A model of the electrically excited human cochlear neuron. II. Influence of the three-dimensional cochlear structure on neural excitability

A simplified spiraled model of the human cochlea is developed from a cross sectional photograph. The potential distribution within this model cochlea is calculated with the finite element technique for an active scala tympani implant. The method in the companion article [Rattay et al., 2001] allows for simulation of the excitation process of selected elements of the cochlear nerve. The bony boundary has an insulating influence along every nerve fiber which shifts the stimulation condition from that of a homogeneous extracellular medium towards constant field stimulation: for a target neuron which is stimulated by a ring electrode positioned just below the peripheral end of the fiber the extracellular voltage profile is rather linear. About half of the cochlear neurons of a completely innervated cochlea are excited with monopolar stimulation at three-fold threshold intensity, whereas bipolar and especially quadrupolar stimulation focuses the excited region even for stronger stimuli. In contrast to single fiber experiments with cats, the long peripheral processes in human cochlear neurons cause first excitation in the periphery and, consequently, neurons with lost dendrite need higher stimuli.     

Computer-aided three-dimensional reconstruction of guinea pig cochlear aqueduct.

A computer-aided method of three-dimensional reconstruction was applied to the determination of the overall spatial configuration of the guinea pig cochlear aqueduct. The rotation function of the reconstructed images was useful in showing the individual small parts of the duct. A semi-translucent display of the segmental reconstruction of the duct demonstrated a difference in the density of the cellular components between the opening to the perilymphatic space and the duct portion. We propose that the cochlear aqueduct serves as a protective mechanism against a sudden change in CSF pressure in the subarachnoid space.     

耳蜗组织连续切片显微结构的三维重建研究

目的:探讨生物组织连续切片显微结构的三维重建使用方法.方法:应用火棉胶切片技术获得耳蜗组织连续切片,显微镜下放大40倍后对耳蜗组织摄像.使用Adobe photoshop 6.0图像处理软件和Amira3.0三维重建软件对内耳结构进行图像拼接、对位和三维重建.结果:三维重建立体真实再现了耳蜗组织的解剖结构,重建效果表明该研究显微结构的数据输入、图像拼接、内定位技术和三维重建方法是可行的.结论:采用内定位技术,借助重建软件,可以在个人计算机上实现生物组织连续切片显微结构的立体重建,是一种实用的重建方法.     

Surface topography of the cochlear nuclei in humans: two- and three-dimensional analysis

We used three-dimensional reconstruction to study the cochlear nuclear complex (CN) in postmortem adult brains. Resulting data show that the largest part of the CN surface, particularly the dorsal cochlear nucleus (DCN), is fully within the lateral recess of the fourth ventricle. The surface of another subdivision, the ventral cochlear nucleus (VCN), is also almost entirely within the recess, except for a narrow zone adjacent to the caudoventral border of the nucleus. The caudal portion of the exposed zone of the VCN is in the vicinity of the rootlets of the glossopharyngeal (IX) nerve, and the ventral portion is close to the terminal part of the vestibulocochlear (VIII) nerve. The border between the intraventricular part of the CN and the extraventricular portion and also the terminal part of the VIII nerve approximately coincides with the line of attachment of the inferior medullary velum of the fourth ventricle (tenia of the choroid plexus). In the narrow strip of this ventral most part of the tenia we did not observe big blood vessels or neurons. Accordingly this could be a reasonably safe surgical route to the intraventricular surface of the CN.     

Computer-aided three-dimensional reconstruction of the cochlear nerve root

Computer-aided three-dimensional reconstruction techniques have been used for reconstruction of the cochlear nuclei and the cochlear nerve root. The cochlear nuclei, the nerve root, and the other adjacent structures were studied in serial frontal and horizontal sections. Boundaries of these structures were digitized into a computer and eight three-dimensional models were produced. Two solid models (reconstructions) showed the spatial surface topography, while the others revealed the appearance of the nerve root within the nuclei. The reconstructions also showed the position of the tenia of the tela choroidea on the surface of the nuclei and its relation to the root. This knowledge makes it possible to use the tenia as a landmark for determination of the root position within the nuclei. As the nerve enters the nucleus and forms the root, there is an appreciable reduction in its thickness. At the same time, the distance between the nuclei surface and the root, which has a cone shape, increases. The cone is curved along its longitudinal axis toward the inferior cerebellar peduncle. The top of the root borders the dorsal cochlear nucleus. This study stems from work in this institution on placement of the central electroauditory prosthesis on the cochlear nuclei surface. The results obtained further knowledge of the anatomy of the nuclei, specifically the areas used for the prosthesis implantation and the underlying tissue.     

