Immunoglobulin deposition in thickened basement membranes of aging strial capillaries

The presence of immunoglobulins in the thickened basement membrane (BM) of aging strial capillaries was investigated as a possible indicator of autoimmunity in the genesis of atypical BM. Cochleas from young and old Mongolian gerbils raised in quiet were examined by immunostaining at the light microscopic level for IgG and IgM and for the BM components laminin (La) and type IV collagen (IV-C). Another age-graded series of cochleas was stained for IgG at the ultrastructural level. No immunoreactive IgG was detected in specimens from animals less than 6 months old. In contrast, 2 of 12 cochleas from 20- to 28-month-old gerbils and 11 of 20 cochleas from gerbils 30 months or older showed positive staining for IgG in strial capillary BM. IgM was not detected at any age. At the electron microscope level, no immunoreactive IgG was detected in the stria of cochleas younger than 30 months. However, labeling demonstrative of IgG was observed in the thickened BM of some strial capillaries in all six cochleas from gerbils older than 33 months. Lysosome-like granules in endothelial cells and the superiormost marginal cells also stained for content of IgG as did fibrillar material in edematous regions in the intrastrial space. In addition to showing accumulation of IgG, the findings confirm our prior demonstration of increased La deposition in the thickened strial capillary BM of all cochleas from old gerbils. The BM alterations appear confined to strial capillaries in old gerbils, since morphological observations and immunostaining for La and IgG failed to detect changes in BMs at any other site in a wide survey of aged gerbil organs including vessels in other regions of the affected cochleas. The results point more towards the development of an age-dependent permeability to IgG selectively in strial capillaries than to autoimmunity as an explanation of the IgG in BM.

Temporal bone pathology of a patient with cochlear implant

A 66-year-old man with profound total deafness received a 22-channel cochlear implant. After surgery, his hearing improved and the postoperative course was uneventful except transient vertigo. The patient died of lung cancer after 2.5 years. Twelve hours after death, the temporal bone was harvested for histological and electron microscopic examination. The mastoid air cells were preserved, although a few mucosal folds were present. The Dacron mesh tie had largely been absorbed, but part remained embedded in fibrous tissue. No inflammation was noted in the tympanic cavity or the round window through which the electrodes were inserted. In the cochlea, the scala tympani was filled with fibrous tissue at the site of electrode insertion. The scala media had collapsed, while the organ of Corti and stria vascularis showed degeneration. Osseous spiral bundles and spiral ganglion cells were noted, though reduced in number.

Time course of nerve-fiber regeneration in the noise-damaged mammalian cochlea

The time course of events which are essential for nerve-fiber regeneration in the mammalian cochlea was determined using a group of chinchillas that had been exposed for 3.5 hr to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. The animals recovered from 40 min (0 days) to 100 days at which times their inner ears were fixed and the organs of Corti prepared for phase-contrast and bright-field microscopy as plastic-embedded flat preparations. Selected areas identified in the flat preparations were semi-thick and thin sectioned at radial or tangential angles for examination by bright-field and transmission electron microscopy. The following time-ordered events appeared critical for nerve-fiber regeneration: (1) The area of the basilar membrane in which regeneration had a possibility of occurring showed signs of severe injury. Outer hair cells degenerated first followed by outer pillars, inner pillars, inner hair cells and other supporting cells; (2) Myelinated nerve fibers in the osseous spiral lamina became fragmented, starting at the distal ends of the fibers. This degeneration gradually extended back to Rosenthal's canal; (3) Fibrous processes, originating from Schwann-like cells in the osseous spiral lamina, extended laterally on the basilar membrane; (4) Schwann cells lined up medial to the habenulae perforata in the areas of severest damage, apparently ready to migrate through the habenulae onto the basilar membrane; (5) Schwann-cell nuclei appeared on the basilar membrane beneath the developing layer of squamous epithelium which was in the process of replacing the degenerated portion of the organ of Corti; (6) Regenerated nerve fibers with thin myelin sheaths or a simple investment of Schwann cell cytoplasm appeared in areas of total loss of the organ of Corti; and (7) The myelin sheaths on the regenerated nerve fibers gradually became thicker.

