The endocochlear potential as an indicator of reticular lamina integrity after noise exposure in mice.

The endocochlear potential (EP) provides part of the electrochemical drive for sound-driven currents through cochlear hair cells. Intense noise exposure (110 dB SPL, 2 h) differentially affects the EP in three inbred mouse strains (C57BL/6 [B6], CBA/J [CBA], BALB/cJ [BALB]) (Ohlemiller and Gagnon, 2007, Hearing Research 224:34-50; Ohlemiller et al., 2011, JARO 12:45-58). At least for mice older than 3 mos, B6 mice are unaffected, CBA mice show temporary EP reduction, and BALB mice may show temporary or permanent EP reduction. EP reduction was well correlated with histological metrics for injury to stria vascularis and spiral ligament, and little evidence was found for holes or tears in the reticular lamina that might 'short out' the EP. Thus we suggested that the genes and processes that underlie the strain EP differences primarily impact cochlear lateral wall, not the organ of Corti. Our previous work did not test the range of noise exposure conditions over which strain differences apply. It therefore remained possible that the relation between exposure severity and acute EP reduction simply has a higher exposure threshold in B6 mice compared to CBA and BALB. We also did not test for age dependence. It is well established that young adult animals are especially vulnerable to noise-induced permanent threshold shifts (NIPTS). It is unknown, however, whether heightened vulnerability of the lateral wall contributes to this condition. The present study extends our previous work to multiple noise exposure levels and durations, and explicitly compares young adult (6-7 wks) and older mice (>4 mos). We find that the exposure level-versus-acute EP relation is dramatically strain-dependent, such that B6 mice widely diverge from both CBA and BALB. For all three strains, however, acute EP reduction is greater in young mice. Above 110 dB SPL, all mice exhibited rapid and severe EP reduction that is likely related to tearing of the reticular lamina. By contrast, EP-versus-noise duration examined at 104 dB suggested that different processes contribute to EP reduction in young and older mice. The average EP falls to a constant level after ∼7.5 min in older mice, but progressively decreases with further exposure in young mice. Confocal microscopy of organ of Corti surface preparations stained for phalloidin and zonula occludens-1 (ZO-1) indicated this corresponds to rapid loss of outer hair cells (OHCs) and formation of both holes and tears in the reticular lamina of young mice. In addition, when animals exposed at 119 dB were allowed to recover for 1 mo, only young B6 mice showed collapse of the EP to ≤5 mV. Confocal analysis suggested novel persistent loss of tight junctions in the lateral organ of Corti. This may allow paracellular leakage that permanently reduces the EP. From our other findings, we propose that noise-related lateral wall pathology in young CBA and BALB mice promotes hair cell loss and opening of the reticular lamina. The heightened vulnerability of young adult animals to noise exposure may in part reflect special sensitivity of the organ of Corti to acute lateral wall dysfunction at younger ages. This feature appears genetically modifiable.

Noise exposure alters cyclooxygenase 1 (COX-1) and 5-lipoxygenase (5-LO) expression in the guinea pig cochlea.

CONCLUSION: Changes in the metabolism of arachidonic acid (AA) might be part of a noise-induced compensatory mechanism with regional specificity. OBJECTIVES: The released imbalance of prostaglandins and leukotrienes, both AA metabolites, might result in altered blood flow regulation in the inner ear and probably contributes to noise-induced hearing loss. The aim of this study was to gain further information about noise-dependent changes in AA metabolism in the mammalian cochlea. METHODS: In this prospective animal study, 10 male guinea pigs were exposed to tone bursts for 1 h at 70 dB sound pressure level (SPL) (n = 5) or 90 dB SPL (n = 5). Five animals were used as controls. Alterations in cyclooxygenase 1 (COX-1) and 5-lipoxygenase (5-LO) expression were determined by quantitative immunohistochemical analysis in 11 cochlear regions. RESULTS: COX-1 expression was decreased after both 70 dB SPL and 90 dB SPL exposure in most cell types of the organ of Corti and increased in the nerve fibers of the osseous spiral lamina. 5-LO was lowered after 90 dB SPL exposure, preferentially in the third cochlear turn in the organ of Corti, in the first and second turn in spiral ganglion cells, and in all turns in the stria vascularis.

Histopathologic changes of contralateral human temporal bone in unilateral Ménière's disease.

