[CENTRIFUGAL INHIBITION OF SINGLE AFFERENT NEURONS OF THE COCHLEAR NUCLEUS BY SOUND (CATS)].
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Cats↗
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Comparison of speech recognition with different speech coding strategies (SPEAK, CIS, and ACE) and their relationship to telemetric measures of compound action potentials in the nucleus CI 24M cochlear implant system.
Speech understanding and subjective preference for three different speech coding strategies (spectral peak coding [SPEAK], continuous interleaved sampling [CIS], and advanced combination encoders [ACE]) were investigated in 11 post-lingually deaf adult subjects, using the Nucleus CI 24M cochlear implant system. Subjects were randomly assigned to two groups in a balanced crossover study design. The first group was initially fitted with SPEAK and the second group with CIS. The remaining strategies were tested sequentially over 8 to 10 weeks with systematic variations of number of channels and rate of stimulation. Following a further interval of 3 months, during which subjects were allowed to listen with their preferred strategy, they were tested again with all three strategies. Compound action potentials (CAPs) were recorded using neural response telemetry. Input/output functions in relation to increasing stimulus levels and inter-stimulus intervals between masker and probe were established to assess the physiological status of the cochlear nerve. Objective results and subjective rating showed significant differences in favour of the ACE strategy. Ten of the 11 subjects preferred the ACE strategy at the end of the study. The estimate of the refractory period based on the inter-stimulus interval correlated significantly with the overall performance with all three strategies, but CAP measures could not be related to individual preference of strategy or differences in performance between strategies. Based on these results, the ACE strategy can be recommended as an initial choice specifically for the Nucleus CI 24M cochlear implant system. Nevertheless, access to the other strategies may help to increase performance in individual patients.
Acoustic Stimulation↗
Meningiomas of the cerebellopontine angle with extension into the internal auditory canal.
OBJECT: Only some meningiomas of the cerebellopontine angle (CPA) extend into the internal auditory canal (IAC) or arise from its dural lining. The authors investigated cases of CPA tumors in which the meningioma was inserted in the dura mater in or at the ICA or infiltrated a cranial nerve. METHODS: The authors reviewed patient charts including surgical and clinical records, intraoperative recordings of auditory evoked potentials, records of postoperative auditory examinations, and imaging studies. In a series of 421 patients harboring CPA meningiomas, 72 patients in whom there was dural involvement of the IAC were investigated. Total tumor resection was achieved in 86.1%. In 34 patients, opening of the IAC was required for total tumor removal; this procedure did not influence the patient functional outcome. Among patients with secondary involvement of the IAC, anatomical preservation of the facial and cochlear nerves was obtained in 94%, whereas among patients in whom the lesion arose from the dura in or at the IAC these values were 80 and 75%, respectively. Functional preservation of the seventh and eighth cranial nerves in cases of tumor extension within the IAC was 86 and 77%, respectively, whereas in cases in which the IAC was involved it was only 60%. In four of five patients in whom the tumor had its origin in the dura mater within the IAC, the seventh or eighth cranial nerve had to be sacrificed to achieve tumor removal because of the lesion's infiltrative behavior. Facial nerve reconstruction by sural grafting was performed in the same operative procedure. CONCLUSIONS: Meningiomas of the CPA involving the IAC require special surgical management. Dural involvement of the IAC requires opening by using a diamond drill, a procedure that does not influence cranial nerve outcome. The increased rate of cranial nerve morbidity is attributed to the infiltrative behavior of these meningiomas. If affected nerve segments have to be sacrificed, immediate reconstruction enables satisfactory long-term results.
Adult↗
Protein kinase C regulates [3H]D-aspartate release in auditory brain stem nuclei.
