Out of Committee: Equilibrium | Vestibular and Balance Dysfunction in Children with Sensorineural Hearing Loss

Sharon L. Cushing, MD, and Robert C. O’Reilly, MD

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Sensorineural hearing loss (SNHL) is the most common congenital sensory impairment, occurring in three out of every 1,000 live births.¹ The prevalence of vestibular impairment in children with SNHL is high, ranging between 20%-70%^2-6 with 35% displaying severe/complete impairment of the vestibular end organs,^2,7 which translates into balance impairments.^8,9 Dysfunction is often missed as most children with vestibular dysfunction will never be vertiginous. Given this high prevalence and the challenges in recognizing these deficits, children presenting with SNHL should be regularly screened for vestibular and balance dysfunction.^3,10-12

Impact of Etiology

The risk of having concurrent cochleovestibular loss is dependent upon the etiology of the SNHL.² The capacity of diagnostic tools, such as imaging, molecular genetic techniques, and virology testing, are enhanced by providing accurate and detailed phenotypic description of the child. This should include vestibular and balance assessment. The most common etiologies of deafness associated with severe vestibular impairment are:

• Genetic causes nonsyndromic mimickers such as Usher Syndrome Type 1¹³ and CHARGE syndrome¹⁴
• Cochleovestibular anomalies such as incomplete partitions¹⁵
• Acquired infections such as meningitis^{16, 17, 18, 19, 20} and congenital cytomegalovirus (cCMV)^21-23,24
• Ototoxicity (i.e., aminoglycosides²⁵ or chemotherapeutics²⁶)

Just as SNHL can present with varying degrees of severity and any rate of progression, so too can vestibular impairment. Although the risk of vestibular impairment is highest in those with the most significant cochlear deficits,^27-31 the co-existence of both sensory deficits with respect to degree and time course can follow any pattern. 

Impact of Cochlear Implantation

No discussion about vestibular impairment in SNHL would be complete without considering the impact of implantation on inner ear function. The literature is heterogeneous as methodologies used to quantify impact are variable. Initial reports of dysfunction following cochlear implantation (CI) were based only on subjective complaints of dizziness, occurring in 2%-49% of patients and more likely in adults with increasing age.^32-40 The risk of losing or significantly diminishing horizontal canal function based on caloric testing following CI ranges between 0 and 77%.^{3, 36, 39, 41-51} The most robust data in the pediatric population comes from carefully obtained pre- and postoperative vestibular function testing. This data was used in order to estimate the risk of total bilateral vestibular loss following bilateral simultaneous CI. Overall that risk was felt to be in the order of 2%.^{6, 20, 52, 53} Although this risk should not be ignored when considering bilateral CI in children, it is important to note that this risk of total bilateral vestibular loss is exceeded by the underlying etiology of the deafness itself. In summary, regarding the risk of vestibular injury from CI:

• Adults and children are different both in their risk of vestibular impairment at baseline and following CI and this primarily reflects differences in etiology.
• The great majority of children receiving a CI have a pre-existing vestibular impairment at baseline prior to surgery.
• Vestibular injury can be induced by CI.

Screening for Vestibular Impairment in Children with SNHL

Identifying vestibular and balance impairment is important in children presenting with SNHL. However, the challenges of doing so present barriers. A screening algorithm, such as the following, can be applied in the clinical setting to better identify those at risk of vestibular impairment who should receive more thorough testing: 

• Review of motor milestones (Table 1)
• Assessment of balance – one-foot standing (Table 2)⁵⁴
• Assessment of horizontal canal function

Screening assessment of horizontal canal function can be done without specialized equipment using a clinical head impulse test. Additionally, infants younger than six months of age display a developmental inability to suppress their vestibulo-ocular reflex response allowing for easy assessment of the horizontal canal. This can be done by spinning the child (and the caregiver on whose lap they sit) on a stool and examining for postrotary nystagmus (fast-phase directed away from the direction of the acceleration).⁵⁵ Ideally, all three items of the screening assessment would be performed; however, completion of any single one may identify a child at risk of vestibular dysfunction.

Functional Impact of Vestibular and Balance Impairment

At minimum, vestibular loss carries a number of safety concerns that should be relayed to patients. Absence of bilateral horizontal canal function (areflexia), saccular dysfunction, and poor balance measured on objective tests of function have been demonstrated to increase the odds of CI device failure 7.6 times,⁵⁶ with failure defined as mechanical or electrical malfunction of the surgically implanted internal component. 