HYPOTHESIS: To disclose the histopathologic findings in the contralateral temporal bone in unilateral Ménière's disease. BACKGROUND: Several functional studies reported abnormal findings in the contralateral ears in patients with unilateral Ménière's disease. METHODS: This study involved quantitative analysis, including the number of spiral ganglion cells, the loss of cochlear hair cells, the area of stria vascularis, and the density of fibrocytes in the spiral ligament. It included 14 temporal bones from 7 subjects with bilateral Ménière's disease, 30 temporal bones from 15 subjects with unilateral Ménière's disease, and 17 age-matched normal control temporal bones from 12 subjects. RESULTS: The mean number of spiral ganglion cells in the contralateral temporal bones in patients with unilateral Ménière's disease was 17,376.0 and was significantly lower than that in normal controls. The mean loss of inner and outer hair cells in the contralateral temporal bones in patients with unilateral Ménière's disease was significantly greater than that in normal controls in all turns. The stria vascularis was severely atrophic and degenerated in patients with Ménière's disease. The mean area of stria vascularis in contralateral temporal bones in patients with unilateral Ménière's disease was significantly smaller than normal controls. There was no significant difference in the density of fibrocytes in the spiral ligament between the diseased side and the contralateral side in patients with unilateral Ménière's disease and between normal control and contralateral side. CONCLUSION: The contralateral inner ear in patients with unilateral Ménière's disease has significantly more damage compared with inner ears of normal controls.

Fast alterations of vascular endothelial growth factor (VEGF) expression and that of its receptors (Flt-1, Flk-1 and Neuropilin) in the cochlea of guinea pigs after moderate noise exposure.

Vascular endothelial growth factor (VEGF) is a vascular permeability regulating, proangiogenic factor with neuroprotective properties. Its expression in the inner ear has been demonstrated, but little is known concerning its subcellular distribution or potential involvement in sound perception and adaptation to noise. Therefore, we determined the expression patterns and levels of VEGF and the three VEGF-receptors FLK, FLT and Neuropilin in the cochlea of guinea pigs, and examined the alterations occurring after noise exposure. After 70 dB exposure, VEGF expression was found to be reduced in all cell types of the organ of Corti, in the stria vascularis and in spiral ganglion cells. Additional down-regulation was observed in the spiral ligament and in interdental cells after 90 dB. In contrast, VEGF showed an in tendency increased level after both intensities in nerve fibers of the osseous spiral lamina. Expression of FLT was affected similarly, showing down-regulation after 70 and 90 dB on spiral ganglion cells, the nerve fibers of the osseous spiral lamina and on Deiters cells. Additionally, down-regulation was observed in the remaining cell types of the organ of Corti, the stria vascularis, the spiral ligament and the interdental cells. The Neuropilin levels remained unchanged by our experiments; apart from the blood vessel endothelium, there was no detectable expression in any of the cell types investigated. The FLK expression pattern was likewise unaffected by exposure to 70 or 90 dB, with the notable exception of an increased level occurring in Schwann cells after 90 dB. We postulate that modulation of VEGF and its receptors may be part of a neuroprotective mechanism in response to noise.

Impact of electrode insertion depth on intracochlear trauma.

OBJECTIVE: To assess the effect of cochlear implant (CI) insertion depth and surgical technique on intracochlear trauma. STUDY DESIGN AND SETTING: Twenty-one fresh human temporal bones were implanted with CI electrodes and underwent histologic processing and evaluation. Specimens were grouped into 3 categories: 1) soft implantation technique and standard electrode; 2) soft implantation technique and flexible prototype array; 3) forceful implantations and standard electrode. Based on the grading system (1 to 4), 2 numeric values were calculated indicating the overall severity of cochlear damage (trauma indices). RESULTS: Mean trauma index values were 13.8, 36.3, and 59.2 for group 1, 2, and 3, respectively. Differences in cochlear trauma (trauma index) were nonsignificant between specimens in groups 1 and 2 but were significant between groups 1 and 3. CONCLUSION: This study gives evidence that intracochlear trauma increases with deep insertions. Thus, in cases where cochlear integrity might be important, limited insertions should be achieved.

[Evaluation of the insertion-trauma of the Nucleus Contour Advance electrode-array in a human temporal bone model]. / Evaluation des Insertionstraumas des Nucleus Contour Advance-Elektrodenträgers im humanen Felsenbeinmodell.

