Anatomy of the Round Window Region With Relation to Selection of Entry Site Into the Scala Tympani.

OBJECTIVES/HYPOTHESIS: The aim of cochlear implantation is to safely insert an electrode array into the scala tympani (ST) while avoiding damage to surrounding structures. There is disagreement on the optimal way of entering the ST-the round window (RW) approach versus cochleostomy. Regardless of the chosen approach, it is vital to understand the regional anatomy, which is complex, difficult to conceptualize, and rarely dissected in temporal bone courses. The goal of this study was to examine the anatomy of the RW to gain more in-depth knowledge on the local relationships of the anatomical structures and propose an approach for entering the ST in cochlear implant surgery tailored to the encountered anatomy. STUDY DESIGN: Cadaveric prevalence study and expert opinion with literature review. METHODS: Cadaveric temporal bone dissection (n = 13) by the first author assessing the RW anatomy. RESULTS: The round window membrane (RWM) and the osseous spiral lamina (OSL) are curved structures, each with a horizontal and a vertical part. The two horizontal portions are very closely apposed. The relationship between the OSL and the RWM determines the best site for a cochleostomy, which if required is best placed anteroinferiorly to the RWM. The distance between the oval window inferior margin and the RW membrane is less than 2 to 3 mm. The ST initially extends inferiorly and medially to the RW. CONCLUSIONS: The findings of our dissection have implications for cochlear implant surgery in aiming to avoid trauma to the OSL and basilar membrane and aid decision making in choosing the safest surgical approach. LEVEL OF EVIDENCE: 5. Laryngoscope, 131:E598-E604, 2021.

Surface microstructure of the perilymphatic space: implications for cochlear implants and cell- or drug-based therapies.

OBJECTIVE: To study the surface microstructure of the scala tympani and scala vestibuli in humans and cats using scanning electron microscopy. DESIGN: Cochleas from 8 humans and 4 cats were harvested and the otic capsule and soft tissue removed before the cochleas were prepared for scanning electron microscopy. Micrographs were taken of the bony surface of both the scala tympani and scala vestibuli in each cochlear turn. The diameter and density of the micropores (canaliculi perforantes) and the thickness of the osseous spiral lamina (OSL) adjacent to Rosenthal's canal was measured. RESULTS: The human cochlea exhibits numerous canaliculi on the surface of the scala tympani, particularly associated with the OSL. There was a large range of diameters in the modiolar region of the OSL (0.2-23.0 micro m). The OSL was also very thin, with a mean thickness of 26.8 micro m in the base, tapering to 8.4 micro m in the apical turn. Far fewer canaliculi were evident in the scala vestibuli. Examination of the cat cochleas showed a similar distribution of canaliculi to that seen in the human; however, they were smaller in diameter and the OSL was thicker than in the human cochleas. CONCLUSIONS: The OSL is a thin and highly porous bony lamina that would appear to provide an open and extensive fluid communication channel between the scala tympani and Rosenthal's canal. These findings have important implications for the design and application of perimodiolar cochlear implant electrode arrays and may provide a potential route for drug- and cell-based cochlear therapies delivered via the scala tympani.

Possible involvement of nitric oxide in the sensorineural hearing loss of bacterial meningitis.

Microperfusion of scala tympani with the NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP), produced marked depression of the compound action potential (CAP) and cochlear microphonic (CM) together with severe and widespread morphological damage to hair cells and supporting cells in the organ of Corti. In addition, direct perfusion of N-methyl-D-aspartate (NMDA) into scala tympani, which probably induces excess stimulation of NMDA receptors within the cochlea and which is known to lead to the release of NO, was found to elicit similar electrophysiological and structural lesions in the cochlea. Pre-perfusion of scala tympani with L-methyl arginine (L-MA), which inhibits the release of NO, or superoxide dismutase (SOD), an O2-scavenger, conferred marked protection upon the cochlea from the lesions caused by NO donors. These observations indicate that enhanced NO production is likely to be an important factor responsible for pathological insult of the cochlea. The possibility is discussed that this factor is involved in the chain of events leading to hearing loss caused by bacterial meningitis. Such hearing loss is a major sequela of bacterial meningitis in children.

Apical cochlear nerve exposed to perilymph in the gerbil and rat.

