Editorial on the Research Topic
Age-Related Vestibular Loss: Current Understanding and Future Research Directions
This Research Topic reflects the collective work of 44 authors from around the world
yielding 11 thoughtful and provocative publications. Several themes have clearly emerged
from this body of work, which help to establish where we are in understanding age-related
vestibular loss and the fundamental research gaps that we must address. First and
foremost, it is clear that we are dealing with a topic of tremendous public health
significance. The global population is aging, and age-related degeneration of the
vestibular system is a widespread phenomenon that occurs as part of the normal aging
process. Older individuals disproportionately experience falls, which are a disastrous
event associated with tremendous morbidity and early mortality. It is known that the
vestibular system contributes to fall risk; however, the extent to which vestibular
loss contributes to falls in older adults is not precisely known (and may differ across
individuals). Moreover, although falls are highly common and age-related vestibular
loss is widespread, vestibular therapies such as vestibular rehabilitation are seldom
offered to the large number of older adults presenting with falls in the primary care
setting. Much work needs to be done to provide a strong, quantitative evidence base
for the causal relationship between vestibular loss and falls in older adults and
the benefit of vestibular therapy.
A second theme that follows from the first is the clear need for efficient clinical
tests that identify clinically meaningful vestibular loss in older adults. The vestibular
system is a highly complex structure that encompasses five peripheral end-organs and
widespread central connections through brainstem nuclei, the cerebellum, the thalamus,
vestibular cortex, and hippocampus. The function of the vestibular system can be probed
at many levels, based on anatomy (e.g., canal vs. otolith, peripheral vs. central),
based on level of analysis (e.g., cellular neurotransmitters vs. cortical networks,
reflex vs. perceptual testing), and based on functional behaviors (e.g., gait vs.
spatial orientation). Moreover, in the context of aging, vestibular loss is typically
one of the multiple concomitant deficits that may be occurring and contributing to
a given clinical phenotype (e.g., dizziness, imbalance, and falls). Specifically,
older adults may also have deficits in proprioception, vision, hearing, and muscle
strength. Further, even if sensorimotor function is relatively intact, older adults
may have deficits in central integration of these various sensory signals to generate
a coherent motor output. Additionally, older individuals may compensate to varying
degrees for their deficits, such that inadequacies of compensation may also contribute
to the clinical picture. It is, therefore, critical for useful clinical tests to disambiguate
the various layers of potential contributing factors (i.e., primary sensorimotor deficits,
deficits in central integration, and deficits in compensation) within a given older
adult, with the goal of providing “personalized” strategies to improve balance function.
The publications in this Issue highlight numerous vestibular assays that differentiate
older from younger individuals. Chau et al. observed reduced vestibulo-ocular reflex
responses to rotational stimulation among older adults with dizziness. Bermúdez Rey
et al. observed increasing vestibular perceptual thresholds beginning at the age of
40, and the authors also found that higher roll tilt thresholds were associated with
poorer postural stability. Chiarovano et al. reported that older adults with postural
instability did not experience a rotation perception during warm caloric irrigation.
They termed this phenomenon “vestibular neglect,” and suggested it arose from reduced
central responsivity to peripheral stimulation. Maheu et al. provide a review of age-related
differences in performance on the standard clinical vestibular tests. Several studies
also considered how aging might lead to deficits in central vestibular processing.
At a molecular level, Smith discussed the differences in neurotransmission occurring
at the level of the vestibular nuclear complex in older vs. younger animals. Xie et
al. showed poorer performance among older adults on a test of spatial navigation,
the triangle completion task, relative to younger individuals. Arshad and Seemungal
reviewed several recent studies that reported reduced connectivity of central vestibular
networks associated with increased age. Two studies specifically evaluated whether
compensation for vestibular loss differs between young and old adults. Vestibular
compensation relies on central mechanisms (cerebellar, brainstem, striatal, etc.)
and may thus be a measure of central nervous system rather than vestibular function.
Scheltinga et al. reported that older adults with acute unilateral vestibular loss
(aUVL) experienced greater balance impairments compared to younger individuals with
aUVL. Moreover, the balance impairments in older individuals took longer to improve
and were less likely to resolve. Anson et al. examined how age influenced the generation
of compensatory saccades following horizontal head impulse testing. They observed
that increased age was associated with larger compensatory saccade amplitude, even
after accounting for underlying VOR gain.
These studies all assess vestibular function at a different level of analysis. However,
the “holy grail” would be a test that provides a comprehensive snapshot of the extent
to which the observed vestibular impairment contributes to the individual’s symptoms
relative to other impairments, and the individual’s level of compensation. As stated
by Fernández et al., “Reaching a complete, meaningful, and treatment-oriented diagnosis
in elderly dizzy patients remains an important challenge for even the most experienced
clinician.” New techniques and assays will likely be needed to accomplish this task.
For example, postural assessment is becoming increasingly powerful through frequency-based
analyses and analyses of complexity. It is possible that a simple postural test can
determine the source of an individual’s sway (e.g., reduced proprioception vs. vestibular
function) based on the sway frequencies observed. Moreover, as discussed by Anson
and Jeka, assays that are more ecologically valid (e.g., accelerometry deployed during
routine daily activities) may be more sensitive at detecting deficits that manifest
dynamically during daily activities but that may not be evident in a clinical laboratory.
Finally, assays that measure central vestibular processing and compensation may prove
useful in understanding the clinical picture and also designing therapy. Currently,
functional imaging studies provide insight into these processes, although other measures
that can be more easily scaled may prove more useful in the clinical setting.
This research topic clearly highlights a growing awareness of age-related vestibular
loss, and its potentially far-reaching impact on the public’s health. Indeed, efforts
are currently underway by the Barany Society’s International Classification of Vestibular
Diseases to codify diagnostic criteria for age-related vestibular loss. As discussed,
age-related vestibular loss is a complex phenomenon to manage clinically and study
scientifically, largely owing to the complexity of the vestibular system as well as
the complexity of the aging process. This is both a challenge and an opportunity to
harmonize our conceptual framework, standardize our assessment tools, and catalyze
innovation and new developments in this field.
Author Contributions
The author confirms being the sole contributor of this work and approved it for publication.
Conflict of Interest Statement
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.