BACKGROUND: The development of intracochlear electrode arrays is aiming at a placement close to the modiolus with an insertion as atraumatic as possible. A new perimodiolar electrode model the Nucleus Contour Advance was to be evaluated regarding the possible intracochlear trauma. METHODS: The implantation of the Contour Advance electrode was performed in 11 frozen native temporal bones. Beneath a regular insertion in 5 temporal bones in 6 cases the insertion was carried out using the "advance-off-stylett" technique with a fixed stylett. The temporal bones were embedded in metacrylate based resin for histomorphological evaluation. The evaluation was performed regarding to the intracochlear placement close to the modiolus and the damage to intracochlear fine structures (basilar membrane, osseus spiral lamina). RESULTS: In 2 out of 11 cases we found a perforation from the scala tympani to the scala vestibuli independent of the insertion-technique. A severe intracochlear trauma was observed in one case with fracture of osseus spiral lamina using the AOS-technique. A close position to the modiolus could be achieved by insertion the scala tympani without perforation of the basilar membrane. CONCLUSIONS: The Nucleus Contour Advance electrode array showed minimal trauma in human temporal bones by using a standard insertion technique. By using the freehand AOS-technique a severe cochlear trauma is possible. Therefore further development in electrode design and the use of an insertion-tool is recommended.

Clarion 1.2 standard electrode array with partial space-filling positioner: radiological and histological evaluation in human temporal bones.

The purpose of this study was to evaluate whether use of a positioner for situating the Clarion 1.29 standard electrode array in close proximity to the modiolus, causes damage to fine intra-cochlear structures, and to provide a comparison with results obtained for insertions of the array performed without a positioner. The study was performed in seven freshly frozen human temporal bones. Electrode location and intra-cochlear trauma was analysed using cross-sectional imaging and histological analysis. Insertion of the Clarion array did not reveal major trauma. The devices inserted with the positioner showed a consistently closer location of the electron array towards the modiolus, however, insertion resulted in significant displacement of both the electrode array and the positioner resulting in severe destruction of the basilar membrane and osseous spiral lamina along the length of the basal and middle turns. The devices inserted with the positioner resulted in major trauma to the basilar membrane and osseous spiral lamina. Therefore, systematic safety studies in larger samples of human temporal bones should be performed and the results carefully evaluated before implantation can be recommended unreservedly.

The effect of inflammation on blood vessel area in Rosenthal's canal in the cat cochlea.

The quantitative analysis of spiral ganglion cells is important. It is conventionally expressed as a cell density, the number of cells within Rosenthal's canal divided by its area. The area of Rosenthal's canal conventionally excludes the areas of blood vessels within it. If blood vessels proliferate in an inflammatory response then excluding their area may result in an under-estimate of cell loss. This study investigates whether blood vessel area increases with inflammation. Eighteen implanted and stimulated cat cochleas were studied. Using computer aided analysis techniques we measured the areas of Rosenthal's canals and its blood vessels. A histologist quantitatively graded the inflammation in each cochlea. Blood vessel area increased with the grade of inflammation. Multiple regression analysis showed this effect was significant (P < 0.01). This increase in blood vessel area may have an effect on density measurements made by excluding that area, leading to an under-estimate of ganglion cell loss.

Cochlear histopathologic characteristics following long-term implantation. Safety studies in the young monkey.

OBJECTIVE: To evaluate the safety of cochlear implantation in children 2 years of age or younger using a non-human primate model. DESIGN: Histopathologic study. SUBJECTS: Five macaque monkeys. INTERVENTION: A commercially produced electrode array was implanted for 3 years. RESULTS: Histologic examination demonstrated the biocompatibility of the scala tympani electrode array. Limited soft-tissue growth and new bone growth were observed in the lower basal turn. There was no evidence of intracochlear infection. Evidence of minimal insertion trauma was seen. Hair cell loss was, in general, confined to the region of the electrode array, with normal hair cell populations toward the apex, although there was evidence of hair cell preservation adjacent to the array. At least 20% of spiral ganglion cells were preserved even in regions with a smaller proportion of hair cells. CONCLUSIONS: Even after extended periods of implantation, the electrode array is well tolerated, and there is substantial preservation of neural elements.

Insertional trauma of multichannel cochlear implants.

