Longitudinal performance of an implantable vestibular prosthesis

The VertiGO! Trial protocol: A prospective, single-center, patient-blinded study to evaluate efficacy and safety of prolonged daily stimulation with a multichannel vestibulocochlear implant prototype in bilateral vestibulopathy patients

BACKGROUND

A combined vestibular (VI) and cochlear implant (CI) device, also known as the vestibulocochlear implant (VCI), was previously developed to restore both vestibular and auditory function. A new refined prototype is currently being investigated. This prototype allows for concurrent multichannel vestibular and cochlear stimulation. Although recent studies showed that VCI stimulation enables compensatory eye, body and neck movements, the constraints in these acute study designs prevent them from creating more general statements over time. Moreover, the clinical relevance of potential VI and CI interactions is not yet studied. The VertiGO! Trial aims to investigate the safety and efficacy of prolonged daily motion modulated stimulation with a multichannel VCI prototype.

METHODS

A single-center clinical trial will be carried out to evaluate prolonged VCI stimulation, assess general safety and explore interactions between the CI and VI. A single-blind randomized controlled crossover design will be implemented to evaluate the efficacy of three types of stimulation. Furthermore, this study will provide a proof-of-concept for a VI rehabilitation program. A total of minimum eight, with a maximum of 13, participants suffering from bilateral vestibulopathy and severe sensorineural hearing loss in the ear to implant will be included and followed over a five-year period. Efficacy will be evaluated by collecting functional (i.e. image stabilization) and more fundamental (i.e. vestibulo-ocular reflexes, self-motion perception) outcomes. Hearing performance with a VCI and patient-reported outcomes will be included as well.

DISCUSSION

The proposed schedule of fitting, stimulation and outcome testing allows for a comprehensive evaluation of the feasibility and long-term safety of a multichannel VCI prototype. This design will give insights into vestibular and hearing performance during VCI stimulation. Results will also provide insights into the expected daily benefit of prolonged VCI stimulation, paving the way for cost-effectiveness analyses and a more comprehensive clinical implementation of vestibulocochlear stimulation in the future.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT04918745. Registered 28 April 2021.

The Next Challenges of Vestibular Implantation in Humans

Patients with bilateral vestibulopathy suffer from a variety of complaints, leading to a high individual and social burden. Available treatments aim to alleviate the impact of this loss and improve compensatory strategies. Early experiments with electrical stimulation of the vestibular nerve in combination with knowledge gained by cochlear implant research, have inspired the development of a vestibular neuroprosthesis that can provide the missing vestibular input. The feasibility of this concept was first demonstrated in animals and later in humans. Currently, several research groups around the world are investigating prototype vestibular implants, in the form of vestibular implants as well as combined cochlear and vestibular implants. The aim of this review is to convey the presentations and discussions from the identically named symposium that was held during the 2021 MidWinter Meeting of the Association for Research in Otolaryngology, with researchers involved in the development of vestibular implants targeting the ampullary nerves. Substantial advancements in the development have been made. Yet, research and development processes face several challenges to improve this neuroprosthesis. These include, but are not limited to, optimization of the electrical stimulation profile, refining the surgical implantation procedure, preserving residual labyrinthine functions including hearing, as well as gaining regulatory approval and establishing a clinical care infrastructure similar to what exists for cochlear implants. It is believed by the authors that overcoming these challenges will accelerate the development and increase the impact of a clinically applicable vestibular implant.

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.

