Effect of the acoustic reflex on inner ear damage induced by industrial noise

Intra-Operative Ossicular Chain Status in Patients with Chronic Otitis Media

The Objective of the study was to assess the ossicular status in chronic otitis media (COM)-mucosal and squamosal type and statistically evaluate the extent of ossicular destruction intraoperatively in COM patients. The findings of this study could help us to predict preoperatively the probability of having ossicular chain destruction in COM ears and thus patients could therefore be properly consented about these potential issues before surgery. The study was carried out in ENT department of tertiary health care hospital, between January 2019 to January 2020. All patients of all age groups and both genders, diagnosed with COM Mucosal and Squamosal Type with complaints of ear discharge and hearing loss with good cochlear reserve and requiring surgery were included in the study, after taking informed written consent in vernacular language. All the patients included in the study were evaluated with detailed history, clinical examination including otomicroscopy, tuning fork tests and pure tone audiometry. The patients were then posted for ear surgery and the middle ear status and ossicular chain status were assessed using a microscope intraoperatively. Out of 98 patients, 45(45.9%) had mucosal and 53 (54.08%) had squamosal disease. Ossicular chain was eroded in 69 cases (70.5%). 23 out of 45 (51.1%) mucosal cases and 46 out of 53 squamosal cases (86.7%) reported ossicular erosion. Most frequently involved was long process of incus > stapes > malleus. From our study, we concluded that there is a significant relationship between type of disease pathology in middle ear and ossicular erosion being higher in Squamosal type of COM, with malleus being the most resistant and incus being the most susceptible ossicle.

Asymmetry of Occupational Noise Induced Hearing Loss: An Electrophysiological Approach

Introduction  The question as to whether occupational noise exposure causes symmetrical or asymmetrical hearing loss is still controversial and incompletely understood. Objective  Two electrophysiological methods (cortical evoked response audiometry: CERA and auditory steady state responses: ASSR) were used to address this issue. Method  156 subjects with a well-documented history of noise exposure, a wide range of noise induced hearing loss (NIHL) and without middle ear pathology underwent both a CERA and an ASSR examination in the context of an exhaustive medicolegal expert assessment intended for possible compensation. Results  Whatever the method (CERA or ASSR), the average electrophysiological hearing thresholds (1-2-3 kHz) are significantly worse in the left ear. The right - left differences in CERA and ASSR thresholds are strongly correlated with each other. No significant effect of frequency is found. No correlation is observed between right - left differences in hearing thresholds and either age or degree of hearing loss. Conclusion  In NIHL, there is an actual average right - left difference of about 2.23 dB, i.e., 3.2%, the left ear being more impaired.

Middle ear muscle and medial olivocochlear activity inferred from individual human ears via cochlear potentials

The peripheral auditory system is influenced by the medial olivocochlear (MOC) and middle ear muscle (MEM) reflexes. When elicited by contralateral acoustic stimulation (CAS), these reflexes reduce cochlear amplification (MOC reflex) and limit low-frequency transmission through the middle ear (MEM reflex). The independent roles of these reflexes on auditory physiology and perception are difficult to distinguish. The amplitude of the cochlear microphonic (CM) is expected to increase or decrease when the MOC and MEM reflexes are elicited by CAS, respectively, which could lead to a straightforward interpretation of what reflex is dominant for a given CAS level. CM and ear canal sound pressure level (SPL) were measured for a 500 Hz, 90 dB SPL probe in the presence of contralateral broadband noise (CBBN) for levels ranging from 45-75 dB SPL. In most subjects, CM amplitude increased for CBBN levels of 45 and 55 dB SPL, while no change in ear canal SPL was observed, consistent with eliciting the MOC reflex. Conversely, CM amplitude decreased, and ear canal SPL increased in the presence of 65 and 75 dB SPL CBBN, consistent with eliciting the MEM reflex. A CM-based test of the MOC reflex may facilitate detection of MEM effects and the assessment of adults with cochlear hearing loss.

