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PubMed2003

Clinical relevance of the distance between the cochlea and the facial nerve in cochlear implantation.

HYPOTHESIS: To elucidate possible mechanisms of facial nerve costimulation after cochlear implantation that are supposed to result from the close cochlea to facial nerve contact. BACKGROUND: One of the postoperative complications of cochlear implantation is facial muscle twitching, which has preferentially been found in otosclerotic patients. It impairs hearing benefits because of deactivation of electrodes and can still not be adequately prevented. METHODS: A total of 13 temporal bones were dissected to quantify where the labyrinthine portion of the facial nerve is closest to the scala tympani, the placement site of the cochlear implantation electrode array. After the typical operative procedures to find out the number of electrodes lying closest to the facial nerve were performed, a cochlear implantation array was inserted into four specimens. The clinical records of 14 otosclerotic patients were investigated to correlate these results with the position of in vivo deactivated electrodes. RESULTS: The closest distance between the scala tympani and the nerve was only 0.33 mm (+/-0.14). On average, after insertion of 23 electrode resp. marking rings, the facial nerve was reached. This is clinically the position of most frequently deactivated electrodes to prevent postoperative facial nerve costimulation. CONCLUSIONS: These investigations support the hypothesis that a direct current spread at the site of the facial nerve crossing the cochlear basal turn is most likely the reason for postoperative facial muscle twitching facilitated in otospongiotic bone. Prevention could therefore be achieved by cochlear implantation designs and surgical techniques that take into consideration the site of closest contact.

Adult↗

PubMed1988

[Intraoperative electrophysiological monitoring for hearing preservation in acoustic neurinoma surgery].

Five acoustic neurinomas have been operated with hearing preservation as a goal. We monitored intraoperative brainstem auditory evoked potentials (BAEP) in all five cases, electrocochleogram (ECoG) using needle electrode in external auditory meatus in four, and compound action potentials directly recorded from the cochlear nerve (CAP VIII) in three. In all five cases the tumor was totally resected and cochlear nerve was anatomically preserved. However, in only one case useful hearing was preserved with preservation of all wave forms of the BAEP. Another patient with preservation of all wave forms of BAEP and the ECoG showed postoperative severe hearing loss. Other three patients showed postoperative severe hearing loss: only Wave I of BAEP and ECoG were preserved without preservation of the CAP VIII in one whose cochlear nerve was thought to be damaged in cerebellopontine angle cistern; Wave I of BAEP, ECoG and CAP VIII were preserved in one in whom it was suggested cochlear nerve near brainstem or cochlear nucleus was damaged; none of the BAEP, ECoG and CAP VIII was preserved in one in whom it was suggested distal cochlear nerve, or internal auditory artery was damaged. These different patterns of changes suggested that different causes for the hearing loss and difficulties in hearing preservation during acoustic neurinoma surgery. Having identified the putative mechanism of the hearing loss by monitoring those potentials, suggestions are made about how such hearing loss might be avoided. For preservation of the hearing in acoustic neurinoma surgery, all of those potentials including all wave forms of BAEP, ECoG and CAP VIII should be preserved during surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1989

The auditory system in methyl mercurial intoxication: a neuropathological investigation on 14 autopsy cases in Niigata, Japan.

The auditory system in 14 autopsy cases of methyl mercurial intoxication in Niigata was examined neuropathologically. In addition to degeneration of the transverse temporal gyrus, there was also either significant loss of small myelinated fibers or decrease of large neurons in the cochlear nerve, ventral cochlear nucleus and inferior colliculus. The degree of decrease was higher in acute patients than in the chronic, and was not correlative to the severity of cerebrovascular sclerosis. The hearing impairment in methyl mercurial intoxication may be induced by the combined degeneration of the neurons or nerve fibers of these structures in both acute and chronic patients.

Adult↗

PubMed1995

Anomaly of the facial nerve and cochlear implantation.

The incidence of preoperative damage to the facial nerve in cochlear implantation is very low. The course of the facial nerve is not routinely evaluated preoperatively, but variations in its course can restrict access to the round window and impede implantation. In 5 of 42 operations, direct visualization of the round window was not possible. In 1 patient there was an obvious congenital bifurcation. Successful implantation was made possible by mobilizing the nerve.