The vestibular implant: Opinion statement on implantation criteria for research

This opinion statement proposes a set of candidacy criteria for vestibular implantation of adult patients with bilateral vestibulopathy (BVP) in a research setting. The criteria include disabling chronic symptoms like postural imbalance, unsteadiness of gait and/or head movement-induced oscillopsia, combined with objective signs of reduced or absent vestibular function in both ears. These signs include abnormal test results recorded during head impulses (video head impulse test or scleral coil technique), bithermal caloric testing and rotatory chair testing (sinusoidal stimulation of 0.1 Hz). Vestibular implant (VI) implantation criteria are not the same as diagnostic criteria for bilateral vestibulopathy. The major difference between VI-implantation criteria and the approved diagnostic criteria for BVP are that all included vestibular tests of semicircular canal function (head impulse test, caloric test, and rotatory chair test) need to show significant impairments of vestibular function in the implantation criteria. For this, a two-step paradigm was developed. First, at least one of the vestibular tests needs to fulfill stringent criteria, close to those for BVP. If this is applicable, then the other vestibular tests have to fulfill a second set of criteria which are less stringent than the original criteria for BVP. If the VI-implantation is intended to excite the utricle and/or saccule (otolith stimulation), responses to cervical and ocular vestibular evoked myogenic potentials must be absent in addition to the above mentioned abnormalities of semicircular canal function. Finally, requirements for safe and potentially effective stimulation should be met, including implanting patients with BVP of peripheral origin only, and assessing possible medical and psychiatric contraindications.

Results From a Second-Generation Vestibular Implant in Human Subjects: Diagnosis May Impact Electrical Sensitivity of Vestibular Afferents

OBJECTIVE

Auditory and vestibular outcomes after placement of a vestibular-cochlear implant in subjects with varying causes of vestibular loss.

STUDY DESIGN

Prospective case study.

SETTING

Tertiary referral center.

PATIENTS

Three human subjects received a vestibular-cochlear implant. Subject 1 had sudden hearing and vestibular loss 10 years before implantation. Subjects 2 and 3 had bilateral Menière's disease with resolution of acute attacks. All subjects had severe-profound deafness in the implanted ear and bilateral vestibular loss.

INTERVENTION

Vestibular-cochlear implant with electrode positions confirmed by CT.

MAIN OUTCOME MEASURES

Electrically-evoked vestibular and cochlear compound action potentials (ECAPs), speech perception, and electrically-evoked slow-phase eye velocities.

RESULTS

Subject 1 had no vestibular ECAP, but normal cochlear ECAPs and cochlear implant function. She had minimal eye-movement with vestibular stimulation. Subject 2 had vestibular ECAPs. This subject had the largest eye velocities from electrical stimulation that we have seen in humans, exceeding 100 degrees per second. Her cochlear implant functions normally. Subject 3 had vestibular and cochlear ECAPs, and robust eye-movements and cochlear implant function.

CONCLUSION

The etiology of vestibular loss appears to have a profound impact on sensitivity of vestibular afferents in distinction to cochlear afferents. If this dichotomy is common, it may limit the application of vestibular implants to diagnoses with preserved sensitivity of vestibular afferents. We speculate it is due to differences in topographic organization of Scarpa's versus the spiral ganglion. In two subjects, the second-generation device can produce higher velocity eye movements than seen in the four subjects receiving the first-generation device.

Fluctuations in Vestibular Afferent Excitability in Menière's Disease

OBJECTIVE

To determine if Menière's disease is associated with fluctuations in afferent excitability in four human subjects previously implanted with vestibular stimulators.

STUDY DESIGN

Longitudinal repeated measures.

SETTING

Tertiary referral center, human vestibular research laboratory.

PATIENTS

Four human subjects with previously uncontrolled Menière's disease unilaterally implanted in each semicircular canal with a vestibular stimulator. One subject had only two canals implanted.

INTERVENTION(S)

Repeated measures of electrically-evoked slow phase eye velocity and vestibular electrically-evoked compound action potentials (vECAP) over 2 to 4 years.

MAIN OUTCOME MEASURE(S)

Slow phase eye velocity and N1-P1 vECAP amplitudes as a function of time.

RESULTS

There were statistically significant fluctuations in electrically evoked slow phase eye velocity over time in at least one semicircular canal of each subject. vECAP N1-P1 amplitudes measured at similar time intervals and stimulus intensities seem to show somewhat correlated fluctuations. One of the subjects had a single Menière's attack during this time period. The others did not.

