19years), all with bilateral pallidal DBS implanted at least 6 months previously. Scalp EEG (10-20 system) was sampled at 5KHz. Contacts from each stimulating electrode were tested separately, using 6Hz bipolar stimulation. Pairs of electrode contacts were tested sequentially using combinations of cathodal stimulation (2 volts) with an adjacent anode, and pulse width at the patient’s therapeutic level. Each combination was recorded for 3 minutes, giving >1000 stimuli per contact pair. Therapeutic settings were restored on study completion. Offline, the DBS stimulus artefact was used to segment EEG data into epochs (10ms pre-stimulus, 150ms post-stimulus). After artefact rejection, 900-1100 epochs were averaged to obtain the DBSEP for each contact pair. Results: Clear DBSEPs were obtained in 12/15 patients on one or both sides, with peak latency of 20ms, and largest amplitude over the ipsilateral central/centro-parietal region, concordant with adult literature. In 3/15 patients no convincing DBSEP was obtained. Conclusion: To our knowledge this is the largest study of DBSEPs in dystonia and confirms the feasibility and tolerability of this technique in children. Future work will investigate potential clinical applications of DBSEPs to inform an individualised approach to DBS parameter selection.

Extraoperative electrical stimulation mapping in epilepsy presurgical evaluation: a proposal and review of the literature

BackgroundElectrical stimulation mapping (ESM) is an important tool for the localization of the seizure onset zone (SOZ) in patients with medically resistant epilepsy (MRE). ESM is the gold standard for the identification of eloquent cortex in epilepsy surgery candidates. However, there is no standard protocol outlining how to perform ESM, to obtain the most useful information possible. The objective of this study, after reviewing the literature concerning ESM, is to propose a unifying technique to validate reliable data across different centers. MethodsIn this manuscript we summarize this technique from its origin to present, and review protocols used in other centers. We also describe a protocol that has been used in our institution, which utilizes depth electrodes. ResultsThe most common type of ESM uses a “close-loop” system, bipolar and high frequency stimulation (50 Hz). We propose to use a pulse width of 300 µs, current spanning 1–6 mA in depth electrodes and 1–11 mA in subdural-grids. Stimulation time of 5 s maximum and at least 10 s break in between the stimulations. ConclusionsESM is a useful tool for understanding eloquent cortex as well as the epilepsy network, although there is no clear consensus regarding how it should be performed.

Transcranial focused ultrasound induces sustained synaptic plasticity in rat hippocampus

Transcranial focused ultrasound (tFUS) neuromodulation provides a promising emerging non-invasive therapy for the treatment of neurological disorders. Many studies have demonstrated the ability of tFUS to elicit transient changes in neural responses. However, the ability of tFUS to induce sustained changes need to be carefully examined. In this study, we use the long-term potentiation/long term depression (LTP/LTD) model in the rat hippocampus, the medial perforant path (mPP) to dentate gyrus (DG) pathway, to explore whether tFUS is capable of encoding frequency specific information to induce plasticity. Single-element focused transducers were used for tFUS stimulation with ultrasound fundamental frequency of 0.5 MHz and nominal focal distance of 38 mm tFUS stimulation is directed to mPP. Measurement of synaptic connectivity is achieved through the slope of field excitatory post synaptic potentials (fEPSPs), which are elicited using bipolar electrical stimulation electrodes and recorded at DG using extracellular electrodes to quantify degree of plasticity. We applied pulsed tFUS stimulation with total duration of 5 min, with 5 levels of pulse repetition frequencies each administered at 50 Hz sonication frequency at the mPP. Baseline fEPSP is recorded 10 min prior, and 30+ minutes after tFUS administration. In N = 16 adult wildtype rats, we observed sustained depression of fEPSP slope after 5 min of tFUS focused at the presynaptic field mPP. Across all PRFs, no significant difference in maximum fEPSP slope change was observed, average tFUS induced depression level was observed at 19.6%. When compared to low frequency electrical stimulation (LFS) of 1 Hz delivered to the mPP, the sustained changes induced by tFUS stimulation show no statistical difference to LFS for up to 24 min after tFUS stimulation. When both the maximum depression effects and the duration of sustained effects are both taken into account, PRF 3 kHz can induce significantly larger effects than other PRFs tested. tFUS stimulation is measured with a spatial-peak pressure amplitude of 99 kPa, translating to an estimation of 0.43 °C temperature increase when assuming no loss of heat. The results suggest the ability of tFUS to encode sustained changes in synaptic connectivity through mechanism which are unlikely to involve thermal changes.