A Time-Course-Based Estimation of the Human Medial Olivocochlear Reflex Function Using Clicks

The auditory efferent system, especially the medial olivocochlear reflex (MOCR), is implicated in both typical auditory processing and in auditory disorders in animal models. Despite the significant strides in both basic and translational research on the MOCR, its clinical applicability remains under-utilized in humans due to the lack of a recommended clinical method. Conventional tests employ broadband noise in one ear while monitoring change in otoacoustic emissions (OAEs) in the other ear to index efferent activity. These methods, (1) can only assay the contralateral MOCR pathway and (2) are unable to extract the kinetics of the reflexes. We have developed a method that re-purposes the same OAE-evoking click-train to also concurrently elicit bilateral MOCR activity. Data from click-train presentations at 80 dB peSPL at 62.5 Hz in 13 young normal-hearing adults demonstrate the feasibility of our method. Mean MOCR magnitude (1.7 dB) and activation time-constant (0.2 s) are consistent with prior MOCR reports. The data also suggest several advantages of this method including, (1) the ability to monitor MEMR, (2) obtain both magnitude and kinetics (time constants) of the MOCR, (3) visual and statistical confirmation of MOCR activation.

Middle Ear Muscle Reflex and Word Recognition in "Normal-Hearing" Adults: Evidence for Cochlear Synaptopathy?

OBJECTIVES

Permanent threshold elevation after noise exposure, ototoxic drugs, or aging is caused by loss of sensory cells; however, animal studies show that hair cell loss is often preceded by degeneration of synapses between sensory cells and auditory nerve fibers. The silencing of these neurons, especially those with high thresholds and low spontaneous rates, degrades auditory processing and may contribute to difficulties in understanding speech in noise. Although cochlear synaptopathy can be diagnosed in animals by measuring suprathreshold auditory brainstem responses, its diagnosis in humans remains a challenge. In mice, cochlear synaptopathy is also correlated with measures of middle ear muscle (MEM) reflex strength, possibly because the missing high-threshold neurons are important drivers of this reflex. The authors hypothesized that measures of the MEM reflex might be better than other assays of peripheral function in predicting difficulties hearing in difficult listening environments in human subjects.

DESIGN

The authors recruited 165 normal-hearing healthy subjects, between 18 and 63 years of age, with no history of ear or hearing problems, no history of neurologic disorders, and unremarkable otoscopic examinations. Word recognition in quiet and in difficult listening situations was measured in four ways: using isolated words from the Northwestern University auditory test number six corpus with either (a) 0 dB signal to noise, (b) 45% time compression with reverberation, or (c) 65% time compression with reverberation, and (d) with a modified version of the QuickSIN. Audiometric thresholds were assessed at standard and extended high frequencies. Outer hair cell function was assessed by distortion product otoacoustic emissions (DPOAEs). Middle ear function and reflexes were assessed using three methods: the acoustic reflex threshold as measured clinically, wideband tympanometry as measured clinically, and a custom wideband method that uses a pair of click probes flanking an ipsilateral noise elicitor. Other aspects of peripheral auditory function were assessed by measuring click-evoked gross potentials, that is, summating potential (SP) and action potential (AP) from ear canal electrodes.

RESULTS

After adjusting for age and sex, word recognition scores were uncorrelated with audiometric or DPOAE thresholds, at either standard or extended high frequencies. MEM reflex thresholds were significantly correlated with scores on isolated word recognition, but not with the modified version of the QuickSIN. The highest pairwise correlations were seen using the custom assay. AP measures were correlated with some of the word scores, but not as highly as seen for the MEM custom assay, and only if amplitude was measured from SP peak to AP peak, rather than baseline to AP peak. The highest pairwise correlations with word scores, on all four tests, were seen with the SP/AP ratio, followed closely by SP itself. When all predictor variables were combined in a stepwise multivariate regression, SP/AP dominated models for all four word score outcomes. MEM measures only enhanced the adjusted r values for the 45% time compression test. The only other predictors that enhanced model performance (and only for two outcome measures) were measures of interaural threshold asymmetry.

