Open-set speech perception in adult cochlear implant users with ossified cochleae

This study describes open-set speech recognition in cochlear implant subjects with ossified cochleae and compares it to a control group with open cochleae. Twenty-one postlingually deafened adults with a Med-El Combi 40/40+GB split- electrode implant were matched to patients using a Med-El cochlear implant with a standard electrode. Speech recognition was assessed over an 18-month period. Split- electrode patients improved significantly over time, but their scores were significantly lower and increased significantly slower than those of controls. Of 14 patients with a duration of deafness less than 20 years, average sentence test scores were 50%, and average monosyllabic word test scores were 31%. This study provides evidence that cochlear implantation is beneficial to patients with ossified cochleae, but early implantation is advisable.

Meningitis following cochlear implantation: pathomechanisms, clinical symptoms, conservative and surgical treatments

Pneumococcal otogenic meningitis is a rare postsurgical complication that can develop following stapedectomy or after cochlear implantation. The bacterial infection can be fatal in some instances. A recent increase in the incidence of otogenic meningitis among cochlear implant wearers is of concern. The majority of meningitis cases are associated with a 2-component electrode manufactured by one cochlear implant company. The device with the added 'positioner' component has been withdrawn from the market (FDA Public Health Web Notification: Cochlear Implant Recipients may be at Greater Risk for Meningitis, Updated: August 29, 2002, www.fda.gov/cdrh/safety/cochlear.html). Not all cases have been subsequent to otitis media and symptoms have developed from less than 24 h up to a few years after implantation. The purpose of this paper is to review and discuss the pathogenesis, pathology/bacteriology and to elaborate on some clinical features of otogenic meningitis in implanted children and adults. Essential aspects of surgery, electrode design, and cochleostomy seal are discussed. Conclusions are drawn from the available data and recommendations are made for good practice in cochlear implantation and follow-up.

Protection and regrowth of the auditory nerve after deafness: neurotrophins, antioxidants and depolarization are effective in vivo

Deafness, with loss of sensory (hair) cells, results in progressive pathophysiological changes ending in the degeneration of most auditory nerve neurons. It is now possible to consider these events in the broader context of anti-apoptotic survival factors in the peripheral and central nervous system. One consequence of deafferentation of a neuron is the loss of neurotrophins that can lead to a change in oxidative state (formation of free radicals), changes in intracellular Ca(2+), and an up-regulation of apoptotic genes. Interventions that can modify availability of neurotrophins, [Ca(2+)](I), and/or free radical formation or their destructive effects, may preserve the auditory nerve. Some interventions (neurotrophins) may also lead to a regrowth of neurites. Studies in this area are of basic value and also of immediate clinical interest for the application of the cochlear prosthesis to the severe and profoundly deaf, since the benefits of this prosthesis are directly dependent on auditory nerve survival and the proximity of stimulating electrode to neuron. We, and others, have found that auditory nerve degeneration can be prevented by chronic electrical stimulation. We have demonstrated in vivo that this effect can be blocked by tetrodotoxin, thus indicating that propagated action potentials are a necessary condition, and by verapamil (Ca(2+) channel blocker), supporting in vitro studies by others, indicating that L-type Ca(2+) channels are necessary for stimulation-induced rescue of the deafferented auditory nerve. The intensities of electrical stimulation required for rescue are at levels sufficient to express the intermediate-early gene c-fos which can initiate transcription of anti-apoptotic genes and pathways, and up-regulate expression of neurotrophins that may act in an autocrine manner to protect the nerve from death. We, and others, have found that chronic local delivery (osmotic pump and microcannulation of the inner ear fluid spaces) of individual neurotrophins and cocktails of factors can also enhance survival of the deafferented nerve, and some can also initiate a regrowth of degenerated peripheral processes of the nerve into the region of the destroyed sensory epitheliae. Recently, we have shown that this rescue can occur with delayed intervention, after degeneration of some neurons has begun, more closely mimicking the human clinical situation. Finally, we have shown that interventions with antioxidants may also be effective in preventing pathophysiological changes of the auditory nerve following deafness. These studies in the auditory periphery support the 'neurotrophic factor hypothesis' as proposed as a general mechanism underlying neurodegenerative and age-related pathology of the central nervous system. Additional animal studies can yield a rational scientific basis to justify human trials, with a goal to maintain auditory cell survival and initiate and direct fiber growth to the next generation of prosthesis. Intimate contact between electrode and a dense population of auditory neurons should greatly enhance the benefits of these devices for the profoundly deaf.

