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Morphologic changes in the guinea pig cochlea following cochleostomy--a preliminary scanning electron microscope study.
Fistulization of the membranous labyrinth via a surgically created endolymphatic-perilymphatic shunt (cochleosacculotomy) is currently being advocated as a treatment for endolymphatic hydrops. As a preliminary study to the investigation of the effects of labyrinthine fistulization on the hydropic condition, we sought first to reproduce the cochleosacculotomy procedure in a series of normal animals. Fistulization of the cochlear duct was performed in 15 normal animals using a tungsten wire directed via the round window through the cochlear partition in the region of the organ of Corti. The animals were sacrificed at regular intervals and the cochleas examined for histopathologic changes with the scanning electron microscope. Observations suggest that discrete permanent fistulization of the cochlear duct in the normal guinea pig results in progressive sensory cell degeneration. The implication of these observations and the apparent difference between these results and the experience thus far reported in humans are discussed.
Threshold sensitivity and frequency selectivity measured with round window whole nerve action potentials in the awake, restrained chinchilla.
Chronic electrodes, placed on the round windows of four adult chinchillas, were used to measure whole-nerve action potential (AP) thresholds and tuning curves. The AP quiet thresholds were within 17 dB of thresholds measured by behavioral methods and within 3 dB of those measured by other evoked response procedures. The AP tuning curves, obtained by a simultaneous masking procedure, were also similar to previously measured tuning curves in the chinchilla. These results indicate that long-term indwelling electrodes can be successfully placed on the chinchilla round window and used to measure threshold sensitivity and frequency selectivity. Recordings from these electrodes could be used in a variety of situations and may be particularly useful in studying the effects of noise on the cochlea.
Chronic perilymphatic fistula: experimental model in the guinea pig.
Chronic perilymphatic fistulas were created in guinea pig cochleas using silicone rubber tubing placed into the scala tympani through the round window. Fistula patency was determined by fluorescein perfusion into cerebral spinal fluid. Fistula were found to be patent in 6 of 6 animals at 7 days and 8 of 13 animals at 28 days. Analysis of ABRs revealed threshold increases of 10 to 15 dB across all frequencies at 1 hour and 7 days. However, thresholds returned to pre-fistula levels by 28 days. Animals with acute fistulas (simple laceration of the round window) had similar threshold increases at 1 hour; however, recovery to baseline levels occurred by day 7. Control animals with intact round windows did not have threshold shifts. Scanning electron microscopy revealed hair cell loss localized to the apical and basal turns of the cochlea. The morphologic changes observed occurred acutely (within 7 days) and were not progressive, despite the presence of a fistula. Hair cell loss or degeneration did not correlate with hearing loss.
Perilymphatic hypertension.
A syndrome termed perilymphatic hypertension is described as being seen in a small subset of patients with sudden sensorineural hearing loss. A patent or semipatent cochlear aqueduct or modiolus are considered precursors to this condition. Perilymphatic hypertension is believed to predispose to perilymphatic fistula, which may be part of the process of resolution. The pathogenesis for perilymphatic hypertension and fistula are hypothesized and discussed. Treatment for this subset of patients consisted of paracentesis of the round window membrane followed by grafting, with improvement of hearing in certain patients. In no instance did hearing subsequently decrease.
Labyrinthitis ossificans: histopathologic consideration for cochlear implantation.
Labyrinthitis ossificans may be a hindrance to cochlear implantation by making electrode insertion difficult. We performed a histopathologic study of 24 temporal bones with labyrinthitis ossificans from multiple causes. The organ of Corti was graphically reconstructed and the degree of obstruction was estimated for each millimeter of the cochlea. Correlations were calculated between the degree of new bone formation and the cause, patient's age and sex, and time from the original temporal bone insult. Our results demonstrate that complete cochlear ossification is rare. The scala tympani in the basal turn of the cochlea is the most frequent area of ossification, regardless of the cause of the labyrinthitis ossificans. Meningogenic labyrinthitis, usually a childhood disease, was associated with the greatest amount of ossification. When ossification resulted from tympanogenic labyrinthitis, the scala tympani was completely ossified near the round window niche in all temporal bones. Neo-ossification of the basal turn associated with otosclerosis was limited to the proximal 6 mm of the scala tympani in all cases. Three temporal bones had a patent round window niche and basal turn, but significant apical and middle-turn ossification. Peripheral sensorineural elements were severely degenerated in the region of the ossification in all specimens, and spiral ganglion cell counts were decreased.
Cochlear implants in children: reliability of computed tomography.