Adult↗

PubMed1987

[Management of acoustic neurinoma with preserved hearing].

Six cases of acoustic neurinoma (AT) with preoperatively preserved hearing are presented. Their clinical features and surgical management for hearing preservation are discussed. Presenting symptoms were hearing decrease in 3 cases (#1, #2, #6), trigeminal neuralgia in 3 cases (#4, #5, #6), and hemifacial spasm in case #2. Case #3 was an incidentally diagnosed case by CT scan. Preoperative decrease of hearing in the affected ears ranged from 10 dB to 60 dB. In all cases except for case #1, CT scans revealed CP angle tumors sized 1 - 4 cm in diameter. A small intracanalicular tumor was detected in case #1 by metrizamide CT scan. Enlargement of the internal auditory meatus was detected in two cases (#4, #6). Through a lateral suboccipital transmeatal approach, all cases underwent radical total resection of tumor with anatomical preservation of both facial and cochlear nerves. Postoperatively, in spite of good morphological preservation of cochlear nerve, useful hearing function was noted in only one case (#3) who had excellent preoperative hearing (10 dB) and whose tumor was very small (1 cm). Facial nerve function was satisfactory in all patients. According to the previous reports, the level of hearing decrease in AT is related to the origin of tumor, direction of its growth, invasions or involvement to cochlear nerve and extension toward the labyrinth. Major factors to achieve preservation of useful hearing in AT surgery are 1) size of the tumor and 2) preoperative hearing acuity. Early detection of small tumor is extremely important and surgery must be precise to preserve cochlear nerve, internal auditory artery and labyrinth.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

[Acoustic neuroma. A review of 50 cases].

A consecutive series of 50 patients undergoing acoustic neurinoma excision between 1985 and 1990 is reported. All patients were operated by the retrosigmoid approach. With no mortality, the most frequent complication was a leak of cerebrospinal fluid (12%). The facial nerve was preserved in 94% of cases and the cochlear nerve in 38% of cases. In our series, the size of the neurinoma is the most important predictive factor for the post-operative facial nerve function. The cochlear nerve could be preserved in 19 patients but only 4 patients retain some hearing after surgery, which represents 8% of the whole series and 21% of the preserved cochlear nerves. 4 patients had a subtotal excision. All 4 had a grade IV neurinoma. Only one patient had a recurrence that necessitated a second surgery. Complete excision is the goal in acoustic neurinoma surgery. However the dilemma can sometimes arise between a complete removal necessitating the sacrifice of the facial nerve and a subtotal extirpation with regular control CT-scans in the follow-up.

Adult↗

PubMed1992

Microsurgical anatomy of acoustic neuroma.

Because acoustic neuromas most frequently arise in the posteriorly placed vestibular nerves, they usually displace the facial and cochlear nerves anteriorly (Figs. 11, 12, and 13). The facial nerve is stretched around the anterior half of the tumor capsule. Variability in the direction of growth of the tumor arising from the vestibular nerves may result in the facial nerve being displaced, not only directly anteriorly, but also anterior-superiorly or anterior-inferiorly. The nerve is infrequently found on the posterior surface of the tumor. Because the facial nerve always enters the facial canal at the anterior-superior quadrant of the lateral margin of the meatus, it is usually easiest to locate it here, rather than at a more medial location where the degree of displacement of the nerve is more variable. The cochlear nerve also lies anterior to the vestibular nerve and is most frequently stretched around the anterior half of the tumor. The strokes of the fine dissecting instruments used in removing the tumor should be directed along the vestibulocochlear nerve from medial to lateral rather than from lateral to medial because traction medially may tear the tiny filaments of the cochlear nerve at the site where these filaments penetrate the lateral end of the meatus to enter the cochlea. The landmarks that are helpful in identifying the facial and vestibulocochlear nerves at the brain stem on the medial side of the tumor have been reviewed. These nerves, although distorted by tumor, can usually be identified on the brain stem side of the tumor at the lateral end of the pontomedullary sulcus, just rostral to the glossopharyngeal nerve and just anterior-superior to the foramen of Luschka, flocculus, and choroid plexus protruding from the foramen of Luschka. After the facial and vestibulocochlear nerves are identified on the medial and lateral sides of the tumor, the final remnants of the tumor are separated from the intervening segment of the nerves. In the three approaches to the meatus and cerebellopontine angle--retrosigmoid, translabyrinthine, and middle fossa--a communication may be established between the subarachnoid space and the mastoid air cells that requires careful closure to prevent a cerebrospinal fluid leak.

