Cochlear Turn Research Articles

Intravenous Delayed Gadolinium-Enhanced MR Imaging of the Endolymphatic Space: A Methodological Comparative Study.

In-vivo non-invasive verification of endolymphatic hydrops (ELH) by means of intravenous delayed gadolinium (Gd) enhanced magnetic resonance imaging of the inner ear (iMRI) is rapidly developing into a standard clinical tool to investigate peripheral vestibulo-cochlear syndromes. In this context, methodological comparative studies providing standardization and comparability between labs seem even more important, but so far very few are available. One hundred eight participants [75 patients with Meniere's disease (MD; 55.2 ± 14.9 years) and 33 vestibular healthy controls (HC; 46.4 ± 15.6 years)] were examined. The aim was to understand (i) how variations in acquisition protocols influence endolymphatic space (ELS) MR-signals; (ii) how ELS quantification methods correlate to each other or clinical data; and finally, (iii) how ELS extent influences MR-signals. Diagnostics included neuro-otological assessment, video-oculography during caloric stimulation, head-impulse test, audiometry, and iMRI. Data analysis provided semi-quantitative (SQ) visual grading and automatic algorithmic quantitative segmentation of ELS area [2D, mm2] and volume [3D, mm3] using deep learning-based segmentation and volumetric local thresholding. Within the range of 0.1–0.2 mmol/kg Gd dosage and a 4 h ± 30 min time delay, SQ grading and 2D- or 3D-quantifications were independent of signal intensity (SI) and signal-to-noise ratio (SNR; FWE corrected, p < 0.05). The ELS quantification methods used were highly reproducible across raters or thresholds and correlated strongly (0.3–0.8). However, 3D-quantifications showed the least variability. Asymmetry indices and normalized ELH proved the most useful for predicting quantitative clinical data. ELH size influenced SI (cochlear basal turn p < 0.001), but not SNR. SI could not predict the presence of ELH. In conclusion, (1) Gd dosage of 0.1–0.2 mmol/kg after 4 h ± 30 min time delay suffices for ELS quantification. (2) A consensus is needed on a clinical SQ grading classification including a standardized level of evaluation reconstructed to anatomical fixpoints. (3) 3D-quantification methods of the ELS are best suited for correlations with clinical variables and should include both ears and ELS values reported relative or normalized to size. (4) The presence of ELH increases signal intensity in the basal cochlear turn weakly, but cannot predict the presence of ELH.

Combination photobiomodulation/N-acetyl-L-cysteine treatment appears to mitigate hair cell loss associated with noise-induced hearing loss in rats.

Sensorineural hearing loss is an intractable disease. Acoustic overstimulation creates hearing loss; many patients exhibit social and emotional dysfunctions. In a model of noise-induced hearing loss (NIHL), low-level laser photobiomodulation (PBM) at a near-infrared wavelength significantly improved auditory brainstem response (ABR) thresholds. In addition, both N-acetyl-L-cysteine (NAC) and acetyl-L-carnitine (ALCAR) attenuated NIHL, reducing the effects of noise trauma in the cochlea and the central auditory system. Here, we combined PBM with antioxidants to explore hearing threshold recovery and morphological hair cell changes after rats were exposed to noise. The average auditory brainstem response thresholds after PBM/NAC combination treatment were reduced from the apex to the basal turn at all of 8, 16, and 32 kHz compared to the noise-only group. The PBM/NAC combination treated group exhibited intact outer hair cells in all turns, and significantly greater hair cell numbers in the middle and basal cochlear turns, than did controls. Thus, PBM/NAC treatment may prevent hearing dysfunction caused by NIHL.

