Detection and quantification of endolymphatic hydrops in the guinea pig cochlea by magnetic resonance microscopy

Visualization of Reissner's membrane in the mouse inner ear using highly sensitive magnetic resonance imaging analysis

Although research on hearing loss, including the identification of causative genes, has become increasingly active, the pathogenic mechanism of hearing loss remains unclear. One of the reasons for this is that the structure of the inner ear of mice, which is commonly used as a genetically modified animal model, is too small and complex, making it difficult to accurately capture abnormalities and dynamic changes in vivo. Especially, Reissner's membrane is a very important structure that separates the perilymph and endolymph of the inner ear. This malformation or damage induces abnormalities in hearing and balance. Until now, imaging analyses, such as magnetic resonance imaging (MRI) and computed tomography, are performed to investigate the inner ear structure in vivo; however, it has been difficult to analyze the small inner ear structure of mice owing to resolution. Therefore, there is an urgent need to develop an image analysis method that can accurately capture the structure of the inner ear of mice including Reissner's membrane, both dynamically and statically. This study aimed to investigate whether it is possible to accurately capture the structure (e.g., Reissner's membrane) and abnormalities of the inner ear of mice using an 11.7 T MRI. By combining two types of MRI methods, in vivo and ex vivo, we succeeded for the first time in capturing the fine structure of the normal mouse inner ear, such as the Reissner's membrane, and inflammatory lesions of otitis media mouse models in detail and accurately. In the future, we believe that understanding the state of Reissner's membrane during living conditions will greatly contribute to the development of research on inner ear issues, such as hearing loss.

Inner ear MRI enhancement based on 3D-real IR sequence in patients with Meniere's disease after intravenous gadolinium injection: comparison of different doses used and exploration of a appropriate dose

OBJECTIVES

Three-dimensional inversion-recovery sequence with real reconstruction (3D-real IR) magnetic resonance imaging (MRI) can detect endolymphatic hydrops of the inner ear. We aimed to explore a appropriate dose for intravenous gadolinium injection.

DESIGN

Observational prospective study.

SETTING

Tertiary referral center.

PARTICIPANTS

We collected 90 unilateral definite Meniere's disease patients.

MAIN OUTCOME MEASURES

All enrolled patients were divided into three groups randomly (patients in group A, B and C received gadolinium injection in 1/1.5/2 times doses, respectively). After 4 hours, inner ear MRI scans were applied.

RESULTS

The signal intensities of B-affected ears and C-affected ears were significantly higher than A-affected ears (p < 0.05), however, no difference was found between B-affected ears and C-affected ears (p=0.267). The same conditions also appeared in the three unaffected-ear groups. Moreover, the signal intensities of affected-ear in group A, B and C were significantly higher than that of the corresponding unaffected-ear groups (p < 0.05). Besides, the subjective visual evaluation scores of group B and C were significantly better than that of group A (p < 0.05).

CONCLUSIONS

Intravenous injection of gadolinium in a single dose may be unbefitting for the inner ear imaging based on 3D-real IR MRI, both the applications of gadolinium in 1.5 times and double doses can have a good perilymphatic enhancement effect of inner ear. In order to minimize the use of dose for avoiding or mitigating the adverse reactions and renal damage, 1.5 times dose may be preferred in clinical practice.

Magnetic resonance imaging with intravenous gadoteridol injection based on 3D-real IR sequence of the inner ear in Meniere's disease patient: feasibility in 3.5-h time interval

BACKGROUND

Intravenous gadoteridol injection can be applied to visualize endolymphatic hydrops (EH).

AIMS/OBJECTIVES

To explore whether 3.5-h time interval was feasible for clinical practice.

MATERIALS AND METHODS

We collected 70 unilateral Meniere's disease (MD) patients who were divided into two groups randomly (group A: 3.5-h time interval; group B: 4-h time interval). Among the two groups, the signal intensity (SI) in perilymphatic area of the basal turn of cochlea, the results of visual evaluations in the vestibule, cochlea and semicircular canal and the detection results of EH were compared.

RESULTS

Regarding the SI, no difference was found between A-affected ears and B-affected ears (p=.499), and no difference was found between A-unaffected ears and B-unaffected ears (p=.111). However, a difference was found between A-affected ears and A-unaffected ears (p=.005), and a difference was found between B-affected ears and B-unaffected ears (p=.012). Besides, no difference was found between the visual evaluations in the vestibule, cochlea, and semicircular canal of the two groups. Regarding the detection results of EH, no difference was found between the two groups (all p>.05).

CONCLUSIONS AND SIGNIFICANCE

In the clinical application of gadoteridol for the inner ear, 3.5-h delayed MR imaging is feasible.