Hair cell loss from acoustic trauma in chloroquine-treated red, black and albino guinea pigs

In order to further elucidate the relationship between noise-induced hearing loss and pigmentation, a two-factor study was designed. Albino, red and black guinea pigs were divided into controls and chloroquine-treated groups and exposed to 1 kHz noise, 105 dB SPL, for 72 hours. One month later the animals were sacrificed and the loss of hair cells evaluated. The red guinea pigs developed a greater hair cell loss (OHC) in all three OHC rows than black or albino animals. Black and albino groups showed equal amounts of OHC loss. A high dosage of chloroquine seemed to reduce the OHC loss in albino, but not in black or red guinea pigs. The greater OHC loss in red compared with black animals is in accordance with the original hypothesis that melanin protects the inner ear against noise trauma. However, as red guinea pigs developed greater OHC loss than albinos, it is obvious that the original hypothesis needs to be modified to consider also the different melanin types, i.e., the black eumelanin and the red pheomelanin. The present results are interpreted as a toxic interaction in the strial melanocytes between pheomelanin and noise. It is suggested that the pathophysiology of noise-induced hearing loss involves cochlear mechanisms related to radical oxygen species (ROS) as melanin both generates and neutralizes ROS. A hypothesis about a linkage between dopamine, noise trauma and the cochlear melanocyte system is discussed.

Does the organ of Corti attempt to differentiate new hair cells after antibiotic intoxication in rat pups?

In the adult mammalian cochlea, post-injury hair cell losses are considered to be irreversible. Recent studies in cochlear explants of embryonic rodents show that the organ of Corti can replace lost hair cells after injury. We have investigated this topic in vivo during the period of cochlear development. Rat pups were treated with a daily subcutaneous injection of 500 mg/kg amikacin for eight consecutive days between postnatal day 9 (PND 9) and PND 16. During this period the organ of Corti is not fully mature, but hair cells are hyper-sensitive to aminoglycoside antibiotics. Scanning and transmission electron microscopy was used to evaluate morphological changes in the organs of Corti during the treatment and at different post-treatment periods, up until PND 90. A massive loss in outer and inner hair cells was observed at least as early as PND 14. A prominent feature in the apical part of cochleas at PND 21 and 35 was the transient presence of small atypical cells in the region of pre-existing outer hair cells. These atypical cells had tufts of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the replacement of degenerating inner hair cells by pear-shaped supporting cells throughout the cochlea. These cells were covered with long microvilli, and their basal pole was contacted by both afferent and efferent fibers, as in the early stages of inner hair cell maturation. At PND 55 and 90, these features were not clearly observed due to further cytological changes in the organ of Corti. It is possible that an attempt at hair cell neodifferentiation could occur in vivo after an amikacin treatment in the rat during the period of cochlear hyper-sensitivity to antibiotic.

MRI of active otosclerosis

Our aim was to determine whether MRI reliably shows pathology in patients with active otosclerosis (otospongiosis). We studied five patients with clinical and audiometric signs of this disorder and positive findings on high-resolution CT and tympanocochlear scintigraphy. Contrast enhancement of otospongiotic lesions was found in all affected ears, and could be topographically related to demineralised otospongiotic foci on CT. In lesions in the lateral wall of the labyrinth MRI sometimes showed the pathology better than CT, where partial-volume effects could be troublesome.

Electrical stimulation of the auditory nerve. I. Correlation of physiological responses with cochlear status

