Cochlear hair cell loss in single-dose versus continuous round window administration of gentamicin

Gentamicin delivery to the inner ear: Does endolymphatic hydrops matter?

Introduction

Middle ear application of gentamicin is a common medical treatment for uncontrolled Ménière’s disease. The objective of the study was to evaluate the impact of endolymphatic hydrops on inner ear delivery.

Methods

Perilymph gentamicin concentrations and correlation with endolymphatic hydrops in an animal model were assessed. A group of 24 guinea pigs was submitted to surgical obstruction of the endolymphatic sac and duct of the right ear. Gentamicin was applied either to the right ear’s round window niche or through a transtympanic injection. Perilymph specimens were collected at different times. Histologic morphometry was used to evaluate both turn-specific and overall hydrops degree.

Results

In animals with endolymphatic hydrops, lower concentrations of gentamicin were observed after 20 or 120 minutes of exposure and in both types of administration, when compared to controls. This difference reached statistical significance in the round window niche application group (Mann-Whitney, p = 0,007). A negative correlation between perilymphatic gentamicin concentration and hydrops degree could be observed in both groups, after 120 minutes of exposure (Spearman correlation, round window niche p<0,001; TT p = 0,005).

Conclusions

The study indicates that the endolymphatic hydrops degree has a negative interference on the delivery of gentamicin into the inner ear following middle ear application.

Cannula-based drug delivery to the guinea pig round window causes a lasting hearing loss that may be temporarily mitigated by BDNF

Sustained local delivery of drugs to the inner ear may be required for future regenerative and protective strategies. The round window is surgically accessible and a promising delivery route. To be viable, a delivery system should not cause hearing loss. This study determined the effect on hearing of placing a drug-delivery microcatheter on to the round window, and delivering either artificial perilymph (AP) or brain-derived neurotrophic factor (BDNF) via this catheter with a mini-osmotic pump. Auditory brainstem responses (ABRs) were monitored for 4 months after surgery, while the AP or BDNF was administered for the first month. The presence of the microcatheter - whether dry or when delivering AP or BDNF for 4 weeks - was associated with an increase in ABR thresholds of up to 15 dB, 16 weeks after implantation. This threshold shift was, in part, delayed by the delivery of BDNF. We conclude that the chronic presence of a microcatheter in the round window niche causes hearing loss, and that this is exacerbated by delivery of AP, and ameliorated temporarily by delivery of BDNF.

Perilymph pharmacokinetics of locally-applied gentamicin in the guinea pig

Intratympanic gentamicin therapy is widely used clinically to suppress the vestibular symptoms of Meniere's disease. Dosing in humans was empirically established and we still know remarkably little about where gentamicin enters the inner ear, where it reaches in the inner ear and what time course it follows after local applications. In this study, gentamicin was applied to the round window niche as a 20 μL bolus of 40 mg/ml solution. Ten 2 μL samples of perilymph were collected sequentially from the lateral semi-circular canal (LSCC) at times from 1 to 4 h after application. Gentamicin concentration was typically highest in samples originating from the vestibule and was lower in samples originating from scala tympani. To interpret these results, perilymph elimination kinetics for gentamicin was quantified by loading the entire perilymph space by injection at the LSCC with a 500 μg/ml gentamicin solution followed by sequential perilymph sampling from the LSCC after different delay times. This allowed concentration decline in perilymph to be followed with time. Gentamicin was retained well in scala vestibuli and the vestibule but declined rapidly at the base of scala tympani, dominated by interactions of perilymph with CSF, as reported for other substances. Quantitative analysis, taking into account perilymph kinetics for gentamicin, showed that more gentamicin entered at the round window membrane (57%) than at the stapes (35%) but the lower concentrations found in scala tympani were due to greater losses there. The gentamicin levels found in perilymph of the vestibule, which are higher than would be expected from round window entry alone, undoubtedly contribute to the vestibulotoxic effects of the drug. Furthermore, calculations of gentamicin distribution following targeted applications to the RW or stapes are more consistent with cochleotoxicity depending on the gentamicin concentration in scala vestibuli rather than that in scala tympani.

