Obliteration of the ductus reuniens

Acute histological reactions in the otolith organs to inner ear drug delivery through a cochlear implant

Introduction

Cochlear implantation is currently regarded as a safe and minimally invasive procedure. However, cochlear implantation can have an impact on vestibular function, despite the lack of correlation between patient symptomatology and damage in vestibular tests. Thus, the present study aims to analyze the presence of hydrops and histological reactions at the level of the vestibule after cochlear implantation with dexamethasone pump delivery in Macaca fascicularis (Mf).

Materials and methods

A detailed histological study was conducted on a total of 11 Mf. All 11 Mf were divided into three groups: 5 Mf were implanted with an electrode array HL-14 connected to a pump delivering FITC-dextran for 24 h (Group A); 4 Mf were implanted with a CI electrode array attached to a pump for FITC-dextran delivery for 7 days (Group B); and 2 Mf were considered the control group, without any kind of cochlear device implantation (Group C). After drug deliver, the selected macaques were euthanized to collect tissue samples for histological analysis. An experienced observer, focusing on the utricle and saccule areas, conducted a blinded inner ear histology analysis.

Results

Surgical procedures were successfully performed in all cases. No signs of cochlear reaction to the device were observed, including neither collapse nor fibrosis. Endolymphatic sinus dilatation was observed in Mf4A and Mf3B, while cochlear hydrops was observed in Mf3A. The mean areas of the utricle and saccule exhibited some statistically significant differences, specifically, in the saccule between groups C and both groups A (p = 0.028) and B (p = 0.029); however, no significant differences were observed between groups A and B or among comparisons of the utricle.

Discussion

A significant concern relates to the safety of cochlear implantation with regard to vestibular preservation and hearing. New advancements in electrode arrays, such as CI devices coupled with delivery pumps, pose a challenge in maintaining minimally traumatic surgical concept-based procedures without affecting the inner ear homeostasis. The implantation of this device may cause vestibular hydrops in the saccule, indicating that the longer the time of substance release, the greater the grade of hydrops evidenced at the saccular level. Apart from this finding, the risk of histological damage to the vestibule is low.

Histopathological reaction in the vestibule after cochlear implantation in Macaca fascicularis

Cochlear implantation surgery (CI) is considered a safe procedure and is the standard treatment for the auditory rehabilitation in patients with severe-to-profound sensorineural hearing loss. Although the development of minimally traumatic surgical concepts (MTSC) have enabled the preservation of residual hearing after the implantation, there is scarce literature regarding the vestibular affection following MTCS. The aim of the study is to analyze histopathologic changes in the vestibule after CI in an animal model (Macaca fascicularis). Cochlear implantation was performed successfully in 14 ears following MTCS. They were classified in two groups upon type of electrode array used. Group A (n = 6) with a FLEX 28 electrode array and Group B (n = 8) with HL14 array. A 6-month follow-up was carried out with periodic objective auditory testing. After their sacrifice, histological processing and subsequent analysis was carried out. Intracochlear findings, vestibular presence of fibrosis, obliteration or collapse is analyzed. Saccule and utricle dimensions and neuroepithelium width is measured. Cochlear implantation was performed successfully in all 14 ears through a round window approach. Mean angle of insertion was >270° for group A and 180-270° for group B. In group A auditory deterioration was observed in Mf 1A, Mf2A and Mf5A with histopathological signs of scala tympani ossification, saccule collapse (Mf1A and Mf2A) and cochlear aqueduct obliteration (Mf5A). Besides, signs of endolymphatic sinus dilatation was seen for Mf2B and Mf5A. Regarding group B, no auditory deterioration was observed. Histopathological signs of endolymphatic sinus dilatation were seen in Mf 2B and Mf 8B. In conclusion, the risk of histological damage of the vestibular organs following minimally traumatic surgical concepts and the soft surgery principles is very low. CI surgery is a safe procedure and it can be done preserving the vestibular structures.

Vestibular dysfunction in pediatric patients with cochlear implantation: A systematic review and meta-analysis

Objective

Vestibular dysfunction may delay the achievement of balance and perception milestones in pediatric patients after cochlear implantation (CIM).

Methods

A strategic literature search was done following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. We searched the PubMed, Medline, Embase, Web of Science, and Cochrane Library databases from inception to July 2022. Studies were included on the otoliths, semicircular canals, and balance function changes in children after CIM. Two reviewers independently assessed the level of evidence, methodological limitations, risk of bias, and characteristics of the cases. Matched pre- and postoperative vestibular functional test data, including ocular and cervical vestibular-evoked myogenic potential (oVEMP and cVEMP), caloric test, video head impulse test (vHIT), and Bruininks-Oseretsky Test 2 (BOT-2), were used to calculate the relative risk of vestibular disorders. Subgroup analyses were performed according to surgical approach, CIM device status, and etiology.

Results

Twenty studies that met the inclusion criteria were selected for the meta-analysis. We observed significant vestibular dysfunction in pediatric patients with CIM. The results showed a statistically significant increase in abnormal cVEMP response (RR = 2.20, 95% CI = 1.87, 2.58, P < 0.0001), abnormal oVEMP response (RR = 2.10, 95% CI = 1.50, 2.94, P < 0.0001), and abnormal caloric test results (RR = 1.62, 95% CI = 1.20, 2.19, P = 0.0018) after implantation. Statistically significant differences were not found in the vHIT test results of all three semicircular canals before and after the operation (P > 0.05). Regarding static and dynamic balance, we found significantly poorer BOT-2 scores in children with CIM than in the normal group (mean difference = −7.26, 95% CI = −10.82, −3.70, P < 0.0001).

Conclusion

The results showed that vestibular dysfunction might occur after CIM in pediatric patients. Some children experience difficulties with postural control and balance. Our results suggest that a comprehensive evaluation of vestibular function should be performed before and after CIM.

Prevalence of Endolymphatic Hydrops in Cochlear Implant Candidates with Idiopathic Profound Sensorineural Hearing Loss

OBJECTIVE

To determine the prevalence of endolymphatic hydrops (EH) in cochlear implant (CI) candidates with idiopathic profound sensorineural hearing loss (SNHL) and its influence on the preservation of audiovestibular function after cochlear implantation.

STUDY DESIGN

Prospective case series.

SETTING

Tertiary referral center.