The purpose of the present study was to evaluate evoked potential and single fibre responses to biphasic current pulses in animals with varying degrees of cochlear pathology, and to correlate any differences in the physiological response with status of the auditory nerve. Six cats, whose cochleae ranged from normal to a severe neural loss (< 5% spiral ganglion survival), were used. Morphology of the electrically evoked auditory brainstem response (EABR) was similar across all animals, although electrophonic responses were only observed from the normal animal. In animals with extensive neural pathology, EABR thresholds were elevated and response amplitudes throughout the dynamic range were moderately reduced. Analysis of single VIIIth nerve fibre responses were based on 207 neurons. Spontaneous discharge rates among fibres depended on hearing status, with the majority of fibres recorded from deafened animals exhibiting little or no spontaneous activity. Electrical stimulation produced a monotonic increase in discharge rate, and a systematic reduction in response latency and temporal jitter as a function of stimulus intensity for all fibres examined. Short-duration current pulses elicited a highly synchronous response (latency < 0.7 ms), with a less well synchronized response sometimes present (0.7-1.1 ms). There were, however, a number of significant differences between responses from normal and deafened cochleae. Electrophonic activity was only present in recordings from the normal animal, while mean threshold, dynamic range and latency of the direct electrical response varied with cochlear pathology. Differences in the ability of fibres to follow high stimulation rates were also observed; while neurons from the normal cochlea were capable of 100% entrainment at high rates (600-800 pulses per second (pps)), fibres recorded from deafened animals were often not capable of such entrainment at rates above 400 pps. Finally, a number of fibres in deafened animals showed evidence of 'bursting', in which responses rapidly alternated between high entrainment and periods of complete inactivity. This bursting pattern was presumably associated with degenerating auditory nerve fibres, since it was not recorded from the normal animal. The present study has shown that the pathological response of the cochlea following a sensorineural hearing loss can lead to a number of significant changes in the patterns of neural activity evoked via electrical stimulation. Knowledge of the extent of these changes have important implications for the clinical application of cochlear implants.

A five-generation family with late-onset progressive hereditary hearing impairment due to cochleosaccular degeneration

Cochleosaccular dysplasia or degeneration (Scheibe degeneration) is considered the most common cause of profound congenital hearing impairment, and accounts for approximately 70% of cases 2 with hereditary deafness. A five-generation family with hereditary hearing impairment associated with cochleosaccular degeneration has recently been identified. The diagnosis of classical Scheibe degeneration was based on histopathological findings in the temporal bones of the proband, a 61-year-old profoundly deaf male. Auditory structures in the brainstem of the proband were also studied. Twenty-two members of the family were contacted for surveys and blood samples. Of these, 6 males and 2 females have hearing impairment. Complete audiological evaluation was done on 12 family members, and prior audiologic records of the proband and affected family members were available for study. Affected family members suffer a mild bilateral high-frequency hearing loss during childhood and adolescence, and progress to moderate-to-profound deafness in the second and third decades of life. The family is suitable for linkage analysis and does not map to previously reported loci harboring autosomal dominant, nonsyndromic hereditary hearing impairment genes. The genetic study of this family will be helpful in identifying the genes which, when mutated, result in Scheibe degeneration.

Co-administration of the neurotrophic ACTH(4-9) analogue, ORG 2766, may reduce the cochleotoxic effects of cisplatin

In this study the effect of the neurotrophic ACTH(4-9) analogue, ORG 2766, on cisplatin cochleotoxicity was investigated with both light- and transmission electron microscopy. Guinea pigs were treated with either cisplatin+ORG 2766 (n = 11) or cisplatin + physiological saline (n = 9). All animals treated with cisplatin + physiological saline showed complete loss of outer hair cells (OHC) and degeneration of the organ of Corti in the basal cochlear turns, while partial OHC loss was found in the middle and apical turns. The inner hair cells (IHC) and other cochlear tissues were not affected. Eight animals from the group treated with cisplatin + ORG 2766 demonstrated similar pathological changes, but to a lesser degree, especially in the middle turns. The three remaining animals demonstrated no cochlear alterations at all, light-microscopically, and only minor subcellular changes in the OHCs at the ultrastructural level. Electrophysiologically, these three animals showed normals compound action potential (CAP) amplitudes at stimulus frequencies from 0.5 to 16 kHz and normal cochlear microphonics (CM) in the frequency range from 0.5 to 8 kHz. The other animals treated with cisplatin + ORG 2766 showed a severe loss in their CAPs and CM, except for one showing intermediate loss. All animals from the group treated with cisplatin alone showed a severe loss in their CAPs and CM. Endolymphatic hydrops was present in all animals from the cisplatin- and the cisplatin + ORG 2766-treated groups. These data indicate that daily, concomitant administration of ORG 2766 may reduce OHC loss and subsequent degeneration of the organ of Corti in cisplatin-treated guinea pig cochleas.

