[Progressive sensorineural hearing loss in cochlear otosclerosis]

Otosclerosis is a bone dysplasia limited to the otic capsule causing abnormal resorption and redeposition of bone. The existence of the entity "pure labyrinthine otosclerosis" or "cochlear otosclerosis" is not accepted by all authors; however, there is clinical and histologic evidence to support the existence of a progressive sensorineural hearing loss due to otospongiotic-otosclerotic lesions of the labyrinthine capsule, although diagnosis of this condition may be difficult. The involvement of the inner ear is described as degenerative changes in the spiral ligament, stria vascularis, organ of Corti, and cochlear neurons. The most frequent audiometric configuration is a "bite-type" curve, but flat or rising shapes can also be observed; speech discrimination appears unusually good for a pure sensorineural hearing loss and recruitment is frequently absent. A cochlear otosclerosis should be suspected when there is a family history of otosclerosis, the onset of the hearing loss occurs from the third to fifth decade, and worsening of the hearing loss is observed during periods of intense hormonal and endocrine activity, a positive Schwartze sign is present and bilateral sensorineural loss is associated with signs of unilateral stapedial ankylosis. A definitive diagnosis of cochlear otosclerosis can be made only with computed tomography, which allows a quantitative assessment of the involvement of the labyrinthine capsule by spongiotic or sclerotic areas. The factors to be considered are: otosclerotic foci 1 mm or more in diameter and a density different from that of the normal otic capsule, partially or completely erased contour of the capsule, double ring effect, bony neoformation in the labyrinthine spaces, and increased thickness of the cochlear capsule. The medical management of cochlear otosclerosis is based on sodium fluoride, in association with calcium and vitamin D; some authors have also proposed diphosphonates as inhibitor agents of bone resorption. Surgery may be useful only in those patients presenting a hearing loss so severe that the bone threshold cannot be evaluated and a gap between air and bone conduction cannot be excluded; in these cases stapes operations can improve hearing to a level that may be useful in hearing aid application.

[Abnormal augmentation of the evoked potential and morphological changes of guinea pig cochlea induced by cisplatin]

OBJECTIVE

To investigate the characteristic of cochlear microphone(CM), summating potential(SP), compound action potential(CAP) and the morphological changes of hair cells after selective damage to the inner ear by acute cochlear perfusion of cisplatin.

METHODS

Dynamic changes of CM, -SP and CAP were recorded by glass electrode from the third turn of the scala media guinea pig's cochlea before and during cisplatin perfusion of the whole cochlea for 2 hours.

RESULTS

It indicated that after one hour of the perfusion, the amplitude of CM, -SP and CAP decreased at stimulation intensity < or = 60 dB SPL, while the amplitude increased at stimulation intensity > or = 70 dB SPL, as compared with those before perfusion (the average of CM increased by 3.6 mV at 90 dB SPL, average of -SP increased by 1.6 mV at 120 dB SPL, average of CAP increased by 0.23 mV at 90 dB SPL). After two hours of perfusion, the amplitude of CM, -SP and CAP decreased in all the stimulation intensity. The succinic dehydrgenase (SDH) staining was decreased in OHCs, while that of IHC's remained normal. Transmission electron microscopic examination of organ of Corti showed morphological changes in OHCs, such as disappearance of nuclear chromosome, denature and reduction number, mitochondria while the structure of IHCs remained normal.

CONCLUSION

The abnormal augmentation phenomenon of CM and -SP may be due to the abnormal modulation of Ca2+ in IHCs and OHCs. The abnormal augmentation of CAP suggests that the suppression effects of OHCs and efferent neurotransmitter on IHCs and afferent neurotransmitter may be reduced after OHCs damage.

[The impact of electrical stimulation on the functional status of cochlear nervous structures in cats with experimentally induced hearing disorders]

Effects of non-invasive electric stimulation on hearing were studied in cats with depressed periphery of the acoustic analyser. Experimental neurosensory hypoacusis was induced by large doses of canamycin (25-30 mg/kg). The hearing was assessed by short-latent stem and cortical long-latent evoked potentials (EP). Consideration was also given to changes in the amplitude of the wave V of short-latent EP in varying intensity of the sound stimulus. When deep suppression was reached, 8-day course of electric stimulation was started with registration of EP. It was found that canamycin elevates thresholds of identification of wave V of short-latent stem EP and thresholds of emergence of long-latent stem EP. The curve amplitude/intensity was also affected. The course of electrical stimulation returned the above parameters of the thresholds and the curve to the baseline levels. This is the evidence of therapeutic action of non-invasive electrical stimulation.