Bipolar subcortical stimulation with tip of ultrasound aspirator and suction device

Bipolar subcortical stimulation with tip of ultrasound aspirator and suction device

Pulsed radiofrequency treatment of the dorsal root ganglion in patients with chronic neuropathic pain: A narrative review

Background: Chronic neuropathic pain (CNP) is a complex condition that has profound impacts on quality of life. Pulsed radiofrequency (PRF) on the dorsal root ganglia (DRGs) is a novel treatment that has shown clinical efficacy in pain relief, however, its mechanism remains unknown. Objectives: The objective of this review is to synthesize the literature on inflammatory markers and clinical pain outcomes in CNP patients treated with PRF. Study Design: A narrative review was conducted. Setting: Eligibility criteria included human trials on adults diagnosed with CNP. Monopolar and bipolar PRF treatments on the DRG were included. Methods: Four peer reviewed electronic databases (Medline, EMBASE, PubMed, and Cochrane) were systematically searched for studies on PRF on the DRG to treat CNP. The primary outcome measures included pain scores and cerebrospinal fluid samples taken pre- and posttreatment measuring inflammatory markers. Results: Thirty-three articles were identified in the database searches. Titles, abstracts, and full-text articles were evaluated, and eight articles met the inclusion criteria. The study designs included five randomized-controlled trials and three quasi-experimental studies. Patients: There were 311 patients pooled with an age range of 35–76 years. Types of CNP included chronic radicular pain, postmastectomy pain syndrome, chronic lumbosacral pain, and postherpetic neuralgia. Intervention: Treatments in included studies included monopolar and bipolar PRF stimulation ranging from 120 s at 2 Hz to 360 s with 5 Hz pulses. Measurement: The main findings revealed that PRF treatment provided significant pain relief (P < 0.05), with the greatest pain reduction at 3 months. Pro-inflammatory markers were found to decrease, whereas anti-inflammatory markers increased post-PRF intervention. Limitations: There were differing PRF procedure standards, and it is uncertain whether a higher frequency or duration is correlated with better outcomes. Studies had small sample sizes increasing the margin of error. Longer duration randomized-controlled trials are needed to understand the optimal therapeutic duration using PRF.

Electrically evoked ABR through cochlear implant in children with auditory neuropathy spectrum disorder

To estimate the applicability of electrically evoked auditory brainstem response (eABR) registration for the estimation of neural integrity after cochlear implantation (CI) in children with auditory neuropathy spectrum disorder (ANSD) and to compare the eABR data with patient's hearing performance. 4 children, Nucleus (Cochlear) CI users, with ANSD were enrolled in the study. Hearing performance in these children ranged from successful to unsatisfied. eABR were recorded via Eclipse EP25 (Interacoustics). Electrical bipolar stimulation was achieved with Custom Sound EP software (Cochlear). EABR were registered with the use of different stimulation parameters (pulse width, stimulated electrodes) in 3 patients with satisfactory results of rehabilitation. eABR thresholds corresponded to maximum comfortable levels of patients stimulation MAP. eABR were absent in the patient with poor hearing performance. EABR measurements in children with ANSD demonstrated restoration of neuronal conduction in the auditory pathway up to the brainstem after cochlear implantation in 3 patients. eABR results were well correlated with hearing performance. Thereby, the study of eABR applicability for clinical practice will be expanded.

Altered sleep intensity upon DBS to hypothalamic sleep-wake centers in rats.