Insertional trauma to the cochlea from three different multichannel cochlear implant electrodes was evaluated in a single-blind controlled study in fresh human temporal bones. Sixteen fresh human temporal bones were implanted with one of three types of multichannel electrodes (Symbion/InnerAid, Cochlear/Nucleus, or Storz/UCSF). Seven temporal bones were used as controls where a cochleostomy only was created. The temporal bones were evaluated histologically and cochlear histograms of the trauma were created. Although the three electrode designs caused damage which was unlikely to hinder implant performance, a distinct pattern of trauma was seen with each of the three electrode types. The least traumatic of the three electrode designs in this study was the Nucleus type. The degree of insertional trauma may be relevant to changing indications for insertion of cochlear implants as well as for patients with device failure who require reimplantation.

Symptomatic versus asymptomatic endolymphatic hydrops: a histopathologic comparison.

One of the unanswered questions in Meniere's disease research is the pathophysiology by which the classic symptoms are produced. A histopathological study was undertaken to identify the pathological features of symptomatic endolymphatic hydrops and their relationship to these symptoms. Two groups of temporal bones were examined, compared, and described. The first group was from patients with symptoms of Meniere's disease (n = 29). Temporal bones in the second group were chosen for the presence of endolymphatic hydrops and the absence of otologic symptoms (n = 13). Significant differences were noted in the severity of hydrops, the frequency of membrane ruptures, the endolymphatic duct, and coexistent pathologic conditions. Ruptures were seen in 38% of temporal bones from symptomatic patients and in only 8% of temporal bones from patients with asymptomatic endolymphatic hydrops. Based on this study and a review of the literature, the authors believe that the symptoms and findings of Meniere's disease are explained best on the basis of both chemical and physical mechanisms being operational intermittently and together.

[Experimental studies of ischemia of the cochlea. Part 3: Pathophysiology]. / Experimentelle Untersuchungen zur Ischämie der Cochlea. Teil 3: Pathohistologie.

Selective interruption of cochlear blood flow is possible without operative manipulations in the middle or inner ear. By fitting a magnet into the external auditory canal and injecting iron filings into the cephalic vein is it possible to obtain an impaired inner ear in otherwise healthy experimental animals that can be kept alive for any length of time. The hearing loss induced by the ischemia normally remains unchanged over a period of weeks. The highly vascularised areas of the cochlea, namely the spiral ligament, the vascular stria, the spiral prominence and the root cells in the external spiral sulcus, as well as the spiral limbus were all clearly degenerated, but to different degrees. Pronounced vacuolisation of cells, degeneration of tissue, reduction of cellular elements, stasis in large and small blood vessels and morphological disorganisation were observed. The organ of Corti showed no changes worthy of note. On revascularisation of the spiral ligament and the spiral limbus with resumption of function, the organ of Corti can return to normal activity again. Repeat blood flow disturbances can also lead to recurrent sudden hearing loss and intermittent loss of hearing.

On the pathomechanism of cochlear dysfunction in experimental perilymph fistulas.

The mechanism leading to hearing impairment in perilymph fistulas was investigated in guinea pigs with perforated round window membranes (RWM) by analyzing alterations of inner ear fluid pressure, changes of auditory function following manipulations to get presumed air bubbles out of the cochlea ("positional audiometry"), and temporal bone sections. The instantaneous loss of normal positive inner ear fluid pressure after RWM perforation had no immediate effect on auditory function. Inner ear pressure was restored 4 days following RWM perforation. "Positional audiometry" was negative in guinea pigs with perforated RWM. All ears in which auditory thresholds had increased had additional iatrogenic lesions at the spiral lamina. Fistulas in the RWM per se do not affect auditory thresholds. The question of the surgical repair of fistulas was not directly addressed; it only can be concluded that there are no direct sequelae of an isolated fistula which induce auditory impairment and which could be prevented by surgical repair of the fistula.

Inner ear morphologic changes resulting from cochlear implantation.

Insertion of electrodes into the cochlea can damage the spiral ligament, organ of Corti, osseous spiral lamina, and dendrites. The amount of damage depends on the method of insertion and the amount of surgical trauma. Damage is limited to the area immediately adjacent to the electrode. Damage in the cochlea and prolonged electrical stimulation do not affect ganglion cell population. The ganglion cell population responds directly to the electrical stimulus. Examination of three auditory nerves from three cases and the cochlear nuclei from one case also reveals no effect of prolonged electrical stimulation.