Uncoiling the Human Cochlea-Physical Scala Tympani Models to Study Pharmacokinetics Inside the Inner Ear

In the field of cochlear implantation, artificial/physical models of the inner ear are often employed to investigate certain phenomena like the forces occurring during implant insertions. Up to now, no such models are available for the analysis of diffusion processes inside the cochlea although drug delivery is playing an increasingly important role in this field. For easy access of the cochlea along its whole profile, e.g., for sequential sampling in an experimental setting, such a model should ideally be longitudinal/uncoiled. Within this study, a set of 15 micro-CT imaging datasets of human cochleae was used to derive an average representation of the scala tympani. The spiral profile of this model was then uncoiled along different trajectories, showing that these trajectories influence both length and volume of the resulting longitudinal model. A volumetric analysis of the average spiral model was conducted to derive volume-to-length interrelations for the different trajectories, which were then used to generate two tubular, longitudinal scala tympani models with volume and length properties matching the original, spiral profile. These models can be downloaded for free and used for reproducible and comparable simulative and experimental investigations of diffusion processes within the inner ear.

Early Detection of Endolymphatic Hydrops using the Auditory Nerve Overlapped Waveform (ANOW)

Endolymphatic hydrops is associated with low-frequency sensorineural hearing loss, with a large body of research dedicated to examining its putative causal role in low-frequency hearing loss. Investigations have been thwarted by the fact that hearing loss is measured in intact ears, but gold standard assessments of endolymphatic hydrops are made postmortem only; and that no objective low-frequency hearing measure has existed. Yet the association of endolymphatic hydrops with low-frequency hearing loss is so strong that it has been established as one of the important defining features for Ménière's disease, rendering it critical to detect endolymphatic hydrops early, regardless of whether it serves a causal role or is the result of other disease mechanisms. We surgically induced endolymphatic hydrops in guinea pigs and employed our recently developed objective neural measure of low-frequency hearing, the Auditory Nerve Overlapped Waveform (ANOW). Hearing loss and endolymphatic hydrops were assessed at various time points after surgery. The ANOW detected low-frequency hearing loss as early as the first day after surgery, well before endolymphatic hydrops was found histologically. The ANOW detected low-frequency hearing loss with perfect sensitivity and specificity in all ears after endolymphatic hydrops developed, where there was a strong linear relationship between degree of endolymphatic hydrops and severity of low-frequency hearing loss. Further, histological data demonstrated that endolymphatic hydrops is seen first in the high-frequency cochlear base, though the ANOW demonstrated that dysfunction begins in the low-frequency apical cochlear half. The results lay the groundwork for future investigations of the causal role of endolymphatic hydrops in low-frequency hearing loss.

Focal Endolymphatic Hydrops as Seen in the Pars Inferior of the Human Inner Ear

HYPOTHESIS

Endolymphatic hydrops of the human inner ear may be localized focally in the pars inferior of the human inner ear.

BACKGROUND

Endolymphatic hydrops may be found in the human inner ear in patients who in life had suffered from Ménière's syndrome or a variety of other disorders. The degree of endolymphatic hydrops may differ based on location in the inner ear.

METHODS

A computer-assisted search of all cases in the collection of the Massachusetts Eye and Ear Infirmary in which endolymphatic hydrops was found in the inner ear yielded 13 specimens in which there was good evidence for focal endolymphatic hydrops in the pars inferior. Temporal bones were prepared for light microscopy. Semi-serial sections were reviewed to generate localization data for endolymphatic hydrops and also to search for evidence of a previous inflammatory process, including fibrosis or new bone formation.

RESULTS

Endolymphatic hydrops was present in the saccule in 10 of 13 specimens. In the cochlear duct, there were segments of the cochlea in which there was no cochlear hydrops juxtaposed to other regions in which there was severe endolymphatic hydrops. Transition between hydropic and non-hydropic status in the cochlear duct was often abrupt.Evidence for a previous inflammation process was found in 6 of 13 specimens including fibrosis because of temporal bone fracture, or traumatic stapedectomy and in those cases in which the cause of hearing loss was idiopathic, fibrosis, and osteoid between the labyrinthine surface of the footplate and the hydropic saccular wall, and/or osteoid in the scala vestibuli, or in the proximate Rosenthal's canal. Evidence of a previous inflammatory process was uniformly seen in the perilymphatic compartment.

CONCLUSIONS

Endolymphatic hydrops of the pars inferior in the human may have a focal distribution. This study suggests that the pathogenesis of endolymphatic hydrops is unlikely to be because of distal obstruction of longitudinal flow and was more consistent with the hypothesis that homeostasis of the endolymphatic and perilymphatic volumes occurs all along the cochlear duct. Other factors including variable distensibility of Reissner's membrane or disturbance of local solute homeostatic mechanisms may be responsible for wide variations in the degree of hydrops. A focal inflammatory process during life may be one cause of focal endolymphatic hydrops as seen histopathologically.