CONCLUSIONS

In these four subjects originally diagnosed with Menière's disease, there was fluctuating electrical excitability of the ampullar nerve of at least one canal in each subject. These fluctuations occurred without active symptoms of Menière's disease.

Interactions between Auditory and Vestibular Modalities during Stimulation with a Combined Vestibular and Cochlear Prosthesis

BACKGROUND

A combined vestibular and cochlear prosthesis may restore hearing and balance to patients who have lost both. To do so, the device should activate each sensory system independently.

OBJECTIVES

In this study, we quantify auditory and vestibular interactions during interleaved stimulation with a combined 16-channel cochlear and 6-channel vestibular prosthesis in human subjects with both hearing and vestibular loss.

METHODS

Three human subjects were implanted with a combined vestibular and cochlear implant. All subjects had severe-to-profound deafness in the implanted ear. We provided combined stimulation of the cochlear and vestibular arrays and looked for interactions between these separate inputs. Our main outcome measures were electrically evoked slow-phase eye velocities during nystagmus elicited by brief trains of biphasic pulse stimulation of the vestibular end organs with and without concurrent stimulation of the cochlea, and Likert scale assessments of perceived loudness and pitch during stimulation of the cochlea, with and without concurrent stimulation of the vestibular ampullae.

RESULTS

All subjects had no auditory sensation resulting from semicircular canal stimulation alone, and no sensation of motion or slow-phase eye movement resulting from cochlear stimulation alone. However, interleaved cochlear stimulation did produce changes in the slow-phase eye velocities elicited by electrical stimulation. Similarly, interleaved semicircular canal stimulation did elicit changes in the perceived pitch and loudness resulting from stimulation at multiple sites in the cochlea.

CONCLUSIONS

There are significant interactions between different sensory modalities during stimulation with a combined vestibular and cochlear prosthesis. Such interactions present potential challenges for stimulation strategies to simultaneously restore auditory and vestibular function with such an implant.

Continuous vestibular implant stimulation partially restores eye-stabilizing reflexes

BACKGROUNDBilateral loss of vestibular (inner ear inertial) sensation causes chronically blurred vision during head movement, postural instability, and increased fall risk. Individuals who fail to compensate despite rehabilitation therapy have no adequate treatment options. Analogous to hearing restoration via cochlear implants, prosthetic electrical stimulation of vestibular nerve branches to encode head motion has garnered interest as a potential treatment, but prior studies in humans have not included continuous long-term stimulation or 3D binocular vestibulo-ocular reflex (VOR) oculography, without which one cannot determine whether an implant selectively stimulates the implanted ear's 3 semicircular canals.METHODSWe report binocular 3D VOR responses of 4 human subjects with ototoxic bilateral vestibular loss unilaterally implanted with a Labyrinth Devices Multichannel Vestibular Implant System vestibular implant, which provides continuous, long-term, motion-modulated prosthetic stimulation via electrodes in 3 semicircular canals.RESULTSInitiation of prosthetic stimulation evoked nystagmus that decayed within 30 minutes. Stimulation targeting 1 canal produced 3D VOR responses approximately aligned with that canal's anatomic axis. Targeting multiple canals yielded responses aligned with a vector sum of individual responses. Over 350-812 days of continuous 24 h/d use, modulated electrical stimulation produced stable VOR responses that grew with stimulus intensity and aligned approximately with any specified 3D head rotation axis.CONCLUSIONThese results demonstrate that a vestibular implant can selectively, continuously, and chronically provide artificial sensory input to all 3 implanted semicircular canals in individuals disabled by bilateral vestibular loss, driving reflexive VOR eye movements that approximately align in 3D with the head motion axis encoded by the implant.TRIAL REGISTRATIONClinicalTrials.gov: NCT02725463.FUNDINGNIH/National Institute on Deafness and Other Communication Disorders: R01DC013536 and 2T32DC000023; Labyrinth Devices, LLC; and Med-El GmbH.

