Reversible conductive hearing loss: restored activity in the central auditory system.

The effect of a reversible, unilateral hearing loss on 2-deoxyglucose (2-DG) uptake in the central auditory system was studied using young gerbils. All animals had a unilateral conductive hearing loss (CHL), induced by atresia, on postnatal day 21 (P21). One week later, on P28, animals had their atresia repaired (CHL/R), or not repaired (CHL/NR), and CHL/NR animals entered the 2-DG experiments. CHL/R animals were allowed a 1-week period of restored binaural hearing experience prior to entering 2-DG experiments on P35. Animals in each group were injected with 2-DG and exposed to ambient sounds for 45 min prior to sacrifice. Uptake of 2-DG was measured in the anteroventral cochlear nucleus (AVCN), the medial superior olive (MSO), and the inferior colliculus (IC) on both sides of the brain. In CHL/NR animals, there were significant differences in uptake between the AVCN, MSO, and IC ipsilateral versus contralateral to the manipulated ear, indicating an imbalance in ascending afferent activity. In CHL/R animals, there were no significant differences, suggesting that 1 week after CHL repair, the appearance of balanced afferent activity had been restored.

Consequences of unilateral hearing loss: cortical adjustment to unilateral deprivation.

The effect of unilateral hearing loss on 2-deoxyglucose (2-DG) uptake in the central auditory system was studied in postnatal day 21 gerbils. Three weeks following a unilateral conductive hearing loss (CHL) or cochlear ablation (CA), animals were injected with 2-DG and exposed to an alternating auditory stimulus (1 and 2kHz tones). Uptake of 2-DG was measured in the inferior colliculus (IC), medial geniculate (MG), and auditory cortex (fields AI and AAF) of both sides of the brain in experimental animals and in anesthesia-only sham animals (SH). Significant differences in uptake, compared to SH, were found in the IC contralateral to the manipulated ear (CHL or CA) and in AAF contralateral to the CHL ear. We hypothesize that these findings may result from loss of functional inhibition in the IC contralateral to CA, but not CHL. Altered states of inhibition at the IC may affect activity in pathways ascending to auditory cortex, and ultimately activity in auditory cortex itself. Altered levels of activity in auditory cortex may explain some auditory processing deficits experienced by individuals with CHL.

Consequences of unilateral hearing loss: time dependent regulation of protein synthesis in auditory brainstem nuclei.

Conductive hearing impairment results in marked changes in neuronal activity in the central auditory system, particularly in young animals [Tucci, D.L., Cant, N.B., Durham, D., 1999. Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil. Laryngoscope 109, 1359-1371]. To better understand the effects of conductive hearing loss (CHL) on cellular metabolism, incorporation of (3)H-leucine was used as a measure of protein synthesis in immature postnatal day 21 gerbils subjected to either unilateral CHL by malleus removal or profound sensorineural hearing loss by cochlear ablation. (3)H-leucine uptake was measured after survival times of 6 or 48h. Protein synthesis values were standardized to measurements from the abducens nucleus and compared with measurements from sham animals at similar age/survival times. Protein synthesis in the medial superior olive (MSO) was found to be significantly down-regulated (bilaterally) after CHL in animals surviving 48h. However, 6h after CHL manipulation, protein synthesis is up-regulated in MSO (bilaterally) and in the ipsilateral medial nucleus of the trapezoid body.

Effects of conductive hearing loss on gerbil central auditory system activity in silence.

Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.

Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil.

OBJECTIVES/HYPOTHESIS: The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2-deoxyglucose (2-DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA. STUDY DESIGN: Sham-controlled study of metabolic effects of CHL during sound stimulation. METHODS: Gerbils (aged 21 days or adult), underwent malleus removal, CA, or a sham procedure. Young animals survived either 48 hours or 3 weeks; adults survived 3 weeks. Each age/survival CHL group contained eight animals; otherwise, each group (CA and sham) contained five animals, for a total number of 54. At the appropriate survival time, animals were given an intracardiac injection of 14C-2-DG, and sacrificed under anesthesia after 45 minutes of exposure to normal laboratory sounds. Tissue sections were prepared for exposure to x-ray film for optical density measurements, and alternate sections stained for identification of nuclei. Measurements from auditory nuclei of experimental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham group. Auditory evoked potential thresholds to both air- and bone-conducted stimuli were obtained in a second group of neonatal and adult animals. RESULTS: Both CHL and CA resulted in a marked decrease in 2-DG uptake in the major ascending projection of the manipulated ear, in both the neonatal and adult animals. In young animals, effects of CHL and CA were similar. Effects of CHL in adult animals were less marked and significantly different from either effects of CHL in young animals or effects of CA in adult animals. HL following malleus removal only was purely conductive and ranged from 38 to 55 dB across frequency. CONCLUSIONS: Results suggest that, particularly in young animals, a unilateral CHL may have profound effects on metabolic activity in the central auditory system.

