A review of neurophysiological effects and efficiency of waveform parameters in deep brain stimulation

Neurostimulation has diverse clinical applications and potential as a treatment for medically refractory movement disorders, epilepsy, and other neurological disorders. However, the parameters used to program electrodes-polarity, pulse width, amplitude, and frequency-and how they are adjusted have remained largely untouched since the 1970 s. This review summarizes the state-of-the-art in Deep Brain Stimulation (DBS) and highlights the need for further research to uncover the physiological mechanisms of neurostimulation. We focus on studies that reveal the potential for clinicians to use waveform parameters to selectively stimulate neural tissue for therapeutic benefit, while avoiding activating tissue associated with adverse effects. DBS uses cathodic monophasic rectangular pulses with passive recharging in clinical practice to treat neurological conditions such as Parkinson's Disease. However, research has shown that stimulation efficiency can be improved, and side effects reduced, through modulating parameters and adding novel waveform properties. These developments can prolong implantable pulse generator lifespan, reducing costs and surgery-associated risks. Waveform parameters can stimulate neurons based on axon orientation and intrinsic structural properties, providing clinicians with more precise targeting of neural pathways. These findings could expand the spectrum of diseases treatable with neuromodulation and improve patient outcomes.

The Safety to Switch from Constant Voltage to Constant Current with a Mixed Internal Pulse Generator in Deep Brain Stimulation

Background

Deep brain stimulation (DBS) is an efficient modality for the treatment of movement disorders. Differing from the constant voltage (CV)-DBS devices, constant current (CC)-DBS devices may allow more precise stimulation of the target brain regions since they are less influenced by impedance. If internal pulse generators (IPGs) of DBS devices are required to be connected with electrodes of different brands, employing proper adapters is necessary. Such connected DBS devices are called mixed or hybrid devices.

Objectives

As there is sparse information about the clinical mixed devices, we studied their safety and efficacy.

Materials and Methods

Clinical scores of 13 patients implanted with mixed DBS devices were determined with the Unified Parkinson's Disease Rating Scale (UPDRS) in Parkinson's disease (PD) (n = 10) and with the Burke-Fahn-Marsden Dystonia Rating Scale (BFMDRS) in dystonia (n = 3). Patient satisfaction was assessed with the Timmerman questionnaire. The Clinical Global Impression Improvement (CGI-I) Scale was also evaluated.

Results

Patients' overall satisfaction was considerably higher with mixed devices. The UPDRS and BFMDRS clinical scores did not significantly differ after switching to a mixed DBS device. Three patients before the DBS switch suffered from side effects under the CV mode. These patients got rid of the side effects in their follow-up with a reduction in pulse width values.

Discussion

Mixed devices working in CC mode are well tolerated with high patient satisfaction.

Conclusion

Besides patient satisfaction, mixed IPGs are also considered safe.

Constant current or constant voltage deep brain stimulation: short answers to a long story

PURPOSE

Recently, the feature of generating constant current output has been added to the implantable pulse generators (IPGs). The efficacy of the conventionally used constant voltage (CV) stimulation has been proved in different movement and psychiatric disorders. In this systematic review, we aimed to discuss the effect of constant current (CC) and constant voltage stimulation on patients with Parkinson's disease (PD) who had subthalamic nucleus deep brain stimulation implantation; we also compared these methods of stimulation with each other.

METHODS

Using the words "Deep brain stimulation", "constant current" and "constant voltage", we developed a broad search strategy and a systematic search was conducted in PubMed, Scopus, Web of Science and Cochrane electronic bibliographic databases. Studies on the Parkinson's disease patients with subthalamic deep brain stimulation, which mentioned constant current or/and constant voltage setting stimulation were included.

RESULTS

After screening of 284 articles, 10 reports were found eligible for this study. The score of unified Parkinson's disease rating scale part 3 was improved compared to the baseline, whether the stimulation was CV at baseline or CC. No significant change in non-motor outcomes was found.