Brain Stem↗

PubMed1980

Changes of acoustic nerve and cochlear nucleus evoked potentials due to repetitive stimulation.

Evoked potential recordings were studied in the acoustic nerve and cochlear nucleus of adult decerebrate cats as a function of the tone duration (1-1500 msec) and the tone repetition rate (0.5/sec-200/sec). Response decrements due to repetitive tonal stimulation were primarily a function of the inter-tone interval. At inter-tone intervals longer than 100 msec, no response decrements were observed in the acoustic nerve or the cochlear nucleus. With shorter inter-tone intervals, reversible response decrements of identical magnitudes ocurred in both the acoustic nerve and the cochlear nucleus evoked potentials. The mechanism of response decrement may be due to depression at the hair cell-acoustic nerve junction.

Acoustic Stimulation↗

PubMed1999

Hearing impairment in patients with acoustic neuroma--analysis by electrocochleography.

OBJECTIVE: In patients with acoustic neuroma, the site and severity of hearing impairment are important in discussing surgical approaches. Since the effectiveness of conventional auditory psychological testing is limited, we studied objectively hearing impairment of the cochlea and the cochlear nerve due to the tumor. METHODS: Electrocochleography (ECochG) was carried out in 21 patients with acoustic neuroma. Cochlear microphonic potential (CM) and action potential (AP) in ECochG evoked with clicks and short tone bursts were recorded through a transtympanic needle electrode technique. Cochlear function was studied using the detection thresholds of CM, and cochlear nerve involvement was analyzed by differences between AP and CM detection thresholds. RESULTS: The 1 kHz CM detection threshold was elevated in 17 (81.0%) of 21 patients indicating cochlear impairment. Of seven patients with normal hearing or mild sensorineural hearing loss in pure tone audiometry, three had a slightly elevated CM detection threshold. Of five patients with pronounced pure tone levels, four showed a CM response and were thought to have mild cochlear dysfunction. Cochlear nerve impairment was confirmed in three of four patients with well-developed CM based on elevated AP detection thresholds. Three patients had CM response but no AP response, suggesting severe cochlear nerve impairment. CONCLUSION: Disorders of the cochlea and the cochlear nerve can be evaluated with ECochG AP and CM measurement. The findings of ECochG are thought to be important information to judge hearing prognosis, thereby enhancing its clinical utility.

Adult↗

PubMed2000

An in vivo quantifiable model of cochlear neuronal degeneration induced by central process injury.

In the available in vivo experimental models for cochlear neuronal degeneration, the peripheral (hair cell side) process of the cochlear nerve has been injured in order to induce neuronal degeneration. However, there has been no dependable experimental model in which cochlear neuronal degeneration begins from the central (brain stem side) process. This lack of a central process injury model has probably been due to the experimental difficulties that had to be overcome in order to reproducibly and selectively injure the central process of the cochlear neurons while maintaining the patency of the internal auditory artery in small experimental animals such as rats. Using rats, we first developed a central process injury model in which the reduction of the spiral ganglion cells due to retrograde degeneration of cochlear neurons can be quantitatively evaluated. In our experimental model, the cochlear nerve was compressed and injured by a compression-recording (CR) electrode placed at the internal auditory meatus. First, the cochlear nerve was compressed until the compound action potentials of the cochlear nerve became flat, and then the CR electrode was advanced by various compression speeds (5, 10, or 200 micrometer/s) to reach the same depth (400 micrometer). In our model, therefore, the reduction of the spiral ganglion cells was caused compression speed dependently. This method made it possible to produce compression injury to the cochlear nerve without evidence of damage to the blood supply to the cochlea via the internal auditory artery. This model gives us the means to obtain knowledge that was previously impossible to derive from the peripheral process injury models.