We previously found that unilateral cochlear ablation altered transmitter release from glutamatergic synaptic endings in several brain stem auditory nuclei. To determine if this release activity could be regulated by protein kinase C (PKC), which has been associated with regulation of transmitter release, the electrically evoked release of [3H]d-aspartate ([3H]d-Asp) was quantified in vitro as an index of exocytosis from glutamatergic presynaptic endings in the major subdivisions of the cochlear nucleus (CN) and in the main nuclei of the superior olivary complex (SOC). Treating dissected tissues with a PKC activator, such as phorbol 12,13-diacetate (PDA) or phorbol 12,13-dibutyrate (PDBu) (3 microM), elevated the evoked release of [3H]d-Asp by 1.5- to 3.3-fold. The PKC inhibitor Ro31-8220 (50 nM) did not alter the evoked release but blocked the stimulatory effects of PDA and PDBu. These findings suggested that PKC could positively regulate transmitter release from glutamatergic presynaptic endings in brain stem auditory pathways. Seven days after unilateral cochlear ablation, when cochlear nerve endings had degenerated in the ipsilateral CN, PDBu elevated the evoked release bilaterally in each CN subdivision and SOC nucleus, implying that PKC could regulate glutamatergic release in the noncochlear pathways remaining in the ipsilateral CN and in the other pathways after unilateral hearing loss. After 145 postlesion days, Ro31-8220 blocked endogenous elevations in the evoked release in the ipsilateral SOC but did not alter the elevated or upregulated release in the other tissues. This suggested that the elevations of glutamatergic release activity in the ipsilateral SOC that appeared after unilateral cochlear ablation depended on endogenous activation of PKC.
Animals↗
Cochlear implantation in adults with prelingual deafness. Part II. Underlying constraints that affect audiological outcomes.
OBJECTIVES/HYPOTHESIS: To discuss the underlying physiological and anatomical constraints on audiological performance of late-implanted prelingually deafened adult cochlear implant patients. STUDY DESIGN: Retrospective review. METHODS: Published literature on the topic of auditory pathway responses to prolonged congenital deafness was reviewed. In particular, the authors sought to identify the anatomical and physiological changes that take place in both the peripheral and central auditory pathways in response to prolonged deafness, as well as how they are altered by chronic electrical stimulation. RESULTS: The currently available evidence suggests that the colonization of the auditory cortex by other sensory modalities is the main limiting factor in postimplantation performance, not the pathological degenerative changes of the auditory nerve, cochlear nucleus, or auditory midbrain. CONCLUSION: The reviewed evidence, although circumstantial, suggests that emphasizing aurally based educational programs before (with hearing aids) and after cochlear implantation could reduce the cortical colonization phenomenon and potentially improve postimplantation audiological performance of patients with long-term prelingual deafness.
Adult↗
Plasticity of response properties of inferior colliculus neurons following acute cochlear damage.
1. The discharge patterns of 40 neurons in the central nucleus of the inferior colliculus (ICC) of the chinchilla were evaluated before and after acute cochlea trauma from intense tone exposure. Single-unit recordings were obtained from neurons in the ICC contralateral to the sound-stimulated ear. Cochlear trauma was induced with a short-duration (15-25 min), high-intensity pure tone (95-115 dB SPL) at a frequency above the neuron's characteristic frequency (CF). The aim of the traumatizing exposure was to damage the peripheral sensory receptors associated with frequencies above the neuron's CF. The damage was expected to attenuate inputs to the neural circuits responsible for activating the inhibitory sidebands above CF. 2. Three types of frequency-threshold tuning curves were observed before the exposure: 1) open V tuning curves (55%) that became wider as sound intensity increased; 2) level-tolerant tuning curves (37.5%) that maintained a very narrow bandwidth even at high sound intensities; and 3) upper-threshold tuning curves (7.5%) in which excitatory responses were elicited at low and moderate intensities, but not at high intensities. The traumatizing exposure caused a dramatic widening of level-tolerant (80% of sample) and upper-threshold tuning curves (100% of sample) at high sound intensities but caused almost no change in the low-threshold tip of the tuning curves. By contrast, tuning curves with an open V configuration were generally unaffected (approximately 90% of sample) by the traumatizing exposure. 3. Discharge rate-level functions in the ICC were of two general types: 1) strongly nonmonotonic (60%) or 2) saturating, monotonic (40%). The traumatizing exposure caused a significant increase in the suprathreshold discharge rates in 70% of all neurons studied. Among the neurons with strongly nonmonotonic discharge rate-level functions, 93% showed a significant increase in discharge rate. 4. The poststimulus time histograms (PSTH) to tone bursts were of three main types: 1) onset, 2) pauser, and 3) sustained responders. The traumatizing exposure had almost no effect on the PSTH of onset or sustained responders. However, pause PSTH frequently (75%) showed a significant decrease in the pause duration and an increase in the sustained discharge rate following the pause after the exposure. 5. The results suggest that the response properties of neurons with extremely narrow tuning curves and nonmonotonic discharge rate-level functions are shaped by an inhibitory circuit that is activated by frequencies above the high-frequency flank of the tuning curve. This inhibitory circuit modifies the excitatory response in the following ways: 1) it narrows the excitatory response area at suprathreshold intensities particularly at frequencies below CF, 2) it alters the shape of the discharge rate-level function by suppressing the discharge rates at suprathreshold intensities, and 3) it modifies the temporal discharge pattern of the pause PSTH by suppressing neural activity that occurs after the onset response. The effects of these inhibitory inputs can be reduced (disinhibition) by damaging the sensory cells in the inner ear that activate this circuit. The exact locus of the inhibitory circuit(s) is unknown but may involve inhibitory inputs located at the level of the cochlear nucleus and/or at levels up to the inferior colliculus.
