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

Effects of experimental round window membrane laceration on the free amino acid profile of perilymph

To evaluate biochemical changes of inner ear fluid following perilymphatic fistula (PLF), free amino acid (FAA) profiles of perilymph in experimental PLF were determined using high performance liquid chromatography (HPLC). Thirty-five guinea pigs were anesthetized and prepared as PLF models by perforating the round window membrane (RWM) of the left ear. Right ears served as controls. Samples (2 microliters) were aspirated from scala tympani through a RWM perforation. Animals were divided into two groups according to time of sampling following PLF induction: 2-week group (n = 17) and 4-week group (n = 18). Compound action potential (CAP) evoked by 1, 2, 4 and 8 kHz tone bursts were measured using a round window electrode from the left ear before PLF induction and from both ears before final sampling. RWM perforations were completely closed at the time of the final sampling in 8 of 17 animals from the 2-week group, and 15 of 18 animals from the 4-week group. In comparison with that in control ears, concentrations of FAA throughout all profiles was dramatically elevated in the PLF ears with a healed RWM perforation. Most PLF ears with persistent RWM perforation showed minimal differences between 2-week and 4-week groups. No remarkable CAP threshold changes were found at any frequencies tested following PLF induction in both the 2-week and 4-week groups. The unchanged 8 kHz threshold suggests that FAA concentration increases only at the basal end of the cochlea. FAAs accumulate within the basal end of scala tympani in ears with a healed RWM.(ABSTRACT TRUNCATED AT 250 WORDS)

Inner ear injury caused by air intrusion to the scala vestibuli of the cochlea

In a previous communication, we demonstrated that the introduction of air into the scala tympani of the cochlea causes a decrease of cochlear potentials; however, the change in endocochlear dc potential (EP) was mild and the decreased cochlear microphonics (CM) and compound action potentials (CAP) were, at least partially, reversible. In contrast, we have now found that air perfusion (3-60 microliters/min) in the scala vestibuli decreased cochlear potentials more drastically than that in the scala tympani. The change in the EP after air perfusion in the scala vestibuli was characterized by a decrease of the negative EP in response to anoxia. The CM drastically decreased upon the initiation of air perfusion and no recovery was observed after refilling of the perilymph. Histological examination showed collapse of Reissner's membrane in 12 out of 17 cochleas examined. The extent and frequency of the collapse increased with an increase in the amount of air perfused in the scala vestibuli. As the minimal amount of air needed to cause inner ear damage by air perfusion in the scala vestibuli is as small as 3 microliters, it is possible that the prognosis is worse in cases with fistula of the oval window compared to that of the round window area, if the pneumolabyrinth is involved in the pathophysiology of perilymphatic fistula. It is also indicated that air inflation of the middle ear is dangerous in cases with fistula in the oval window.

Functional changes associated with experimentally induced endolymphatic hydrops

A guinea pig model with surgically induced endolymphatic hydrops of the inner ear has been developed and studied over the past thirty years. The aim of such studies is to obtain insight into physiological processes associated with endolymphatic hydrops in man and in particular in Menière's disease where endolymphatic hydrops is systematically encountered at post-mortem examination of the temporal bones. The present review attempts to draw together the data pertaining to functional modifications of inner ear function in the animal model. For simplicity the data are categorised under five main titles: electrochemical modifications, electrophysiological modifications, pressure and hydrops, sensitivity to other insults and vestibular dysfunction. One of the most striking observations that can be made is that the data originating from different authors are very variable. There is, however, some evidence suggesting that the evolution of the auditory dysfunction could be considered as consisting of a series of different phases. This kind of information could serve as a basic framework for future research on the animal model.