Nonhuman primate vestibuloocular reflex responses to prosthetic vestibular stimulation are robust to pulse timing errors caused by temporal discretization

Electrical stimulation of vestibular afferent neurons to partially restore semicircular canal sensation of head rotation and the stabilizing reflexes that sensation supports has potential to effectively treat individuals disabled by bilateral vestibular hypofunction. Ideally, a vestibular implant system using this approach would be integrated with a cochlear implant, which would provide clinicians with a means to simultaneously treat loss of both vestibular and auditory sensation. Despite obvious similarities, merging these technologies poses several challenges, including stimulus pulse timing errors that arise when a system must implement a pulse frequency modulation-encoding scheme (as is used in vestibular implants to mimic normal vestibular nerve encoding of head movement) within fixed-rate continuous interleaved sampling (CIS) strategies used in cochlear implants. Pulse timing errors caused by temporal discretization inherent to CIS create stair step discontinuities of the vestibular implant's smooth mapping of head velocity to stimulus pulse frequency. In this study, we assayed electrically evoked vestibuloocular reflex responses in two rhesus macaques using both a smooth pulse frequency modulation map and a discretized map corrupted by temporal errors typical of those arising in a combined cochlear-vestibular implant. Responses were measured using three-dimensional scleral coil oculography for prosthetic electrical stimuli representing sinusoidal head velocity waveforms that varied over 50-400°/s and 0.1-5 Hz. Pulse timing errors produced negligible effects on responses across all canals in both animals, indicating that temporal discretization inherent to implementing a pulse frequency modulation-coding scheme within a cochlear implant's CIS fixed pulse timing framework need not sacrifice performance of the combined system's vestibular implant portion. NEW & NOTEWORTHY Merging a vestibular implant system with existing cochlear implant technology can provide clinicians with a means to restore both vestibular and auditory sensation. Pulse timing errors inherent to integration of pulse frequency modulation vestibular stimulation with fixed-rate, continuous interleaved sampling cochlear implant stimulation would discretize the smooth head velocity encoding of a combined device. In this study, we show these pulse timing errors produce negligible effects on electrically evoked vestibulo-ocular reflex responses in two rhesus macaques.

Nonhuman Primate Studies to Advance Vision Science and Prevent Blindness

Most primate behavior is dependent on high acuity vision. Optimal visual performance in primates depends heavily upon frontally placed eyes, retinal specializations, and binocular vision. To see an object clearly its image must be placed on or near the fovea of each eye. The oculomotor system is responsible for maintaining precise eye alignment during fixation and generating eye movements to track moving targets. The visual system of nonhuman primates has a similar anatomical organization and functional capability to that of humans. This allows results obtained in nonhuman primates to be applied to humans. The visual and oculomotor systems of primates are immature at birth and sensitive to the quality of binocular visual and eye movement experience during the first months of life. Disruption of postnatal experience can lead to problems in eye alignment (strabismus), amblyopia, unsteady gaze (nystagmus), and defective eye movements. Recent studies in nonhuman primates have begun to discover the neural mechanisms associated with these conditions. In addition, genetic defects that target the retina can lead to blindness. A variety of approaches including gene therapy, stem cell treatment, neuroprosthetics, and optogenetics are currently being used to restore function associated with retinal diseases. Nonhuman primates often provide the best animal model for advancing fundamental knowledge and developing new treatments and cures for blinding diseases.

