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Application of thiamine (1 X 10(-11)-1 X 10(-3) mol/l) on the membrane of cochlear round window increased the amplitude and decreased the latency of the auditory nerve action potential, waves I and II of brainstem auditory-evoked electrical potentials in response to acoustic clicks of different intensity. The mechanism of thiamine action on auditory-evoked electric potentials is discussed.
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Rehabilitation of patients with total hearing loss through surgical implantation of intracochlear devices is warranted only in cases for which deafness is absolutely not relieved by conventional hearing prostheses. Furthermore, it is required that the patient respond positively to the electrostimulation test of the round window, be strongly motivated, as well as present a high index of socialization. These three parameters will also determine the type of implant to be used, i.e. mono- or multielectrode, as well as particular indications relating to children.
The structure of the internal and external tissue of the cochlear opening of the cochlear aquaeduct was examined by light microscopy on semithin sections and by scanning electron microscopy. The whole area is filled with a net of mesenchymal cells. The cell axes are randomly orientated inside the aquaeduct. On the outside of the cochlear aquaeduct fibrocytic tissue fills a space which is triangular in cross-section between the basal part of the cochlea wall of the tympanic scala and the middle portion of the round window membrane. In this area the direction of the net is uniform and it gives the impression of anchorage of the round window membrane on the perilymphatic side. The rim bordering the perilymphatic space is a dense net but not fully closed. The scanning electron microscopic pictures taken perpendicular to this border structure show clearly a texture of mesenchymal cells with open spaces. No closed "membrana limitans" was found. The possible function of the fixation of the round window membrane to the perilymphatic space giving rise to an asymmetric perilymph movement is discussed with regard to the physiology of sound transmission.
Two patients with combined perilymphatic fistulas of both the round window and the lateral semicircular canal are presented. They became asymptomatic only when both fistulas were closed. In both cases the hearing improved concurrently. It is recommended that when a traumatic perilymphatic fistula is not cured by closing window fistulas, the lateral semicircular canal be explored.
Some clinical observations suggest that sudden hearing loss or dizziness can be due to rupture of the round window membrane. Experiments on 16 guinea pigs showed that after injection of fluid the pressure in the scala tympani of the basal turn increases. This leads to rupture of the round window membrane. The pressure was recorded via a piezoresistant transducer. Morphological changes were visualised by scanning electron microscopy.
For more than 15 years, rupture of the round window membrane has been discussed as a possible cause of sudden sensineural hearing loss. With regard to the pathophysiological basis for such a rupture, an abnormality in the width of the aquaeductus cochlearis as well as anatomical variations of the round window membrane itself are presumed as causative factors. The authors report on six cases concerning this incidence. The indication for tympanoscopy and the technique for the closure of the round window fistula are reported. In 5 of the 6 patients treated in this manner, surgery the resulted in a significant and lasting hearing gain.
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 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.
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More than 90% of cases of patients suffering from acquired or congenital deafness may be provided with some sound sensation owing to the electrical stimulation of the few cochlear nerve fibers that are still present in the deaf cochlea. In order to obtain some speech discrimination without lip reading it is necessary to elicite different frequency sound sensations. Thus electrically proofs compartments must be performed all along the cochlear keyboard. The placement of 12 intra-cochlear electrodes is greatly facilitated by using an electrodes-bearer which is introduced through a very simple and classical surgical procedure. The indications for implanting depend on undiscussible criteria (total deafness, auditory response to electrical stimulation of the round window, patient's motivation). Clinical results consist of psychoaffective improvement and auditory possibilities. All patients are able to discriminate homosyllabic words in closed lists without lips reading, but the percentages of these performances greatly vary from one patient to the other. Some patients, generally suffering from acquired total deafness, are able to discriminate words or sentences without reference list and without help of lips reading. These results depend on the patient's socialization level, on the pre or post-lingual status of the deafness, on an eventual cochlear ossification, and on the value of several pre operative electro physiological data. Associated handicaps (autism, blindness, tetraplegia) are not contra indications and often represent supplementary reasons for implanting. The value of these post-operative results may be predicted owing to a series of electrical pre-operative tests and clinical data. Thus it is possible to determine the respective indications of multi and single channel cochlear implants.
Sensorineural hearing loss is generally felt to be an untreatable medical condition. However, in some cases, prompt diagnosis and treatment of the underlying condition may reverse the deafness. This article summarizes various treatable forms of sensorineural hearing loss and provides illustrative cases histories of patients who have had sensorineural hearing losses that were improved by medical or surgical intervention. Patients with reversible sensorineural deafness due to inadvertent aminoglycoside over-dosage, congenital cholesteatoma, Meniere's syndrome, blood coagulopathy, and perilymphatic fistula all had improvements in auditory function after medical or surgical intervention. Recent experimental studies on animals may explain the basic mechanisms behind hearing loss and recovery. Aminoglycoside ototoxicity appears to have an initial reversible step, followed by a permanent process. Early endolymphatic hydrops and fistulas may cause mechanical effects in the cochlea which can be corrected. Coagulopathy may cause hypoxia which reverses after anticoagulation. These observations reveal that animal experiments can be useful in explaining human auditory dysfunction of the reversible type.
The effect of the duration of click stimuli on the compound action potential recorded from the round window in the rat and the effect of low-pass filtering of short click sounds were studied. Thus the intensity functions of the round window N1 potential have a two-segment course and there is a difference in the response to rarefaction and condensation clicks, depending upon the content of low-frequency components in the click stimulus. The intensity function of the rat's response to broadband clicks does not show the same two-segment course as has been reported in experiments in other animals, and there is little difference between the response to condensation clicks and that to rarefaction clicks in this animal. However, when the duration of the click is increased or when broadband clicks are subjected to low-pass filtering, the intensity functions in response to condensation clicks do show a change in course, while the response to rarefaction clicks remains essentially unchanged. A similar change in the response to a broadband click can be induced by adding a low-pass-filtered click to the broadband click. The response to such a combination is not only a linear summation of the neural response to the individual components of the stimuli and the cochlear microphonics, but the low-frequency components that are added also affect the response to the broadband click, mainly by reducing the amplitude of the response.
Computed tomography has replaced conventional multidirectional tomography in the imaging of patients with the presumed diagnosis of otosclerosis. In addition to obliterative disease, which was easily diagnosed with multidirectional tomography, anterior and posterior foci of varying sizes can now also be visualized preoperatively. Active cochlear otosclerosis (otospongiosis) is diagnosed in the presence of foci of demineralization in the otic capsule. Diagnosis of these entities has been of great interest to our referring otologic surgeons.
The parameters of the cochlear round window responses to stimulation by acoustic clicks of different intensity, polarity and frequency were studied in experiments on anesthetized cats. The intensity function reflecting the dependence of the amplitude of potential change upon the level of stimulating click included two parts differing in slope. The dependence of the electric responses on the stimulus intensity and frequency was also studied. A specific form of the reaction was found in some cases. The study of all these parameters gives us some reasons to make a conclusion on the function of the auditory nerve.
Labyrinthine fistula of the round window should be considered in the differential diagnosis of vertigo and sudden hearing loss occurring in patients, who are not only divers, during physical stress or exertion. Surgical exploration and closure of the fistulae are the only means of diagnosis and treatment of this condition. Since these fistulae may heal spontaneously in most instances early surgery is recommended after a short interval of conservative therapy. Five patients, of whom 4 underwent surgery and one was seen for an expert opinion are presented.
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