In Vivo Visualization of Endolymphatic Hydrops in Guinea Pigs: Magnetic Resonance Imaging Evaluation at 4.7 Tesla

In order to find out whether it is possible to visualize experimental endolymphatic hydrops in the cochlea with magnetic resonance imaging (MRI) at 4.7 T, we used 11 guinea pigs. Five normal guinea pigs were used as controls. Early manifestation of endolymphatic hydrops was evaluated in endolymphatic sac (ES)-intact animals (n = 6), and advanced manifestation in ES-damaged animals (n = 5) by means of MRI with gadolinium-diethylenetriaminepentaacetate-bismethylamide (Gd-DTPA-BMA) contrast agent. Hearing was tested with electrocochleography. The surface area of 3 partitions of the cochlea was used to quantify endolymphatic hydrops. The fine structure of the 3 partitions of the cochlea was visualized with MRI in all animals, as Gd-DTPA-BMA appeared mainly in the scala tympani and scala vestibuli. As early as 5 days after endolymphatic sac surgery, endolymphatic hydrops started to appear as visualized by MRI and also verified with histology. Severe damage to the inner ear barrier with Gd-DTPA-BMA leakage into the scala media was detected with MRI in 1 ES-damaged animal that had a 60-dB hearing loss. To conclude, endolymphatic hydrops can be visualized with high-resolution MRI by means of Gd-DTPA-BMA, and it is possible to quantify the extent of endolymphatic hydrops. Damage to the inner ear barrier or possible rupture of membranes can be shown with the assistance of Gd-DTPA-BMA.

Three-dimensional histological specimen preparation for accurate imaging and spatial reconstruction of the middle and inner ear

PURPOSE

This paper presents a highly accurate cross-sectional preparation technique. The research aim was to develop an adequate imaging modality for both soft and bony tissue structures featuring high contrast and high resolution. Therefore, the advancement of an already existing micro-grinding procedure was pursued. The central objectives were to preserve spatial relations and to ensure the accurate three-dimensional reconstruction of histological sections.

METHODS

Twelve human temporal bone specimens including middle and inner ear structures were utilized. They were embedded in epoxy resin, then dissected by serial grinding and finally digitalized. The actual abrasion of each grinding slice was measured using a tactile length gauge with an accuracy of one micrometre. The cross-sectional images were aligned with the aid of artificial markers and by applying a feature-based, custom-made auto-registration algorithm. To determine the accuracy of the overall reconstruction procedure, a well-known reference object was used for comparison. To ensure the compatibility of the histological data with conventional clinical image data, the image stacks were finally converted into the DICOM standard.

RESULTS

The image fusion of data from temporal bone specimens' and from non-destructive flat-panel-based volume computed tomography confirmed the spatial accuracy achieved by the procedure, as did the evaluation using the reference object.

CONCLUSION

This systematic and easy-to-follow preparation technique enables the three-dimensional (3D) histological reconstruction of complex soft and bony tissue structures. It facilitates the creation of detailed and spatially correct 3D anatomical models. Such models are of great benefit for image-based segmentation and planning in the field of computer-assisted surgery as well as in finite element analysis. In the context of human inner ear surgery, three-dimensional histology will improve the experimental evaluation and determination of intra-cochlear trauma after the insertion of an electrode array of a cochlear implant system.

Endolymphatic hydrops in Meniere's disease detected by MRI after intratympanic administration of gadolinium: comparison with sudden deafness

CONCLUSION

The detection rate of endolymphatic hydrops was significantly higher in patients with Meniere's disease compared with those with sudden deafness, indicating that 3 T magnetic resonance imaging (MRI) with intratympanic gadolinium injection was effective in diagnosing endolymphatic hydrops.

OBJECTIVES

To compare the detection rate of endolymphatic hydrops between patients with Meniere's disease and sudden deafness as controls by 3 T MRI after intratympanic gadolinium injection with conventional pulse sequence such as two-dimensional fluid-attenuated inversion recovery.

METHODS

Ten patients with unilateral Meniere's disease and eight with sudden deafness underwent inner ear MRI 24 h after intratympanic gadolinium injection.

RESULTS

The endolymphatic space was detected as a low signal intensity area, while the perilymphatic space showed high intensity by gadolinium enhancement. Due to faint enhancement, images could not be evaluated in 1 of 10 patients with Meniere's disease. However, the other nine patients together with two of the eight with sudden deafness were diagnosed as having hydrops. The difference in detection rates between the two diseases was statistically significant. Two hydrops-positive cases with sudden deafness were considered to be of the secondary type of hydrops, because images were taken after partial recovery from hearing loss several months after the onset of the disease.

Endolymphatic hydrops: pathophysiology and experimental models

It is well established that endolymphatic hydrops plays a role in Ménière disease, even though the precise role is not fully understood and the presence of hydrops in the ear does not always result in symptoms of the disease. It nevertheless follows that a scientific understanding of how hydrops arises, how it affects the function of the ear, and how it can be manipulated or reversed could contribute to the development of effective treatments for the disease. Measurements in animal models in which endolymphatic hydrops has been induced have given numerous insights into the relationships between hydrops and other pathologic and electrophysiological changes, and how these changes influence the function of the ear. The prominent role of the endolymphatic sac in endolymph volume regulation, and the cascade of histopathological and electrophysiological changes that are associated with chronic endolymphatic hydrops, have now been established. An increasing number of models are now available that allow specific aspects of the interrelationships to be studied. The yclical nature of Ménière symptoms gives hope that treatments can be developed to maintain the ear in permanent state of remission, possibly by controlling endolymphatic hydrops, thereby avoiding the rogressive damage and secondary pathologic changes that may also contribute to the patient's symptoms.

