Differential passage of gadolinium through the mouse inner ear barriers evaluated with 4.7T MRI

Magnetic Targeting of Gadolinium Contrast to Enhance MRI of the Inner Ear in Endolymphatic Hydrops

OBJECTIVES

1. Determine the feasibility and efficiency of local magnetic targeting delivery of gadolinium (Gad) contrast to the inner ear in rodents. 2. Assess any potential ototoxicity of magnetic targeting delivery of Gad in the inner ear. 3. Study the utility of magnetic targeting delivery of Gad to visualize and quantify endolymphatic hydrops (EH) in a transgenic mouse model.

STUDY DESIGN

Controlled in vivo animal model study.

METHODS

Paramagnetic Gad was locally delivered to the inner ear using the magnetic targeting technique in both rat and mouse models. Efficiency of contrast delivery was assessed using magnetic resonance imaging (MRI). Ototoxicity of Gad was examined with histology of the cochlea and functional audiological tests. The Phex mouse model was used to study EH, hearing loss, and balance dysfunction. Magnetic targeting delivery of Gad contrast was used in the Phex mouse model to visualize the effects of EH using MRI.

RESULTS

Magnetic targeting improved the delivery of Gad to the inner ear and the technique was reproducible in both rat and mouse models. The delivery method did not result in microstructural damage or any significant hearing loss in a normal animal. Magnetic targeting of Gad in the Phex mouse model allowed detailed visualization and quantification of EH.

CONCLUSION

This study provided the first evidence of the effectiveness and efficiency of the local magnetic targeting delivery of gadolinium contrast to the inner ear and its application to the visualization and quantification of EH. Laryngoscope, 133:914-923, 2023.

Refining surgical techniques for efficient posterior semicircular canal gene delivery in the adult mammalian inner ear with minimal hearing loss

Hearing loss is a common disability affecting the world's population today. While several studies have shown that inner ear gene therapy can be successfully applied to mouse models of hereditary hearing loss to improve hearing, most of these studies rely on inner ear gene delivery in the neonatal age, when mouse inner ear has not fully developed. However, the human inner ear is fully developed at birth. Therefore, in order for inner ear gene therapy to be successfully applied in patients with hearing loss, one must demonstrate that gene delivery can be safely and reliably performed in the mature mammalian inner ear. In this study, we examine the steps involved in posterior semicircular canal gene delivery in the adult mouse inner ear. We find that the duration of perilymphatic leakage and injection rate have a significant effect on the post-surgical hearing outcome. Our results show that although AAV2.7m8 has a lower hair cell transduction rate in adult mice compared to neonatal mice at equivalent viral load, AAV2.7m8 is capable of transducing the adult mouse inner and outer hair cells with high efficiency in a dose-dependent manner.

New Frontiers in Managing the Dizzy Patient

Despite progress in vestibular research in the last 20 years, much remains poorly understood about vestibular pathophysiology and its management. A shared language is a critical first step in understanding vestibular disorders and is under development. Telehealth will continue for patients with dizziness, and ambulatory monitoring of nystagmus will become a diagnostic tool. In the next 2 decades, it is anticipated that vestibular perceptual threshold testing will become common in tertiary centers, imaging with improved spatial resolution will yield better understanding of vestibular pathophysiology, and that vestibular implants will become a part of clinical practice.

MRI With Gadolinium as a Measure of Blood-Labyrinth Barrier Integrity in Patients With Inner Ear Symptoms: A Scoping Review

Objective: Capillaries within the inner ear form a semi-permeable barrier called the blood-labyrinth barrier that is less permeable than capillary barriers elsewhere within the human body. Dysfunction of the blood-labyrinth barrier has been proposed as a mechanism for several audio-vestibular disorders. There has been interest in using magnetic resonance imaging (MRI) with intravenous gadolinium-based contrast agents (GBCA) as a marker for the integrity of the blood labyrinth barrier in research and clinical settings. This scoping review evaluates the evidence for using intravenous gadolinium-enhanced MRI to assess the permeability of the blood-labyrinth barrier in healthy and diseased ears.

Methods: A systematic search was conducted of three databases: PubMed, EMBASE, CINAHL PLUS. Studies were included that used GBCA to study the inner ear and permeability of the blood-labyrinth barrier. Data was collected on MRI protocols used and inner ear enhancement patterns of healthy and diseased ears in both human and animal studies.

Results: The search yielded 14 studies in animals and 53 studies in humans. In healthy animal and human inner ears, contrast-enhanced MRI demonstrated gradual increase in inner ear signal intensity over time that was limited to the perilymph. Signal intensity peaked at 100 min in rodents and 4 h in humans. Compared to controls, patients with idiopathic sudden sensorineural hearing loss and otosclerosis had increased signal intensity both before and shortly after GBCA injection. In patients with Ménière's disease and vestibular schwannoma, studies reported increased signal at 4 h, compared to controls. Quality assessment of included studies determined that all the studies lacked sample size justification and many lacked adequate control groups or blinded assessors of MRI.

Conclusions: The included studies provided convincing evidence that gadolinium crosses the blood-labyrinth barrier in healthy ears and more rapidly in some diseased ears. The timing of increased signal differs by disease. There was a lack of evidence that these findings indicate general permeability of the blood-labyrinth barrier. Future studies with consistent and rigorous methods are needed to investigate the relationship between gadolinium uptake and assessments of inner ear function and to better determine whether signal enhancement indicates permeability for molecules other than gadolinium.

Third Window Syndrome: Surgical Management of Cochlea-Facial Nerve Dehiscence

Objective: This communication is the first assessment of outcomes after surgical repair of cochlea-facial nerve dehiscence (CFD) in a series of patients. Pre- and post-operative quantitative measurement of validated survey instruments, symptoms, diagnostic findings and anonymous video descriptions of symptoms in a cohort of 16 patients with CFD and third window syndrome (TWS) symptoms were systematically studied.

