Vestibular ototoxicity in the chick: effects of streptomycin on equilibrium and on ampullary dark cells

Hair cell toxicology: With the help of a little fish

Hearing or balance loss are disabling conditions that have a serious impact in those suffering them, especially when they appear in children. Their ultimate cause is frequently the loss of function of mechanosensory hair cells in the inner ear. Hair cells can be damaged by environmental insults, like noise or chemical agents, known as ototoxins. Two of the most common ototoxins are life-saving medications: cisplatin against solid tumors, and aminoglycoside antibiotics to treat infections. However, due to their localization inside the temporal bone, hair cells are difficult to study in mammals. As an alternative animal model, zebrafish larvae have hair cells similar to those in mammals, some of which are located in a fish specific organ on the surface of the skin, the lateral line. This makes them easy to observe in vivo and readily accessible for ototoxins or otoprotective substances. These features have made possible advances in the study of the mechanisms mediating ototoxicity or identifying new potential ototoxins. Most importantly, the small size of the zebrafish larvae has allowed screening thousands of molecules searching for otoprotective agents in a scale that would be highly impractical in rodent models. The positive hits found can then start the long road to reach clinical settings to prevent hearing or balance loss.

Intratympanic Treatment in Menière's Disease, Efficacy of Aminoglycosides Versus Corticosteroids in Comparison Studies: A Systematic Review

OBJECTIVE

To compare the functional outcomes and complications of intratympanic gentamicin (ITG) versus intratympanic corticosteroids (ITC) in Menière's disease.

DATA SOURCES

An electronic search was conducted in the Cochrane Library, PubMed, and Embase databases on February 3, 2019. Articles written in English, Dutch, German, French, or Turkish language were included.

STUDY SELECTION

Study inclusion criteria were: 1) patients diagnosed with definite Menière's disease according to the criteria of the American Academy of Otolaryngology-Head and Neck Surgery, 2) treated with ITG or ITC in a comparison study, and 3) reported subjective and objective outcomes concerning Menière's disease.

DATA EXTRACTION

The quality of eligible studies was assessed according to an adjusted version of the Cochrane Risk of Bias tool. The extracted data were study characteristics (study design, publication year, and number of relevant patients), patient's characteristics (sex and age), disease characteristics (uni or bilateral and duration of Menière's disease), treatment protocol, and different therapeutic outcomes (vertigo, tinnitus, aural fullness, and hearing loss).

DATA SYNTHESIS

A total of eight articles were included for data extraction and analysis. For subjective outcomes, ITG was slightly favored compared to intratympanic corticosteroids. This was significant only in three studies (p < 0.05). For objective outcomes and complications, no significant differences were seen.

CONCLUSIONS

The result of this systematic review shows some benefit of ITG over ITC for subjective outcomes and no difference regarding objective outcomes or complication rate. However, this superiority of ITG is rather weak. Both interventions can be effectively and safely used in controlling Menière's disease in acute situations.

Uptake of fluorescent gentamicin by peripheral vestibular cells after systemic administration

Objective

In addition to cochleotoxicity, systemic aminoglycoside pharmacotherapy causes vestibulotoxicity resulting in imbalance and visual dysfunction. The underlying trafficking routes of systemically-administered aminoglycosides from the vasculature to the vestibular sensory hair cells are largely unknown. We investigated the trafficking of systemically-administered gentamicin into the peripheral vestibular system in C56Bl/6 mice using fluorescence-tagged gentamicin (gentamicin-Texas-Red, GTTR) imaged by scanning laser confocal microscopy to determine the cellular distribution and intensity of GTTR fluorescence in the three semicircular canal cristae, utricular, and saccular maculae at 5 time points over 4 hours.