CONCLUSIONS

Although CC stimulation has shown a significant improvement in both motor and non-motor symptoms of PD, switching from CV to CC did not result in a significant change in the score of these items based on UPDRS. To sum up, implantation of constant current devices is safe and significantly improves motor function; it also maintains an acceptable safety profile in patients with PD.

Technology of deep brain stimulation: current status and future directions

Deep brain stimulation (DBS) is a neurosurgical procedure that allows targeted circuit-based neuromodulation. DBS is a standard of care in Parkinson disease, essential tremor and dystonia, and is also under active investigation for other conditions linked to pathological circuitry, including major depressive disorder and Alzheimer disease. Modern DBS systems, borrowed from the cardiac field, consist of an intracranial electrode, an extension wire and a pulse generator, and have evolved slowly over the past two decades. Advances in engineering and imaging along with an improved understanding of brain disorders are poised to reshape how DBS is viewed and delivered to patients. Breakthroughs in electrode and battery designs, stimulation paradigms, closed-loop and on-demand stimulation, and sensing technologies are expected to enhance the efficacy and tolerability of DBS. In this Review, we provide a comprehensive overview of the technical development of DBS, from its origins to its future. Understanding the evolution of DBS technology helps put the currently available systems in perspective and allows us to predict the next major technological advances and hurdles in the field.

Constant Current versus Constant Voltage: Clinical Evidence Supporting a Fundamental Difference in the Modalities

BACKGROUND

Deep brain stimulation (DBS) is an effective surgical treatment for movement disorders. Early versions of implantable systems delivered stimulation with constant voltage (CV); however, advances in available and newer platforms have permitted programming in constant current (CC). From a treatment management perspective, there are theoretical advantages of CC stimulation. In this case series, we present clinical evidence supporting the maintenance of current regardless of changes to impedance.

MATERIALS AND METHODS

This case series included 3 patients with Parkinson's disease status post-bilateral subthalamic nucleus DBS. Patients in this series self-reported intermittent diplopia with pressure applied to the scalp. Patients were subsequently examined and converted from CV to CC and re-examined. Impedances were checked prior to and after conversion from CV to CC as well as while applying pressure to the scalp that induced the adverse effects.

RESULTS

Across patients, we observed that compression of the scalp overlying the connector, while patients were maintained in CV, consistently and objectively induced unilateral adduction of an eye. In addition, during scalp compression, while in CV, impedance was reduced, which would increase current delivery. Converting the patients to CC stimulation without changing other stimulation parameters eliminated diplopia and objective findings of eye deviation with compression of the scalp overlying the hardware despite changes in impedance.

CONCLUSIONS

In this case series, we provide clinical support for the principal differences between CV and CC stimulation.

Feasibility of changing for a rechargeable constant current neurostimulator in Parkinson's disease

BACKGROUND

Little is known about outcome and settings adaptations after replacement of constant-voltage non-rechargeable implantable pulse generator (CV-nrIPG) by constant-current rechargeable IPG (CC-rIPG).

OBJECTIVE

To determine the feasibility and safety of replacing a CV-nrIPG by a CC-rIPG in Parkinson's disease (PD) and the subsequent outcome.

METHODS

A prospective cohort of thirty PD patients, whose CV-nrIPG was replaced by a CC-rIPG in University Hospital of Lyon between January 2017 and December 2018 (rIPG group) and 39 PD patients, who underwent the replacement of a CV-nrIPG by the same device in 2016 (nrIPG group), were enrolled in this study. Three surgeons performed the operations. Duration of hospitalization for the replacement as well as the number of in or outpatient visits during the first 3 months after the surgery were recorded. In the rIPG group, we compared preoperative DBS settings and the theoretical amplitude estimated using Ohm's law to the amplitude used at the end of follow-up. We assessed patients' and clinicians' opinion on the patient global functioning after the replacement using Clinical Global Impression score.