Study design: Observational analytic case-control study.

Setting: Quaternary referral center.

Patients: Group 1 had 8 patients (5 children and 3 adults) with CFD and TWS who underwent surgical management using a previously described round window reinforcement technique. Group 2 had 8 patients (2 children and 6 adults) with CFD who did not have surgical intervention.

Interventions: The Dizziness Handicap Inventory (DHI) and Headache Impact Test (HIT-6) were administered pre-operatively and post-operatively. In addition, diagnostic findings of comprehensive audiometry, cervical vestibular evoked myogenic potential (cVEMP) thresholds and electrocochleography (ECoG) were studied. Symptoms before and after surgical intervention were compared.

Main outcome measures: Pre- vs. post-operative DHI, HIT-6, and audiometric data were compared statistically. The thresholds and amplitudes for cVEMP in symptomatic ears, ears with cochlea-facial nerve dehiscence and ears without CFD were compared statistically.

Results: There was a highly significant improvement in DHI and HIT-6 at pre- vs. post-operative (p < 0.0001 and p < 0.001, respectively). The age range was 12.8–52.9 years at the time of surgery (mean = 24.7 years). There were 6 females and 2 males. All 8 had a history of trauma before the onset of their symptoms. The mean cVEMP threshold was 75 dB nHL (SD 3.8) for the operated ear and 85.7 dB (SD 10.6) for the unoperated ear. In contrast to superior semicircular canal dehiscence, where most ears have abnormal ECoG findings suggestive of endolymphatic hydrops, only 1 of 8 operated CFD ears (1 of 16 ears) had an abnormal ECoG study.

Conclusions: Overall there was a marked improvement in DHI, HIT-6 and symptoms post-operatively. Statistically significant reduction in cVEMP thresholds was observed in patients with radiographic evidence of CFD. Surgical management with round window reinforcement in patients with CFD was associated with improved symptoms and outcomes measures.

Comprehensive comparison of MR image quality between intratympanic and intravenous gadolinium injection using 3D real IR sequences

Background: Intratympanic and intravenous gadolinium administration is used to visualise endolymphatic hydrops. Aims/objectives: The goal of this study was to compare the image quality between intratympanic (IT-method) and intravenous (IV-method) gadolinium administration using three-dimensional inversion recovery with real reconstruction (3D real IR) sequences. Materials and methods: A number of 152 patients with Meniere's disease were included. The 3D real IR sequence was performed 24 h after IT administration or 4 h after IV administration. The detection rate of endolymphatic hydrops, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of the two methods were compared. Specifically, the average image scores of the two methods were evaluated by two radiologists. Results: The SNR_ROI and CNRs of the IT-method were higher than those of the IV-method, whereas no significant difference between the IT-method and IV-method with regard to the SNR_B was found. The average image scores were 3.49 ± 0.12 and 3.30 ± 0.12 for the IT-method and IV-method, respectively (p = .229). No statistically significant difference was found between two methods in terms of the detection rate of endolymphatic hydrops. Conclusions and significance: IT-method images can display endolymphatic hydrops more precisely than IV-method images. The IV-method can be used as an alternative to the IT-method in clinical applications to some extent.

Investigation of inner ear anatomy in mouse using X-ray phase contrast tomography

A thorough understanding of inner ear anatomy is important for investigators. However, investigation of the mouse inner ear is difficult due to the limitations of imaging techniques. X-ray phase contrast tomography increases contrast 100-1,000 times compared with conventional X-ray imaging. This study aimed to investigate inner ear anatomy in a fresh post-mortem mouse using X-ray phase contrast tomography and to provide a comprehensive atlas of microstructures with less tissue deformation. All experiments were performed in accordance with our institution's guidelines on the care and use of laboratory animals. A fresh mouse cadaver was scanned immediately after sacrifice using an inline phase contrast tomography system. Slice images were reconstructed using a filtered back-projection (FBP) algorithm. Standardized axial and coronal planes were adjusted with a multi-planar reconstruction method. Some three-dimensional (3D) objects were reconstructed by surface rendering. The characteristic features of microstructures, including otoconia masses of the saccular and utricular maculae, superior and inferior macula cribrosae, single canal, modiolus, and osseous spiral lamina, were described in detail. Spatial positions and relationships of the vestibular structures were exhibited in 3D views. This study investigated mouse inner ear anatomy and provided a standardized presentation of microstructures. In particular, otoconia masses were visualized in their natural status without contrast for the first time. The comprehensive anatomy atlas presented in this study provides an excellent reference for morphology studies of the inner ear.

Ultra-high-field (9.4 T) MRI Analysis of Contrast Agent Transport Across the Blood-Perilymph Barrier and Intrastrial Fluid-Blood Barrier in the Mouse Inner Ear

HYPOTHESIS

Effective paramagnetic contrast agent for the penetration of the perilymphatic spaces of the scala tympani, scala vestibuli, and scala media of the mouse inner ear can be determined using intravenous injection of various gadolinium (Gd) complexes and ultra-high-field magnetic resonance imaging (MRI) at 9.4 Tesla.

BACKGROUND

A number of contrast agents have been explored in experimental high-field MRI to determine the most effective Gd complex for ideal signal-to-noise ratio and maximal visualization of the in vivo mammalian inner ear in analyzing the temporal and spatial parameters involved in drug penetration of the blood-perilymph barrier and intrastrial fluid-blood barrier in the mouse model using MRI.