PATIENTS

CI candidates with idiopathic progressive SNHL, but without classic EH-associated symptoms.

INTERVENTIONS

Delayed intravenous gadolinium-enhanced inner ear fluid-attenuated inversion recovery magnetic resonance imaging as well as pure-tone audiograms, video head impulse tests, and vestibular evoked myogenic potentials before and 4 weeks after cochlear implantation.

MAIN OUTCOME MEASURES

Prevalence of EH before cochlear implantation, audiovestibular function before and after surgery in hydropic and nonhydropic ears.

RESULTS

Thirty-two ears in 16 CI candidates were included. Nine ears (28%) with EH were detected. Although preoperative hearing thresholds, utricular function, and semicircular canal function were not different between the two groups, saccular function was reduced in hydropic ears. Ten subjects received a unilateral CI. Of these, 3 (30%) showed EH on the implanted side. There was no difference regarding postoperative hearing loss between the two groups, but the results point toward a higher vulnerability of hydropic ears with respect to loss of otolith function after cochlear implantation.

CONCLUSIONS

This is the first study showing that EH can be assumed in about one third of CI candidates with idiopathic profound SNHL, but no classic EH-associated symptoms. Preliminary results suggest that EH has no influence on the preservation of cochlear function but could be a risk factor for loss of otolith function after cochlear implantation.

Pathogenesis and Etiology of Ménière Disease: A Scoping Review of a Century of Evidence

Importance

Ménière disease is a rare chronic benign disorder of the inner ear with a natural history of multiple clinical phenotypes of variable severity and a tendency to burnout with time. Although multiple treatment modalities have been shown to improve the disease process-some adversely affecting cochleovestibular function-it remains uncertain whether one, several separate, or a combination of pathophysiologic mechanisms affect the disease process. A scoping review of the evidence underlying proposed pathophysiologic mechanisms of Ménière disease is needed to determine which processes are most likely to be etiopathogenic factors.

Observations

Of the 4602 relevant articles found through Embase, Ovid, and PubMed, 444 met inclusion criteria. The most common reported causes of Ménière disease were autoimmune or immune-mediated, genetic, or structural dysfunction of the inner ear. During the study period from inception to March 2021, etiologic theories shifted from structural dysfunction to autoimmune and genetic causes of Ménière disease.

Conclusions and Relevance

This scoping review found that Ménière disease is a multifactorial disease with lifelong comorbidities and loss of quality-associated life-years whose most commonly reported causes were structural dysfunction, immunologic damage, and genetic susceptibility. Recent studies have examined how autoinflammatory processes and vestibular migraine may be associated with Ménière disease. Large heterogeneity among studies may be explained by historical differences in the clinical understanding of the disease, as well as evolving intervention methodologies and practitioner expertise. Ménière disease is a multifactorial disease with lifelong comorbidities and loss of quality-associated life-years; therefore, future studies of reliable biomarkers of endolymphatic hydrops and real-time imaging are warranted to improve understanding and treatment.

Ductus Reuniens and Its Possible Role in Menière's Disease

OBJECTIVE

After 160 years the true underlying cause of Meniere's disease remains enigmatic. The aim of our study is to discuss the possible implication of an obstruction of the ductus reuniens as a cause in Menière's disease.

METHODOLOGY

We first conducted an historical study of the description of the ductus reuniens. We then reviewed the literature regarding ductus reuniens obstruction in animal experiments, human post-mortem studies and living ear imaging. We completed its description by modern microCT imaging. Limited knowledge on the fate of dislodged saccular otoconia is summarized. The possible implications for Meniere's attacks are discussed.

RESULTS

Victor Hensen was the first to describe the ductus reuniens in 1863. He described its length and width and predicted that saccular otoconia might enter the ductus and the cochlea. On microCT the narrowest width of the human ductus reuniens was 0.14 mm. The literature reports cochlear endolymphatic hydrops occurring after animal experimental obstruction of the duct. Human postmortem studies have confirmed saccular otoconial clumps entering the ductus and the cochlea. A postmortem study has shown sites of endolymphatic obstruction, and imaging speculates on blockages in ears with Meniere's disease. Dislodged utricular otoconia can be in clumps of otolithic membranes.

CONCLUSION

Blockages of the ductus reuniens and at other endolymphatic system sites appear to be a feature in Meniere's disease ears. The blockages have been postulated to be saccular otoconia either causing or aggravating hydrops. This could be consistent with observed nystagmus reversals during attacks as the endolymphatic sac attempts to clear the hydrops and the otoconia.

A Synchrotron and Micro-CT Study of the Human Endolymphatic Duct System: Is Meniere's Disease Caused by an Acute Endolymph Backflow?

Background: The etiology of Meniere's disease (MD) and endolymphatic hydrops believed to underlie its symptoms remain unknown. One reason may be the exceptional complexity of the human inner ear, its vulnerability, and surrounding hard bone. The vestibular organ contains an endolymphatic duct system (EDS) bridging the different fluid reservoirs. It may be essential for monitoring hydraulic equilibrium, and a dysregulation may result in distension of the fluid spaces or endolymphatic hydrops. Material and Methods: We studied the EDS using high-resolution synchrotron phase contrast non-invasive imaging (SR-PCI), and micro-computed tomography (micro-CT). Ten fresh human temporal bones underwent SR-PCI. One bone underwent micro-CT after fixation and staining with Lugol's iodine solution (I2KI) to increase tissue resolution. Data were processed using volume-rendering software to create 3D reconstructions allowing orthogonal sectioning, cropping, and tissue segmentation. Results: Combined imaging techniques with segmentation and tissue modeling demonstrated the 3D anatomy of the human saccule, utricle, endolymphatic duct, and sac together with connecting pathways. The utricular duct (UD) and utriculo-endolymphatic valve (UEV or Bast's valve) were demonstrated three-dimensionally for the first time. The reunion duct was displayed with micro-CT. It may serve as a safety valve to maintain cochlear endolymph homeostasis under certain conditions. Discussion: The thin reunion duct seems to play a minor role in the exchange of endolymph between the cochlea and vestibule under normal conditions. The saccule wall appears highly flexible, which may explain occult hydrops occasionally preceding symptoms in MD on magnetic resonance imaging (MRI). The design of the UEV and connecting ducts suggests that there is a reciprocal exchange of fluid among the utricle, semicircular canals, and the EDS. Based on the anatomic framework and previous experimental data, we speculate that precipitous vestibular symptoms in MD arise from a sudden increase in endolymph pressure caused by an uncontrolled endolymphatic sac secretion. A rapid rise in UD pressure, mediated along the fairly wide UEV, may underlie the acute vertigo attack, refuting the rupture/K+-intoxication theory.