RESULTS

Duration of hospitalization (P=0.47) and need for additional hospitalizations (P=0.73) or consultations (P=0.71) to adapt DBS parameters did not differ between the two groups. Neurological condition (CGI score) was considered as unchanged by both patients and neurologists. Final amplitude of stimulation using CC-rIPG was not predicted by Ohm's law in most cases.

CONCLUSIONS

Replacing CV-nrIPG by CC-rIPG is safe and well tolerated but require neurological expertise to set the new parameters of stimulation.

Deep Brain Stimulation in Parkinson's Disease

Deep brain stimulation (DBS) has become an established therapeutic tool for treating patients with Parkinson's disease (PD) who have troublesome motor fluctuations and dyskinesias refractory to best medical therapy. In addition to its proven efficacy in patients with late PD, the EARLYSTIM trial not only demonstrated the efficacy of DBS in patients with early motor complications but also showed that it did not lose its therapeutic efficacy as the years passed by. However, like all other therapies for PD, DBS is not offered to patients either as a cure for this disease nor is it expected to stop the progression of the neurodegenerative process underlying PD; these important issues need to be highlighted to patients who are considering this therapy. This article aims to provide an introduction to residents or trainees starting a career in movement disorders of the technical aspects of this therapy and the evidence base for its use. For the latter objective, PUBMED was searched from 1946 to 2017 combining the search terms "deep brain stimulation" and "Parkinson's disease" looking for studies demonstrating the efficacy of this therapy in PD. Inclusion criteria included studies that involved more than 20 patients with a physician confirmed diagnosis of PD and a follow-up of greater than or equal to at least 12 months. The findings from those studies on motor symptoms, medication requirements, quality of life, and independence in activities of daily living in PD patients are summarized and presented in tabulated form in this paper at the end.

Deep Brain Stimulation Programming for Movement Disorders: Current Concepts and Evidence-Based Strategies

Deep brain stimulation (DBS) has become the treatment of choice for advanced stages of Parkinson's disease, medically intractable essential tremor, and complicated segmental and generalized dystonia. In addition to accurate electrode placement in the target area, effective programming of DBS devices is considered the most important factor for the individual outcome after DBS. Programming of the implanted pulse generator (IPG) is the only modifiable factor once DBS leads have been implanted and it becomes even more relevant in cases in which the electrodes are located at the border of the intended target structure and when side effects become challenging. At present, adjusting stimulation parameters depends to a large extent on personal experience. Based on a comprehensive literature search, we here summarize previous studies that examined the significance of distinct stimulation strategies for ameliorating disease signs and symptoms. We assess the effect of adjusting the stimulus amplitude (A), frequency (f), and pulse width (pw) on clinical symptoms and examine more recent techniques for modulating neuronal elements by electrical stimulation, such as interleaving (Medtronic®) or directional current steering (Boston Scientific®, Abbott®). We thus provide an evidence-based strategy for achieving the best clinical effect with different disorders and avoiding adverse effects in DBS of the subthalamic nucleus (STN), the ventro-intermedius nucleus (VIM), and the globus pallidus internus (GPi).

A Fully Integrated RF-Powered Energy-Replenishing Current-Controlled Stimulator

This paper presents a fully-integrated current-controlled stimulator that is powered directly from on-chip coil antenna and achieves adiabatic energy-replenishing operation without any bulky external components. Adiabatic supply voltages, which can reach a differential range of up to 7.2 V, are directly generated from an on-chip 190-MHz resonant LC tank via a self-cascading/folding rectifier network, bypassing the losses that would otherwise be introduced by the 0.8 V system supply-generating rectifier and regulator. The stimulator occupies 0.22 mm ² in a 180 nm silicon-on-insulator process and produces differential currents up to 145  μA. Using a charge replenishing scheme, the stimulator redirects the charges accumulated across the electrodes to the system power supplies for 63.1% of stimulation energy recycling. To benchmark the efficiency of stimulation, a figure of merit termed the stimulator efficiency factor (SEF) is introduced. The adiabatic power rails and energy replenishment scheme enabled our stimulator to achieve an SEF of 6.0.

