Temporal bone studies of the human peripheral vestibular system. Normative vestibular hair cell data

Age-related differences in parameters of vestibular evoked myogenic potentials

CONCLUSIONS

The statistically significant correlations between vestibular evoked myogenic potential (VEMP) parameters and age may be due to hair cell loss of the otolith organ and/or to degenerative changes of the vestibular neural pathway. These findings indicate that age should be taken into account when interpreting VEMP results. It is also important to determine a standard method for performing VEMP and a universal index for comparison among laboratories.

OBJECTIVES

VEMP, which measures the surface electric potential from the cervical muscle evoked by sufficiently loud sounds, is a useful tool to evaluate vestibule-colic reflex function. We have assayed the effect of age on VEMP results.

SUBJECTS AND METHODS

After excluding subjects with a previous history of dizziness, middle ear pathology, or other inner ear symptoms, a total of 97 healthy volunteers (194 ears) were included. All VEMP parameters were analyzed to find differences related to side and gender, as well as the relationship between age and each VEMP parameter.

RESULTS

Age was correlated with all VEMP parameters. Latency of p13, n23 showed a negative correlation and amplitude of p13-n23 showed a positive correlation with age. Differences between the right and left sides were not significant.

Quantal and nonquantal transmission in calyx-bearing fibers of the turtle posterior crista

Intracellular recordings were made from nerve fibers in the posterior ampullary nerve near the neuroepithelium. Calyx-bearing afferents were identified by their distinctive efferent-mediated responses. Such fibers receive inputs from both type I and type II hair cells. Type II inputs are made by synapses on the outer face of the calyx ending and on the boutons of dimorphic fibers. Quantal activity, consisting of brief mEPSPs, is reduced by lowering the external concentration of Ca2+ and blocked by the AMPA-receptor antagonist CNQX. Poisson statistics govern the timing of mEPSPs, which occur at high rates (250-2,500/s) in the absence of mechanical stimulation. Excitation produced by canal-duct indentation can increase mEPSP rates to nearly 5,000/s. As the rate increases, mEPSPs can change from a monophasic depolarization to a biphasic depolarizing-hyperpolarizing sequence, both of whose components are blocked by CNQX. Blockers of voltage-gated currents affect mEPSP size, which is decreased by TTX and is increased by linopirdine. mEPSP size decreases severalfold after impalement. The size decrease, although it may be triggered by the depolarization occurring during impalement, persists even at hyperpolarized membrane potentials. Nonquantal transmission is indicated by shot-noise calculations and by the presence of voltage modulations after quantal activity is abolished pharmacologically. An ultrastructural study shows that inner-face inputs from type I hair cells outnumber outer-face inputs from type II hair cells by an almost 6:1 ratio.

Structural and functional changes in the cristae ampullares of aged gerbils

We have reported that calretinin and calbindin staining of calyxes in the apical region of the cristae is reduced or absent in old gerbils (>or=35 months) that had normal numbers of hair cells [Kevetter GA, Leonard RB (2002) Decreased expression of calretinin and calbindin in the labyrinth of old gerbils. Brain Res 957:362-365]. Here we examine the ability of primary afferents in aged gerbils to carry a tracer injected into the vestibular nuclear complex to their terminals in the cristae. Calyxes throughout the cristae were well labeled in a young animal with such an injection. In the aged animals, many calyxes were only partially filled or not filled at all. In some cases labeled axons were also missing from the stroma underlying the missing calyxes. There is a strong correspondence between the region where the calyxes were not filled and the absence of calretinin immunostaining. To determine if afferents from the cristae are functionally abnormal, we recorded from their axons and attempted to activate them with natural stimulation. Among afferents that could be activated, we encountered many afferents that had spontaneous activity but could not be modulated with natural stimulation. When tested, the firing rate of these afferents could be modulated with galvanic stimulation, and/or they could be activated by pulsed electrical stimulation. We also encountered afferents that had no spontaneous activity. The presence of these axons was revealed by an injury discharge that could not be modulated with natural stimulation. When tested, these axons could be activated with pulsed electrical stimulation. In some instances we encountered two or more such afferents in a row, an event we have not seen in young animals. We suggest that the simplest explanation for these observations is that calyxes are being lost in old animals.