METHODS

Gadoteric acid (Dotarem), Gadobutrol (Gadovist), Gadodiamide (Omniscan), Gadopent acid (Magnevist), and Mangafodipir (Teslascan) were administered intravenously using the tail vein of 60 Balb/C mice. High-resolution T1 images of drug penetration were acquired with a horizontal 9.4 T Agilent magnet after intravenously injection. Signal intensity was used as a metric of temporal and spatial parameters of drug delivery and penetration of the perilymphatic and endolymphatic spaces.

RESULTS

ANOVA analysis of the area under the curve of intensity enhancement in perilymph revealed a significant difference (p < 0.05) in the scalae uptake using different contrast agents (F (3,25) = 3.54, p = 0.029). The Gadoteric acid complex Dotarem was found to be the most effective Gd compound in terms of rapid, morphological enhancement for analysis of the temporal, and spatial distribution in the perilymphatic space of the inner ear.

CONCLUSION

Gadoteric acid (Dotarem) demonstrated efficacy as a contrast agent for enhanced visualization of the perilymphatic spaces of the inner ear labyrinthine in the mouse, including the scala tympani and scala vestibuli of the cochlea, and the semicircular canals of the vestibular apparatus. These findings may inform the clinical application of Gd compounds in patients with inner ear fluid disorders and vertigo.

Challenges and opportunities in developing targeted molecular imaging to determine inner ear defects of sensorineural hearing loss

The development of inner ear gene carriers and delivery systems has enabled genetic defects to be repaired and hearing to be restored in mouse models. Today, promising advances in translational therapies provide confidence that targeted molecular therapy for inner ear diseases will be developed. Unfortunately, the currently available non-invasive modalities, such as Computerized Tomography scan or Magnetic Resonance Imaging provide insufficient resolution to identify most pathologies of the human inner ear, even when the current generation of contrast agents is utilized. The development of targeted contrast agents may play a critical role in determining the cause of, and treatment for, sensorineural hearing loss. Such agents should be able to pass through the cochlea barriers, possess minimal cytotoxicity, and easily conjugate to a targeting agent, without distorting the anatomic details. This review focuses on a series of contrast agents which may fit these criteria for potential clinical application.

Magnetic resonance imaging findings in Ménière's disease

OBJECTIVES

To identify and evaluate cranial magnetic resonance imaging findings associated with Ménière's disease.

METHODS

Seventy-eight patients with a documented diagnosis of Ménière's disease and 35 controls underwent 1.5 T or 3 T magnetic resonance imaging of the brain. Patients also underwent otological, vestibular and audiometric examinations.

RESULTS

Lack of visualisation of the left and right vestibular aqueducts was identified as statistically significant amongst Ménière's disease patients (left, p = 0.0001, odds ratio = 0.02; right, p = 0.0004, odds ratio = 0.03). Both vestibular aqueducts were of abnormal size in the Ménière's disease group, albeit with left-sided significance (left, p = 0.008, odds ratio = 10.91; right, p = 0.49, odds ratio = 2.47).

CONCLUSION

Lack of vestibular aqueduct visualisation on magnetic resonance imaging was statistically significant in Ménière's disease patients compared to the general population. The study findings suggest that magnetic resonance imaging can be useful to rule out retrocochlear pathology and provide radiological data to support the clinical diagnosis of Ménière's disease.

Rhesus Cochlear and Vestibular Functions Are Preserved After Inner Ear Injection of Saline Volume Sufficient for Gene Therapy Delivery

Sensorineural losses of hearing and vestibular sensation due to hair cell dysfunction are among the most common disabilities. Recent preclinical research demonstrates that treatment of the inner ear with a variety of compounds, including gene therapy agents, may elicit regeneration and/or repair of hair cells in animals exposed to ototoxic medications or other insults to the inner ear. Delivery of gene therapy may also offer a means for treatment of hereditary hearing loss. However, injection of a fluid volume sufficient to deliver an adequate dose of a pharmacologic agent could, in theory, cause inner ear trauma that compromises functional outcome. The primary goal of the present study was to assess that risk in rhesus monkeys, which closely approximates humans with regard to middle and inner ear anatomy. Secondary goals were to identify the best delivery route into the primate ear from among two common surgical approaches (i.e., via an oval window stapedotomy and via the round window) and to determine the relative volumes of rhesus, rodent, and human labyrinths for extrapolation of results to other species. We measured hearing and vestibular functions before and 2, 4, and 8 weeks after unilateral injection of phosphate-buffered saline vehicle (PBSV) into the perilymphatic space of normal rhesus monkeys at volumes sufficient to deliver an atoh1 gene therapy vector. To isolate effects of injection, PBSV without vector was used. Assays included behavioral observation, auditory brainstem responses, distortion product otoacoustic emissions, and scleral coil measurement of vestibulo-ocular reflexes during whole-body rotation in darkness. Three groups (N = 3 each) were studied. Group A received a 10 μL transmastoid/trans-stapes injection via a laser stapedotomy. Group B received a 10 μL transmastoid/trans-round window injection. Group C received a 30 μL transmastoid/trans-round window injection. We also measured inner ear fluid space volume via 3D reconstruction of computed tomography (CT) images of adult C57BL6 mouse, rat, rhesus macaque, and human temporal bones (N = 3 each). Injection was well tolerated by all animals, with eight of nine exhibiting no signs of disequilibrium and one animal exhibiting transient disequilibrium that resolved spontaneously by 24 h after surgery. Physiologic results at the final, 8-week post-injection measurement showed that injection was well tolerated. Compared to its pretreatment values, no treated ear's ABR threshold had worsened by more than 5 dB at any stimulus frequency; distortion product otoacoustic emissions remained detectable above the noise floor for every treated ear (mean, SD and maximum deviation from baseline: -1.3, 9.0, and -18 dB, respectively); and no animal exhibited a reduction of more than 3 % in vestibulo-ocular reflex gain during high-acceleration, whole-body, passive yaw rotations in darkness toward the treated side. All control ears and all operated ears with definite histologic evidence of injection through the intended site showed similar findings, with intact hair cells in all five inner ear sensory epithelia and intact auditory/vestibular neurons. The relative volumes of mouse, rat, rhesus, and human inner ears as measured by CT were (mean ± SD) 2.5 ± 0.1, 5.5 ± 0.4, 59.4 ± 4.7 and 191.1 ± 4.7 μL. These results indicate that injection of PBSV at volumes sufficient for gene therapy delivery can be accomplished without destruction of inner ear structures required for hearing and vestibular sensation.