Results

Low intensity GTTR fluorescence was detected at 0.5 hours as both discrete puncta and diffuse cytoplasmic fluorescence. The intensity of cytoplasmic fluorescence peaked at 3 hours, while punctate fluorescence was plateaued after 3 hours. At 0.5 and 1 hour, higher levels of diffuse GTTR fluorescence were present in transitional cells compared to hair cells and supporting cells. Sensory hair cells typically exhibited only diffuse cytoplasmic fluorescence at all time-points up to 4 hours in this study. In contrast, non-sensory cells rapidly exhibited both intense fluorescent puncta and weaker, diffuse fluorescence throughout the cytosol. The numbers and size of fluorescent puncta in dark cells and transitional cells increased over time. There is no preferential GTTR uptake by the five peripheral vestibular organs’ sensory cells. Control vestibular tissues exposed to Dulbecco’s phosphate-buffered saline or hydrolyzed Texas Red had negligible fluorescence.

Conclusions

All peripheral vestibular cells rapidly take up systemically-administered GTTR, reaching peak intensity 3 hours after injection. Sensory hair cells exhibited only diffuse fluorescence, while non-sensory cells displayed both diffuse and punctate fluorescence. Transitional cells may act as a primary pathway for trafficking of systemic GTTR from the vasculature to endolymph prior to entering hair cells.

Temporal and spatial distribution of gentamicin in the peripheral vestibular system after transtympanic administration in guinea pigs

BACKGROUND AND OBJECTIVE

Transtympanic administration of gentamicin is effective for treating patients with intractable vertigo. This study explored the spatial and temporal distribution of gentamicin in vestibular end-organs after transtympanic administration.

METHODS

Thirty guinea pigs were transtympanically injected with gentamicin conjugated to Texas Red (GTTR) and their vestibular end-organs examined after various survival periods. Another 9 guinea pigs received GTTR at different doses. Nine animals received Texas Red only and served as controls. We used confocal microscopy to determine the cellular distribution of GTTR in semicircular canal cristae, as well as the utricular and saccular maculae.

RESULTS

The most intense GTTR labeling was present in the saccule compared to other vestibular end-organs. GTTR fluorescence was detected predominantly in type I hair cells, type II hair cells and transitional cells after a single transtympanic dose of GTTR (0.1 mg/ml, 0.05 ml), while only weak fluorescence was observed in non-sensory cells such as supporting cells, dark cells and lumenal epithelial cells. Transitional cells displayed intense GTTR fluorescence in the supra-nuclear regions 24 h after transtympanic injection that was retained for at least 4 weeks. A decreasing spatial gradient of GTTR fluorescence was observed sensory epithelial regions containing central type I to peripheral type I and then type II hair cells in the crista ampullaris, and from striolar to extra-striolar hair cells within the vestibular macula. GTTR fluorescence extended from being restricted to the apical cytoplasm at lower doses to the entire cell body of type I hair cells with increasing dose. GTTR fluorescence reached peak intensities for individual regions of interest within the cristae and maculae between 3 and 7 days after transtympanic injection.

CONCLUSION

The saccular uptake of GTTR is greater than other vestibular end-organs after transtympanic injection in the semicircular canals.

The Effect of Intratympanic Methylprednisolone and Gentamicin Injection on Ménière’s Disease

Objectives

To compare the efficacy of intratympanic injections of methylprednisolone (ITMP) and intratympanic injections of gentamicin (ITG) to control the symptoms of Ménière’s disease and to evaluate their effect on hearing level.

Study Design

A historical cohort study.

Setting

Tertiary referral center.

Subjects and Methods

Eighty-nine patients affected by Ménière’s disease were included in this study, of whom 47 were treated with ITG and 42 were treated with ITMP. Two periods of follow-up were considered: 0 to 6 months and 6 to 12 months after the intratympanic injections (ITI). Mean outcome measurements consisted of control of vertigo attacks, tinnitus, and aural fullness; pure-tone average (PTA); and speech discrimination score (SDS).