Age-Related Changes in Vestibular Evoked Myogenic Potentials

Vestibular evoked myogenic potentials (VEMP) in response to sound stimulation (500 Hz tone burst, 129 dB SPL) were studied in 1000 consecutive patients. VEMP from the ear with the larger amplitude were evaluated based on the assumption that the majority of the tested patients probably had normal vestibular function in that ear. Patients with known bilateral conductive hearing loss, with known bilateral vestibular disease and those with Tullio phenomenon were not included in the evaluation. It was found that there was an age-related decrease in VEMP amplitude and an increase in VEMP latency that appeared to be rather constant throughout the whole age span. The VEMP data were also compared to an additional group of 10 patients with Tullio phenomenon. Although these 10 patients did have rather large VEMP, equally large VEMP amplitudes were observed in a proportion of unaffected subjects of a similar age group. Thus, the finding of a large VEMP amplitude in response to a high-intensity sound stimulation is not, per se, distinctive for a significant vestibular hypersensitivity to sounds.

Regional estimates of hair cells and supporting cells in the human crista ampullaris

Regional estimates of type I and type II vestibular hair cells (HC) and supporting cell (SC) numbers were obtained from the horizontal crista ampullaris by using design-based stereology in human. Cristae were microdissected from temporal bones obtained post-mortem (N=16, age range 26-98 years). Three groups were made according to age: group 1, n=5, ages between 26 and 67 years, average age 51 years; group 2, n=4, average age 84 years; and group 3, n=7, average age 94 years. For group 1, the average total HC number was 8,005+/-214, corresponding to 4,119+/-107 type I HC, 3,886+/-117 type II HC, and 10,274+/-224 SC. The type I:type II HC ratio was 1.06+/-0.01, and HC density was 0.80 cells/100 microm2. For group 2, the average total HC number was 7,074+/-489, corresponding to 3,733+/-212 type I HC, 3,341+/-314 type II HC, and 9,321+/-858 SC. The type I:II HC ratio was 1.12+/-0.06, and HC density was 0.75 cells/100 microm2. For group 3, the average HC number was 6,009+/-327, corresponding to 3,380+/-223 type I HC, 2,628+/-235 type II HC, and 10,185+/-182 SC. The type I:II HC ratio was 1.34+/-0.10, and HC density was 0.63 cells/100 microm2. A significant decline in type I, type II, and total HC number and density was found in groups 2 and 3, with individuals exceeding the average human life span.

Normative data for P1/N1-latencies of vestibular evoked myogenic potentials induced by air- or bone-conducted tone bursts

OBJECTIVE

The response characteristics of acoustically elicited vestibular evoked myogenic potentials (VEMPs) largely depend on the stimuli applied. A tone-burst stimulation of 500 Hz seems to be clinically most appropriate because those VEMPs can be elicited at the lowest stimulus intensity possible. The aim of the present paper was to describe normative data for tone-burst evoked VEMPs.

METHODS

VEMPs of 64 healthy subjects were recorded ipsilaterally during air- or bone-conducted tone burst stimulation. The EMG of the tonically activated sternocleidomastoid muscle was recorded ipsilaterally by surface electrodes. Averages were taken for P1/N1-latencies and -amplitudes of male and female volunteers within 3 different age groups.

RESULTS

The latencies did not show any significant differences between female and male volunteers or between air- and bone-conducted stimulation. The latencies did also not show any significant difference among the 3 age groups. The limits for normal latencies (mean + 2 SD) are, therefore, 20.3 ms for P1 and 28.0 ms for N1. Although the P1/N1-amplitudes were decreased with increasing age, the tonic muscle activity was not significantly different between the age groups.

CONCLUSIONS

The present findings strongly suggest the evaluation of VEMP latencies by using normative values obtained exactly with the same stimulus parameters.

SIGNIFICANCE

Normative data as described in the present study are required to detect isolated saccular defects which are indicative of a vestibular disorder.