Effects of ionomer cement on guinea pig cochleas

The inner ear toxicity of ionomeric cement (Ionocem) when used as a reconstructive material in ear surgery was evaluated in 47 guinea pigs used in a morphological study. The cement was implanted on either the promontory, the round window membrane or the stapes. There was no significant hair cell loss compared with the control ear at three weeks, two months or three months after implantation. Using electrocochleography, the hearing thresholds before, and three weeks, two and three months after the implantation were compared in an additional seven animals. No evident hearing loss was observed in any animal during the study period. No morphological or functional evidence for an ototoxic effect of the ionomeric cement was found in this study.

Use of aminoglycosides in elderly patients. Pharmacokinetic and clinical considerations

Aminoglycosides still represent a mainstay in the treatment of serious infections caused by Gram-negative bacilli in elderly patients. The aging process is accompanied by various physiological changes (e.g. alterations in body composition, impairments in certain organ functions), which may affect drug disposition and, subsequently, drug action. For aminoglycosides that are eliminated by the renal route, kidney function is the key parameter that should be taken into account when dosage regimens are calculated. Because there is a progressive decline in renal function with aging, the glomerular filtration rate should be estimated for each patient. Any change in creatinine clearance (CLCR) should result in a proportional correction of the dosage regimen. Such individualised dosage of aminoglycosides is particularly important because of their narrow therapeutic indices. There are no conclusive data which indicate that age per se affects the elimination of aminoglycoside antibiotics. Overdosage may result from overestimation of renal function if crude serum creatinine (SCr) levels are used as a guide. Nomograms for the relationship between SCr and CLCR have been developed. However, nomograms should be used with caution because substantial interindividual variability in the plasma concentration-clearance relationship is still observed. Therefore, the choice of a maintenance dose based on an assessment of renal function, which change rapidly, should always be considered as preliminary, and verification by serum concentration measurements is necessary. As a result, the use of aminoglycoside serum concentration monitoring during therapy as the most important guide for dosage adjustment is particularly important in the elderly, and is indispensable in conjunction with frequent assessment of renal function. Although a matter of debate, the value of serum concentration monitoring has been demonstrated. With traditional multiple daily dosage, monitoring peak and trough concentrations has been recommended. For once daily dosage, however, no guidelines relating to therapeutic and/or toxic concentrations are available yet. In the meantime, we recommend monitoring at least trough concentrations. Once daily administration of aminoglycosides has emerged as a new mode of treatment. Compared with multiple daily administration, once daily dosage may have a number of advantages, and many clinical trials comparing the efficacy or safety of both modes have shown either superiority or equivalence of the new mode in most indications. At present, however, no data from studies of once daily administration in young compared with elderly adults are available.

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.

Middle ear overpressure treatment of endolymphatic hydrops in guinea pigs

Guinea pigs placed outside or inside a pressure chamber and exposed to 49.2 cm H2O overpressure continuously for 24 h immediately after blockage of the endolymphatic duct showed no significant difference in the magnitude of endolymphatic hydrops when compared to controls, although there was a slight indication of a volume decrease in the outside-treatment group and an increase in the inside-treatment group. A pressure increase of 49.2 cm H2O in the external auditory canal for 1 h twice a day for 2 weeks outside the chamber significantly inhibited the development of hydrops. The latter result supports the merit of pressure application through the external auditory canal as a treatment for Meniere's disease.

Cochlear implant deep electrode insertion: extent of insertional trauma

We have recently undertaken deep insertions of the Combi-40 cochlear implant electrode (Med-E1 Corp., Innsbruck, Austria) into apical regions of the scala tympani using a cochleostomy approach. In order to examine the extent of the insertional trauma, 12 fresh human temporal bones were implanted with original Combi-40 electrodes. The specimens were histologically processed with the implants in place by employing a sawing and grinding technique. In most cases, only very discrete distortions of the epithelium of the spiral ligament occurred within the middle cochlear turns. Furthermore, a slight displacement of the basilar membrane caused by the electrode was occasionally seen. However, in 2 cases more severe damage such as basilar membrane rupture and electrode displacement was found. Attempts to insert the electrode beyond the point of first resistance resulted in electrode kinking within the basal cochlear turn with subsequent fracture of the osseous spiral lamina. According to our results, deep electrode insertions do not aggravate the insertional trauma provided no force is applied when resistance is felt.