Spike Generators and Cell Signaling in the Human Auditory Nerve: An Ultrastructural, Super-Resolution, and Gene Hybridization Study

Background: The human auditory nerve contains 30,000 nerve fibers (NFs) that relay complex speech information to the brain with spectacular acuity. How speech is coded and influenced by various conditions is not known. It is also uncertain whether human nerve signaling involves exclusive proteins and gene manifestations compared with that of other species. Such information is difficult to determine due to the vulnerable, "esoteric," and encapsulated human ear surrounded by the hardest bone in the body. We collected human inner ear material for nanoscale visualization combining transmission electron microscopy (TEM), super-resolution structured illumination microscopy (SR-SIM), and RNA-scope analysis for the first time. Our aim was to gain information about the molecular instruments in human auditory nerve processing and deviations, and ways to perform electric modeling of prosthetic devices. Material and Methods: Human tissue was collected during trans-cochlear procedures to remove petro-clival meningioma after ethical permission. Cochlear neurons were processed for electron microscopy, confocal microscopy (CM), SR-SIM, and high-sensitive in situ hybridization for labeling single mRNA transcripts to detect ion channel and transporter proteins associated with nerve signal initiation and conductance. Results: Transport proteins and RNA transcripts were localized at the subcellular level. Hemi-nodal proteins were identified beneath the inner hair cells (IHCs). Voltage-gated ion channels (VGICs) were expressed in the spiral ganglion (SG) and axonal initial segments (AISs). Nodes of Ranvier (NR) expressed Nav1.6 proteins, and encoding genes critical for inter-cellular coupling were disclosed. Discussion: Our results suggest that initial spike generators are located beneath the IHCs in humans. The first NRs appear at different places. Additional spike generators and transcellular communication may boost, sharpen, and synchronize afferent signals by cell clusters at different frequency bands. These instruments may be essential for the filtering of complex sounds and may be challenged by various pathological conditions.

A Micro-CT and Synchrotron Imaging Study of the Human Endolymphatic Duct with Special Reference to Endolymph Outflow and Meniere's Disease

Meniere’s disease remains enigmatic, and has no treatment with sufficient evidence. The characteristic histopathological finding is endolymphatic hydrops, suggesting either an overproduction or decreased reabsorption of endolymph in the human inner ear. This study presents the first analysis of the vascular plexus around the human endolymphatic duct using micro computed tomography and coherent synchrotron radiation with phase contrast imaging. Using a software program, data were processed by volume-rendering with scalar opacity mapping to create transparent three-dimensional reconstructions. A rich vascular plexus was discovered around the endolymphatic duct that drained into collecting channels, linked to the vestibular venous outflow system. This network is believed to make up the principal route for endolymph outflow, and its associated malfunction may result in endolymphatic hydrops and Meniere’s disease.

Lermoyez syndrome revisited: 100-year mystery

OBJECTIVE

This study revisited seven patients with Lermoyez syndrome over the past 25 years using an inner ear test battery to elucidate its mechanism.

METHODS

From 1992 to 2017, we have experienced 4096 patients with Meniere's disease (MD) and seven patients (5 males and 2 females, 8 ears) with Lermoyez syndrome. Two of the Lermoyez patients were elderly, aged 71 and 85 years. An inner ear test battery comprising audiometry, ocular and cervical vestibular-evoked myogenic potential (oVEMP and cVEMP) tests, and caloric test were performed.

RESULTS

Significant improvement of mean hearing levels (MHLs) was identified at low and middle frequencies after vertiginous attack, but not at high frequencies. Inner ear deficits in Lermoyez patients ran from abnormal hearing (100%) to abnormal cVEMP (43%), caloric (38%) and oVEMP (0) tests, exhibiting a significantly declining sequence. This declining sequence differed from that in MD.

CONCLUSIONS

Lermoyez syndrome is extremely rare, with prevalence relative to MD of 0.2%. The mechanism is considered as blockage in the ductus reuniens caused by dislodged saccular otoconia. Aging and trauma are two precipitating factors for the dislodged saccular otoconia, which may explain why Lermoyez syndrome occurs most frequently in males and some elderly.

Saccular otoconia as a cause of Ménière's disease: hypothesis based on two theories

BACKGROUND

The cause of Ménière's disease remains enigmatic after 156 years. Schuknecht's rupture and potassium intoxication theory of attacks was based on histological studies.

OBJECTIVES

This paper aimed to: present the most contemporary evidence indicating that ruptures do not usually occur, and discuss the possibility that detached saccular otoconia are the main cause of Ménière's disease; and to establish an unequivocal definition of the age of Ménière's disease onset.

METHOD

The paper reviews the electrophysiological basis of the Gibson-Arenberg drainage theory used to explain vertigo attacks. The current, limited knowledge of the likely fate of detached saccular otoconia is discussed.

RESULTS

Electrophysiological studies during attacks do not support endolymph ruptures, but rather endolymph flowing in one direction and then in the opposite direction. Age of onset for Ménière's disease parallels that for benign paroxysmal positional vertigo.

CONCLUSION

The similarity of age of onset spectrum for Ménière's disease and benign paroxysmal positional vertigo raises the possibility that the two conditions have the same fundamental cause.

Vitamin D supplementation may improve symptoms in Meniere's disease

In the last 4 years the authors observed a trend that correcting vitamin D deficiency in newly diagnosed cases of Meniere's disease decreased the necessity of the ablative therapy with intratympanic gentamicin. According to their hypothesis, vitamin D supplementation may indeed have a beneficial effect in Meniere's disease if the symptoms are caused by a local postviral autoimmune reaction. Vitamin D has a strong immunomodulatory role, one of which is the regulation of the expression of pro-inflammatory mediators. The authors suggest further epidemiological studies to decide if there is a connection between vitamin D deficiency and Meniere's disease.

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.

