Round window membrane permeability. An in vitro model

The round window membrane is regarded to be the main route for passage of potentially ototoxic substances from the middle ear cavity into the inner ear. This may be of clinical importance in acute otitis media and chronic otitis media, where sensorineural hearing impairment sometimes develops. The accuracy and reliability of an in vitro round window membrane permeability model was studied. The round window membrane of the mongolian gerbil was resected, together with its bony niche. The preparation was mounted between two glass chambers representing the middle ear and the inner ear. Passage through the round window membrane did not occur within 3 h for low density lipoprotein with a molecular weight of 2,300 kD. Only minute amounts of highly density lipoprotein, of molecular weight 115-350 kD, passed the round window membrane. The passage rate of horseradish peroxidase, which has previously been shown to pass the round window membrane in vivo, was estimated. The design of the present model is considered to make feasible controlled permeability studies on the round window membrane. Passage rates for different substances through the round window membrane can be calculated under controlled conditions by using this type of in vitro model.     

Permeability of the normal round window membrane to Haemophilus influenzae type b endotoxin.

Sensorineural hearing loss associated with otitis media may be due to passage of ototoxic substances such as bacterial toxins and antibiotics, from the middle ear into the inner ear. The round window membrane is the most likely route for such transport. The aim of this study was to analyze the extent of endotoxin passage through the normal round window membrane. The round window membranes of 19 chinchillas were exposed in vivo to Gelfoam soaked in purified Haemophilus influenzae type b endotoxin at a concentration of 45,000 endotoxin units per ml (EU/ml) during 3 to 24 h. Endotoxin levels in the perilymph were measured with Limulus Amaebocyte Lysate or Quantitative Chromogenic Limulus Amaebocyte Lysate. Endotoxin was detected in half of the inner ears at concentrations close to the detection limit (approximately 4 EU/ml). The results suggest that the normal round window membrane efficiently protects the inner ear against the passage of bacterial endotoxins from the middle ear cavity. It is unlikely that endotoxin at concentrations found in the middle ear secretion during otitis media can traverse the round window membrane in sufficient amount to cause inner ear deterioration.     

Round window membrane and labyrinthine pathological changes: an overview

The round window membrane is considered the most likely pathway from the middle to the inner ear. Various substances placed in the middle ear have been seen to pass through the round window membrane. Once toxic substances or inflammatory mediators such as cytokines and nitric oxide enter the inner ear, various inner ear sequelae such as labyrinthitis, endolymphatic hydrops, sensorineural hearing loss or more insidious diseases can occur.     

Round window membrane and labyrinthine pathological changes: an overview

The round window membrane is considered the most likely pathway from the middle to the inner ear. Various substances placed in the middle ear have been seen to pass through the round window membrane. Once toxic substances or inflammatory mediators such as cytokines and nitric oxide enter the inner ear, various inner ear sequelae such as labyrinthitis, endolymphatic hydrops, sensorineural hearing loss or more insidious diseases can occur.     

Clinical Aspects of Round Window Membrane Permeability Under Normal and Pathological Conditions

Current research and an overall review of 25 years of round window membrane studies are presented. The approach, rationale and concepts that have evolved from these studies are described. Ultrastructural studies of the round window membrane of humans, monkeys, felines and rodents have disclosed three basic layers: an outer epithelium, a middle core of connective tissue and an inner epithelium. Interspecies variations are mainly in terms of thickness, being thinnest in rodents and thickest in humans. Morphologic evidence suggests that the layers of the round window participate in resorption and secretion of substances to and from the inner ear, and that the membrane could play a role in the defense system of the ear. Different substances, including antibiotics and tracers, when placed in the middle ear side traverse the membrane. Tracers placed in perilymph become incorporated into the membrane by the inner epithelial cells. Permeability is selective and factors affecting permeability include size, concentration, electrical charge, thickness of the membrane and facilitating agents. Passage of substances through the membrane is by different pathways, the nature of which is seemingly decided at the outer epithelium of the membrane. Round window membrane studies have provided increased knowledge of the anatomy and function of this structure, as well as new insights into pathology and pathogenesis. The concepts that have evolved from these studies are potentially useful for understanding middle and inner ear interactions, and for eventual drug delivery (based on permeability) to the inner ear.     