It is expected that children who are deaf and have concurrent vestibular impairments will be delayed in their motor milestones.^{9, 19, 20, 57} However, given the vestibular system’s far-reaching projections throughout the brain, impairment leads to deficits beyond locomotion⁵⁸ and plays a role in neurocognition, including perceptual and visuospatial ability,⁵⁹ memory, and executive function. Neuroanatomically bilateral vestibular deficits are correlated with decreased hippocampal volume.⁶⁰ The bulk of the literature examining the vestibular-cognitive relationship focuses on deficits resulting from acquired vestibular loss, but congenital absence of vestibular function may lead to a distortion of typical brain development as demonstrated in congenital hearing or other sensory losses. There is likely a critical period to develop accurate spatial representations akin to those for linguistic development.⁶¹ Much of the resulting cognitive deficits may occur from brain changes at the level of the hippocampus that result from an absence of vestibular input during that critical time period. The direct impact of this sensory deprivation could be a failure to develop a construct for the relative representation of both the body and other objects in space. Poor hippocampal development may contribute to broader issues with learning, memory, and executive function.⁶¹

Balance deficits also occur in children with unilateral profound SNHL.⁶² Multiple factors are likely to contribute to their poorer balance skills including the following: 

• Unrehabilitated unilateral hearing loss in early development that promotes an “aural preference syndrome” where hearing is biased to one ear, compromising spatial hearing.^{63, 64}
• Combined impairment of both hearing and vestibular function with the prevalence of end organ specific dysfunction (otoliths and horizontal canal) ranging from 17%-48%.⁶⁵

This high prevalence of vestibular impairment again is likely related to the etiologies leading to unilateral hearing loss (i.e., cochlear nerve aplasia, cCMV, cochleovestibular anomalies, and sudden SNHL).^{66, 67} It is becoming clear that the role of this dual sensory impairment is underestimated in children with unilateral SNHL and needs to be considered as we measure outcomes following intervention (i.e., CI for single-sided deafness).

Exploring Rehabilitative Strategies

Different therapeutic approaches can be used for the rehabilitation of children with loss of vestibular sensitivity by capitalizing on the developing brain’s remarkable plasticity.

Children with SNHL and reduced/absent vestibular function may benefit from practicing balance strategies in various environmental contexts to prime their visual and somatic senses facilitating compensation. More specifically, a 10-day exercise program focused on activities of static balance can lead to significant improvement in standing balance duration in children with SNHL compared with untreated hearing-impaired controls.⁶⁸

While initial concerns were that CI may negatively impact vestibular function and balance, evidence exists that CI may actually positively influence balance function. Small improvements in performance have been documented on computerized dynamic posturography (CDP) in some individuals following CI activation.^{45, 3} In addition, children with SNHL and implants perform better on standardized test of balance function (BOT-2 balance subset) with their CI on versus off⁵⁷, and this benefit is achieved both in settings of directional sound as well as nondirectional white noise.⁶⁹ Although we recognize the importance of visual, somatosensory, and vestibular cues in the maintenance of balance, the contribution of hearing is rarely considered. A theoretical possibility is that balance improvement is related to extra-cochlear spread of current. Electrical activity from the CI is known to reach the vestibular end organs. Vestibular evoked myogenic potentials (VEMPs), a measure of otolithic function, can be elicited with electrical stimulation in children with CI. The presence of electric VEMPs in acoustically nonresponsive ears, along with their shorter latencies, suggests that electrical current can bypass the otoliths and directly stimulate vestibular neural elements.⁷⁰ Additionally, the perception of visual vertical can be improved in the presence of CI stimulation.⁷¹ Building on this principle, the functionality of the intracochlear electrode array may be used to provide head-referenced cues to improve balance.⁷² Others have been working to more directly activate the vestibular end organs using separate electrode arrays targeting the posterior labyrinth.^{73-75, 76-81} The hope is that ongoing advancements in all strategies will provide a number of potential therapeutic options to address the balance impairment due to vestibular dysfunction.

Summary

Vestibular impairment and balance deficits are the most frequent co-existing clinical features in children with SNHL. These dual sensory impairments often result from a shared etiology but can also occur as a side effect of CI. Recognition of vestibular deficits in children who are deaf is vitally important as they have impact on motor development, balance skills, and safety. These vestibular deficits increase the risk of implant failure and negatively affect cognitive development. Early identification and rehabilitation of these vestibular and balance deficits are necessary to optimize function over the course of a lifetime. 

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