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PubMedDate not supplied

Electrocochleography and cochlear pathology.

In experimentally damaged inner ears the structural alterations were correlated to electrocochleographic responses of the ear. Sectioning of the cochlear nerve with degeneration of the type I neurons but intact sensory cells results in normal cochlear microphonics but very weak and atypical nerve responses. By contrast, damage of the organ of Corti with retrograde degeneration abolished primarily the cochlear microphonics, whereas the compound VIII nerve action potential is barely affected when only the outer hair cells are gone and even a small number of surviving inner hair cells is still compatible with a relatively strong compound action potential of the cochlear nerve.

Action Potentials↗

PubMed2001

Postoperative vestibular-evoked myogenic potentials in cases with vestibular schwannomas.

Although still the subject of discussion, vestibular-evoked myogenic potentials (VEMPs) have been considered to reflect the function of the saccular and, more recently, the cochlear tracts. To accurately determine the precise afferent pathway carrying VEMPs, we studied the outcomes of VEMPs and other examinations in patients with unilateral vestibular schwannomas. Eleven patients with unilateral vestibular schwannomas resected using a middle cranial fossa approach were included in the study. Patients underwent pure-tone threshold audiometry, caloric tests and analysis of auditory brainstem responses (ABRs) and VEMPs pre- and postoperatively. The results were compared with those obtained in patients with intact superior or inferior vestibular and cochlear nerves. Among the 11 patients studied, 4 retained their VEMPs postoperatively. Three of the 10 patients with inferior vestibular schwannomas exhibited normal VEMPs, preserved hearing levels (20 dB HL) and anatomically intact superior vestibular nerves. In all of these cases, ABRs more closely correlated with VEMPs than with caloric responses. In one of the cases with inferior vestibular schwannomas, VEMPs were preserved postoperatively and VEMP latencies were shortened, which indicates the preoperative presence of a conduction block in either the cochlear or superior vestibular nerve. VEMPs may be conducted in both the superior vestibular and cochlear nerves, as well as in the inferior vestibular nerve. Thus, evaluation of saccular nerve function should be performed carefully, especially in cases where hearing is preserved. It appears that cochlear conduction may proceed along two pathways, one direct and the other via the brainstem, but this remains to be verified.

Adult↗

PubMed1993

Responses to speech signals in the normal and pathological peripheral auditory system.

The responses to single (/a/ and /i/) and double vowel (/a,i/) stimuli of normal guinea pig cochlear nerve fibres are compared with those from animals with a cochlear hearing loss. When the threshold losses are sufficient to exclude the higher harmonics of the /i/, the temporal representation of the second and higher formants is lost. Smaller threshold elevations allow a representation of the second formant when the vowel /i/ is presented alone. However, under double vowel stimulation wider auditory filters allow the capture of the synchrony of high characteristic frequency fibres by lower frequencies thereby losing the higher formants of the /i/ and also much of the information about its fundamental frequency.

Animals↗

PubMed2000

Advantages of the retrosigmoid approach in auditory brain stem implantation.

From April 1997 to December 1999, six patients (five men and one woman), ranging in age from 22 to 37 years with neurofibromatosis type 2 (NF2) were operated on via the classic retrosigmoid-transmental (RS-TM) approach for removal of a vestibular schwannoma (VS) (tumor size from 12 to 40 mm) and for auditory brain stem implantation (ABI). After tumor removal, the floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleos were reached, and the ABI was inserted. More recently, an ABI was implanted via the retrosigmoid approach in a 4-year-old boy with a cochlear malformation (common cavity) associated with cochlear nerve aplasia. Electrically evoked auditory brain stem responses (EABRs) and neural response telemetry (NRT) were performed to verify the correct positioning of the inserted electrodes. No major complications related to ABI were observed. ABI has been activated to date in five of the NF2 patients. Auditory sensations with various numbers of electrodes were evoked in all patients. We consider the RS-TM approach the route of choice for ABI insertion in patients with NF2 and good hearing, offering a chance of hearing preservation, and in patients with complete cochlear ossification, severe head trauma and cochlear fracture, or nerve disruption, or a combination of these. A new indication for ABI implantation via the RS approach is presented by patients with bilateral cochlear nerve aplasia.

Journal Article↗

PubMed2004

Preoperative imaging protocol for cochlear implant candidates.

OBJECTIVE: To formulate and test a CT imaging protocol for preoperative scanning of the temporal bone in cochlear implant candidates. MATERIAL AND METHODS: A human head was scanned in three CT planes: axial, axiopetrosal and semilongitudinal. Multiplanar reformats (MPRs), based on axial slices, were created and compared with the corresponding images obtained by direct scanning in the respective planes. All scans were analyzed on a viewing workstation. RESULTS: The axial plane image allowed for an overview of the temporal bone. The width of the facial recess and the cochlear nerve canal could be studied on combined axial and axiopetrosal images. Cochlear patency could be evaluated using combined axial and semilongitudinal images. Axiopetrosal and semilongitudinal MPRs were able to replace the images obtained by direct scanning in the respective planes. CONCLUSION: The combination of the axial CT plane image and MPRs was found to be sufficient for preoperative analysis of the temporal bone morphology.