Results

The 2 groups had the same number of vertigo spells per month before ITI ( P = .883). Six to 12 months after ITI, 82.9% of the ITG group and 48.1% of the ITMP group achieved complete control of vertigo ( P = .004). There was better control of tinnitus and aural fullness with ITG than with ITMP ( P ≤ .002). The 2 groups had a statistically significant difference in hearing level before ITI ( P ≤ .001). This difference was no longer present 6 to 12 months after ITI ( P > .05).

Conclusion

Intratympanic injections of gentamicin are more efficient than ITMP in controlling the symptoms of Ménière’s disease. The 2 groups ended up without a difference in hearing level after ITI. According to these findings, administrating ITMP to control Ménière’s disease seems to be less beneficial than ITG.

Intratympanic gentamicin injections for the treatment of Ménière's disease

Objective

To assess the success rate and complications of treatment with injections of intratympanic gentamicin to the middle ear of patients with Ménière's disease.

Study Design

Case series with chart review.

Setting

A tertiary otologic private ENT clinic.

Subjects and Methods

Patients with Ménière's disease, who had not responded to medical treatment. Included in the sample were patients with Ménière's disease who had been treated with injections of gentamicin into the middle ear. After each gentamicin application, patients were monitored for their symptoms and hearing. If symptoms persisted, they received another intratympanic injection of gentamicin. This method is referred to as the variable titration method. A retrospective chart review was performed, and questionnaires were used to assess hearing, functional status, tinnitus, ear fullness sensation, and the control of vertigo attacks in response to treatment.

Results

Nineteen patients were sampled. Eighteen patients (94%) had complete or substantial control of vertigo. Five patients (26%) had worse hearing results on their post-treatment audiogram, averaging 13 dB hearing loss (range, 5-25 dB). In response to the questionnaires, all patients reported a significant improvement in the quality of life after treatment.

Conclusion

The treatment was found to be highly effective. The variable titration method of injection prevents unnecessary injections for patients whose symptoms have already subsided.

Treatment of Ménière's Disease by Low-Dosage Intratympanic Gentamicin Application: Effect on Otolith Function

OBJECTIVES

The intratympanic application of a low dosage of gentamicin is increasingly favored as treatment for Ménière's disease. While posttreatment observations have confirmed a long-term success of the therapy of vertigo attacks, clear differences in the posttreatment recovery interval can be observed. In addition to differences in central-vestibular compensation, the degree of peripheral vestibular damage, i.e., to the saccule, utricle, and semicircular canal ampullae, varies among patients. This study provides comprehensive pre- and posttreatment results from unilateral functional tests of the individual vestibular receptors and of the cochlea in patients with Ménière's disease.

STUDY DESIGN

Prospective clinical study.

METHODS

Nineteen patients with unilateral Ménière's disease were treated by intratympanic application of gentamicin by injection of 0.3 mL (12 mg) through the tympanic membrane under local anesthesia. Tests were performed immediately previous to treatment and subsequently in the periods 4 to 8 weeks and 12 to 16 weeks after treatment. Unilateral saccular function was tested by means of acoustic-click, vestibular-evoked myogenic potentials (VEMP), and unilateral utricular function by subjective visual vertical (SVV) during unilateral centrifugation. Bithermal caloric testing was performed to assess unilateral semicircular canal function.

RESULTS

Prior to gentamicin treatment, the caloric response from the diseased ear was normal in 3 patients, below normal in 14 patients, and in 2 cases almost completely absent. VEMP responses could be recorded bilaterally in 13 patients; while in 6, no VEMPs could be measured from the diseased ear. Utricular function measured by SVV estimation was found to be normal in 11 patients and marginally abnormal in 2 patients. In six cases, the SVV was clearly underestimated during centrifugation of the diseased side. The posttreatment findings demonstrate that VEMPs were absent in all treated patients, and the caloric response was abnormally low in all but one case. In contrast, only 12 of 19 patients produced abnormal SVV responses.

CONCLUSION

The results demonstrate that incremental, intratympanic application of gentamicin effectively eliminates semicircular canal and saccular function. In contrast, utricular function appears to be maintained in 30 to 40% of cases.