Low-frequency modulation of compound action potential in experimental perilymphatic fistula and endolymphatic hydrops

We have tested the hypothesis that the cause of cochlear dysfunction associated with perilymphatic fistula (PLF) is closely related to endolymphatic hydrops (ELH). Using guinea pigs, we studied the tone-burst elicited compound action potential (CAP) and its modulation as caused by a 50 Hz biasing tone in experimental PLF. We compared these results with those of experimental ELH. Following perilymph aspiration through the perforated round window membrane, mild but significant elevations of CAP thresholds at tested frequencies were found. A reduction in the amplitude of cochlear microphonics (CM) for a 50 Hz sine wave appeared to correlate with these CAP threshold changes. However, there were no significant changes in the modulation effect of the 50 Hz biasing tone on the CAP elicited by an 8 kHz tone burst. This finding differed from that in ears with experimental ELH, in which significant reductions of both 50 Hz CM and the degree of CAP modulation were consistently observed. We concluded that it is unlikely that the underlying mechanisms of a modification to the low frequency response of the base of the cochlea following perilymph aspiration is linked to that of experimental ELH.

Electrocochleography in the presence and absence of perilymphatic fistula

The purpose of the present study was to evaluate the effect of an acute perilymphatic fistula on the amplitude ratio (SP/AP) of the summating potential (SP) and action potential (AP). The effect of the acute fistula on AP threshold was also addressed. Electrocochleographic recordings were obtained before and immediately after surgical laceration of the round window membrane in 19 guinea pigs. Stimuli comprised clicks and 2,000-Hz and 8,000-Hz tone bursts, presented initially at 100 dB peak equivalent sound pressure level and in descending 10-dB steps. After fistula induction the SP/AP significantly increased for the click and 8,000-Hz tone burst stimuli but not for the 2,000-Hz tone burst stimuli. No significant change in AP threshold occurred. These findings suggest that the SP/AP may be sensitive to perilymphatic fistula, at least in guinea pigs. The changes in the SP/AP do not appear to be related to changes in threshold.

Chronic perilymph fistula in the guinea pig with implications in the human

To improve understanding of the pathophysiology of perilymph fistulas, a predictable animal model of a chronic fistula was developed. Our findings suggest that guinea pig fistulas do not remain patent for prolonged periods. By extrapolating these findings to humans, we postulate that the symptoms and signs of perilymph fistula are possibly due not to one prolonged constant fistula, but rather a series of "blowouts" from an inherent congenital or posttraumatic weak spot in either the round or oval window. We feel that a diagnosis of perilymph fistula must be considered in any patient presenting with a Meniere's-like symptom set concomitant with a congenital inner ear deformity or a history of inner ear trauma.

On the pathomechanism of cochlear dysfunction in experimental perilymph fistulas

The mechanism leading to hearing impairment in perilymph fistulas was investigated in guinea pigs with perforated round window membranes (RWM) by analyzing alterations of inner ear fluid pressure, changes of auditory function following manipulations to get presumed air bubbles out of the cochlea ("positional audiometry"), and temporal bone sections. The instantaneous loss of normal positive inner ear fluid pressure after RWM perforation had no immediate effect on auditory function. Inner ear pressure was restored 4 days following RWM perforation. "Positional audiometry" was negative in guinea pigs with perforated RWM. All ears in which auditory thresholds had increased had additional iatrogenic lesions at the spiral lamina. Fistulas in the RWM per se do not affect auditory thresholds. The question of the surgical repair of fistulas was not directly addressed; it only can be concluded that there are no direct sequelae of an isolated fistula which induce auditory impairment and which could be prevented by surgical repair of the fistula.

The reaction of the guinea pig cochlea to perforations of the round window membrane with and without perilymph aspiration

The influence of simple opening of the round window (RW) membrane and the effect of aspiration of perilymph on the electrophysiological characteristics of the cochlea was tested in guinea pigs by measurement of the compound action potentials. We found that perforations of the RW membrane failed to lead to either short-term or long-term damage in cochlear function. There was only a slight spontaneous escape of perilymph but without measurable functional loss. Additional aspiration of perilymph led to entry of air into the basal turn and to an immediate loss of function of the cochlea. This regressed within 4 weeks in the middle- and low-frequency ranges. Measurable long-term damage persisted only in the high-frequency ranges. We attribute contradictory results of other authors to methodological errors which we avoided by a specific selection of healthy animals and the development of standardized operation, recording and measurement procedures.

