Electrode insertion trauma in cochlear implantation

Residual hearing conservation may be important in cochlear implantation of children and of adults with disabling tinnitus responsive to extracochlear stimulation. Damage to the neural elements of the cochlea during electrode insertion may have a negative impact on residual hearing conservation. Histologic analysis of eight temporal bones with cochlear implants reveals trauma at essentially two locations: the round window insertion site and along the basal turn of the cochlea. In four of the bones, insertion at the round window resulted in damage to the osseous spiral lamina and the electrode was inserted through the scala media into the scala vestibuli. Evidence of secondary reactive osseous formation was also noted in these bones. This paper relates the surgical anatomy of the round window to histologic findings and microanatomical dissections. Recommendations for implantation surgery include creation of a cochleostomy by removal of the floor of the round window niche and a superior-to-inferior angle of electrode insertion.

[Recurrence of perilymphatic fistula]

Recurrence of the perilymphatic fistula is not rare and may be a tough problem for surgical treatment. This is because a graft is usually applied on the ruptured window(s) from the middle ear and therefore the perilymphatic pressure directly acts on the graft. The recurrence may be caused by a technical failure, use of an unsuitable graft material, poor postoperative bedrest, trauma, increased inner ear pressure, etc. In our clinic, the recurrence occurred in 7 of 48 cases surgically treated. Vertigo accompanied with spontaneous or positional nystagmus was seen in all 7 recurrent cases, while only 2 of them complained of worsening of the existing hearing loss. Re-operation was carried out in two patients. In the first case, closure of the round window by the previous operation was found incomplete, and the perilymph leaked through the gap around the graft. In the second case, closure of the round window was complete, but perilymph leaked from the oval window. In order to prevent the recurrence, the operation should be carefully performed by using strong and adhesive tissue as a graft material, applying a glue between the graft and the inner ear window(s), and keeping strict postoperative bedrest.

Histopathologic study of the perilymph-suctioned labyrinth

A small amount of perilymph was suctioned from the scala tympani through the round window membrane of guinea pigs. The animals either were killed immediately after the suctioning or were kept alive for 1 to 3 months. For morphologic study, the conventional celloidin embedding method was employed, and the temporal bones were serially sectioned. Various kinds and degrees of changes in the membranous labyrinth were observed: hydrops, collapse, and rupture. Loss of outer hair cells in the upper turns of the cochlea was noted in the 1- to 3-month recovery specimens. These findings are identical to those found in animals in which artificial perilymph was injected into the subarachnoid space in order to produce experimental perilymphatic fistula. After comparing morphologic changes of the membranous labyrinth following use of each method, we concluded that perilymph suctioning from the round window can be used as an animal model of perilymphatic fistula.

Role of perilymphatic fistula in sudden hearing loss: an animal model

The electrophysiologic response of the guinea pig cochlea was monitored after sequential lesions to Reissner's membrane and the round window (RW). Action potential (AP) responses to click stimuli were recorded from the RW before and after discrete puncture-type lesions were created in the cochlear partition of the second turn. Observed decrements were typically minor, comparable to no greater than 10 dB attenuation of stimulus intensity. The RW membranes then were perforated to create perilymphatic fistulas. Further monitoring demonstrated a rapid (within 5 to 10 minutes), severe decrement in AP amplitude and latency, with complete loss of the AP within 1 hour. Control animals with RW perforations alone did not show these decrements. Correct placement of the second turn lesions was documented by histology. We conclude that discrete lesions in the cochlear duct are not reflected in the AP input-output functions unless there is a fluid leak from the RW, and thus present a possible model for idiopathic sudden hearing loss.

Maintenance of hydrostatic pressure gradients in the membranous labyrinth

In normal guinea pig ears, hydrostatic perilymphatic pressure is equal to endolymphatic pressure. Alterations of perilymphatic pressure induced, for example, by laceration of the round window membrane are transmitted immediately to the endolymphatic compartment, probably via Reissner's membrane. In guinea pigs with experimental endolymphatic hydrops, however, pressure gradients between the endolymph and perilymph remained preserved after rupture of the round window membrane. This is considered as further evidence that after long-standing distention of Reissner's membrane the membranous labyrinth loses its ability to equalize endolymphatic and perilymphatic pressure.