Comparison of inner ear MRI enhancement in patients with Meniere’s disease after intravenous injection of gadobutrol, gadoterate meglumine, or gadodiamide

PurposeTo compare the enhancement results of three gadolinium contrast agents in the inner ear of patients with Meniere’s disease 4 h after intravenous injection of gadobutrol, gadoterate meglumine, or gadodiamide. MethodsWe enrolled 60 patients with a definitive diagnosis of unilateral Meniere’s disease and divided them into three groups of 20 patients; each group received a double dose of gadobutrol, gadoterate meglumine, or gadodiamide. The postcontrast signal intensity of the basal cochlear turn was scored quantitatively, and qualitative visual evaluation of the cochlea, vestibule and semi-circular canals was also performed. The results of both evaluations were compared between the three patient groups. ResultsThe cochlear basal turn signal intensity of the gadobutrol group was significantly higher than that of the gadoterate meglumine and gadodiamide groups; however, no significant difference was observed between the gadoterate meglumine and gadodiamide groups. The intensity of visualization of the semi-circular canals was significantly better in the bilateral gadobutrol group than in the gadoterate meglumine and gadodiamide groups; however, there was no significant difference in terms of the intensity of visualization of the semi-circular canals between the gadoterate meglumine and gadodiamide groups. There were no significant differences in the intensity of visualization of the cochlea and vestibule among the three groups. ConclusionsCompared with gadoterate meglumine and gadodiamide, gadobutrol can provide a higher degree of perilymphatic enhancement and better anatomical details of the semi-circular canals in the ears of patients with Meniere’s disease.

Channel Interaction During Infrared Light Stimulation in the Cochlea.

The number of perceptually independent channels to encode acoustic information is limited in contemporary cochlear implants (CIs) because of the current spread in the tissue. It has been suggested that neighboring electrodes have to be separated in humans by a distance of more than 2 mm to eliminate significant overlap of the electric current fields and subsequent interaction between the channels. It has also been argued that an increase in the number of independent channels could improve CI user performance in challenging listening environments, such as speech in noise, tonal languages, or music perception. Optical stimulation has been suggested as an alternative modality for neural stimulation because it is spatially selective. This study reports the results of experiments designed to quantify the interaction between neighboring optical sources in the cochlea during stimulation with infrared radiation. In seven adult albino guinea pigs, a forward masking method was used to quantify the interaction between two neighboring optical sources during stimulation. Two optical fibers were placed through cochleostomies into the scala tympani of the basal cochlear turn. The radiation beams were directed towards different neuron populations along the spiral ganglion. Optically evoked compound action potentials were recorded for different radiant energies and distances between the optical fibers. The outcome measure was the radiant energy of a masker pulse delivered 3 milliseconds before a probe pulse to reduce the response evoked by the probe pulse by 3 dB. Results were compared for different distances between the fibers placed along the cochlea. The energy required to reduce the probe's response by 3 dB increased by 20.4 dB/mm and by 26.0 dB/octave. The inhibition was symmetrical for the masker placed basal to the probe (base-to-apex) and the masker placed apical to the probe (apex-to-base). The interaction between neighboring optical sources during infrared laser stimulation is less than the interaction between neighboring electrical contacts during electrical stimulation. Previously published data for electrical stimulation reported an average current spread in human and cat cochleae of 2.8 dB/mm. With the increased number of independent channels for optical stimulation, it is anticipated that speech and music performance will improve. Lasers Surg. Med. © 2020 Wiley Periodicals LLC.

Cochlear duct length and cochlear distance on preoperative CT: imaging markers for estimating insertion depth angle of cochlear implant electrode.

Preoperative estimation of the insertion depth angle of cochlear implant (CI) electrodes is essential for surgical planning. The purpose of this study was to determine the cochlear size using preoperative CT and to investigate the correlation between cochlear size and insertion depth angle in morphologically normal cochlea. Thirty-five children who underwent CI were included in this study. Cochlear duct length (CDL) and the diameter of the cochlear basal turn (distance A/B) on preoperative CT and the insertion depth angle of the CI electrode on postoperative radiographs were independently measured by two readers. Correlation between cochlear size and insertion depth angle was evaluated. Interobserver agreement was calculated using the intraclass correlation coefficient (ICC). The mean CDL, distance A, and distance B of 70 ears were 36.20 ± 1.57 mm, 8.67 ± 0.42 mm, and 5.73 ± 0.32 mm, respectively. The mean insertion depth angle was 431.45 ± 38.42°. Interobserver agreements of CDL, distance A/B, and insertion depth angle were fair to excellent (ICC 0.864, 0.862, 0.529, and 0.958, respectively). Distance A (r = - 0.7643) and distance B (r = - 0.7118) showed a negative correlation with insertion depth angle, respectively (p < 0.0001). However, the correlation between CDL and insertion depth angle was not statistically significant (r = - 0.2333, p > 0.05). The CDL and cochlear distance can be reliably obtained from preoperative CT. Distance A can be used as a predictive marker for estimating insertion depth angle during CI surgery.