Phonological representation and speech understanding with cochlear implants in deafened adults

In the present study cognitive performance in 15 deafened adult cochlear implant candidates was examined and related to level of speech understanding after 12 months of experience with the implant. The implant group performed on par with normal hearing controls in all cognitive tasks used in the study with one exception: Performance was significantly lower in cognitive tasks where use of a phonological representation of sound is a key task-demand. Observations of the implanted individuals' level of speech understanding indicate that only those individuals who, pre-operatively, were in possession of phonological representations comparable to that of normal hearing could follow and understand a speaker that was out of sight. The results are discussed with respect to (a) deterioration in the phonological representation of sounds as a function of absence of external auditory stimulation, and (b) the role of cognitive factors in predicting success in speech understanding with the implant.

Effects of mastoid cavity obliteration on the growth of experimentally produced residual cholesteatoma

The effects of obliteration of the mastoid cavity on the growth of residual cholesteatoma were histologically studied in an animal model. A dermal cyst was produced by grafting a piece of autologous auricular skin in the otic bulla of 11 guinea-pigs. Three weeks after grafting, part of the cyst wall facing the cavity was removed and the debris accumulated inside was inserted into the surrounding granulation using a micropick. This procedure simulates the growth mechanism of cholesteatoma residue which sometimes occurs after middle ear surgery in human subjects. In six animals, the bulla was then obliterated with plaster of Paris. The remaining five animals were used as controls. Animals were killed for histological study at 2, 4 or 8 weeks postoperatively. Microscopic examinations revealed that in the obliteration group, severe inflammatory reactions were induced in the otic bulla, although the graft epithelium survived there; dermal cyst reformed in only one of six animals. In the controls, cyst reformation was recognized in all animals. This indicated that severe inflammation induced by plaster prevented growth of the graft epithelium in the otic bulla.

Intraoperative electrical stimulation of the stapedius reflex in children

OBJECTIVE

To describe the use of intraoperative electrical stimulation on the stapedius reflex in children.

SETTING

Hospital cochlear implantation center.

PATIENTS AND METHODS

Eighteen children under the age of 10 years, who have undergone cochlear implantation, have been examined during surgery with the electrically elicited stapedius reflex in the monopolar stimulation mode.

RESULTS

A stapedius reflex could visually be detected for all subjects.

CONCLUSION

The responses serve to test the implant, provide guidance in avoiding overstimulation of the child when the device is switched on, and estimate the optimal C-levels.

Some experiences with the nucleus 20 + 2 cochlear implant in adults and children

OBJECTIVE

To evaluate the effectiveness of the Nucleus 20 + 2 implant in a group of patients.

PATIENTS

Fifteen children and 11 adults who have received the Nucleus 20 + 2 implant since late 1993.

RESULTS

The outcome in most patients has been favorable.

Effects of lonomer Cement on Guinea Pig Cochleas

The inner ear toxicity of ionomeric cement (lonocem®) 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.

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.

Postmeningitic labyrinthine ossification primarily affecting the semicircular canals

In a series of six cochlear-implant candidates, including three small children, labyrinthine ossification in various stages of development was observed at CT. In four of the candidates the ossifying process was more advanced in the semicircular canals than in the cochleae, and in two equally distributed. The ossifying process developed during a period of 4-5 months in two of the children. Asymmetry of its extension was found in four patients. The causative organisms were Hemophilus influenzae and Streptococcus pneumoniae. The radiologic assessment of cochlear-implant candidates should include the semicircular canals where the ossification may start, and herald the development of cochlear ossification.

Factors affecting auditory performance of postlinguistically deaf adults using cochlear implants

A model of auditory performance and a model of ganglion cell survival in postlinguistically deafened adult cochlear implant users are suggested to describe the effects of aetiology, duration of deafness, age at implantation, age at onset of deafness, and duration of implant use. The models were compared with published data and a composite data set including 808 implant users. Qualitative agreement with the model of auditory performance was found. Duration of deafness had a strong negative effect on performance. Age at implantation had a slight negative effect on performance, increasing after age 60 years. Age at onset of deafness had little effect on performance up to age 60. Duration of implant use had a positive effect on performance. Aetiology had a relatively weak effect on performance.

