Patterns of neural degeneration in the human cochlea and auditory nerve: implications for cochlear implantation

Although the identity of all the variables that may influence speech recognition after cochlear implantation is unknown, the degree of preservation of spiral ganglion cells is generally considered to be of primary importance. A series of experiments in our laboratories, directed at quantification of surviving spiral ganglion cells in the profoundly deaf, evaluation of the predictive value of a variety of clinical parameters, and the evaluation of the consequences of implantation in the inner ear, is summarized. Histologic study of the inner ears of patients who were deafened during life demonstrated that the cause of deafness accounted for 57% of the variability of spiral ganglion cell counts. Spiral ganglion cell counts were highest in individuals deafened by aminoglycoside toxicity or sudden idiopathic deafness and lowest in those deafened by postnatal viral labyrinthitis, congenital or genetic deafness, or bacterial meningitis. Study of the determinants of degeneration of the spiral ganglion revealed that degeneration is most severe in the basal compared with the apical turn and more severe when both inner and outer hair cells are absent. Unlike the findings in some experimental animal studies, no survival advantage of type II ganglion cells could be identified. There was a strong negative correlation between the degree of bony occlusion of the cochlea and the normality of the spiral ganglion cell count. However, even in specimens in which there was severe bony occlusion, significant numbers of spiral ganglion cells survived. A strong positive correlation between the diameter of the cochlear, vestibular, and eighth cranial nerves with the total spiral ganglion cell count (p < 0.001) was found. This would suggest that modern imaging techniques may be used to predict residual spiral ganglion cell population in cochlear implant candidates. Trauma from implantation of the electrode array was studied in both cadaveric human temporal bone models and temporal bones from individuals who received implants during life. A characteristic pattern of damage to the lateral cochlear wall and basilar membrane was identified in the upper basal turn. New bone formation and perielectrode fibrosis was common after cochlear implantation. Despite this significant trauma and reaction, there is no firm evidence that further degeneration of the spiral ganglion can be predicted as a consequence.

Clinical aspects of round window membrane permeability under normal and pathological conditions

Current research and an overall review of 25 years of round window membrane studies are presented. The approach, rationale and concepts that have evolved from these studies are described. Ultrastructural studies of the round window membrane of humans, monkeys, felines and rodents have disclosed three basic layers: an outer epithelium, a middle core of connective tissue and an inner epithelium. Interspecies variations are mainly in terms of thickness, being thinnest in rodents and thickest in humans. Morphologic evidence suggests that the layers of the round window participate in resorption and secretion of substances to and from the inner ear, and that the membrane could play a role in the defense system of the ear. Different substances, including antibiotics and tracers, when placed in the middle ear side traverse the membrane. Tracers placed in perilymph become incorporated into the membrane by the inner epithelial cells. Permeability is selective and factors affecting permeability include size, concentration, electrical charge, thickness of the membrane and tacilitating agents. Passage of substances through the membrane is by different pathways, the nature of which is seemingly decided at the outer epithelium of the membrane. Round window membrane studies have provided increased knowledge of the anatomy and function of this structure, as well as new insights into pathology and pathogenesis. The concepts that have evolved from these studies are potentially useful for understanding middle and inner ear interactions, and for eventual drug delivery (based on permeability) to the inner ear.

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.

Effects of Type 2 Diabetes Mellitus on Cochlear Structure in Humans

OBJECTIVE

To evaluate the effects of type 2 diabetes mellitus on cochlear elements in humans.

DESIGN

Comparative study of the histopathologic characteristics of human temporal bones.

SETTING

Otopathology laboratory in a tertiary academic medical center.

PATIENTS

Temporal bones from 18 patients with type 2 diabetes mellitus were divided into 2 groups according to the method of management of diabetes: insulin in 11 patients (mean age, 51.9 years; age range, 44-65 years) and oral hypoglycemic agents in 7 patients (mean age, 54.4 years; age range, 45-64 years). The diabetic groups and 26 age-matched controls (mean age, 52.9 years) were examined using light microscopy, and the cochlear changes were compared between groups.