Deep brain stimulation (DBS) has been scarcely investigated in the field of sleep research. We hypothesize that DBS onto hypothalamic sleep- and wake-promoting centers will produce significant neuromodulatory effects and potentially become a therapeutic strategy for patients suffering severe, drug-refractory sleep–wake disturbances. We aimed to investigate whether continuous electrical high-frequency DBS, such as that often implemented in clinical practice, in the ventrolateral preoptic nucleus (VLPO) or the perifornical area of the posterior lateral hypothalamus (PeFLH), significantly modulates sleep–wake characteristics and behavior. We implanted healthy rats with electroencephalographic/electromyographic electrodes and recorded vigilance states in parallel to bilateral bipolar stimulation of VLPO and PeFLH at 125 Hz and 90 µA over 24 h to test the modulating effects of DBS on sleep–wake proportions, stability and spectral power in relation to the baseline. We unexpectedly found that VLPO DBS at 125 Hz deepens slow-wave sleep (SWS) as measured by increased delta power, while sleep proportions and fragmentation remain unaffected. Thus, the intensity, but not the amount of sleep or its stability, is modulated. Similarly, the proportion and stability of vigilance states remained altogether unaltered upon PeFLH DBS but, in contrast to VLPO, 125 Hz stimulation unexpectedly weakened SWS, as evidenced by reduced delta power. This study provides novel insights into non-acute functional outputs of major sleep–wake centers in the rat brain in response to electrical high-frequency stimulation, a paradigm frequently used in human DBS. In the conditions assayed, while exerting no major effects on the sleep–wake architecture, hypothalamic high-frequency stimulation arises as a provocative sleep intensity-modulating approach.

Apical Reference Stimulation: A Possible Solution to Facial Nerve Stimulation.

Postimplantation facial nerve stimulation is a common side-effect of intracochlear electrical stimulation. Facial nerve stimulation occurs when electric current intended to stimulate the auditory nerve, spread beyond the cochlea to excite the nearby facial nerve, causing involuntarily facial muscle contractions. Facial nerve stimulation can often be resolved through adjustments in speech processor fitting but, in some instances, these measures exhibit limited benefit or may have a detrimental effect on speech perception. In this study, apical reference stimulation mode was investigated as a potential intervention to facial nerve stimulation. Apical reference stimulation is a bipolar stimulation strategy in which the most apical electrode is used as the reference electrode for stimulation on all the other intracochlear electrodes. A person-specific model of the human cochlea, facial nerve and electrode array, coupled with a neural model, was used to predict excitation of auditory and facial nerve fibers. These predictions were used to evaluate the effectiveness in reducing facial nerve stimulation using apical reference stimulation. Predictions were confirmed in psychoacoustic tests by determining auditory comfort and threshold levels for the apical reference stimulation mode while capturing electromyography data in two participants. Models predicted a favorable outcome for apical reference stimulation, as facial nerve fiber thresholds were higher and auditory thresholds were lower, in direct comparison to conventional monopolar stimulation. Psychophysical tests also illustrated decreased auditory thresholds and increased dynamic range during apical reference stimulation. Furthermore, apical reference stimulation resulted in lower electromyography energy levels, compared to conventional monopolar stimulation, which suggests a reduction in facial nerve stimulation. Subjective feedback corroborated that apical reference stimulation alleviated facial nerve stimulation. Apical reference stimulation may be a viable strategy to alleviate facial nerve stimulation considering the improvements in dynamic range and auditory thresholds, complemented with a reduction in facial nerve stimulation symptoms.

Vagus nerve stimulation activates nucleus of solitary tract neurons via supramedullary pathways.