GDNF protects the cochlea against noise damage

Glial-derived neurotrophic factor (GDNF) was tested for its ability to prevent hearing and sensory cell loss in guinea pigs exposed to acoustic trauma. Hearing was measured prior to any treatment. Animals were exposed to damaging levels of noise either before or after local application of GDNF to one ear. Four weeks later, hearing and sensory cell loss was greater in the control ear than in the ear receiving GDNF before acoustic trauma or 2 h after trauma, but not 4 or 6 h after trauma. The results indicate that GDNF treatment in vivo can prevent cochlear sensory cell damage and hearing loss if present during or shortly after acoustic trauma.

Effects of nimodipine on noise-induced hearing loss

The effects of nimodipine, a calcium channel blocker, on noise-induced hearing loss were examined in gerbils. Animals were implanted subcutaneously with a timed-release pellet containing either nimodipine (approximately 10 mg/kg/day) or placebo and exposed to either 102 or 107 dBA noise. Serum levels were tested in two subjects and were in the range known to protect humans from cerebral artery vasospasm and ischemia-related neurologic deficits. Nimodipine and control groups had similar amounts of noise-induced (a) permanent threshold shift; (b) reductions in distortion product otoacoustic emissions; (c) reductions in tuning and suppression of the compound action potential; and (d) loss of outer hair cells. The results suggest that nimodipine, at a dose which results in clinically relevant serum levels, does not provide protection from the effects of moderately intense noise exposures.

Cochlear neuromas

OBJECTIVE

This study aimed to review the clinicoradiographic findings in 16 patients with unilateral cochlear neuromas.

STUDY DESIGN

The data included in this presentation were obtained from a retrospective case review.

SETTING

The study was conducted at a tertiary care academic institution.

PATIENTS

Patients of any age, gender, or race with a unilateral cochlear neuroma participated.

INTERVENTIONS

Standard audiometric and radiographic assessment was performed.

MAIN OUTCOME MEASURES

Progressive sensorineural hearing loss was measured.

RESULTS

Sensorineural hearing loss is out of proportion to the size of the tumor (< 8 mm in all 16 patients).

CONCLUSIONS

Gadolinium-enhanced magnetic resonance imaging in the axial and coronal planes can help identify cochlear neuromas in selected patients with unilateral sensorineural hearing loss.

The etiology of idiopathic sudden sensorineural hearing loss. Experimental herpes simplex virus infection of the inner ear

HYPOTHESIS

Experimentally induced herpes simplex virus type 1 (HSV-1) labyrinthitis provides a suitable model for idiopathic sudden sensorineural hearing loss (ISSHL).

BACKGROUND

Viral labyrinthitis has been postulated to play a role in the pathophysiology of ISSHL. Circumstantial evidence is pointing at members of the herpes virus family. Experimental viral labyrinthitis elicited by various virus families leaves a virus-specific pattern of cochlear damage. Herpes viruses provide the best matching pattern in the distribution of cochlear damage when compared with ISSHL postmortem cochlear histopathology.

METHODS

Herpetic viral labyrinthitis was induced in guinea pigs using perilymphatic inoculation with HSV-1. A control group was inoculated with the culture medium only. Infection was confirmed by the measurement of HSV antibodies. Hearing was monitored. Cochlear damage was evaluated by light and electron microscopy.

RESULTS

In all HSV-1 inoculated animals, rapid loss of hearing occurred. Seroconversion took place, but no systemic manifestations of herpetic infection were observed. The control group showed no cochlear or systemic symptoms. When comparing cochlear histopathology in ISSHL to experimental viral HSV-1 labyrinthitis, strong similarities were found: degeneration of the stria vascularis, destruction of the organ of Corti, loosening of the tectorial membrane, and inflammatory changes in neural structures.