Animals↗
Preservation of vestibular, cochlear, and facial nerves during microsurgical removal of acoustic tumors. Report of two cases.
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Cochlear Nerve↗
Profile of electrodiagnostic abnormalities in cats with GM1 gangliosidosis.
OBJECTIVE: To determine which electrodiagnostic tests yield abnormal findings in cats with GM1 gangliosidosis, and to determine the approximate age of onset of electrodiagnostic abnormalities. ANIMALS: Cats (28 to 335 days old) affected with GM1 gangliosidosis (n = 11) and unaffected controls (n = 14). PROCEDURE: Cats were grouped by age: group 1, < or = 90 days, group 2, 91 to 200 days; and group 3, > 200 days. Electrodiagnostic tests were conducted, including needle electromyography, motor and sensory nerve conduction velocity, spinal evoked potentials, and brainstem auditory evoked potentials. Results for control and affected cats were compared, using the general linear model for ANOVA and Scheffé's test for multiple comparisons. RESULTS: Needle electromyography did not reveal abnormal spontaneous activity in skeletal muscles of any cat; furthermore, statistical analysis did not indicate significant difference between affected and control groups for nerve conduction velocity, confirming that degeneration of peripheral nerve fibers is not a feature of this disease. However, spinal evoked potentials were abnormal in group-3 cats; conduction velocity within sensory pathways in the cranial part of the spinal cord was significantly slower in GM1-affected cats (P = 0.0002). Brainstem auditory evoked responses also were abnormal: wave V (generated in the region of the pons) had prolonged latency in cats of groups 2 and 3 (P = 0.0003 and 0.0001, respectively, at 90 decibels sound pressure level). In the oldest cats, latencies for earlier waves within the auditory pathway also were prolonged; wave I (generated by the cochlear nerve) was prolonged in group-3 cats (P = 0.0423). CONCLUSIONS: Motor and sensory nerve conduction velocities remained within normal limits in GM1-affected cats. However, spinal evoked potentials indicated slowing in conduction velocity along the cranial part of the spinal cord in group 3 cats. Brainstem auditory evoked responses indicated prolonged latencies in cats of groups 2 and 3.
Animals↗
[Cerebellopontine angle surgery. Part 1: General remarks].
OBJECTIVES: Cerebellopontine angle tumors, although uniform in location, are diverse with regard to the site of tumor origin and displacement of the neurovascular structures. With the increasing use of magnetic resonance imaging in the diagnosis of brain lesions, the treatment of tumors of the cerebellopontine angle has changed extraordinarily. An appreciation of the vascular and cranial nerve microanatomy and the intimate relationship between neurovascular structures and the tumor is essential for achieving optimal surgical results METHODS: A review was made of the current literature. RESULTS AND CONCLUSIONS: The different treatment options are compared with microsurgical tumor resection with respect to neurological outcome and long-term follow-up. As microsurgical removal is still the treatment of choice for such tumors, the indication for surgical treatment is analyzed and the different surgical approaches to the cerebellopontine angle are described in terms of their frequency of involvement in the surrounding neural and vascular structures with varying tumor size. The etiology of facial and cochlear nerve injury, as well as complications involving other cranial nerves, are discussed with emphasis on pathophysiological and anatomical considerations. In conclusion, the great variation in the anatomical location and the involvement of neurovascular structures in the cerebellopontine angle is demonstrated.