Animals↗

PubMed2001

Narrow internal auditory meatus: an idiopathic case confirming the origin and pathway of vestibular evoked myogenic potentials in humans.

OBJECTIVE: To confirm the origin and pathway of vestibular evoked myogenic potentials (VEMPs) in humans. DESIGN: Case study. SETTING: University hospital. PATIENT: A patient with a narrow internal auditory meatus (IAM). MAIN OUTCOME MEASURES: Imaging studies and functional studies concerning the seventh and eighth cranial nerves. RESULTS: Of the 4 nerves in the IAM, all but the cochlear nerve had normal function and normal courses, despite the pronounced narrowing of the IAM. The facial nerve had a normal diameter, but the vestibular nerves were thinner. Imaging revealed that the cochlear nerve was absent or extremely thinned. Both the cochlea and the cochlear nerve showed no function in the affected ear, although the VEMPs were evoked normally. CONCLUSION: Our results definitively support the vestibular origin of VEMPs in humans.

Adult↗

PubMed2002

The origin of the 900 Hz spectral peak in spontaneous and sound-evoked round-window electrical activity.

We have monitored the spectrum of the (spontaneous) neural noise at the round window (RW) and on the surface of the antero-ventral cochlear nucleus (CN) and the dorsal CN (DCN) of anaesthetised guinea pigs. We have also obtained the average gross extracellular waveform evoked by 20 kHz tone-bursts (0.25 ms and 25 ms) at each of these recording sites, and calculated the spectrum of the average waveforms (SAW). With these tone-bursts, only a small population of neurones in the extreme basal turn of the cochlea near the RW electrode responds, presumably with only a single action potential for each 0.25 ms tone-burst. The RW waveforms recorded between 20 dB and 60 dB SPL were very similar, and are therefore presumably a simple estimate of the shape of the contribution of the firing of a single neurone to the gross RW signal (the unitary potential or UP). In normal animals, the SNN and the SAW were remarkably similar, with peaks at 900 Hz and at 2400 Hz, suggesting that they are not due to neural synchronisation (as suggested previously by others), but are due to an oscillatory waveform produced by each single fibre action potential. Abolition of all spike activity by RW tetrodotoxin left a waveform with only a summating potential and a dendritic potential, and no 900 Hz peak in the SAW or SNN, indicating that the spectral peak is due to neural spiking only. Abolition of the CN contribution to the RW waveforms by CN application of lignocaine or sectioning of the cochlear nerve at the internal meatus (by focal aspiration of the DCN and underlying cochlear nerve) showed that the 900 Hz peak was not simply due to the addition of a delayed and inverted CN contribution: mathematical modelling shows that this would produce a broad spectral peak at about 1200 Hz. Moreover, the 900 Hz spectral peak remains after complete abolition of the CN contribution, although reduced in amplitude. This residual 900 Hz peak can be traced to an oscillation in the gross waveform due to the presence of two peaks (P(1)* and N(2)*) which follow the intact N(1) peak. The P(1)* and N(2)* peaks were present at the RW, but not at the cochlear nerve as it exits the internal meatus, suggesting that they were not due to double-spiking of some of the neurones, but were probably due to a sub-threshold electrical resonance in the peripheral dendrites. We have successfully modelled the production of the SNN and the compound action potential and SAW in response to 0.25 ms and 25 ms tone-bursts at 20 kHz by including only a damped 900 Hz resonance in the UP, without refractory effects, preferred intervals or synchronisation in the timing of neural spike generation. Such resonances in other neurones are known to be due to the activation kinetics of the voltage-controlled sodium (Na(+)) channels of these neurones. The presence of such sub-threshold oscillations probably indicates that the peripheral dendrites are devoid of stabilising potassium (K(+)) channels. We also discuss the role of this membrane resonance in generating burst-firing of the cochlear nerve (as with salicylate) and the role of such burst-firing in generating tinnitus.

Action Potentials↗

PubMed2005

Auditory brainstem implant (ABI): new frontiers in adults and children.