Cochlear implant studies at Research Triangle Institute and Duke University Medical Center.

Examples from several areas of cochlear implant research are presented, with emphasis on the continuous interleaved sampling (CIS) approach to speech processor design. Within-subject comparisons of such processors with the compressed analog (CA) approach of the clinical Ineraid device are reviewed, and ongoing similar comparisons with the clinical Nucleus spectral peak (SPEAK) strategy are outlined. Correlations between chronic performance levels with clinical CA processors and initial performance levels with CIS, data on further improvements in performance with chronic use of CIS, and instances of substantial benefit from custom fitting of CIS parameters are presented as examples of findings with immediate clinical implications. New studies are described, involving the measurement of intracochlear evoked potentials in response to cochlear implant stimulation, and the integration of such work with computer modeling studies.

Vestibular rehabilitation strategies in Meniere's disease.

Once the episodic spells of vertigo associated with Meniere's disease have abated, vestibular rehabilitation exercises play an important role in promoting adaptation to the decreased vestibular input. Outlines of different treatment approaches for three types of patients with Meniere's disease are presented. The exercise approaches advocated here consist of exercises to foster adaptation of the vestibular system in cases of unilateral loss or hypofunction and exercises to promote the substitution of alternative strategies and to enhance remaining function in cases of bilateral vestibular hypofunction. The selection of the appropriate exercises is based on the nature of the vestibular loss, the patient's symptoms, and the functional capabilities of the patient. Based on an understanding of the vestibular system, the balance system, and normal functional capabilities, appropriate rehabilitative exercises can be designed for this group of patients.

Evaluation of MRI compatibility of the modified nucleus multichannel auditory brainstem and cochlear implants.

Magnetic resonance imaging (MRI) has been contraindicated for users of cochlear implants because of the internal magnet and other possible interactions. Users of the Nucleus Mini-22 Cochlear Implant (CI) or the experimental Multichannel Auditory Brainstem Implant (ABI) may have other disorders that are best diagnosed by MRI. The CI and ABI were modified by replacing the internal magnet and integrated circuit lid with nonmagnetic material. Tests were conducted in a 1.5-T MRI machine. Safety tests for force, heating, induced current, unintentional implant output, and implant damage were conducted by using various phantom models. Image distortion was evaluated in two subjects with implants. The maximum force measured was 2,818 dynes. There was < 0.1 degree C temperature increase in the vicinity of the implant. The maximum induced charge was > or = 667 times less than the minimum charge for auditory stimulation. There was no unintentional output during MRI scans and no change in implant function after 10 repeated scans. Image distortion consisted primarily of darkening and was worst in the axial plane, where it extended 1-2 cm medially and inferiorly from the receiver/stimulator. Compatibility-test results were acceptable, with a large margin of safety. Image distortion is limited to darkening in the immediate vicinity of the implant.

Cochlear implantation in patients with cochlear malformations.

OBJECTIVE: To report operative findings, postoperative course, and postimplantation performance in patients with cochlear malformations who underwent cochlear implantation. DESIGN: Case study and intervention study (before-after trial). Minimum follow-up of 12 months; average follow-up of 24 months. SETTING: Academic tertiary referral center. PATIENTS: Six patients, including five children who underwent implantation at ages 3.5 to 13 years and one adult who underwent implantation at age 27 years. malformations included common cavity deformity (n = 1), cochlear hypoplasia (n = 2), and incomplete partition (n = 3). All patients with cochlear malformations who underwent implantation at the University of Michigan, Ann Arbor, are included, selected from a group of 196 patients so treated since 1986. INTERVENTION: Implantation with a standard multichannel cochlear implant. MAIN OUTCOME MEASURES: Operative findings described include round window and facial nerve anatomy and cerebrospinal fluid leak. Postoperative roentgenographic findings, electrode activation, and reason for non-use of electrodes were investigated. Standard tests of speech perception were used to compare preoperative and postoperative performance for each subject. RESULTS: Operative findings included round window abnormalities (three patients), anomalous facial nerve (one patient), and cerebrospinal fluid leak (three patients). No surgical complications occurred. A minimum of 10 electrodes were activated for all patients. Electrode thresholds and discomfort levels were variable for several months after implantation. All patients demonstrated improved performance after implantation. Four subjects demonstrated open-set speech perception. Two other subjects, whose poor language skills precluded administration of standard tests, showed increased awareness of environmental sounds and increased vocalization after implantation. CONCLUSIONS: Cochlear implantation can be a successful method of rehabilitation in patients with congenital deafness who have cochlear malformations.