Clinical aspects of round window membrane permeability under normal and pathological conditions

Current research and an overall review of 25 years of round window membrane studies are presented. The approach, rationale and concepts that have evolved from these studies are described. Ultrastructural studies of the round window membrane of humans, monkeys, felines and rodents have disclosed three basic layers: an outer epithelium, a middle core of connective tissue and an inner epithelium. Interspecies variations are mainly in terms of thickness, being thinnest in rodents and thickest in humans. Morphologic evidence suggests that the layers of the round window participate in resorption and secretion of substances to and from the inner ear, and that the membrane could play a role in the defense system of the ear. Different substances, including antibiotics and tracers, when placed in the middle ear side traverse the membrane. Tracers placed in perilymph become incorporated into the membrane by the inner epithelial cells. Permeability is selective and factors affecting permeability include size, concentration, electrical charge, thickness of the membrane and tacilitating agents. Passage of substances through the membrane is by different pathways, the nature of which is seemingly decided at the outer epithelium of the membrane. Round window membrane studies have provided increased knowledge of the anatomy and function of this structure, as well as new insights into pathology and pathogenesis. The concepts that have evolved from these studies are potentially useful for understanding middle and inner ear interactions, and for eventual drug delivery (based on permeability) to the inner ear.     

Ciprofloxacin and the inner ear--a morphological and round window membrane permeability study.

This study was conducted in order to evaluate ototoxic potential and passage through the round window membrane of ciprofloxacin, a newly developed antimicrobial drug with an antibacterial spectrum well suited for the treatment of suppurative otitis media. Ciprofloxacin was injected intraperitoneally in 40 guinea pigs in doses ranging from 25 to 150 mg/kg body weight for 14 consecutive days. Sixteen inner ears of chinchillas were exposed to ciprofloxacin at a concentration of 0.1 mg/ml applied directly to the intact round window membrane. Despite severe deterioration of the animals injected with the highest doses, no behavioral or definite morphological evidence of inner ear or vestibular damage could be seen. Ciprofloxacin concentrations in the perilymph were determined with high-performance liquid chromatography. Direct passage of ciprofloxacin, presumably via the round window membrane, was demonstrated. After 1 h and 15 min, an arithmetic mean concentration of 0.165 micrograms/ml (95% confidence interval +/- 0.053) could be demonstrated in the perilymph. It can be concluded that ciprofloxacin has no ototoxic effect when administered systemically. Furthermore, when locally applied into the middle ear, it has the capacity to pass into the inner ear.     

Blood flow of the round window

Summary Motion pictures of the blood flow of the round window in guinea pigs were taken. In this part the blood flow of the inner ear could be distinguished from that of the middle ear. Our motion pictures show that the blood flow of the inner ear can be observed through the round window membrane without an operation on the inner ear.     

Cochlear Aqueduct Flow Resistance Depends on Round Window Membrane Position in Guinea Pigs

The resistance for fluid flow of the cochlear aqueduct was measured in guinea pigs for different positions of the round window membrane. These different positions were obtained by applying different constant pressures to the middle ear cavity. Fluid flow through the aqueduct was induced by small pressure steps superimposed on these constant pressures. It was found that the resistance for fluid flow through the aqueduct depended on the round window position but not on flow direction. The results can be explained by special fibrous structures that connect the round window with the entrance of the aqueduct. It was also found that the equilibrium inner ear pressure depends on middle ear pressure, indicating that the aqueduct does not connect the inner ear with a cavity with constant pressure.     

Ultrastructure of the round window membrane

The round window membrane presents the only non-osseous wall of the labyrinth. Electron microscopic study of the round window membrane was undertaken in order to determine whether it could act as a route of transmission from the middle to the inner ear. In the cat round window membrane three layers can be identified. The tympanic surface is composed of a single layer of flat cells resting on the basal lamina. Multiple desmosomes and tight cell junctions are present. The intermediate layer contains supporting elements collagen and elastic tissues. The labyrinthine layer is composed of a single layer of long squamous cells which lie end to end on the basal lamina. The absence of intracellular organelles and microvilli, and the lack of basal lamina involutions into the epithelial cells preclude the presence of an active transport mechanism. The tight cell junctions also seem to present a barrier to free diffusion across the membrane. The chemical agents which have been observed to pass from the middle ear into the inner ear may have done so by diffusion through the round window membrane after altering the cell junctions; however, a readily accessible route of transmission through the round window membrane does not appear to be present.     