MAIN OUTCOME MEASURES

Morphometric measurements of vessel wall thickness in the basilar membrane and stria vascularis were made in all turns of the cochlea at the midmodiolar level. Area measurements of the stria vascularis were made in all turns of the cochlea at the midmodiolar level. Cochlear reconstructions and standard cytocochleograms were prepared using an oil immersion objective. The number of spiral ganglion cells was determined for each segment of the cochlea. Comparisons were made in each segment between diabetic and control groups.

RESULTS

In the insulin group, walls of the vessels of the basilar membrane and stria vascularis in all turns were significantly thicker than those of controls. Walls of the vessels of the stria vascularis in the basal turn were also significantly thicker in the oral hypoglycemic group than in controls. Atrophy of the stria vascularis in most turns of the insulin group and the lower middle turn of the oral hypoglycemic group was significantly greater than in the controls. Loss of cochlear outer hair cells was significantly greater in the lower and upper basal turns in both diabetic groups. No significant difference was found in the number of spiral ganglion cells or inner hair cells between groups.

CONCLUSION

This study demonstrates that cochlear microangiopathy and degeneration of the stria vascularis and cochlear outer hair cells are found in patients with type 2 diabetes mellitus.

Blast overpressure induced structural and functional changes in the auditory system

Blast overpressure of sufficient intensity can produce injury to various organ systems. Unprotected ears result in the auditory system being the most susceptible. The injuries to the auditory system include: rupture of the tympanic membrane, dislocation or fracture of the ossicular chain, and damage to the sensory structures on the basilar membrane. All these injuries can be characterized as a form of mechanical damage to the affected structure. Injury to the sensory structures on the basilar membrane leads to temporary and permanent loss of hearing sensitivity. The temporary component of the hearing loss shows a time course after removal from the noise which frequently will include an initial increase in hearing loss followed by a recovery period during which threshold may return to preexposure levels or stabilize at a higher level which represents a permanent loss of hearing sensitivity. This type of recovery function suggests that there are damage processes which continue after the traumatic event and that intervention might mitigate some of the damage and hearing loss.

The Nucleus Contour electrode array: a radiological and histological study

OBJECTIVES

To evaluate the handling and insertion trauma of the recently developed Nucleus perimodiolar Contour electrode array (Cochlear Ltd., Pty, Lane Cove, New South Wales, Australia) in human temporal bones compared with the Nucleus standard straight electrode array.

STUDY DESIGN

E-perimental control group.

METHODS

Twenty-nine fresh-frozen bones were implanted with different electrode arrays by an experienced cochlear implant surgeon, and evaluated both radiologically and histologically.

RESULTS

Intracochlear insertion of the standard Nucleus straight electrode array was found to be atraumatic, confirming previous findings in the literature. Insertion of the Nucleus Contour electrode array resulted in instances of localized basilar membrane penetration causing the electrode array to move from the scala tympani into the scala vestibuli. However, this trauma did not result in any observable damage to the osseous spiral lamina or the modiolus. Basilar membrane penetration was observed in six of eight cochlear bones when a standard cochleostomy size (approximately 0.8 mm) and site (anterior and superior to the round window) were used. However, when the surgical technique was modified to use a slightly larger cochleostomy ( approximately 1.8 mm) situated closer to the round window and employ a partial stylet withdrawal technique during electrode insertion, the frequency of penetrations was restricted to two of seven bones. This trauma rate is comparable to that observed with other cochlear implants designs.

CONCLUSIONS

Following our results, the design of the Nucleus Contour electrode appears to fulfill the safety requirements for an intracochlear electrode array, provided that the surgical insertion technique is modified in the manner outlined.

Multichannel intracochlear electrodes: mechanism of insertion trauma

Cochlear damage from electrode insertion, a potential cause of further neural degeneration, is a major concern in the use of intracochlear electrodes. A study was undertaken to evaluate mechanisms by which damage may occur. Fresh human temporal bone preparations were created to allow direct intracochlear observation during round window insertion of a free-fitting multichannel intracochlear electrode array. The path taken by the electrode, point of first resistance, and any resulting damage were documented. Tips of study electrodes in this preparation tended to embed in the outer wall of the scala tympani. The integrity of the basilar partition was largely maintained during insertion when this was stopped at the point of first resistance. However, insertion beyond the point of first resistance typically resulted in widespread damage to intracochlear structures.