Quantitative evaluation of cochlear neurons and computer-aided three-dimensional reconstruction of spiral ganglion cells in humans with a peripheral loss of nerve fibres

Quantitative data on human cochlear neuronal elements were collected from various regions in five patients with high-tone hearing loss due to presbycusis and in two patients with normal hearing. The number of nerve fibers was assessed in the spiral lamina and in the inner acoustic meatus together with counts of spiral ganglion cells. The results show that the number of neurons decreased peripherally, i.e., with increasing distance from the central nervous system in patients with high-tone hearing loss due to presbycusis. In two patients with normal hearing no significant difference in the number of neurons was found in the lamina spiralis as compared to the inner acoustic canal. Computer-aided 3-dimensional reconstruction of the human spiral ganglion displayed large bipolar neurons (type I cells), but also large ganglion cells with one missing axon. The results may indicate that a slow retrograde degeneration occurs from the periphery towards the spiral ganglion in presbycusis. Transmission electron microscopy analysis of freshly fixed human spiral ganglions displayed interneural connections. It is speculated whether a trophic supply from other neurons at the level of the spiral ganglion can prevent or delay further degeneration of the central axon.

Chronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study

A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 microseconds/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms off) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program. All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P = 0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P = 0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.

The role of the cochlear efferent system in acquired resistance to noise-induced hearing loss

Previous work has shown that the cochlear efferent system may play a role in protecting the ear from noise-induced temporary threshold shifts (TTS) following exposures to a single tone or series of moderate-level noises ('toughening'). However, whether the olivocochlear bundle (OCB) is important in decreasing noise-induced permanent threshold shifts (PTS) remains an open question. The importance of the OCB in decreasing the ear's susceptibility to noise, as reflected by 2f1-f2 distortion product otoacoustic emissions, was assessed by sectioning both the ipsilateral and contralateral divisions of the efferent system and exposing chinchillas while awake to an octave band noise (4 kHz) at a low level (85 dB SPL) for 10 days (6 h/day) and then at a high level (95 dB SPL) for 48 h. Complete de-efferentation was verified by cochlear acetylcholinesterase staining. The ears that were de-efferent showed substantially more TTS, greater PTS and larger cochlear lesions of outer hair cells. The results suggest that the efferent system may influence the ear's ability to develop resistance to noise trauma.

Histopathologic observations of the aging gerbil cochlea

Age-related histopathologic changes were examined in cochleas from 17 gerbils born and kept in a quiet environment until near the end of their life expectancy. Hearing loss varied greatly as did the loss of outer hair cells (OHC). Inner hair cells (IHC) were seldom missing even in cochleas with severe hearing losses. Flask- and spherical-shaped OHCs were frequently seen in the apical turn. Stereocilia were usually present and orderly on OHCs, but the tallest row of stereocilia on IHCs was often disarrayed and sometimes missing. Alterations in supporting cells were sometimes present in regions of extensive OHC loss. Although pillar cells were seldom missing, the nuclei of outer pillar cells were commonly displaced from their normal basal position. The density of radial fibers appeared similar to that in young gerbils except in the apical turn of one old ear where a marked loss of radial fibers occurred without an attendant loss of IHCs. All of the quiet-aged cochleas showed a characteristic clustering of epithelial cells lining the scala media surface of Reissner's membrane. This structural rearrangement was not accompanied by a significant decrease in the total number of cells forming Reissner's membrane and did not appear to be associated with hearing loss. The findings confirm and extend earlier work showing that several different types of cells are susceptible to histopathologic changes in old ears. The extent of histopathologic changes varied widely as did the degree of hearing loss in animals with a restricted genetic background and maintained under carefully controlled environmental conditions. It was not possible, based on these initial findings, to relate specific structural to specific functional changes in the aging cochlea. Further light and electron microscopic analysis of other regions from these aged cochleas may provide more conclusive data.