Effect of perilymphatic air perfusion on cochlear potentials

Perilymphatic fistula is now widely recognized to cause acute profound hearing loss. It is still controversial, however, which mechanism it is that causes the reversible hearing loss. Recently, it has been suggested by two groups of researchers that the intrusion of air bubbles into the perilymphatic space (a condition called pneumolabyrinth or aerolabyrinth) through the ruptured labyrinthine window(s) may be one of the causes. In order to examine the mechanism underlying the hearing loss associated with pneumolabyrinth, the perilymphatic space of the guinea pig cochlea was perfused with air and cochlear potentials were recorded. Although perfusion of the scala tympani with air at a rate as high as 200 microliter/min caused an immediate and drastic decrease of the cochlear microphonics (CM) and the compound action potential (AP), it had little effect on the endocochlear dc potential (EP) during perfusion for 20 min. A decline in EP was seen in half the ears, but only when the duration of perfusion exceeded 30 min. These results show that the EP has an amazing resistance to air trapped in the scala tympani of the cochlea and that the initial decrease of hearing acuity after the elimination of perilymph from the scala tympani (or introduction of air into the scala tympani) is probably due to interference in CM and AP generation mechanisms rather than to strial dysfunction.

Perilymphatic fistula in the guinea pig

Clinical reports on perilymphatic fistulae (PLF) of the round window membrane (RWM) have shown different degrees of hearing loss. However, the hearing can also be entirely unaffected. Experimentally induced PLF of the RWM in animals showed results similar to those of the clinical reports, although some researchers have concluded that perforation of the RWM per se does not necessarily induce significant hearing loss. The purpose of the present study was to clarify if, and how, PLF of RWM in the guinea pig actually affects the auditory action potentials. During 1 h of observation following perforation of the RWM, the immediate and continuous effect of the PLF was evaluated. In more than 50% of the animals, different amounts of threshold shift were obtained. In another group of guinea pigs an attempt was made to clarify the findings by reducing the cerebrospinal fluid pressure before perforating the RWM.

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.

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.

An electrophysiologic study of experimental perilymphatic fistula

This study was undertaken to elucidate the mechanism that causes sensorineural hearing loss in clinical cases with perilymphatic fistula. Perilymph was experimentally aspirated through the round window membrane in 17 guinea pigs. The extent of cochlear damage was examined electrophysiologically as well as histopathologically. Immediately after aspiration, several types of changes in summating potential (SP) were observed. Two animals without a polarity change of the SP showed only slight threshold changes in both cochlear microphonic and action potentials, and no specific histopathologic changes in the cochlea. Reversed polarity of the SP was observed in three animals, of which one showed a high-amplitude negative SP followed by rapidly progressive hearing loss. Bulging of Reissner's membrane was confirmed histopathologically in this case. The SP disappeared in the remaining 12 animals. In animals with profound electrophysiologic changes, bulging or rupture of Reissner's membrane and damaged hair cells were observed. These findings suggest that an abrupt change in perilymphatic pressure produces morphologic changes in the membranous labyrinth, causing changes in the vibration function of the cochlear partition and in the function of the organ of Corti. Abrupt pressure imbalance may be a causative factor of sensorineural hearing loss in the case of perilymphatic fistula.

Experimental perilymphatic fistula

Perilymphatic fistula was produced in guinea pigs by injecting artificial perilymph into the subarachnoid space of the posterior fossa. Rupture of the round window was confirmed by direct observation of the round window membrane under a surgical microscope during the injection. The animals were either vitally fixed or kept alive for one to three months before fixation. Conventional celloidin embedding method was used for serial sectioning. In immediate observation, nine cochleas showed hydrops, seven showed collapse, and 20 of 36 cochleas showed no change. Changes in the vestibular apparatus varied, with collapse of the membranous vestibular labyrinth as the main change. Delayed observation revealed 16 normal cochleas and four collapses and no hydrops out of 20 ears. Of these 20, nine ears showed normal vestibular apparatus, nine collapse, and two saccule hydrops. Loss of the outer hair cells was observed in five of 20 cochleas. Marked compression of the organ of Corti was seen in both immediate and delayed observations.

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.

Endolymphatic hydrops: can it be controlled?

While endolymphatic shunt procedures are conceptually logical, there are characteristics of underlying pathological conditions and host response that severely limit the probabilities of achieving functional shunts.