Electron microscopic findings in presbycusic degeneration of the basal turn of the human cochlea

Three human temporal bones with presbycusis affecting the basal turn of the cochlea were studied by light and electron microscopy. Conditions in two ears examined by light microscopy were typical of primary neural degeneration, with a descending audiometric pattern, loss of cochlear neurons in the basal turn, and preservation of the organ of Corti. Ultrastructural analysis revealed normal hair cells and marked degenerative changes of the remaining neural fibers, expecially in the basal turn. These changes included a decrease in the number of synapses at the base of hair cells, accumulation of cellular debris in the spiral bundles, abnormalities of the dendritic fibers and their sheaths in the osseous spiral lamina, and degenerative changes in the spiral ganglion cells and axons. These changes were interpreted as an intermediate stage of degeneration prior to total loss of nerve fibers and ganglion cells as visualized by light microscopy. In the third ear the changes observed were typical of primary degeneration of hair and supporting cells in the basal turn with secondary neural degeneration. Additional observations at an ultrastructural level included maintenance of the tight junctions of the scala media despite loss of both hair and supporting cells, suggesting a capacity for cellular "healing" in the inner ear. Degenerative changes were found in the remaining neural fibers in the osseous spiral lamina. In addition, there was marked thickening of the basilar membrane in the basal turn, which consisted of an increased number of fibrils and an accumulation of amorphous osmiophilic material in the basilar membrane. This finding supports the concept that mechanical alterations may occur in presbycusis of the basal turn.

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.

Round window membrane and labyrinthine pathological changes: an overview

The round window membrane is considered the most likely pathway from the middle to the inner ear. Various substances placed in the middle ear have been seen to pass through the round window membrane. Once toxic substances or inflammatory mediators such as cytokines and nitric oxide enter the inner ear, various inner ear sequelae such as labyrinthitis, endolymphatic hydrops, sensorineural hearing loss or more insidious diseases can occur.

Noise-induced transient microlesions in the cell membranes of auditory hair cells

Several types of nonauditory cells recover from transitory mechanically induced microlesions in their cell membranes. We report evidence that hair cells in the auditory papilla of the alligator lizard suffered similar membrane wounding when exposed to noise loud enough to induce a temporary threshold shift. Lucifer yellow, a molecular marker that does not normally penetrate through the cell membrane into the cytoplasm, was introduced into the extracellular fluid bathing the basolateral membrane of the hair cells. We assessed the effect of loud noise on the function of the ear by measuring compound action potentials of the auditory nerve before exposure to the noise, immediately after cessation of the noise, and after recovering overnight. Hair cells that were exposed to the noise took up much more Lucifer yellow than hair cells that were not exposed. We propose that the Lucifer yellow entered the hair cells via noise-induced lesions in their cell membranes, and that the cells were able to survive and recover functionally.

Comparison of cochlear histopathology following two implant designs for use in scala tympani

Two designs of intracochlear implants were evaluated for their histopathologic effects in the monkey ear. A molded electrode designed to fit the contour of the basal turn of the scala tympani tended to create basilar membrane fistulas and osseous spiral lamina fracture, along with relatively extensive loss of spiral ganglion cells in sites adjacent to the implant. A delicate, free-fit electrode induced local encapsulation but little or no mechanical damage and limited degeneration of spiral ganglion cells. Both electrode types occasionally induced changes in stria vascularis.