CONCLUSIONS

Results suggest that, among normal-hearing subjects, there is a significant peripheral contribution to diminished hearing performance in difficult listening environments that is not captured by either threshold audiometry or DPOAEs. The significant univariate correlations between word scores and either SP/AP, SP, MEM reflex thresholds, or AP amplitudes (in that order) are consistent with a type of primary neural degeneration. However, interpretation is clouded by uncertainty as to the mix of pre- and postsynaptic contributions to the click-evoked SP. None of the assays presented here has the sensitivity to diagnose neural degeneration on a case-by-case basis; however, these tests may be useful in longitudinal studies to track accumulation of neural degeneration in individual subjects.

Paired measurements of cochlear function and hair cell count in Dutch-belted rabbits with noise-induced hearing loss

The effects of noise-induced hearing loss have yet to be studied for the Dutch-belted strain of rabbits, which is the only strain that has been used in studies of the central auditory system. We measured auditory brainstem responses (ABRs), 2f₁-f₂ distortion product otoacoustic emissions (DPOAEs), and counts of cochlear inner and outer hair cells (IHCs and OHCs, respectively) from confocal images of Myo7a-stained cochlear whole-mounts in unexposed and noise-overexposed, Dutch-belted, male and female rabbits in order to characterize cochlear function and structure under normal-hearing and hearing-loss conditions. Using an octave-band noise exposure centered at 750 Hz presented under isoflurane anesthesia, we found that a sound level of 133 dB SPL for 60 min was minimally sufficient to produce permanent ABR threshold shifts. Overexposure durations of 60 and 90 min caused median click-evoked ABR threshold shifts of 10 and 50 dB, respectively. Susceptibility to overexposure was highly variable across ears, but less variable across test frequencies within the same ear. ABR and DPOAE threshold shifts were smaller, on average, and more variable in male than female ears. Similarly, post-exposure survival of OHCs was higher, on average, and more variable in male than female ears. We paired post-exposure ABR and DPOAE threshold shift data with hair cell count data measured in the same ear at the same frequency and cochlear frequency location. ABR and DPOAE threshold shifts exhibited critical values of 46 and 18 dB, respectively, below which the majority of OHCs and IHCs survived and above which OHCs were wiped out while IHC survival was variable. Our data may be of use to researchers who wish to use Dutch-belted rabbits as a model for the effects of noise-induced hearing loss on the central auditory system.

Pre-operative Indicators of Ossicular Necrosis in Tubotympanic CSOM

Ossicular necrosis and erosion is commonly seen in CSOM with cholesteatoma but can also occur in tubotympanic CSOM. This usually comes as a surprise during surgery. Preoperative knowledge of ossicular necrosis helps the surgeon to be better prepared for the surgery and ossicular construction if required. Hence, this study is intended to identify the pre-operative indicators of ossicular necrosis. This cross sectional study was conducted in Father Muller Medical College in Mangalore. Sixty nine consecutive patients with tubotympanic CSOM underwent a detailed clinical examination with pure tone audiometry and subsequently tympanoplasty with or without cortical mastoidectomy. All clinical findings, pure-tone audiometry, and intraoperative otomicroscopic observations were recorded. Incidence of ossicular necrosis was 23 %. The incus was most frequently eroded followed by malleus and then stapes. On bivariate analysis patients age above 30 years (p = 0.05), duration of CSOM more than 10 years (p = 0.02), presence of granulation (p < 0.05), absence of tympanosclerosis (p = 0.01), moderately severe (45-60 dB) hearing loss (p = 0.01) and an air bone gap of more than 55.7 dB in the right ear and 63.95 dB in the left ear was found to be statistically significant (p < 0.05). On multivariate analysis only presence of middle ear granulation tissue was found to be the significant predicator [(p = 0.005), OR 14.37, 95 % CI 2.26-90.0]. The presence of granulation tissue and a wide air bone gap on pure tone audiometry were the best indicators of ossicular necrosis. Preoperative identification of these indicators can help the surgeon to be better prepared for the surgery.