Reporting Guidelines and Issues to Consider for Using Intracranial Brain Stimulation in Studies of Human Declarative Memory

Participants with stimulating and recording electrodes implanted within the brain for clinical evaluation and treatment provide a rare opportunity to unravel the neuronal correlates of human memory, as well as offer potential for modulation of behavior. Recent intracranial stimulation studies of memory have been inconsistent in methodologies employed and reported conclusions, which renders generalizations and construction of a framework impossible. In an effort to unify future study efforts and enable larger meta-analyses we propose in this mini-review a set of guidelines to consider when pursuing intracranial stimulation studies of human declarative memory and summarize details reported by previous relevant studies. We present technical and safety issues to consider when undertaking such studies and a checklist for researchers and clinicians to use for guidance when reporting results, including targeting, placement, and localization of electrodes, behavioral task design, stimulation and electrophysiological recording methods, details of participants, and statistical analyses. We hope that, as research in invasive stimulation of human declarative memory further progresses, these reporting guidelines will aid in setting standards for multicenter studies, in comparison of findings across studies, and in study replications.

The development of neural stimulators: a review of preclinical safety and efficacy studies

OBJECTIVE

Given the rapid expansion of the field of neural stimulation and the rigorous regulatory approval requirements required before these devices can be applied clinically, it is important that there is clarity around conducting preclinical safety and efficacy studies required for the development of this technology.

APPROACH

The present review examines basic design principles associated with the development of a safe neural stimulator and describes the suite of preclinical safety studies that need to be considered when taking a device to clinical trial.

MAIN RESULTS

Neural stimulators are active implantable devices that provide therapeutic intervention, sensory feedback or improved motor control via electrical stimulation of neural or neuro-muscular tissue in response to trauma or disease. Because of their complexity, regulatory bodies classify these devices in the highest risk category (Class III), and they are therefore required to go through a rigorous regulatory approval process before progressing to market. The successful development of these devices is achieved through close collaboration across disciplines including engineers, scientists and a surgical/clinical team, and the adherence to clear design principles. Preclinical studies form one of several key components in the development pathway from concept to product release of neural stimulators. Importantly, these studies provide iterative feedback in order to optimise the final design of the device. Key components of any preclinical evaluation include: in vitro studies that are focussed on device reliability and include accelerated testing under highly controlled environments; in vivo studies using animal models of the disease or injury in order to assess efficacy and, given an appropriate animal model, the safety of the technology under both passive and electrically active conditions; and human cadaver and ex vivo studies designed to ensure the device's form factor conforms to human anatomy, to optimise the surgical approach and to develop any specialist surgical tooling required.

SIGNIFICANCE

The pipeline from concept to commercialisation of these devices is long and expensive; careful attention to both device design and its preclinical evaluation will have significant impact on the duration and cost associated with taking a device through to commercialisation. Carefully controlled in vitro and in vivo studies together with ex vivo and human cadaver trials are key components of a thorough preclinical evaluation of any new neural stimulator.

Chinese expert consensus on programming deep brain stimulation for patients with Parkinson's disease

Background

Deep Brain Stimulation (DBS) therapy for the treatment of Parkinson’s Disease (PD) is now a well-established option for some patients. Postoperative standardized programming processes can improve the level of postoperative management and programming, relieve symptoms and improve quality of life.

Main body

In order to improve the quality of the programming, the experts on DBS and PD in neurology and neurosurgery in China reviewed the relevant literatures and combined their own experiences and developed this expert consensus on the programming of deep brain stimulation in patients with PD in China.

Conclusion

This Chinese expert consensus on postoperative programming can standardize and improve postoperative management and programming of DBS for PD.

