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.

Unified cochlear model for low- and high-frequency mammalian hearing.

The spatial variations of the intricate cytoarchitecture, fluid scalae, and mechano-electric transduction in the mammalian cochlea have long been postulated to provide the organ with the ability to perform a real-time, time-frequency processing of sound. However, the precise manner by which this tripartite coupling enables the exquisite cochlear filtering has yet to be articulated in a base-to-apex mathematical model. Moreover, while sound-evoked tuning curves derived from mechanical gains are excellent surrogates for auditory nerve fiber thresholds at the base of the cochlea, this correlation fails at the apex. The key factors influencing the divergence of both mechanical and neural tuning at the apex, as well as the spatial variation of mechanical tuning, are incompletely understood. We develop a model that shows that the mechanical effects arising from the combination of the taper of the cochlear scalae and the spatial variation of the cytoarchitecture of the cochlea provide robust mechanisms that modulate the outer hair cell-mediated active response and provide the basis for the transition of the mechanical gain spectra along the cochlear spiral. Further, the model predicts that the neural tuning at the base is primarily governed by the mechanical filtering of the cochlear partition. At the apex, microscale fluid dynamics and nanoscale channel dynamics must also be invoked to describe the threshold neural tuning for low frequencies. Overall, the model delineates a physiological basis for the difference between basal and apical gain seen in experiments and provides a coherent description of high- and low-frequency cochlear tuning.

Atraumatic Scala Tympani Cochleostomy; Resolution of the Dilemma.

OBJECTIVES: While an accurate placement in cochleostomy is critical to ensure appropriate insertion of the cochlear implant (CI) electrode into the scala tympani (ST), the choice of preferred cochleostomy sites widely varied among experienced surgeons. We present a novel technique for precise yet readily applicable localization of the optimum site for performing ST cochleostomy. MATERIAL AND METHODS: Twenty fresh frozen temporal bones were dissected using the mastoidectomy-posterior tympanotomy approach. Based on the facial nerve and the margins of the round window membrane (RWM), the cochleostomy site was chosen to insert the electrode into the ST while preserving the surrounding intracochlear structures. RESULTS: There is a limited safe area suitable for the ST implantation in the area inferior and anterior to the RWM. There is a higher risk of scala vestibuli (SV) insertion anterior to that area. Posterior to that area, the cochlear aqueduct (CA) and inferior cochlear vein (ICV) are liable for the injury. CONCLUSION: For atraumatic CI, precise and easy localization of the site of cochleostomy play a pivotal role in preserving intracochlear structures. Accurate setting of the vertical and horizontal orientations is mandatory before choosing the site of cochleostomy. The facial nerve and the margins of the RWM offer a very helpful clue for such localization; meanwhile, it is readily identifiable in the surgical field.

The Human Cochlear Aqueduct and Accessory Canals: a Micro-CT Analysis Using a 3D Reconstruction Paradigm.

OBJECTIVE: We sought to study the anatomic variations of the cochlear aqueduct and its accessory canals in human temporal bones using micro-CT and a 3D reconstruction paradigm. More knowledge about the anatomic variations of these structures, particularly at the basal turn of the cochlea and round window niche, may be important to better preserve residual hearing as well as the neural supply during cochlear implant surgery. METHODS: An archival collection of 30 human temporal bones underwent micro-CT and 3D reconstruction. A surface enhancement paradigm was applied. The application displays reconstructed slices as a 3D object with realistic 3D visualization of scanned objects. Virtual sectioning or "cropping" of the petrous bone presented subsequent areas. Thereby, the bony canals could be followed from inside the basal turn of cochlea and middle ear to the jugular foramen. RESULTS: The cochlear aqueduct was always paralleled by an accessory canal containing the inferior cochlear vein. It ran from the basal turn of the cochlea and exited laterally in the jugular foramen. In 70% of the cases, a secondary accessory canal was observed and it derived mostly from a depression or infundibulum located in the floor of the round window niche. This canal also exited in the jugular foramen. The secondary accessory canal occasionally anastomosed with the primary accessory canal suggesting that it contains a vein that drains middle ear blood to the cranial sinus. CONCLUSION: Micro-CT with 3D surface reconstruction paradigm offers new possibilities to study the topographic anatomy of minor details in the human inner ear. The technique creates simulated transparent "castings" of the labyrinth with a coinciding surface view through enhancement of contrast between boundaries. Accessory canals that drain blood from the cochlea, spiral ganglion, and middle ear could be characterized three-dimensionally.