A New Theory for Ménière's Disease: Detached Saccular Otoconia

Ménière's disease is an inner ear disorder characterized by vertigo attacks, fluctuating and progressive hearing loss, tinnitus, and aural fullness in the affected ear. The pathophysiology of Ménière's disease remains elusive. Theories so far are anatomical variation in the size or position of the endolymphatic sac and duct, viral inflammation or autoimmune involvement of the sac, or a genetically determined abnormality of endolymph control. Animal studies on blocking the ductus reuniens and endolymphatic duct have produced hydrops in the cochlea, saccule, and utricle. Cone beam computed tomography images show a similar pattern with apparent obstruction of the ductus reuniens, saccular duct, and endolymphatic sinus. New studies documenting the age of onset of Ménière's disease show a pattern similar to benign paroxysmal positional vertigo, raising the possibility that the fundamental cause of Ménière's disease might be detached saccular otoconia.

Practical 3DCT imaging of the vestibular aqueduct for Meniere's disease

CONCLUSION

Radiographic visualization of the vestibular aqueduct (VA) from a lateral inside view was effective in assessing patients with Meniere's disease (MD). There were no VA shapes specific to MD on radiography, except for an obliterated VA. This technique could yield more accurate images and functional assessment of the VA for MD evaluation in a clinically useful and convenient manner, without requiring morphologic measurement.

OBJECTIVE

To visualize the detailed images of the VA using three-dimensional (3D) computed tomography (CT) and discuss its clinical utility in assessing MD.

SUBJECTS AND METHODS

The VAs in 13 healthy subjects and 25 MD patients, who were definitely diagnosed according to criteria described by the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology, Head and Neck Surgery (AAO-HNS), were imaged from the medial and lateral sides using 3DCT and compared to conventional CT images.

RESULTS

Examination of the VA from both the lateral outside and inside views on 3DCT yielded more precise images than generated by conventional CT and could be useful to estimate the VA function. The estimated VA function in the MD ears was significantly abnormal compared to the function in healthy ears. An obliterated VA was characteristic of affected MD ears.

Three-dimensional cone beam computed tomography imaging of the membranous labyrinth in patients with Meniere's disease

CONCLUSION

Three-dimensional cone beam computed tomography (3DCT) images revealed characteristic malformations of the membranous labyrinth of the inner ear in Meniere's disease (MD). The morphology of the membranous region between the vestibular cecum of the cochlea and the saccule of ears with MD was compared to that of healthy ears. The present study supports the hypothesis proposed earlier that reuniting duct blockade is a result of the dislodgement of saccular otoconia.

OBJECTIVE

To visualize the membranous labyrinth using 3DCT and to investigate the pathology of MD.

METHODS

A preparatory study was conducted to determine the optimal 3DCT window settings for the detection of water, muscle, calcium carbonate (CaCO3), and bone. Based on this preparatory study, the ears of 13 healthy volunteers and 25 MD patients definitely diagnosed according to the criteria issued by the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology, Head and Neck Surgery (AAO-HNS), were visualized.

RESULTS

The differences in the membranous labyrinth between MD ears and healthy ears could be visualized using 3DCT. The images were classified into three types based on their morphological pattern. The ears of patients with MD were different from normal ears in terms of this classification.

Reliability of magnetic resonance imaging performed after intratympanic administration of gadolinium in the identification of endolymphatic hydrops in patients with Ménière's disease

OBJECTIVE

To evaluate the reliability of magnetic resonance imaging performed after intratympanic gadolinium administration in evidencing endolymphatic hydrops in patients with Ménière's disease (MD).

PATIENTS

A total of 26 patients (18 male and 8 female subjects, aged 25-78 yr; median age, 56 yr) with definite MD and 12 subjects (8 male and 4 female subjects, aged 31-75 yr; median age, 51 yr) with various unilateral non-MD disorders of the inner ear were examined.

INTERVENTION

A 0.6-ml solution of gadobutrol (1 mmol/ml), diluted 1:7 in saline, was injected in the affected ear through the inferior-posterior quadrant of the tympanic membrane, using a 22-gauge spinal needle. In 9 MD patients, the contralateral ear also was injected. The patient was kept with the head rotated 45 degrees contralaterally for 30 minutes after each injection. Twenty-four hours later, a 3-dimensional fluid-attenuated inversion recovery magnetic resonance imaging using a 3 Tesla unit was performed.

MAIN OUTCOME MEASURE

Perilymphatic enhancement was evaluated in different portions of the labyrinth in MD ears and compared with the outcomes obtained in the non-MD ears.

RESULTS

All MD ears showed impaired perilymphatic enhancement of variable degrees. No enhancement defects could be observed in all examined contralateral unaffected ear of the patients with MD, as well as in 11 of the 12 ears of the subjects with various unilateral non-MD disorders.

CONCLUSION

Perilymphatic enhancement defect of variable degrees is observed in the pathologic ear of every patient with MD. The consistency of this phenomenon in MD ears and the complete enhancement in most of the ears without MD safely enable to attribute these findings to endolymphatic hydrops. It is likely in the near future that imaging may be used to achieve a certain diagnosis of MD in life.

A model analysis of tensile stress in the toadfish vestibular membranes

Background. A theoretical model analysis of stress in the vestibular membranes has identified a geometrical stress factor incorporating shape, size, and thickness that can be used to assess peak stress in the various chambers. Methods. Using published measurements of the toadfish vestibular membranes made during surgery, the geometrical stress factor can be evaluated for each chamber based on the model. Results. The mean geometrical stress factor is calculated to be the lowest in the semicircular canal (4.4), intermediate in the ampulla (6.0), and the highest in the utricle (17.4). Conclusions. The model predicts that substantial hoop stress disparities exist in the toadfish vestibular labyrinth. Stress is least in the semicircular canal, which therefore appears to be the structure with greatest stability. The utricle is found to be the most stress prone structure in the vestibular labyrinth and therefore appears to be the chamber most vulnerable to distention and potential modification.

Blockage of reuniting duct in Meniere's disease

CONCLUSION

The bony saccular orifice to the bony groove of the ductus reuniens (reuniting duct) could not be visualized in the Meniere's ear with significantly greater frequency compared with normal subjects, which suggests that the reuniting duct is affected by radio-opaque substances in CT findings.

OBJECTIVE

This study investigated a more specific, objective, and simpler strategy to diagnose Meniere's disease by assessing the reuniting duct.