Radiologic anatomy of the round window relevant to cochlear implantation and inner ear drug delivery.

ObjectiveTo determine anatomic relationships and variation of the round window membrane to bony surgical landmarks on computed tomography.Study designRetrospective imaging review.Methods100 temporal bone images were evaluated. Direct measurements were obtained for membrane position. Vector distances and angulation from umbo and bony annulus were calculated from image viewer software coordinates.ResultsThe angle of round window membrane at junction with cochlear basal turn was (42.1 ± 8.6)°. The membrane's position relative to plane of the facial nerve through facial recess was (14.7 ± 5.2)° posterior from a reference line drawn through facial recess to carotid canal. Regarding transtympanic drug delivery, the round window membrane was directed 4.1 mm superiorly from the inferior annulus and 5.4 mm anteriorly from the posterior annulus. The round window membrane on average was angled superiorly from the inferior annulus (77.1 ± 27.9)° and slightly anteriorly from the posterior annulus (19.1 ± 11.1°). The mean distance of round window membrane from umbo was 4 mm and posteriorly rotated 30° clockwise from a perpendicular drawn from umbo to inferior annulus towards posterior annulus. Together, these measurements approximate the round window membrane in the tympanic membrane's posteroinferior quadrant.ConclusionsThese radiologic measurements demonstrate normal variations seen in round window anatomy relative to facial recess approach and bony tympanic annulus, providing a baseline to assess round window insertion for cochlear implantation and outlines anatomic factors affecting transtympanic drug delivery.

Dendritic Degeneration of Human Auditory Nerve Fibers and Its Impact on the Spiking Pattern Under Regular Conditions and During Cochlear Implant Stimulation.

Due to limitations of human in vivo studies, detailed computational models enable understanding the neural signaling in the degenerated auditory system and cochlear implants (CIs). Four human cochleae were used to quantify hearing levels depending on dendritic changes in diameter and myelination thickness from type I of the auditory nerve fibers (ANFs). Type I neurons transmit the auditory information as spiking pattern from the inner hair cells (IHCs) to the cochlear nucleus. The impact of dendrite diameter and degree of myelination on neural signal transmission was simulated for (1) synaptic excitation via IHCs and (2) stimulation from CI electrodes. An accurate three-dimensional human cochlear geometry, along with 30 auditory pathways, mimicked the CI environment. The excitation properties of electrical potential distribution induced by two CI were analyzed. Main findings: (1) The unimodal distribution of control dendrite diameters becomes multimodal for hearing loss cases; a group of thin dendrites with diameters between 0.3 and 1 μm with a peak at 0.5 μm appeared. (2) Postsynaptic currents from IHCs excite such thin dendrites easier and earlier than under control conditions. However, this advantage is lost as their conduction velocity decreases proportionally with the diameter and causes increased spike latency and jitter in soma and axon. Firing probability reduces through the soma passage due to the low intracellular current flow in thin dendrites during spiking. (3) Compared with dendrite diameter, variations in myelin thickness have a small impact on spiking performance. (4) Contrary to synaptic excitation, CIs cause several spike initiation sites in dendrite, soma region, and axon; moreover, fiber excitability reduces with fiber diameter. In a few cases, where weak stimuli elicit spikes of a target neuron (TN) in the axon, dendrite diameter reduction has no effect. However, in many cases, a spike in a TN is first initiated in the dendrite, and consequently, dendrite degeneration demands an increase in threshold currents. (5) Threshold currents of a TN and co-stimulation of degenerated ANFs in other frequency regions depend on the electrode position, including its distance to the outer wall, the cochlear turn, and the three-dimensional pathway of the TN.

Access to the Apical Cochlear Modiolus for Possible Stem Cell-based and Gene Therapy of the Auditory Nerve.

