Due to the increased fall risk in the elderly population, the assessment of balance function is of high concern. The cause of a patient's balance problem may not be due to the vestibular system exclusively. However, there is an age-related decline in the vestibular system which makes stable balance more difficult. Vestibular hair cells begin to deteriorate at birth and there is a 20% reduction in the number of hair cells by age 70–95 [1▪▪]. Even though we are steadily losing vestibular hair cells as we age, there is a reduced capacity by the time an individual enters the elderly population. This article aims to provide insight into these age-related declines and how they are represented in various vestibular tests.
As an individual progress from middle age to elderly, they are more likely to develop balance problems. Although the vestibular system plays a role in overall balance function, it is not the only system to contribute. Multiple systems such as vision, proprioception, postural motor, and vestibular, need to be effectively integrated in order for an individual to be balanced [1▪▪]. One vestibular test used to examine the functional balance capability of an individual is computerized dynamic posturography (CDP). This balance assessment examines the sway of a patient while they perform a variety of conditions. It evaluates the functional capacity of balance which includes the vestibular pathway and also visual and proprioceptive contributions to overall balance .
A patient's ability to integrate the information of these systems can be influenced by sub-optimal central processing. While performing balance testing, the elderly population attends more to visual information due to their reduced proprioception and vestibular input [3▪]. They attend more to visual information, even when this information is altered, such as in CDP testing. The middle-aged population does not show this increased attention on visual information for sensory integration [4▪▪]. There was no difference between the middle-aged and elderly population when examining the sway of individuals [4▪▪]. However, there was a greater activation of cortical structures in the elderly, which indicate more resources were needed for sensory integration [4▪▪]. This may be due to reduced proprioceptive input, which required the individual to rely on other cortical structures to maintain a similar balance as the middle-aged population [4▪▪]. During this sensory reweighting process for stable balance, more cortical regions of the brain were more distributed for the elderly population [4▪▪]. This greater distribution of cortical activation indicates more attention was required from the individual when performing balance tasks [4▪▪]. Older adults may not show more sway during balance testing, but they require increased concentrated and brain activation to the balance tasks, than a younger population.
White matter changes, which can be seen in the elderly population, also influence balance function. These white matter changes, such as white matter hyperintensity, are a general finding in patients over the age of 60 and may disrupt the cortical–subcortical connections that are needed for motor control and balance . CDP was determined to be more effective than videonystagmography (VNG) in determining the balance dysfunction for individuals with central white matter changes . The CDP test may be more effective in this regard as it evaluates the integration of multiple senses, whereas VNG testing only evaluates the function of the horizontal semicircular canal. For the elderly population, CDP or other balance function tests are important in the vestibular test battery to determine the overall functional balance capability instead of only evaluating the physical capabilities of the vestibular system.
VESTIBULAR EVOKED MYOGENIC POTENTIAL
The vestibular evoked myogenic potential (VEMP) can be used to evaluate the function of the otolith structures and corresponding neural pathways. The cervical VEMP (cVEMP) evaluates the saccule and inferior vestibular nerve function, whereas the ocular VEMP (oVEMP) evaluates the utricle and superior vestibular nerve function. These responses are obtained by presenting an air or bone conducted sound and measuring the ipsilateral sternocleidomastoid muscle (cVEMP) or contralateral inferior oblique muscle (oVEMP) electromyography response. This is a fairly new test of vestibular function which was approved by the US Food and Drug administration in 2015 [6▪]. There have been many studies evaluating the various vestibular disorders which can be identified with these tests and the responses obtained in the elderly population. One study examined cVEMP and found the threshold for this response increased with increasing age, particularly for individuals aged 50 and above . They also determined the cVEMP amplitude decreased with increased age, which was corroborated with other studies . Bilaterally reduced or absent VEMP responses have been identified in patients over 60 years of age; however, there can be a large range of responses which can muddy the clinical picture of the patient's vestibular function . Even with a variety of responses obtained from the patient, a significant asymmetry between ears can be used to determine if there is otolith dysfunction . The latencies of VEMPs have been determined to be stable in the elderly population [1▪▪]. With the use of vibration induced VEMP responses, the reduction in amplitude will occur at a later age in the elderly population, compared with air conducted responses [1▪▪].