Effect of age on detecting a loss of balance in a seated whole-body balancing task

BACKGROUND

Most falls are attributed to a loss of balance without a quantitative definition of the term. It has been proposed that a loss of balance is detectable as an unusually large (anomalous) value of the system control error. The hypotheses were tested that age will not affect the detection of control error anomaly, or prediction of the associated compensatory response, in a challenging balancing task.

METHODS

Twenty healthy older adults were asked to sit and balance a chair over its rear legs for as long as possible. The dominant foot's ground reaction force and the chair's sagittal-plane acceleration represented the system input and output, respectively. Control error was the difference between actual and expected acceleration output from a self-identified forward internal model of the system. A control error anomaly was detected once the error crossed a threshold set at three standard deviations (3-Sigma) above the mean of baseline data. Results from five trials were compared to published results in 20 healthy young adults. FINDINGs. A control error anomaly was successfully detected in 91% of 91 older adult trials, statistically similar to the 92% success rate obtained previously in young adults. A response was predicted in 57% of the 77 older adult trials with responses, significantly less than the 92% obtained in the young adult trials (age effect significant: P<0.005).

INTERPRETATION

The condition leading to uncontrolled backward acceleration of the chair was reliably detected in both groups. While the young waited to respond to this condition, older subjects responded prematurely.

Estimation of the number of nerve fibers in the human vestibular endorgans using unbiased stereology and immunohistochemistry

The objective of this study was to obtain estimates of the number of nerve fibers in the human crista ampullaris and utricular macula from normal individuals using unbiased stereology and immunohistochemistry. Vestibular endorgans with the attached vestibular nerve stump were microdissected from the temporal bones. Specimens were divided into two groups. The first group (group 1, N = 8, age range, 68-98 years old, mean = 87 years) was fixed with paraformaldehyde and post-fixed with osmium tetroxide. The second group (group 2, N = 5, age range, 80-98 years old, mean = 86.6 years) was fixed with paraformaldehyde, immunoreacted with monoclonal antibodies against neurofilaments, and post-fixed with osmium tetroxide. The endorgans of both groups were embedded in resin and 2-mum thick sections were made. Estimates of the number of nerve fibers were obtained using an unbiased stereological method, the fractionator. The diameter distribution of nerve fibers was also obtained. The average number of fibers in the horizontal, posterior and superior cristae of individuals in group 1 (N = 14 cristae) was 1424+/-320 (CV = 0.22). The average percentage of small (less than 3 microm), medium (between 3 and 5 microm) and large (more than 5 microm) size fibers was 22.4%, 51.5% and 26.1%, respectively. In group 2 (N = 12), there was an average of 1792+/-99 (CV = 0.05) nerve fibers. The average percentage of small, medium and large size fibers was 22%, 51.2% and 26.8%. In the macula utricle from group 1, there was an average of 3026 nerve fibers (N = 2, ages 80 and 96 years old). There was an average 30.75% small, 56% medium and 13.2% large size fibers. In the utricular macula from group 2 (N = 3, ages 84, 92 and 96 years old), there was an average of 3715 nerve fibers. The average percentage of small, medium and large size fibers was 33.2%, 51.7% and 15.1%. The nerve fiber number in both groups is within the range of previous studies, however, the number of fibers in group 2 was significantly higher than that in group 1 (p = 0.01). This difference is likely due to increased sensitivity gained by the immunohistochemical staining of the axoplasm of nerve fibers in group 2. Results from the present study demonstrate the use of unbiased stereology and immunohistochemistry in human vestibular endorgans, as a reliable and efficient method to estimate the number of nerve fibers. These methods can be applied for studies of normal aging and pathological conditions of the vestibular periphery.

Quantitative Study Of The Vestibular Sensory Epithelium In Cochleosaccular Dysplasia

BACKGROUND

Cochleosaccular dysplasia is the most common pathologic finding seen in children with profound congenital sensorineural hearing loss. There has been no quantitative study on the peripheral vestibular system in cochleosaccular dysplasia.