Cochlear Implantation↗

PubMed1992

[Inner ear damage due to hyperlipidemia in the young and old guinea pigs].

Nineteen guinea pigs of one month and 11 of 30 months were given high fat diet for 3 months to produce hyperlipidemia; 16 of them followed by 3 months' normal diet to resume blood lipid level; 27 for control. The results showed that: The serum cholesterol level of the experimental groups was markedly elevated (P < 0.01) with fatty degeneration of liver. Damages in OHC, IHC, cells of stria vascularis and few myelin sheaths of cochlear nerve were seen in all experimental animals, and the reduction of cochlear damages was not seen in those animals with blood lipid level resumed to normal. Auditory dysfunction was very marked as shown by ABR in the old animals with hyperlipidemia as compared with controls (P < 0.01).

Animals↗

PubMed1994

Facial nerve outcome after acoustic neuroma surgery.

Factors influencing facial nerve preservation and function in acoustic neuroma surgery were studied in 108 cases. Anatomic preservation of the nerve was inversely related to tumor size and improved as the series progressed. When the nerve was saved, normal postoperative function was inversely related to tumor size and was more common if the cochlear nerve was also saved. Most intact nerves eventually recovered some function, but late function was seldom completely normal unless there was some early recovery. The results demonstrate the importance of tumor size, operator experience, and ease of dissection on facial nerve outcome.

Cochlear Nerve↗

PubMed1993

[MRI enhancement of the facial nerve with Gd-DTPA--second report--investigation of enhanced nerve portions in patients with facial palsy].

We performed enhanced MRI using Gd-DTPA in 84 patients with facial palsy. After assessing enhancement of the normal facial nerve, we examined enhancement in patients with Bell's palsy and Ramsay Hunt syndrome. The following results were obtained. 1. In 95% of patients with Bell's palsy, enhancement was obtained in the distal IAC and labyrinthine portions. In 72%, enhancement was significant from the distal IAC portion through the vertical portion. In some of the patients who underwent enhanced MRI twice, increased signal intensity was observed in distal portions such as the vertical portion. 2. In many cases with Ramsay Hunt syndrome, enhancement was seen extensively in the IAC portion through the vertical portion. In the subjects with internal auditory symptoms such as vertigo and tinnitus, enhancement of the IAC portion was seen not only in the facial nerve but also in the vestibular and the cochlear nerves. These results suggest that the vascular permeability of lesions in Bell's palsy may be increased from the distal IAC portion to the vertical portion. Judging from the present findings with Ramsay Hunt syndrome, symptoms related to the enhanced portions suggest that accompanying internal auditory symptoms occur due to inflammation of the IAC portions of cochlear and vestibular nerves.

Adult↗

PubMed1995

Temporal modulation transfer functions in the European Starling (Sturnus vulgaris): II. Responses of auditory-nerve fibres.

The temporal resolution of cochlear-nerve fibres in the European starling was determined with sinusoidally amplitude-modulated noise stimuli similar to those previously used in a psychoacoustic study in this species (Klump and Okanoya, 1991). Temporal modulation transfer curves (TMTFs) were constructed for cochlear afferents allowing a direct comparison with the starling's behavioural performance. On average, the neuron's detection of modulation was less sensitive than that obtained in the behavioural experiments, although the most sensitive cells approached the values determined psychophysically. The shapes of the neural TMTFs generally resembled low-pass or band-pass filter functions, and the shapes of the averaged neural functions were very similar to those obtained in the behavioural study for two different types of stimuli (gated and continuous carrier). Minimum integration times calculated from the upper cut-off frequency of the neural TMTFs had a median of 0.97 ms with a range of 0.25 to 15.9 ms. The relations between the minimum integration times and the tuning characteristics of the cells (tuning curve bandwidth, Q10 dB-value, high- and low-frequency slopes of the tuning curves) are discussed. Finally, we compare the TMTF data recorded in the starling auditory nerve with data from neurophysiological and behavioural observations on temporal resolution using other experimental paradigms in this and other vertebrate species.

Acoustic Stimulation↗

PubMed1995

The limitations of hearing preservation in acoustic neuroma surgery: histological study of the interface between the eighth cranial nerve and the tumor.

