Model-based Vestibular Afferent Stimulation: Modular Workflow for Analyzing Stimulation Scenarios in Patient Specific and Statistical Vestibular Anatomy

Vestibular prosthesis: from basic research to clinics

Balance disorders are highly prevalent worldwide, causing substantial disability with high personal and socioeconomic impact. The prognosis in many of these patients is poor, and rehabilitation programs provide little help in many cases. This medical problem can be addressed using microelectronics by combining the highly successful cochlear implant experience to produce a vestibular prosthesis, using the technical advances in micro gyroscopes and micro accelerometers, which are the electronic equivalents of the semicircular canals (SCC) and the otolithic organs. Reaching this technological milestone fostered the possibility of using these electronic devices to substitute the vestibular function, mainly for visual stability and posture, in case of damage to the vestibular endorgans. The development of implantable and non-implantable devices showed diverse outcomes when considering the integrity of the vestibular pathways, the device parameters (current intensity, impedance, and waveform), and the targeted physiological function (balance and gaze). In this review, we will examine the development and testing of various prototypes of the vestibular implant (VI). The insight raised by examining the state-of-the-art vestibular prosthesis will facilitate the development of new device-development strategies and discuss the feasibility of complex combinations of implantable devices for disorders that directly affect balance and motor performance.

Computer Simulation of the Electrical Stimulation of the Human Vestibular System: Effects of the Reactive Component of Impedance on Voltage Waveform and Nerve Selectivity

The vestibular system is responsible for our sense of balance and spatial orientation. Recent studies have shown the possibility of partially restoring the function of this system using vestibular implants. Electrical modeling is a valuable tool in assisting the development of these implants by analyzing stimulation effects. However, previous modeling approaches of the vestibular system assumed quasi-static conditions. In this work, an extended modeling approach is presented that considers the reactive component of impedance and the electrode-tissue interface and their effects are investigated in a 3D human vestibular computer model. The Fourier finite element method was employed considering the frequency-dependent electrical properties of the different tissues. The electrode-tissue interface was integrated by an instrumental electrode model. A neuron model of myelinated fibers was employed to predict the nerve responses to the electrical stimulus. Morphological changes of the predicted voltage waveforms considering the dielectric tissue properties were found compared to quasi-static simulations, particularly during monopolar electrode configuration. Introducing the polarization capacitance and the scar tissue around the electrode in combination with a power limitation leads to a considerable current reduction applied through the active electrode and, consequently, to reduced voltage amplitudes of the stimulus waveforms. The reactive component of impedance resulted in better selectivity for the excitation of target nerves compared to the quasi-static simulation at the expense of slightly increased stimulus current amplitudes. We conclude that tissue permittivity and effects of the electrode-tissue interface should be considered to improve the accuracy of the simulations.

Fitting the determined impedance in the guinea pig inner ear to randles circuit using square error minimization in the range of 100 Hz to 50 kHz

OBJECTIVE

A number of lumped and distributed parameter models of the inner ear have been proposed in order to improve the vestibular implant stimulation. The models should account for all significant physical phenomena influencing the current propagation: electrical double layer (EDL) and medium polarization. The electrical properties of the medium are reflected in the electrical impedance, therefore the aim of this study was to measure the impedance in the guinea pig inner ear and construct its equivalent circuit.

APPROACH

The electrical impedance was measured from 100 Hz to 50 kHz between a pair of platinum electrodes immersed in saline solution using sinusoidal voltage signals. The Randles circuit was fitted to the measured impedance in the saline solution in order to estimate the EDL parameters (C, W, and R_ct) of the electrode interface in saline. Then, the electrical impedance was measured between all combinations of the electrodes located in semicircular canal ampullae and the vestibular nerve in the guinea pig in vitro. The extended Randles circuit considering the medium polarization (R_i, R_e, C_m) together with EDL parameters (C, R_ct) obtained from the saline solution was fitted to the measured impedance of the guinea pig inner ear. The Warburg element was assumed negligible and was not considered in the guinea pig model.

MAIN RESULTS

For the set-up used, the obtained EDL parameters were: C=27.09*10^-8F, R_ct=18.75 kΩ. The average values of intra-, extracellular resistances, and membrane capacitance were R_i=4.74 kΩ, R_e=45.05 kΩ, C_m=9.69*10^-8F, respectively.