Image evaluation of endolymphatic space in fluctuating hearing loss without vertigo

The objective of the present study was to investigate endolymphatic space images in patients with fluctuating hearing loss without vertigo, and to elucidate its underlying pathophysiology. Eight patients with fluctuating hearing loss without vertigo were included in this study. 3T MRI was taken, 24 h after intratympanic injection of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA). Electrocochleography and VEMP tests were performed to evaluate cochlear and vestibular functions. Endolymphatic hydrops were observed both in the cochlea and in the vestibule of all eight patients. Three patients out of six whose summating potential/action potential (SP/AP) ratio was recordable showed an elevation of SP/AP ratio. In the two patients with remarkable endolymphatic hydrops in the vestibule, VEMP was absent from the affected ear. In conclusion, 3T MRI after intratympanic injection of Gd-DTPA revealed endolymphatic hydrops both in the cochlea and in the vestibule in the patients with fluctuating hearing loss without vertigo.

Visualization of Reissner membrane and the spiral ganglion in human fetal cochlea by micro-computed tomography

PURPOSE

Although visualization of fine structures in the cochlea such as Reissner membrane (vestibular membrane) is important for elucidation of the mechanism and the establishment of therapy for inner ear diseases, they cannot be visualized by even the most advanced high-resolution medical computed tomography (CT) and magnetic resonance imaging. Visualization of Reissner membrane in dissected animals by micro-magnetic resonance imaging has been reported, but bone could not be visualized. We attempted to visualize human fetal Reissner membrane and the spiral ganglion by micro-focus x-ray CT (micro-CT), which has a spatial resolution several hundred times greater than the conventional medical CT.

MATERIALS AND METHODS

Serial tomograms of a dissected pyramis, including the cochlea of human fetuses (stillborn specimens), were obtained by micro-CT, and 3-dimensional reconstruction was performed by a volume-rendering method.

RESULTS

Clear tomograms (theoretical spatial resolution, 12.2 x 12.2 microm; slice thickness 77.5 microm) and 3-dimensional reconstructed images (theoretical spatial resolution, 6.8 x 6.8 microm; slice thickness, 40.0 microm) of Reissner membrane and the spiral ganglion with a bony labyrinth (cochlear bone) were successfully obtained for the first time. The thickness of Reissner membrane obtained by the tomogram was 12 microm, which corresponds to the optical macroscopic value from resin-embedded histologic sections.

CONCLUSIONS

This study showed that micro-CT enables us to visualize the internal fine structure of the human cochlea. As the success rate of the visualization of Reissner membrane is not high, it is necessary to improve the image quality and contrast resolution of micro-CT to enable stable visualization of fine structures. The development of imaging equipment such as micro-CT for medical use should play an important role in the elucidation of the mechanism and the establishment of therapy for inner ear diseases.

Displacements of the organ of Corti by gel injections into the cochlear apex

In order to transduce sounds efficiently, the stereocilia of hair cells in the organ of Corti must be positioned optimally. Mechanical displacements, such as pressure differentials across the organ caused by endolymphatic hydrops, may impair sensitivity. Studying this phenomenon has been limited by the technical difficulty of inducing sustained displacements of stereocilia in vivo. We have found that small injections (0.5-2 microL) of Healon gel into the cochlear apex of guinea pigs produced sustained changes of endocochlear potential (EP), summating potential (SP) and transducer operating point (OP) in a manner consistent with a mechanically-induced position change of the organ of Corti in the basal turn. Induced changes immediately recovered when injection ceased. In addition, effects of low-frequency bias tones on EP, SP and OP were enhanced during the injection of gel and remained hypersensitive after injection ceased. This is thought to result from the viscous gel mechanically limiting pressure shunting through the helicotrema. Cochlear microphonics measured as frequency was varied showed enhancement below 100 Hz but most notably in the sub-auditory range. Sensitivity to low-frequency biasing was also enhanced in animals with surgically-induced endolymphatic hydrops, suggesting that obstruction of the perilymphatic space by hydrops could contribute to the pathophysiology of this condition.

Visualizing soft tissue in the mammalian cochlea with coherent hard X-rays

This paper concerns an important aspect of current developments in medical and biological imaging: the possibility for imaging soft tissue at relatively high resolution in the micrometer range or better, without tedious and/or entirely destructive sample preparation. Structures with low absorption contrast have been visualized using in-line phase contrast imaging. The experiments have been performed at the Advanced Photon Source, a third generation source of synchrotron radiation. The source provides highly coherent X-ray radiation with high photon flux (>10(14) photons/s) at high photon energies (5-70 keV). Thick gerbil cochlear slices have been imaged and were compared with those obtained by light microscopy. Furthermore, intact gerbil cochleae have been imaged to identify the soft tissue structures involved in the hearing process. The present experimental approach was essential for visualizing the inner ear structures involved in the hearing process in an intact cochlea.