Inner ear barriers to nanomedicine-augmented drug delivery and imaging

There are several challenges to inner ear drug delivery and imaging due to the existence of tight biological barriers to the target structure and the dense bone surrounding it. Advances in imaging and nanomedicine may provide knowledge for overcoming the existing limitations to both the diagnosis and treatment of inner ear diseases. Novel techniques have improved the efficacy of drug delivery and targeting to the inner ear, as well as the quality and accuracy of imaging this structure. In this review, we will describe the pathways and biological barriers of the inner ear regarding drug delivery, the beneficial applications and limitations of the imaging techniques available for inner ear research, the behavior of engineered nanomaterials in inner ear applications, and future perspectives for nanomedicine-based inner ear imaging.

Dexamethasone Is One of the Factors Minimizing the Inner Ear Damage from Electrode Insertion in Cochlear Implantation

The aim of this study was to investigate the efficacy of preoperative and intraoperative steroid administration for inner ear protection in cochlear implantation (CI). Nineteen subjects who underwent CI were included in the study, and 10 subjects were enrolled as controls (steroid-administered group, n = 19; control group, n = 10). Dexamethasone (dexamethasone sodium phosphate, 5 mg/ml) was systemically administered preoperatively (1 ml) and topically applied during CI (0.5 ml). The extent of hearing preservation (HP) after CI and the change in the bithermal caloric response were evaluated. Hearing level was calculated using mean thresholds [(250 Hz + 500 Hz + 1,000 Hz + 2,000 Hz)/4]. Preoperative hearing thresholds were similar in the steroid-administered and control groups (100.92 ± 12.60 vs. 103.29 ± 14.39 dB, p = 0.650). The mean thresholds significantly increased in both groups after surgery (108.46 ± 14.08 dB, p = 0.006, for the steroid-administered group; 117.50 ± 6.34 dB, p = 0.027, for the control group), and the difference between the groups was also significant (p = 0.027). The postoperative shift in the hearing thresholds at frequencies of 500 and 1,000 Hz was significant in the steroid-administered group and that at the frequencies of 500, 1,000 and 2,000 Hz was significant in the control group. However, the extent of the shift in hearing threshold levels at each frequency was not significantly different between the groups. Preservation of hearing thresholds was compared between the groups, and there were significantly more subjects with complete and partial HP in the steroid-administered group than in the control group (p = 0.008). The preoperative caloric response was maintained after CI in the steroid-administered group. This study suggests that the perioperative use of a steroid could minimize the inner ear damage after CI.

Postaurical injection is a systemic delivery supported by symmetric distribution of Gd-DOTA in both the ipsilateral and contralateral ears

Postaurical injection of therapeutics was recently applied in clinical practice to treat inner ear diseases based on supposed existence of a direct channel from the postaurical area to the inner ear. Doubting on the associated reports and aiming to provide evidence on the inner ear uptake mechanism, the present study tracked the dynamic distribution of gadolinium-tetra-azacyclo-dodecane-tetra-acetic acid (Gd-DOTA) in rat inner ears after postaurical injection using MRI. A targeted tympanic medial wall delivery was utilized as control. The results showed that, at the early time points after postaurical injection, Gd-DOTA distributed mainly in tissues surrounding the bulla, temporal bone and skull and neck space. In the inner ear, there was gradual uptake of Gd-DOTA on both the ipsilateral and contralateral sides with equal signal intensities. There was no sign of direct channel carrying the agent from the postaurical area to the inner ear. Targeted tympanic medial wall delivery induced significantly greater uptake of Gd-DOTA in the inner ear than did postaurical injection. At 30 min post-administration, targeted tympanic medial wall delivery yielded 4.6-folds higher signal intensity than did postaurical injection. The total dose of Gd-DOTA delivered by the targeted tympanic medial wall approach was only 0.1% of that delivered by postaurical injection. In conclusion, postaurical injection is a systemic administration, which is similar to hypodermic injection, rather than a focal delivery method. By contraries, targeted tympanic medial wall delivery induces fast and abundant uptake of Gd-DOTA in the ipsilateral inner ear without significant distribution in unwanted areas.

An Overview of Nanoparticle Based Delivery for Treatment of Inner Ear Disorders

Nanoparticles offer new possibilities for inner ear treatment as they can carry a variety of drugs, protein, and nucleic acids to inner ear. Nanoparticles are equipped with several functions such as targetability, immuno-transparency, biochemical stability, and ability to be visualized in vivo and in vitro. A group of novel peptides can be attached to the surface of nanoparticles that will enhance the cell entry, endosomal escape, and nuclear targeting. Eight different types of nanoparticles with different payload carrying strategies are available now. The transtympanic delivery of nanoparticles indicates that, depending on the type of nanoparticle, different migration pathways into the inner ear can be employed, and that optimal carriers can be designed according to the intended cargo. The use of nanoparticles as drug/gene carriers is especially attractive in conjunction with cochlear implantation or even as an inclusion in the implant as a drug/gene reservoir.

