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Introduction and Human Target Population. Vestibular disorders are very common and often very debilitating. They affect the lives of people of all ages and backgrounds. Loss or disruption of normal vestibular function results in vertigo, loss of balance and orientation, and falls. 10% of patients with vestibular complaints are disabled by their vestibular loss. Vestibular disorders can be idiopathic. Drugs and environmental toxins can adversely affect the inner ear, as can viral infection and a host of genetic disorders. The social cost of vestibular disorders is great. 95 million Americans, or about 36% of the population, will visit their physician with a primary complaint of dizziness at least once in their lifetime. Society must bear the burden of lost work and productivity and the cost of treatment for these individuals. This cost has been estimated to exceed $1 billion per year in the United States. 5.4 million Americans visit their physician with a balance complaint annually, placing balance disorders among the top 25 most common reasons that Americans seek health care. With an aging population, the incidence and cost of balance disorders will increase since balance disorders increase in prevalence with increasing age. By age 75, balance disorders are one of the most common reasons for seeking health care. Indeed, the majority of individuals over the age of 70 report problems with dizziness and imbalance and 1/3 of individuals aged 65 to 75 report that balance problems adversely affect their quality of life (NAMCS, 1991; NIDCD, 1989).
Despite advances in our understanding of the vestibular system due to basic research sponsored by the NIH, NASA, and NSF, treatment of vestibular disorders is largely based on therapies developed decades ago. With the exception of vestibular rehabilitation, which has expanded greatly over the past 10 years, most drugs and surgical therapies have been in use for over 20 years. Vestibular disorders have been the subject of many meetings, seminars, publications, and research grants. These activities have reflected and supported substantial increases in knowledge about the effects of various diseases on balance, in particular, the effects of aging. Nonetheless, no substantively new therapeutic directions have appeared in clinical practice. For example, we still rely on destructive procedures to correct the irreversibly diseased inner ear. Unlike the auditory system, where there are now developing notions of factors governing vulnerability to the major causes of deafness, most of the common vestibular disorders – e.g. Meniere’s disease and vestibular neuronitis – remain etiologic and clinical enigmas.
A new direction in the treatment of vestibular disorders has recently emerged. First, with the successful introduction of implantable cochlear prostheses for hearing loss (Gates and Miyamoto, 2003; Francis and Niparko, 2003, Rubinstein, 2004; Spitzer et al., 2003) work has begun in earnest on an implantable vestibular prosthesis of essentially the same design. The device would, in theory, provide an analog of the missing vestibular information by stimulating intact nerves that are no longer receiving input from vestibular hair cells (Lewis et al., 2003; Rubinstein and Della-Santina, 2003; Wall et al., 2003). Although this strategy holds great promise, such a device presents many challenges in its design and implementation. It is clear, however, that many vestibular patients will potentially benefit from the introduction of such a device.
Bilateral loss: Patients who have bilateral vestibular areflexia would benefit from stimulation strategies that can provide some reliable head orientation information via the vestibular system. Such patients experience not only disorientation but also oscillopsia as a consequence of their loss of vestibular information. Some, such as patients who have iatrogenically induced vestibular loss from aminoglycoside ototoxicity would benefit primarily from the reduction in oscillopsia and return of dynamic visual acuity resulting from restoration of the vestibulo-ocular reflex (VOR). Others, such as children with Type I Usher's syndrome, will benefit primarily from the restoration of orientation cues as their other primary sensory processes (hearing then vision) progressively deteriorate. For such patients, restoration of vestibular information early in life prior to progressive retinal deterioration would allow an opportunity to acquire orientation strategies based on artificial vestibular information that could be used throughout life. This information could presumably be presented unilaterally or bilaterally, since as is discussed below, the brain has adaptive strategies to allowing successful utilization of unilaterally presented vestibular information.
Unilateral Loss: Unilateral loss of vestibular function typically results in a series of central changes that begin with a restoration of balanced tonic activity in the secondary vestibular neurons that receive vestibular information, and then a slower adaptive process that allows reinterpretation of vestibular input. In many patients these processes result in a recovery of vestibular function that, while imperfect across the full frequency range of the normal vestibular system, is quite acceptable to them and requires no further intervention or treatment. In other patients, these processes are inadequate, leading in the worst case to a loss of vestibular tone and a chronic disequilibrium. Such patients may well benefit from the restoration of stochastically determined tonic afferent input from the damaged vestibular end organ. It is possible, although unknown, that the restoration of background activity would allow the slower vestibular adaptation mechanism in such patients to more fully compensate for their vestibular loss.
Other Populations: The forgoing discussions presume that a loss of vestibular function results from a loss of vestibular input. This is often not the case, because the primary loss that defeats vestibular compensation and adaptive change is a loss of reliable correlation between sensory input and changes in orientation. This can be produced by peripheral disease processes such as Meniere's disease, which in its early incarnation is both unilateral and fluctuating. It can also be produced by a reduction or elimination in the central adaptive mechanisms, notably those of the vestibulocerebellum, that maintain a reliable correlation by fine tuning brainstem input-output relationships based on movement performance (e.g., retinal slip for the VOR). Such a reduction could result from direct damage to the neural machinery that underlies these mechanisms, or from the progressive deterioration of motor learning that accompanies aging. In this way, young and old alike may benefit from augmentation of vestibular input. Indeed, in pediatric patients for whom surgical treatment of posterior fossa tumors results in spatial disorientation and movement deficits, it is the central processing of vestibular sensory input and not the input itself that is compromised. For these patients, augmentation of sensory information with peripheral stimulation strategies based on movement performance may provide a restoration of function that would be otherwise unobtainable (Phillips et al, 2004).