Loss of spiral ganglion neurons (SGN) is permanent and responsible for a substantial number of patients suffering from hearing impairment. It can derive from the degeneration of SGNs due to the death of sensory hair cells as well as from auditory neuropathy. Utilizing stem cells to recover lost SGNs increasingly emerges as a possible therapeutic option, but access to human SGNs is difficult due to their protected location within the bony impacted cochlea. Aim of this study was to establish a reliable and practicable approach to access SGNs in the human temporal bone for possible stem cell and gene therapies. In seven human temporal bone specimen a transcanal approach was used to carefully drill a cochleostomy in the lateral second turn followed by insertion of a tungsten needle into the apical modiolus to indicate the spot for intramodiolar injections. Subsequent cone beam computed tomography (CBCT) served as evaluation for positioning of the marker and cochleostomy size. The apical modiolus could be exposed in all cases by a cochleostomy (1.6 mm2, standard deviation ±0.23 mm2) in the lateral second turn. 3D reconstructions and analysis of CBCT revealed reliable positioning of the marker in the apical modiolus, deviating on average 0.9 mm (standard deviation ±0.49 mm) from the targeted center of the second cochlear turn. We established a reliable, minimally invasive, transcanal surgical approach to the apical cochlear modiolus in the human temporal bone in foresight to stem cell-based and gene therapy of the auditory nerve.

BDNF Outperforms TrkB Agonist 7,8,3'-THF in Preserving the Auditory Nerve in Deafened Guinea Pigs.

In deaf subjects using a cochlear implant (CI) for hearing restoration, the auditory nerve is subject to degeneration, which may negatively impact CI effectiveness. This nerve degeneration can be reduced by neurotrophic treatment. Here, we compare the preservative effects of the naturally occurring tyrosine receptor kinase B (TrkB) agonist brain-derived neurotrophic factor (BDNF) and the small-molecule TrkB agonist 7,8,3′-trihydroxyflavone (THF) on the auditory nerve in deafened guinea pigs. THF may be more effective than BDNF throughout the cochlea because of better pharmacokinetic properties. The neurotrophic compounds were delivered by placement of a gelatin sponge on the perforated round window membrane. To complement the histology of spiral ganglion cells (SGCs), electrically evoked compound action potential (eCAP) recordings were performed four weeks after treatment initiation. We analyzed the eCAP inter-phase gap (IPG) effect and measures derived from pulse-train evoked eCAPs, both indicative of SGC healthiness. BDNF but not THF yielded a significantly higher survival of SGCs in the basal cochlear turn than untreated controls. Regarding IPG effect and pulse-train responses, the BDNF-treated animals exhibited more normal responses than both untreated and THF-treated animals. We have thus confirmed the protective effect of BDNF, but we have not confirmed previously reported protective effects of THF with our clinically applicable delivery method.

Cervical and Ocular Vestibular-Evoked Myogenic Potentials in Patients With Intracochlear Schwannomas.

Objective: To evaluate ocular and cervical vestibular evoked myogenic potentials (oVEMPs and cVEMPs) in patients with solely intracochlear localization of an intralabyrinthine schwannoma (ILS).Study Design: Retrospective analysis of a series of cases.Setting: Monocentric study at a tertiary referral center.Patients: Patients with intracochlear schwannoma (ICS) and VEMP measurements.Outcome Measures: Signed asymmetry ratio (AR) of cVEMPs and oVEMPs to air conducted sound with AR cut-offs considered to be asymmetrical when exceeding ±30% for cVEMPs and ±40% for oVEMPs with respect to the side affected by the tumor (reduced amplitudes on the affected side indicated by negative values, enhanced amplitudes by positive values); VEMP amplitudes and latencies; tumor localization in the cochlear turn and scala.Results: Nineteen patients with a solely intracochlear tumor (ICS patients) [10 males, 9 females, mean age 57.1 (SD: 13.4) years] were included in the study. On the affected side, cVEMPs were absent or reduced in 47% of the patients, normal in 32%, and enhanced in 21%. Ocular VEMPs on the affected side were absent or reduced in 53% of the patients, normal in 32% and enhanced in 15%. Latencies for cVEMPs and oVEMPs were not significantly different between the affected and non-affected side. In all patients with enhanced VEMPs, the tumor was located in the scala tympani and scala vestibuli.Conclusions: As a new and unexpected finding, VEMP amplitudes can be enhanced in patients with intracochlear schwannoma, mimicking the third window syndrome.