CONCLUSIONS

Based on clinical and histopathologic characteristics, experimental HSV-1 labyrinthitis provides a suitable model of ISSHL.

Acute effects of semicircular canal destruction on the cochlea, with and without preceding Pseudomonas aeruginosa exotoxin A treatment

Acute electrophysiological and morphological changes in the cochlea following destruction of the semicircular canals (SCCs) were investigated to elucidate differences in cochlear vulnerability to surgical procedure under two conditions: normal healthy condition and a pathological condition induced by Pseudomonas aeruginosa exotoxin A (PaExoA). Frequency-specific auditory brainstem response recording and examination by light microscopy (LM) and scanning electron microscopy (SEM) were performed in the acute stage, immediately following, and 2, 5, and 10 days after the intervention. All normal healthy rats showed noticeable hearing loss immediately after the operation, predominantly in lower frequency areas, followed by complete recovery within 5 days. LM revealed rupture and slight distension of Reissner's membrane. SEM revealed considerable disarray of the stereocilia, especially on the outer hair cells (OHCs) of the third row in the upper half-turns of the cochlea. By contrast, all rats under pathological conditions showed delayed and incomplete hearing recovery from postoperative hearing loss. LM revealed various kinds of cochlear reaction, such as distension of Reissner's membrane, infiltration of inflammatory cells into the cochlea, and severe inflammatory change. Damage both of inner hair cells in the basal turn and of OHCs in the upper half-turns was more noticeable, compared with that under normal conditions. These findings indicate that SCC destruction under pathological conditions induced by PaExoA is detrimental to postoperative hearing recovery, even if the preoperative hearing level had appeared electrophysiologically normal.

[DNA replication and cellular proliferation in the noise damaged basilar papillar of chicks]

An acoustic trauma model for study of the repair of the auditory epithelia was established in postnatal 8-14 days chicks. The animals were continuously exposed to the wide band noise at 115 dB (A) for 72 hours. The tracer of DNA replication-[3H] thymidine (3H-dR) was given in vivo or in vitro during or after exposure. The basilar papilla were cultured and processed with autoradiographic technique. The results demonstrated that at 48 hours following initial labelling of tracer, the incorporation of 3H-TdR was found over the nucli of supporting cells (or precursor cells) in the injured region of basilar papilla. The labelled hair cells appeared at 96 hours near the labelled supporting cells. Until the fifth to sixth days of culturing, the regenerated hair cells were still immature morphologically. No sign of labelled hair cells were detected in the control animals. Our findings indicated the potential capability of cellular proliferation and DNA replication in damaged auditory epithelia in vitro. Some kind of supporting cells in damaged region could be the precursors for regenerating hair cells. The positive cells labelled with 3H-TdR migrated to the surface of basilar papilar and differentiated into hair cells eventually. Therefore, it could be speculated that a homogeneous relationship may exist between labelled supporting cells and labelled hair cells.

Hearing in the MRL/lpr mouse as a possible model of immune-mediated sensorineural hearing loss

In order to clarify the possible mechanism of hearing loss in immune-mediated sensorineural hearing loss, basic research needed includes animal model studies. In the present investigation, we examined hearing thresholds and cochlear histologies of the MRL/lpr mouse which is now well-known as a model for pathology consistent with systemic lupus erythematosis (SLE). Present findings demonstrated that there were no statistically significant differences in auditory brainstem response (ABR) thresholds between 4- to 6-week-old "young" and 20- to 25-week-old "old" MRL mice. These differences were not sex-dependent. Under light microscopy, there were no abnormal morphological findings in the cochleas of either young or old MRL mice. With immunohistochemistry, mouse IgG was detected around the capillary walls in the stria vascularis in both young and old MRL mice. Serum IgG level of the MRL mice significantly decreased after predonisolone (PSL) administration. However, expression of mouse IgG in the stria vascularis was not observed in the MRL mice after PSL administration. From these results, we speculate that the hearing of the MRL mouse does not always deteriorate, and the deposition of mouse IgG on the capillary wall in the stria vascularis is not a sufficient factor to induce hearing loss. At this point, we conclude that the MRL mouse should not be considered a useful model for immune-mediated sensorineural hearing loss.