Computer-aided three-dimensional reconstruction and measurement for multiple-electrode cochlear implant

To assist surgeons in performing multiple-electrode cochlear implant operations, the spatial relations between middle and inner ear structures in six normal temporal bones were studied using the computer-aided 3-D reconstruction method. When viewed through the external auditory canal, the approach used for electrode insertion, and the relation of the malleus to the helicotrema or the scala vestibuli in the second turn of the cochlea were found to vary considerably. In contrast, when viewed from the same transmeatal approach, both the direction and the distance from the stapes footplate to the scala vestibuli of the second turn of the cochlea and helicotrema were found to be quite stable. We concluded that the stapes footplate was a better landmark than the malleus when approaching the helicotrema and scala vestibuli of the second turn of the cochlea during multiple-electrode cochlear implant surgery.     

Morphometric analysis of the cochlear nerve in man.

Fiber diameters were analyzed in the meatal segment of the cochlear nerve from 7 temporal bones obtained from 7 patients. Two patients had normal hearing for their age. Two had sustained noise exposure and one had presbyacusis of predominantly neural type. The cochleae displayed characteristic degeneration patterns. The other two manifested hearing loss of unspecified type. The fiber diameters ranged from 0.5 to 11 microns. The diameter distribution was unimodal in all seven nerves. The means of the diameters ranged from 4.2 to 5.5 microns. They were significantly different between patients with age-related normal hearing on the one hand and patients with noise induced hearing loss and neural presbyacusis on the other. The findings are discussed in relation to changes in nerve conduction speed and hearing loss; a possible correlation between the fiber diameter distribution and the tonotopical arrangement of the cochlea is suggested.     

The fine structure of the cochlear plexus

Summary Ultrathin sections and freeze-fracture replicas of the guinea pig cochlear plexus were studied under the electron microscope. Generally, the capillaries possess a continuous endothelial cell layer. The endothelial cells are connected by tight junctions. Occasionally, fenestrated capillaries can be found. Nonmyelinated nerves are intimately related to smooth muscle cells of arterioles as well as pericytes and endothelial cells of capillaries. The axonal varicosities contain clear synaptic-type vesicles. The cochlear plexus cells are connected by desmosomes, gap junctions, and tight junctions. The latter are thought to be part of the blood-perilymph-barrier in this region. There is evidence that the cochlear plexus derives from the meninges.Supported by grants of the Deutsche Forschungsgemeinschaft, Ja 205/6     

The cost and analysis of nonuse of cochlear implants.

OBJECTIVE: Analysis of the cost implications and reasons for nonuse of cochlear implants in an established cochlear implant unit. STUDY DESIGN: Clinical data were analyzed retrospectively to construct a table of cochlear implant use over time to identify nonuse and to suggest the reasons for this. SETTING: Yorkshire Cochlear Implant Service is a tertiary referral center. PATIENTS: Three hundred forty consecutively implanted patients from 1990 to 2005. MAIN OUTCOME MEASURES: Life table analysis showed that most children used their implant (p = 0.7 during 11 yr). However, 11 of 155 children and 2 of 185 adults became nonusers during the period of study. The 11 children stopped because of age at implant, educational placement, and family support. Two adults stopped because of psychological issues and inability to adapt to the signal. Surgical and implant costs have initial impact, with subsequent years' costs reflecting programming issues and maintenance. When considering nonuse, there are 2 effects: first, no more costs are incurred, and second, no more years of use are accumulated. Thus, nonuse reduces both costs and years. Costs of gaining a year of use as a function of time showed that there was little financial impact from the 11 children nonusers. As a ratio of "no nonuse" and observed "nonuse" in children, the ratio is 1.07 by 13 years of implantation (7%). The adult group was too few to analyze. CONCLUSION: The nonuse added 7% to the average cost. Retrospective audit identifies that patient selection by a multidisciplinary team is crucial to reducing nonuse.     

On the structure of phoneme categories in listeners with cochlear implants.