OBJECTIVE

To investigate quantitatively the extent of pathologic changes of the vestibular sensory epithelium in cochleosaccular dysplasia.

SUBJECTS AND METHODS

Thirteen temporal bones with congenital deafness from 10 individuals were selected for this study from the temporal bone collection of University of Minnesota that showed suitable pathologic findings for the histopathologic criteria of cochleosaccular dysplasia. Age-matched normal control temporal bones were also selected. The vestibular hair cells including types I and II hair cells were counted separately in the saccular macula, utricular macula, and three cristae of the semicircular canals using Nomarski microscopy.

RESULTS

The hair cell densities of types I and II hair cells in the macula of the saccule in cochleosaccular dysplasia were significantly decreased compared with the data of normal subjects. Both types I and II hair cells in the utricular macula and the cristae of the three semicircular canals in cochleosaccular dysplasia were well preserved, and no significant difference was observed between findings of cochleosaccular dysplasia and normal controls in the utricle and the three semicircular canals.

CONCLUSIONS

In cases with cochleosaccular dysplasia, the neurosensorial hair cells of the saccule were affected; however, the osseous labyrinth, the membranous utricle, and the semicircular canals were normal. Further studies should be performed to establish the pathogenesis of cochleosaccular dysplasia in humans.

Comparative Morphology of Rodent Vestibular Periphery. II. Cristae Ampullares

We made flattened neuroepithelial preparations of horizontal and vertical (anterior and posterior) cristae from mouse, rat, gerbil, guinea pig, chinchilla, and tree squirrel. Calretinin immunohistochemistry was used to label the calyx class of afferents. Because these afferents are restricted to the central zone of the crista, their distribution allowed us to delineate this zone. In addition to calyx afferents, calretinin also labels ∼5% of type I hair cells and 20% of type II hair cells throughout the mouse and rat crista epithelium. Measurements of the dimensions of the cristae and counts of hair cells and calyx afferents were determined on all species. Numbers of calyx afferents, hair cells, area, length, and width of the sensory epithelium increase from mouse to tree squirrel. As in the companion paper, we obtained additional data on vestibular end organ dimensions from the literature to construct a power law function describing the relationship between crista surface area and body weight. The vertical cristae of the mouse, rat, and gerbil have an eminentia cruciatum, a region located transversely along the midpoint of the sensory organ and consisting of nonsensory cells. Apart from this eminentia cruciatum, there are no statistical differences between horizontal and vertical cristae with regard to area, width, length, the number and type of hair cells, and number of calretinin-labeled calyx afferents.

Hair cell numbers do not decrease in the crista ampullaris of geriatric gerbils

Among the geriatric population, dizziness and falling are serious problems. One system involved in balance that may change with age is the vestibular system. A common assertion is that the number of vestibular hair cells decreases as age increases. Our goal was to quantitate the number of hair cells in young and old gerbils and document the decrease. We used physical dissector design-based stereological procedures on serial 2-microm sections through the crista ampullaris. Between young and aged gerbils, there were no quantitative differences in the number, density, or types of hair cells or the length of the crista ampullaris. This lack of change in the number of hair cells suggests that the cause for vestibular dysfunction during aging must lie elsewhere.

Stereological study of postnatal development in the mouse utricular macula

This study describes the morphometric changes taking place in the utricular macula of mice with ages in geometric progression from 1 to 512 days after birth. By using design-based stereological methods, the total volume and surface area of the sensory epithelium as well the total number of the hair cells and supporting cells were estimated. Finally, the numerical density, volume density, and mean volume of the individual cell types were determined. The major changes were found in the number of the individual cell types during the first couple of weeks, and a mature composition of cell types was not attained until 16 days after birth. There was no change in the total number of cells and no decline in the number of hair cells within the time period studied.

Aging Effect on Vestibular Evoked Myogenic Potential

OBJECTIVE

Vestibular evoked myogenic potential (VEMP) is applied to explore the integrity of sacculocollic reflex. Although tests to evaluate vestibular-ocular reflex pathway have shown that vestibular function is adversely affected by aging, VEMP, in this study, is used as a novel test to define how aging influences sacculocollic reflex pathway.