Neuronal and transneuronal degeneration of auditory axons in the brainstem after cochlear lesions in the chinchilla: cochleotopic and non-cochleotopic patterns

Terminal axonal degeneration in the brain following cochlear lesions was studied with the Nauta-Rasmussen method. Losses of hair cells and myelinated cochlear fibers were assessed. The cochleotopic map projected, from apex to base, on the ventral-to-dorsal axes of the cochlear nuclei. The cochleotopic correspondence was better for loss of cochlear nerve fibers and inner hair cells, than for outer hair cells. Cochlear fibers were traced to all parts of the cochlear nucleus, including the small-cell shell, also to cell-group Y and the flocculus. Terminal axonal degeneration in nuclei of the superior olivary complex, lateral lemniscus, and inferior colliculus was interpreted as transynaptic, since degenerated axons could not be traced to these locations from the cochlear nerve or trapezoid body. Moreover, biotinylated dextran amine injection in the basal turn of scala media of a normal cochlea labeled cochlear nerve fibers projecting to the high-frequency regions of the cochlear nuclei and to the flocculus, but not to more central auditory nuclei. This is the first detailed account of transynaptic degeneration in the ascending auditory pathway resulting from cochlear damage in an adult mammal. These findings are consistent with a dystrophic process depending on hair-cell loss and/or direct damage to cochlear nerve fibers.

Osteoma of the promontory: a case report

Osteomas of the middle ear are rare lesions. We report the case of a 20-year-old male with osteoma originating from the promontory coincident with tympanic membrane perforation. Histopathological diagnosis was performed by means of a superficial curettage during surgery, to avoid a cochlear fenestration. Postoperative closure of the air-bone gap after myringoplasty confirmed the presumed asymptomatic nature of these lesions.

Degeneration of axons in the brainstem of the chinchilla after auditory overstimulation

The patterns of axonal degeneration following acoustic overstimulation of the cochlea were traced in the brainstem of adult chinchillas. The Nauta-Rasmussen method for axonal degeneration was used following survivals of 1-32 days after a 105 min exposure to an octave-band noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. Hair-cell and myelinated nerve-fiber loss were assessed in the cochlea. The cochleotopic pattern of terminal degeneration in the ventral cochlear nucleus correlated with the sites of myelinated fiber and inner-hair-cell loss: this correlation was less rigorous with outer-hair-cell loss, especially in the dorsal cochlear nucleus. These results are consistent with a dystrophic process with a slow time course depending on hair-cell loss and/or direct cochlear nerve-fiber damage. However, in a number of cases with no damage in the apical cochlea, fine fiber degeneration occurred with a faster course in low-frequency regions in the dorsal cochlear nucleus and, transynaptically, in a non-cochleotopic pattern in the superior olive and inferior colliculus. These findings suggest that neuronal hyperactivity plays a role in the central degeneration following acoustic overstimulation, possibly by an excitotoxic process.

The spontaneous appearance of hair cell-like cells in the mammalian cochlea following aminoglycoside ototoxicity

Young rats (in vivo) and cochleas from neonatal rats (in vitro) were treated with ototoxic antibiotics. Scanning electron microscope observations of the cicatricial epithelium of the former outer hair cell region revealed cells with a tuft of microvilli at their apical surface that could contain actin filaments, as observed by phalloidin staining. The apical organization of these hair cell-like cells was reminiscent of fetal hair cells topped with a bundle of microvilli. During both in vivo and in vitro observations, and despite the use of several growth factors in vitro, these hair cell-like cells did not differentiate into mature sensory cells. These hair cell-like cells might represent an attempt by the former sensory epithelium to regenerate.

N-methyl-D-aspartate antagonists limit aminoglycoside antibiotic-induced hearing loss

The use of aminoglycoside antibiotics is limited by ototoxicity that can produce permanent hearing loss. We report that concurrent administration of N-methyl-D-aspartate (NMDA) antagonists markedly attenuates both the hearing loss and destruction of cochlear hair cells in guinea pigs treated with aminoglycoside antibiotics. These findings indicate that aminoglycoside-induced hearing loss is mediated, in part, through an excitotoxic process. The high correlation (Spearman correlation coefficient: 0.928; P < 0.01) obtained between the relative cochleotoxicities of a series of aminoglycosides in humans and the potencies of these compounds to produce a polyamine-like enhancement of [3H]dizocilpine binding to NMDA receptors is consistent with this hypothesis, and provides a simple in vitro assay that can predict this aspect of aminoglycoside-induced ototoxicity.