21 Implantable Vestibular Devices

Since the first vestibular implant (VI) prototype was described in 2000,1 numerous advancements have been made. Clinical trials are now underway in humans and several devices are on their second or third design iteration. While challenges remain, many of the early results are encouraging. Vestibular disorders are common and their incidence will increase as the population ages. Pathologies are diverse and it is likely that only a subset of conditions could be treated with vestibular implantation. The most obvious target disorder is bilateral vestibular hypofunction, where subjects have no vestibulo-ocular reflex (VOR) and any head movement produces visual blurring (oscillopsia). However, several other conditions might be treated as well, such as chronic uncompensated unilateral hypofunction and vertiginous Meniere’s attacks.

Medical treatments for chronic or episodic vestibular disorders are often ineffective.^2,3 Most procedural treatments are ablative⁴ or unproven.⁵ For the hypofunctional disorders, such as uncompensated unilateral loss or bilateral vestibular dysfunction, no treatment exists beyond rehabilitation. These conditions can be severely incapacitating and can impose great economic burdens.⁶

The widespread success of cochlear implantation has led to intense interest in creating an equivalent technology for vestibular pathology. This chapter provides an overview of relevant vestibular physiology, potential VI indications, device design, and early clinical trial results.

Relevant Anatomy and Physiology

A detailed description of the anatomy and physiology of the vestibular system is presented in Chapter 5 of this book. In this section, we review anatomy relevant to the field of vestibular implantation. The vestibular system is comprised of five sensory organs. The three semicircular canals lie in what are essentially X, Y, and Z planes. Each canal detects rotation in its respective plane. The canals are named after their relative location in the temporal bone: superior (anterior), horizontal (lateral), and posterior. The remaining two organs, the utricle and saccule, detect horizontal and vertical linear movements, respectively.

The soft membranous labyrinth divides the vestibular system into two compartments. The outer compartment is the perilymphatic space. The inner compartment is the endolymphatic space. Angular acceleration of the head induces movement of fluid in the semicircular canals. This is detected in the ampullae through deflection of hair cell stereocilia. Depending on the direction of fluid movement, either depolarization or hyperpolarization occurs. This signal is then encoded by the primary afferent neurons as either an increase or a decrease in spike rate, respectively.

Within each semicircular canal ampulla, all hair cells are oriented in the same direction. For this reason, each canal encodes motion about one axis (because all hair cells of a particular canal are stimulated equivalently). In comparison, the hair cells of both the utricle and the saccule are oriented in numerous directions. This property is relevant to VI design, and explains why all major VI devices currently focus on semicircular canal implantation.

In the absence of movement, the output from the vestibular organs is not quiescent, but rather has a baseline tonic neural firing rate. A change in motion will either increase or decrease the firing frequency of this resting signal depending on the direction. This is termed rate encoding.^1,7

Each semicircular canal has a contralateral pair that detects motion information in a particular plane. The two horizontal semicircular canals are paired. The left superior (or anterior) semicircular canal is paired with the right posterior semicircular canal. This plane is often abbreviated LARP. The right superior (or anterior) semicircular canal is paired with the left posterior semicircular canal. This plane is abbreviated RALP. The two canals in each pair encode motion within the same plane but in the opposite direction. This system of redundant pairs is called “push–pull,” and explains why patients with no vestibular function on one side are usually minimally symptomatic. It also predicts that a unilateral VI should be able to restore vestibular function in a patient with severe bilateral impairment.8,9,10

Relevant Functions of the Vestibular System

One of the challenges of understanding the vestibular system is that its function is less obvious than that of the auditory system. While not considered a bona fide sense, such as hearing, vestibular perception essentially acts as a “sixth sense.” For example, with your eyes closed and sense of touch masked, the vestibular system alone allows you to consciously perceive head movements in space.¹¹ Pathologic derangement of the vestibular system is dramatic and severe, showcasing its importance. While previous chapters address the function of the vestibular system, a brief discussion of two key functions relevant to vestibular implantation is useful.

The first function is control of balance and posture. This is accomplished through the vestibulospinal tract, comprised of neuronal projections from the brainstem vestibular nuclei to the spinal cord and motor neurons innervating limb muscles. The bodywide system to maintain balance and posture is highly redundant (likely because of its importance) and can be divided into three separate contributing systems: the vestibular system, proprioception, and visual input. Impairment of any of these three systems can perturb balance in challenging situations.