Excitotoxicity and repair of cochlear synapses after noise-trauma induced hearing loss

Guinea-pigs were exposed to a traumatic sound inducing up to 80 dB hearing loss. Beside the well described mechanical damage to outer hair cells, a total disruption of inner hair cell (IHC)-auditory nerve synapses was acutely observed within the traumatized area. To test the hypothesis that synaptic damage is due to an excessive release of glutamate by the IHCs, we examined the protective effect of the glutamate antagonist kynurenate on noise-induced hearing loss. The high degree of protection observed with kynurenate attests that dendritic damage is an important component in noise-induced hearing loss. Moreover, we demonstrate that a synaptic repair mechanism occurring within the first few days post-exposure is partly responsible for the recovery of temporary threshold shifts after an acoustic trauma.

Intracochlear factors contributing to psychophysical percepts following cochlear implantation

The performance of cochlear implant patients may be related to intracochlear, histopathological factors. We have performed detailed post-mortem examinations of five human, implanted cochleas and for each electrode correlated the psychophysical threshold, comfortable level and dynamic range with spiral ganglion cell survival, presence of fibrous tissue and/or new bone, and distance between the centers of the electrode bands and Rosenthal's canal. The psychophysical parameters were strongly interrelated. Threshold and comfort levels correlated with the distance between the electrodes and Rosenthal's canal. Threshold levels also correlated with the presence of intracochlear fibrous tissue and new bone, especially with the former. The dynamic range showed a negative correlation with intracochlear pathology, especially with new bone. Comfort levels and dynamic range were related to spiral ganglion cell survival. The distance between the electrodes and the modiolus increased with increasing levels of fibrous tissue and new bone. Spiral ganglion cell survival was decreased with increasing levels of fibrous tissue and new bone.

Effects of inner hair cell loss on inferior colliculus evoked potential thresholds, amplitudes and forward masking functions in chinchillas

The effects of outer hair cell (OHC) loss on evoked potential (EVP) thresholds, amplitudes and forward masking (FWM) functions have been fairly well characterized. In contrast, the effects of inner hair cell (IHC) losses are largely unknown, primarily due to the difficulty of producing selective IHC lesions. Recent studies have shown that IHCs of the chinchilla are preferentially damaged by the anticancer drug, carboplatin. In this study, we administered a single 100 mg/kg dose of carboplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and amplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker probe intervals of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplatin treatment. Histology revealed IHC lesions ranging from approximately 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massive IHC lesions were not associated with elevations of IC-EVP thresholds. IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increased susceptibility to FWM, particularly at intermediate (10-20 ms) masker-probe intervals, without significantly changing the overall time course of FWM. The results provide new perspectives on the contribution of IHCs to FWM, and on the ability of the central auditory system to adapt to a significant reduction of neural input from the cochlea.

[The technique of chronic infusion of drugs into the cochlea by an osmotic pump]

OBJECTIVE

To introduce a method of chronic infusion of drugs into the cochlea using a osmotic pump.

METHODS

A stainless steel tube was surgically implanted into the basal turn of the chinchilla cochlea and connected to the pump. To assess the reliability of the procedure, and the pump, Hank's solution or L-buthionine-[S,R]-sulfoximine (BSO) was infused into the cochlea by this pump and inferior colliculus (IC) potential and hair cell morphology were examined.

RESULTS

In three Hank's solution infused cochleas, the thresholds of IC potential were kept in the normal level and no hair cell loss was found 14 days after the beginning of the infusion, suggested that the pump implantation and Hank's solution infusion had no significant effects on the cochlear functions. In contrast, threshold shifts of IC potentials and hair cell loss were found in two BSO infused animals, suggested that the infused solution had distributed into the whole cochlea.

CONCLUSION

This study indicates that the osmotic pump can be used in chronic delivery of experimental drugs into the cochlea.

Microangiopathy of the brain, retina, and cochlea (Susac syndrome). A report of five cases and a review of the literature

OBJECTIVE

This study reports five new cases of microangiopathy of the brain, retina, and cochlea (Susac syndrome) and reviews the world's literature.