STUDY DESIGN

Prospective study.

SETTING

Academic tertiary referral center.

SUBJECTS

Eighty normal subjects, equally divided into four groups according to their age, were enrolled to this study. Group I included patients aged <20 years, Group II patient ages ranged from 21 to 40 years, Group III patients were 41 to 60 years, and Group IV included patients older than 60 years.

INTERVENTIONS

Recordings of VEMP responses.

MAIN OUTCOME MEASURES

The response rate and parameters of VEMP, including p13 latency, n23 latency, amplitude, and interaural difference ratio.

RESULTS

The VEMP response rates from Groups I to IV was 98%, 98%, 90%, and 60%, respectively, disclosing a significant difference only between Group IV and other groups (p < 0.05). The amplitude was negatively correlated with age in contrast to the n23 latency, correlating positively with age; both reached a significant difference (p < 0.05). Although the p13 latency had a trend to prolong as age increased, no significant correlation existed (p < 0.06). Moreover, the interaural difference ratio was also not significantly correlated with age.

CONCLUSIONS

As age increased over 60 years, the VEMP response rate decreased dramatically. While age increased, the VEMP amplitude decreased in comparison to n23 latency prolonged. These findings might suggest that aging could deteriorate the saccular and corresponding neural functions. When interpreting the VEMP parameters, it should be kept in mind that aging could affect VEMP responses. Based on this study, we suggest establishing different reference values according to different age groups when evaluating VEMP response in patients with vestibular diseases.

Unbiased Quantification of the Microdissected Human Scarpa???s Ganglion Neurons

OBJECTIVE

The objective of the present study was to obtain unbiased estimates of the total number of Scarpa's vestibular ganglion neurons in individuals with normal vestibular function.

STUDY DESIGN

Application of unbiased stereology using microdissected human temporal bone specimens.

METHODS

Postmortem temporal bones were obtained from five young subjects with no history of audiovestibular disease (age range 42-49 years). The vestibular nerve containing the Scarpa's ganglion was microdissected, embedded in paraffin, and cut into 40 microm serial sections. Unbiased estimates of the total number of neurons were obtained using the optical fractionator technique of stereology.

RESULTS

An average of 23,599 (coefficient of variation [CV] = 0.11) vestibular ganglion neurons was obtained. There was no significant difference between the results obtained from the microdissected specimens and results that had been obtained from an age-matched group derived from a previously published report from our laboratory using archival human temporal bone specimens.

CONCLUSIONS

This study represents the first report to demonstrate the combination of the microdissection technique and the unbiased stereologic technique in the human temporal bone. This study demonstrates the reliability of the microdissection technique as an alternative method of human temporal bone processing for unbiased stereology. The utility of the microdissection technique is that specimens can be used for quantification, immunohistochemistry, and other powerful applications.

Comparative morphology of rodent vestibular periphery. I. Saccular and utricular maculae

Calyx afferents, a group of morphologically and physiologically distinct afferent fibers innervating the striolar region of vestibular sensory epithelia, are selectively labeled by antibodies to the calcium-binding protein calretinin. In this study, the population of calretinin-stained calyx afferents was used to delineate and quantify the striolar region in six rodent species: mouse, rat, gerbil, guinea pig, chinchilla, and tree squirrel. Morphometric studies and hair cell and calyx afferent counts were done. Numbers of hair cells, area, length, and width of the sensory epithelium increase from mouse to tree squirrel. In the mouse and rat, calretinin is found in 5-9% of all type I hair cells, 20-40% of striolar type II hair cells, and 70-80% of extrastriolar type II hair cells. Numbers of calyx afferents increase from mouse to squirrel, with more complex calyx afferents in larger species. About 10% of calyx afferents are branched. Based on our counts of total numbers of calyx afferents in chinchilla maculae and in comparison to fiber counts in the literature, the proportion of calyx afferents is greater than previously described, constituting nearly 20% of the total. Because morphometric measures increase with body weight, we obtained additional data on vestibular end organ surface areas from the literature and used this to construct a power law function describing this relationship. The function holds for species with body weights less than approximately 4 kg. Greater than 4 kg, the surface area of the sensory epithelia remains constant even with increasing body weight.