PURPOSE: To describe cochlear implant users' phoneme labeling, discrimination, and prototypes for a vowel and a sibilant contrast, and to assess the effects of 1 year's experience with prosthetic hearing. METHOD: Based on naturally produced clear examples of "boot," "beet," "said," and "shed" by 1 male and 1 female speaker, continua with 13 stimuli were synthesized for each contrast. Seven hearing controls labeled those stimuli and assigned them goodness ratings, as did 7 implant users at 1-month postimplant. One year later, these measures were repeated, and within category discrimination, d', was assessed. RESULTS: Compared with controls, implant users' vowel and sibilant labeling slopes were substantially shallower but improved over 1 year of prosthesis use. Their sensitivity to phonetic differences within phoneme categories was about half that of controls. The slopes of their goodness rating functions were shallower and did not improve. Their prototypes for the sibilant contrast (but not the vowels) were closer to one another and did not improve by moving apart. CONCLUSIONS: Implant users' phoneme labeling and within-category perceptual structure were anomalous at 1-month postimplant. After 1 year of prosthesis use, phoneme labeling categories had sharpened but within category discrimination was well below that of hearing controls.     

A three-dimensional nonlinear active cochlear model analyzed by the WKB-numeric method

A physiologically based nonlinear active cochlear model is presented. The model includes the three-dimensional viscous fluid effects, an orthotropic cochlear partition with dimensional and material property variation along its length, and a nonlinear active feed-forward mechanism of the organ of Corti. A hybrid asymptotic and numerical method combined with Fourier series expansions is used to provide a fast and efficient iterative procedure for modeling and simulation of the nonlinear responses in the active cochlea. The simulation results for the chinchilla cochlea compare very well with experimental measurements, capturing several nonlinear features observed in basilar membrane responses. These include compression of response with stimulus level, two-tone suppressions, and generation of harmonic distortion and distortion products.     

Fine structure of cochlear innervation in the cat

Examination of adult and juvenile cat cochleas by electron microscopy and semi-serial sections permitted identification of the cytological features characteristic of the afferent and efferent nerve fiber populations identified in Golgi impregnations of the cochlea. This study demonstrated the distribution of synaptic contacts made by these fiber populations. As in the Golgi findings, radial and outer spiral afferent fibers were identified in well separated zones of the inner spiral bundle. The trunks of the outer spiral fibers, containing many microtubules and few neurofilaments, at first coursed spirally below the inner hair cells on the proximal face of the inner pillar, turned abruptly between adjacent pillar cells and entered the tunnel without branching. Radial afferents, containing many neurofilaments and a few microtubules, coursed through the inner spiral bundle, maintaining a radial or oblique orientation and proceeded directly toward the inner hair cells. Efferent fibers in the region of the inner spiral bundle were distinguishable by size, by orientation, and, to a lesser extent, by cytology. Small (1 micron) efferent fibers, containing few neurofilaments, an occasional microtubule, and mitochondria, occurred in the inner and tunnel spiral bundles and formed large varicosities, which contacted radial afferents. A separate population of much thicker efferents, containing many neurofilaments, mitochondria and dense-cored vesicles, but no microtubules, did not enter the inner spiral bundle but coursed directly to the level of the tunnel spiral bundle on the proximal face of the inner pillar cells. These fibers crossed the tunnel at the level of the tunnel spiral bundle and, upon reaching the outer hair cells, formed large synaptic contacts on outer hair cells and on outer spiral fibers as well. Some of these efferent fibers also synapse on afferent fibers while crossing the tunnel. The findings agree with previous observations with the Golgi method showing that entirely separate populations of spiral ganglion cells innervate the inner and outer hair cells. Likewise, there are efferent fibers innervating only inner or outer hair cells, but the probability of efferent fibers to both inner and outer hair cells cannot be ruled out.     

Rapid disruption of cochlear function and structure by trimethyltin in the guinea pig.

Trimethyltin (TMT) is a potent ototoxicant which acutely disrupts generation of the action potential evoked by a broad range of tone frequencies and subsequently produces selective high frequency impairment and outer hair cell (OHC) damage in the extreme basal turn of the cochlea. We investigated the development of TMT ototoxicity in the guinea pig 6-48 h following treatment using the compound action potential (CAP), cochlear microphonic (CM), endocochlear potential (EP) and light and electron microscopic examinations. At all time intervals studied, TMT reduced CAP sensitivity and CM amplitude. The effect was relatively broad across test frequencies at 6 h and subsequently became restricted to higher frequencies. No disruption of the EP was observed between 6 and 24 h following TMT. OHC pathology in the basal turn of the cochlea 12 h following TMT consisted of vacuolization in the supranuclear region and disruption of the cuticular plate; some mitochondria exhibited dark inclusions. Type 1 spiral ganglion cells appeared swollen at 24 h with separation of myelin from the cell bodies. No pathological changes were observed in the inner hair cells (IHC). The present data identify the OHC as targets responsible for the loss of CM sensitivity after TMT as the EP was unaffected. These data suggest that CAP and CM recovery at low and middle frequencies following acute TMT administration is accompanied by recovery of neurotransmission at the IHC or Type 1 SGC level and OHC recovery at apical regions of the cochlea.     