Experimental Fusion of Contrast Enhanced High-Field Magnetic Resonance Imaging and High-Resolution Micro-Computed Tomography in Imaging the Mouse Inner Ear

Objective:

Imaging cochlear, vestibular, and 8th cranial nerve abnormalities remains a challenge. In this study, the membranous and osseous labyrinths of the wild type mouse inner ear were examined using volumetric data from ultra high-field magnetic resonance imaging (MRI) with gadolinium contrast at 9.4 Tesla and high-resolution micro-computed tomography (µCT) to visualize the scalae and vestibular apparatus, and to establish imaging protocols and parameters for comparative analysis of the normal and mutant mouse inner ear.

Methods:

For in vivo MRI acquisition, animals were placed in a Milleped coil situated in the isocenter of a horizontal 9.4 T Varian magnet. For µCT examination, cone beam scans were performed ex vivo following MRI using the µCT component of a nanoScan PET/CT in vivo scanner.

Results:

The fusion of Gd enhanced high field MRI and high-resolution µCT scans revealed the dynamic membranous labyrinth of the perilymphatic fluid filled scala tympani and scala vestibule of the cochlea, and semicircular canals of the vestibular apparatus, within the µCT visualized contours of the contiguous osseous labyrinth. The ex vivo µCT segmentation revealed the surface contours and structural morphology of each cochlea turn and the semicircular canals in 3 planes.

Conclusions:

The fusion of ultra high-field MRI and high-resolution µCT imaging techniques were complementary, and provided high-resolution dynamic and static visualization of the complex morphological features of the normal mouse inner ear structures, which may offer a valuable approach for the investigation of cochlear and vestibular abnormalities that are associated with birth defects related to genetic inner ear disorders in humans.

Vasopressin induces endolymphatic hydrops in mouse inner ear, as evaluated with repeated 9.4 T MRI

From histopathological specimens, endolymphatic hydrops has been demonstrated in association with inner ear disorders. Recent studies have observed findings suggestive of hydrops using MRI in humans. Previous studies suggest that vasopressin may play a critical role in endolymph homeostasis and may be involved in the development of Ménière's disease. In this study we evaluate the effect of vasopressin administration in vivo in longitudinal studies using two mouse strains. High resolution MRI at 9.4 T in combination with intraperitoneally delivered Gadolinium contrast, was performed before and after chronic subcutaneous administration of vasopressin via mini-osmotic pumps in the same mouse. A development of endolymphatic hydrops over time could be demonstrated in C57BL6 mice (5 mice, 2 and 4 weeks of administration) as well as in CBA/J mice (4 mice, 2 weeks of administration; 6 mice, 3 and 4 weeks of administration). In most C57BL6 mice hydrops developed first after more than 2 weeks while CBA/J mice had an earlier response. These results may suggest an in vivo model for studying endolymphatic hydrops and corroborates the future use of MRI as a tool in the diagnosis and treatment of inner ear diseases, such as Ménière's disease. MRI may also be developed as a critical tool in evaluating inner ear homeostasis in genetically modified mice, to augment the understanding of human disease.

Diverse patterns of perilymphatic space enhancement in the rat inner ear after intratympanic injection of two different types of gadolinium: a 9.4-tesla magnetic resonance study

OBJECTIVE

To compare the quality of perilymphatic enhancement in the rat inner ear after intratympanic injection of two types of gadolinium with a 9.4-tesla micro-MRI.

MATERIALS AND METHODS

Gadolinium was injected into the middle ear in 6 Sprague-Dawley rats via the transtympanic route. The left ear was injected with Gd-DO3A-butrol first, and then the right ear was injected with Gd-DOTA. MR images of the inner ear were acquired 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4 h after intratympanic (IT) injection using an Agilent MRI system 9.4T/160/AS. The normalized signal intensity was quantitatively analyzed at the scala vestibuli (SV), scala media, and scala tympani (ST) using a Marosis M-view system. Then the normalized signal intensities (SIs) were compared between the two contrast agents.

RESULTS

For Gd-DO3A-butrol, the SI was as low as 1.0-1.5 throughout 1-4 h at the SV and ST of the basal turn. The maximum SI was 1.5 ± 0.5 at the SV (2 h) and 1.3 ± 0.5 at the ST (2 h). For Gd-DOTA, the 1-hour postinjection SI at the basal turn was 2.5 ± 0.5 at the SV, 1.6 ± 0.3 at the ST, and 1.2 ± 0.3 at the scala media. In the apical turn, the maximum SI was reached after 2.5 h. The maximum SI in the apical turn was 1.8 ± 0.4 at the SV (3.5 h), 1.8 ± 0.4 at the ST (4 h), and 1.4 ± 0.3 at the scala media (4 h).

CONCLUSIONS

We were able to clearly visualize and separate the ST and SV using IT Gd and 9.4-tesla micro-MRI. We recommend using Gd-DO3A-butrol over Gd-DOTA and to perform the MRI 2.5 h after using IT Gd in the rat inner ear.

Toxicity of silver nanoparticle in rat ear and BALB/c 3T3 cell line

Background

Silver nanoparticles (AgNPs) displayed strong activities in anti-bacterial, anti-viral, and anti-fungal studies and was reportedly efficient in treating otitis media .The potential impact of AgNPs on the inner ear was missing.

Objective

Attempted to evaluate the potential toxicity of AgNPs in the inner ear, middle ear, and external ear canal after transtympanic injection in rats.