Enhanced local bioavailability of single or compound drugs delivery to the inner ear through application of PLGA nanoparticles via round window administration

In this paper, the potential of poly(D,L-lactide-co-glycolide acid) (PLGA) nanoparticles (NPs) for carrying single or compound drugs traversing the round window membrane (RWM) was examined after the round window (RW) administration of different NPs to guinea pigs. First, coumarin-6 was incorporated into PLGA NPs as a fluorescent probe to investigate its ability to cross the RWM. Then, PLGA NPs with salvianolic acid B (Sal B), tanshinone IIA (TS IIA), and total panax notoginsenoside (PNS) including notoginsenoside R1 (R1), ginsenoside Rg1 (Rg1), and ginsenoside Rb1 (Rb1) were developed to evaluate whether NPs loaded with compound drugs would pass through the RWM and improve the local bioavailability of these agents. PLGA NPs loaded with single or compound drugs were prepared by the emulsification solvent evaporation method, and their particle size distribution, particle morphology, and encapsulation efficiency were characterized. In vitro release study showed sustained-release profiles of Sal B, TS IIA, and PNS from the NPs. The pharmacokinetic results showed that NPs applied to the RWM significantly improved drug distribution within the inner ear. The AUC0-t of coumarin-6 in the perilymph (PL) following RW administration of NPs was 4.7-fold higher than that of coumarin-6 solution, and the Cmax was 10.9-fold higher. Furthermore, the AUC(0-t) of R1, Rg1, and Rb1 were 4.0-, 3.1-, and 7.1-fold greater, respectively, after the application of NPs compared to the compound solution, and the Cmax were, respectively, 14.4-, 10.0-, and 16.7-fold higher. These findings suggest that PLGA NPs with unique properties at the nanoscale dimensions have a powerful ability to transport single or compound drugs into the PL through the RWM and remarkably enhance the local bioavailability of the encapsulated drugs in the inner ear. The use of PLGA NPs as nanoscale delivery vehicles to carry drugs across the RWM may be a promising strategy for the treatment of inner ear diseases.

Gentamicin concentration gradients in scala tympani perilymph following systemic applications

It has been shown in prior studies that round window membrane (RWM) application of gentamicin produced a robust basal-apical concentration gradient in the perilymph of scala tympani (ST) with peak concentrations in the basal turn of ST. These gradients potentially contribute to the clinical efficacy and safety of intratympanic gentamicin applications for the treatment of Ménière's disease. The present study aimed to establish the distribution of gentamicin along ST perilymph after systemic applications. Gentamicin sulfate was applied intravenously in the amounts of 100, 300 and 600 mg/kg body weight (BW) over a period of 3 h or as a 300 mg/kg BW subcutaneous bolus injection. At 3 and 5 h after the start of the application perilymph of ST was aspirated from the cochlea apex of the right and left cochlea, respectively, and 10 sequential 1-µl perilymph samples from the apex of each cochlea were quantitatively analyzed using a fluorescence polarization immunoassay. In contrast to local RWM delivery, systemic application of gentamicin resulted in the highest perilymph levels in the apex of the cochlea with decreasing concentrations towards the basal regions of ST. The absolute gentamicin concentrations increased with the amount of drug applied and time before sampling. While it is likely that the basal-apical gradient measured after local drug applications to the round window niche is the result of the direct uptake of drugs into the perilymph of the ST, distribution by diffusion and a very low perilymph flow towards the cochlear apex, computer simulations suggested that the apical-basal gradient observed with these systemic applications can be explained by higher entry rates of gentamicin in the apex compared to the basal turns of the cochlea. It is also possible that gentamicin enters perilymph indirectly from the blood via the endolymph. In this case the faster kinetics in apical turns could be due to the smaller cross-sectional area of ST relative to endolymph in the apical turns.

Intra-tympanic delivery of short interfering RNA into the adult mouse cochlea

Trans-tympanic injection into the middle ear has long been the standard for local delivery of compounds in experimental studies. Here we demonstrate the advantages of the novel method of intra-tympanic injection through the otic bone for the delivery of compounds or siRNA into the adult mouse cochlea. First, a fluorescently-conjugated scrambled siRNA probe was applied via intra-tympanic injection into the middle ear cavity and was detected in sensory hair cells and nerve fibers as early as 6 h after the injection. The fluorescent probe was also detected in other cells of the organ of Corti, the lateral wall, and in spiral ganglion cells 48 h after the injection. Furthermore, intra-tympanic delivery of Nox3 siRNA successfully reduced immunofluorescence associated with Nox3 in outer hair cells 72 h after injection by 20%. Drug or siRNA delivery via intra-tympanic injection does not compromise the tympanic membrane or interfere with noise-induced hearing loss, while trans-tympanic injections significantly altered the cochlear response to noise exposure. In summary, intra-tympanic injection through the otic bone into the middle ear cavity provides a promising approach for delivery of compounds or siRNA to cochlear hair cells of adult mice, relevant for the study of mechanisms underlying inner ear insults and, specifically, noise-induced hearing loss.