Auditory brainstem responses predict auditory nerve fiber thresholds and frequency selectivity in hearing impaired chinchillas

Noninvasive auditory brainstem responses (ABRs) are commonly used to assess cochlear pathology in both clinical and research environments. In the current study, we evaluated the relationship between ABR characteristics and more direct measures of cochlear function. We recorded ABRs and auditory nerve (AN) single-unit responses in seven chinchillas with noise-induced hearing loss. ABRs were recorded for 1-8 kHz tone burst stimuli both before and several weeks after 4 h of exposure to a 115 dB SPL, 50 Hz band of noise with a center frequency of 2 kHz. Shifts in ABR characteristics (threshold, wave I amplitude, and wave I latency) following hearing loss were compared to AN-fiber tuning curve properties (threshold and frequency selectivity) in the same animals. As expected, noise exposure generally resulted in an increase in ABR threshold and decrease in wave I amplitude at equal SPL. Wave I amplitude at equal sensation level (SL), however, was similar before and after noise exposure. In addition, noise exposure resulted in decreases in ABR wave I latency at equal SL and, to a lesser extent, at equal SPL. The shifts in ABR characteristics were significantly related to AN-fiber tuning curve properties in the same animal at the same frequency. Larger shifts in ABR thresholds and ABR wave I amplitude at equal SPL were associated with greater AN threshold elevation. Larger reductions in ABR wave I latency at equal SL, on the other hand, were associated with greater loss of AN frequency selectivity. This result is consistent with linear systems theory, which predicts shorter time delays for broader peripheral frequency tuning. Taken together with other studies, our results affirm that ABR thresholds and wave I amplitude provide useful estimates of cochlear sensitivity. Furthermore, comparisons of ABR wave I latency to normative data at the same SL may prove useful for detecting and characterizing loss of cochlear frequency selectivity.

Auditory brainstem circuits that mediate the middle ear muscle reflex

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

Stapedius muscle fibre characterization in the noise exposed and auditory deprived rat

In skeletal muscle, interventions that unload the muscle cause slow-to-fast myosin heavy chain (MHC) conversions, whereas fast-to-slow conversions are seen when the muscles are engaged in resistance training and endurance exercise. The stapedius muscle (SM) is reported to prevent cochlear damage by noise. This theory may be supported by showing comparable changes of muscle fibre composition when ears are exposed to longstanding noise (SM training). Comparable changes after sound deprivation (SM unloading) would suggest that the SM needs a certain degree of daily activity evoked by environmental sound to sustain its normal composition. We investigated the difference in myosin composition of SM fibres from rats exposed to noise, from auditory deprived rats and from rats exposed to low level ambient noise (control group). Consecutive complete SM cross-sections were processed by enzymehistochemistry to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) and by immunohistochemistry using MHC antibodies. Fibres were assigned to mATPase type I, IIA, IIX or 'Miscellaneous' categories. Per mATPase category, the fibres were attributed to groups with specific MHC isoform compositions. Auditory deprivation lasting nine weeks was accomplished by closure of the external meatus at the age of three weeks. A slow-to-fast shift was seen in these rats when compared to the control group. The noise exposed group was exposed to 65-90dB sound pressure level during a period lasting nine weeks from the age of three weeks onwards. A shift from an overwhelming presence of type mATPase IIX, as seen in the control group, to type mATPase IIA occurred in the noise exposed group. Also, more MHC IIA/IIX hybrid fibres were found in the mATPase IIX category. An adaptive response to the acoustic environment in the characteristics of the fibres of the SM, comparable to the response in skeletal muscles on unloading and training activity, can be ascertained. This supports the theory that the SM plays an active role in modulating external acoustic energy on entry to the cochlea. Our results are also in favour of another postulated function of the SM, the unmasking of high-frequency signals in low-frequency background noise.

Central auditory pathways mediating the rat middle ear muscle reflexes

The middle ear muscle (MEM) reflexes function to protect the inner ear from intense acoustic stimuli and to reduce acoustic masking. Sound presented to the same side or to the opposite side activates the MEM reflex on both sides. The ascending limbs of these pathways must be the auditory nerve fibers originating in the cochlea and terminating in the cochlear nucleus, the first relay station for all ascending auditory information. The descending limbs project from the motoneurons in the brainstem to the MEMs on both sides, causing their contraction. Although the ascending and descending pathways are well described, the cochlear nucleus interneurons that mediate these reflex pathways have not been identified. In order to localize the MEM reflex interneurons, we developed a physiologically based reflex assay in the rat that can be used to determine the integrity of the reflex pathways after experimental manipulations. This assay monitored the change in tone levels and distortion product otoacoustic emissions within the ear canal in one ear during the presentation of a reflex-eliciting sound stimulus in the contralateral ear. Preliminary findings using surgical transection and focal lesioning of the auditory brainstem to interrupt the MEM reflexes suggest that MEM reflex interneurons are located in the ventral cochlear nucleus.