Intratympanic gentamicin: a 1-day protocol treatment for unilateral Meniere's disease

PURPOSE

The objective of this study was to assess the usefulness of low-dose intratympanic gentamicin administration in patients with Meniere's disease (MD).

MATERIALS AND METHODS

We carried out a retrospective review of the charts of 55 patients with definite MD in accordance with the AAO-HNS (American Academy of Otolaryngology-Head and Neck Surgery) Diagnostic Scale who were treated at Gruppo Otologico (Piacenza, Rome, Italy). The treatment protocol consisted of 3 intratympanic injections of gentamicin at a 12-hour interval between each administration.

RESULTS

Results were resumed in accordance with AAO-HNS criteria and showed complete control of vertigo spells in 60% (class A), good control in 16% (class B), moderate control in 4% (class C), poor control in 16% (class D), and no control in 4% (class E) of the patients. Audiological assessment showed unchanged thresholds in 61%, improvement in 22%, and worsening in 17% of the cases.

CONCLUSION

Administration of gentamicin is a good alternative to medical and surgical procedures in the treatment of intractable MD. One-day treatment allows for adequate control of vertigo attacks without compromising hearing and quality of life and with reduced costs.

Angular vestibulo-ocular reflex gains correlate with vertigo control after intratympanic gentamicin treatment for Meniere's disease

OBJECTIVES

The objective of our study was to determine whether angular vestibulo-ocular reflex (aVOR) gains correlated with vertigo control after intratympanic gentamicin treatment for Meniere's disease.

METHODS

We conducted a prospective study of 18 subjects with unilateral Meniere's disease treated with intratympanic gentamicin injection and followed all subjects for 1 year. We measured the gain of the aVOR elicited by rapid rotary head thrusts in each of the canal planes for each subject before and after treatment with intratympanic gentamicin by using magnetic search coils to record eye movements.

RESULTS

During the follow-up period, 11 subjects ("single-treatment group"; 61%) had control of their vertigo with a single gentamicin injection. The remaining 7 subjects ("multiple-treatment group"; 39%) experienced recurrent vertigo that required a second injection of gentamicin at a mean of 6 months after the first treatment. The 11 subjects in the single-treatment group had significantly greater reduction of labyrinthine function after the first treatment, as measured by change in ipsilateral horizontal canal gain, than did the 7 subjects with vertigo recurrence. Changes in caloric asymmetry did not correlate with vertigo control.

CONCLUSIONS

Our results suggest that successful treatment of Meniere's disease is closely related to attenuation of semicircular canal function as measured by horizontal canal aVOR gains.

Intratympanic gentamicin for the treatment of Ménière's disease and other forms of peripheral vertigo

Titration therapy with intratympanic gentamicin offers class A or B control of vertigo in 87% (range, 75%-100%) of patients with unilateral Ménière's disease. The risk of additional hearing loss is about 21% (range,0-37%). Vertigo may recur, however, in nearly one third of patients over time. These recurrences can also be treated by intratympanic gentamicin with a similar risk of hearing loss. The salient effect of intratympanic gentamicin is probably the reduction of vestibular function through damage to hair cells, but a complete ablation of function does not seem to be necessary for vertigo control.

Inner ear changes with intracochlear gentamicin administration in Guinea pigs

OBJECTIVES/HYPOTHESIS

Transtympanic administration of gentamicin is reported to be a useful treatment for vertigo in such conditions as Meniere's disease, and determining appropriate clinical dosage of gentamicin is difficult. The authors examined the relation between gentamicin dosages and inner ear function in guinea pigs.

STUDY DESIGN

This study is a basic science project designed to examine cochlear and vestibular function in animal models.

METHODS

Various concentrations of gentamicin solution were infused into the right inner ear of guinea pigs by osmotic pumps. Caloric nystagmus as a marker of vestibular function and the change in auditory brainstem response (ABR) threshold as a marker of cochlear function were observed.