Distribution of gentamicin in inner ear after local administration via a chitosan glycerophosphate hydrogel delivery system

OBJECTIVES

We examined the distribution of gentamicin sulfate in the inner ear after delivery via a chitosan glycerophosphate (CGP) hydrogel system and examined the change in morphology of the hair cells so as to determine how gentamicin affected the function of the inner ear.

METHODS

A matrix of CGP hydrogel loaded with gentamicin conjugated with Texas Red (GTTR) was injected into the round window niche of the left ear of C57/BL6 mice. The mice were painlessly killed on day 1 or day 7 after injection. Confocal fluorescence microscopy was used to locate the gentamicin in the cochlear and vestibular systems.

RESULTS

In the vestibule, the intensity of GTTR staining in the hair cells of the macula of the saccule on day 1 was significantly stronger than that on day 7, and the number of hair cell bundles on top of the cuticular plate on day 7 was obviously decreased in comparison to that on day 1. In the cochlea, the intensity of GTTR staining in the basal turn was significantly stronger than that in the medial turn on both day 1 and day 7. Negligible fluorescence was observed in the apical turn on both day 1 and day 7. Less-intense GTTR staining was detected on day 7 than on day 1 in both the basal turn and the medial turn. There was some outer hair cell loss in the basal turn on day 7, and no hair cell loss in the medial and apical turns at any time point.

CONCLUSIONS

Gentamicin is distributed in the inner ear via the CGP hydrogel delivery system in a time-dependent and basal-to-apical manner. This finding implies that the vestibule and the basal turn may retain more gentamicin for a longer period than do other sites in the inner ear. These two characteristics may account for the high-frequency hearing loss and vestibular dysfunction seen with use of this system to deliver gentamicin into the inner ear.

Efficacy of different routes of administration for Coenzyme Q10 formulation in noise-induced hearing loss: systemic versus transtympanic modality

CONCLUSION

The effectiveness of a coenzyme Q10 formulation, Q-ter, given via transtympanic injection is interesting for the future application of this minimally invasive procedure in the treatment of reactive oxygen species (ROS)-induced hearing loss.

OBJECTIVE

We focused on antioxidant therapy in noise-induced hearing loss (NIHL). Our study was designed to evaluate the effectiveness of Q-ter for different schedules of drug administration to establish the best modality for treatment.

METHODS

Rats were exposed to acoustic trauma (10 kHz at 120 dB for 60 min) and received Q-ter according to two modalities: systemic (Q-ter 100 mg/kg for 4 days 1 h before and 3 days post noise exposure) and transtympanic (Q-ter 20 and 40% concentration 1 h before noise exposure). Auditory brainstem response (ABR), immunohistochemical and morphological studies were performed.

RESULTS

Q-ter administration significantly decreased NIHL at day 21 from noise exposure. The improvement of auditory function by Q-ter was paralleled by a significant reduction in oxidative stress. The transtympanic and systemic routes of drug administration showed a similar degree of protection.

Effects of NasoPore Packing in the Middle Ear Cavity of the Guinea Pig

Objective. To evaluate the effects of NasoPore after packing of the middle ear in guinea pigs.

Study Design. A randomized, prospective, controlled animal study.

Setting. University laboratory.

Subjects and Methods. Forty-one guinea pigs were divided into 3 groups. In group 1 (n = 12), the middle ears of animals were unilaterally implanted with NasoPore, leaving the contralateral middle ears packed with absorbable gelatin sponge soaked in a solution containing kanamycin and furosemide as an ototoxicity-positive control; group 2 (n = 17) underwent the same experimental protocol as group 1, except the gelatin sponge was unsoaked; in group 3 (n = 12), NasoPore was inserted unilaterally and no packing material was placed into the contralateral ear. Auditory brainstem responses (ABRs) were performed preoperatively and 3 months after the procedure. The surface preparation and scanning electron microscopy (SEM) were assessed 3 months postoperatively, whereas pathology of middle ear was analyzed in 5 samples of group 2.