Micro-CT study of the human cochlear aqueduct.

OBJECTIVE: The anatomic structure of the cochlear aqueduct (CA) in human temporal bone specimens was observed using micro-computed tomography (CT). MATERIALS AND METHODS: Micro-CT scanning of 18-µm-thick slices was performed on 30 slides of human temporal bone specimens to observe the CA structure and its relationship with its surroundings. The length, internal and external apertures, and the narrowest width of the CA were measured. The differences in CAs were compared between high jugular bulb (HJB) specimens and normal specimens. RESULTS: A large number of CA images were acquired using Micro-CT scanning, which clearly displayed the basic anatomic structures, stereotactic localizations, and adjacent relationships of the CAs. The whole course of a CA was 12.31 ± 3.60 mm, the diameter of the internal aperture was 465 ± 242 µm, the diameter of the external aperture was 2.88 ± 1.06 mm, the narrowest diameter was 601 ± 335 µm, the diameter of the opening of inferior cochlear vein (ICV) was 151 ± 50 µm, the distance between the internal aperture and ICV was 270 ± 197 µm, and the distance between the inferior margin of the internal acoustic meatus (IAM) and the top most part of the external aperture of the CA was 6.783 ± 2.15 mm. No bony obstruction of the CA or CA enlargement was observed in the specimens. A total of 28 CAs had one accompanying bony canal in the surroundings. The length and travelling of the CA were not affected by the level of the jugular bulb (JB). The variation of the travelling of the ICV was larger than that of the CA. CONCLUSION: Micro-CT adequately displayed the bony CA canal and provided a new method for anatomical studies of the CA and a basis for functional studies.

Anatomical variations of the temporal bone on high-resolution computed tomography imaging: how common are they?

OBJECTIVES: This study aimed to evaluate the prevalence of normal variations of temporal bone anatomy on high-resolution computed tomography imaging and report their clinical importance. METHODS: A retrospective review was conducted of high-resolution temporal bone computed tomography imaging performed at NHS Greater Glasgow and Clyde over an eight-year period. The presence of five variants was determined. These variants were: a high dehiscent jugular bulb, an anteriorly located sigmoid sinus, a deep sinus tympani, an enlarged cochlear aqueduct and a large internal auditory meatus. RESULTS: A total of 339 temporal bones were examined. The incidences of a high dehiscent jugular bulb, anteriorly located sigmoid sinus, deep sinus tympani, enlarged cochlear aqueduct and an enlarged internal auditory meatus were 2.76 per cent, 2.94 per cent, 5.01 per cent, 0.58 per cent and 1.76 per cent respectively. CONCLUSION: Anatomical variations of the temporal bone are not uncommon and it is important for the investigating otologist to be aware of such variations prior to undertaking surgery.

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

OBJECTIVE: To compare the quality of perilymphatic enhancement in the rat inner ear after intratympanic injection of two types of gadolinium with a 9.4-tesla micro-MRI. MATERIALS AND METHODS: Gadolinium was injected into the middle ear in 6 Sprague-Dawley rats via the transtympanic route. The left ear was injected with Gd-DO3A-butrol first, and then the right ear was injected with Gd-DOTA. MR images of the inner ear were acquired 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4 h after intratympanic (IT) injection using an Agilent MRI system 9.4T/160/AS. The normalized signal intensity was quantitatively analyzed at the scala vestibuli (SV), scala media, and scala tympani (ST) using a Marosis M-view system. Then the normalized signal intensities (SIs) were compared between the two contrast agents. RESULTS: For Gd-DO3A-butrol, the SI was as low as 1.0-1.5 throughout 1-4 h at the SV and ST of the basal turn. The maximum SI was 1.5 ± 0.5 at the SV (2 h) and 1.3 ± 0.5 at the ST (2 h). For Gd-DOTA, the 1-hour postinjection SI at the basal turn was 2.5 ± 0.5 at the SV, 1.6 ± 0.3 at the ST, and 1.2 ± 0.3 at the scala media. In the apical turn, the maximum SI was reached after 2.5 h. The maximum SI in the apical turn was 1.8 ± 0.4 at the SV (3.5 h), 1.8 ± 0.4 at the ST (4 h), and 1.4 ± 0.3 at the scala media (4 h). CONCLUSIONS: We were able to clearly visualize and separate the ST and SV using IT Gd and 9.4-tesla micro-MRI. We recommend using Gd-DO3A-butrol over Gd-DOTA and to perform the MRI 2.5 h after using IT Gd in the rat inner ear.