Cerebellopontine Angle↗
Long-term effects of cochlear implants in children.
OBJECTIVE: Since 1987, when the use of multichannel cochlear implants was initiated in children, candidacy has expanded; many thousands of children have received these devices, and results have revealed a wide range of performance. However, few long-term studies exist on a large population of these children. There have been concerns expressed that cochlear implant function might degrade over time, that devices and electrodes might migrate and extrude in the growing child, or that there might even be a deleterious effect of long-term stimulation of the cochlear nerve. The purpose of this study was to explore the long-term effects of implantation as a function of performance over time, reimplantation, and educational factors. STUDY DESIGN AND SETTING: We studied 81 children who received implants at a major academic medical center and were followed for 5 to 13 years. RESULTS: Results revealed significant gains in speech perception, use of oral language, and ability to function in a mainstream environment. There was no decrease in performance over time and no significant incidence of device or electrode migration or extrusion, and device failure did not cause a deterioration in long-term outcome. CONCLUSIONS: Multichannel cochlear implants in children provide perception, linguistic, and educational advantages, which are not adversely affected by long-term electrical stimulation.
Adolescent↗
The influence of potentially limiting factors on paediatric outcomes following cochlear implantation.
OBJECTIVE: Children with isolated hearing impairment who have received cochlear implant at the optimal age mostly achieve remarkable results that are assessed by objective speech perception and production measurements. Different outcomes may be expected in case of conditions which may have a negative impact on postoperative performance. The aim of this study was to assess the influence of potentially limiting factors on postimplant outcomes. METHODS: Four groups of examinees (11 in total) were involved by the study: (A) four children with additional disabilities, (B) three children with cochlear malformation and/or ossification, (C) three reimplanted children, (D) a child with retrocochlear (cochlear nerve) pathology. Hearing, speech perception and production were examined by pure tone audiogram, speech audiogram, categories of auditory performance, speech intelligibility rating, listening progress profile, and monosyllabic trochee polysyllable test. Postoperative positive life changes were assessed by a questionnaire. RESULTS: Group A: perception skills better than expected, less satisfactory speech development. Group B: good sound perception, poor understanding. Group C: after reimplantation undisturbed conditions for continuation of optimal rehabilitation course. Group D: unsatisfactory results of pure tone hearing as well as speech perception and production. CONCLUSION: In spite of unfavourable conditions all examinees, except a child with retrocochlear pathology, were found to have a considerable benefit after cochlear implantation (with regard to obvious heterogeneity within each group). Evaluation of success, especially in children with multiple handicaps, also has to include subjective indicators of positive life changes, even those not directly associated with hearing.
Audiometry, Pure-Tone↗
A modified retrosigmoid approach for direct exposure of the fundus of the internal auditory canal for hearing preservation in acoustic neuroma surgery.
OBJECTIVE: This is a clinical report on a modified retrosigmoid approach with direct exposure of the fundus of the internal auditory canal for hearing preservation in acoustic neuroma surgery. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral center of an ear, nose, and throat department in a public hospital. PATIENTS: One hundred fifty consecutive procedures were reviewed, including 61 males and 89 females with an age range of 13 to 69 years and a mean age of 47 years. There were 15 patients with tumor occupying solely the internal auditory canal and 135 patients with extension into the cerebellopontine angle with an extrameatal diameter of up to 52 mm and a mean of 11.5 mm. INTERVENTION: The retrosigmoid approach included a wide craniotomy, a perimeatal petrous bone removal up to the blue line of the labyrinth, and a direct exposure of the fundus at the orifices of the facial and cochlear nerves. The quadrant of the superior vestibular nerve remained unexposed. MAIN OUTCOME MEASURES: Hearing was measured according to the American Academy of Otolaryngology-Head and Neck Surgery criteria for reporting results of hearing preservation and by comparison with the preoperative level. Facial nerve function was measured using the House-Brackmann grading. The radicality of tumor removal was investigated with mid- to long-term magnetic resonance imaging (MRI). RESULTS: Measurable hearing was preserved in 45.3%, and in 32.4% of these cases, it was within 15 dB/15% discrimination. Grade 1 or 2 facial function was preserved in 85.3%. MRI follow-up revealed a 3.3% tumor residual or regrowth in the complete series. No residual tumor was found at the 3-year MRI in the last series of patients operated on with direct control of the fundus. CONCLUSIONS: This modified retrosigmoid approach permits the direct exposure of the facial and cochlear quadrants of the fundus. This allows tumor dissection under direct visual control. Removing the tumor from the vestibular quadrant of the fundus is done blindly in a minority of cases and carries a minimal risk of residual tumor. This technique requires only conventional equipment and skills of neurotology.