The interface between the eighth cranial nerve and acoustic neuroma was investigated by light microscopy and immunohistochemistry in paraffin sections in 13 patients, and in Epon sections in 19 patients. The cochlear nerve was severely invaded by tumor cells in two of six specimens examined, moderately invaded in two specimens, and not invaded in two specimens. Gliosis was frequently found within the cochlear and vestibular nerve. Hemangioma-like tissue was occasionally found attached to the cochlear and vestibular nerve adjacent to the tumor. These findings were considered to be related to difficulty in hearing preservation in some patients although acoustic neuroma surgery was successful.

Adult↗

PubMed2004

An immunogold investigation of the distribution of GABA and glycine in nerve terminals on the somata of spherical bushy cells in the anteroventral cochlear nucleus of guinea pig.

Spherical bushy neurons in the anteroventral cochlear nucleus receive glutamatergic primary terminals from the cochlear nerve and terminals of noncochlear (i.e. nonprimary) origin, many of which colocalize gamma-aminobutyric acid (GABA) and glycine. Here the relationship between GABA and glycine in these terminals has been investigated using postembedding immunogold labelling. A significant negative correlation was found between the density of terminal labelling for GABA and for glycine in four guinea pigs. Terminals could be divided into three categories, GABA-only, glycine-only, or colocalizing depending on whether they had a significantly higher labelling density for either amino acid than the primary terminals. The overall labelling density in all four animals was significantly greater for GABA in GABA-only terminals than colocalizing ones but similar for glycine in both. Within the terminals, the labelling density over synaptic vesicles, nonvesicular regions of cytoplasm and mitochondria was also investigated. No significant difference was detected in the labelling density of vesicles compared with nonvesicular regions for either amino acid. However, a significant difference was found between the overall labelling density over mitochondria and nonvesicular regions for both. There was also significantly more mitochondrial GABA labelling in GABA-only terminals compared to colocalizing terminals but mitochondrial glycine labelling was similar in glycine-only and colocalizing terminals. Thus the level of GABA is higher in single than in colocalizing terminals, particularly in the mitochondria, but similar for glycine in both. It is possible therefore that the presence of glycine affects the level of GABA in the nonprimary terminals but that the presence of GABA does not affect the level of glycine.

Animals↗

PubMed1999

Additive noise can enhance temporal coding in a computational model of analogue cochlear implant stimulation.

Conventional analogue multichannel cochlear implants are unlikely to convey formant information by the fine time structure of evoked discharges. Theoretically, however, the addition of noise to the channel outputs could enhance the representation of formants by time coding. In this study, the potential benefit of noise in analogue coding schemes was investigated using a computer model of cochlear implant stimulation. The cochlear nerve was modelled by the Frankenhauser-Huxley equations. For all five vowels investigated, the optimal addition of noise to the first channel of the simulated implant (200-671 Hz) caused enhancement of the first formant representation (as seen in amplitude spectra of the simulated discharges). For vowels with a low-frequency second formant, clear enhancement of the second formant resulted from the optimal addition of noise to the third channel (1200-2116 Hz). On the basis of the present computational study, additive noise would be expected to enhance the coding of temporal information by the discharges of a single nerve fiber.

Cochlear Implants↗

PubMedDate not supplied

Tectorial membrane: a possible sharpening effect on the frequency analysis in the cochlea.

Excitation of cochlear hair cells is believed to be produced by radial deflection of their stereocilia as a result of shear motion between the reticular lamina and the tectorial membrane. The shear motion must depend on the mechanical coupling between the organ of Corti and the tectorial membrane. It is shown that, because longitudinal stiffness of the tectorial membrane, the shear motion must be enhanced as the wavelength decreases toward the cochlear vibration maximum. The resulting sharpening of the vibration maximum agrees with the frequency selectivity found in the inner hair cells and cochlear-nerve afferents.

Basilar Membrane↗

PubMed2003

Facial nerve paralysis following cochlear implant surgery.

OBJECTIVES: Facial nerve paralysis is a rare but devastating complication of cochlear implant surgery. The aims of the study were to define the incidence of facial nerve paralysis in our series and understand possible mechanisms of injury. STUDY DESIGN: Retrospective chart review and case reports. METHODS: Charts were reviewed of all 705 patients implanted between 1980 and 2002 at the authors' institutions to identify those with postoperative facial nerve weakness and determine incidence. For patients with facial nerve weakness, onset, degree, and timing of paralysis were noted; clinical findings were correlated to operative report findings. The method of treatment was noted, and the final facial nerve function outcome was recorded. RESULTS: Five patients (one child and four adults) were found to have postoperative facial nerve weakness, for an incidence of 0.71%. This complication was delayed in all cases, ranging from 18 hours to 19 days postoperatively. All patients were treated with steroids or steroids combined with antiviral medication, and all ultimately recovered normal facial function. CONCLUSIONS: In the study series, the incidence of facial nerve paralysis following cochlear implant surgery was 0.71%. Possible mechanisms of injury included heating injury and viral reactivation. All patients presented with a delayed facial nerve paralysis and did recover normal facial nerve function.