SUBJECTS AND METHODS

We examined the ears of 12 patients with definitely diagnosed unilateral Meniere's disease in stage 3 based on Meniere's disease criteria proposed by the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS), and the ears of 12 normal control subjects using three-dimensional (3D) cone beam CT.

RESULTS

The bony groove of the reuniting duct between the saccule and cochlea was visualized in all control subjects. However, the bony saccular orifice to the bony groove could not be visualized in the lesional ear of Meniere's patients with significantly greater frequency compared with those of the contralateral non-lesional ears and control ears (p < 0.01). This orifice was not patent in 66.7% (8 of 12 ears) on the lesional side but all contralateral non-lesional ears of the patients and normal control ears were patent.

Assessment of the reuniting duct by three-dimensional CT rendering

CONCLUSION

The rendering strategy sometimes induces misunderstanding of the image. We demonstrated a more accurate image of the bony groove of the reuniting duct using three-dimensional (3D) cone beam CT image, which was less affected by artifacts created by the rendering effect.

OBJECTIVE

To obtain a suitable image of the groove of the reuniting duct for future morphological study.

MATERIALS AND METHODS

The grooves of reuniting ducts in 10 healthy human subjects were analyzed by cone beam CT in comparison with a cadaver study.

RESULTS

We could obtain more accurate 3D CT images of the bony groove in human subjects by checking the landmarks of 3D CT images.

Three-dimensional images of the reuniting duct using cone beam CT

CONCLUSION

There is a bony groove under the course of the reuniting duct of the inner ear. Cone beam CT could show three-dimensional (3D) reconstruction images of this groove in a cadaver and living human subjects.

OBJECTIVE

To obtain simple and universal images of the reuniting duct in humans for clinical use.

MATERIALS AND METHODS

We investigated the reuniting duct macroscopically by observing the temporal bone in cadavers and living human subjects using cone beam CT. 3D reconstruction images of the duct were analyzed by the rendering software IVIEW.

RESULTS

The reuniting duct showed a bony groove between the saccule and cecum of the hook portion of the cochlea that could be three-dimensionally visualized by cone beam CT. A bony groove image in a living human subject could also be visualized by cone beam CT.

Meniere's disease

Meniere's disease is a chronic illness that affects a substantial number of patients every year worldwide. The disease is characterised by intermittent episodes of vertigo lasting from minutes to hours, with fluctuating sensorineural hearing loss, tinnitus, and aural pressure. Although there is currently no cure, more than 85% of patients with Meniere's disease are helped by either changes in lifestyle and medical treatment, or minimally invasive surgical procedures such as intratympanic steroid therapy, intratympanic gentamicin therapy, and endolymphatic sac surgery. Vestibular neurectomy has a very high rate of vertigo control and is available for patients with good hearing who have failed all other treatments. Labyrinthectomy is undertaken as a last resort and is best reserved for patients with unilateral disease and deafness.

Vestibular evoked myogenic potentials evoked by multichannel cochlear implant - influence of C levels

CONCLUSIONS

This study showed that vestibular evoked myogenic potentials (VEMPs) evoked by cochlear implant (CI), could be related to the comfortable level (C level), particularly in the channels that are closer to the apical turn of the cochlea.

OBJECTIVE

The purpose of this study was to investigate the correlation between VEMPs and C level of each channel.

SUBJECTS AND METHODS

We investigated 24 children who underwent cochlear implantation. VEMPs were recorded from the operated ears with the CI switched 'off' or 'on'. To investigate the correlation between VEMPs and C level, we selected 13 patients with Nucleus 24 (SPrint), and divided them into group A (normal VEMPs) and B (absence of VEMPs). In these children, all the 22 electrodes were active, and were mapped in the same frequency range for each channel.

RESULTS

Twenty children (83%) showed no VEMPs with the CI 'off'. Among them, 10 elicited VEMPs with the CI 'on', but the other 10 did not. In all channels, the mean C levels of CI were higher in group A than in group B. The p values in channels 1-12 were >0.10, in channels 13-16 were 0.06-0.09, and in channels 17-22 were 0.05-0.06, which were lower but not statistically significant.

Soluble megalin is accumulated in the lumen of the rat endolymphatic sac

The endolymphatic sac (ES) is believed to play an important role in maintaining homeostasis in the inner ear by the absorption and endocytosis of endolymph. Megalin is a 600-kDa multiligand endocytic receptor expressed in certain types of absorptive epithelia including kidney proximal tubules. We analyzed the immunoreactivity for megalin in rat ES by immunofluorescence, immunogold electron microscopy, and immunoblotting. With immunostaining, the luminal substances of the ES were strongly stained for megalin. Megalin was also localized in luminal macrophage-like cells and both types of epithelial cell (mitochondria-rich cells and ribosome-rich cells). In these cells, the megalin was localized in the lumen of endosomes, but was not membrane associated. This localization pattern indicates that the megalin in these cells is not a membrane receptor, but merely one of the constituents that are endocytosed from the lumen of the ES. Immunoblotting indicated that the megalin in the ES is a 210-kDa molecule lacking a cytoplasmic domain. This suggests that the megalin in the ES may be a soluble form, different from the 600-kDa membrane-bound receptor expressed in kidneys. Taken together, it is likely that the megalin in the ES lumen is a soluble component and may be endocytosed by the ES epithelial cells. Furthermore, we found that the tectorial membrane, an acellular structure in the cochlea, gave a strong megalin immunoreaction. Since the cochlea is connected to the ES, the megalin may be transported alone or with the components of the tectorial membrane from the cochlea to the ES lumen through longitudinal flow.

Histopathology of the Peripheral Vestibular System after Cochlear Implantation in the Human

OBJECTIVES

The objective of this study was to describe the histology of the peripheral vestibular system in temporal bones from patients who in life had undergone cochlear implantation and to correlate the findings with previous reports of vestibular dysfunction after cochlear implantation. This is the first quantitative report of the impact of implantation on the vestibular neuronal end organ. METHODSThere were 19 temporal bones available for histologic study. Of these, 17 were suitable for the description of the morphology of the membranous labyrinth, 8 for counting Scarpa's ganglion cells, and 6 for measuring the densities of vestibular hair cells. The bones were fixed, cut, and stained according to previously published methods. Preferably, the implanted electrode was left in situ. Vestibular hair cells were counted with Nomarski's optics.