VEMP testing examines the vestibular system but also extends to cortical structures. VEMP responses have been shown to activate the cortical regions of the bilateral superior and temporal gyri, posterior insular gyri, and parietal operculum [1▪▪]. This study found an age dependent decline in the vestibulocollic response and how the frontal and cerebellar deactivation patterns decrease with age [1▪▪]. This information furthers the understanding of VEMP testing as more research is needed to determine what is stimulated with VEMP responses and how far that stimulation extends cortically.
As the VEMP response is sensitive to the intensity of the sound of stimulation, the degree of hearing loss of the patient is a factor to be considered. Patients with conductive loss can have absent VEMP responses using an air conducted sound. The elderly population is more likely to have hearing loss, mainly in the high frequencies, due to presbycusis. One study analyzed the relationship between degree of hearing loss and the presence of cVEMP responses. They determined latencies were delayed when the patient had hearing loss, with the most delayed latency occurring for the individuals with more severe hearing loss [9▪▪]. The amplitude also decreased based on the patient's increased hearing loss [9▪▪]. This study was able to observe vestibular deficits before an individual presented with hearing loss, but had a history of noise exposure [9▪▪]. These individuals had a delayed VEMP latency with normal hearing [9▪▪]. Based on this research, the saccule can be damaged and an abnormal VEMP can be observed with more than 40-dB HL hearing loss at 4000 Hz [9▪▪]. In addition, as the elderly population already has hearing loss or is likely to develop it soon, they are at increased risk for further damaging their hearing due to the very loud air-conducted sound needed to elicit the response [10▪]. There may not be much diagnostic yield in performing this test in patients over the age of 70, as they may not have a response and the clinician may be causing further hearing loss due to exposure to loud sounds in an already damaged cochlea.
VIDEO HEAD IMPULSE TESTING
The video head impulse test (vHIT) is used to evaluate the vestibular ocular reflex (VOR) and all six semicircular canals. The function of the semicircular canals is determined by examining the VOR gain and presence of catch-up saccades during head movements of 150–300°/s. There are some changes in VOR gain with increased age. When testing the horizontal semicircular canal, there is decreased VOR gain after age 71 [11▪]. During testing of the posterior semicircular canal, there is a significant decrease in VOR gain after age 80 [12▪▪]. For testing of the anterior semicircular canal, there is a gradual decrease in VOR gain after age 80, but it was not statistically significant [12▪▪]. However, all vHIT VOR gains for all semicircular canals were still within normal limits for ages 71 and above [12▪▪]. For individuals aged over 80, there is an age-related decline in VOR gain, but they do not have any balance complaints . This indicates that even though the elderly population has decreased VOR gain, indicating reduced semicircular canal functioning, it does not decrease to an abnormal level.
Recently, there has been another use for the vHIT, specifically for older adults. One study evaluated the function of the 10 s tandem leg stand with catch-up saccade amplitude and latency during vHIT [14▪▪]. The study revealed catch-up saccades during vHIT can be used to determine low levels of vestibular impairment in older adults, by comparing the participants performance on the 10 s tandem leg stance [14▪▪]. The tandem walking test can be used as a screening for older adults with vestibular loss [14▪▪]. Results revealed higher saccade amplitude and shorter saccade latency were associated with an inability to perform the tandem leg stance [14▪▪]. These compensatory saccades are part of the occulomotor gaze-stabilization strategy which compensates for a deficient VOR and vestibular system [14▪▪]. The study introduces a new way to use vHIT specifically for older adults; however, more research is needed to adapt it for clinical use.