RESULTS

After 14 days of treatment, high gentamicin concentrations of 40 mg/mL caused canal paralysis and a rapid shift in ABR threshold. Animals exposed to low gentamicin concentrations of 4 mg/mL showed no obvious change in either vestibular or cochlear function. Animals exposed to moderate gentamicin concentrations of 12 mg/mL showed a moderate shift in ABR threshold and caloric malfunction. Histopathological examination revealed that after 14 days of treatment with 40 mg/mL gentamicin, severe cytoplasmic damage occurred in both vestibular and cochlear end organs. In animals treated with 12 mg/mL gentamicin, hair cells remained in the cochlear third turn and ampulla of the lateral semicircular canal.

CONCLUSION

The authors established an animal model that showed the moderate damage of inner ear with moderate-dose gentamicin. The study results indicated that the appropriate administration of gentamicin could establish a stable effect on the inner ear. It may be important to select the protocol that delivers a stable dosage of gentamicin to treat patients with Meniere's disease safely and effectively.

Transfection of young guinea pig vestibular cells in vitro with an adenovirus vector

This study shows distributions of lacZ-positive cells in the vestibular labyrinthine explants of young guinea pigs with mature ears. When adenovirus lacZ vectors were administered to the vestibular labyrinth following removal of the otoconial membrane, lacZ-positive cells were observed in the mesothelial cells in the perilymphatic space, in the sensory and supporting cells in the utricle and ampulla, and in the transitional and dark cells in the ampulla. When the otoconial membrane was preserved, lacZ-positive cells were not distributed in the utricular sensory epithelium. These findings suggest that adenovirus vectors can transform a variety of vestibular epithelial cells, but that it is difficult for adenovirus vectors to pass through the otoconial membrane.

Round window gentamicin mu-catheter--a new therapeutic tool in Ménière's disease

The vestibulotoxic effect of gentamicin is well documented; however, there is no general agreement on the gentamicin dose needed to control vertigo attacks without affecting the hearing. In the current study, 14 patients with Ménière's disease refractory to medical treatment were treated by small doses of gentamicin delivered via a mu-catheter, placed into the round window niche. An electronic micropump delivered 10 microliters (100 micrograms)/h. The patients received a total dose of 4-14 mg. The effects on the vestibular symptoms were most encouraging, with cessation of vertigo in 13 of the 14 patients, control of Tumarkin attacks in 4 out of 6 cases and release of aural pressure and fullness in 2 out of 4 cases. According to the recommendations of the Committee on Hearing and Equilibrium, the patients in this material belonged to functional levels 5 and 6. At the last clinical follow-up, five patients were classified as level 1, eight patients as level 2 and one patient as level 3. A significant hearing loss was seen in one patient; however, this was most likely due to the natural development of the disease in this particular case. The results look promising, but must be investigated further in a placebo-controlled study.

Intratympanic gentamicin for the treatment of unilateral Meniere's disease

OBJECTIVE

To determine the efficacy of intratympanic gentamicin instillation as treatment of incapacitating unilateral Meniere's disease, using a predetermined regimen with a fixed dose.

STUDY DESIGN

A prospective study from a single institution between 1988 and 1998.

METHODS

One hundred fourteen patients were enrolled in this study. Gentamicin (26.7 mg/mL) was administered three times daily for 4 consecutive days. The Committee on Hearing and Equilibrium Guidelines for Reporting Treatment Results in Meniere's Disease of the American Academy of Otolaryngology and Head and Neck Surgery (1985) were used.

RESULTS

Comprehensive data were available for 90 individuals. Complete control of vertigo was achieved in 76 (84.4%), substantial control in 8 (9.0%), limited control in 2 (2.2%), and insignificant control in 4 (4.4%) patients. Disability scores at the end of 2 years were as follows: 76 patients (84.4%) had no disability, 5 (5.6%) had mild disability, 2 (22%) had moderate disability, and 7(7.8%) had severe disability. Caloric testing responses, as determined using electronystagmography, were as follows: 71% of the patients had an absent ice-water response, 16% had a positive ice-water response, and in 13% there continued to be present a bithermal response. Hearing was worse in 22 patients (25.6%), unchanged in 41 (48.2%), and improved in 22 (25.6%).