Age-Related Changes in the Vestibular-Evoked Myogenic Potentials

OBJECTIVE: We sought to investigate the effects of gender and age on the vestibular-evoked myogenic potential (VEMP).

STUDY DESIGN AND SETTINGS: Sixty healthy adult volunteers (28 male and 32 female) were investigated in this study. The subject age was distributed between 20 and 77 years old.

RESULTS: No gender-related differences were detected in the VEMP. There was a significant correlation between age and both the evoking threshold and the pInII amplitude of the VEMP, whereas no significant correlation was observed between age and left-right differences of the VEMP.

CONCLUSIONS: The correlation between age and the parameters of the VEMP is presumably secondary to age-related functional changes in the sensory and neural elements of the VEMP. It is safe to evaluate the VEMP using the value of the non-affected side when assessing unilateral lesions. However, it may be necessary to take age into account in evaluating the VEMP when bilateral lesions are suspected.

Unbiased stereologic type I and type II hair cell counts in human utricular macula

OBJECTIVE

The objective of the study was to obtain unbiased estimates of the total number of type I and type II hair cells in human utricular macula from individuals with documented normal vestibular function.

STUDY DESIGN

Application of unbiased stereology using microdissected human temporal bone specimens was conducted in an observational study.

METHODS

Postmortem temporal bones were obtained from 10 normal patients (age range, 42-96 y; mean age, 82 y). The utricular maculae were microdissected, embedded in plastic, and cut into serial 2-microm sections. Unbiased estimates of the total number of type I and type II hair cells were obtained using the physical fractionator technique of stereology.

RESULTS

The average total number of hair cells was 27,508 (CV = 11%) consisting of 17,326 (coefficient of variation [CV] = 11%) type I hair cells and 10,182 (CV = 13%) type II hair cells. The ratio of type I to type II hair cells was 1.70:1. In the age range of the study, there was no statistically significant correlation between hair cell counts and age.

CONCLUSIONS

Morphometric studies of the human utricular sensory epithelium can be accomplished using unbiased stereology on microdissected specimens. There was no effect of age on total hair cell counts or on the ratio of type I to type II hair cells in the age range of the study. Further studies on younger subjects are needed to establish the effect of age. The results from the present study are closely aligned with prior studies that estimated total hair cell counts using surface mount preparations. The current data represent the first total type I and type II hair cell counts in human utricular neuroepithelium.

Aging attenuates the vestibulorespiratory reflex in humans

Activation of the vestibular system changes ventilation in humans. The purpose of the present study was to investigate whether aging alters the vestibulorespiratory reflex in humans. Because aging attenuates the vestibulosympathetic reflex, it was hypothesized that aging would attenuate the vestibulorespiratory reflex. Changes in ventilation during engagement of the semicircular canals and/or the otolith organs were measured in fourteen young (26 +/- 1 years) and twelve older subjects (66 +/- 1 years). In young subjects, natural engagement of the semicircular canals and the otolith organs by head rotation increased breathing frequency during dynamic upright pitch at 0.25 Hz (15 cycles min-1) and 0.5 Hz (30 cycles min-1) (delta2 +/- 1 and delta4 +/- 1 breaths min-1, respectively; P < 0.05) and during dynamic upright roll (delta2 +/- 1 and delta4 +/- 1, respectively; P < 0.05). In older subjects, the only significant changes in breathing frequency occurred during dynamic pitch and roll at 0.5 Hz (delta2 +/- 1 and delta2 +/- 1 for pitch and roll, respectively). Stimulation of the horizontal semicircular canals by yaw rotation increased minute ventilation in young but not older subjects. Selective engagement of the otolith organs during static head-down rotation did not alter breathing frequency in either the young or older subjects. The results of this study indicate that the vestibulorespiratory reflex is attenuated in older humans, with greater vestibular stimulation needed to activate the reflex.