Results

In in vitro studies, the IC₅₀ for AgNPs in neutral red uptake assay was lower than that in NAD(P)H-dependent cellular oxidoreductase enzyme assay (WST-1) and higher than that in total cellular ATP and nuclear membrane integrity (propidium iodide) assessments. In in vivo experiments, magnetic resonance imaging (MRI) showed that significant changes in the permeability of biological barriers occurred in the middle ear mucosa, the skin of the external ear canal, and the inner ear at 5 h post-transtympanic injection of AgNPs at concentrations ranging from 20 μg/ml to 4000 μg/ml. The alterations in permeability showed a dosage-response relationship, and were reversible. The auditory brainstem response showed that 4000 μg/ml AgNPs induced hearing loss with partial recovery at 7 d, whereas 20 μg/ml caused reversible hearing loss. The functional change in auditory system was in line with the histology results. In general, the BALB/c 3T3 cell line is more than 1000 times more sensitive than the in vivo studies. Impairment of the mitochondrial function was indicated to be the mechanism of toxicity of AgNPs.

Conclusion

These results suggest that AgNPs caused significant, dose-dependent changes in the permeability of biological barriers in the middle ear mucosa, the skin of the external ear canal, and the inner ear. In general, the BALB/c 3T3 cell line is more than 1000 times more sensitive than the in vivo studies. The rat ear model might be expended to other engineered nanomaterials in nanotoxicology study.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-014-0052-6) contains supplementary material, which is available to authorized users.

Nanoparticle based inner ear therapy

Synthetic nanoparticles can be used to carry drugs, genes, small interfering RNA (siRNA) and growth factors into the inner ear, to repair, restore and induce cellular regeneration. Nanoparticles (NPs) have been developed which are targetable to selected tissue, traceable in vivo, and equipped with controlled drug/gene release. The NPs are coated with a ‘stealth’ layer, and decorated with targeting ligands, markers, transfection agents and endosomal escape peptides. As payloads, genes such as the BDNF-gene, Math1-gene and Prestin-gene have been constructed and delivered in vitro. Short-hairpin RNA has been used in vitro to silence the negative regulator of Math1, the inhibitors of differentiation and DNA binding. In order to facilitate the passage of cargo from the middle ear to the inner ear, the oval window transports gadolinium chelate more efficiently than the round window and is the key element in introducing therapeutic agents into the vestibule and cochlea. Depending upon the type of NPs, different migration and cellular internalization pathways are employed, and optimal carriers should be designed depending on the cargo. The use of NPs as drug/gene/siRNA carriers is fascinating and can also be used as an intraoperative adjunct to cochlear implantation to attract the peripheral processes of the cochlear nerve.

MRI evidence of endolymphatic impermeability to the gadolinium molecule in the in vivo mouse inner ear at 9.4 tesla

OBJECTIVE

Previous in vivo experimental magnetic resonance imaging (MRI) investigations of the mammalian inner ear at 4.7 Tesla have indicated that intravenously injected gadolinium (Gd) penetrates the perilymphatic labyrinth, but not the endolymphatic membranous labyrinth. In the present study, high field MRI at 9.4T was used to visualize the in vivo mouse vestibulo-cochlea system, and to determine whether the endolymphatic system is permeable to a Gd complex.

METHODS

A 9.4 T Varian magnet equipped with a 12 cm inner diameter gradient system with maximum gradient strength of 600 mT/m, a millipede coil (Varian design) and a Gd contrast agent were used for image acquisition in the normal C57 BL-6 mouse.

RESULTS

High-resolution 2D and 3D images of the mouse cochlea were acquired within 80 minutes following intravenous injection of Gd. Gd initially permeated the perilymphatic scala tympani and scala vestibuli, and permitted visualization of both cochlear turns from base to apex. The superior, inferior and lateral semicircular canals were subsequently visualized in 3 planes. The membranous endolymphatic labyrinth was impermeable to intravenously injected Gd, and thus showed no apparent uptake of Gd at 9.4T.

CONCLUSION

The 9.4T field strength MRI permitted acquisition of high resolution images of anatomical and physiological features of the normal, wild type mouse perilymphatic inner ear in vivo, and provided further evidence that the endolymphatic system is impermeable to intravenously injected Gd.

Markers of cochlear inflammation using MRI

PURPOSE

To quantify spatial and temporal inflammation-induced changes in vascular permeability and macrophage infiltration in guinea-pig (GP) cochlea using MRI.

MATERIALS AND METHODS

GPs were injected with lipopolysaccharide (LPS) to induce cochlear inflammation. One group was injected with a gadolinium based contrast agent (GBCA) and dynamic contrast enhanced (DCE)-MRI was performed at 4, 7, and 10 days after LPS treatment. A two-compartment pharmacokinetic model was used to determine the apparent rate constant of GBCA extravasation (K(trans) ). A second group was injected with ultrasmall superparamagnetic iron oxide particles (USPIOs) and studied at 2, 3, and 7 days after LPS treatment to detect tissue USPIO uptake and correlate with histology. For both groups, control GPs were scanned similarly.

RESULTS

The signal enhancement increased substantially and more rapidly at day 4 in LPS-treated than in control cochlea shortly following GBCA injection. K(trans) of LPS-treated cochlea was maximum on day 4 at 0.0218 ± 0.0032 min(-1) and then decreased to control level at 0.0036 ± 0.0004 min(-1) by day 10. In the second group, the relative signal intensity and T2 in cochlear perilymphatic spaces on day 2 decreased, on average, by 54% and 45%, respectively, compared with baseline and then remained under control levels by day 7. This suggests the infiltration of inflammatory cells, although unconfirmed by histology.

CONCLUSION

This provides the first measurement of cochlear vascular permeability using MRI and a quantitative evaluation of the development of cochlear inflammation. MRI holds considerable potential for the assessment of disease processes such as clinical diagnosis of conditions such as labyrinthitis.

Contrast enhancement of the inner ear in magnetic resonance images taken at 10 minutes or 4 hours after intravenous gadolinium injection

CONCLUSION

Contrast enhancement of the inner ear by three-dimensional fluid-attenuated inversion recovery magnetic resonance imaging (3D-FLAIR MRI) taken 4 h after intravenous gadolinium (Gd) injection was better than when taken at 10 min. Using heavily T2-weighted 3D-FLAIR MRI, visualization of endolymphatic hydrops (EH) was possible in the vestibule and the cochlea after a standard intravenous dose of Gd.