Migration of hyaline cells into the chick basilar papilla during severe noise damage

Severe acoustic damage in the chick cochlea causes a destruction of both hair cells and supporting cells in a localized area on the basilar papilla. In this region, the sensory cells are replaced by a layer of flattened epithelial cells. We have employed scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) to examine the structure and cytoskeletal changes involved in this process. Immunocytochemical staining for actin indicates that the flattened cells are derived from the hyaline cells normally located along the inferior edge of the basilar papilla. In control cochleae the hyaline cells contain dense bundles of actin filaments that anchor into the basal surface of the cells. The hyaline cells appear to redistribute into the severely damaged region by extending the actin bundles at their basal surfaces. Moreover, the efferent nerves that normally form a network among the hyaline cells move into the severely damaged area along with the hyaline cells. In moderately damaged cochleae, where only hair cells are lost, the hyaline cells do not spread into the damaged region. The functional role of this hyaline cell migration is unknown, but it may be involved in maintenance or repair of the severely damaged cochlea.

Morphometric analysis of age-related changes in the human basilar membrane

The histopathologic correlates of presbycusis suggest several categories, including degeneration of sensory cells, neurons, or the stria vascularis. Lack of clear-cut histopathologic changes in some cases has suggested an "indeterminate category"; however, several studies have suggested that a disorder of the basilar membrane (BM) may underlie indeterminate presbycusis. The objective of the present study was to quantify age-related changes in the human BM and correlate them with audiometric patterns. Under high-resolution light microscopy, BM thickness was calculated, and the number of tympanic mesothelial cells (TMCs) lining the BM was counted, at 4 cochlear locations in 92 temporal bones. The control group (n = 80) included subjects from 10 decades of age with normal hearing and/or histopathologic findings. The indeterminate group (n = 12) consisted of elderly patients (ages 64 to 91 years) with hearing loss and no apparent histopathologic changes. Age-related BM thickening was seen in both groups, but only in the most basal cochlear region. The BM thickness in the indeterminate group was not significantly different from that of age-matched controls. Counts of TMCs showed age-related decreases in all cochlear regions in both groups; however, TMC counts in the indeterminate group were not different from those of age-matched controls. The results suggest that BM histopathology is not a common cause of presbycusis. Although age-related BM thickening, seen in both groups, could contribute to hearing loss, the extreme basal region, to which the thickening was confined, is not tested in routine audiometry.

Recovery of the endocochlear potential and the K+ concentrations in the cochlear fluids after acoustic trauma

Intense noise stimulation (142 dB, 1/3-octave-band noise centred at 1 kHz for 1 h) causes damage mainly in the second turn of the cochlea. Several hours (3-5) after the noise exposure, the endocochlear potential (EP) was found to be very low (5.7 +/- 6.0 mV). Similarly, the K+ concentration in the endolymph (Ke+) had decreased to low values (18.9 +/- 9.5 mM). The return of EP and Ke+ to normal values took 5-20 days. In contrast to the Ke+ changes, the perilymph K+ concentration (Kp+) increased slightly after the noise exposure to 4.5 +/- 1.7 mM and returned to normal values one day after the exposure. Differences were found in the time course of the EP, Ke+ and Kp+ changes after the arrest of ventilat ion when animals with acoustic trauma were com,ared with normal healthy individuals. The anoxic EP in noise-exposed animals never decreased to values more negative than -20 mV. The results imply that the inner ear mechanisms maintaining positive EP, Ke+ and Kp+ are severely damaged after acoustic trauma and that their function is restored in 5-20 days. With respect to some parameters (decrease of the EP during anoxia, the value of anoxic negative EP, EP overshoot after reventilation) the inner ear mechanisms are, however, still abnormal.

Electrophysiological and morphological evaluation of the acute ototoxicity of sodium nitroprusside

Nitric oxide (NO) is a messenger molecule that mediates several physiological functions and pathological processes. Sodium nitroprusside (SNP), a potent vasodilator, when given clinically as an anti-hypertension agent, exerts its function by releasing NO. It was reported recently that SNP causes a loss of auditory nerve compound action potential (CAP) after topical application of SNP on guinea pig round window membrane (RWM). The current study was designed to investigate the ototoxic target of SNP through both electrophysiological and morphological approaches. The CAP threshold at frequencies ranging from 2 to 36 kHz, the cochlear microphonic quadratic distortion product (cmQDP, F2-F1, where F1 = 17.1 kHz; F2 = 18 kHz), and the cochlear microphonic (CM) at the frequency of F1 were recorded via a round window electrode before and up to 2 h after RWM application of 1 microliter of drug solution. Cochlear blood flow (CBF) and arterial blood pressure were monitored. The cochleae were then processed for morphological examination. The effect of SNP on endocochlear potential (EP) was also studied. Results showed that cmQDP, CM, and CAP, as well as EP, were suppressed in varying amounts, while CBF was substantially increased following drug application. Morphological evaluations showed swelling of the afferent inner radial dendrites within the basal cochlear turn in the higher concentration groups of SNP, while the hair cells presented no evidence of damage at the light microscopic level. The results indicate that SNP has an acute ototoxic effect in a concentration- and time-dependent manner. The targets of SNP ototoxicity are at least the afferent dendrites and stria vascularis.