Inner ear drug delivery via a reciprocating perfusion system in the guinea pig

Rapid progress in understanding the molecular mechanisms associated with cochlear and auditory nerve degenerative processes offers hope for the development of gene-transfer and molecular approaches to treat these diseases in patients. For therapies based on these discoveries to become clinically useful, it will be necessary to develop safe and reliable mechanisms for the delivery of drugs into the inner ear, bypassing the blood-labyrinthine barrier. Toward the goal of developing an inner ear perfusion device for human use, a reciprocating microfluidic system that allows perfusion of drugs into the cochlear perilymph through a single inlet hole in scala tympani of the basal turn was developed. The performance of a prototype, extracorporeal reciprocating perfusion system in guinea pigs is described. Analysis of the cochlear distribution of compounds after perfusion took advantage of the place-dependent generation of responses to tones along the length of the cochlea. Perfusion with a control artificial perilymph solution had no effect. Two drugs with well-characterized effects on cochlear physiology, salicylate (5 mM) and DNQX (6,7-Dinitroquinoxaline-2,3-dione; 100 and 300 microM), reversibly altered responses. The magnitude of drug effect decreased with distance from the perfusion pipette for up to 10 mm, and increased with dose and length of application.

Evidence of tectorial membrane radial motion in a propagating mode of a complex cochlear model

Knowledge of vibratory patterns in the cochlea is crucial to understanding the stimulation of mechanosensory cells. Experiments to determine the motion of the cochlear partition and surrounding fluid are extremely challenging. As a result, the motion data are incomplete and often contradictory. The bending mechanism of hair bundles, thought to be related to the shear motion and endolymphatic flow between the tectorial membrane (TM) and reticular lamina (RL), is controversial. We, therefore, extend the frequency range of our previous hybrid analytical-finite-element approach to model the basal as well as apical regions of the guinea pig cochlea. We solve the fluid-solid interaction eigenvalue problem for the axial wavenumber, fluid pressure, and vibratory relative motions of the cochlear partition as a function of frequency. A simple monophasic vibratory mode of the basilar membrane is found at both ends of the cochlea. However, this simple movement is associated with a complex frequency-dependent relative deformation between the TM and the RL. We provide evidence of a radial component of TM motion that is out of phase with the RL and that facilitates the bending of outer hair cell stereocilia at appropriate frequencies at both the cochlear base and apex.

Calibrated magnetic resonance hydrometry: an in vitro study

PURPOSE

To demonstrate a quantitative approach to measuring fluid volumes with standard single shot RARE sequences.

MATERIALS AND METHODS

In phantom experiments, magnetic resonance hydrometry (MRH), in combination with various calibration phantoms (5 mL up to 500 mL) as internal standards, was used to quantify fluid volumes. In total, 16 volume phantoms were investigated with six different calibration phantoms. In addition, noise correction was implemented to correct the quantification results and to avoid the influence of random noise in the image.

RESULTS

All MR measurements show significant correlations of up to r = 0.99 (P <.05) with the real applied volume in the investigated phantoms. However, measurements of large volumes were more precise with large calibration phantoms. Noise reduction did not change the correlation between measured and real applied volumes, but did reduce the error of the measured volumes. Calibrated magnetic resonance hydrometry (cMRH) is able to quantify volumes of fluid fast and noninvasively. The volumes of the used calibration phantoms have to be at least in the order of magnitude of the volumes that are to be measured.

CONCLUSION

In vitro, cMRH using a single-shot rapid acquisition with refocused echoes (ssRARE) sequence and calibration phantoms is a fast and accurate method of quantifying steady amounts of fluid.

Quantitative anatomy of the guinea pig endolymphatic sac

The endolymphatic sac is believed to represent one of the primary loci for endolymph volume regulation in the inner ear. Quantitative analysis of physiologic measurements from the endolymphatic sac requires knowledge of the anatomy of the structure, specifically the luminal volume and the variation of cross-sectional area with distance along the sac. Recently techniques have become available to make these measurements. In the present study, fixed, isolated specimens of the guinea pig endolymphatic sac were imaged by high-resolution magnetic resonance microscopy (MRM) or by histological serial sections. Structures were reconstructed and quantified using image analysis software. In specimens imaged by MRM the endolymphatic sac volume, including tissue and lumen, was 359 nl for the intraosseous region and 106 nl for the extraosseous region, totaling 465 nl for the entire structure. The luminal volumes were 131 nl for the intraosseous region and 13 nl for the extraosseous region, totaling 144 nl. In histological specimens the volume, including tissue and lumen, was 414 nl for the intraosseous region and 121 nl for the extraosseous region, totaling 535 nl for the entire structure. The luminal volumes were 152 nl for the intraosseous region and 26 nl for the extraosseous region, totaling 179 nl. Differences in volume estimates obtained by the two methods were not statistically significant and variation was dominated by inter-specimen variation. Pooling the data, the total volume of the endolymphatic sac in the guinea pig including tissue and lumen was 506 nl (S.D. 100, n=17) and the volume of the lumen was 169 nl (S.D. 48, n=14).

Analysis of gentamicin kinetics in fluids of the inner ear with round window administration

HYPOTHESIS

That a theoretical basis for quantifying drug distribution in the inner ear with local applications can be established.

BACKGROUND

As methods of local drug delivery to the inner ear gain wider clinical acceptance it becomes important to establish how drugs are distributed in the ear as a function of time and for different delivery methods.

METHODS

The time course of gentamicin concentration in the inner ear fluids was simulated with a program that considered general pharmacokinetic principles and incorporated inner ear dimensions and drug dispersal processes, including diffusion, clearance, and intercompartmental exchange.