Afferent innervation patterns of the saccule in pigeons

The innervation patterns of vestibular saccular afferents were quantitatively investigated in pigeons using biotinylated dextran amine as a neural tracer and three-dimensional computer reconstruction. Type I hair cells were found throughout a large portion of the macula, with the highest density observed in the striola. Type II hair cells were located throughout the macula, with the highest density in the extrastriola. Three classes of afferent innervation patterns were observed, including calyx, dimorph, and bouton units, with 137 afferents being anatomically reconstructed and used for quantitative comparisons. Calyx afferents were located primarily in the striola, innervated a number of type I hair cells, and had small innervation areas. Most calyx afferent terminal fields were oriented parallel to the anterior-posterior axis and the morphological polarization reversal line. Dimorph afferents were located throughout the macula, contained fewer type I hair cells in a calyceal terminal than calyx afferents and had medium sized innervation areas. Bouton afferents were restricted to the extrastriola, with multi-branching fibers and large innervation areas. Most of the dimorph and bouton afferents had innervation fields that were oriented dorso-ventrally but were parallel to the neighboring reversal line. The organizational morphology of the saccule was found to be distinctly different from that of the avian utricle or lagena otolith organs and appears to represent a receptor organ undergoing evolutionary adaptation toward sensing linear motion in terrestrial and aerial species.

Decreased expression of calretinin and calbindin in the labyrinth of old gerbils

Calretinin and calbindin staining were compared in the vestibular periphery of old (35-48 months) and young (4-12 months) animals. Both stain calyx-only afferents; calbindin stains additional terminals in the apex [Brain Res. 928 (2002) 8-17]. In six of seven pairs of animals, calretinin and calbindin staining was diminished or absent in the old animals. These changes suggest that a reduction in certain calcium-binding proteins may be a characteristic of aging animals.

Effects of hypergravity on the morphological properties of the vestibular sensory epithelium. II. Life-long exposure of rats including embryogenesis

Rats were exposed to a hypergravity (HG) level of 2.5 x g from conception until the age of 14 weeks. The vestibular epithelia of four of these animals and four control animals were immunohistochemically labeled for actin and tubulin. The apical cross-sectional area of epithelial cells of HG exposed rats appeared to be larger in all end organs. Area increase was 7.0% in the utricle (p<0.005) and 8.2% in the crista (p<<0.001). Hair cells and supporting cells appeared to be intact. The cellular arrangement and the proportion of different cell types within the epithelia was normal.

Influence of vision on head stabilization strategies in older adults during walking

BACKGROUND

Maintaining balance during dynamic activities is essential for preventing falls in older adults. Head stabilization contributes to dynamic balance, especially during the functional task of walking. Head stability and the role of vision in this process have not been studied during walking in older adults.

METHODS

Seventeen older adults (76.2 +/- 6.9 years) and 20 young adults (26.0 +/- 3.4 years) walked with their eyes open (EO), with their eyes closed (EC), and with fixed gaze (FG). Participants performed three trials of each condition. Sagittal plane head and trunk angular velocities in space were obtained using an infrared camera system with passive reflective markers. Frequency analyses of head-on-trunk with respect to trunk gains and phases were examined for head-trunk movement strategies used for head stability. Average walking velocity, cadence, and peak head velocity were calculated for each condition.

RESULTS

Differences between age groups demonstrated that older adults decreased walking velocity in EO (p =.022). FG (p = .021), and EC (p = .022). and decreased cadence during EC (p = .007). Peak head velocity also decreased across conditions (p < .0001) for older adults. Movement patterns demonstrated increased head stability during EO. diminished head stability with EC, and improved head stability with FG as older adult patterns resembled those of young adults.

CONCLUSIONS

Increased stability of the lower extremity outcome measures for older adults was indicated by reductions in walking velocity and cadence. Concomitant increases in head stability were related to visual tasks. Increased stability may serve as a protective mechanism to prevent falls. Further, vision facilitates the head stabilization process for older adults to compensate for age-related decrements in other sensory systems subserving dynamic balance.