Combining perimodiolar electrode placement and atraumatic insertion properties in cochlear implantation -- fact or fantasy?

CONCLUSIONS

Except for basal cochlear traumatization, all specimens implanted into scala tympani showed atraumatic insertion properties and good perimodiolar electrode positioning. Cochleostomy preparation and placement can have a significant impact on levels of basal cochlear trauma.

OBJECTIVE

In the past, perimodiolar cochlear implant electrodes increased the risk for intracochlear traumatization when compared to free-fitting arrays. Recently, however, clinical evidence for atraumatic perimodiolar implantations with preservation of residual hearing has been described. The aim of this paper was to histologically evaluate a perimodiolar cochlear implant array for its insertion properties in cadaver human temporal bones. Surgical and electrode factors, as well as preparation artifacts influencing intracochlear trauma, were considered in the evaluation.

MATERIALS AND METHODS

Sixteen human temporal bones were harvested up to 24 hours post mortem and implanted immediately with the Nucleus 24 Contour Advance cochlear implant electrode array. Implantations were either performed using a regular caudal approach cochleostomy or through the round window membrane. After implantation, all bones underwent special histological processing, which allowed sectioning of undecalcified bone. Insertion properties were evaluated according to a grading system.

RESULTS

Fourteen specimens were implanted into scala tympani and only two exhibited basal trauma attributable to electrode insertion characteristics. Two bones were implanted into scala vestibuli after causing trauma in the region of the cochleostomy. Insertion depths ranged from 180 degrees to 400 degrees. All bones showed good perimodiolar electrode positioning. Basal trauma due to surgical issues and histological artifacts was present in 10 of 16 bones.

Reversible and irreversible damage to cochlear afferent neurons by kainic acid excitotoxicity

Kainic acid (KA) selectively damages afferent synapses that innervate, in chickens, mainly tall hair cells. To better understand the nature of KA-induced excitotoxic damage to the cochlear afferent neurons, KA, at two different concentrations (0.3 or 5 mM), was injected directly into the inner ear of adult chickens. Pathologic changes in the afferent nerve ending and cell body were evaluated with light and transmission electron microscopy at various time points after KA application. The compound action potential (CAP) and cochlear microphonic (CM) potential were recorded to monitor the physiologic status of the afferent neurons and hair cells, respectively. Hair cell morphology and function were essentially normal after KA treatment. However, afferent synapses beneath tall hair cells were swollen within 30 minutes after KA at both low (KA-L) and high (KA-H) doses. In the KA-L group, the swelling disappeared within 1 day and the morphology of the postsynaptic region returned to near normal condition. In the KA-H group, by contrast, the vacant region beneath tall hair cells remained evident even 20 weeks after KA. The number of cochlear ganglion neurons in the KA-H group decreased progressively from 1 to 8-20 weeks, whereas hair cells in the basilar papilla remained morphologically intact out to 20 weeks after KA. There was no significant change in neuron number in the KA-L group. Temporal changes in the CAP amplitude paralleled the anatomic changes, although the CAP only partially recovered. These results suggest that KA induces partially reversible damage to cochlear afferent neurons with low KA concentration; above this level, KA triggers irreversible, progressive neurodegeneration.