The Dynamics of Prosthetically Elicited Vestibulo-Ocular Reflex Function Across Frequency and Context in the Rhesus Monkey

Electrical vestibular neurostimulation may be a viable tool for modulating vestibular afferent input to restore vestibular function following injury or disease. To do this, such stimulators must provide afferent input that can be readily interpreted by the central nervous system to accurately represent head motion to drive reflexive behavior. Since vestibular afferents have different galvanic sensitivity, and different natural sensitivities to head rotational velocity and acceleration, and electrical stimulation produces aphysiological synchronous activation of multiple afferents, it is difficult to assign a priori an appropriate transformation between head velocity and acceleration and the properties of the electrical stimulus used to drive vestibular reflex function, i.e., biphasic pulse rate or pulse current amplitude. In order to empirically explore the nature of the transformation between vestibular prosthetic stimulation and vestibular reflex behavior, in Rhesus macaque monkeys we parametrically varied the pulse rate and current amplitude of constant rate and current amplitude pulse trains, and the modulation frequency of sinusoidally modulated pulse trains that were pulse frequency modulated (FM) or current amplitude modulated (AM). In addition, we examined the effects of differential eye position and head position on the observed eye movement responses. We conclude that there is a strong and idiosyncratic, from canal to canal, effect of modulation frequency on the observed eye velocities that are elicited by stimulation. In addition, there is a strong effect of initial eye position and initial head position on the observed responses. These are superimposed on the relationships between pulse frequency or current amplitude and eye velocity that have been shown previously.

A dual-axis single-proof-mass angular accelerometer for a vestibular prosthesis

A dual-axis single-proof-mass angular accelerometer has been developed for a vestibular prosthesis. Designed to sense head rotations both in the yaw and the pitch planes, the output of the inertial sensor may be coded as amplitude or rate modulated biphasic current pulses to stimulate vestibular nerves. Fabricated with a high aspect ratio commercial process, a sensor with small form factor (1.4 mm × 0.8 mm) is achieved with a scale factor of 95.5 μV/rad/sec2 and 145.8 μV/rad/sec2 in the yaw and the pitch planes, respectively. Superior linear acceleration rejection was demonstrated for both rotating axis, and an overall power consumption of 296 μW was estimated including sensor and interface circuit.

Wireless inertial measurement of head kinematics in freely-moving rats

While miniature inertial sensors offer a promising means for precisely detecting, quantifying and classifying animal behaviors, versatile inertial sensing devices adapted for small, freely-moving laboratory animals are still lacking. We developed a standalone and cost-effective platform for performing high-rate wireless inertial measurements of head movements in rats. Our system is designed to enable real-time bidirectional communication between the headborne inertial sensing device and third party systems, which can be used for precise data timestamping and low-latency motion-triggered applications. We illustrate the usefulness of our system in diverse experimental situations. We show that our system can be used for precisely quantifying motor responses evoked by external stimuli, for characterizing head kinematics during normal behavior and for monitoring head posture under normal and pathological conditions obtained using unilateral vestibular lesions. We also introduce and validate a novel method for automatically quantifying behavioral freezing during Pavlovian fear conditioning experiments, which offers superior performance in terms of precision, temporal resolution and efficiency. Thus, this system precisely acquires movement information in freely-moving animals, and can enable objective and quantitative behavioral scoring methods in a wide variety of experimental situations.

Loss of Afferent Vestibular Input Produces Central Adaptation and Increased Gain of Vestibular Prosthetic Stimulation