For example, an individual with cataracts may have no difficulty walking until a crack in the pavement causes his/her body to suddenly tilt to one side. Because of the visual impairment, the correlating visual shift in the surroundings may not be appreciated. A fall may then result if the posture derangement is not quickly recognized by the other two systems. Thus, if one system is dysfunctional, then appropriate function of the other two systems becomes paramount. A patient with impairment of the vestibular system may perform adequately until the lights are turned off (impairment of visual input) or until walking on a plush surface, such as soft carpet or sand (impairment of proprioception). Impairment beyond a critical level will cause falls and injury.^12,13 Vestibular implantation might improve balance through restoration of one of these three critical systems.

The second key function of the vestibular system is less obvious, but equally important: stabilization of images when the head is moving. This is accomplished through the vestibulo-ocular reflex (VOR), which is composed of a neural arc from the vestibular nuclei to the extraocular muscles. The VOR prevents visual blurring when the head is moving. Because the head is often in motion (i.e., nearly all situations except quietly sitting or lying), the VOR can be thought of as “image stabilization” for the retina.

The VOR produces compensatory eye movements that are equal and opposite to head movements. One can test the impressive and underappreciated function of the VOR by holding printed text at a comfortable reading distance in front of the face. While keeping the head still, moving the text rapidly to the left and right causes blurring of the words. In contrast, if the text is kept still and the head is rotated at the same rate, the words will be much sharper. This is because head rotation invokes the powerful VOR, which fine-tunes eye movements to maintain alignment of images on the retina. The VOR confers obvious evolutionary advantages, such as keeping an object (e.g., another animal) sharply visible while the head is constantly moving (e.g., while running on a hunt).

Two additional systems help with image stabilization: the smooth pursuit and optokinetic pathways.¹⁴ However, these systems are slower and their inadequacy is demonstrated by the abovementioned example of the moving printed text. Without vestibular input from at least one functioning ear, the VOR is absent. The correlating clinical symptom is oscillopsia, detailed further later.

Disease Processes That May Be Treated by a Vestibular Implant

Symptoms from vestibular disease depend more on their time course and sidedness than the underlying condition. In addition, because vestibular implantation replaces the function of the diseased organ rather than reversing the root cause, the underlying disease is less relevant. It is thus useful to organize pathology into six groups based on their timing (acute versus recurrent/episodic versus chronic) and their location (unilateral versus bilateral). Because VI research, at least currently, focuses on disorders peripheral to the vestibular nerve, discussion is limited to peripheral disorders. Central disorders that cause vestibular symptoms are beyond the scope of this chapter. Table 21.1 provides an overview of this organizational scheme. In the following subsections, we explain how VIs might be employed to treat vestibular pathology based on these categories.

Acute Unilateral Disorders

The cardinal symptom of acute, unilateral vestibular dysfunction is vertigo. This results from the sudden asymmetry in neural output between the two vestibular systems (with the diseased system’s output typically reduced compared with the contralateral system’s). The brain interprets the mismatch as vertigo, the illusory perception of spinning. Underlying etiologies of acute, unilateral disease include trauma as well as inflammatory/infectious causes, such as vestibular neuronitis or labyrinthitis.^15,16

Table 21.1 Categories of peripheral vestibular disorders* based on time course and sidedness

BPPV, benign paroxysmal positional vertigo

* Disorders that are potential targets for vestibular implantation are in italics.

† Multiple underlying etiologies, e.g., ototoxicity.

In diseases with recurrent episodes, the brain cannot compensate. Vertigo is then experienced anew with every attack. The most common example is benign paroxysmal positional vertigo (BPPV). Because medical (and more rarely surgical) treatment is straightforward and effective,17,18 vestibular implantation would likely not be relevant.

Meniere’s disease is the second most common cause of recurrent vertigo. Symptoms include a tetrad of fluctuating sensorineural hearing loss, tinnitus, vertigo attacks, and aural fullness. Patients are often incapacitated by the unpredictable vertigo episodes. Meniere’s disease is also relatively common, with a prevalence of ~ 43 per 100,000.^19,20,21

Treating Meniere’s has been challenging due to the poorly understood root cause. It has been theorized that acute ruptures in the membranous labyrinth cause mixing of perilymph and endolymph. This reduces the endolymphatic potential, blocking neural transmission. The acute unilateral loss of tonic vestibular output then results in vertigo, manifested by an “attack.”²²

While the initial management for Meniere’s disease is medical, ~ 15% of patients do not respond. The only nonablative procedures are endolymphatic sac surgery and myringotomy with the Meniett device, both of which may have limited long-term efficacy.^19,20,23 Intratympanic steroids have also been used but more controlled data are needed.²⁴ The remaining interventions are all destructive, including intratympanic gentamicin, vestibular nerve section, and labyrinthectomy. These ablative techniques carry risks, including hearing loss and chronic unsteadiness due to poor central compensation.^19,20,21

Because vertigo in Meniere’s disease is thought to be triggered by a sudden unilateral loss of tonic vestibular output, attacks could potentially be aborted by a vestibular “pacemaker.” This type of device would simply provide the absent vestibular output by electrically stimulating the vestibular nerve. Patients would turn on the device only during the onset of a vertigo attack. Details of a pacemaker-based VI are further discussed later in the chapter.