OBJECTIVES

To define a suitable time point for imaging Gd uptake in the inner ear acquired with heavily T2-weighted 3D-FLAIR MRI after standard intravenous Gd administration.

METHODS

Using a 3 Tesla MRI unit, heavily T2-weighted 3D-FLAIR MRI images were taken twice at approximately 10 min (conventional timing) and 4 h after intravenous gadodiamide (0.1 mmol/kg) injection in 10 patients with inner ear diseases including Ménière's disease.

RESULTS

The 4 h delay increased Gd enhancement of the 3D-FLAIR MRI images of the perilymphatic space in both symptomatic and asymptomatic ears. The increase in Gd enhancement was greater in symptomatic than in asymptomatic ears. Using this heavily T2-weighted 3D-FLAIR technique, EH was observed in both the cochlea and vestibule in images taken 4 h after the intravenous Gd injection.

The role of preoperative, intratympanic glucocorticoids for hearing preservation in cochlear implantation: a prospective clinical study

BACKGROUND

Hearing Preservation is becoming increasingly important in cochlear implantation as there is growing evidence that preserving the residual hearing, especially in the low frequencies in combination with the electric stimulation can significantly improve hearing and speech outcomes in noise. Besides the ongoing development of atraumatic implant electrodes and insertion techniques, the implementation of pharmacologic hair cell protection is thought to increase hearing preservation. This study investigates the effects of preoperative intratympanic glucocorticoid application on hearing preservation rates in cochlear implantation.

STUDY DESIGN

Prospective interventional study.

SETTING

Tertiary neurotology referral center.

PATIENTS

Patients undergoing cochlear implantation with measurable preoperative hearing thresholds using either a Flex soft electrode or a Flex EAS electrode depending on the degree of residual low frequency hearing.

INTERVENTION

Preoperative intratympanic steroid application during cochlear implantation via round window insertion.

MAIN OUTCOME MEASURES

Level of hearing preservation after cochlear implantation; electrode- and frequency-specific hearing preservation rates.

RESULTS

Preoperative hearing thresholds were comparable in the control group and the interventional Flex soft group (70.5 db±12.5 dB vs. 73.5 dB±10.5 dB, P=.27). As per selection criteria the low-frequency hearing thresholds were significantly lower in interventional Flex EAS groups when compared to the control group. Hearing preservation was significantly better in the interventional group with no case of complete hearing loss in this group (11 dB±2.5 dB vs. 19.5 dB 3.5 dB, P<.05). The interventional group displayed a higher stability of hearing preservation after implantation (r=.8, P=.03). Level of hearing preservation was higher when a specific hearing preservation electrode was used (r=.85, P<.05). Hearing preservation in the low frequencies was significantly higher than in the high frequencies.

CONCLUSIONS

Our study suggests that the additional preoperative use of intratympanic glucocorticoids improves and stabilizes hearing preservations rates in round window cochlear implantation for adults and children with residual hearing.

Transport augmentation through the blood-inner ear barriers of guinea pigs treated with 3-nitropropionic acid and patients with acute hearing loss, visualized with 3.0 T MRI

OBJECTIVE

To visualize the permeability changes in the blood-inner ear barriers of guinea pigs with acute mitochondria dysfunction and in patients with acute hearing loss using contrast agent-enhanced MRI.

MATERIALS AND METHODS

An animal model of acute mitochondria dysfunction-induced hearing loss was created by introducing 3-nitropropionic acid (3-NP) intratympanically in guinea pigs. Vestibular disorder and hearing loss were evaluated. An MRI was performed at 2 h after either intravenous (IV) or intratympanic administration of dimeglumine gadopentetate (Gd-DTPA), using 3D fast-recovery fast spin-echo (FRFSE) and 3D fluid-attenuated inversion recovery (FLAIR) sequences. The inner ears of patients with acute hearing loss were imaged using a 3D-FLAIR sequence with a 3 T MRI machine at 2 h post-IV injection with Gd-DTPA at a routine dosage.

RESULTS

Guinea pigs treated with 3-NP showed severe hearing loss and vestibular dysfunction. MR imaging with a 3D-FLAIR sequence at 2 h post-IV injection of Gd-DTPA was an optimal method for visualizing transport augmentation through the blood-inner ear barriers. Apoptosis appeared in the stria vascularis and Reissner's membrane of cochleae treated with 3NP. Similar MRI changes were observed in patients with SSHL and Ménière's disease 2 h post-IV injection with Gd-DTPA using the 3D-FLAIR sequence.

CONCLUSION

Variations of Gd-DTPA transport through the blood-inner ear barriers induced by mitochondria toxin was visualized in guinea pigs using a clinical 3.0 T machine. IV injection of Gd-DTPA with 2 h of waiting time and imaging with 3D-FLAIR are optimal methods. The MRI observation of the inner ear in the animal model was translatable to patients with acute hearing loss, using an IV injection of Gd-DTPA at the routine dosage.