Implanted vestibular neurostimulators are effective in driving slow phase eye movements in monkeys and humans. Furthermore, increases in slow phase velocity and electrically evoked compound action potential (vECAP) amplitudes occur with increasing current amplitude of electrical stimulation. In intact monkeys, protracted intermittent stimulation continues to produce robust behavioral responses and preserved vECAPs. In lesioned monkeys, shorter duration studies show preserved but with somewhat lower or higher velocity behavioral responses. It has been proposed that such changes are due to central adaptive changes in the electrically elicited vestibulo-ocular reflex (VOR). It is equally possible that these differences are due to changes in the vestibular periphery in response to activation of the vestibular efferent system. In order to investigate the site of adaptive change in response to electrical stimulation, we performed transtympanic gentamicin perfusions to induce rapid changes in vestibular input in monkeys with long-standing stably functioning vestibular neurostimulators, disambiguating the effects of implantation from the effects of ototoxic lesion. Gentamicin injection was effective in producing a large reduction in natural VOR only when it was performed in the non-implanted ear, suggesting that the implanted ear contributed little to the natural rotational response before injection. Injection of the implanted ear produced a reduction in the vECAP responses in that ear, suggesting that the intact hair cells in the non-functional ipsilateral ear were successfully lesioned by gentamicin, reducing the efficacy of stimulation in that ear. Despite this, injection of both ears produced central plastic changes that resulted in a dramatically increased slow phase velocity nystagmus elicited by electrical stimulation. These results suggest that loss of vestibular afferent activity, and a concurrent loss of electrically elicited vestibular input, produces an increase in the efficacy of a vestibular neurostimulator by eliciting centrally adapted behavioral responses without concurrent adaptive increase of galvanic afferent activation in the periphery.

Vestibular Implants: 8 Years of Experience with Electrical Stimulation of the Vestibular Nerve in 11 Patients with Bilateral Vestibular Loss

BACKGROUND

The concept of the vestibular implant is primarily to artificially restore the vestibular function in patients with a bilateral vestibular loss (BVL) by providing the central nervous system with motion information using electrical stimulation of the vestibular nerve. Our group initiated human trials about 10 years ago.

METHODS

Between 2007 and 2013, 11 patients with a BVL received a vestibular implant prototype providing electrodes to stimulate the ampullary branches of the vestibular nerve. Eye movements were recorded and analyzed to assess the effects of the electrical stimulation. Perception induced by electrical stimulation was documented.

RESULTS

Smooth, controlled eye movements were obtained in all patients showing that electrical stimulation successfully activated the vestibulo-ocular pathway. However, both the electrical dynamic range and the amplitude of the eye movements were variable from patient to patient. The axis of the response was consistent with the stimulated nerve branch in 17 out of the 24 tested electrodes. Furthermore, in at least 1 case, the elicited eye movements showed characteristics similar to those of compensatory eye movements observed during natural activities such as walking. Finally, diverse percepts were reported upon electrical stimulation (i.e., rotatory sensations, sound, tickling or pressure) with intensity increasing as the stimulation current increased.

CONCLUSIONS

These results demonstrate that electrical stimulation is a safe and effective means to activate the vestibular system, even in a heterogeneous patient population with very different etiologies and disease durations. Successful tuning of this information could turn this vestibular implant prototype into a successful artificial balance organ.

Vestibular implantation and longitudinal electrical stimulation of the semicircular canal afferents in human subjects

Animal experiments and limited data in humans suggest that electrical stimulation of the vestibular end organs could be used to treat loss of vestibular function. In this paper we demonstrate that canal-specific two-dimensionally (2D) measured eye velocities are elicited from intermittent brief 2 s biphasic pulse electrical stimulation in four human subjects implanted with a vestibular prosthesis. The 2D measured direction of the slow phase eye movements changed with the canal stimulated. Increasing pulse current over a 0-400 μA range typically produced a monotonic increase in slow phase eye velocity. The responses decremented or in some cases fluctuated over time in most implanted canals but could be partially restored by changing the return path of the stimulation current. Implantation of the device in Meniere's patients produced hearing and vestibular loss in the implanted ear. Electrical stimulation was well tolerated, producing no sensation of pain, nausea, or auditory percept with stimulation that elicited robust eye movements. There were changes in slow phase eye velocity with current and over time, and changes in electrically evoked compound action potentials produced by stimulation and recorded with the implanted device. Perceived rotation in subjects was consistent with the slow phase eye movements in direction and scaled with stimulation current in magnitude. These results suggest that electrical stimulation of the vestibular end organ in human subjects provided controlled vestibular inputs over time, but in Meniere's patients this apparently came at the cost of hearing and vestibular function in the implanted ear.