Hearing loss following ventriculoperitoneal shunt in communicating hydrocephalus patients: a pilot study.

OBJECTIVES/HYPOTHESIS: Hearing loss can be associated with a decrease in cerebrospinal fluid (CSF) pressure because changes in CSF pressure induce changes in perilymph pressure. Hearing loss after neurosurgical procedures have been reported, but clinical information on hearing loss after the placement of ventriculoperitoneal (VP) shunts, the most commonly used CSF shunt for hydrocephalus patients, is limited. This study is aimed to show the relationship between VP shunt and hearing loss. STUDY DESIGN: Prospective study. METHODS: Pure tone threshold and electrocochleography were preoperatively performed in nine patients (18 ears) undergoing elective VP shunt placement. Five-day and 1-month post-shunt placement hearing thresholds were compared with baseline data. A correlation analysis was conducted between the threshold and summating potential/action potential (SP/AP) ratio changes at 5 days and 1 month after shunt placement. Cochlear aqueduct dimensions measured by high-resolution CT were compared between ears with and without hearing loss. RESULTS: About 40% of subject ears showed hearing loss with a threshold elevation of at least 15 dB in one or more frequencies. After VP shunt placement, the mean threshold of all ears showed a significant increase in most frequencies and the pure tone average. The change in the SP/AP ratios was significantly correlated with the change in the pure tone average at both 5 days and 1 month after shunt placement. Cochlear aqueduct dimensions were not correlated with hearing loss occurrence. CONCLUSIONS: Hearing thresholds may increase following VP shunt placement, possibly due to secondary endolymphatic hydrops.

Hearing loss after lumbar puncture.

We report a 63-year-old man who developed transient acute on chronic hearing loss after a lumbar puncture performed to investigate raised intracranial pressure. He was subsequently found to have stenosis of the distal left transverse sinus, which was treated with stenting.

Petromastoid canal and cochlear aqueduct in cochlear implant candidates.

OBJECTIVE: To present temporal bone fine channels in cochlear implantation candidates. METHODS: Review of the axial sections of 108 temporal bone CTs. In type I, the petromastoid canal (PMC) was invisible but appeared as channels in type II and type III, <0.5 and 0.5-1 mm wide, respectively, and in type IV was >1 mm wide. The cochlear aqueduct (CA) was visualized up to the vestibule in type 1, the medial two thirds in type 2, the external aperture and/or the medial third in type 3, and was undetectable in type 4. RESULTS: The PMC size and shape differed significantly between the young (aged <5 years) and older (aged 5-16 years) children and between the young children and adults. A wide PMC (>2 mm) was found in only children younger than 2 years. Children up to 2 years of age and those aged 2 to 5 years demonstrated similar findings. The CA types differed among the pediatric and adult CI candidates, with the main difference appearing after the age of 16 years. There was no correlation between CA and PMC types. CONCLUSIONS: It is likely that the age-related changes in CA and PMC are attributable to the developmental or age-related changes in skull base growth.

Magnetic resonance cisternography: comparison between 3-dimensional driven equilibrium with sensitivity encoding and 3-dimensional balanced fast-field echo sequences with sensitivity encoding.