Subthalamic Nucleus Deep Brain Stimulation in Parkinson's Disease: The Effect of Varying Stimulation Parameters

Subthalamic Nucleus Deep Brain Stimulation (STN DBS) is a well-established and effective treatment modality for selected patients with Parkinson's disease (PD). Since its advent, systematic exploration of the effect of stimulation parameters including the stimulation intensity, frequency, and pulse width have been carried out to establish optimal therapeutic ranges. This review examines published data on these stimulation parameters in terms of efficacy of treatment and adverse effects. Altering stimulation intensity is the mainstay of titration in DBS programming via alterations in voltage or current settings, and is characterised by a lower efficacy threshold and a higher side effect threshold which define the therapeutic window. In addition, much work has been done in exploring the effects of frequency modulation, which may help patients with gait freezing and other axial symptoms. However, there is a paucity of data on the use of ultra-short pulse width settings which are now possible with technological advances. We also discuss current evidence for the use of novel programming techniques including directional and adaptive stimulation, and highlight areas for future research.

Voltage adjustment improves rigidity and tremor in Parkinson's disease patients receiving deep brain stimulation

Deep brain stimulation of the subthalamic nucleus is recognized as the most effective treatment for moderate and advanced Parkinson's disease. Programming of the stimulation parameters is important for maintaining the efficacy of deep brain stimulation. Voltage is considered to be the most effective programming parameter. The present study is a retrospective analysis of six patients with Parkinson's disease (four men and two women, aged 37–65 years), who underwent bilateral deep brain stimulation of the subthalamic nucleus at the First Affiliated Hospital of Sun Yat-sen University, China, and who subsequently adjusted only the stimulation voltage. We evaluated motor symptom severity using the Unified Parkinson's Disease Rating Scale Part III, symptom progression using the Hoehn and Yahr scale, and the levodopa equivalent daily dose, before surgery and 1 and 2 years after surgery. The 2-year follow-up results show that rigidity and tremor improved, and clinical symptoms were reduced, while pulse width was maintained at 60 μs and frequency at 130 Hz. Voltage adjustment alone is particularly suitable for patients who cannot tolerate multiparameter program adjustment. Levodopa equivalent daily dose was markedly reduced 1 and 2 years after surgery compared with baseline. Our results confirm that rigidity, tremor and bradykinesia can be best alleviated by voltage adjustment. The trial was registered at ClinicalTrials.gov (identifier: NCT01934881).

Glial responses to implanted electrodes in the brain

The use of implants that can electrically stimulate or record electrophysiological or neurochemical activity in nervous tissue is rapidly expanding. Despite remarkable results in clinical studies and increasing market approvals, the mechanisms underlying the therapeutic effects of neuroprosthetic and neuromodulation devices, as well as their side effects and reasons for their failure, remain poorly understood. A major assumption has been that the signal-generating neurons are the only important target cells of neural-interface technologies. However, recent evidence indicates that the supporting glial cells remodel the structure and function of neuronal networks and are an effector of stimulation-based therapy. Here, we reframe the traditional view of glia as a passive barrier, and discuss their role as an active determinant of the outcomes of device implantation. We also discuss the implications that this has on the development of bioelectronic medical devices.

Management of Elevated Therapeutic Impedances on Deep Brain Stimulation Leads

CLINICAL VIGNETTE

A 64-year-old male with a history of essential tremor with bilateral thalamic ventralis intermedius deep brain stimulation implants had elevated therapeutic impedance values despite normal lead integrity impedances and good response to stimulation.

CLINICAL DILEMMA

Do elevated therapeutic impedance values indicate a sign of hardware malfunction? What are the guidelines to approach deep brain stimulation hardware malfunction?

CLINICAL SOLUTION

Lead integrity impedance values are a better evaluation of hardware integrity. The discrepancy between therapeutic and lead-integrity impedance values can arise when using low voltage settings.

GAPS IN KNOWLEDGE

There are no established guidelines for the management of possible hardware malfunction in deep brain stimulation. The recommended approach is to distinguish between open and short circuit problems followed by an "inching" evaluation, assessing the structures from the implantable and programmable generator to the intracranial leads. Constant-current devices will deliver a more stable stimulation but the effect of their adoption is still not clear.

EXPERT COMMENTARY

This case emphasizes the need for clinicians to understand fundamental differences in lead integrity and therapeutic impedance while utilizing a methodical approach in treating hardware malfunction. It highlights future avenues of investigation regarding the utility of constant current DBS technology.