DESIGN

Five cases were systematically studied by the authors. The cases in the literature were identified through Medline searches for Susac syndrome; microangiopathy of the brain, retina, or ear; and cross-referencing the indexes of each retrieved article.

PARTICIPANTS

The number of new patients studied in this report was five. An additional 41 patients were culled from the literature.

INTERVENTION

Patients were treated with corticosteroids, antineoplastic agents, and other methods in a noncontrolled, nonrandomized fashion.

MAIN OUTCOME MEASURES

With respect to therapeutic intervention, the main clinical outcome measures were return of vision, improvement of neurologic and psychiatric manifestations, and recovery of auditory function. Alterations of abnormalities observed by cranial magnetic resonance imaging also were monitored.

RESULTS

Of 46 identified patients, 39 were women. The mean age of the patients was 30 years. Forty-one patients (89%) had arterial occlusions, which were bilateral in 60%. Thirty-one patients (67%) reported hearing loss. Twenty patients (44%) had a global encephalopathy, but other neurologic manifestations were common. The mean duration of the illness was 46.7 months.

CONCLUSION

This rare syndrome is more common than previously thought, has a strong female preponderance, and often can be identified at an early stage with a careful history and physical examination. Magnetic resonance imaging of the brain often shows lesions suggestive of multiple sclerosis. Fluorescein angiography may show arteriolar wall hyperfluorescence. Early treatment with corticosteroids often is, although not invariably, associated with a good prognosis. The disease appears to be self-limited in most patients.

Genetics of age-related hearing loss in mice. IV. Cochlear pathology and hearing loss in 25 BXD recombinant inbred mouse strains

The effects of three putative genes which contribute to age-related hearing loss (AHL genes) were evaluated using auditory brainstem response (ABR) thresholds and post-mortem cochlear histopathology in 25 recombinant BXD inbred mouse strains, originally derived from C57BL/6J (B6) and DBA/2J (D2) progenitor strains. All BXD strains showed substantial elevation of ABR thresholds and loss of spiral ganglion cells (SGCs) during the first year of life. The findings are consistent with our genetic model in which D2 and B6 inbred strains both possess the Ahl (age-related hearing loss) gene, whereas D2 possesses two additional chromosomal loci with AHL genes (Ahl2 and Ahl3). The between-strain distribution in the severity of SGC loss and ABR threshold elevations suggests that the severity of hearing loss is determined in large part by the number of AH L genes an animal possesses and by additional genetic background effects. The present findings also demonstrate that, because BXD strains vary substantially in the rate and severity of progressive hearing loss (but are genetically closely related), they can provide powerful animal models for developmental studies of AHL.

Clonal expansion of T-cell receptor beta gene segment in the retrocochlear lesions of EAE mice

It has been reported that the T cell receptor V beta 8.2 (TcrbV8.2) gene segment is predominantly expressed in encephalomyelitic T cells responding to myelin basic protein (MBP) in experimental allergic encephalomyelitis (EAE) mice. We have demonstrated retrocochlear hearing loss in EAE mice in previous studies. Administration of a monoclonal antibody specific to the T cell receptor V beta 8 (TcrbV8) subfamily prevented both this type of hearing loss and the central nerve disease. In this study, we examined the role of the TcrbV8.2 gene segment in the retrocochlear lesions of EAE mice. A clonal expression of T cell receptor beta chain gene segment (TcrbV8.2-TcrbD2-TcrbJ2.7) was identified in the retrocochlear lesions. The TcrbV8.2 gene segment appears to recombine only with TcrbJ2.1 (32.1%) and TcrbJ2.7 (67.9%) gene segments. The TcrbJ2.7 gene segment has also been previously identified as the dominant TcrbJ gene in the lymph nodes of EAE mice. Only TcrbD2, with a length of 4 amino acids, was observed recombining with these TcrbV8.2 sequences. G and C nucleotides are predominantly expressed at the N regions between the V-D and D-J junctions. This dominant TcrbV gene segment (TcrbV8.2-TcrbD2-TcrbJ2.7) observed in the retrocochlear lesions has been identified in the MBP-specific T cells from the lymph nodes of EAE mice. These results suggest that a small subset of antigen-specific T cells migrate to, and expand at, the retrocochlear lesions, which leads to hearing loss.