Vagus nerve stimulation (VNS) treats patients with drug-resistant epilepsy, depression and heart failure, but the mechanisms responsible are uncertain. The mild stimulus intensities used in chronic VNS suggest activation of myelinated primary visceral afferents projecting to the nucleus of the solitary tract (NTS). Here, we monitored the activity of second and higher order NTS neurons in response to peripheral vagal activation using therapeutic VNS criteria. A bipolar stimulating electrode activated the left cervical vagus nerve, and stereotaxically placed single tungsten electrodes recorded unit activity from the left caudomedial NTS of chloralose-anaesthetized rats. High-intensity single electrical stimuli established vagal afferent conduction velocity (myelinated A-type or unmyelinated C-type) as well as synaptic order (second vs. higher order using paired electrical stimuli) for inputs to single NTS neurons. Then, VNS treatment was applied. A mid-collicular knife cut (KC) divided the brainstem from all supramedullary regions to determine their contribution to NTS activity. Our chief findings indicate that the KC reduced basal spontaneous activity of second-order NTS neurons receiving myelinated vagal input by 85%. In these neurons, acute VNS increased activity similarly in Control and KC animals. Interestingly, the KC interrupted VNS activation of higher order NTS neurons and second-order NTS neurons receiving unmyelinated vagal input, indicating that supramedullary descending projections to NTS are needed to amplify the peripheral neuronal signal from VNS. The present study begins to define the pathways activated during VNS and will help to better identify the central nervous system contributions to the therapeutic benefits of VNS therapy. KEY POINTS: Vagus nerve stimulation is routinely used in the clinic to treat epilepsy and depression, despite our uncertainty about how this treatment works. For this study, the connections between the nucleus of the solitary tract (NTS) and the higher brain regions were severed to learn more about their contribution to activity of these neurons during stimulation. Severing these brain connections reduced baseline activity as well as reducing stimulation-induced activation for NTS neurons receiving myelinated vagal input. Higher brain regions play a significant role in maintaining both normal activity in NTS and indirect mechanisms of enhancing NTS neuronal activity during vagus nerve stimulation.

SURG-09. SUCCESSFUL SAFETY AND EFFICACY OF GAMMATILE INTRACRANIAL BRACHYTHERAPY IMPLANTED DURING AWAKE CRANIOTOMY

Abstract INTRODUCTION GammaTile intracranial brachytherapy utilizing cesium-131 seeds has demonstrated encouraging safety and local control results in a single-arm precommercial study, and recently received Food and Drug Administration clearance. The authors report the first case of GammaTile intraoperative brachytherapy performed during an awake craniotomy. METHODS A 50-year-old man suffered a generalized tonic-clonic seizure while a vehicle passenger. MRI revealed a 2.8 cm left lateral frontal lesion nearing his Broca’s area. Open brain biopsy revealed IDH-wildtype MGMT unmethylated GBM. Unfortunately, despite several interventions (awake craniotomy, adjuvant chemoradiation with temozolomide, Avastin, tumor treating fields) he suffered tumor progression near the left parietal resection cavity. Due to the location of this tumor, re-resection was planned with awake craniotomy and language mapping. A preoperative planning session involving Radiation Oncology and Neurosurgery identified the area of the expected postoperative bed, and consequently five GammaTiles were ordered, each containing 4 cesium-131 3.5 U seeds. RESULTS During surgery, once the tumor was mapped, bipolar stimulation was performed while the patient spoke in complete sentences until going into speech arrest on the posterior edge of the gyrus, indicative of language cortex. Following this mapping, microsurgical maximal safe resection occurred, after which areas at risk for tumor residual/recurrence were determined in consultation with Radiation Oncology. Subsequently, Neurosurgery placed all five GammaTiles (20 cesium-131 seeds total) in customized forward fashion to optimally cover the dosimetric needs of these areas. Following GammaTile placement, closure was completed and radioactive surveys of the room remained within state statue. Postoperative dosimetry yielded excellent coverage, and speech function was maintained. CONCLUSIONS The first reported case of GammaTile intraoperative brachytherapy during awake craniotomy supports the safety and feasibility of this treatment strategy. This case indicates that for patients with tumors adjacent to eloquent cortex, awake craniotomy can allow for maximum safe resection and implantation of intraoperative brachytherapy.

Bipolar Directional Deep Brain Stimulation in Essential and Parkinsonian Tremor.