Adult↗

PubMed2000

Functional magnetic resonance imaging may avoid misdiagnosis of cochleovestibular nerve aplasia in congenital deafness.

OBJECTIVE: To investigate a narrow internal auditory canal (IAC) syndrome using functional magnetic resonance imaging (fMRI) of the auditory cortex. STUDY DESIGN: The study design was a case report. The follow-up period lasted 18 months. SETTING: The study was carried out in the audiology clinic of an ear, nose, and throat department and in the department of pediatric neuroradiology at a university hospital. MAIN OUTCOME MEASURES: Age-appropriate observational audiometry, objective audiovestibular tests, computed tomography (CT), magnetic resonance imaging (MRI), and (fMRI) of the auditory cortex were performed to analyze in detail the profound deafness of a young child. RESULTS: Audiovestibular examination demonstrated both measurable hearing and normal vestibulo-ocular reflex, and CT showed narrow IACs combined with normal labyrinths. Axial MR images completed by sagittal sections perpendicular to the IAC delineated a single nerve that was initially supposed to be the facial nerve. No cochleovestibular nerve was identified. However, fMRI performed with the patient under general anesthesia demonstrated activation of the primary auditory cortex during 1-kHz monaural stimulation on the left side. CONCLUSIONS: The absence of cochleovestibular nerve on MR studies cannot exclude connections between the inner ear and the central auditory pathways. This might be caused by a lack of spatial resolution of anatomical MR studies. The single nerve delineated within the IAC might also carry both facial and cochleovestibular fibers. Functional MRI can assess the cortical response to acoustic stimuli when aplasia of the cochleovestibular nerve is suspected. This case study illustrates a novel and atypical presentation of cochlear nerve dysplasia.

Acoustic Stimulation↗

PubMed1980

Surgical management of cerebellopontine angle tumors.

Of the 50 cerebellopontine angle tumors diagnosed during a three and one half year period, 36 were removed (all totally) using one of the three approaches to the cerebellopontine angle. Six patients, all elderly (over age 70) with obstructive hydrocephalus and complicating medical problems, had ventriculoperitoneal shunts inserted. One of these subsequently required tumor excision 10 months later. Of the 35 patients with normal functioning facial nerves pre-operatively, 34 (97 per cent) had the nerve spared surgically. Of 12 patients in whom hearing conservation was attempted, the cochlear nerve was spared in eight. However, useful hearing was preserved in one, with sound appreciated only in another.

Adult↗

PubMed1988

Induction of aberrant functional afferents to the chick cochlear nucleus.

Surgical extirpation of the otocyst on embryonic day (E) 3 in chick embryos prevents formation of the cochlear nerve and results in development of an aberrant axonal projection from the contralateral cochlear nucleus (nucleus magnocellularis, NM) to the deafferented NM. We have studied the morphology of this projection using horseradish peroxidase injections in NM axons and light and electron microscopy. The ability of the projection to activate its target neurons synaptically was assessed by means of extracellular microelectrode recording from in vitro preparations of the chick brainstem. The aberrant projection arises as a vertically directed branch from the contralaterally traveling NM axon at the medial border of nucleus laminaris (NL). This axonal branch forms boutonal endings that may terminate anywhere in NM but are most common in its ventral and medial regions. In our experiments, this projection is not seen on the unoperated side of experimental animals or in normal controls from E11 onward but is found on the operated sides of all experimental animals, including those with bilateral removal of the otocysts. The aberrant projection persists at least from E11 through hatching and has essentially identical features in unilaterally and bilaterally lesioned animals. The endings of the aberrant projection are boutonal in form and, in the electron microscope, exhibit all of the elements associated with normal synapses. Electrophysiological studies confirm that stimulation of the aberrant axons can elicit postsynaptic responses in NM and suggest that these synapses use an excitatory amino acid neurotransmitter.

Animals↗

PubMed1981

"Onion bulb" formation associated with a solitary neoplasm of the eighth nerve sheath.

Audiometric patterns associated with retrocochlear disorders include abnormal adaptation, delay of auditory evoked potential latencies, and characteristic abnormalities of speech intelligibility functions. The anatomic substrate of these psychoacoustic and electrophysiologic abnormalities is unknown. This paper describes the incidental observation of "onion bulbs" in a solitary neoplasm involving the eighth nerve sheath and in the cochlear nerve lateral to the neoplasm. Such hypertrophic neuropathy in the eighth nerve has not been described previously. This occurrence raises the question of sequential demyelination and remyelination in a neoplasm-bearing nerve as a possible histopathologic correlate to retrocochlear dysfunction.

Cranial Nerve Neoplasms↗