Healing of experimentally produced round window membrane rupture

The healing mechanism of experimentally produced ruptures of the round window was histologically studied in guinea pigs. Surgically created rupture of the round window resulted in spontaneous closure within 2 weeks. Light microscopical examination indicated that the rupture closed with proliferation of connective tissue which usually appeared to protrude into the middle ear cavity, each case showing a unique shape. In one case, the wound was covered with granulation tissue at the site of closure on the middle ear surface. Although the rupture was closed by connective tissue, the elastic fibres of the middle layer remained disrupted at the early stage of healing.

Perilymph fistula: the Stanford experience

Seventy-eight tympanotomies were performed to determine the presence of perilymph fistulas (PLFs); of these, 51% were found. The oval and round windows of all patients were grafted, whether PLFs were present or not. Of those patients with PLFs, 64% had resolution of their major symptom; when no PLF was found, 44% had a similar outcome. We concluded that 1) PLFs often behave as if they are epiphenomena in relation to hearing and balance, 2) PLFs can be intermittent, 3) PLF surgery is disappointing for restoration of hearing in sudden hearing loss when compared to the rate of spontaneous recovery, 4) stabilizing a fluctuating or progressive loss is a more realistic goal, and 5) establishing preoperative criteria for exploration is still a problem.

[Microperforation and removal of the round window membrane. Short- and long-term study in animal experiments using electrocochleography and evoked response audiometry]

Our earlier animal experiments on guinea pigs showed that instrumental perforation of the round window membrane by a 0.2 mm platinum wire leads to an instant loss of the inner ear functions. The membrane defect healed in a few days, the cochlear structures remained intact, and the compound action potential of the auditory nerve and the brain-stem responses could be evoked again with normal latency times within 2 weeks. 1. In the studies reported here we first carried out microperforations with a 1 micron needle electrode, which caused no changes of the hearing potentials (cochlear microphonics, compound action potential of the auditory nerve, brain-stem responses), and no visible defect of the round window membrane and no perilymph outflow. 2. The removal of the round window membrane and the withdrawal of the perilymph led to a loss of the cochlear microphonics and to a considerable increase of the latency times of wave I (Jewett). The hearing potential regained their original values after 2 weeks without closure of the round window niche. The round window membrane had regenerated spontaneously and the scala tympani was again filled with perilymph. After covering the round window niche with a connective tissue graft, the hearing potentials regained their original values after 2 weeks, as they had done without cover of the round window niche. The round window membrane regenerated below the tissue graft and the scala tympani was also filled again with perilymph.(ABSTRACT TRUNCATED AT 250 WORDS)

Surgery of the round window

Tympanotomy is routinely performed in our institution to control the round window membrane for patients with recurrent sudden deafness, with Meniere-like attacks, and those with Meniere's disease. In a large number of these patients, especially those with vertigo, perilymph fistula has been diagnosed. Most patients are free of attacks after the fistula has been closed, and in those with a short history of illness, hearing can become normal. In the future, closure of the round window membrane should be considered an important procedure in the treatment of inner ear dysfunctions.

Pressure transmission properties from the externa ear canal to the inner ear. An experimental study using guinea pigs

The inner ear pressure (PIE) in response to pressure changes in the external ear canal was measured in guinea pigs while alternatively opening and closing the perforation of the otic bulla. When the bulla was opened, only a transient degree of applied pressure was transmitted to the inner ear and the amplitude of the PIE was smaller than that of the corresponding PIE when the bulla was closed. This was because the applied pressure was exclusively transmitted to the inner ear via the ossicular chain. When the otic bulla was closed, the pressure was transmitted not only via the ossicular chain but also via the round window (RW) through the middle ear cavity. When the bulla was closed, the amplitude of PIE was larger by a positive pressure load than by the corresponding negative one. The amplitude of PIE showed a linear relationship to ear canal pressure of at least within the +/- 200 mmH2O range, as long as pressure was slowly applied to the ear canal. When the loading pressure was abruptly changed, a bouncing response, possibly reflecting elasticity of the RW, was evoked, which diminished or disappeared when the round window was artificially ruptured.