Magnetic resonance imaging of the inner ear in Meniere's disease

Recent magnetic resonance imaging (MRI) techniques have made it possible to examine the compartments of the cochlea using gadolidium-chelate (GdC) as a contrast agent. As GdC loads into the perilymph space without entering the endolymph in healthy inner ears, the technique provides possibilities to visualize the different cochlear compartments and evaluate the integrity of the inner ear barriers. This critical review presents the recent advancements in the inner ear MRI technology, contrast agent application and the correlated ototoxicity study, and the uptake dynamics of GdC in the inner ear. GdC causes inflammation of the mucosa of the middle ear, but there are no reports or evidence of toxicity-related changes in vivo either in animals or in humans. Intravenously administered GdC reached the guinea pig cochlea about 10 minutes after administration and loaded the scala tympani and scala vestibuli with the peak at 60 minutes. However, the perilymphatic loading peak was 80 to 100 minutes in mice after intravenous administration of GdC. In healthy animals the scala media did not load GdC. In mice in which GdC was administered topically onto the round window, loading of the cochlea peaked at 4 hours, at which time it reached the apex. The initial portions of the organ to be filled were the basal turn of the cochlea and vestibule. In animal models with endolymphatic hydrops (EH), bulging of the Reissner's membrane was observed as deficit of GdC in the scala vestibuli. Histologically the degree of bulging correlated with the MR images. In animals with immune reaction-induced EH, MRI showed that EH could be limited to restricted regions of the inner ear, and in the same inner ear both EH and leakage of GdC into the scala media were visualized. More than 100 inner ear MRI scans have been performed to date in humans. Loading of GdC followed the pattern seen in animals, but the time frame was different. In intravenous delivery of double-dose GdC, the inner ear compartments were visualized after 4 hours. The uptake pattern of GdC in the perilymph of humans between 2 hours and 7 hours after local delivery needs to be clarified. In almost all patients with probable or suspected Ménière's disease, EH was verified. Specific algorithms with a 12-pole coil using fluid attenuation inversion recovery sequences are recommended for initial imaging in humans.

MRI manifestation of novel superparamagnetic iron oxide nanoparticles in the rat inner ear

AIM

Superparamagnetic iron oxide nanoparticles hierarchically coated with oleic acid and Pluronic F127 copolymers (POA@SPION) have shown exceptional T2 contrast enhancement. The aim of the present work was to investigate the MRI manifestation of POA@SPION in the inner ear.

MATERIALS & METHODS

A total of 26 male Wister rats were selected for testing POA@SPION administered through intracochlear, intratympanic and intravenous routes. MRI was performed with a 4.7 T MR scanner.

RESULTS & CONCLUSION

POA@SPION can be introduced into the perilymph space, after which it becomes widely distributed and can demonstrate the integrity of the perilymph-endolymph barrier. Positive highlighting of the endolymph compartment against the darkened perilymph was visualized for the first time. POA@SPION passed through the middle-inner ear barriers in only small amounts, but stayed in the perilymph for 3 days. They did not traverse the blood-perilymph barrier or blood-endolymph barrier. The inner ear distribution of POA@SPION was confirmed by histology. POA@SPION is a promising T2 negative contrast agent.

Gadolinium uptake in the rat inner ear perilymph evaluated with 4.7 T MRI: a comparison between transtympanic injection and gelatin sponge-based diffusion through the round window membrane

OBJECTIVE

To investigate the inner ear uptake of the contrast agent gadolinium administered intratympanically through transtympanic injection or using a gelatin sponge placed on the round window.

MATERIALS AND METHODS

The T1 contrast agent Gadolinium-tetra-azacyclo-dodecane-tetra-acetic acid (Gd-DOTA) was administrated to the middle ear cavities of 12 male Wistar rats by a transtympanic injection technique (TTI group; 40 microl), gelatin sponge diffusion (GelD40 group; 40 microl), or gelatin sponge diffusion (GelD8 group; 8 microl). Magnetic resonance imaging was performed with a 4.7-T scanner using a T1-weighted 2-dimensional rapid acquisition with relaxation enhancement sequence and a high-resolution T1-weighted 3-dimensional rapid acquisition with relaxation enhancement sequence.

RESULTS

The uptake of Gd-DOTA into the perilymph was more pronounced at 3 than at 1 hour after intratympanic administration for all methods studied. Transtympanic injection and GelD40 induced similar uptake of Gd-DOTA in the lower turns of the rat cochlea. Transtympanic injection induced less efficient Gd-DOTA uptake in the apex than GelD40. GelD8 was less efficient at Gd-DOTA uptake than either TTI or GelD40.

CONCLUSION

Both TTI and GelD40 are able to efficiently deliver substances to inner ear destinations. Considering its simplicity, TTI is more practical for use in the clinic for the administration of substances to the inner ear, although it provided less efficient uptake in the apex than GelD40.

Improving the visualization of fluorescently tagged nanoparticles and fluorophore-labeled molecular probes by treatment with CuSO(4) to quench autofluorescence in the rat inner ear

Fluorescent tags and fluorophore-conjugated molecular probes have been extensively employed in histological studies to demonstrate nanoparticle distribution in inner ear cell populations. However, autofluorescence that exists in the rodent cochleae disturbs visualization of the fluorescent tags and fluorophore labeling. In the present work, we aimed to improve the visualization of fluorescently tagged nanoparticles and fluorophore-labeled molecular probes by treatment with CuSO(4) to quench autofluorescence in the rat inner ear. The in vivo study was performed on eight- to nine-month-old rats using confocal laser scanning microscopy, and the in vitro study was carried out with DiI-tagged poly(ethylene glycol) and poly(capro-lactone) polymersomes and different fluorescent-labeling agents using a spectrofluorometer. The nanoparticles were intratympanically administered using either an osmotic pump or transtympanic injection. Abundant autofluorescence was detected in spiral ganglion cells (SGCs), stria marginal cells, spiral ligament fibrocytes (SL) and the subcuticular cytoplasm of inner hair cells (IHCs). Sparsely distributed faint autofluorescence was also visualized in outer hair cells (OHCs). The autofluorescence was eliminated by treatment with 1 mM CuSO(4) (in 0.01 M ammonium acetate buffer) for 70-90 min, while the fluorescent tag in the nanoparticle was absolutely preserved and the labeling fluorescence signals of the molecular probes were mostly retained.