CONCLUSIONS

Intratympanic gentamicin administration using this particular protocol is an effective treatment option for patients with disabling unilateral Meniere's disease. Hearing loss is a distinct possibility, and patients should be advised accordingly.

Direct round window membrane application of gentamicin in the treatment of Meniere's disease

OBJECTIVE

To evaluate the effectiveness of the round window membrane (RWM) Gelfoam gentamicin technique in patients with Meniere's disease who were unresponsive to medical management or in whom surgical therapy failed.

STUDY DESIGN

Protocol 1, single intratympanic gentamicin infusion; protocol 2 (the best method), 2 infusions, 5 days apart with reevaluation at 1 month; and protocol 3, multiple infusions 1 to 4 weeks apart.

PATIENTS

In total, 32 patients (19 male, 13 female) were enrolled in the study. The mean age was 65 years (range 34 to 94 years). Seven of these patients were surgical salvage cases.

INTERVENTIONS

Laser-assisted otoendoscopy with a 1.7-mm otoendoscope (Smith-Nephew Richards, Memphis, TN) was performed first. If the RWM was obscured by mucosa or adhesions, these were cleared before placing a 2 x 3 mm piece of dry Gelfoam against the RWM. Buffered gentamicin (26.7 mg/mL) was then injected into the middle ear (0.2 to 0.3 mL).

RESULTS

Overall, vertigo was controlled in 75% of the patients after the completion of the treatment, with subtotal vestibular ablation in two thirds of patients. Hearing was preserved in 90% of the patients (within 15 dB pure-tone average or 15% speech discrimination score), tinnitus improved in 48%, and aural pressure improved in 62.5%.

Kinetics of gentamicin uptake in the inner ear of Chinchilla langier after middle-ear administration in a sustained-release vehicle

The search for a safe, effective treatment for the vertigo associated with Meniere's disease has long been an important topic in otolaryngology. In recent years many groups have begun using intratympanic gentamicin to treat this vertigo. Although reported cure rates are as high as 90%, many questions remain regarding this type of treatment. Current limitations are the necessity for repeated treatments and a lack of clear dosing guidelines. In addition, the gentamicin must be delivered in a manner that allows maximal vestibulotoxic effect without injury to hearing. Until investigators can control the exact amount of medicine that is placed in the ear and have an understanding of the kinetics of gentamicin absorption, adequate dosing guidelines will be difficult to establish, and therapy will continue to rely on empiric data. We describe the use of a fibrin-based sustained-release vehicle, impregnated with gentamicin, injected into the middle ear of chinchillas. This allows for a prolonged effect without repeated dosing. Using this model, we studied the absorption kinetics of gentamicin at time points ranging from 8 hours to 1 week after injection. We used our findings to create a kinetics curve of gentamicin absorption. We discuss the shape and characteristics of this kinetics curve and examine the effects of the fibrin-based sustained-release vehicle and gentamicin on the middle ear. We noted no absorption in the contralateral (untreated ear) or blood. Through better understanding of the actions of gentamicin in this animal model, we hope to facilitate safer use of intratympanic medicines in our patient population and initiate programs for the use of this sustained-release vehicle in human beings.