Verbal information-processing capabilities and cochlear implants: implications for preoperative predictors of speech understanding

In this study, we examined preoperative verbal cognitive capacity in 11 deafened adults who were cochlear implant candidates and reexamined level of speech understanding after 6-8 months' experience with the implant. Verbal cognitive performance in the implant group was compared in a group of normal hearing subjects and in nonimplanted group of deafened adults. The three groups performed on par with each other with an exception: The individuals in the cochlear implant group and the nonimplanted group of deafened adults performed significatly worse than those of normal hearing in tasks in which use of internal speech is a key feature (i.e., rhyme judgement and lexical decision tasks). Postoperative observations of the implanted individuals' level of speech understanding suggest that it is possible to predict the level of speech understanding by means of a properative cognitive assessment. The characteristics of three verbal cognitive abilities prove to be critical indicators of 6 - 8 months' postoperative outcome: internal speech functioning, speed of verval information processing, and working memory capacity - the first factor proved the most decisive. We discuss the results with respect to direct versus indirect predictors of outcomes from cochlear implant operations and the effect of auditory deprivation on deafened adults' capability to process auditory information.

Electromagnetic stimulation of the auditory system of deaf patients

Electromagnetically induced auditory perception was investigated in 18 deaf patients who were candidates for cochlear implants. In the extracranial magnetic stimulation (EMS) procedure, patients were stimulated with time-varying magnetic field brief pulses from a coil positioned at the i) auricle, ii) the mastoid, and iii) the temporal lobe area. EMS elicited auditory sensations in 26 ears (of 14 patients/subjects). The lowest threshold of auditory sensation (TAS) was found to be at the 20% EMS level, with a range of 20-50% of the maximum level (2.0 Tesla), and approximately equal sensitivity in each coil position. Eleven of the subjects hearing EMS-induced sound perceived changes in pitch while 6 heard "clicks" or clicks and tones. Spearman Rho correlation analysis showed a mild negative correlation between the EMS/TAS and the pre-implant FFA, best tone threshold (BTT), and direct promontorial electrical stimulation (ES) thresholds at 250 Hz and 500 Hz. No correlation was found between EMS or ES and performance on the pre-implant or post-implant psychacoustic tests (MAC VIII or 3-Digit speech tests) or the measurements of the thickness of cutaneous and osseous tissue from the stimulation sites at the mastoid and ear canal to the cochlear and 8th nerve. A fair positive correlation was found between the EMS/TAS and the post-implant (6 months) ES threshold when the electrodes allocated the 500 Hz frequency range were stimulated. A mild positive correlation between the pre-cochlear-implant promontorial electrical stimulation (ES) at 250 Hz and the four frequency tone average (FFA: 0.5, 1, 2, 4 kHz) was also found.(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine-induced changes in human cochlear blood flow

Cochlear blood flow (CBF) was monitored over the basal turn stria vascularis using laser Doppler flowmetry in five human subjects during middle ear surgery. The effects of systemically administered epinephrine (0.3 microgram/kg) and topically applied epinephrine (1:10,000) on the round window membrane (RWM) were examined. Topical epinephrine caused a mean reduction of 60 percent in CBF (maximum peak reduction 65-85% across subjects), which slowly recovered ( > 10 min) toward baseline following epinephrine removal from the RWM. The changes in CBF are similar to those found in animal studies, but are much larger, indicating a relatively more pronounced role of adrenergic agents in CBF control in humans. Systemic epinephrine caused a 40 percent decrease in skin blood flow, a 90 percent increase in blood pressure (BP), above a resting hypotensive mean level of 65 mmHg, and a 50 percent increase in CBF. The CBF change followed the change in BP, but recovered toward baseline more slowly. The dramatic and somewhat prolonged decreases in CBF with RWM application of epinephrine may compromise sensory function and could account for the occasional unexplained sensorineural hearing loss or tinnitus associated with middle ear procedures that use topical epinephrine. The semipermeability of the RWM may, on the other hand, offer a route for therapeutic increases in CBF with vasodilative agents and provide an appropriate treatment for some cases of sensorineural hearing loss.