Progressive sensorineural hearing loss and a widened vestibular aqueduct in Pendred syndrome

Pendred syndrome is an autosomal recessive inherited disorder. Obligatory features are profound deafness in childhood and defective organic binding of iodine in the thyroid gland. Therefore, goiter is a common symptom. Hypoplasia of the cochlea is another feature. Recently, the gene for Pendred syndrome was identified. We describe a boy whose sensorineural hearing loss in both ears progressed rapidly from about 50 to 60 dB at the age of 3 years and 3 months to more than 100 dB at the age of 4 years and 4 months. This loss was preceded by a medical history of a progressive hearing loss. The progressive nature of the hearing loss motivated a search for the cause. Dysplasia of the cochlea and a widened vestibular aqueduct were found. The results of thyroid function tests were normal, but he had an elevated level of thyroglobulin. The diagnosis of Pendred syndrome was confirmed by the positive results of a potassium perchlorate test, indicating defective organic binding of iodine in the thyroid gland. It is possible that the widened vestibular aqueduct was responsible for the increase in the hearing impairment. Aside from the branchio-otorenal syndrome, Pendred syndrome is the only other known genetic disorder with a widened vestibular aqueduct. If a child has progressive sensorineural deafness and a widened vestibular aqueduct, it is important to consider a diagnosis of Pendred syndrome. A widened vestibular aqueduct may help to elucidate the pathophysiologic characteristics of hearing loss in these genetic types of deafness in childhood.

Expression of adeno-associated virus integrated transgene within the mammalian vestibular organs

HYPOTHESIS

Adeno-associated virus (AAV) is a suitable viral vector for transgene expression within the mammalian vestibular organs.

BACKGROUND

In vivo introduction and expression of a foreign gene within the cochlear tissues have been established using a variety of viral vectors and guinea pig as the animal model. However, the vestibular neuroepithelia of the mammalian inner ear as a potential target for transgene expression remain to be investigated.

METHODS

Transgene expression was assessed within the vestibular neuroepithelia of guinea pigs after intracochlear infusion of the recombinant AAV vector with the aid of an osmotic minipump. Evaluation of the transgene within the vestibular apparatus focused on its duration of expression from 2-24 weeks after intracochlear AAV infusion using immunohistochemistry.

RESULTS

In the AAV-beta-galactosidase (beta-gal)-infused animals, the sensory hair cells as well as the supporting epithelial cells of cristae and maculae were positive for the transgene expression. The relative level of beta-gal expression was noted to decrease progressively over time. Transduction of the vestibular neuroepithelia also was observed in the contralateral ear, a finding that has been documented previously in AAV-integrated transgene expression in the cochlea.

CONCLUSION

This study reports the first demonstration of introduction and long-term transgene expression within the vestibular neuroepithelia. The ability to express a foreign gene with the vestibular system allows the possibility of experimental and therapeutic application of gene therapy technology to address vestibular function and dysfunction.

Immunohistochemical analysis of proliferating cells in a sterile labyrinthitis animal model

Inflammatory reactions within the cochlea lead to the formation of fibrotic tissue and bone. To determine which cells are involved in the proliferation of the inflammatory response within the cochlea, sterile labyrinthitis was created by inoculating keyhole limpet hemocyanin (KLH) into the scala tympani of systemically sensitized animals. Cellular proliferation was assessed immunohistochemically using the monoclonal antibody Ki-67. Proliferating cells were identified among inflammatory cells and fibroblasts within the matrix, as well as in endosteal cells lining the scala tympani. Inflammatory and potential osteoprogenitor cells were labeled as late as 6 weeks after inoculation, suggesting the absence of a strong immunosuppressive mechanism. Endosteal cells may proliferate and secrete the extracellular matrix used by the inflammatory cells to move within the cochlear scalae. They may also participate in the ossification of the inflammatory matrix.