Short-duration space flight and hearing loss

OBJECTIVES: The study goal was to determine whether a single Space Shuttle mission can induce decrements in astronaut hearing.

STUDY DESIGN: We retrospectively compared audiogram information obtained from Space Shuttle astronauts at 10 days preflight, day of return (R + 0), 3 days after landing return (R + 3), and at a mean delayed postflight follow-up of 8 months.

RESULTS: Temporary threshold shift (mean, 4.6 dB) was noted in R + 0 versus preflight conditions ( P < 0.01). Small permanent threshold shifts (mean, 0.83 dB) were found at R + 3 and postflight follow-up compared with preflight in the lower frequencies (500 to 2000 Hz), and corresponding pure tone average ( P < 0.001).

CONCLUSIONS AND SIGNIFICANCE: The data indicate that a single Shuttle flight can induce a substantial temporary threshold shift and a small but statistically significant permanent threshold shift, particularly in the frequencies involved in speech reception. Although single-mission effects are small, cumulative effects over several missions may ultimately produce clinically significant hearing loss.

Effects of industrial noise exposure on distortion product otoacoustic emissions (DPOAEs) and hair cell loss of the cochlea--long term experiments in awake guinea pigs

Distortion product otoacoustic emissions (DPOAEs), a sensitive detector of outer hair cell (OHC) function, cochlear microphonics (CM), and hair cell loss have been monitored in 12 awake guinea pigs before and after 2 h exposure to specific, played-back industrial noise (105 dB SPL maximal intensity). All animals had stable DPOAE levels before noise exposure. In the first hours after noise exposure DPOAE levels were reduced significantly. In about 70% a partial recovery of the DPOAEs was found within 4 months after noise exposure. In 16% of the investigated ears no recovery of DPOAEs was observed. However, in a few ears increased DPOAEs were observed after noise exposure. Exposure to industrial noise caused both morphological changes in the middle turns of the cochlea and electrophysiological changes in the middle frequency range. A close correlation existed between reduced DPOAE levels, loss in CM potentials, and area of damaged or lost OHCs, but not with the numbers of damaged or lost OHCs in the cochlea. It can be concluded that continuous industrial noise causes a damage to OHCs which differs form the damage caused by impulse noise.

Frequency summation observed in the human acoustic reflex

It is known that the threshold of an acoustically induced middle-ear-muscle (MEM) reflex can be lowered by the simultaneous presentation of a second tone (facilitator), which is presented to the ipsilateral or contralateral ear at a level below the acoustic reflex threshold (ART) of the facilitator itself (Sesterhenn and Breuninger, 1976; Blood and Greenberg, 1981). In the present study, a primary elicitor and a facilitator were presented to the ear contralateral to that used for measurement of the acoustic reflex (AR), and the effects of changing frequencies and sound levels of the facilitator were investigated in human subjects with normal ears. The sound levels of facilitators, which caused a significant reduction of ART for the primary elicitors (facilitation thresholds), showed an asymmetrical pattern as a function of frequency of the facilitators. The facilitation thresholds tended to be lower when a facilitator with a frequency lower than the frequency of the elicitor (1 kHz) was used. In addition, effects of the elicitor on the masked thresholds of the facilitator were examined to observe the possible interaction between elicitor and facilitator from the viewpoint of 'spread of excitation'. The underlying mechanism of summation effects of two tones are discussed based on the possible input mechanism involved in the acoustically induced MEM reflex are.

