Role of age and middle ear transmission characteristics in the frequency tuning of cVEMP and oVEMP

Project Description Ageing causes a decline in the functioning of most sensory systems, including the vestibular system. Decrements in vestibular function are an independent risk factor for falling. Therefore, there is a need for efficient vestibular diagnostic tests that identify clinically meaningful vestibular impairments in older patients. Background: Age related degenerative changes of the vestibular system occur in the vestibular end organs as well as the central vestibular structures. The vestibular end organs: saccule and utricle can be assessed using cervical evoked myogenic potential (cVEMP) and ocular vestibular evoked myogenic potential (oVEMP) respectively. The cVEMP and oVEMP are some of the most widely used vestibular diagnostic assessments; however previous works have showed that both responses, when elicited using a conventional and widely used stimulus of 500 Hz, are often bilaterally absent in older adults yielding results that are difficult to interpret. Therefore, there is a need to identify a stimulus that could best assess the older adult’s saccular and utricular functioning. Objective and methods: The purpose of this study was: 1) to assess age-related changes on various stimuli that can be used to assess cVEMP and oVEMP, and 2) to determine the optimal stimulus frequency in the young adult, middle age, and older adult populations. Additionally, we performed middle ear evaluation, to delineate age related changes in the middle ear from changes occurring in the inner ear of balance. A total of 107 participants consented to be in the study. Participants were divided in three groups: young adults (18-30 years), middle aged adults (41-59 years) and older adults (60-80 years). All of the participants underwent cVEMP and oVEMP testing using 500 Hz, 750 Hz and 1000 Hz air conducted stimuli. Middle ear functioning was also calculated for each of the participants. Results and Conclusion: It was found that age did not cause any change in measures of middle ear functioning. However, age did alter the saccular (cVEMP) and utricular (oVEMP) responses and this age effect was dependent on the stimulus frequency. The strength/amplitude of the cVEMP decreased with increasing age for the more conventional frequency (500 Hz), but the age effect was not observed at the higher frequency of 1000 Hz. Further, for the older adults, cVEMP was largest in amplitude and more often present at 1000 Hz. This differed from the young adult group where 500 Hz was the best frequency to elicit a cVEMP. For the oVEMP, the strength/amplitude decreased with increasing age for all the three stimuli/frequencies and the best stimulus frequency in all the three age groups was 1000 Hz. We also found that middle ear did not contribute towards shaping the response of cVEMP and oVEMP for various stimuli. Thus, the changes in the cVEMP and oVEMP frequency tuning observed in the older adults may not be stemming from middle ear but could be due to age related changes in the vestibular system

Amplitude-modulated-cVEMP: A novel method to assess vestibular functioning in aging population

The aging population is one of the pressing health concerns today. A major concern with increasing age is risk of falls, as falls are one of the leading causes of injury and death in older adults with an associated large economic cost. Vestibular system (ear related balance functioning) is an independent risk factor for falls among older adults. The primary role of the vestibular system is to maintain head and eye coordination, upright posture and balance, and conscious realization of spatial orientation and motion. The field of hearing and vestibular (ear related balance functioning) science has shown tremendous advancement in the last 30 years, however, our ability to diagnose and treat vestibular impairment is limited. Limitation in the assessment of vestibular system leaves 20-30% of patients with vestibular complaints undiagnosed. Thus, there is a need to develop techniques to study vestibular mechanisms that cannot be tapped using existing methods. Our study is an attempt to explore a novel aspect of vestibular functioning that cannot be done using existing assessment techniques. Our study utilized a novel stimulation (amplitude modulated tones) and recording paradigm: amplitude-modulated-cVEMP (AMcVEMP) to characterize untapped mechanisms (phase synchrony of vestibular nerve and non-linearity of vestibular system) of the vestibular functioning. We aimed to study the effects of age on the phase synchrony and the non-linearity of the vestibular system using AMcVEMP. We included 73 human participants from three different age groups (young = 30 participants, middle age = 22 participants, older adults = 21 participants) to understand effects of age on the phase synchrony and non-linearity of the vestibular system. Our data showed evidence of phase synchrony and non-linearity in the human vestibular system of all age groups. Our study also showed alteration in phase synchrony and non-linearity of vestibular system as an effect of aging. Phase synchrony of vestibular nerve and nonlinear processing in the vestibular system has been reported in many animal models, but there was no non-invasive method to assess this mechanism in human vestibular system. AMcVEMP is the only non-invasive tool to assess these mechanisms from human vestibular system. Ability to record phase synchrony and non-linearity from vestibular system can advance our understanding of the mechanisms of vestibular functioning in healthy and vestibular malfunctioning in disordered populations. Overall, the outcomes from the study have improved our understanding of age-related changes in the vestibular system and has opened areas for exploration that will eventually lead to better standard of care for older adults and patients with dizziness