Objetivo: Comparar la estimulación cerebral profunda (ECP) talámica direccional monopolar, bipolar y direccional bipolar en pacientes con temblor. Métodos: Se seleccionaron catorce pacientes con temblor (7 con temblor esencial y 7 con Enfermedad de Parkinson) implantados con electrodos direccionales de ECP en el núcleo intermedio ventral del tálamo (VIM). Se compararon los umbrales de efectos adversos de la estimulación monopolar direccional (DIREC) con la ECP circular y con un diseño randomizado, con dos programaciones bipolares diferentes (BIPOLAR = ánodo circular; BI-DIREC = ánodo direccional). También se evaluó la supresión del temblor (Escala de Evaluación del Temblor, EET), justo por debajo del umbral de efectos adversos. Resultados: La ECP direccional en la mejor dirección individualizada mostró un umbral de efectos adversos más alto que la ECP circular (p = 0.0063). Los umbrales se elevaron aún más utilizando cualquiera de los programas de estimulación bipolar (BIPOLAR p = 0.0029, BI-DIREC p = 0.0022). No hubo diferencias en los umbrales de efectos adversos de los dos programas de estimulación bipolar, pero hubo menor frecuencia de efectos adversos con BI-DIREC. Tampoco hubo diferencias en los puntajes de la EET con la estimulación justo por debajo del umbral de efectos adversos de todas las condiciones de estimulación. Conclusiones: Los umbrales de efectos adversos de la estimulación monopolar direccional y bipolar con ánodos circulares y direccionales fueron más elevados comparados con la estimulación monopolar circular en el VIM. La ECP bipolar con ánodos direccionales provocó efectos adversos con menor frecuencia que la estimulación bipolar y monopolar direccional. Todos los programas de estimulación tuvieron efectos comparables en la supresión del temblor justo por debajo del umbral de efectos adversos. Por ende, se debería explorar la programación direccional y diferentes opciones de estimulación bipolar en pacientes con efectos adversos por la estimulación talámica, cuando la programación monopolar no resulte satisfactoria. Se requiere un mayor número de estudios para explorar la eficacia de los diferentes paradigmas de estimulación bipolar.

Phantom Stimulation for Cochlear Implant Users With Residual Low-Frequency Hearing.

Objective:In cochlear implants (CIs), phantom stimulation can be used to extend the pitch range toward apical regions of the cochlea. Phantom stimulation consists of partial bipolar stimulation, in which current is distributed across two intracochlear electrodes and one extracochlear electrode as defined by the compensation coefficient σ. The aim of this study was, (1) to evaluate the benefit of conveying low-frequency information through phantom stimulation for cochlear implant (CI) subjects with low-frequency residual hearing using electric stimulation alone, (2) to compare the speech reception thresholds obtained from electric-acoustic stimulation (EAS) and electric stimulation in combination with phantom stimulation (EPS), and (3) to investigate the effect of spectrally overlapped bandwidth of speech conveyed via simultaneous acoustic and phantom stimulation on speech reception thresholds.Design:Fourteen CI users with ipsilateral residual hearing participated in a repeated-measures design. Phantom stimulation was used to extend the frequency bandwidth of electric stimulation of EAS users towards lower frequencies without changing their accustomed electrode-frequency allocation. Three phantom stimulation configurations with different σ’s were tested causing different degrees of electric field shaping towards apical regions of the cochlea that may affect the place of stimulation. A baseline configuration using a moderate value of σ () for all subjects, a configuration that was equivalent to monopolar stimulation by setting σ to 0 () and a configuration that used the largest value of σ for each individual subject (). Speech reception thresholds were measured for electric stimulation alone, EAS and EPS. Additionally, acoustic stimulation and phantom stimulation were presented simultaneously (EAS+PS) to investigate their mutual interaction. Besides the spectral overlap, the electrode insertion depth obtained from cone-beam computed-tomography scans was determined to assess the impact of spatial overlap between electric and acoustic stimulation on speech reception.Results:Speech perception significantly improved by providing additional acoustic or phantom stimulation to electric stimulation. There was no significant difference between EAS and EPS. However, two of the tested subjects were able to perform the speech perception test using EAS but not using EPS. In comparison to the subject’s familiar EAS listening mode, the speech perception deteriorated when acoustic stimulation and phantom stimulation conveyed spectrally overlapped information simultaneously and this deterioration increased with larger spectral overlapConclusions:(1) CI users with low-frequency acoustic residual hearing benefit from low-frequency information conveyed acoustically through combined EAS. (2) Improved speech reception thresholds through low-frequency information conveyed via phantom stimulation were observed for EAS subjects when acoustic stimulation was not used. (3) Speech perception was negatively affected by combining acoustic and phantom stimulation when both stimulation modalities overlapped spectrally in comparison to the familiar EAS.