Stability of hearing preservation following acoustic neuroma surgery.

Identification of small acoustic neuromas has become commonplace. Frequently, affected individuals are identified prior to the development of significant hearing loss. Whereas many studies have focused on hearing preservation surgery, few have reported on stability of hearing results after resection of acoustic neuroma. Between 1985 and 1991, 36 patients underwent resection of an acoustic neuroma via a retrosigmoid, internal auditory canal approach with attempted hearing preservation. Hearing was preserved in 24 patients; 17 were available for testing for the present study. Follow-up ranged from 1.5 to 8 years. All patients underwent complete audiologic assessment; most patients also underwent auditory brainstem response testing. There was an average 6 dB increase in pure-tone average between early (1 month) postoperative and long-term postoperative test results. A binomial single subject statistic was used to assess for significant changes in speech recognition scores over time. In two subjects there was a significant decrease; however, speech recognition also improved significantly in two subjects. Five of the 17 subjects demonstrated either a significant (at least 15 dB) increase in pure-tone average or a significant decrease in speech recognition over the time-course of the study. All patients maintained usable hearing. We conclude that long-term hearing preservation is a realistic goal in selected acoustic neuroma operations.

The cochlear nuclear complex: MR location and abnormalities.

PURPOSE: To determine the usefulness of MR imaging in locating known structural landmarks of the cochlear nuclear complex (CNC), and to determine the frequency of CNC abnormalities, based on these landmarks, in patients referred for MR evaluation of unilateral sensorineural hearing loss. SUBJECTS AND METHODS: We studied 12 consecutive months of temporal bone/posterior fossa MR images retrospectively to find four known structural landmarks of the CNC: the vestibulocochlear nerve root entry zone; the cerebellar flocculus; the curvilinear choroid plexus along and protruding from the foramen of Luschka; and the bulge of the CNC into the lateral recess of the fourth ventricle and the foramen of Luschka. We tabulated the number of landmarks located per CNC and the number and type of CNC MR abnormalities. Medical records were then reviewed to tabulate clinical indication for MR imaging, type of hearing deficit, and etiology of the hearing deficit and clinical-pathologic proof. RESULTS: 175 patients (ages 15-75 years) provided 350 CNCs for study. All 350 CNCs showed at least three of the four landmarks. Thirteen of the 175 patients (7.4%) had focal CNC MR abnormalities; 136 of these 175 patients had been referred for MR evaluation of unilateral sensorineural hearing loss. In 10 of these 136 patients (7.4%), the CNC abnormalities shown on MR proved to be the cause of the sensorineural hearing loss. CONCLUSIONS: MR reliably delineates the CNC. Over 1 year, approximately 7% of patients referred for MR evaluation of unilateral sensorineural hearing loss had MR-determined focal CNC abnormalities that caused the sensorineural hearing loss.

Primary lymphoma of the temporal bone.

Involvement of the temporal bone by lymphoreticular neoplasm is rare; all reported cases have been of secondary involvement. This article presents what we believe to be the first two reported cases of primary temporal bone lymphoma. The patients, an elderly man and a boy, both presented with infection of the ear, hearing loss, and facial nerve paresis. In both cases, facial paresis resolved after appropriate chemotherapeutic treatment. Patient presentation and clinical course are discussed in light of published work on temporal bone malignancy. Further investigation, including computed tomography and biopsy, should be considered for patients who present with an apparent middle ear infection unresponsive to medical therapy. The development of facial paralysis in such a patient warrants heightened suspicion of malignancy.

Anatomical correlates of functional recovery in the avian inner ear following aminoglycoside ototoxicity.

Tucci and Rubel have demonstrated functional recovery of the chick cochlea following aminoglycoside ototoxicity. The cochleae of these same animals were examined by scanning electron microscopy (SEM) in order to further understand this recovery process. Hatchling chicks were given daily doses of gentamicin for 10 days. Auditory-evoked potential measurements and examination of the cochlea by scanning electron microscopy were performed after survival periods of 5 days to 20 weeks. After 5 days of gentamicin exposure, there was near complete basal hair cell loss associated with a high-frequency hearing loss. Apical progression of damage with a broad-band hearing loss occurred over 4 weeks. At 20-weeks, hair cell counts were normal with a small high-frequency hearing loss. Hair cell regeneration played a major role in the functional recovery of the cochlea.

Physiologic status of regenerated hair cells in the avian inner ear following aminoglycoside ototoxicity.