UNLABELLED: Previous studies have considered only patients with neurofibromatosis type 2 (NF2) older than 12 years as candidates for an auditory brainstem implant (ABI). Our study expands the potential criteria to include both children and adult subjects with other cochlear or cochlear nerve malfunctions who either would not benefit at all from a cochlear implant (eg, cochlear nerve aplasia or avulsion) or whose benefit was or would be severely compromised (eg, cochlear ossification, cochlear fracture). STUDY DESIGN: In our department, over the period from April 1997 to September 2002, 29 patients, 20 adults and 9 children, were fitted with ABIs. Their ages ranged from 14 months to 70 years. Thirteen subjects had tumors, 10 NF2 and 3 solitary vestibular schwannoma, and 16 patients had a variety of nontumor (NT) cochlear or cochlear nerve diseases. A retrosigmoid-transmeatal approach was used in T and a retrosigmoid approach in NT patients. The electrode array was inserted into the lateral recess of the fourth ventricle and correct electrode positioning was monitored with the aid of electrically evoked auditory brainstem responses (EABRs). RESULTS: Correct implantation was achieved in all patients. No complications were observed due to implantation surgery or related to ABI activation or long-term use. Auditory sensations were induced in all patients with various numbers of electrodes (from 5 to 15). Different pitch sensations were identifiable with different electrode stimulation. Closed-set word recognition, open-set sentence recognition, and speech tracking scores achieved by the patients are reported in detail. The auditory performance of the patients showed significantly better outcomes than controls (Multicentric European clinical investigations on ABI with NF2). CONCLUSION: We have shown that the indications for the ABI can be extended to include NT patients with severe cochlear and/or cochlear nerve abnormalities. The degree of auditory benefit varies as a function of the underlying pathological conditions, with NT subjects exhibiting significantly better outcomes than the T patients.

Adolescent↗

PubMed1985

Uptake and release of gamma-aminobutyric acid in the guinea pig cochlear nucleus after axotomy of cochlear and centrifugal fibers.

This study attempts to determine if gamma-aminobutyric acid (GABA) may be a transmitter of cochlear nerve fibers projecting from the cochlea to the cochlear nucleus, and of centrifugal fibers projecting to the cochlear nucleus via the trapezoid body and the acoustic striae of the medulla. The uptake and the electrically evoked release of exogenous [14C]GABA were measured, in vitro, in the three major subdivisions of the guinea pig cochlear nucleus; the anteroventral, posteroventral, and dorsal cochlear nuclei. These activities were compared using unlesioned animals, animals with bilateral cochlear ablations, and animals whose trapezoid body and acoustic striae were interrupted on the right side of the medulla. Subdivisions from unlesioned animals took up [14C]GABA, achieving concentrations in the tissues that were 11-19 times that in the medium. Electrical stimulation evoked a Ca2+-dependent release of [14C]GABA from each subdivision. Bilateral cochlear ablation, which presumably destroyed the cochlear nerve fibers, had no effect on [14C]GABA uptake and release. Section of the trapezoid body and the acoustic striae on the right side of the medulla typically severed all known connections of the right posteroventral and dorsal cochlear nuclei with the rest of the brain, but left intact many connections involved with the right anteroventral cochlear nucleus. This lesion partially depressed [14C]GABA uptake and release in the right posteroventral and dorsal cochlear nuclei, but not in the right anteroventral cochlear nucleus. These findings suggest that one or more of the centrifugal tracts projecting to the cochlear nucleus may be GABAergic, 88% or more of the cochlear nerve fibers probably are not GABAergic, and some neurons of the cochlear nucleus are probably GABAergic.

Animals↗

PubMed2002

Auditory brainstem implantation: the University of Verona experience.