Toward a method for programming balanced bilateral cochlear implant stimulation levels in children

Abstract

Objectives

In the present study, we investigated differences in stimulation levels between bilateral cochlear implants (CIs) in children and compared these to previously recorded measures of balanced input. We hypothesized that differences in stimulation levels between two CI: (1) will be observed both in behavioral and objective measures, and (2) can help to provide bilateral stimulation levels that are perceived as balanced.

Method

Participants were 19 children who received bilateral CIs sequentially. Behaviorally determined minimum and maximum stimulation levels from the first (CI-1) and second (CI-2) CIs at an apical electrode (#20) were compared with thresholds of the electrically evoked compound action potential (ECAP) of the auditory nerve and electrically evoked stapedius reflex (ESR).

Results

All four measures were found to be greater when evoked from CI-1 than CI-2 and the dynamic range between behavioral measures was reduced in CI-2 relative to CI-1. Significant correlations were found between differences in behavioral and objective measures; as differences in T-levels between CI-1 and CI-2 increased, so did the differences in ECAP thresholds and a similarly positive relationship was found between C-level differences and ESR threshold differences. Differences in ECAP thresholds between CI-1 and CI-2 were significantly correlated with balanced bilateral levels.

Discussion

Children using two different Nucleus CIs often require different stimulation levels on either side. Although lower levels were needed on the side more recently implanted with newer technology, the dynamic range of input provided on the second side was reduced relative to the more experienced side. Differences in behavioral measures between CI-1 and CI-2 can be partially predicted by objective measures. ECAP thresholds show the best promise for helping to provide balanced bilateral CI input in children.     

Evaluation of the implanted cochlear implant electrode by CT scanning with three-dimensional reconstruction

Olfactory disorders are common in the general population, but research into the consequences of these disorders has been lacking. Not until recently, when specific tools for assessing changes in quality of life due to olfactory disorders have become available, have systematic investigations been conducted. In this article we present the most important roles that olfaction plays in humans and review the scientific literature on the consequences of olfactory disorders. This review suggests that quality of life, regarding safety issues and interpersonal relations, as well as eating habits and nutritional intake are severely altered in a large proportion of patients with olfactory disorders.     

Multichannel cochlear implantation in children with cochlear ossification.

OBJECTIVE: To compare results of post-meningitic children who had cochlear implantation with partial or complete drill-out to those who had no drill-out. STUDY DESIGN: This study is a retrospective case review. SETTING: The Atlanta Cochlear Implant Group is a private, tertiary, outpatient clinic. PATIENTS: Eligibility included all our post-meningitic patients, 2-17 years, having a cochlear implant between June 1990 and July 1997. INTERVENTIONS: All subjects had a Nucleus 22 cochlear implant surgically implanted, speech processor programming and follow-up testing in our center, and aural rehabilitation in a variety of therapy settings. MAIN OUTCOME MEASURE: Open and closed set speech discrimination tests. RESULTS: Test performance for speech understanding was highest in the non-ossified group and lowest, but above chance, in the ossified group with complete drill-out. CONCLUSIONS: While children with non-ossified cochleas performed best, even children with extensive ossification requiring complete drill-out benefited from cochlear implantation.     

人工耳蜗植入手术并发症及临床分析

目的 探讨人工耳蜗植入手术并发症的预防及处置.方法 回顾分析2000年12月至2010年8月在我科由单一术者完成的416例人工耳蜗植入手术,发生并发症29例(29/416),占7%,随访时间1个月～10年,并对其原因进行分析,提出预防及处置的措施.结果 29例并发症中,按轻重程度分为:轻度并发症23例,占5.5%,包括术后皮下血肿13例,急性中耳炎2例,术后迟发性切口感染2例,术后迟发性面瘫1例,面肌抽搐1例,术后重度眩晕4例;重度并发症6例,占1.5%,包括脑脊液漏2例、切口感染致植入体外露1例、鼓膜穿孔致电极外露1例、耳蜗骨化至电极未能全部植入1例,磁铁移位1例.无电极耳蜗内扭曲、未植入耳蜗、植入体移位及脑膜炎、永久性面瘫发生.轻度并发症经处置后均痊愈,脑脊液漏经二次手术修补痊愈.电极、植入体外露2例病人,均取出植入体,行对侧耳蜗植入.1例磁铁移位经二次手术予以复位.结论 如何降低人工耳蜗植入手术严重并发症的发生,仍是人工耳蜗植入亟待解决的问题.