The "toughening" phenomenon in rat's auditory organ

In audiological "toughening" or "conditioning" phenomenon prior exposure to moderate noise reduces the extent of hearing deterioration caused by the subsequent exposure to traumatic test noise known to cause inner ear damage. "Toughening" has been demonstrated in many mammalian laboratory animals such as guinea pig and chinchilla but not in rat or mouse. Our aim was to study the occurrence of this phenomenon in the rat. Ninety-one white male Wistar rats were divided into four groups: unexposed control group (U, n = 10), "conditioning" only (C, n = 32), "conditioning" plus test noise (C + T, n = 36) and test noise only (T, n = 13). Groups C and C + T were "conditioned" for 10 hours with 4.0 kHz OBN between 55 and 95 dB sound pressure levels (SPLs). After 10 hours rest groups C + T and T were exposed to the same noise at 105 dB SPL for 13 hours. The hearing thresholds were determined by auditory brainstem response audiometry (ABR) either immediately after or 3 weeks after the exposures. After that the animals were sacrificed. The cochleas were removed and perilymphatically fixed and further processed for quantitative cytocochleograms. Both the temporary (TTS) and the permanent threshold shifts (PTS) were smaller in animals which had been "conditioned" prior exposure to traumatic noise. Yet only 95 dB SPL "conditioning" gave statistically significant difference (p < 0.05) in PTS. From our results we conclude that "conditioning" effect seems to be present also in the rat. However to confirm this, further experiments are needed. The mechanisms behind "conditioning" are still unknown and also to clarify them, further efforts are needed.

The role of middle ear muscles in the development of resistance to noise induced hearing loss

The role of middle ear muscles (MEMs) in the development of increased resistance to noise induced hearing loss (NIHL) was studied using monaural chinchillas. Animals with severed MEMs as well as those with intact MEMs were exposed to an octave band noise (OBN) centered at 0.5 kHz at 95 dB for six hours/day for ten consecutive days. Results indicated that animals with severed MEMs showed greater initial threshold shifts (TS) than the animals with intact MEMs. Both the groups showed a decrease in TS over the ten days of exposure. The subjects were given five days of recovery and then re-exposed to the same noise at 106 dB for 48 h. Permanent threshold shifts (PTS) in each group was compared against those in a control group exposed to the noise only at the higher level. Interestingly, both the 'conditioned' groups incurred substantially less PTS than the control group exposed only to the higher level.

Time lapse effects of impulse noise on cochlear microphonics in rabbits observed in chronic experiments

Cochlear microphonics (CM) were recorded from awake rabbits with chronically implanted electrodes. Test frequencies used were 1,250, 2,500, 5,000 and 10,000 Hz, with intensities increased from 40 to 100 dB SPL. The rabbits were exposed to ten noise impulses of 144 dB SPL, which were then followed by impulse intensities of 153 and 164 dB SPL for the same animal. Input-output functions before and after each noise exposure were recorded, and recovery processes of the CM were tracked. After 144 dB SPL impulses, complete recovery of CM occurred; the effects of 153 dB SPL impulses varied from restitution up to complete CM loss. Impulses of 164 dB SPL caused an irreversible CM loss in all cases. Despite inter-individual differences, the threshold for irreversible CM loss in rabbits may occur between impulse noise intensities of 153 and 164 dB SPL.

Fusion of stereocilia on inner hair cells in man and in the rabbit, rat and guinea pig

The complexity of the problem of sensorineural hearing loss is illustrated by the well-known poor correlation between pure tone thresholds and loss of sensory cells in the organ of Corti and between pure tone thresholds and other auditory functions such as discrimination of speech and thresholds of the intra-aural muscle reflex (MER). The present work demonstrates that two types of inner ear pathology are present in several mammalian species, including man. Both types can be induced by noise. In addition to the earlier well-known loss of outer hair cells, it is shown that abnormal sensory hairs on inner hair cells are an important and frequent finding in acoustically damaged ears. Fusion and inclination of inner hair cell cilia were predominant in rabbits with moderate hearing loss and a rather common finding in rats and humans. In guinea pigs, damage to inner hair cell stereocilia was not common and was seen only in conjunction with considerable loss of outer hair cells. In summary, fusion of the stereocilia of inner hair cells was a common finding in several mammalian species.