Age and vestibular function

Timothy C. Hain, MD

Return to Index. • modeling bilateral damage page •Last revision: March 7, 2021

It is well accepted that inner ear function -- including both hearing and balance declines with age. The reason for this is that the ear wears out and has no repair mechanism. It contains moving parts - -hair cells -- which gradually die off with age. It is estimated that half of inner ear function -- and in particular vestibular ganglion cells -- are lost by the age of 80. Counts of vestibular ganglion cells probably oversimplifies the entire functional deficit.

Model

This page is an attempt to document the effect of age on vestibular responses, as documented in the literature. There are also other pages on this site that comment on the effect of age, and we will reference them when appropriate.

Vestibular testing

There are presently 3 well established measures of vestibular function - - caloric testing, rotatory chair testing, VEMP testing, and VHIT. One of these measures otolith function (VEMP), while the other all measure semicircular canal function.

By total vestibular output we mean: the total ocular response for a given change in head velocity. This is not the same as the peak eye velocity, as the peak is a response at a particular time, that does not account for responses prior to and following the peak. The total response requires adding up all of the eye movement output over time.

Lets consider how well current tests do in estimating total vestibular function as people age

Caloric total response --is it the same as the total vestibular function ?

The main caloric parameter, peak slow phase velocity, is very little affected by age. Furthermore, there is a caloric parameter called the "total response", consisting of the sum of 4 peak velocities. . One might argue that the total response of caloric esting provides a “total vestibular output” parameter, However, this is a peak velocity measurement, occuring roughly at a single time point, and thus is not the same as the entire caloric output, which would require considering all eye movement elicited by the caloric stimulus. Or to put this in another way, when lifting weights, one would not confuse the maximum speed with which someone lifts a weight to the amount of energy that one expends while lifting the weight up and putting it back.. Furthermore, the caloric input corresponds to very low frequencies of vestibular stimulation, analogous to 0.003 Hz (Jacobson, Newman, & Peterson, 1997). Thus the VNG doesn’t cover the entire frequency range of the vestibular response.

Nevertheless, a normal young person has roughly 100 deg/sec of total response. This corresponds roughly to 100% vestibular function (abeit for low frequencies alone). Here the term "function" is an important one, as this presumably reflects the combination of hair cells, nerve, and central plasticity all cascaded. One might lose one ear, dropping one's total response from 100 to 50, and then have a compensatory process proceed that raises the response on the remaining ear to 75. Thus the caloric total response might reasonably underestimate peripheral vestibular damage. From a systems perspective, this is oversimplified. Caloric responses probably are mainly a measure of the low-frequency vestibular response. We don't understand entirely the mapping between total caloric responses, and vestibular loss as quantified by the product of [hair cell][vestibular nerve]. This could be worked out by simply comparing total responses in patients with well documented unilalteral loss, such as due to tumors or severe vestibular neuritis.

Rotatory chair -- gain and time constant. Moderately affected by age.

This subject is discussed on the page of the age dependence of norms for rotatory chair testing. Both gain and time constant decline with age, the gain just slightly, and the time constant more substantially.

VHIT testing: Little affected by age, and not a logical proxy for age either.

The VHIT test is little affected by age. It has a number of other limitations making it intrinsicially different than the caloric total response or the Rotatory chair Gain-TC product. The VHIT device has no total vestibular response parameter. This is because it measures velocity rather than position. Furthermore the VHIT is limited to high frequencies of vestibular stimulation, predominantly at 2.5 hz (McGarvie, Curthoys, MacDougall, & Halmagyi, 2015). VHIT testing doesn't correlate very well with vestibular loss. It is a reasonable test for unilateral loss however.

VEMP testing: Greatly affected by age.

VEMPs assess the otoliths, not the semicircular canals, and thus are "something completely different". As VEMPs have an "apples/oranges" type input-output relationship -- sound input, muscle contraction output, VEMPs have no direct relationship between vestibular function in the sense of matching a stimulus to a response. The most you can say is that they are something measurable related to the ear that can be quantified at different ages. Both cVEMP tests and oVEMP tests decline greatly with age.

Vestibular reserve and age

As a general rule, most sensory and neural systems peak in humans at the age of 2, and it is downhill from there on. There is some integration and connections between neurons that gets adjusted, but the number of cells slowly declines. Furthermore, because most neurons in the human brain (and inner ear) do not regenerate, when you lose a neuron or hair cell, it is gone forever. Discouraging but reality.