Multicentre evaluation of the temporal bones obtained from a patient with suspected Menière's disease

A multicentre study of the inner ears of an 88-year-old patient with vertiginous spells and severe hearing loss in the left ear was performed, employing regular and block surface preparations, light and electron microscopy with qualitative and quantitative evaluation of the cochlear and vestibular nerves. There was severe hydrops of the left cochlea and saccule. Reissner's membrane extended into the vestibule and herniated into the perilymphatic space of the non-ampullated end of the horizontal canal. Furthermore, the short canal connecting the posterior ampulla with the utricle had a small, exceedingly thin balloon-like expansion. Only slight hydrops limited to the cochlea was found in the right ear. Sensorineural degeneration was much more pronounced in the left cochlea than in the right. The number of cochlear and vestibular nerve fibres was greatly reduced in the left ear where more fibres with degenerative changes were present. In both specimens the number of myelinated nerve fibres in osseous spiral lamina was smaller than that in the cochlear nerve in the internal auditory canal. Changes occurred in the endolymphatic sacs but were considered non-specific. In this case severe, apparently progressive hydrops and sensorineural degeneration, characteristic of Menière's disease, were associated with atypical onset of clinical symptoms at a late age.

Measurement of human cochlear blood flow

Cochlear blood flow (CBF) was measured with a laser-Doppler (L-D) flowmeter (Periflux PR2-B) in four unanesthetized human subjects with chronic tympanic membrane perforations and nine anesthetized human subjects undergoing middle ear operations. The L-D recordings were made over the promontory and/or the round window membrane during carbogen breathing and direct electrical stimulation of the cochlea in both groups and with warm water irrigation of the external ear canal in the anesthetized subjects. Carbogen led to little or no change in CBF as monitored with either measurement approach in either subject group. Electrical stimulation yielded an increase (15% to 25%) in CBF as recorded from the promontory in seven of the nine subjects tested. Warm (44 degrees C to 49 degrees C) water irrigation produced changes of 20% to 60% in CBF that were partially recoverable in the 10 minutes available for study. This study demonstrated the feasibility of direct CBF measurement in humans with the L-D method. Moreover, the data indicate that carbogen has little influence on CBF and that electrical stimulation at relatively safe levels and warm water irrigation of the ear canal produce increases in human CBF.

Central nervous system involvement in patients with facial palsy due to borrelial infection

Patients with a peripheral facial palsy due to an underlying Ixodes-borne borreliosis often have cerebrospinal fluid findings indicating meningeal involvement. The aim of the present study was to identify signs of CNS involvement by means of brain-stem evoked response audiometry (BSER) in patients with a facial palsy due to borreliosis, as well as in patients with a facial palsy of unknown aetiology. Bell's palsy. Abnormalities in BSER were found to be significantly more common along the borrelial patients. Furthermore, a reversal of these abnormalities following treatment with benzyl-penicillin was found in the majority of patients with borreliosis. The results of the present study emphasize the importance of diagnosing and treating an underlying borreliosis in patients with a peripheral facial palsy.

Brainstem response audiometry in chronic Lyme borreliosis

Auditory brainstem responses (ABR) were investigated in 26 patients with acrodermatitis chronic atrophicans, which is a late manifestation of Lyme borreliosis. Nine of the patients showed pathological ABR, four of them unilaterally and five bilaterally. The main pathological findings were: 1) Poor reproducibility of waves IV-V or of wave V; 2) Increased latency of wave V. After antibiotic treatment, ABR was improved in eight of the nine patients, and in three of them it was normal. In the five patients who did not completely recover, the improvement consisted in better reproducibility and a tendency towards normal wave V latencies. The results of this study indicate that the central nervous system may become involved in patients with acrodermatitis chronica atrophicans.

Plaster of Paris and hair cell morphology. A scanning electron microscopic study of an alternative implant material for ear surgery

Many techniques have been proposed for reconstruction of the posterior canal wall and/or obliteration of the mastoid bowl after radical mastoidectomy. The variety of materials that have been used, biological as well as foreign materials of different kinds, indicates that the ideal solution has yet to be found. Plaster of Paris, a biocompatible, degradable ceramic material prepared from CaSO4, may have an osteogenic property and become an alternative implant material for ear surgery. However, its possible ototoxicity has not been studied previously. Plaster was implanted in the tympanic cavity of guinea pigs. On investigation in the scanning electron microscope after 4.5 to 8 months, no toxic damage to the hair cell morphology was observed. Thus, from an ototoxic point of view, plaster of Paris would seem suitable for use as an implant material in ear surgery. Studies concerning the possible osteogenic property of plaster of Paris and its clinical application in human ear surgery are in progress.