Deep Brain Stimulation Emergencies: How the New Technologies Could Modify the Current Scenario

After 25 years of deep brain stimulation (DBS) for the treatment of Parkinson's disease, it has become increasingly recognized that a range of postoperative urgent situations and emergencies may occur. In this review we describe the possible scenarios of DBS-related emergencies: perioperative (intraoperative and early postoperative) and postoperative settings and issues from suboptimal control of motor and nonmotor symptoms in the early programming phase and during long-term follow-up. We also outline potential advantages in the management of these emergencies offered by the newest devices, emerging technologies, and new possibilities in programming.

Shifting from constant-voltage to constant-current in Parkinson's disease patients with chronic stimulation

The study aimed to evaluate safety and efficacy of shifting stimulation settings from constant-voltage (CV) to constant-current (CC) programming in patients with Parkinson's disease (PD) and chronic subthalamic nucleus deep brain stimulation (STN DBS). Twenty PD patients with chronic STN DBS set in CV programming were shifted to CC and followed for 3 months; the other stimulation settings and the medication regimen remained unchanged. Side effects, motor, non-motor, executive functions, and impedance were assessed at baseline and during follow-up. No adverse events were observed at time of shifting or during CC stimulation. Motor and non-motor measures remained unchanged at follow-up despite impedance decreased. Compared to baseline, inhibition processes improved at follow-up. The shifting strategy was well tolerated and the clinical outcome was maintained with no need to adjust stimulation settings or medications notwithstanding a decrease of impedance. Improvement of inhibition processes is a finding which needed further investigation.

Evaluation of the axonics modulation technologies sacral neuromodulation system for the treatment of urinary and fecal dysfunction

INTRODUCTION

Sacral neuromodulation (SNM) remains one of the few effective treatments for refractory bladder and bowel dysfunction. However, SNM is associated with frequent need for surgical intervention, in many cases because of a failed battery. A rechargeable SNM system, with a manufacturer-reported battery life of 15 years or more, has entered post-market clinical testing in Europe but has not yet been approved for clinical testing in the United States. Areas covered: We review existing neuromodulation technologies for the treatment of lower urinary tract and bowel dysfunction and explore the limitations of available technology. In addition, we discuss implantation technique and device specifications and programming of the rechargeable SNM system in detail. Lastly, we present existing evidence for the use of SNM in bladder and bowel dysfunction and evaluate the anticipated trajectory of neuromodulation technologies over the next five years. Expert commentary: A rechargeable system for SNM is a welcome technological advance. However surgical revision not related to battery changes is not uncommon. Therefore, while a rechargeable system would be expected to reduce costs, it will not eliminate the ongoing maintenance associated with neuromodulation. No matter the apparent benefits, all new technologies require extensive post-market monitoring to ensure safety and efficacy.

Proceedings of the Fourth Annual Deep Brain Stimulation Think Tank: A Review of Emerging Issues and Technologies

This paper provides an overview of current progress in the technological advances and the use of deep brain stimulation (DBS) to treat neurological and neuropsychiatric disorders, as presented by participants of the Fourth Annual DBS Think Tank, which was convened in March 2016 in conjunction with the Center for Movement Disorders and Neurorestoration at the University of Florida, Gainesveille FL, USA. The Think Tank discussions first focused on policy and advocacy in DBS research and clinical practice, formation of registries, and issues involving the use of DBS in the treatment of Tourette Syndrome. Next, advances in the use of neuroimaging and electrochemical markers to enhance DBS specificity were addressed. Updates on ongoing use and developments of DBS for the treatment of Parkinson's disease, essential tremor, Alzheimer's disease, depression, post-traumatic stress disorder, obesity, addiction were presented, and progress toward innovation(s) in closed-loop applications were discussed. Each section of these proceedings provides updates and highlights of new information as presented at this year's international Think Tank, with a view toward current and near future advancement of the field.