Chronic Unilateral Disorders

Central compensation is a phenomenon that allows the brain to acclimate to asymmetric vestibular input. Because of compensation, disorders that begin with an acute phase of vertigo ordinarily cease to be symptomatic with time. Even when a fixed deficit persists, patients are usually unaffected because only one functioning ear is required to maintain adequate vestibular function, as previously explained. Chronic unilateral vestibular deficits are thus typically asymptomatic. In fact, slowly progressive unilateral disorders that lack an acute phase (such as a vestibular schwannoma) may not ever produce obvious vestibular symptoms. These well-compensated conditions would not benefit from vestibular implantation.

The goal of vestibular implantation is to restore vestibular functionality and/or reduce symptoms through electrical stimulation of the vestibular nerve. It has been known since the 1960s that stimulation of the vestibular nerve can induce the VOR, thus providing the basis for the field of vestibular neurostimulation.25,26 At the present, VIs can be divided into two types of designs. The first design would serve as a complete prosthetic vestibular system, replacing the diseased motion-sensing end-organs with angular gyroscopes and linear accelerometers. A schematic of such a design is illustrated in Fig. 21.1a. The three components would be motion sensors, a signal processor, and a nerve stimulator.²⁷ This design is equivalent to the microphone, speech processor, and nerve stimulator (receiver/stimulator) of a cochlear implant. This device is referred to as a sensor-based vestibular implant.

The second design is simpler and eschews a motion sensor. This type of device would use a preprogrammed signal to replace absent tonic activity from vestibular afferents (Fig. 21.1b). In some cases, the signal could instead attempt to supplement afferent input to increase VOR gain by providing an elevated baseline around which the remaining depressed natural input could modulate central neurons (Fig. 21.1c). This device is referred to as a pacemaker-based vestibular implant.²⁸

In both design types, the stimulator component creates a biphasic charge-balanced pulsatile current delivered to branches of the vestibular nerve at their end-organ terminals. Each electrode array is implanted into an end-organ, and each array may contain multiple electrodes. The stimulation signal may be modulated (modified) by varying the amplitude of the current (µA) or the frequency of the pulse (pulse rate, Hz). Increasing either variable will result in increased velocity input to the central nervous system, resulting in higher slow-phase velocity eye movements (a component of the VOR, explained later). Conversely, lowering these parameters would produce the reverse effect.¹

Unlike the auditory system, which contains only a single organ (the organ of Corti), the vestibular system contains five sensory organs. This makes prosthetic implantation of the vestibular system more complicated than cochlear implantation.

Each of the three semicircular canals encodes rotational motion in only one axis. Stimulating a single canal results in stereotyped eye movements in a corresponding axis. The utricle and saccule, however, each encode motion in multiple directions because their hair cells are oriented in multiple directions. Stimulating the utricle or saccule does not result in predictable eye movements.^{29,30,31,32,33} For this reason, current VI designs are semicircular canal prostheses only.¹⁰

Sensor-Based Vestibular Implant

When symptoms result from not being able to detect motion, a sensor-based VI is needed. The model condition is bilateral vestibular dysfunction, usually due to dysfunction of the vestibular mechanosensory hair cells. Electromechanical components that detect a change in motion, such as angular gyroscopes and linear accelerometers, would replace the nonfunctioning semicircular canals and (possibly) the utricle and saccule.

Fig. 21.1 Schematics of various vestibular implant designs. Several recent devices additionally incorporate an integrated cochlear implant for concomitant sensorineural hearing loss (not pictured). (a) Sensor-based device that would restore the VOR and the ability to perceive 3D movement. (b) Pacemaker-based device that would be turned on during the onset of acute vertigo attacks in recurrent conditions such as Meniere’s disease. Turning on the stimulus would theoretically abort the attack. (c) Pacemaker-based device that would be chronically kept on to increase VOR gain in uncompensated chronic unilateral disorders. Used with permission from Waltzman SB, Roland JT Jr. Cochlear Implants. 3rd ed. New York, NY: Thieme; 2014.

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