RESULTS

Cochlear fluid space dimensions of the chinchilla were derived from three-dimensional magnetic resonance images and were incorporated into the simulator. The published time course of gentamicin in vestibular perilymph of chinchillas was closely approximated by the adjustment of parameters defining round window membrane permeability, clearance, and interscala exchange. To simulate the time course, it was necessary for drug entry into the vestibule to be dominated by interscala exchange rather than longitudinal spread through the helicotrema. The effects of different round window delivery methods were also calculated. Perilymph drug levels and spatial distribution in the ear were shown to be markedly influenced by the time the applied drug remained in the middle ear.

CONCLUSION

The development of local inner ear drug application strategies requires consideration of inner ear pharmacokinetic characteristics, delivery methods, and therapeutic range of the drug.

High-resolution magnetic resonance imaging of human cochlea

OBJECTIVES

High-resolution MRI (MRI) of human inner ear structures provides several advantages over other imaging modalities. High-resolution visualization of inner ear ultrastructure in a noninvasive manner may provide important information about inner ear disease that is not obtainable in other ways. The study was performed to demonstrate the capabilities of MRI at high resolution on the human cochlea, vestibular structures, and facial nerve. Comparative analyses of MRI anatomy with that seen on histological dissection were made. The aim of the study was to define the anatomy of human cadaveric cochlea using a 9.4-Tesla magnetic resonance scanner, currently the most powerful magnetic resonance magnet available.

STUDY DESIGN

Experimental pilot study of cadaveric human cochleae.

METHODS

Serial scanning using a 9.4-Tesla magnetic resonance imager on normal preserved and fresh cadaveric inner ears was performed in different planes.

RESULTS

The images revealed detailed anatomy of the modiolus, utricle, saccule, semicircular canals, and facial nerve. Specifically, identifiable structures within the cochlea included the osseous spiral lamina, Reissner's membrane, membranous spiral lamina, spiral ligament, and others.

CONCLUSIONS

Data established through the acquisition of images from cadaver cochlea, facial nerve, and vestibular complex provide a foundation for developing steps for testing temporal bones and, eventually, patients with Meniere's disease and other inner ear disease. The present ongoing project will provide information on baseline images of the inner ear using high-resolution MRI.

Quantitative anatomy of the round window and cochlear aqueduct in guinea pigs

In order to analyze the entry of solutes through the round window membrane, a quantitative description of round window anatomy in relationship to scala tympani is required. High-resolution magnetic resonance microscopy was used to visualize the fluid spaces and tissues of the inner ear in three dimensions in isolated, fixed specimens from guinea pigs. Each specimen was represented as consecutive serial slices, with a voxel size of approximately 25 microm(3). The round window membrane, and its relationship to the terminal portion of scala tympani in the basal turn, was quantified in six specimens. In each image slice, the round window membrane and scala tympani were identified and segmented. The total surface area of the round window membrane averaged 1.18 mm(2) (S.D. 0.08, n=6). The length and variation of cross-sectional area as a function of distance for the cochlear aqueduct was determined in five specimens. The cochlear aqueduct was shown to enter scala tympani at the medial limit of the round window membrane, which corresponded to a distance of approximately 1 mm from the end of the scala when measured along its mid-point. These data are of value in simulating drug and other solute movements in the cochlear fluids and have been incorporated into a public-domain simulation program available at http://oto.wustl.edu/cochlea/.

Diagnosis of endolymphatic hydrops in vivo with magnetic resonance imaging

HYPOTHESIS

High-resolution magnetic resonance imaging (MRI) at 1.5 T preferentially enhances the perilymph over endolymph after administration of contrast with gadodiamide, which allows for differentiation of the membranous labyrinth. Furthermore, this imaging allows for the detection of endolymphatic hydrops.

BACKGROUND

Endolymphatic hydrops is believed to be associated with a number of ear diseases, including Ménière's disease. Although the pathologic changes of overaccumulation of endolymph in the inner ear are obvious on postmortem histologic examination of the temporal bone, they have yet to be observed in a living organism. Previous attempts to visualize this condition with high-resolution contrast-enhanced computed tomographic imaging and MRI have been unsuccessful.

METHODS

Healthy pigmented guinea pigs underwent a unilateral surgical ablation of the endolymphatic sac to create endolymphatic hydrops in the ear. High-resolution temporal bone imaging was performed by use of a 1.5-T MRI system. Two-dimensional images were acquired by a spin-echo technique with and without contrast enhancement by gadodiamide.

RESULTS

T1-weighted gadodiamide contrast-enhanced MRI of the midmodiolar level of the cochlea demonstrated that the perilymph appeared to be preferentially enhanced relative to the endolymph, resulting in a clear distinction between the scalae of the inner ear. The contrast-enhanced T1-weighted MRI of the midmodiolar level of the hydropic cochlea demonstrated a significantly enlarged scala media in comparison with the normal cochlea in the same animal.

CONCLUSION

The demonstration of endolymphatic hydrops has been possible for the first time in vivo by the use of a standard 1.5-T MRI system. This research has important clinical implications: Specifically, this technique may allow for the noninvasive diagnosis of Ménière's disease.