Safety and efficacy of GammaTile intracranial brachytherapy implanted during awake craniotomy

GammaTile intracranial brachytherapy (cesium-131 seeds) has demonstrated encouraging safety and local control results, and recently received Food and Drug Administration clearance for newly diagnosed and recurrent brain tumors. The authors present the first reported case of GammaTile intraoperative brachytherapy performed during an awake craniotomy. A 50-year-old man had a biopsy-proven, 2.8 cm left lateral frontal glioblastoma lesion nearing Broca's area on MRI. Despite several interventions (craniotomy, adjuvant chemoradiation, tumor treating fields) tumor progression occurred near the left parietal resection cavity. Re-resection was planned with awake craniotomy and language mapping. A preoperative planning session involving Radiation Oncology and Neurosurgery identified the area of the expected postoperative bed, and consequently five GammaTiles were ordered, each containing 4 cesium-131 3.5 U seeds. During surgery, tumor mapping and bipolar stimulation were performed while the patient spoke in complete sentences. Speech arrest occurred upon stimulation at the posterior edge of the gyrus, indicative of language cortex. Microsurgical maximal safe resection subsequently occurred, and areas at risk for residual/recurrence disease were determined in consultation with Radiation Oncology. Subsequently, Neurosurgery placed all five GammaTiles (20 cesium-131 seeds total) after which closure was completed and radioactive surveys of the room remained within state statue. Postoperative dosimetry yielded excellent coverage. The first reported case of GammaTile intraoperative brachytherapy during awake craniotomy supports the safety and feasibility of this treatment strategy. This case indicates that for patients with tumors adjacent to eloquent cortex, awake craniotomy can allow for custom implantation of intraoperative brachytherapy following maximum safe resection.

Modeling the Impact of Electrode/Tissue Geometry on Electrical Stimulation in Stereo-EEG.

Electrical stimulation through depth electrodes is used to map function and seizure onset during stereoelectroencephalography in patients undergoing evaluation for epilepsy surgery. Factors such as electrode design, location, and orientation are expected to impact effects of electrical stimulation. We developed a steady-state finite element model of brain tissue including five layers (skull through white matter) and an implanted electrode to explore the impact of electrode design and placement on the activation of brain tissue by electrical stimulation. We calculated electric potentials, current densities, and volume of tissue activated ( Volact ) in response to constant current bipolar stimulation. We modeled two depth electrode designs (3.5- and 4.43-mm intercontact spacing) and varied electrode location and orientation. The electrode with greater intercontact spacing produced 8% to 23% larger Volact (1% to 16% considering only gray matter). Vertical displacement of the electrodes by half intercontact space increased Volact for upward displacement (6% to 83% for all brain tissue or -5% to 96% gray matter only) and decreased Volact (1% to 16% or 24% to 49% gray matter only) for downward displacement. Rotating the electrode in the tissue by 30° to 60° with respect to the vertical axis increased Volact by 30% to 90% (20%-48% gray matter only). Location and orientation of depth electrodes with respect to surrounding brain tissue have a large impact on the amount of tissue activated during electrical stimulation mapping in stereoelectroencephalography. Electrode design has an impact, although modest for commonly used designs. Individualization of stimulation intensity at each location remains critical, especially for avoiding false-negative results.