Hair cell distributions in the normal human cochlea. A report of a European working group

Cochlear hair cell counts from individuals who had clinically normal hearing prior to their death have been plotted for various age bands as a function of the number of hair cells per millimetre against their position in the cochlea. Position has been expressed as the distance of that observation of hair cell density from the base of the cochlea, divided by the total length of the cochlea, thereby giving a proportional representation of the cochlea in the range of 0.0 to 1.0 with 20 subdivisions of 0.05. There is an age-related decrease in the number of hair cells in the normal population, and this is more marked for the outer hair cells.

Ultrastructural evaluation of the microslicing method for the study of temporal bone pathology

Microslices 3 mm thick from undecalcified human temporal bones were prepared with a special cutting machine and then processed for SEM and TEM in order to evaluate advantages and disadvantages of the microslicing technique for the study of the temporal bone pathology. In the examined microslices there was some mechanical distortion of the membranous labyrinth, detachment of soft tissues from bone and a considerable amount of contamination by bone dust and debris which are circulated during sectioning. For SEM the method therefore has limited value. For TEM a relatively contamination free area can be found some distance from the cutting surface of each microslice.

Plaster of Paris: thermal properties and biocompatibility. A study on an alternative implant material for ear surgery

There is no single technique for reconstruction of the canal wall following radical mastoidectomy. Autograft tissues may be unsuitable because of resorption and limited availability. An ideal foreign implant material should be bone-like in quality, non-ototoxic, readily available and malleable. Plaster of Paris, a degradable ceramic powder of CaSO4, may be regarded as a bone mineral. But a certain amount of heat develops during its solidifying process and may present a risk for injury to the facial nerve and the labyrinth. We studied the temperature rise with respect to added solution, to volume, and to surroundings, and investigated its biocompatibility when implanted in soft tissue in guinea pigs. The investigations suggested no risk of thermal injury and the biocompatibility was excellent. Hence plaster of Paris may constitute an alternative implant material for mastoid obliteration and reconstruction of the canal wall in ear surgery.

Ototoxicity of kanamycin in albino and pigmented guinea pigs. I. A morphologic and electrophysiologic study

Ototoxic drugs of the aminoglycoside type have been shown to accumulate to melanin, suggesting a possible mechanism for their ototoxicity. The present study was undertaken by combining electrophysiologic and morphologic methods to investigate whether the ototoxicity of kanamycin is different in pigmented and albino guinea pigs. In pigmented animals a kanamycin dose of 200 mg per kilogram of body weight per day resulted in hearing loss together with loss of both inner and outer hair cells. The albino animals in the same dose group showed significantly less hearing loss and hair cell degeneration. With daily doses of 20 and 60 mg/kg/day, no difference in ototoxicity was found between the pigmented and albino animals. The results support the hypothesis that affinity of kanamycin to inner ear melanin might be responsible for the difference in ototoxicity between albino and pigmented guinea pigs.

Functional histopathology of the human audio-vestibular organ. Euro-Data-Hearing

The present paper describes the setting-up of a new project, sponsored by the Commission of the European Communities, to promote the study of functional histopathology of the human audio-vestibular organ. The project has the purpose of co-ordinating in Europe morphological research on the hearing and equilibrium organ and promoting the comparison of data from clinical, physiological and morphological studies in the field of hearing and equilibrium impairment. A manual (Iurato et al., 1982) has been prepared with the objectives of (1) stimulating clinicians to participate in the project, (2) encouraging the collection of audiological data for comparison with pathology, and (3) supplying clinicians with the necessary information about the technique of fixation and preparation of the specimens. The fixation procedures for the two major techniques of studying ear pathology: (1) microdissection and electron microscopy, and (2) embedding of the whole block for serial sectioning and light microscopy, are described in detail. Special paragraphs deal with the technique of fixation of the brain and with the technique of removal of the temporal bones and brain at autopsy. The instructions for delivery of the specimens and the publication rules are enclosed in the main body of the manual. Appendix 1 contains a list of the participating laboratories. Appendix 2 deals with autopsy legislation in the different European countries. Appendix 3 contains the sender's data sheet which supplies the instructions for collecting the clinical history of the patient and the audiological and vestibular data.