Adolescent↗
Differential expression of cytoskeletal genes in the cochlear nucleus.
The relationship between structure and function is clearly illustrated by emerging evidence demonstrating the role of the neuronal cytoskeleton in physiological processes. For example, alterations in axonal caliber, a feature of the cytoskeleton, have been shown to affect reflex arc latencies and are prominent features of several neuropathological disorders. Even in the nonpathologic situation, however, axonal diameter may be a crucial element for the normal function of specialized auditory neurons. To investigate this relationship, we used serial analysis of gene expression and microarray analyses to characterize the expression of cytoskeletal genes in the central auditory system. These data, confirmed by real-time RT-PCR, identified differential expression of intermediate neurofilament transcripts (i.e., Nefh, Nef3, and Nfl) among the subdivisions of the cochlear nucleus. In situ hybridization was used to identify specific classes of neurons within the cochlear nucleus expressing these neurofilament genes. Robust neurofilament expression was seen in bushy cells and cochlear nerve root neurons, suggesting an association between cytoskeletal structure and rapid conduction velocities. Gene expression data were also identified for other classes of cytoskeletal and structural genes important in neuronal function. These results may help to explain some causes of hearing loss in hereditary neuropathies and provide an anatomic basis for understanding normal neuronal function in the central auditory system.
Animals↗
Reconnecting neuronal networks in the auditory brainstem following unilateral deafening.
When we disturbed the auditory input of the adult rat by cochleotomy or noise trauma on one side, several substantial anatomical, cellular, and molecular changes took place in the auditory brainstem. We found that: (1) cochleotomy or severe noise trauma both lead to a considerable increase of immunoreactivity of the growth-associated protein GAP-43 in the ventral cochlear nucleus (VCN) of the affected side; (2) the expression of GAP-43 in VCN is restricted to presynaptic endings and short fiber segments; (3) axon collaterals of the cholinergic medial olivocochlear (MOC) neurons are the path along which GAP-43 reaches VCN; (4) partial cochlear lesions induce the emergence of GAP-43 positive presynaptic endings only in regions tonotopically corresponding to the extent of the lesion; (5) judging from the presence of immature fibers and growth cones in VCN on the deafened side, at least part of the GAP-43 positive presynaptic endings appear to be newly formed neuronal contacts following axonal sprouting while others may be modified pre-existing contacts; and (6) GAP-43 positive synapses are formed only on specific postsynaptic profiles, i.e., glutamatergic, glycinergic and calretinin containing cell bodies, but not GABAergic cell bodies. We conclude that unilateral deafening, be it partial or total, induces complex patterns of reconnecting neurons in the adult auditory brainstem, and we evaluate the possibility that the deafness-induced chain of events is optimized to remedy the loss of a bilaterally balanced activity in the auditory brainstem.
Animals↗
Chronic cluster headache managed by nervus intermedius section.
Cluster headache sufferers who become candidates for surgical treatment are those relatively rare patients who are refractory to all attempts at pharmacological relief. Ablative surgical procedures have been directed against either the trigeminal nerve or the nervus intermedius/greater superficial petrosal (NI/GSP) pathway. Both carry nociceptive impulses from the head and face, and the NI also carries parasympathetic fibres which appear to be responsible for the autonomic concomitants of cluster headache. Trigeminal operative procedures are not consistently helpful in chronic cluster headache, while NI section has been shown to give potentially long lasting relief but carries the potential risks of cerebellopontine angle surgery. In eight selected cases of chronic cluster headache we have demonstrated a high early success rate for pain relief, with few complications, in the performance of NI section, combined, when indicated, with microvascular decompression of the trigeminal main sensory root. We believe that cochlear nerve monitoring helps prevent postoperative hearing impairment. An intimate relationship between the NI and arterial loops of the anterior inferior cerebellar artery (AICA) or the internal auditory artery has been frequently observed in our chronic cluster headache patients.