Current issues in cochlear gene transfer

Cochlear gene therapy represents a potential experimental and therapeutic tool to understand and treat deafness. In designing cochlear gene transfer studies, the chosen route of delivery of vector and the choice of gene therapy vector have to be given careful consideration. Several different routes of delivery have been tested in our laboratory including infusion with osmotic minipump, direct microinjection into the cochlea and application of vector-transgene complex-soaked Gelfoam((R)) into the direct contact with the round window membrane. In our experience, the latter is an easy, safe and atraumatic technique to deliver gene into the cochlea. A number of different gene transfer vectors have been investigated in vivo for their efficacy, utility and safety in intracochlear gene transfer. Vectors successfully studied include cationic liposomes, adeno-associated virus, adenovirus, lentivirus, herpes simplex virus and vaccinia virus. While the viral vectors offer clear experimental advantages, human gene therapy in the future will likely utilize nonviral vectors to maximize safety. Finally, safety issues regarding dissemination of gene transfer vectors beyond the target cochlea will need to be adequately addressed.

Temporal Bone Histopathology in Alport Syndrome

OBJECTIVE

To determine the histopathologic abnormalities within the cochlea in Alport syndrome.

BACKGROUND

Alport syndrome, which manifests as hereditary nephritis and sensorineural hearing loss (SNHL), is caused by mutations in genes that code for the proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen. The proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen are present in the basement membrane of the organ of Corti. Previous temporal bone studies have failed to identify histopathologic correlates for the SNHL.

METHODS

We examined temporal bones from nine individuals with a clinical diagnosis of Alport syndrome. One of our cases also had genetic testing that showed a mutation in the type IV collagen proportional, variant5 chain gene.

RESULTS

By light microscopy, eight of nine cases demonstrated two unique pathologic changes: 1) a "zone of separation" between the basilar membrane and overlying cells of the organ of Corti and 2) presence of cells filling the tunnel of Corti and extracellular spaces of Nuel. The cytologic losses of hair cells, stria vascularis, and cochlear neuronal cells were insufficient to account for the observed SNHL in our cases. Electron microscopy was performed in four cases; all four demonstrated the following: 1) the zone of separation that was observed at light microscopy occurred between the basement membrane and the basilar membrane, 2) the cells within the tunnel of Corti and spaces of Nuel were morphologically similar to supporting cells, and 3) the basement membrane of strial capillaries and the spiral vessel (under the basilar membrane) were normal.

CONCLUSIONS

The histopathologic correlates of cochlear involvement in Alport syndrome are abnormalities of the basement membrane of cells of the organ of Corti and dysmorphogenesis (cellular infilling of the tunnel and extracellular spaces) of the organ of Corti. We hypothesize that these abnormalities result in SNHL by altering cochlear micromechanics.

Morphologic changes in round window membrane after topical hydrocortisone and dexamethasone treatment

HYPOTHESIS

Are all glucocorticoids supposed to have the same effect on the round window membrane?

BACKGROUND

Interest in glucocorticoids for topical treatment of inner ear diseases is increasing. The safety of such treatment should therefore be an important consideration before clinical use.

METHODS

In this study the authors investigated the morphology of the round window membrane after topical instillation of dexamethasone or hydrocortisone into the middle ear. Twenty Sprague-Dawley rats were used. Five rats received 5 doses, and five rats 10 doses, of 1 microg (20 microl) dexamethasone in the right ear, and five others were given 5 doses, and five rats 10 doses, of 2% (20 microl) hydrocortisone solution, also in the right ear. Membrane morphology was studied in both light microscopy and transmission electron microscopy. The thickness of exposed membranes was measured and compared with that of control membranes.

RESULTS

Thickening and microscopically signs of inflammation were observed in hydrocortisone-exposed membranes but not in dexamethasone-exposed membranes, which did not differ morphologically from those in control ears.

CONCLUSION

Although hydrocortisone has anti-inflammatory properties, it seems to provoke inflammation in the round window membrane after topical instillation. Dexamethasone had no such effects, however.