OBJECTIVE: We sought to describe the advantages of the retrosigmoid-transmeatal (RS-TM) approach in the application of auditory brainstem implants (ABIs) in adults with monolateral and bilateral vestibular schwannoma (VS) and in children with cochlear nerve aplasia. STUDY DESIGN: We conducted a retrospective case review. SETTING: The study was conducted at the ENT Department of the University of Verona, Italy. PATIENTS: Six adult patients (5 men and 1 woman) with neurofibromatosis type 2 (NF2) were operated on for VS removal with ABI. An additional patient had a unilateral VS in the only hearing ear. Tumor size ranged from 12 to 40 mm. In addition, 2 children received ABIs for bilateral cochlear nerve aplasia. INTERVENTION: An RS-TM approach was used in all VS patients, and an RS approach was used in the subjects with cochlear nerve aplasia. After tumor excision, landmarks (VII, VIII and IX cranial nerves, choroid plexus) for the foramen of Luschka were carefully identified. The choroid plexus was then partially removed and the tela choroidea divided and bent back; the floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleus became visible. In the 2 subjects with no cochlear nerve, the choroid plexus and VII and IX cranial nerves were used as landmarks. The electrode array was then inserted into the lateral recess and the correct position was monitored with the aid of electrically evoked auditory brainstem responses (EABR) and neural response telemetry (NRT). RESULTS: Correct implantation was possible in all patients. Auditory sensations were induced in all patients with various numbers of electrodes. Different pitch sensations could be identified with different electrode stimulation. CONCLUSIONS: We believe that the RS approach is the route of choice for patients who are candidates for ABI due to the easy and clear access to the cochlear nucleus area. This route avoids some of the drawbacks of the translabyrinthine approach, such as mastoidectomy, labyrinthectomy, sealing of the cavity and posterior fossa with abdominal fat, and contamination from the middle ear. For this reason, it is the route of choice in children with cochlear nerve aplasia or severe cochlear malformation and in adults with complete ossification of the cochlea or cochlear nerve disruption due to cranial trauma.

Acoustic Stimulation↗

PubMed1980

Pressure sensation in Meniere's disease.

This article reports a case of Meniere's disease accompanied by a persistent feeling of pressure in the ear. The patient had undergone a series of surgical procedures to alleviate his symptoms. The pressure sensation was relieved only by translabyrinthine cochlear nerve section. The relationships of the result of translabyrinthine cochlear nerve section with the mechanism and pathway of the pressure sensation are discussed. Cochlear nerve section is recommended at the same time as vestibular nerve section for patients with unilateral Meniere's disease with severe pressure sensation in the ear.

Adult↗

PubMed1980

Brainstem responses to electrical stimulation of cochlea.

We performed a brainstem evoked response (BSER) study to evaluate the extent to which electrical stimulation of the cochlea was conducted centrally by facial, vestibular, and cochlear nerves. Short-term experiments were performed in three monkeys: via a postauricular approach to the round window, a molded Silastic multielectrode prosthesis was placed in the scala tympani. The BSER was recorded to threshold and suprathreshold biphasic electrical pulses delivered to the implant electrodes. A middle cranial fossa dissection was then carried out, exposing the nerves of the internal auditory canal from above. Facial and vestibular neurotomy had no significant effect on BSER, while cochlear nerve section abolished the response. In one animal, blunt pressure on the cochlear nerve caused a reversible loss of electrically evoked BSER. Electrically evoked BSER probably depends on propagated impulses in the cochlear nerve.

Animals↗

PubMed1987

Long-term hearing preservation after acoustic neuroma surgery.

The past two decades have evidenced a remarkable improvement in the capacity of otologic surgeons to treat cerebellopontine angle tumors. Advances in instrumentation because of technologic changes, coupled with better surgical training, have contributed to a highly successful rate of tumor removal. The capability to extirpate the tumor now permits the otologic surgeon to attempt simultaneous preservation of the facial and cochlear nerve functions. Just as middle ear surgery improved to the point in the 1950s that reconstruction as well as disease eradication became possible, so can surgeons now consider preservation of function of the inner ear. It is now technically feasible--in some instances--to attempt to preserve hearing in tumors of the eighth nerve. Several studies have provided evidence that the cochlear nerve can be anatomically and functionally preserved when neuromas involve either the inferior or superior vestibular nerves. However, other studies assert that grossly intact nerves may contain microscopic islands of tumor cells intermingled with nerve fibers. If viable tumor cells were allowed to remain behind, one would expect them to grow; this would result in loss of hearing function and tumor recurrence, as evidenced by computed tomography (CT) or magnetic resonance imaging (MRI). This article will discuss the issues of cochlear nerve preservation and tumor excision, and review our experience over the past decade.

Cerebellar Neoplasms↗