The Correlation Between Endolymphatic Hydrops and blood-labyrinth barrier Permeability of Meniere Disease.

This study was designed to assess the correlation between the grades of endolymphatic hydrops and the blood-labyrinth barrier permeability in the affected ear in Meniere's disease, following the administration of intravenous gadolinium contrast. Prospective study. The quantitative values of endolymphatic hydrops were determined after intravenous injection of a double-dose of gadobutrol in 39 patients with unilateral definite Meniere's disease. Additionally, the signal intensity ratio of bilateral cochlear basal turns was evaluated and analyzed; The correlation between the grades of the endolymphatic hydrops and the signal intensity ratio of the cochlear basal turns in the affected ear was examined. The grades of the endolymphatic hydrops can be quantitatively evaluated using magnetic resonance imaging (MRI). The signal intensity ratio of the cochlear basal turns in the affected ear was significantly higher than in the unaffected ear (P = .001); there was a positive correlation between the signal intensity ratio of the cochlear basal turn and the grades of cochlear (r = 0.634, P = 0.000) and vestibular(r = 0.559, P = .000) hydrops in the affected ear. The increased permeability of the blood-labyrinth barrier may play a role in the process of endolymphatic hydrops in Meniere's disease.

Anti-PD-1 Therapy Does Not Influence Hearing Ability in the Most Sensitive Frequency Range, but Mitigates Outer Hair Cell Loss in the Basal Cochlear Region.

The administration of immune checkpoint inhibitors (ICIs) often leads to immune-related adverse events. However, their effect on auditory function is largely unexplored. Thorough preclinical studies have not been published yet, only sporadic cases and pharmacovigilance reports suggest their significance. Here we investigated the effect of anti-PD-1 antibody treatment (4 weeks, intraperitoneally, 200 μg/mouse, 3 times/week) on hearing function and cochlear morphology in C57BL/6J mice. ICI treatment did not influence the hearing thresholds in click or tone burst stimuli at 4–32 kHz frequencies measured by auditory brainstem response. The number and morphology of spiral ganglion neurons were unaltered in all cochlear turns. The apical-middle turns (<32 kHz) showed preservation of the inner and outer hair cells (OHCs), whilst ICI treatment mitigated the age-related loss of OHCs in the basal turn (>32 kHz). The number of Iba1-positive macrophages has also increased moderately in this high frequency region. We conclude that a 4-week long ICI treatment does not affect functional and morphological integrity of the inner ear in the most relevant hearing range (4–32 kHz; apical-middle turns), but a noticeable preservation of OHCs and an increase in macrophage activity appeared in the >32 kHz basal part of the cochlea.

Prevalence of occult cochlear basal turn patency

Background Perilymph gusher (PLG) is a rare complication of otologic surgery attributed to a communication between the cochlea and the internal auditory canal (IAC). Subtle patency between the cochlear basal turn and IAC has recently been identified on computed tomography (CT) as a risk factor, specifically when the defect is > 0.75 mm. Objectives Investigate the prevalence of radiographic cochlear basal turn patency. Materials and Methods Patients with CT of the temporal bones and inner ears interpreted as “normal” were included. An otologist and a radiologist independently reviewed CTs to measure radiographic dehiscence in an oblique plane along the interface of the cochlea and IAC. Known PLGs were excluded. Results Two hundred and ten ears were included (88 conductive or mixed hearing loss, 62 sensorineural hearing loss, 41 audiometrically normal ears). 71 ears (33.8%) were radiographically patent. Mean defect width was 0.41 mm (0.15-0.7 mm). Defect width was not associated with type of hearing loss, age, or gender. No defects were wider than 0.75 mm. Conclusions Radiographic patency of the cochlear basal turn may be present in patients with hearing loss and normal hearing, but patency > 0.75 mm (i.e. risk for PLG) is rare.