Effects of round window membrane rupture on cochlear blood flow and inner ear pressures

Effects of round window membrane rupture on cochlear blood flow and inner ear pressures were investigated using non-radioactive microspheres and a servo-micropipet system in guinea pigs. When perilymphatic pressure was raised through a glass capillary tube inserted into the perilymphatic space, both endolymph pressure and perilymph pressure rose. When these inner ear pressures were raised to relatively high levels, cochlear blood vessels could be compressed and endocochlear potential decreased to a negative value due to the interruption of cochlear blood flow. However, in the case when the round window membrane was ruptured by further increase in perilymph pressure, the inner ear pressures decreased and restoration of cochlear blood flow and endocochlear potential was observed. When the round window membrane was ruptured with a fine needle under ordinary inner ear pressures, cochlear blood flow did not change significantly.

The effect of round window membrane rupture on endolymphatic and perilymphatic pressures

We used a guinea pig model to investigate the effect of round window membrane rupture on endolymphatic and perilymphatic pressures under conditions known to increase these pressures: anoxia, hypercapnia, increased intracranial pressure, and occlusion of the vein of the cochlear aqueduct. When the round window membrane was not ruptured, increases in endolymphatic pressure paralleled the perilymphatic pressure following exposure to each of the experimental conditions. After the round window membrane had been ruptured, however, no increases in perilymphatic pressure or endolymphatic pressure were seen. These results suggest that endolymphatic pressure is dependent on perilymphatic pressure, even when the round window membrane is ruptured.

Pneumolabyrinth in perilymphatic fistula: report of three cases

In three patients with perilymphatic fistula, exploratory tympanotomy revealed air bubbles emerging through the ruptured round window membrane. Review of the literature disclosed three case reports in which air was demonstrated in the labyrinth in association with perilymphatic fistula. Experimental evidence that air could enter the labyrinth through a defect of the round window membrane was described in two articles. In our patients, the perilymphatic fistula was produced by implosive force. When a perilymphatic fistula was produced by implosive force, or in the case of a pre-existing perilymphatic fistula, we assume, air may enter the scala tympani through the defect of the round window membrane if the middle ear pressure rises beyond a certain limit. Sudden onset of deafness and reversibility of hearing in perilymphatic fistula could be attributable to the presence of air bubble in the scala tympani--pneumolabyrinth--which might disturb propagation of the traveling wave of the basilar membrane.

Labyrinthine window rupture with round window predominance: a long-term review of 32 cases

A retrospective series is presented of 32 cases of spontaneous labyrinthine window rupture proven at tympanotomy. There was no prior history of stapedectomy or other middle ear surgery. Seventy-eight per cent of cases involved the round window, in contrast to previous series on the subject. The pattern of hearing loss and vestibular symptoms varied widely and followed different antecedent factors, the commonest of which was head injury (46.9%). Including cases misdiagnosed initially, 92.9% of patients with vestibular symptoms experienced improvement following surgery, but only 20.7% of patients had improved hearing; these figures changed with long-term follow-up, which has been rarely reported previously. Seven patients, the majority oval window ruptures, required re-exploration, mainly for vertigo, and prolonged follow-up is required.

[Pressure measurements of the round window membrane in the guinea pig cochlea]

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.

[The "window-fistula symptom" in lesions of the round and oval window]

22 tympanoscopies in suspected perilymph fistulas with clinical symptoms such as sudden hearing loss, tinnitus, vertigo, fluctuating hearing etc., histories and intraoperative findings were registered. In some cases a remarkable interval between the implicated event and the clinical manifestation of the symptoms was noticed. Besides on case history and audiometric data the diagnosis was based on vestibular investigations. In 14 of 22 cases the window-fistula-symptom was positive. The window-fistula-symptom is defined as a provocation nystagmus with maximal intensity in the lateral head position lying on the affected ear and in the head hanging position with the head rotated to the affected side. After abandoning these positions the nystagmus is decreased. Mostly the nystagmus is directed to the healthy side. With regard to the preoperative and postoperative findings a generous indication for tympanoscopy in suspected fistulas of the round and oval window membrane is recommended.