As people develop better balance up through early adulthood, clearly there are other factors at work. People become better at using what they have -- learning and plasticity. There is also considerable redundancy in sensory systems. We have two of several important sensors-- two eyes, two ears, two feet -- and often we can lose one of them without entirely losing our ability to work and function in the community. Similarly, with the inner ear balance function, most people continue to work after losing half of their vestibular function to a process such as vestibular neuritis . Thus there may be a 50% vestibular reserve -- up to which people do fairly well, and after losing more function, they perform more poorly in their work and activities of daily life.

Against this idea is the well known observation that the timing of their vestubular responses changes - -persons who have lost half of their vestibular input perform worse with low-frequency stimuli. An example of this is here (look at rotatory chair responses).

Practically however, most persons who age continue to have close to normal rotatory chair responses (and caloric responses). On the other hand, otolith related responses (e.g. VEMPs) deteriorate much more rapidly. This may have to do with redundancy in the vestibular apparatus -- all of the sensors in the canals "do the same thing". On the other hand, the otolithic sensors do something different things as they are "circularly polarized", and are also far more vulnerable. If we say (roughly) that the otoliths have to resolve 360 degrees of motion while the canals only have a single axis of rotation to deal with, the otoliths have far less neural circuitry to lose.

Following along with this line of thought, the semicircular canals should be resiliant to damage and age because each of the 3 semicircular canals has lots of hair cells that do roughly the same thing. You can lose 50% of your one of the three canals hair cells, and after some readjustment, carry on fairly well with the other 50%, because they all do the same thing (this is an oversimplification).

On the other hand, the otoliths should be vulnerable to damage and age, because the utricle and saccule have to resolve 360 degrees of rotation. Their amount of redundancy is reduced by a factor of 360. A second reason that the otoliths should degrade is that their design is much more vulnerable -- hair cells with stones on the top, are subject to a lot more force (f=ma), than vestibular hair cells that are being pushed back and forth by fluid pressure.

Model

Pathology of vestibular aging.

Counting cells in the inner ear and in the vestibular ganglion is a method of estimating the input side of the Hair Cell and Vestibular Nerve in the graphic above. Ishiyama (2009) reviewed this question. Estimates of hair cell density suggest roughly a 50% decline in the total hair cells in the crista ampullaris comparing fetal cristae to those of older adults aged 71-95. This may be an overestimate, as unbiased stereology suggested a decrease of roughly 25% of hair cells in 90 year olds compared to a younger group (42-67). According to this author, animal studies have documented an age related decline of about 30% (in mice).

Ishiyama also discussed age-related loss of primary vestibular afferents and nerve fibers. They reported an age-related decline of 25% to 37%, comparing persons older than 75 to those 35 and younger.

If we apply this data to the diagram at the top, hair cells decrease in number to 75%, and vestibular nerve fibers also decline to 75%, with then the simple expectation that the "throughput" would be 0.75*0.75, or roughly 0.56. Lets just call it 50% reduction in sensory transmission, assuming that there is no redundancy. This assumption is likely wrong, as it would predict that (for example), caloric responses would be reduced by 50%. Other factors at work may be redundancy and plasticity (in the central circuitry).

There presumably is also some dimunition in central and oculomotor or other output systems with age, but this is difficult to quantify.

Compensation for aging

As the normal central vestibular system has an ability to partially compensate for about 50% loss, by the age of 80, a very rough guess would be that the vestibular ganglion and vestibular nucleus processes can be completely compensated for. In other words, if the vestibular nerve+hair cell transduction drops to 0.5, the central gain goes up to 2.0, making the whole process a wash. This fits well with the general observation that vestibular rotatory chair responses are modestly affected by age, and the gain is little affected at all.

This would leave the "healthy" 80 year old the problem of dealing with hair cell deterioration (counts are roughly 30% down by the age of 80). In other words, an entirely healthy person should start to develop functional vestibular problems (for high frequencies) a little before the age of 80. There are some logical problems -- while the system does "compensate" for high frequency gain, the low-frequency responses are lost. Thus a "compensated" person should not expect restoration of normal function. It is always better to have the original equipment.

How rapidly does it decline ? While there are many studies of hearing function, little is understood about vestibular decline. Cell counts of both nerve cells and hair cells are generally found lessened in older persons (see above). As neither hair cells or vestibular neurons regenerate, and both are subject to wear and tear, it seems highly likely that there is a gradual decline in vestibular function.

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