Editorial: Age-Related Vestibular Loss: Current Understanding and Future Research Directions

摘要

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 vestibu