Vestibulocollic reflexes: normal values and the effect of age

OBJECTIVES

To define normal values and examine the influence of ageing on vestibulocollic reflexes (VCR).

METHODS

Vestibulocollic responses to 100 dB (normal hearing level; NHL) clicks, forehead taps and galvanic stimulation were measured in 70 healthy adults aged 25-85 years.

RESULTS

Click- and galvanic-evoked responses were present bilaterally in all subjects below 60. Average click-evoked response amplitudes decreased with age, with a pronounced decline of 25-30% per decade from the 6th decade. The average click thresholds increased from 85 dB in the third decade to 96.5 dB in the 8th and 9th decades. Average galvanic-evoked VCR amplitudes decreased sharply from the seventh decade. Tap-evoked reflex amplitudes showed a milder decrease. When side to side differences in amplitude were expressed as asymmetry ratios (AR) in subjects below the age of 60, values of up to 35 and 46% were obtained for click amplitudes corrected and uncorrected for background electromyogram (EMG), up to 61% for both corrected and uncorrected tap response amplitudes, and up to 41 and 55% for corrected and uncorrected galvanic-evoked responses.

CONCLUSIONS

A normative range of values can be specified for click- and galvanic-evoked VCRs for subjects up to the age of 60. Click- and galvanic-evoked VCR amplitudes decrease rapidly thereafter while tap-evoked responses are less affected. These changes are probably due to morphological changes in the vestibular system occurring with ageing and are more marked than in several previous reports of age-related changes in caloric responses and vestibulo-ocular reflexes.

Decreasing hair cell counts in aging humans

Deterioration of balance with advancing age is a well-known fact of life. Some investigators have reported a 50% prevalence of dizziness in the elderly. Clinically, progressive dysequilibrium of aging presents as gradually worsening balance due to age-related decline in function of the peripheral vestibular system, central nervous system, vision, and musculoskeletal system. Vestibular function testing has shown clear evidence of age-related changes in peripheral and central sites. Histopathologic changes in the vestibular sensory organs include progressive hair cell degeneration, otoconial degeneration in the otolith organs, and decreasing number of Scarpa's ganglion neurons. Recently, a new quantitative method of assessing vestibular otopathology has been described, utilizing Nomarski differential interference contrast microscopy. This technique has been applied to 67 human temporal bones of individuals from birth to age 100 to create a normative database of total, type I, and type II hair cell counts as a function of age. Results show a highly significant continuous decrease in all counts from birth to age 100, best fit by a linear regression model. Type I hair cell counts in all three semicircular canal cristae decrease at a similar rate, significantly faster than the degeneration observed in type I hair cells of the maculae. Type II hair cell counts decline at the same rate for all 5 sensory epithelia. These normative data provide the basis for comparisons to hair cell counts made in temporal bones from subjects with known vestibular disorders. They also provide a basis for drawing correlations between vestibular function testing and vestibular otopathology.

Balance prosthesis based on micromechanical sensors using vibrotactile feedback of tilt

A prototype balance prosthesis has been made using miniature, high-performance inertial sensors to measure lateral head tilt and vibrotactile elements mounted on the body to display head tilt to the user. The device has been used to study the feasibility of providing artificial feedback of head tilt to reduce postural sway during quiet standing using six healthy subjects. Two vibrotactile display schemes were used: one in which the individual vibrating elements, called tactors, were placed on the shoulders (shoulder tactors); another in which columns of tactors were placed on the right and left sides of the trunk (side tactors). Root-mean-square head-tilt angle (Tilt) and center of pressure displacement (Sway) were measured for normal subjects standing in a semi-tandem Romberg position with eyes closed, under four conditions: no balance aids; shoulder tactors; side tactors; and light touch. Compared with no balance aids, the side tactors significantly reduced Tilt (35%) and Sway (33%). Shoulder tactors also significantly reduced Tilt (44%) and Sway (17%). Compared with tactors, light touch resulted in less Sway, but more Tilt. The results suggest that healthy normal subjects can reduce their lateral postural sway using head tilt information as provided by a vibrotactile display. Thus, further testing with balance-impaired subjects is now warranted.