Regeneration of avian inner ear hair cells has been demonstrated after administration of aminoglycoside and after acoustic trauma. However, no published study to date has documented functional recovery of these regenerated sensory receptor cells. Newborn chicks were treated with gentamicin sulfate (50 mg/kg/day) for a total of either 5 (n = 10) or 10 (n = 76) days. Evoked potential thresholds were obtained one day after the 5-day treatment, or at intervals between one day and 20 weeks after the 10-day treatment course, and compared to thresholds of age-matched control animals. A significant hearing loss, predominantly in the high frequencies, was present after as few as 5 days of drug administration. The magnitude of hearing loss continued to increase, especially at lower frequencies, as survival increased from 1 day to 5 weeks after gentamicin treatment. Sixteen-to-20 weeks after treatment, partial recovery of thresholds was evident. These findings demonstrate that functional recovery does occur in the avian inner ear following aminoglycoside administration. Recovery occurs at all frequencies, but predominantly at low and middle frequencies, leaving significant residual high-frequency threshold elevation. Recovery lags 14 to 18 weeks behind anatomic evidence of hair cell regeneration, which was demonstrated in one study by 2 weeks after comparable administration of gentamicin.

Trends in rehabilitation after cochlear implantation.

Although postimplant rehabilitation is generally considered to be an important aspect of a cochlear implant program, the literature reports widely varying practices in the implementation of such programs. The National Institutes of Health consensus statement on cochlear implantation (May 1988), while recognizing that aural rehabilitation facilitates maximal use of the implant, offers no specific guidelines or recommendations. A questionnaire survey was made of over 200 otolaryngologists to determine current practices in a variety of aspects of their cochlear implant programs. Results were obtained from a total of more than 1400 patients, including adults, adolescents, and children. Rehabilitation is an integral part of the cochlear implant program in the vast majority of active programs surveyed. General aspects of cochlear implant programs, as well as those of rehabilitation programs in particular, are discussed.

Use of the bone conduction ABR Wave I response in determination of cochlear reserve.

Interest in bone-conducted stimulus auditory brainstem response (BC-ABR) testing has centered around evaluation of cochlear integrity in neonates and in patients who are difficult to evaluate audiometrically. The present study was designed to evaluate the Wave I component of the BC-ABR and to determine the utility of this response in assessment of cochlear reserve. The source of Wave I has been shown to be the distal eighth nerve. It was postulated that the presence or absence of this component would provide ear specific information useful for determination of cochlear integrity. In order to test this hypothesis, patients with a documented unilateral hearing loss were studied. Stimulus presentation was via the Radioear B-70 bone vibrator used in conventional audiometric assessment. Evoked potential responses were recorded at four presentation levels. Subjects had either normal hearing bilaterally or normal hearing in one ear and a mild-to-profound sensorineural hearing loss in the opposite ear. Our data indicate that the Wave I response, when measured in this fashion, is ear specific. Ear specificity was shown to be aided by good waveform morphology, as typically observed in younger subjects, and by a relatively large discrepancy in hearing thresholds between the normal and hearing-impaired ears. This technique may be of value in determination of cochlear reserve in patients with problematic masking dilemmas.

Coherence analysis of scalp responses to amplitude-modulated tones.

Magnitude squared coherence is a method of auditory evoked potential (AEP) frequency-domain analysis, measuring the degree to which the AEP is determined by the stimulus as a function of frequency. In 10 normal-hearing human subjects, scalp responses to an amplitude-modulated (AM) tone (500 Hz carrier modulated with a 40 Hz envelope) were recorded. Critical value criteria were utilized in the statistical analysis of coherence-intensity functions for determination of threshold responses. When compared with subjective determination of AEP threshold from time-domain waveforms, coherence analysis provided a significantly (p less than 0.001) more sensitive threshold measure. Coherence analysis provides information on the spectral content of the response, and allows for the objective determination of threshold which may potentially be utilized in a more expeditious threshold measurement scheme.

Changes in spontaneous activity and CNS morphology associated with conductive and sensorineural hearing loss in chickens.

The effects of a conductive or mixed (conductive and sensorineural) hearing loss on anatomical and physiological properties of the chicken auditory system were examined. Animals used in the anatomical studies underwent either a columella (ossicle) removal, which produced a moderate conductive hearing loss, or an oval window puncture, which produced a severe mixed hearing loss, at 4 days posthatch. In a companion study, multiunit spike counts were obtained from 3-week-old chickens before, during, and after consecutive tympanic membrane puncture, columella removal, and oval window puncture. Tympanic membrane puncture and columella removal (conductive hearing loss) are not associated with either cell area changes in the nucleus magnocellularis or changes in spontaneous neuronal activity. Conversely, an oval window puncture (sensorineural damage) is associated with a cell area reduction of 20%, as well as a marked decline in activity within auditory nuclei.