SIGNIFICANCE

The obtained values of the model parameters can serve as a good estimation of the EDL for modelling work. The EDL, together with medium polarization, plays a significant role in the electrical impedance of the guinea pig inner ear, therefore, they should be considered in electrical conductivity models to increase the credibility of the simulations.

Dominant parameter of galvanic vestibular stimulation for the non-associative learning processes

Electrical stimulus is one of the common stimulating methods, and Galvanic vestibular stimulation (GVS) is the oldest form as an electrical stimulation. Nevertheless, GVS is still considered as a secondary stimulating tool for the medical purposes. Even though some unarguable findings have made using GVS, its use has been limited because of its ambiguity as an input source. For better understanding, many previous studies mainly focused on its functional effects, like the ocular reflexes. However, its fundamental effects on the neural activities are still elusive, such as the dominant influences by different parameters of GVS. Here we compared the effects on the neuronal responses by applying two different parameters, strength and rate, of GVS. To assess the dominance on the neuronal responses to these parameters, we designed three independent stimuli. Those stimuli were multiply applied to obtain the responding slopes based on the mechanism of non-associative learning processes, and the effects on the neurons were calculated as an inner angle between two responding slopes. Out of 23 neurons, 15 (65.2%) units were affected more by the strength with a statistical significance (p = 0.047). The ranges of the inner angles also implied the strength (- 3.354°~2.063°) mainly modulated by the neuronal responses comparing with those by the rate (- 2.001°~1.975°). The dominance of the parameters was closely related with the neuronal sensitivity to stimulation (SE) (p = 0.018), while there were few relations with the neuronal regularity, directional preference (DP), and the physiological response (PR) (p > 0.059). Thus, the neural information related with the dominance was delivered by the irregular neurons, and these types of neurons should be the targets for the stimulation. Graphical abstract.

Vestibular implant: does it really work? A systematic review

Introduction

People with vestibular loss present a deficit in the vestibular system, which is primarily responsible for promoting postural control, gaze stabilization, and spatial orientation while the head moves. There is no effective treatment for a bilateral loss of vestibular function. Recently, a vestibular implant was developed for people with bilateral loss of vestibular function to improve this function and, consequently, the quality of life of these patients.

Objective

To identify in the scientific literature evidence that vestibular implants in people with vestibular deficit improves vestibular function.

Methods

One hundred and forty six articles were found from five databases and 323 articles from the gray literature mentioning the relationship between vestibular implant and vestibular function in humans. The PICOS strategy (Population, Intervention, Comparison and Outcome) was used to define the eligibility criteria. The studies that met the inclusion criteria for this second step were included in a qualitative synthesis, and each type of study was analyzed according to the bias risk assessment of the Joanna Briggs Institute through the critical assessment checklist Joanna Briggs institute for quasi-experimental studies and the Joanna Briggs institute critical assessment checklist for case reports.

Results

Of the 21 articles included in reading the full text, 10 studies were selected for the qualitative analysis in the present systematic review. All ten articles analyzed through the critical assessment checklist Joanna Briggs institute showed a low risk of bias. The total number of samples in the evaluated articles was 18 patients with vestibular implants.

Conclusions

Taken together, these findings support the feasibility of vestibular implant for restoration of the vestibulo-ocular reflex in a broad frequency range and illustrate new challenges for the development of this technology.

Concepts and Physiological Aspects of the Otolith Organ in Relation to Electrical Stimulation

BACKGROUND

This paper discusses some of the concepts and major physiological issues in developing a means of electrically stimulating the otolithic system, with the final goal being the electrical stimulation of the otoliths in human patients. It contrasts the challenges of electrical stimulation of the otolith organs as compared to stimulation of the semicircular canals. Electrical stimulation may consist of trains of short-duration pulses (e.g., 0.1 ms duration at 400 Hz) by selective electrodes on otolith maculae or otolithic afferents, or unselective maintained DC stimulation by large surface electrodes on the mastoids - surface galvanic stimulation.

SUMMARY

Recent anatomical and physiological results are summarized in order to introduce some of the unique issues in electrical stimulation of the otoliths. The first challenge is that each otolithic macula contains receptors with opposite polarization (opposing preferred directions of stimulation), unlike the uniform polarization of receptors in each semicircular canal crista. The puzzle is that in response to the one linear acceleration in the one macula, some otolithic afferents have an increased activation whereas others have decreased activation. Key Messages: At the vestibular nucleus this opposite receptor hair cell polarization and consequent opposite afferent input allow enhanced response to the one linear acceleration, via a "push-pull" neural mechanism in a manner analogous to the enhancement of semicircular canal responses to angular acceleration. Within each otolithic macula there is not just one uniform otolithic neural input to the brain - there are very distinctly different channels of otolithic neural inputs transferring the neural data to the brainstem. As a simplification these channels are characterized as the sustained and transient systems. Afferents in each system have different responses to stimulus onset and maintained stimulation and likely different projections, and most importantly different thresholds for activation by electrical stimulation and different adaptation rates to maintained stimulation. The implications of these differences are considered.