RESULTS

Differences in Scarpa's ganglion cell counts and hair cell densities between the implanted and nonimplanted sides were not statistically significant. In 59% of the implanted bones, the cochlea was hydropic, and in the majority of these bones the saccule was collapsed.

CONCLUSION

Cochlear implantation does not cause deafferentation of the peripheral vestibular system. Cochlear hydrops accompanied by saccular collapse is common and may cause attacks of vertigo of delayed onset, similar to Ménière's syndrome as previously reported in several clinical series. Hydrops could be caused by obstruction of endolymphatic flow in the ductus reuniens or in the hook portion of the cochlea or by damage to the lateral cochlear wall caused by implantation.

Vestibular dysfunction of patients with mutations of Connexin 26

The gap junctional network of the inner ear plays an important role in cochlear ionic homoeostasis. Mutations of connexin 26 can induce different types of hearing loss and even deafness. Therefore, it is hypothesized that gap junctions of the human vestibular organ are functionally impaired by mutations of connexin 26. In a prospective, nonrandomized study, the functional status of the semicircular canals and the otolith organs was assessed in one homozygous and six heterozygous carriers of connexin 26 mutations. Five out of seven patients (71.4%) had pathological vestibular evoked myogenic potentials, indicating a loss of saccular function. The utricular function (as tested by subjective haptic vertical) and the function of the semicircular canals (as tested by recording the vestibuloocular reflex) were largely normal. Thus, connexin 26 mutations can be associated with saccular defects of the vestibular receptors.

Activation of caspase-3 is associated with oxidative stress in the hydropic guinea pig cochlea

The aim of this study was to investigate the involvement of oxidative stress and apoptosis in an animal model of Meniere's disease. Endolymphatic hydrops (ELH) is generally accepted as the decisive histological characteristic of Meniere's disease. Closure of the endolymphatic duct (Kimura's method) was used to induce endolymphatic hydrops in guinea pigs. Sham-operated animals served as controls. After 4 weeks the animals operated showed a significant elevation of the hearing thresholds as measured by audiometric brainstem responses (ABR) pre- and postoperatively. Immediately after the second ABR measurement, the animals were sacrificed for further immunohistological examinations of the inner ear with specific antibodies to active caspase-3 (cas-3) as a marker for apoptosis and antibodies to 8-isoprostane (8-iso) and nitrotyrosine (NT) as indicators of oxidative stress. Compared with the sham-operated controls, hydropic cochleae showed strong immunostaining for both oxidative stress markers in spiral ganglion cells, in the blood-vessels and fibrocytes of the lateral wall, as well as in supporting cells of the organ of Corti. Activation of cas-3 in spiral ganglion cells and the lateral wall was found exclusively in hydropic cochleae. Our findings suggest that oxidative stress is involved in the development of endolymphatic hydrops and may lead to cellular damage which induces apoptosis by activation of cas-3. Apoptotic cell death might contribute to the sensorineural hearing loss found in later stages of Meniere's disease.

Etiology, pathophysiology of symptoms, and pathogenesis of Meniere's disease

Endolymphatic hydrops is the pathologic feature associated with Meniere's disease. The development of endolymphatic hydrops appears to arise from multifactorial inheritance with alteration of endolymphatic homeostasis. Various factors associated with the phenomenon of hydrops include functional or anatomic obstruction of endolymphatic flow, malabsorption of endolymph, genetic anomalies, vasodilation, allergy, viral infection, and autoimmunity.

History of Meniere's disease and its clinical presentation

The term Meniere's disease is used to define either the classic triad of vestibular and cochlear symptoms and aural pressure from known or unknown causes or its clinical variants, vestibular and cochlear Meniere's disease. Some variants evolve after years into typical forms, whereas others do not. Some symptoms (positional vertigo) have been long underestimated in previous reports. The more we study our patients and correlate clinical findings and the natural history with pathologic studies on temporal bones and laboratory research, the more we will understand Meniere's disease and its causes. Some causes have already been identified as most probable. Extrinsic factors (inflammation, trauma, otosclerosis, autoimmunity, endocrine disorders, and such) interact with congenital (genetic) and developmental intrinsic factors (primary or secondary, acquired) into a multifactorial inheritance that is, to date, the best explanation for the basis of Meniere's disease. Endolymphatic hydrops is widely accepted as the pathologic substrate, but not all hydrops seems to be progressive or becomes clinically manifest. Endolymphatic hydrops is the result of a dysfunction in the mechanism of production/absorption of endolymph, which is mainly due to defective absorptive activity of the endolymphatic duct and sac. Hyperproduction of endolymph cannot be excluded in some cases. Ruptures of the labyrinthine membranes do not satisfactorily substantiate the multiform duration, recurrence, and repetitiveness of attacks of Meniere's disease, nor do they explain the entire complex of symptoms. It seems reasonable to explain symptoms of Meniere's disease on the basis of mechanical factors (as observed in temporal bone studies) associated with biologic and biochemical factors.

The endolymphatic sac in patients with Ménière's disease: correlation between the MRI and the surgical findings

The endolymphatic sac (ES) is thought to have close relations to pathogenesis of endolymphatic hydrops. Here is reported a retrospective study of 41 patients (42 ears) with Meniere's disease who underwent MRI prior to endolymphatic sac surgery. Based on proton-density weighted image (PDI) and T2-weighted image (T2), the ES including an endolymphatic duct (ED) were estimated whether it was detectable or not. Fourteen ESs were detected on both images (Group A), 14 ESs were detected only on PDI (Group B), and the remaining 14 ESs were not detected on either image (Group D). The actual shape of the sac, obtained from surgical findings, was classified into three (normoplastic, atrophic, invisible). Seventeen ears showed normoplastic ESs and 14 showed atrophic ESs. ES was not detected in 11 ears during surgery, and these findings were compared with image classification. From the study data, normoplastic ESs tend to be observed on both images whereas atrophic or invisible sacs were hardly observed on T2. This classification of ES on MRI was thought to correlate with surgical findings and this correlation was statistically significant (Spearman's rank correlation coefficient. r(s) = 0.58, p < 0.01).