PURPOSE: To assess the detectability of the inner ear structures using both 3-dimensional (3D) balanced fast-field echo (bFFE) and 3D driven equilibrium (DRIVE) sequences in conjunction with parallel imaging and to compare the image quality of those sequences. MATERIALS AND METHODS: Thirty-eight healthy volunteers were examined using a 1.5-T magnetic resonance unit. The 3D bFFE and the 3D DRIVE sequences were compared. The relative contrast between the cranial nerves and the cisternal space was calculated. The detectability of the cranial nerves and the cochlear and semicircular canals was graded on a 3-point scale (1, poor; 2, fair; 3, excellent). RESULT: The relative contrast for the cranial nerve in 3D bFFE and 3D DRIVE was 4.31 +/- 1.53 and 5.73 +/- 4.60, respectively. The 2.5 turns of the cochlea, spiral lamina, and all 3 semicircular canals were better visualized using the 3D DRIVE. CONCLUSION: The 3D DRIVE is superior to the 3D bFFE in evaluation of the structures of the inner ear.

The relationship of the round window membrane to the cochlear aqueduct shown in three-dimensional imaging.

The round window membrane and cochlear aqueduct complex in the guinea pig are reconstructed with 3D-imaging, using orthogonal plane fluorescence optical sectioning (OPFOS). The 3D-images show that the periotic duct and the aqueduct are connected to a pouch-like extension of the round window. The function of this may be regulation of aqueduct flow resistance under the influence of a pressure difference between inner ear fluid and middle ear.

Imágenes en anatomía coclear TC y RM / Cochlear anatomy CT and MR imaging

Los autores ofrecen un breve repaso de anatomía normal de la cóclea con imágenes de TC y RM a fin de posibilitar una más acabada identificación de hallazgos patológicos en pacientes con hipoacusia perceptiva (AU)

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

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

A radiologico-anatomical comparative study of the cochlear aqueduct.

AIM: A comparative radiologico-anatomical study of the cochlear aqueduct (CA) was performed. MATERIALS AND METHODS: Eight cadavers and 23 dry temporal bones were studied. High-resolution computed tomography (HRCT) was carried out on each cadaver before microdissection. Microdissection was carried out in a plane parallel to the HRCT sections. RESULTS: The CA was found to be located an average of 7 mm inferior to the internal acoustic meatus and at the superior edge of the jugular foramen. The external aperture of the CA was triangular in shape in 18 bones (78.3%). The petrosal fossa was located just inferior to the external aperture and housed the glossopharyngeal nerve, which had an incomplete bony canal in four bones (17.4%) and a complete bony canal in three bones (13%). It was possible to demonstrate the petrosal portion of the CA on both coronal and axial HRCT. The otic capsule segment of the CA was impossible to demonstrate on coronal sections. CONCLUSION: The CA cannot be visualized in only one section of the plane in HRCT. Both the otic capsule and petrosal segments can be demonstrated on axial HRCT.

Anatomy of the normal human cochlear aqueduct with functional implications.

There is great variation in published descriptions of the shape, size, and patency of the human cochlear aqueduct. The first part of this paper describes the anatomy of the normal human cochlear aqueduct as determined from a study of 101 temporal bones. Nineteen bones aged 0-1 years and approximately 10 bones per decade of life until age 100 years were examined. The aqueduct was found to have a funnel shaped aperture at the cranial end with a dural sheath extending into it for a varying distance. The rest of the aqueduct was filled with a meshwork of loose connective tissue, often with a central lumen within it. Four types of patencies were noted: central lumen patent throughout length of aqueduct (34%), lumen filled with loose connective tissue (59%), lumen occluded by bone (4%), and obliteration of the aqueduct (3%). The mean value (+/- SD) of the narrowest portion was 138 (+/- 58) microns which occurred 200-300 microns from the cochlear end of the aqueduct. There was no correlation between age and narrowest diameter, or between age and category of patency. In the second part of this paper, we propose quantitative models of aqueduct function, based on measurements of ductal dimensions and known acoustical properties of the inner ear. Our model analyses suggest that in normal ears, the aqueduct (1) cannot support fluid flows large enough to explain stapedectomy gushers, (2) does filter out cardiac- and respiration-induced pulses in CSF and prevents them from affecting cochlear function, and (3) has little effect on normal ossicular transmission of sound for frequencies above 20 Hz. In pathological ears, such as those with ossicular disruption or after a type IV tympanoplasty, a patent aqueduct might affect hearing for frequencies below 150 Hz.

The cochlear aqueduct in pediatric temporal bones.