Vestibular function in Belgian Waterslager canaries (Serinus canarius)

The purpose of this study was to measure vestibular function in Belgian Waterslager canaries using short latency vestibular evoked potentials (VsEPs) elicited by linear acceleration stimuli. Responses were recorded with vertex to mastoid leads using traditional signal averaging. Response thresholds, latencies, and amplitudes were quantified and compared to non-Waterslager controls. Cochlear and vestibular organs were also processed for scanning electron microscopy. Results indicated that vestibular response thresholds were slightly, but significantly, higher for Belgian Waterslager canaries and response amplitudes at 0 dBre: 1.0 g/ms were significantly reduced compared to non-Waterslagers. Response peak latencies were not significantly different. The most striking morphological finding was that the stereociliary bundles of Waterslager saccular hair cells showed no common orientation. Previous reports have also found significantly less hair cells in Waterslager saccules (Weisleder and Park, Hear. Res. 80 (1994) 64-70); however, the present study did not confirm this finding. The utricle and ampullae appeared normal. The present results indicate that vestibular neural function, as measured with VsEPs, is affected in Belgian Waterslager canaries. The results also suggest that one structural correlate of the functional loss is the disordered stereociliary bundles in the sacculus.

Insensitivity of the chicken embryo to the ototoxicity of aminoglycoside antibiotics and a loop diuretic

Guinea pigs are routinely used in the histological evaluation of the cochlea as a method of testing for ototoxicity, but the procedures are very time-consuming. Because the avian cochlea is easier to examine and newly hatched chicks are sensitive to the ototoxic effects of gentamicin, birds may be useful in testing for ototoxicity. The use of chicken embryos would be even better for testing, but whether or not chicken embryos are sensitive to ototoxicants is unknown. In an attempt to determine whether or not chicken embryos may be used instead of guinea pigs in screening tests for ototoxicity, aminoglycoside antibiotics and a loop diuretic, ethacrynic acid, were administered to chicken embryos. A maximum-tolerated dose of gentamicin, kanamycin, streptomycin, ethacrynic acid, or a combination of gentamicin and ethacrynic acid was administered to fertile eggs of White Leghorn chickens on incubation days 10-17. To compare the effect of route of exposure on ototoxicity, gentamicin was administered by injection into the allantoic space, yolk sac, and air cell as well as by submerging the egg in gentamicin solution. With the preferred air cell route the effects of the ototoxic drugs kanamycin, streptomycin, ethacrynic acid, and a combination of ethacrynic acid and gentamicin were compared. On incubation day 18, cochleas were removed from the chicken embryos. Serial sections of these avian cochleas were examined and hair cells were counted. No significant difference was seen between the number of hair cells in cochleas of control chicken embryos and those from chicken embryos treated with drugs. Therefore, the chicken embryo appears to be insensitive to the ototoxicity of aminoglycoside antibiotics and a loop diuretic.

Intratympanic gentamycin in severe Ménière's disease

From 1982 to 1987, 20 patients with disabling attacks of vertigo and severe hearing loss due to unilateral Ménière's disease were treated with local intratympanic gentamycin. In 18 patients control of vertiginous attacks was achieved. Hearing deteriorated in 8 patients. In spite of complete control of attacks a disabling ablation type of unsteadiness and dizziness affected 7 patients. Two patients required additional treatment after 5-8 months remission. Intratympanic gentamycin seems to be a less useful method of treating severe unilateral Ménière's disease than previously stated. It is, however, a possible alternative to surgical labyrinthectomy with a fair chance of residual hearing preservation.

The use of streptomycin to induce unilateral ablation of vestibular function in the rat: a preliminary report

The goal of our research is to develop a method of applying streptomycin to the middle ear so as to destroy vestibular function unilaterally while avoiding damage to auditory function. In this preliminary experiment, a total of 42 mg of streptomycin was infused continuously over a seven-day period into one middle ear cavity of seven rats, with the opposite ear serving as a control. Histologic study revealed a moderate to severe degree of vestibular neuroepithelial damage in all of the infused ears. However, significant damage also occurred in the basal turn of the cochlea in a majority of cases.