Extraneous round window membranes and plugs: possible effect on intratympanic therapy

Recently there has been increasing interest in the possibility of treating inner ear disorders by application of medication into the middle ear on the premise that it will diffuse through the round window membrane into the inner ear. We examined 202 temporal bones from 117 patients to determine the frequency of round window niche obstruction. Patients ranged in age at the time of death from 31 to 97 years. Eleven percent of the ears were found to have fibrous tissue or a fat plug, and 21% had an extraneous (false) round window membrane. Of the 85 patients from whom both temporal bones were examined, 56% had no obstruction in either ear, while 22% had obstruction in both ears. We conclude that anatomic variations of the round window niche may explain the wide variations found in dosage of medication required to produce a clinical result.     

Inner ear damage and passage through the round window membrane of Pseudomonas aeruginosa exotoxin A in a chinchilla model.

By the use of computer-assisted morphometric analysis of the organ of Corti and/or measurements of action potential threshold changes, inner ear changes in chinchillas were evaluated 4 weeks after application on the round window membrane of a Pseudomonas aeruginosa exotoxin A solution. Severe inner ear damage was detected after application of 50 ng (5 microL at a concentration of 10 micrograms/mL) exotoxin A, whereas application of 5 ng exotoxin did not cause measurable inner ear damage. Perilymph concentrations of exotoxin A were measured with an enzyme-linked immunosorbent assay 1.5 to 19 hours after 50 ng, 0.5 micrograms, or 5 micrograms of exotoxin A was applied on the round window membrane. Only the highest concentration produced measurable levels of exotoxin in the inner ear fluids. It is concluded that exotoxin A present on the round window membrane of the chinchilla has the ability to penetrate into the inner ear and cause irreversible inner ear changes.     

Use of the malleus handle as a landmark for localizing the round window membrane

Localization of the round window membrane is desirable in order to allow a more direct application of medication to the inner ear. A new procedure has been developed to deliver direct, near-continuous inner ear perfusion to the round window with the Silverstein MicroWick. In this office procedure, the wick is inserted through a tympanostomy tube into the round window niche. Accurate localization of the round window is a necessary component of this procedure. In an effort to ascertain the precise location of the round window, we examined 25 cadaveric human temporal bones and measured the distance from the umbo to the round window in each sample. We found that the round window was an average of 3.44 mm (+/- 0.68) from the umbo and was situated at an average angle of 113.2 degrees (+/- 9.8) from the long process of the malleus. Our simple and reliable determination of the relationship between the malleus handle and the round window niche allows for the accurate placement of the Silverstein MicroWick and other devices.     

Cochlear hair cell loss in single-dose versus continuous round window administration of gentamicin

CONCLUSIONS: Gentamicin-induced cochlear hair cell loss depends on local middle ear administration kinetics and the total drug dose. Single-dose gentamicin instillation in the middle ear is associated with a high variation in hair cell loss. OBJECTIVE: To compare the effects of single-dose and continuous round window administration of gentamicin on cochlear hair cell loss in a guinea pig model. MATERIAL AND METHODS: Two methods for drug administration to the inner ear were used. In groups of five animals, a total dose of 0.8 or 3.2 mg of gentamicin was either instilled as a single dose directly into the round window niche or administered continuously over a 1-week period using a pump-catheter system. Continuous administration was achieved by means of a posterior tympanotomy and subcutaneous placement of an osmotic pump fitted with a catheter. The tip of the catheter was fixed in the round window niche. One group of five animals served as controls and received a saline infusion. The animals were sacrificed after 1 week and hair cell loss was determined microscopically after dissection and phalloidin labelling of the basilar membrane and organ of Corti. RESULTS: Quantitation of cochlear hair cell loss revealed a dose-dependent effect of gentamicin. With both treatment modalities the higher dose induced a higher percentage of hair cell loss. There was inner and outer hair cell loss in all four groups that received gentamicin. With the single-dose instillation, hair cell loss was distributed irregularly from the round window membrane towards the cochlear apex, whereas continuous administration induced hair cell loss close to the round window membrane. Single-dose instillation induced greater hair cell loss than continuous administration at the same dose. The inter-individual variation in hair cell loss was highest following single-dose instillation.     

Gas permeability of round window membrane

The perilymphatic oxygen tension in the scala tympani was measured with a polarographic technique after various gases were insufflated into the middle ear cavity of 7 guinea pigs. Pure oxygen, insufflated into the middle ear cavity, easily permeated the round window membrane and elevated the perilymphatic oxygen tension. Oxygen transport to the inner ear, other than by the inner ear vessels, was confirmed. The inner ear function may partially be maintained by oxygenation through this route.     