Adult↗
[Epidermoid cysts of the cerebellopontile angle. 6 cases].
6 cases of cerebello-pontine angle (CPA) epidermoid cysts were encountered from 1970 up to 1983. This series accounts for 3,3% of 177 CPA tumours operated on during the same period. Epidermoid cysts result from heterotopia of ectoblastic tissue and CPA is their most frequent site of development. The wall of the cyst is composed of a stratified squamous epithelium. The inside of the cyst is filled with soft waxy material resulting from the desquamation of keratin of the cyst wall. Macroscopically, the cyst is extensive and usually closely attached to the brain stem, cranial nerves and vessels. The commonest clinical picture is that of a trigeminal neuralgia usually typical of tic douloureux. Plain x-ray films of the skull disclose no abnormalities. Air encephalography was performed in an early case: it showed an obliteration of the cerebello pontine cistern by the cyst mass, with no air injection of the tumor as mentioned elsewhere. Computerized tomography made other procedures obsolete in our 4 latest cases: it showed a non-enhancing low-density mass filling the CPA. Hypodensity characteristics can make differentiation from dermoïd and arachnoid cysts. In a patient, auditory evoked response were lengthened bilaterally and suggested a brain stem compression rather than a cochlear nerve involvement. Operative treatment was performed in all of our 6 patients through a suboccipital craniotomy with the aid of the operative microscope. The aim of surgery should be, whenever possible, total removal of the capsule. A partial section of the fifth nerve is not needed. No patient died postoperatively.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Protective effect of the cochlear efferent system during noise exposure.
The aim of the present study was to confirm the hypothesis that the cochlear efferent system is involved in the mechanisms underlying the "toughening" effect at high frequencies. The toughening effect is defined as a progressive threshold shift reduction when repeated exposures to the same noise are applied. Vestibular neurectomy was performed through a posterior cranial fossa approach in six healthy pigmented guinea pigs, and it assured the interruption of both crossed and uncrossed olivocochlear bundles to one ear only, before their entrance in the internal auditory meatus. The animals were then implanted with permanent electrodes for the electrocochleographic findings. Ten days after the operation the animals were exposed to octave-band toughening noise, centered at 4 kHz, at 85-dB SPL, for 10 consecutive days, 6 hours on/18 hours off. The hearing threshold was registered before and at the end of each exposure session. The behavior of the hearing threshold in the operated ears was then compared to that of the controlateral, nonoperated ears. Complete recovery from TS in the control ear began after four days of exposure, whereas in the operated ear hearing loss increased to day 7 (55 dB), with only a partial reduction (45 dB) beyond ten days of exposure. The results of the present study clearly demonstrated that sectioning of the OCB in guinea pigs causes persistent hearing loss during noise-exposure conditioning, in comparison to the contralateral, nonoperated ear. Thus, one can assume that the lack of decrease of TS during intermittent noise exposure could be due to the loss of the protective effect of the efferent fibers, perhaps mediated by the lateral OC neurons that synapse beneath the IHCs.
Animals↗
[Narrow-band action potentials of the guinea-pig cochlea as compared with the ordinary electrocochleograms under normal and pathological conditions].
The narrow-band action potentials (NAP) and the ordinary electrocochleograms were recorded from the guinea-pig cochlea under normal and pathological conditions in order to study whether the NAP could be a useful measure to detect cochlear dysfunctions. The cochlea damaged either by the administration of kanamycin or by a mechanical lesion of the round window served as pathological materials. Recordings showed that the threshold and amplitude measured for the N1 potential of NAPs ran in parallel with those of the cochlear microphonic potentials (CM), under both normal and pathological conditions of the cochlea. This implies that the CM could be replaced by the NAP when difficulties were present in recording CMs. It may be inferred that the NAP reflects responses of the inner hair-cells and cochlear nerves, while the CM would mainly be derived from responses of the outer hair-cells to the frequency-specific movements of the basilar membrane. If so, the NAP should offer a good means for the objective audiometry. Recording NAPs is also superior to the ordinary electrocochleography in that the method makes it possible to obtain responses generated near the apex of the cochlea, i.e., responses to low-pitch sound stimuli.
Action Potentials↗