[The cochlea: modern physiologic insights into sensorineural hearing loss]

The cochlea is organized tonotopically: each frequency between 20 Hz and 20 kHz has its own location on the basilar membrane. In addition the cochlea has a large dynamic range (sound intensity). Active movements of the outer hair cells of the sensory organ of Corti play a crucial part in determining hearing acuity and frequency selectivity. Long-known properties such as combination tones and more recent observations like spontaneous and evoked otoacoustic emissions (the cochlea as sound source) fit in the same concept of a cochlea acting as a non-linear fine-tuned amplifier. Most sensorineural losses in people with diminished hearing are caused by injury to the hair cells, e.g. by medicines. Damage of outer hair cells impairs the cochlear amplifier, leading to loss of sensitivity to soft sounds and loss of frequency selectivity. Damage to the inner hair cells has much the same effect as conductive hearing loss. Tinnitus is not caused by otoacoustic emissions, but probably by denervation hypersensitivity following loss of cochlear input.

Noise-induced threshold shift dynamics measured with distortion-product otoacoustic emissions and auditory evoked potentials in chinchillas with inner hair cell deficient cochleas

Chinchillas (n = 6) were treated with carboplatin and, following a 30-day recovery period, were exposed to a 115 dB peak SPL impact noise presented at a rate of l/s for 6 h/day for 10 days. A second group (n = 6) received only the noise treatment. Cubic distortion product otoacoustic emissions (2f1-f2) and auditory evoked potential (AEP) detection thresholds in response to tone bursts were measured before and 30 days after drug treatment and following the first and 10th day of the noise exposure. Thirty days after the final exposure day, permanent changes in AEP detection thresholds and emissions were measured and cochleograms constructed. The drug treatment eliminated over 80% of the inner hair cells (IHC) in the cochlea, leaving the outer hair cell (OHC) population essentially intact prior to the interrupted noise exposure. The drug treatment alone had very little or no effect on AEP detection thresholds and emission metrics. Following the noise exposure, the IHC-deficient animals showed clear 'toughening' effects in the AEP and emission measures which were the same as measured in the group receiving only the noise. After a 30-day post-exposure recovery period. AEP thresholds were elevated about 10 dB at the low frequencies in the drug-noise group whereas emissions returned to near normal despite the massive IHC losses. These results are consistent with the idea that an intact OHC population is required for toughening. However, sound-evoked efferent pathways activated by the few remaining IHCs (approximately 20%) which, in this preparation, are distributed throughout the cochlea, may still contribute significantly to the toughening phenomena.

Changes in immunostaining of inner ears after antigen challenge into the scala tympani

To study the mechanisms of immune responses and immune injuries in inner ears, labyrinthitis was induced by inoculation of keyhole limpet hemocyanin (KLH) into the scala tympani of systemically sensitized guinea pigs. Inner ears were then immunostained for KLH, immunoglobulin G (IgG), albumin, connexin26 (Cx26), and sodium-potassium adenosine triphosphate (Na,K-ATPase). Inflammatory cells containing KLH were observed in the scala tympani and in the collecting venule of the spiral modiolar vein (SMV). Spiral ligament, spiral limbus, and blood vessels including the SMV were diffusely positive for IgG and albumin. Immunoreactivity for Cx26 and Na,K-ATPase was decreased compared with the normal ears in the fibrocytes of the spiral ligament. These results suggest that inflammatory cells and blood constituents could extravasate into the cochlea from blood vessels and that fibrocyte damage in the spiral ligament could cause cochlear dysfunction.

Acoustic trauma causes reversible stiffness changes in auditory sensory cells

A common cause of hearing impairment is exposure to loud noise. Recent research has demonstrated that the auditory mechanosensory cells are essential for normal hearing sensitivity and frequency selectivity. However, little is known about the effect of noise exposure on the mechanical properties of the auditory sensory cells. Here we report a significant reduction in the stiffness and cell length of the outer hair cells after impulse noise exposure, suggesting that mechanical changes at the cellular level are involved in noise-induced hearing loss. There is a recovery of the cellular stiffness and cell length over a two-week period, indicating an activation of cellular repair mechanisms for restoring the auditory function following noise trauma. The reduced stiffness observed at the cellular level is likely to be the cause for the downward shift of the characteristic frequency seen following acoustic trauma. The deterioration and the recovery of the mechanical properties of outer hair cells may form important underlying factors in all kinds of noise-induced hearing loss.