Electrophysiological and morphological changes in the guinea pig cochlea following mechanical trauma to the organ of Corti

Small discrete lesions were produced in the organ of Corti of the guinea pig cochlea using fine probes to produce direct mechanical insult. The electrophysiological state of the cochlea was assessed using N1 electrocochleography and loss of receptor cells determined by scanning electron microscopy. Principal findings were: 1) Excellent agreement between the location of hair cell losses and the frequency of maximum sensitivity change in the N1 audiogram; 2) The spatial extent of the mechanically induced lesion appears to be more important than the total number of hair cells lost, in determining the magnitude of N1 sensitivity loss; 3) Hair cell losses extending over only 72 micrometers could be detected as significant changes in N1 sensitivity. These results further emphasize the accuracy and usefulness of the N1 electrocochleogram for assessing the functional status of the cochlea; 4) Lesions involving only outer hair cell loss also produced marked elevations of N1 threshold.

Response properties of spiral ganglion neurons in cochleas damaged by direct mechanical trauma

Electrical activity of single neurons in the spiral ganglion of the guinea pig cochlea was correlated with receptor hair cell damage produced by local mechanical lesions of the organ of Corti. Damage to or loss of hair cells was assessed using scanning electronmicroscopy. Principal finds were (1) neurons emanating from damaged regions showed changes in their tuning curves such as loss of sensitivity, broadening of tuning curves; (2) in some cases, the loss of sharp tuning tip resulted in an apparent shift of the tuning curve to lower frequencies; and (3) apparently pure outer hair cell lesions, of limited spatial extent, could result in the same effects as mixed inner and outer hair cell lesions.

Scanning electron microscopy of the nerves within the organ of Corti

A method of studying the innervation inside the organ of Corti is presented. Rabbits, guinea pigs and chinchillas were fixed with perfusion of the perilymphatic spaces. The cochleas were dissected according to the surface specimen technique and the specimens critical point dried, coated with gold and studied in the SEM. The final dissection into the planes of the fluid spaces of the organ of Corti was done in the dry state using especially sharpened watch makers forceps and razor blade knives. The course of the afferent and efferent innervation is described and differences between the species illustrated. Small high nerve endings on the outer hair cell degenerate after cutting the efferent nerve supply by dividing the vestibular nerve indicating their efferent nature.

The innervation of the organ of Corti. A scanning electron microscopic study

The innervation within the organ of Corti of guinea pigs, chinchillas and rabbits was studied in the scanning electron microscope. Specimens were fixed, dissected, critical point dried and coated with gold. Final dissection of the specimens was done in the dry state in the planes of the former fluid spaces. The courses of the efferent and afferent nerve fibres are described. The efferent fibres reach the outer hair cells by crossing the tunnel space, run between the outer pillars and divide several times in the space of Nuel. The afferent fibres cross the floor of the tunnel and turn basalwards in the outer spiral bundles where they run for at least 0.5-0.8 mm before connecting with the outer hair cells. Small nerve endings on every fifth to tenth outer hair cell disappear after cutting the efferent nerves, thus indicating their efferent nature.

Inner ear damage and hearing loss after exposure to tones of high intensity

Experimental animals (cats) were exposed to tones of 125, 1000, 2000, and 4000 Hz at sound pressure levels in the range 120 to 157.5 dB, and for durations of one hour (1000, 2000, 4000 Hz) or four hours (125 Hz). Pure tone audiograms were obtained for each animal before and after exposure. Post-exposure tests were continued until complete recovery of hearing had occurred or until a stable permanent threshold shift had been measured. Cochleas of animals were examined by phase-contrast microscopy; condition of all hair cells was recorded. Extent of inner-ear damage and range of frequencies for which hearing loss occurred increased as exposure tone was decreased in frequency. For example, exposure to 4000 Hz produced damage in a restricted region of the cochlea and hearing loss for a relatively narrow range of frequencies; exposure to 125 Hz produced wide-spread inner ear damage and hearing loss throughout the frequency range 125 to 6000 Hz.

Scanning electron microscopy of the organ of Corti

With the scanning electron microscope we have examined normal cochlear sensory epithelium of the guinea pig and cat and that damaged by noise. The studies demonstrate how the regular surface architecture of the organ of Corti is altered after exposure to noise. The changes include loss of sensory hairs, formation of giant hairs, and complete degeneration of circumscribed areas of the organ of Corti. Our method greatly reduces the artifacts.