Quantitative analysis of cochlear sensory cells and neuronal elements in man.

The myelinated radial fibres in the osseous spiral lamina and the myelinated fibres in the cochlear nerve in the internal auditory canal as well as the sensory cells were counted in cochleae from 15 dissected temporal bones from 8 patients. Light microscopy was carried out on semithin sections of epoxy resin embedded tissue. Audiometry had been performed within 6 months prior to death. Three patients had normal hearing for their age group, 2 had slight presbyacusis and the remaining 3 had sustained noise injury. All specimens clearly had fewer fibres in the spiral lamina than in the internal auditory canal. The cochleae from patients with normal hearing for their age group had a difference in the nerve fibre counts of up to 34%. A case of sensorineural presbyacusis showed 31%, and a case of neural presbyacusis, 47% difference. The greatest difference was found in a case of acoustic trauma, the range in this group being between 25% and 55%. The lower the number of fibres in the spiral lamina, the greater was the difference in all but two specimens. A slow retrograde degeneration, i.e. beginning in the peripheral process of the cochlear nerve, could be an explanation for these findings.

A new model for photochemically induced thrombosis in the inner ear microcirculation and the use of hearing loss as a measure for microcirculatory disorders.

A new photochemical method was employed to cause disorders in the inner ear's microcirculation, using the rat as an animal model. Hearing loss was used as a measure for establishing the altered microcirculation. Under pentobarbital anesthesia, the middle ear was opened by a ventral approach. The lateral wall of the cochlea was then illuminated with a filtered xenon lamp (wavelength 540 nm) while rose bengal was infused intravenously. Photoactivated rose bengal produces oxygen radicals and oxygen singlets, which subsequently damage the vascular epithelium to cause the adhesion and aggregation of platelets in the small vessels. Disintegration of the inner ear hair cells at the irradiated site became evident 24 h after the illumination. These findings further suggest that the photochemical occlusion in the inner ear's microcirculation led to ischemic damage of the stria vascularis and the hair cells in the inner ear. When the action potential (AP) of the cochlea was measured with an electrocochleogram a gradual decrease occurred after the illumination. When acetylsalicylic acid was injected intravenously before treatment, the time required to completely suppress the AP was prolonged in a dose-dependent manner. Findings indicate that our method causes a photochemically induced occlusion in the inner ear's microcirculation and is therefore potentially useful for evaluating the various effects of drugs on the ear.

Degeneration patterns in the organ of Corti and spiral lamina.

The relation between organ of Corti degeneration and radial nerve fibre degeneration in the osseous spiral lamina was studied in two human cochleas.

Distribution of cochlear damage caused by the removal of the round window membrane.

The distribution of damage that occurs in the cochlea after removal of the round window membrane was examined in the apical, middle and basal regions with light and electron microscopy. The damage resembles that seen after acoustic trauma in many respects. The outer hair cells are often disrupted in damaged zones, and the radial afferent fibers to the inner hair cells swell enormously to form large vacuoles. 16 h after opening of round window, there is conspicuous swelling of myelinated axons in the osseous spiral lamina of the apical region. This swelling is associated with large vacuoles underneath the inner hair cells. 10 h after opening the round window, much smaller vacuoles are seen in the apical region. The distribution of the damage is not uniform throughout the cochlea. Damage is usually less severe and is not uniform in the middle region but is pronounced in the base. The nature of the damage is also variable in different animals. For example, sharply delimited, discontinuous damage to the inner hair cells was occasionally observed in the apical region. The most likely cause for the damage to the cochlea is a pressure differential across the organ of Corti that appears after removing the round window membrane. The damage apparently causes low frequency random movements of the basilar membrane that are observed in the experimental cochleas using a reflected laser beam.

Stria vascularis in acoustic trauma.

Mechanical energy of noise destroys not only the organ of Corti, Reissner's membrane, and the basilar membrane but also the lateral wall of cochlear duct. The damage is characterized by ruptures running parallel to the attachment of the basilar membrane in the prominentia spiralis or sulcus spiralis externus. The other signs of injury after acoustic overstimulation are blisterlike detachments of the stria vascularis from the spiral ligament. The mechanism of injury is interpreted as the result of interaction of given mechanical properties of exposed tissues and of the kinetic energy of sound in narrow segments of cochlear duct.