Electronystagmographic findings in a case of Lermoyez's syndrome

A 40-year-old female patient with Lermoyez's syndrome is presented. Prior to the vertiginous attack, right low-tone hearing loss and tinnitus were noted. During the vertiginous attack, spontaneous nystagmus was directed to the right (lesioned) side, with improvement of cochlear symptoms. After the vertiginous attack, nystagmus was positional rather than spontaneous, and was directed to the left (opposite) side, with the subsidence of cochlear symptoms. One week later, follow-up audiometry revealed bilateral normal hearing.

Characteristic ionic composition of endolymph is maintained in cultured inner ear

Inner ear anlagen from mouse were explanted on the 16th gestational day (gd) and cultured for 5 days, i.e. corresponding to the time of birth. By using energy dispersive X-ray technique an elemental composition characteristic for endolymph was found within the membranous labyrinth of the explants. The sodium to potassium ratio in the endolymphatic space of the cultured inner ears corresponded to endolymph of the 16th gd fetus in vivo. There was no difference in the endolymph compartment between the cochlear and vestibular halves of the in vitro specimens. Differences in Na to K ratio between endolymph of the inner ears and the surrounding medium were statistically significant. Thus, endolymph-regulating mechanisms are active also under organ culture conditions, although not fully optimal.

High-resolution MR imaging of the inner ear: findings in Menière's disease

Symptoms in Menière's disease are characterized by hydrops of the endolymphatic system with recurrent rupture of the membranous labyrinth. The primary cause of the increased endolymphatic volume appears to be an imbalance between secretion and resorption of the endolymph which may be due to an obstruction of the membranous endolymphatic duct and sac, located in the vestibular aqueduct. The membranous endolymphatic duct and sac are not expected to be visualized using conventional tomography and high-resolution computed tomography (HR-CT), whereas, these are identified with high resolution MRI (HR-MRI). By HR-MRI, we proposed to demonstrate morphological alternation in 12 patients with Menière's disease, this group was compared with a group of 20 healthy subjects. The degree of visualization on HR-MRI of the membranous endolymphatic duct and sac running through the vestibular aqueduct in the bony canal was assessed. There was a distinct decrease in visualization of the membranous endolymphatic duct and sac in the Menière's group. The results confirm the value of the HR-MRI technique to identify an anatomical abnormality, which is directly correlated with the lesion in cases of unilateral Menière's disease.

Endocytotic activity of the free floating cells and epithelial cells in the endolymphatic sac: an electron microscopic study

The fine structure and its functional properties of both free floating cells and epithelial cells in the endolymphatic sac after injections of India ink particles or polystyrene latex beads, 0.24 micron in diameter, into the endolymphatic space, were studied using light and electron microscopy. Twenty-four to 48 hours after injections, these foreign materials had accumulated in the lumen of the endolymphatic sac and a large number of them were ingested into free floating cells, most of which appear to be macrophages. Granular leucocytes taking up the foreign materials into the cytoplasm were also recognized in the sac lumen. A few of these leucocytes passed through the epithelium and migrated to the subepithelial connective tissue, while the others were degenerated and phagocytosed by the free floating cells in the lumen of the sac. In addition to the free floating cells, ink particles, latex beads, and degenerated leucocytes were endocytosed into some of the epithelial cells. As we reported previously, the epithelial cells were clearly classified into two types; type-1 epithelial cells (cytoorganelle-rich cells) and type-2 epithelial cells (filament-rich cells). The foreign materials and degenerated cells were taken up mainly into the type-1 epithelial cells, while the type-2 epithelial cells did not show so marked endocytotic activity in comparison with the type-1 epithelial cells. Thus, it becomes clear that the endolymphatic sac plays an important role in the endocytotic activity for foreign materials and waste products, and both the free floating cells and the type-1 epithelial cells of the sac have strong endocytotic activity.

Pathogenesis and pathophysiology of Meniére's disease

Meniére's disease, neither spontaneous nor inducible in animals, is studied only in patients. Natural history (epidemiology) shows a triad of major symptoms: vestibular, auditory, and aural pressure. One in 3 patients has bilateral Meniére's; over full lifespans, bilaterality approaches 50%. Aural pressure (74.1%) and positional vertigo during/between attacks (85.9%) are common. Clinical variants can persist for 25+ years. All forms have delayed onset and can occur years after incitement, after otosclerosis, infections like otitis media, syphilis, or trauma. Endolymphatic hydrops is found in all, most importantly in pars inferior (cochlear duct and saccule). Some (not most) cases show ruptures. The saccule can distend into the lateral semicircular canal. Symptomatic attacks are explained on physical/biochemical bases. Both longitudinal (slow) and radial (fast) flow seem operational, longitudinal in advanced Meniére's where membranous labyrinth replaces perilymph in scala vestibuli and vestibule. All forms result from endolymphatic absorptive dysfunction (in duct and sac), with mastoid and periaqueductal hypocellularity, hypodevelopment of Trautmann's triangle, and anterior displacement of lateral sinus. Secondary obstructions in ductus reuniens or utricolo-endolymphatic valve may explain atypical Meniére's (vestibular or cochlear alone).

Otosclerosis involving the vestibular aqueduct and Menière's disease

The coexistence of otosclerosis and endolymphatic hydrops in the temporal bone have been described; however, the mechanism for the development of endolymphatic hydrops in otosclerosis remains unknown. Among 128 temporal bones with otosclerosis, involvement of the vestibular aqueduct by otosclerosis was observed in four temporal bones from two patients. In all four, the vestibular aqueduct was filled with active otosclerotic foci; the lumen of the endolymphatic duct and sac was narrowed as a result of fibrosis, and endolymphatic hydrops, more severe in the pars inferior than the pars superior, was observed. Collapse of the ductus reuniens and dilated saccule was seen in three temporal bones. Our study indicates that otosclerotic obstruction of the vestibular aqueduct may create a disturbance of the outflow and/or absorption of endolymph, leading to the development of endolymphatic hydrops and Meniere's disease, thus supporting the theory of longitudinal flow of endolymph.