Preoperative temporal bone computed tomography (CT) can demonstrate anatomic details relevant to surgical management and is therefore essential in the presurgical evaluation of patients receiving cochlear implants. The purpose of this study was to evaluate preoperative CT studies and compare them to surgical findings in 34 children who received the Nucleus multichannel cochlear implant. The focus of this report is to discuss the dependability of CT scans in predicting surgical findings at the time of cochlear implantation. Results indicate that agreement of CT interpretations with surgical findings is partially related to the etiology of hearing loss and the experience of the surgeon and neuroradiologist. Advantages and limitations of the CT scans in predicting surgical findings are discussed.
A review of the radiological anatomy of the round window.
Experimental studies on dried temporal bones in various projections combined with studies of the anatomical details marked with metal wires and/or barium paste have been performed to define the round window and its anatomical relation to surrounding structures. The exact plane of the round window was not found to be tomographically demonstrable and the term to describe the dehiscence in the lateral wall of the promontory below and posterior to that of the oval window should be the niche of the round window. Previous radiologically described pathological changes at the round window should be reviewed.
Occlusion of the round window by otosclerosis.
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Criteria for otosclerosi surgery and further experiences with round window surgery.
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Anatomical measurements of the cochlear aqueduct, round window membrane, round window niche, and facial recess.
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Clinical and experimental results with focused ultrasound.
Focused ultrasound was applied at the lateral semicircular canal in 19 patients and at the round window in 12 patients, all of whom had intractable vertigo caused by Meniere's disease. Postoperatively, the vertigo was relieved or diminished in 75% of the lateral canal cases and in 25% of the round window cases. The results indicate that the lateral canal approach may have some therapeutic value, but the treatment is arduous for the patient and the risk of facial palsy is always nettlesome. Utilizing similar techniques, ultrasound irradiation was given to the lateral canals of 6 squirrel monkeys and to the round windows of 15 cats. After survival times of up to 3 months, light microscopic studies of the temporal bones of these animals show no morphological changes which can be attributed to the ultrasound irradiation. In general, these findings are in concert with the reports of other investigators and show that selective ablation of the vestibular sense organs cannot be achieved by current techniques of ultrasound irradiation. It seems doubtful that prolonged relief of vertigo can be expected to occur without a more complete morphological ablation of the vestibular sense organs than is possible with the current methods of ultrasound treatment.
Stapedectomy and round window closure.
Round window closure is an uncommon complication of advanced otosclerosis found in 1% of 30,000 stapedectomy cases. A review of 81 patients representing 110 operation cases was made to determine what factors may alert the otologic surgeon to the patient with round window closure and what factors may identify the patient who will achieve the greatest benefit from stapedectomy.
Current clinical and pathological features of round window diseases.
The round window niche and membrane can be involved in clinical problems including perilymphatic fistulas, sensorineural hearing loss in otitis media, and a variety of others. The background of these problems is documented by a review of the literature and recent experimental evidence on round window grafting and tracer studies. Clinical pathological correlates including recent clinical experience are discussed for perilymphatic fistulas (sudden deafness, chronic progressive fluctuating sensorineural hearing loss, and association with Meniere's disease), sensorineural hearing loss in acute and chronic otitis media, and findings in round window membrane including absent round window reflex in cases of exploratory tympanotomy and otosclerosis. In addition, a new syndrome termed perilymphatic hypertension is described as seen in a subset of patients with sudden sensorineural hearing loss.
Round window and conservative fenestration.
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Bone conduction and otosclerosis of the round window.
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Observation of click-evoked compound VIII nerve responses before, during, and over seven months after kanamycin treatment in the guinea pig.
Compound VIII nerve action potential responses to clicks and filtered-clicks were recorded regularly over a period of 7 months following Kanamycin treatment (8 days, 400 mg/kg), in 3 awake guinea pigs each with an implanted electrode on the round window (left ear). From the end of the treatment the responses to the click became dissociated in various degrees within three major phases: (1) a rapid phase (8 to 10 days) where the responses were dramatically altered, (2) a recovery phase which lasted about 2 months, (3) a slow phase over the subsequent 5 months where the responses to the click decreased slowly but gradually while the N 1 component, as well as the responses to the 8 000 Hz filtered-click (which became recruiting at the end of the treatment), disappeared progressively. Responses to the low frequency filtered-clicks were less affected. Cochlear cytograms showed almost complete degeneration of the inner and outer hair cells of the first turn only.
An investigation of sympathetic effects on hearing.
Absolute and masked thresholds were measured behaviourally in the cat before and after lesion of the superior cervical ganglion and sympathetic trunk. No changes were found, even though testing was continued for up to 2 1/2 years afterwards. However in acute experiments in the anaesthetised animal small increases in the neural N1 potential of the cochlea, without any effect on the microphonic, were found after stimulation of the sympathetic trunk.