New Classification of Cochlear Hypoplasia Type Malformation: Relevance in Cochlear Implantation.

This paper attempts to create a new classification type of cochlear hypoplasia (CH)-type malformation taking into consideration of vestibular section and internal auditory canal (IAC). Preoperative computed-tomography (CT) scans of cochlear implant (CI) candidates (N=31) from various clinics across the world with CH type malformation were taken for analysis. CT dataset were loaded into 3D-slicer freeware for three-dimensional (3D) segmentation of the inner-ear by capturing complete inner-ear structures from the entire dataset. Cochlear size in terms of diameter of available cochlear basal turn and length of cochlear lumen was measured from the dataset. In addition, structural connection between IAC and cochlear portions was scrutinized, which is highly relevant to the proposed CH classification in this study. CH group-I has the normal presence of IAC leading to cochlear and vestibular portions, whereas CH group-II is like CH group-I but with some degree of disruption in vestibular portion. In CH group-III, a disconnection between IAC and the cochlear portion irrespective of other features. Within all these three CH groups, the basal turn diameter varied between 3.1 mm and 9.6 mm, and the corresponding cochlear lumen length varied between 3 mm and 21 mm for the CI electrode array placement. A new classification of CH mainly based on the IAC connecting the cochlear and vestibular portions is presented in this study. CI electrode array length could be selected based on the length of the cochlear lumen, which can be observed from the 3D image.

Chloroquine kills hair cells in zebrafish lateral line and murine cochlear cultures: Implications for ototoxicity

Hearing and balance deficits have been reported during and following treatment with the antimalarial drug chloroquine. However, experimental work examining the direct actions of chloroquine on mechanoreceptive hair cells in common experimental models is lacking. This study examines the effects of chloroquine on hair cells using two common experimental models: the zebrafish lateral line and neonatal mouse cochlear cultures. Zebrafish larvae were exposed to varying concentrations of chloroquine phosphate or hydroxychloroquine for 1 h or 24 h, and hair cells assessed by antibody staining. A significant, dose-dependent reduction in the number of surviving hair cells was seen across conditions for both exposure periods. Hydroxychloroquine showed similar toxicity. In mouse cochlear cultures, chloroquine damage was specific to outer hair cells in tissue from the cochlear basal turn, consistent with susceptibility to other ototoxic agents. These findings suggest a need for future studies employing hearing and balance monitoring during exposure to chloroquine and related compounds, particularly with interest in these compounds as therapeutics against viral infections including coronavirus.

Cochlear turns measurements in patients with meningitis: A histopathological study.

ObjectiveTo demonstrate the cochlear turns area changes among patients with a history of meningitis, through otopatologic study.MethodsWe performed an analysis of the area of the bony cochlear turns and the cochlear lumen of the horizontal sections containing the modiolus and the area of the basal turn at the level of round window, in temporal bones obtained from patients with a history of meningitis and compared to a nondiseased control group.ResultsThe mean area of the bony walls and the lumen of all cochlear turns are reduced within the meningitis group. Patients who presented a time from the diagnosis of meningitis to death longer than 30 days had a significant reduction in the cochlear turns area, as compared to the control group.ConclusionFuture studies may further correlate audiologic outcomes, cochlear volume, and cochlear area among patients with meningitis.

Shape of the Cochlear Basal Turn: An Indicator for an Optimal Electrode-to-Modiolus Proximity With Precurved Electrode Type.

The objective of this study was to determine the shape of cochlear basal turn through basic cochlear parameters measurement. The secondary aim was to overlay an image of the precurved electrode array on top of the three-dimensional (3D) image of the cochlea to determine which shape of the cochlear basal turn gives optimal electrode-to-modiolus proximity. Computed tomography (CT) preoperative image-data sets of 117 ears were made available for the measurements of cochlear parameters retrospectively. Three-dimensional slicer was used in the visualization and measurement of cochlear parameters from both 3D and 2D (2-dimensional) images of the inner ear. Cochlear parameters including basal turn diameter (A), width of the basal turn (B), and cochlear height (H) were measured from the appropriate planes. B/A ratio was made to investigate which ratios correspond to round and elliptical shape of the cochlear basal turn. The cochlear size as measured by A value ranged between 7.4 mm and 10 mm. The B value and the cochlear height (H) showed a weak positive linear relation with A value. The ratio between the B and A values anything above or below 0.75 could be an indicator for a more round- or elliptical shaped cochlear basal turn, respectively. One sized/shaped commercially available precurved electrode array would not offer a tight electrode-to-modiolus in the cochlea that has an elliptical shaped basal turn as identified by the B/A ratio of <0.75. Accurate measurement of cochlear parameters adds value to the overall understanding of the cochlear geometry before a cochlear implantation procedure. The shape of cochlear basal turn could have clinical implications when comes to electrode-to-modiolus proximity.