The cochlear aqueduct is a bony channel which contains the fibrous periotic duct and connects the perilymphatic space of the basal turn of the cochlea with the subarachnoid space of the posterior cranial cavity. Previous histological studies suggested that patency depended on age, whereas a more recent study showed no statistical correlation between age and patency. To clarify patency in pediatric cochlear aqueducts, we selected 21 temporal bones from 12 infants and children, varying in age from birth to 9 years, in which the cochlear aqueduct was fully visible on one histological section. Photographs were taken for documentation and the length and width of the orifice of the external aperture of the aqueduct at the scala tympani were measured and followed to the internal aperture at the subarachnoid space. The lumen of the duct was examined for mononucleated cells, blood cells and fibrous tissue. Measurements revealed that the mean length of the cochlear aqueduct was 4.6 mm (range, 2.4-10.7 mm), mean width of the external aperture was 484 microm (range, 225-869 microm), and mean width of the internal aperture was 1293 microm (range, 699-2344 microm). The mean diameter of the narrowest part (isthmus) was 151 microm (range, 75-244 microm). In all temporal bones the cochlear aqueduct was patent, with one exception. This latter temporal bone was from a 2-month-old girl with multiple intralabyrinthine anomalies, with the missing cochlear aqueduct believed to be due to an aplasia. Our results support prior measurements of the cochlear aqueduct and demonstrate a short and patent cochlear aqueduct in newborns. With growth, a significant increasing length of the duct was found.

High-resolution T2-weighted MR imaging of the inner ear using a long echo-train-length 3D fast spin-echo sequence.

The purpose of this study was to assess the value of a long echo-train-length 3D fast spin-echo (3D-FSE) sequence in visualizing the inner ear structures. Ten normal ears and 50 patient ears were imaged on a 1.5T MR unit using a head coil. Axial high-resolution T2-weighted images of the inner ear and the internal auditory canal (IAC) were obtained in 15 min. In normal ears the reliability of the visualization for the inner ear structures was evaluated on original images and the targeted maximum intensity projection (MIP) images of the labyrinth. In ten normal ears, 3D surface display (3D) images were also created and compared with MIP images. On the original images the cochlear aqueduct, the vessels in the vicinity of the IAC, and more than three branches of the cranial nerves were visualized in the IAC in all the ears. The visibility of the endolympathic duct was 80%. On the MIP images the visibility of the three semicircular canals, anterior and posterior ampulla, and of more than two turns of the cochlea was 100%. The MIP images and 3D images were almost comparable. The visibility of the endolymphatic duct was 80% in normal ears and 0% in the affected ears of the patients with Meniere's disease (p < 0.01). In one patient ear a small intracanalicular tumor was depicted clearly. In conclusion, the long echo train length T2-weighted 3D-FSE sequence enables the detailed visualization of the tiny structures of the inner ear and the IAC within a clinically acceptable scan time. Furthermore, obtaining a high contrast between the soft/bony tissue and the cerebrospinal/endolymph/ perilymph fluid would be of significant value in the diagnosis of the pathologic conditions around the labyrinth and the IAC.

Three-dimensional reconstruction of the cochlea from two-dimensional images of optical sections.

This paper describes a methodology for three-dimensional (3D) computer-aided reconstruction of the guinea pig cochlea using orthogonal-plane fluorescence optical sectioning. Specimens are sectioned optically, allowing them to remain intact during observation. Equations to correct the data for specimen translation and rotation are developed and 3D reconstructions of the scala tympani, round window membrane, and cochlear aqueduct are presented. The error associated with the reconstruction is estimated to be < 19 microns.

Fine structure of the human cochlear aqueduct: a light and transmission electron microscopic study of decalcified temporal bones.

The morphologic features of the human cochlear aqueduct were examined using both light and electron microscopy. The lumen of the cochlear aqueduct was observed to be filled with dense, irregular connective tissue corresponding to dura mater. At the entrance to the cerebrospinal fluid space, the dense connective tissue in the ductal lumen was covered with a thin layer of a few flattened cells, which was contiguous with the arachnoid membrane of the brain. A simple low cuboidal epithelium also separated the perilymphatic space from the lumen of the duct. Our observations confirm the presence of a barrier membrane at the opening to the perilymphatic space, and suggest that no transport occurs in the human cochlear aqueduct.