Normal and pathologic features of the limbus spiralis and its functional significance

The normal and pathologic morphology of the limbus spiralis was studied by light and electron microscopy. Beneath the interdental cells are stellate, mesothelial, osmiophilic, and light types of fibrocytes. Morphologic evidence suggests that the stellate type cells occupying the main body of the limbus are actively engaged in the recycling of ground substance. The loosely interwoven mesothelial cells (dark cells) continuous with those of Reissner's membrane may participate in the secretion and maintenance of perilymph. Below this superficial cell lining are the osmiophilic and light cells, which appear to serve as a partial barrier to the exchange of substances between the scala vestibuli and the limbus proper. The collective data from experiments on vascular occlusion, drug toxicity, acoustic trauma, and perilymph and endolymph fistulae indicate that these connective tissue cells, particularly the stellate type of cells, are highly susceptible to injury. The lesions are often total or nonexistent, and are common in the basal and apical turns. When the lesion is small, the region adjacent to the inner sulcus cells is most frequently affected. In severe lesions, the adjacent interdental cells and sensory cells often remain normal. When the organ of Corti is concomitantly affected, the outer hair cells are usually missing. A common factor associated with limbus cell atrophy appears to be changes in ground substance due to the contamination of perilymph. Another factor associated with fluid contamination is the permanent or temporary cessation of blood flow to the limbus capillary arcades. The limbus spiralis may serve as one of the most sensitive indicators of alterations in the chemical composition of perilymph.

Imbalanced calcium homeostasis and endolymphatic hydrops

In this paper the current state of knowledge of the development in experimental endolymphatic hydrops (EEH) is summarized, with particular emphasis on calcium. An imbalanced Ca2+ homeostasis in the inner ear is demonstrated using EEH as an animal model for Meniere's disease. The possibility of a receptor-mediated Ca2+ transport across the epithelial layer, especially the light cells, and of 'chemical signal' as an initiating modulating factor in the disturbance of Ca2+ homeostasis was suggested. It is pointed out that melanin is capable of binding calcium and may act as a buffering system. Finally, the possible malfunction of the Ca-overloaded melanocytes on the inner ear function is discussed.

Ultrastructure of the stria vascularis and Reissner's membrane in experimental hydrops

A time-sequence study was made of the early ultrastructural changes of the stria vascularis and Reissner's membrane in the guinea pig after obliteration of the endolymphatic sac and duct. Pathological alterations of both the stria vascularis and Reissner's membrane were found to start in the apex of the cochlea. The morphological changes of the stria vascularis were characterized by an increase of vesicles in the marginal cells and by intercellular edema, followed by vacuolization and atrophy of marginal and intermediate cells. In Reissner's membrane extensive gaps in the mesothelial cell layer were observed together with intracellular pathology of the epithelial cells. The significance of these ultrastructural changes in the stria vascularis and Reissner's membrane with regard to the pathophysiology of the endolymphatic hydrops is discussed.

Effects of high intensity pure tone stimulation on the endolymphatic sac. Correlations between cochlear morphology and endolymphatic sac response

A systematic study of the effects of acoustic overstimulation on the endolymphatic sac (ES) in the guinea pig was performed. The ES was studied with light and transmission electron microscopy after exposure of the animals to a 3.85 kHz pure tone of 108 dB SPL or 120 dB SPL for 22.5 min (sound energy 9.4 and 150 Pa2 X h, respectively). The damage pattern in the organ of Corti was studied after various post-exposure times with SEM and correlated with the morphological characteristics of the ES in the same ear. This was made possible by using a modified technique for histological processing. In ears with induced structural abnormalities to the organ of Corti, the ES displayed few morphological changes without obvious signs of accumulation of cell debris within the lumen. Initially an increase in the amount of freely floating cells was found which persisted for at least 24 h. The role of the ES for disposal and digestion of locally produced degeneration products within the cochlea after acoustically generated structural damage is discussed.

Longitudinal flow of macromolecules in the endolymphatic space of the rat. An autoradiographical study

Secretion and resorption of sulphated glycoproteins by the epithelial lining of the endolymphatic space of the rat were studied autoradiographically with the use of 35SO4 as a selective marker. This isotope was found to be incorporated into the sensory area of cristae and maculae and a small area of epithelial cells adjacent to the sensory epithelium. Thereafter 35S-labelled glycoproteins were secreted into cupulae and otolithic membranes. No such process could be established in the tectorial membrane. From the gradual disappearance of 35S-labelled glycoproteins from cupulae and otolithic membrane coupled with an accumulation of this substance in the lumen of the endolymphatic sac, the existence of a longitudinal flow under physiological conditions could be concluded. Resorption of glycoproteins by the saccus epithelium was found to be very slow. Accumulation of 35S-labelled glycoproteins in the endolymphatic duct after saccus obstruction indicates that, in addition to the endolymphatic sac, also the duct contributes to the mechanism underlying the longitudinal flow of macromolecules.

Direct measurement of longitudinal endolymph flow rate in the guinea pig cochlea

The rate of longitudinal endolymph flow in the guinea pig cochlea has been measured with a novel tracer technique. The tracer we utilized was the tetramethylammonium (TMA) ion, the movement of which was monitored by ion-sensitive microelectrodes. Extremely small amounts of tracer were required as the electrodes could readily detect TMA concentrations in endolymph as low as 10 microM. TMA was introduced into scala media in the form of a small bolus, varying from 2-20 nl in volume. To examine whether longitudinal flow affects the dispersion of TMA in endolymph, we compared the characteristics of TMA spread to turn I following injection into turn II, with those of TMA spread to turn II following injection into turn I. The comparison of these data with an analytical model combining the processes of diffusion and volume flow demonstrates that the spread of tracer is dominated by passive diffusion processes with very little contribution from longitudinal endolymph flow. The rate of longitudinal endolymph flow between turn I and turn II was estimated to be less than 0.01 mm/min directed towards the basal turn. This value is considerably lower than recently published estimates using other techniques.

Ionic environment of cochlear hair cells

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

An experimental study of inner ear pathology due to NaCl on the round window

An animal experiment was performed to evaluate structural and ultrastructural changes to the inner ear as a result of placing 3-4 crystals of reagent grade sodium chloride (NaCl) on the round window membrane. Chinchillas were sacrificed at 8 and 24 hours after treatment and the cochlear and vestibular tissues were examined by light microscopy and scanning electron microscopy. Inner ear pathology consisted of destruction of both sensory and supporting cells in the basal turn of the organ of Corti, atrophy of the stria vascularis and alterations to the otoconia and the maculae and ampullae of the vestibular system. This study demonstrates that NaCl in the middle ear does not provide a model for Meniere's disease as previously suggested by Arslan. It may, however, be utilized in the destructive treatment of selected inner ear disorders.