Experimental grafting of the round window membrane: Part I

The histology and permeability of cats' round window membranes were described following surgical grafting with gelatin foam or fascia. An increase in the thickness of the round window membrane due to fibrosis, neovascularization, and epithelial metaplasia was observed 2 months following grafting with gelatin foam. Grafting with fascia did not increase the thickness of the round window membrane, but did form a thick layer of granulation tissue adjacent to and overlying the round window membrane, with occasional adhesions between. The layer of granulation tissue consisted of cuboidal or columnar ciliated epithelial cells, fibrous tissue, lipid, and undigested grafted material. The granulation tissue did not always completely cover the round window membranes. Horseradish peroxidase placed in the round window niche of cats with grafts on the round window membrane remained a significant distance further from the inner ear when compared to its migration through nongrafted membranes.     

A novel controlled local drug delivery system for inner ear disease

PURPOSE: Our goal is to develop a novel drug delivery system that can potentially improve clinical outcomes compared to current methods of dosing drugs such as dexamethasone or gentamicin. This system focuses on a single local application to the inner ear via the round window membrane. HYPOTHESIS: A chitosan-glycerophosphate (CGP)-hydrogel based drug delivery system can be engineered to provide local and sustained drug release to the inner ear. STUDY DESIGN: In vitro: drug release and (CGP)-hydrogel matrix degradation were characterized using dexamethasone as a model drug. In vivo: dexamethasone laden CGP-hydrogel was placed in the round window niche of mice. Perilymph samples were obtained from the oval window and analyzed for dexamethasone. The impact of CGP-hydrogel on auditory function was evaluated. RESULTS: In vitro: A CGP-hydrogel was designed to release 92% of the dexamethasone load over 4 consecutive days with concurrent degradation of the hydrogel matrix. In vivo: After surgical placement of CGP-hydrogel to the round window niche, we detected elevated levels of dexamethasone in perilymph for 5 days. Auditory function testing revealed a temporary hearing loss in the immediate postoperative period, which resolved by the 10th postoperative day. CONCLUSIONS: We report the development of CGP-hydrogel, a biodegradable matrix that achieves local, sustained delivery of dexamethasone to the inner ear. There were no significant complications resulting from the surgical procedure or the administration of CGP-hydrogel to our murine model.     

The pathway enabling external sounds to reach and excite the fetal inner ear

The human fetus in utero is able to respond to sounds in the amniotic fluid enveloping the fetus after about 20 weeks gestation. The pathway by which sound reaches and activates the fetal inner ear is not entirely known. It has been suggested that in this total fluid environment, the tympanic membrane and the round window membrane become 'transparent' to the sound field, enabling the sounds to reach the inner ear directly through the tympanic membrane and the round window membrane. It is also possible that sounds reach the inner ear by means of tympanic membrane--ossicular chain--stapes footplate conduction (as in normal air conduction). There is also evidence that sounds reach the fetal inner ear by bone conduction. Several animal and human models of the fetus in utero were studied here in order to investigate the pathway enabling sounds to reach and activate the fetal inner ear. This included studying the auditory responses to sound stimuli of animals and humans under water. It was clearly shown in all the models that the dominant mechanism was bone conduction, with little if any contribution from the external and middle ears. Based on earlier experiments on the mechanism and pathway of bone conduction, the results of this study lead to the suggestion that the skull bone vibrations induced by the sound field in the amniotic fluid enveloping the fetus probably give rise to a sound field within the fetal cranial cavity (brain and CSF) which reaches the fetal inner ear through fluid communication channels connecting the cranial cavity and the inner ear.     

Permeability of the round window membrane to middle-sized molecules in purulent otitis media

The effect of pneumococcal otitis media on the permeability of the round window membrane was evaluated using tetraethylammonium ions as a tracer. Round window membrane permeability is reduced significantly at resolved stages of purulent otitis media. In contrast, measurements of round window membrane permeability indicate that acute purulent otitis media has a tendency to facilitate such permeability. Moreover, histologic observations of the round window membrane following bacterial inoculation further support the evidence of functional changes in round window membrane permeability. These findings indicate that the round window membrane in resolved stages of purulent otitis media plays a protective role in preventing the penetration of harmful substances into the inner ear.