Model-Based Vestibular Afferent Stimulation: Evaluating Selective Electrode Locations and Stimulation Waveform Shapes

A dysfunctional vestibular system can be a severe detriment to the quality of life of a patient. Recent studies have shown the feasibility for a vestibular implant to restore rotational sensation via electrical stimulation of vestibular ampullary nerves. However, the optimal stimulation site for selective elicitation of the desired nerve is still unknown. We realized a finite element model on the basis of μCT scans of a human inner ear and incorporated naturally distributed, artificial neural trajectories. A well-validated neuron model of myelinated fibers was incorporated to predict nerve responses to electrical stimulation. Several virtual electrodes were placed in locations of interest inside the bony labyrinth (intra-labyrinthine) and inside the temporal bone, near the target nerves (extra-labyrinthine), to determine preferred stimulation sites and electrode insertion depths. We investigated various monopolar and bipolar electrode configurations as well as different pulse waveform shapes for their ability to selectively stimulate the target nerve and for their energy consumption. The selectivity was evaluated with an objective measure of the fiber recruitment. Considerable differences of required energy and achievable selectivity between the configurations were observed. Bipolar, intra-labyrinthine electrodes provided the best selectivities but also consumed the highest amount of energy. Bipolar, extra-labyrinthine configurations did not offer any advantages compared to the monopolar approach. No selective stimulation could be performed with the monopolar, intra-labyrinthine approach. The monopolar, extra-labyrinthine electrodes required the least energy for satisfactory selectivities, making it the most promising approach for functional vestibular implants. Different pulse waveform shapes did not affect the achieved selectivity considerably but shorter pulse durations showed consistently a more selective activation of the target nerves. A cathodic, centered triangular waveform shape was identified as the most energy-efficient of the tested shapes. Based on these simulations we are able to recommend the monopolar, extra-labyrinthine stimulation approach with cathodic, centered triangular pulses as good trade-off between selectivity and energy consumption. Future implant designs could benefit from the findings presented here.

Visualization of the Membranous Labyrinth and Nerve Fiber Pathways in Human and Animal Inner Ears Using MicroCT Imaging

Design and implantation of bionic implants for restoring impaired hair cell function relies on accurate knowledge about the microanatomy and nerve fiber pathways of the human inner ear and its variation. Non-destructive isotropic imaging of soft tissues of the inner ear with lab-based microscopic X-ray computed tomography (microCT) offers high resolution but requires contrast enhancement using compounds with high X-ray attenuation. We evaluated different contrast enhancement techniques in mice, cat, and human temporal bones to differentially visualize the membranous labyrinth, sensory epithelia, and their innervating nerves together with the facial nerve and middle ear. Lugol’s iodine potassium iodine (I₂KI) gave high soft tissue contrast in ossified specimens but failed to provide unambiguous identification of smaller nerve fiber bundles inside small bony canals. Fixation or post-fixation with osmium tetroxide followed by decalcification in EDTA provided superior contrast for nerve fibers and membranous structures. We processed 50 human temporal bones and acquired microCT scans with 15 μm voxel size. Subsequently we segmented sensorineural structures and the endolymphatic compartment for 3D representations to serve for morphometric variation analysis. We tested higher resolution image acquisition down to 3.0 μm voxel size in human and 0.5 μm in mice, which provided a unique level of detail and enabled us to visualize single neurons and hair cells in the mouse inner ear, which could offer an alternative quantitative analysis of cell numbers in smaller animals. Bigger ossified human temporal bones comprising the middle ear and mastoid bone can be contrasted with I₂KI and imaged in toto at 25 μm voxel size. These data are suitable for surgical planning for electrode prototype placements. A preliminary assessment of geometric changes through tissue processing resulted in 1.6% volume increase caused during decalcification by EDTA and 0.5% volume increase caused by partial dehydration to 70% ethanol, which proved to be the best mounting medium for microCT image acquisition.