There is considerable controversy regarding the origin and composition of perilymph in the scala vestibuli and scala tympani and the barriers and transport mechanisms between them. To elucidate the anatomical separation between perilymph and the extracellular fluid which surrounds the cochlear nerve in Mongolian gerbils (Meriones unguiculatus) and Sprague Dawley rats (Rattus rattus) we examined the modiolus, osseous spiral lamina and the bone of the perilymphatic scalae using light and electron microscopy. Although the cochlear nerve, spiral ganglion and their extracellular fluid are separated from perilymph by the bone of the modiolus in the middle and basal turns, no bone separates these neural structures from perilymph in the apical turn in the two species examined. Instead, the spiral ganglion and axonal elements of the apical turn were covered by a continuation of the bone lining cell layer of the scala tympani. Gaps between lining cells appeared to provide direct communication between the perilymphatic fluid and the extracellular fluid investing the cochlear nerve. Various authors have described openings in modiolar bone in both the scala vestibuli and the scala tympani. While the bone and cellular covering of the modiolus in the basal middle turns of the cochlea is not presumed to be a complete barrier to fluid exchange between the two scalae and the modiolar canal it can be expected to impose some limitation on the rate of passive diffusion. Therefore our data indicates that in the apical turn of the Mongolian gerbil and Sprague-Dawley rat there may be a more significant communication between perilymph and the extracellular fluid and neural elements of the apical modiolar canal than previously reported.

The cochlear glomeruli in the modiolus of the guinea pig.

The cochlear glomeruli were studied in guinea pigs using scanning electron microscopy of vascular corrosion casts and transmission electron microscopy of tissue sections. Two types of coiled vessels forming the cochlear glomeruli were found in the bony wall of the modiolus. First, upper glomeruli were seen situated in the bony wall next to the scala vestibuli; second, lower glomeruli were located in the osseous spiral lamina just above the spiral ganglion. Upper glomeruli gave rise to radiating arterioles which supplied capillaries of the stria the spiral lamina and limbus. Unlike the main supplying arteries, smooth muscle cells were not present in the walls of the arterioles forming the glomeruli and a peculiar layer of lamellar pericytes was found. The arterioles were strikingly longer than their parent vessels and no autonomic nerves were found in close spatial relationship. Hence, these findings indicate that the cochlear glomeruli serve as efficient devices for reducing cochlear blood pressure.

Ultrastructure of the guinea pig cochlear aqueduct. An electron microscopic study of decalcified temporal bones.

The ultrastructure of the guinea pig cochlear aqueduct was examined using semi-thin and thin sections. The lumen of the cochlear aqueduct was occupied by a sparse meshwork of fibroblasts and delicate connective tissue trabeculae. The periotic tissue lining the bony wall of the aqueduct was composed of multiple layers of both elongated cells and densely arranged laminae of collagen fibrils. These structures were identical to those of the dura mater and the arachnoid. The opening to the perilymphatic space of the scala tympani also contained connective tissue trabeculae, but the arrangement of fibroblasts was more compact here than in the main part of the duct. These structural features suggest that fluid can move freely through cochlear aqueduct, and that the effects of sudden pressure changes in the CSF may be protected against by the densely and perpendicularly arranged fibroblast at the opening to the perilymphatic space.

Scanning electron microscopic studies of the capillaries crossing the scala tympani.

Scanning electron microscopic examinations were carried out on the perilymphatic space bordering the round window in guinea pigs. A number of small vessels were found crossing free through the lumen of the scala tympani in this area. The larger number (5-10) of these capillaries are suspended between the bony cochlear wall and the terminal net of fibrocytes which covers the cochlear opening of the cochlear aqueduct and spreads onto the basal third of the round window membrane. Other capillaries (3-5) cross the lumen of the scala tympani from the outer cochlear wall to the modiolus. All of these capillaries have a thin endothelium and a very thin pericytic covering. These findings suggest that the capillaries crossing the perilymphatic space may give rise to a different chemical composition of the perilymph bordering the round window in comparison with the more upper parts of the scala tympani.

Histological study of chronic electrode implantation through the round window of the guinea pig.

A platinum-iridium electrode with a Teflon cover tube was chronically implanted through the round window into the scala tympani of the guinea pig. Animals were sacrificed at 30 to 40 weeks after electrode implantation, and the cochleae were observed by scanning electron microscopy or with light microscopy. The results revealed that the electrode was firmly enclosed with fibrous tissue on the inside of the round window and with epithelial tissue on the outside. These tissues were considered to act as a barrier to the entry of infection while preventing leakage of the perilymph.

Morphological changes following cochlear implantation in the animal model.

Examination of the effects of cochlear implantation in the animal model has made it increasingly clear that biological changes may be induced by the device. In a preliminary study we established a damage threshold at 400 microA RMA (70 microCoul/cm2/phi) after a single three-hour exposure to continuous 1 kHz sinusoidal stimulation. By systematic modification of stimulation parameters in the present study we have sought to establish damage thresholds and patterns of histological change in the chronically stimulated animal and further elucidate the mechanisms underlying the histopathological change in a second acute animal preparation. In part one, chronic stimulation at 1 000 Hz for a total of 12 hours distributed over a four-week period resulted in a lower damage threshold at 15-20 microCoul/cm2/phi, the lowest value tested. In addition to sensory and supporting cell degeneration osteoneogenesis and spiral ganglion cell degeneration were also observed. These findings are of some relevance under conditions of prolonged implantation and stimulation in the human. In part two of the study, acute stimulation for a three-hour period at 100 Hz reduced the damage threshold from 400 microA to 200 microA RMS. This inverse relationship between threshold and frequency suggests that electrophoretic effects are, at least in part, responsible for the histological changes observed. The threshold shift was less than predicted, however, making it less likely that a single factor is responsible for the stimulation-induced damage.