Magnitude estimates of angular motion: Perception of speed and displacement across vestibular and visual modalities

Both the vestibular system (e.g. VOR) and optokinetic system (e.g. OKN) generate conjugate eye movements in response to either movement of the head or movement of the visual surround. Both systems help to maintain gaze stability. While the VOR is most sensitive to input frequencies above .2 Hz, the oculomotor system helps maintain gaze stability at lower frequencies. Psychophysics is the study of the relationship between perception and a physical stimulus. Previous research on perceptual thresholds across the two sensory modalities shows that there are frequency-dependent differences between vestibular and visual perception. The purpose of this study is to extend previous vestibular psychophysics work by measuring magnitude estimates of speed and angular displacement in response to suprathreshold stimulation from either vestibular or visual stimuli. Participants were exposed to 12 experimental conditions of angular rotation of varying frequencies, peak velocities, and acceleration rates across both sensory modalities. Vestibular stimulation was provided with a rotary chair and visual stimulation with equivalent conditions but under a virtual reality headset. Participants provided magnitude estimates of their speed and angle of displacement. Results reveal that vestibular and visual perception of suprathreshold motion stimuli differ as a function of frequency and agree with perceptual threshold data

Age predicts the absence of caloric-induced vertigo

Introduction: The absence of vertigo during the caloric test, despite a robust response, has been suggested to represent a central vestibular system phenomenon. The purpose of this investigation was to determine the prevalence of absent caloric-induced vertigo perception in an unselected group of patients and to assess possible predicting variables. Methods: Prospective investigation of 92 unselected patients who underwent caloric testing. Inclusion criteria were that each patient generate a maximum slow phase velocity (maxSPV) ≥ 15 deg/sec and a caloric asymmetry of ≤10%. Following the caloric, patients were asked, “Did you have any sensation of motion?” Results: Results showed 75% of patients reported motion with a mean age of 56.51 years compared to a mean age of 66.55 in the 25% of patients reporting an absence of motion. A logistic regression was performed and the overall model was statistically significant accounting for 29% of the variance in caloric perception. The significant predictor variables were patient age and maxSPV of the caloric response. The effect size for both variables was small with an odds ratio of .9 for maxSPV and 1.06 for age. Conclusions: The current investigation showed that both age and maxSPV of the caloric response were significant predictors of vertigo perception during the caloric exam. However, the association between age and caloric perception is not conclusive. Although there is evidence to suggest that these findings represent age-related changes in the central processing of vestibular system stimulation, there are additional unmeasured factors that influence the perception of caloric-induced vertigo

The Clinical Significance of the Failure to Perceive Vertigo in the Postcaloric Period Despite a Robust Caloric Response

Purpose The purpose of this project was to explore the association between the perception of motion during caloric testing and two tasks associated with central vestibular processing: postural stability and visuospatial memory. Method This was a prospective study of 25 patients who were found to have nonvestibular etiologies of their symptoms and normal vestibular function test results and who underwent caloric testing with a mean maximum slow phase eye velocity for each irrigation of 15° or greater. Following each caloric irrigation, patients were asked whether they had any sensation of movement. Patients were grouped based on the presence or absence of motion during the caloric exam (motion perception vs. absent perception). Postural stability was assessed using computerized dynamic posturography, and visuospatial memory was assessed using a memory match card game application. Results There were no significant differences between groups on any measures of peripheral vestibular function. However, the Absent Perception Group showed greater postural instability during Condition 5 of posturography and performed significantly worse on a task of visuospatial working memory. Both age and absence of motion perception predicted abnormal performance on measures of postural stability and visuospatial working memory. Conclusions There appears to be clinical implications to a lack of motion perception during the caloric exam in patients with an otherwise normal peripheral vestibular system. Based on the current findings, we are unable to determine whether differences in postural stability and visuospatial memory were due to age or a central vestibular processing deficit