Characterisation of the static offset in the travelling wave in the cochlear basal turn

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

Histology of the Cochlear Outer Sulcus Cells in Normal Human Ears, Presbycusis, and Menière's Disease.

Outer sulcus cell features and distribution are hypothesized to differ throughout regions of the human cochlea and between diseased and normal specimens. Outer sulcus cells play a role in inner ear fluid homeostasis. However, their anatomy and distribution in the human are not well described. Temporal bone specimens with normal hearing (n = 10), Menière's disease (n = 10), presbycusis with flat audiograms (n = 4), and presbycusis with sloping audiograms (n = 5) were examined by light microscopy. Outer sulcus cells were assessed quantitatively and qualitatively in each cochlear turn. One specimen was stained for tubulin immunofluorescence and imaged using confocal microscopy. Outer sulcus cells interface with endolymph throughout the cochlea, with greatest contact in the apical turn. Mean outer sulcus cell counts in the upper apical turn (8.82) were generally smaller (all p < 0.05) than those of the upper basal (17.71), lower middle (18.99) upper middle (18.23), and lower apical (16.42) turns. Mean outer sulcus cell counts were higher (p < 0.05) in normal controls (20.1) than in diseased specimens (15.29). There was a significant correlation between mean cell counts and tonotopically expected hearing thresholds in the upper basal (r = -0.662, p = 0.0001), lower middle (r = -0.565, p = 0.0017), and upper middle (r = -0.507, p = 0.0136) regions. Other differences in cell morphology, distribution, or relationship with Claudius cells were not appreciated between normal and diseased specimens. Menière's specimens had no apparent unique features in the cochlear apex. Immunofluorescence staining demonstrated outer sulcus cells extending into the spiral ligament in bundles forming tapering processes which differed between the cochlear turns in morphology. Outer sulcus cells vary throughout the cochlear turns and correlate with hearing status, but not in a manner specific to the underlying diagnoses of Menière's disease or presbycusis.

Evaluation of Cochlear Duct Length Measurements From a 3D Analytical Cochlear Model Using Synchrotron Radiation Phase-Contrast Imaging.

Evaluating the accuracy of cochlear duct length (CDL) measurements from a published three-dimensional (3D) analytical cochlear model using Synchrotron Radiation Phase-Contrast Imaging (SR-PCI) data will help determine its clinical applicability and allow for model adjustments to increase accuracy. Accurate CDL determination can aid in cochlear implant sizing for full coverage and frequency map programming, which has the potential to improve hearing outcomes in patients. To overcome problems with the currently available techniques for CDL determination, a novel 3D analytical cochlear model, dependent on four basal turn distances, was proposed in the literature. SR-PCI data from 11 cadaveric human cochleae were used to obtain reference measurements. CDL values generated by the analytical cochlear model were evaluated in two conditions: when the number of cochlear turns (NCT) were automatically predicted based on the four input distances, and when the NCT were manually specified based on SR-PCI data. When the analytical cochlear model automatically predicted the NCT, the mean absolute error was 2.6 ± 1.6 mm, with only 27% (3/11) of the samples having an error in the clinically acceptable range of ±1.5 mm. When the NCT were manually specified based on SR-PCI data, the mean absolute error was reduced to 1.0 ± 0.6 mm, with 73% (8/11) of the samples having a clinically acceptable error. The 3D analytical cochlear model introduced in the literature is effective at modeling the 3D geometry of individual cochleae, however tuning in the NCT estimation is required.