Morphological changes in the normal and neomycin-perfused guinea pig cochlea following chronic prosthetic implantation.

The effects of chronic prosthetic implantation and interval electrical stimulation were studied in the normal and neomycin-perfused cochlea of the guinea pig. One group of guinea pigs was implanted with a multiple-electrode prosthesis in the scala tympani. During a 4-week period, the device was stimulated for 3 hours weekly with a continuous, 1 kHz sinusoidal current of constant intensity. A second group of guinea pigs underwent identical implantation and stimulation except that cochlear perfusion with .1 M neomycin was performed at the time of implantation. Current intensities ranged from .1 to .6 mA RMS. Two complementary control groups were implanted but not stimulated. The animals were sacrificed, the cochleae were perfused with a fixative, and the temporal bones were prepared for examination under a light, transmission, or scanning electron microscope. In the electrically stimulated cochleae, degenerative changes occurred in both the inner and outer hair cells and supporting elements. A decrease was apparent in spiral ganglion cell and nerve fiber populations in areas of inner hair cell depletion and did not seem to correspond to the survival of supporting cells. The electrically active electrodes were uniformly surrounded by a connective tissue matrix and areas of immature bone. These changes occurred at all the intensities tested, and did not monotonically relate in severity to current intensity. None of the changes was apparent in the normal control ears. Morphological changes induced by the ototoxic drug neomycin were so severe is both stimulated and unstimulated cochleae that comparison was not possible; this form of pretreatment is apparently unsuitable for use in studies of electrically induced damage. It appears that in the normal animal, chronic implantation with interval electrical stimulation results in a cumulative form of sensory and neural damage histologically similar to that proceeding from chronic noise exposure as well as other ototoxic agents. Such effects should be minimized if surviving sensorineural and supporting elements in the functionally compromised cochlea are to be preserved.

Cochlear pathology with chronically implanted scala tympani electrodes.

In summary, these results with chronic implantation of three types of scala tympani electrode indicate the critical importance of two factors in reducing the risk of additional damage to cochlear structure by implantation surgery: (1) The shape and mechanical characteristics of the array must be precisely controlled such that insertion can be performed with an acceptably low incidence of trauma; and (2) the specific electrode materials and fabrication procedures must be demonstrated to be highly biocompatible in in vivo animal control studies. It appears that the neuronal elements of a prior normal cochlea (in the cat) can withstand chronic implantation of scala tympani electrodes (for at least 1 year) if these two prerequisites are met. The effects of chronic electrical stimulation with these arrays at current levels appropriate for the operation of such devices in patients is not known at present.

Lymphocytic streaming as an indicator of inner ear compartmentalization.

Despite a great deal of anatomic and physiologic data in animals, controversy still exists over whether or not the perilymphatic space in man is directly connected to the intracranial space via a patent cochlear aqueduct or other fluid channel. Human physiologic data are limited, indirect, and conflicting. Anatomic and pathologic data have heretofore been inadequate for answering the question convincingly. The temporal bones of a 19-year-old woman with central nervous system lymphoblastic leukemia are discussed. The passive-appearing movement of lymphoblasts between cerebrospinal fluid and perilymphatic spaces suggests both a functionally patent cochlear aqueduct and alternate pathways.

A scanning electron microscopic study of the guinea pig cochlear aqueduct. With emphasis on fracture preparation.

The cochlear aqueduct is the duct that connects the perilymphatic space of the scala tympani with the subarachnoid space. However, its functions are not yet fully elucidated. In the present study on the cochlear aqueduct of the guinea pig, a fracture preparation method was used to obtain new findings by scanning electron microscopy.

Ultrastructure of the turkey's basilar papilla and the damping elements in the scala media.

The ultrastructure of the turkey's inner ear was investigated by use of scanning and transmission electron microscopes. It was shown that hair cells of the basilar papilla are separated from each other by microvilli of the supporting cells. Organized compartments in the lower surface of the tectorial membrane of the turkey are arranged in polarized fashion as are the hair bundles. Kinocilia were not identified in the hair bundles examined in this study. Well-organized filamentous elements were observed in the cutucular plates. The continuation of these filamentous elements into the stereocilia indicates that the theory of adjustable resistor in generating cochlear potential may be closely associated with these elements. Membrane bound homogeneous material was found in the scala media between the tegmentum vasculosum and the tectorial membrane. The presence of this homogeneous damping material is believed to be essential in offsetting the sensitivity and enhancing the time resolution ability of the relatively short avian cochlea.