Selective lesions of the vestibular labyrinth

In attempts to decrease gentamicin ototoxicity, the round windows of guinea pigs were covered with fat before gentamicin was given through the middle ear cavity. Although ototoxicity to the cochlea was reduced, the desired effect on the vestibular sense organs also was reduced, because of the oval window's resistance to drug penetration. In another series of guinea pigs, simple opening of the lateral semicircular canal and covering it with a pledget of Gelfoam soaked with gentamicin was an excellent method for producing a selective vestibular lesion. It is expected to reduce the function of all vestibular sense organs without impairing cochlear function.

Ultrastructural changes of statoconia after segmentation of the otolithic membrane

The chick vestibule transformed from a homogeneous epithelial layer at day 2 (stage 15) into a pseudo-stratified epithelial layer at day 4 (stage 24). The apical columnal appearance of sensory cells was evident by day 6 (stage 29). In the supporting cells of the saccule and utricle large rough endoplasmic reticulum cisterns filled with material similar to the primitive organic matrix. Fibrillar material of the otolithic membrane remained attached to the supporting cells and accumulated over the saccule and utricle. The primitive otolithic membrane acquired stress-like lines and statoconial units emerged from the upper surface without a central core. Statoconia thickened at the periphery and a central core formed. Calcium was deposited between the fibrils of older statoconia which were located on top of the segmenting membrane. DIAMOX inhibited statoconia formation and/or prevented calcium and the matrix from associating. Large statoconia (100-200 microns diameter) were formed in embryos injected with this carbonic anhydrase inhibitor. Gel electrophoresis of immature statoconial complexes yielded at least 5 major protein bands between 25 and 210 kDa. Ouabain-sensitive potassium-dependent p-nitrophenylphosphatase activity was demonstrated in the endolymphatic sac of newly hatched chicks.

Gentamicin tympanoclysis: effects on the vestibular secretory cells

Gentamicin application to the middle ear may relieve Meniere's disease, presumably by reducing endolymph secretion by the dark cells. To explore this possibility, the ears of adult cats were treated with daily intratympanic infusions of gentamicin until ataxia occurred. The temporal bones of these cats were then examined using electron microscopy. A 3 per cent solution of gentamicin resulted in ataxia after four treatments. Acute dark cell damage to basal infoldings seen after one month persisted at six months. Treatment with a 0.3 per cent solution required 15 and 21 days to effect ataxia and resulted in a similar damage pattern. Treatment with the 0.3 per cent solution for 13 days (i.e., before ataxia developed) resulted in subtle early lesions in the basal infoldings. Such lesions might affect the rate of endolymph secretion.

Further observations of vestibular ototoxicity in the chick: effects of streptomycin on the ampullary sensory epithelium

Starting eight days after hatching, chicks received daily subcutaneous injections of streptomycin sulfate, either 400 mg/kg for 30 days or 1,200 mg/kg for 15 days. Randomly selected chicks from each group were killed at intervals during the injection period, and the ampullae were examined for signs of vestibulotoxicity. Ampullary cell types differed in sensitivity to streptomycin. First, dark cell processes withered. Second, vacuoles formed in the apices of the light cells of the planum semilunatum. Third, nerve terminals swelled, and their organelles and ground substance clumped together, creating cleared areas. Last, hair cells and supporting cells became slightly vacuolated. However, these cells showed less overt damage than the other cell types. The onset of damage was earlier and the damage more severe with the 1,200-mg than with the 400-mg dosage. There was no evidence of hair cell loss during the experiment.

Glutaraldehyde fixatives for preserving the chick's inner ear

We preserved the inner ears of chicks in various concentrations of glutaraldehyde (2 to 3.5%) and cacodylate buffers (0.025 to 0.1 M). Buffer concentrations below 0.1 M caused osmotic damage that higher glutaraldehyde concentrations only partially counteracted. The combination of 3.5% glutaraldehyde and 0.1 M cacodylate buffer optimally preserved the different cell types and also eliminated problems of swelling and shrinkage. We further improved cellular preservation by immediately immersing the dissected specimen into chilled (4 degrees C), aerated fixative. The improved fixation greatly increased the retention of cytoplasmic ground substances, particularly in supporting cells and nerve terminals.