Another use of vHIT is in the emergency department (ED). Ischemic stroke can mimic vestibular disease in the acute stage [11▪]. vHIT has been gaining use in the acute setting to determine if a patient's symptoms are due to peripheral or central causes. This is useful with posterior inferior cerebellar artery strokes as the patient will present to the ED with vestibular symptoms but have a normal vHIT [15▪]. During the first 48 h, an MRI has a sensitivity of 80–85% in determining ischemic stroke, whereas the vHIT has a sensitivity of 88% [11▪]. This would allow providers to more quickly identify if a patient's symptoms are peripheral or central in nature. MRI testing is also more expensive and can be uncomfortable for the patient. The one weakness with vHIT testing in the ED is that to perform the test accurately, the tester needs to be trained and practiced in delivering accurate head impulses.
There has been discussion as to whether vHIT can replace other testing in the vestibular test battery. When determining a bilateral vestibular loss, both vHIT and rotary chair can be used. For patients with abnormal low-frequency loss, the vHIT was normal [16▪]. However, when the vestibular loss extended to the high frequencies, the vHIT then became abnormal [16▪]. The combination of rotary chair and vHIT can help to determine the degree of bilateral vestibular loss and ensure no patient is left undiagnosed [16▪]. There are also differences between vHIT and caloric testing. vHIT is abnormal if there is a 40% caloric weakness or greater [11▪]. Performing vHIT alone will miss patients with a caloric weakness of 25–40%. The vHIT cannot eliminate the caloric test as calorics are needed if the vHIT is normal, to determine the presence of a milder weakness [17▪]. vHIT does have the ability to test vestibular function over a wider range of frequencies which are more typical during everyday head movement, whereas caloric testing evaluates a very low frequency. Although vHIT adds more information on the vestibular system of the elderly patient, which may overlap with other vestibular tests, it cannot eliminate them and adds more information to the clinical picture.
There are other vestibular assessments which have been used for patients for many years and are more firmly established. Rotary chair is one vestibular test used to determine the presence of vestibular weakness across a range of frequencies (0.01–0.32 Hz). This test is widely used to determine bilateral vestibular weakness and if the weakness is due to a central or peripheral dysfunction. There have been some differences in the sinusoidal harmonic acceleration test between different patient populations, with the elderly population having an increased phase compared with younger populations . However, rotary chair can provide some advantages for the elderly population. During rotary chair testing, the patient is strapped into a chair and fixed firmly in one position. There are no sudden jerks of the patient's head, but instead the entire body is rotated. This position may be more comfortable for elderly patients, especially those with cervical limitations, where vHIT could not be performed . This would allow the clinician to determine the presence of a bilateral vestibular weakness, but rotary chair testing alone would not able to determine if there is a unilateral vestibular dysfunction. This weakness of rotary chair testing leads into the most wide-known and ‘gold standard’ test for clinicians, caloric testing.
The caloric test is the oldest vestibular test to determine a unilateral peripheral dysfunction. It compares one horizontal semicircular canal to the other at one frequency (0.01 Hz). This test is still widely used and is a staple of vestibular testing because it has been extensively researched and is a widely recognized way of identifying a peripheral vestibular dysfunction. Caloric testing also helps the clinician determine how severe the dysfunction can be at one test frequency. While performing this test will not result in any decrease of response for the elderly population , it can still be very uncomfortable and anxiety provoking for the patient. Although the results of this test are heavily relied on for further management and treatment of the patient, it is still only providing information on two vestibular structures (right and left horizontal semicircular canal).
Each of the vestibular tests has their strengths and weaknesses, but they all are used to provide a picture of the patient's vestibular functioning. As more research and vestibular testing have been completed over the past few years, clinicians are able to evaluate more vestibular structures than in the past. Some tests provide more diagnostic yield when testing elderly patients. Balance function testing provides information on the functional balance capabilities for the patient which is important to test on the elderly patient due to their increased fall risk. VEMP testing can assess the saccule and utricle, but will not be as beneficial for the clinician as most elderly patients will have decreased or absent responses. vHIT, rotary chair, and caloric testing all provide information on vestibular structures which is not redundant, but instead complimentary to each other. The vestibular test battery should include all vestibular testing available based on the patient's specific complaint and limitations. Each test provides insight into various structures and processing capabilities which are needed to formulate the best treatment plan and enable the elderly patient to lead a safe and active lifestyle.
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Conflicts of interest
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REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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