Inner-ear abnormalities and their functional consequences in Belgian Waterslager canaries (Serinus canarius)

Recent reports of elevated auditory thresholds in canaries of the Belgian Waterslager strain have shown that this strain has an inherited auditory deficit in which absolute auditory thresholds at high frequencies (i.e. above 2.0 kHz) are as much as 40 dB less sensitive than the thresholds of mixed-breed canaries and those of other strains. The measurement of CAP audiograms showed that the hearing deficit is already present at the level of the auditory nerve (Gleich and Dooling, 1992). Here we show gross abnormalities in the anatomy of the basilar papilla of Belgian Waterslager canaries at the level of the hair cell. The extent of these abnormalities was correlated with the amount of hearing deficit as measured behaviorally.

Two modes of auditory hair cell loss following acoustic overstimulation in the avian inner ear

To determine the type of cell death occurring and how the removal of damaged cells proceeds following overstimulation, we examined chick basilar papillae using an in situ DNA nick end labeling method and transmission electron microscopy. Two distinct modes of hair cell loss were identified. First, hair cells which had not progressed into typical cell death processes, apoptosis or necrosis, were deleted by extrusion from the epithelium just after sound exposure. Second, hair cells manifested degeneration through the process of apoptosis, then further deterioration within the epithelium after the beginning of the process of hair cell regeneration. The latter mode may contribute to the following repair processes.

Conflicting electrophysiological and anatomical data from drug-impaired guinea pig cochleas

In guinea pigs that had been treated with very large doses of the aminoglycoside amikacin (14 x 450 mg/kg/day, i.m.) clear, short-latency responses to various click stimuli could be recorded at the round window. When the same cochleas were examined histologically, no outer or inner hair cells could be found along the entire length of the basilar membrane, save for a very few outer hair cells remaining at the apex. The response patterns resembled that of the compound action potential, and various characteristics suggest that they were of neural origin. Vestibular function, investigated by electronystagmography during rotation, appeared normal, as did most of the saccular and utricular hair cells. Transmission electron microscopy revealed a significant number of cochlear nerve fibres still innervating the remnants of Corti's organ. In other cochleas with similarly extensive destruction induced by another aminoglycoside (sisomycin, 14 x 125 mg/kg/day, 15 days as well as 3 months post-Rx), no responses could be recorded from the round window. Cochleas that were less affected, with the upper turns preserved, gave only small, long-latency responses. These preliminary observations are confirmed by further experiments now in progress. They suggest that unless a considerable number of inner hair cells remained undetected in the lower basal turn, a possibility that appears highly unlikely, there was either a direct mechanical excitation of cochlear nerve fibres, or an acoustical stimulation of vestibular sense organs.

Cochlear hair cell loss in single-dose versus continuous round window administration of gentamicin

CONCLUSIONS

Gentamicin-induced cochlear hair cell loss depends on local middle ear administration kinetics and the total drug dose. Single-dose gentamicin instillation in the middle ear is associated with a high variation in hair cell loss.

OBJECTIVE

To compare the effects of single-dose and continuous round window administration of gentamicin on cochlear hair cell loss in a guinea pig model.

MATERIAL AND METHODS

Two methods for drug administration to the inner ear were used. In groups of five animals, a total dose of 0.8 or 3.2 mg of gentamicin was either instilled as a single dose directly into the round window niche or administered continuously over a 1-week period using a pump-catheter system. Continuous administration was achieved by means of a posterior tympanotomy and subcutaneous placement of an osmotic pump fitted with a catheter. The tip of the catheter was fixed in the round window niche. One group of five animals served as controls and received a saline infusion. The animals were sacrificed after 1 week and hair cell loss was determined microscopically after dissection and phalloidin labelling of the basilar membrane and organ of Corti.

RESULTS

Quantitation of cochlear hair cell loss revealed a dose-dependent effect of gentamicin. With both treatment modalities the higher dose induced a higher percentage of hair cell loss. There was inner and outer hair cell loss in all four groups that received gentamicin. With the single-dose instillation, hair cell loss was distributed irregularly from the round window membrane towards the cochlear apex, whereas continuous administration induced hair cell loss close to the round window membrane. Single-dose instillation induced greater hair cell loss than continuous administration at the same dose. The inter-individual variation in hair cell loss was highest following single-dose instillation.