Quantification of solute entry into cochlear perilymph through the round window membrane

The administration of drugs to the inner ear via the round window membrane is becoming more widely used for both clinical and experimental purposes. The actual drug levels achieved in different regions of the inner ear by this method have not been established. The present study has made use of simulations of solute movements in the cochlear fluids to describe the distribution of a marker solute in the guinea pig cochlear fluid spaces. Simulation parameters were derived from experimental measurements using a marker ion, trimethylphenylammonium (TMPA). The distribution of this ion in the cochlea was monitored without volume disturbance using TMPA-selective microelectrodes sealed into the first and second turns of scala tympani (ST). TMPA was applied to perilymph by irrigation of the intact round window membrane with 2 mM solution. At the end of a 90 min application period, TMPA in the first turn, 1.4 mm from the base of ST, reached an average concentration of 330 microM (standard deviation (S.D.) 147 microM, n = 8). TMPA in the second turn, 7.5 mm from the base of ST reached a concentration of 15 microM (S.D. 33 microM, n = 5). The measured time courses of TMPA concentration change were interpreted using the Washington University Cochlear Fluids Simulator (V 1.4), a public-domain program available on the internet at http ://oto.wustl.edu/cochlea/. Simulations with parameters producing concentration time courses comparable to those measured were: (1) round window permeability: 1.9 x 10(-80 cm/s; (2) ST clearance half-time: 60 min; (3) longitudinal perilymph flow rate: 4.4 nl/min, directed from base to apex. Solute concentrations in apical regions of the cochlea were found to be determined primarily by the rate at which the solute diffuses, balanced by the rate of clearance of the solute from perilymph. Longitudinal perilymph flow was not an important factor in solute distribution unless the bony otic capsule was perforated, which rapidly caused substantial changes to solute distribution. This study demonstrates the basic processes by which substances are distributed in the cochlea and provides a foundation to understand how other applied substances will be distributed in the ear.

Volumetric and dimensional analysis of the guinea pig inner ear

The objective of this study was to provide accurate volumetric data on the fluid spaces and soft tissue in the guinea pig inner ear by measuring all histologic serial sections by means of Metamorph Imaging Software at 400x to 1,000x magnification. The total endolymph volume of the inner ear was 4.691 mm3, of which 1.501 mm3 was in the cochlea, 3.090 mm3 in the vestibular labyrinth, and 0.100 mm3 in the endolymphatic duct and sac. The total perilymph volume was 15.938 mm3, of which 8.867 mm3 was in the cochlea and 7.071 mm3 in the vestibular labyrinth. The volume of the organ of Corti per millimeter length increased toward the apex, but the volumes of the stria vascularis, spiral ligament, and spiral limbus decreased. The volume of the macula utriculi was larger than that of the macula sacculi. The measurement of the luminal surface area of the stria vascularis was 3.944 mm2, and that of the vestibular dark cells was 5.772 mm2.

Hydrops in the cochlea can be induced by sound as well as by static pressure

The Reissner's membrane (RM) was visualised by confocal microscopy in the isolated temporal bone of the guinea pig. The function of the organ was followed by measuring its physiological response. Static pressure applied in the basal coil caused a distention of the RM in the apical coil into the scala vestibuli. The sensitivity to a test tone was reduced. When the pressure was relieved, the RM returned to its original position and the response recovered. If the increased pressure was maintained, the RM would bulge further. The RM could then be reversibly stretched and return gradually, with a delay, to its original position. Alternatively, it could be over-stretched and return with an over-shoot past its original position toward the organ of Corti. In response to repetitive tone pulses of above 80 dB, hydrops of the RM also developed. This was accompanied by a reduced sensitivity. A slow recovery to the original position, or over-shoot, and return of responsiveness could be seen. Above 106 dB sustained loss was generally seen. It is concluded that the RM can accommodate increased scala media pressure by distention. This will relieve the organ of Corti from part of the pressure and may protect the organ from trauma.

P2X receptor-mediated changes in cochlear potentials arising from exogenous adenosine 5'-triphosphate in endolymph

Our previous studies have determined the presence of adenosine 5'-triphosphate (ATP) in the cochlear fluids and shown that extracellular ATP introduced into the endolymphatic compartment of the guinea pig cochlea has a significant dose-dependent suppressive effect on both endocochlear potential (EP) and cochlear microphonic (CM), which is mediated via P2 receptors. In the present study, the influence of P2 receptor agonists and antagonists on this suppressive effect was investigated to characterise the subtypes of P2 receptor mediating the ATP-induced effect on cochlear function. Using a double-barreled pipette attached to a pressure injector, small volumes (2-10 nl) of ATP (0.01-1 mM) and P2 receptor agonists or P2 receptor antagonists in artificial endolymph were introduced into the scala media of the first (basal) and third turns of the guinea pig cochlea, while the EP and CM were monitored. ATP and P2 receptor agonists (5x10(-14)-1x10(-11)cibacron blue. Neither adenosine nor uridine 5'-triphosphate (2x10(-13)-2x10(-11) moles) nor the P2 receptor antagonists on their own had any effect on EP and CM. The ATP effect on the potentials was greater at the third cochlear turn when compared to the first turn. These results provide evidence that in the endolymphatic compartment of the guinea pig, the extracellular ATP effect on cochlear function is likely mediated through an interaction with P2 receptors which assemble as ATP-gated ion channels.