[Clinical and etiologic aspects of rupture of the membrane of the round window]

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.

[Sudden deafness caused by rupture of the round window membrane. Surgical indications, course and prognosis]

Tympanotomy was performed on 39 of 247 patients with sudden hearing loss between 1980 and 1982. In 23 cases a round window membrane defect was found. Evaluation of the indications, intraoperative findings and postoperative course allows the following conclusions to be drawn. Only in patients treated in the second week was the rate of success significantly higher than the spontaneous remission rate in patients without perilymphatic fistula. If there is no improvement in the hearing level in the first week after covering a round window membrane defect the prognosis is poor. Stabilization of the hearing is complete in the first three weeks after treatment. Prognosis in extensive membrane defects with large perilymphatic fistulas is definitely worse than in small round window membrane defects. The best results were achieved for the following groups: physical exertion/barotrauma, sudden deafness, progressive hearing loss despite drug therapy.

Long-term study after perforation of the round window. Animal experiments using electric response audiometry

The round-window membrane of the inner ear of the guinea pig was perforated with a platinum wire under ketamine-xylazine anaesthesia. The latency times of waves I and V (Jewett) increased to 0.6 ms at 100 dB click HL stimulus loudness. The interpeak latencies did not change (4.0-4.2 ms). At 60 dB CHL stimulus loudness, no responses were discernible. Closure of the membrane damage by adhesive fibrin tissue had no effect on the auditory nerve potentials or the brain-stem responses. Normal latency times of waves I-V were seen 7 days after perforation. There was no difference between the animals with repaired and unrepaired membrane damage. We observed spontaneous healing of the round-window membrane 7 days after perforation, and a normal organ of Corti.

Instrumental perforation of the round window. Animal experiments using cochleography and ERA

The round window membrane of the inner ear of the guinea pig was perforated under Ketanest anaesthesia. A very rapid and almost total loss of cochlear microphonics, auditory nerve action potential and brain-stem response developed. Replacement of the perilymph with Ringer's solution and surgical closure of the membrane damage had no effect on auditory nerve action potential or brain-stem response. Further decline in the amplitude of cochlear microphonics was halted, however. When perfusion of the round window membrane was carried out whilst the bulla was full with Ringer's solution, no essential decline was seen in the amplitudes of microphonics and auditory nerve action. Only brain-stem response was reduced--temporarily by 40% of the original amplitudinal level--but this showed subsequent recovery. Preservation of cochlear microphonics and auditory nerve action after perforation in Ringer's solution, i.e. excluding air, indicates that air entering the scala tympani is the cause of the sudden hearing loss after round window perforation. Using a simple physical model, an attempt is made to demonstrate this process. The anatomic connections between the round window, cochlear aqueduct and scala tympani are discussed, as are the different ways in which the round window membrane can be perforated.

Labyrinthine membrane rupture caused by elevated intratympanic pressure during general anesthesia

General anesthesia is not considered as a potential cause of hearing loss. However, three patients who sustained sensorineural hearing loss due to labyrinthine membrane rupture following general anesthesia with nitrous oxide have come to the authors' attention. These cases are presented and the mechanism causing labyrinthine membrane rupture is discussed.

Microscopic characteristics of round window problems in otology

Round window membrane ruptures and perilymph leakage can present the complication or concomitant of sudden deafness and barotrauma. Pathological involvement of the round window membrane can occur in many otological diseases such as otosclerosis, otitis media, tumors, etc. Microscopic manipulations might result in an inadvertent round window membrane lesion and cochlear deafness. In recent years purposeful surgical lesions, including procedures for vertigo and cochlear implant surgery, are being made in the round window membrane. Examples of pathology involving the round window in human temporal bones are described. To understand better the clinical features of early round window rupture in humans,, controlled punctuate lesions were made in 36 chinchilla ears and studied sequentially. Round window membrane healing occurred as early as 3 days post-lesion, and was complete in all ears but 1 of the combined 7, 9 and 11-day group. Healing occurred primarily as a result of middle ear epithelial ingrowth and mesothelial cell reaction in the adjacent perilymphatic space of the scala tympani. These cellular events are described and discussed from an histological and clinical point of view.