Cochlear fluid space dimensions for six species derived from reconstructions of three-dimensional magnetic resonance images

OBJECTIVES

To establish the dimensions and volumes of the cochlear fluid spaces.

STUDY DESIGN

Fluid space volumes, lengths, and cross-sectional areas were derived for the cochleas from six species: human, guinea pig, bat, rat, mouse, and gerbil.

METHODS

Three-dimensional reconstructions of the fluid spaces were made from magnetic resonance microscopy (MRM) images. Consecutive serial slices composed of isotropic voxels (25 microm3) representing the entire volume of fixed, isolated cochleas were obtained. The boundaries delineating the fluid spaces, including Reissner's membrane, were resolved for all specimens, except for the human, in which Reissner's membrane was not consistently resolved. Three-dimensional reconstructions of the endolymphatic and perilymphatic fluid spaces were generated. Fluid space length and variation of cross-sectional area with distance were derived by an algorithm that followed the midpoint of the space along the length of the spiral. The total volume of each fluid space was derived from a voxel count for each specimen.

RESULTS

Length, volume, and cross-sectional areas are provided for six species. In all cases, the length of the endolymphatic fluid space was consistently longer than that of either perilymphatic scala, primarily as a result of a greater radius of curvature. For guinea pig specimens, the measured volumes of the fluid spaces were considerably lower than those suggested by previous reports based on histological data.

CONCLUSIONS

The quantification of cochlear fluid spaces provided by this study will enable the more accurate calculation of drug and other solute movements in fluids of the inner ear during experimental or clinical manipulations.

Three-dimensional reconstruction of the membranous vestibular labyrinth in the toadfish, Opsanus tau

Membranous vestibular labyrinths from the oyster toadfish, Opsanus tau, were fixed, dissected from the animal, stained, and embedded in rectangular blocks of clear histological resin. Photomicrographs of complete embedded labyrinths were taken from six orthogonal directions and used to construct three-dimensional (3D) geometrical models of the semicircular canals, ampullae, utricular vestibule and common crus. Membraneous ducts and ampullae were modeled using a set of cross-sectional elliptical curves laced together to generate curved tubular models of each structure. The ensemble of these curved tubes was used to generate a complete 3D reconstruction of the outside surface of the membranous labyrinth. When viewed from six orthogonal directions, reconstructions closely matched the embedded tissue. Dimensions of the reconstruction and histological sections were compared to measurements of fresh tissue taken from the same animals prior to fixation and used to correct the reconstructions for tissue shrinkage. Results provide estimates of the endolymphatic volumes, local cross-sectional areas and elliptical eccentricities as well as 3D orientations of the geometric canal planes relative to the skull. Ten micrometer histological sections of the material were also prepared to measure wall thickness in various regions of the labyrinth.

Fixation-induced shrinkage of Reissner's membrane and its potential influence on the assessment of endolymph volume

The quantification of endolymph volume by histological techniques or by magnetic resonance (MR) microscopy requires the inner ear to be first treated with chemical fixatives. If the fixative induces soft-tissue shrinkage, it would tend to return a distended Reissner's membrane towards a straight position, since this membrane is anchored to bone at its medial and lateral edges. The goal of this study was to determine the degree of Reissner's membrane shrinkage induced by different fixation protocols to establish methods which minimize tissue shrinkage. Fragments of fresh Reissner's membrane were dissected from isolated cochleae in an artificial perilymph. Specimens were viewed with an inverted microscope during infusion of fixatives, and changes recorded on video tape. Size changes of the specimen were quantified, usually over a 20 min period. Heidenhain-Susa, a fixative which is widely used in histological studies of hydropic cochleae, caused substantial shrinkage of Reissner's membrane, decreasing the length of specimens by an average of 15.1%. Other fixation procedures induced far less shrinkage. The use of 3.1% glutaraldehyde in Hanks' balanced salt solution produced a mean length decrease of only 0.3%. The inclusion in the fixation medium of 4.5% mercuric chloride, corresponding to the concentration which is present in Heidenhain-Susa and which acts to increase the contrast of Reissner's membrane in MR microscopy, contributes significantly to specimen shrinkage. We can conclude that the degree of endolymphatic hydrops may be underestimated in specimens fixed with media containing high levels of mercuric chloride.

Micro-magnetic resonance imaging of the inner ear in the guinea pig

We applied a magnetic resonance microscopy at 7.05 T with a gradient coil unit to image the fine structure of the guinea pig cochlea. First, a three-dimensional MR image of the surface of the cochlea was obtained to select the location of cross-sectional images. Then, cross-sectional images of the basal turn, the second turn and the apical turn of the cochlea were obtained. Based upon the different protein concentrations of the endolymph vs the perilymph, the scala vestibuli, scala tympani and the cochlear duct could be clearly distinguished. This allowed a determination of the location of both the basilar membrane and Reissner's membrane. We raise the possibility that MRM may become useful in the diagnosis of endolymphatic hydrops (Meniere's disease).