Results. ABR thresholds of the contralateral ear significantly increased in group 1 and were slightly shifted in group 2 compared with the NasoPore-packed and nonpacked ears 3 months postoperatively. The NasoPore-packed middle ears were found to have less fibrosis and inflammation and less thickened bone and tympanic membranes than Gelfoam-packed ears. Surface preparations and SEM showed no ototoxicity in the inner ear of NasoPore-packed ears.

Conclusion. NasoPore appears to be effective for use in otosurgery. It caused less fibrosis in the middle ear than conventional packing agents and no ototoxicity to the inner ear.

Histological effects of intratympanic gentamicin on the vestibular organ of guinea pigs

Background:

Transtympanic administration of gentamicin may be suitable to achieve unilateral vestibular ablation, in order to control unilateral Ménière's disease. In low doses, gentamicin appears to affect selectively the vestibular system, with relative sparing of the cochlea. An experimental study on guinea pigs was conducted to determine what single dose of gentamicin would produce a unilateral vestibular organ lesion when applied to the middle ear.

Study design:

Experimental and prospective.

Methods:

Four groups of guinea pigs received different gentamicin doses (1, 5, 10 and 25 mg) administered to the middle ear. The animals' vestibular organs were then assessed by scanning electron microscopy, in order to quantify the level of vestibular damage.

Results:

Study of the utricular macula and the ampullar crista of the lateral semicircular canal revealed vestibular neuroepithelial lesions in all infused ears.

Conclusions:

The severity of the vestibular neuroepithelial lesions was dose-dependent. Lower gentamicin doses were observed to damage vestibular structures more than cochlear structures.

Chronic reduction of endocochlear potential reduces auditory nerve activity: further confirmation of an animal model of metabolic presbyacusis

Gerbils aged in quiet show a decline of the endocochlear potential (EP) and elevated auditory nerve compound action potential (CAP) thresholds. However, establishing a direct relationship between an age-related reduction in the EP and changes in the activities of primary auditory neurons is difficult owing to the complexity of age-related histological changes in the cochlea. To address this issue, we developed a young gerbil model of "metabolic" presbyacusis that uses an osmotic pump to deliver furosemide into the round window niche for 7 days, resulting in a chronically reduced EP. In this model, the only major histopathologic changes were restricted to the hook region of the cochlea and consisted of loss of strial intermediate cells and massive edema in the lateral wall. The morphological and physiological evidence suggests that the cochlea can adapt to furosemide application over time. The morphology of spiral ganglion cells and hair cells appeared normal throughout the cochlea. CAP responses and EP values in this model are similar to those of quiet-aged ears. The spontaneous activity of single auditory fibers (n = 188) was assessed in 15 young gerbils treated with furosemide for 7 days. The percentage of recorded low-spontaneous rate (SR) fibers at characteristic frequencies (CFs) > or = 6 kHz was significantly lower in furosemide-treated than in control ears. Recovery function tests of CAP responses after prior stimulation also showed a decline in activity of the low-SR population with CFs > or = 6 kHz in the treated cochleas. A similar loss in the activity of low-SR fiber has been previously shown in quiet-aged gerbils. These results suggest that dysfunction of the cochlear lateral wall and subsequent chronic reduction in the EP can directly affect the activity patterns of primary auditory neurons in a manner similar to that seen in aged gerbils.

Intratympanic gentamicin treatment of patients with Ménière's disease with normal hearing

Objective: Understand the safety and outcomes of intratympanic gentamicin treatment in patients with Ménière's disease with normal hearing.

Study Design: Case series with chart review.

Setting: Tertiary referral center.

Subjects and Methods: A total of 224 patients with disabling Ménière's disease treated between May 1996 and June 2007 were grouped according to the American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS) 1995 Committee on Hearing and Equilibrium staging guidelines: stage 1 (<25 dB pure-tone average [PTA]); stage 2 through 4 (>25 dB PTA). Patients underwent self-treatment with intratympanic gentamicin (10 mg/mL) three times daily for one to eight weeks. Outcome measures included pre- and post-treatment speech discrimination score (SDS), PTA, electronystagmography, vertigo relief, and statistical analysis utilizing the Pearson χ2 test.

Results: Twenty-two (88%) of 24 patients with stage 1 Ménière's disease showed unchanged or improved SDS. All 24 patients showed a mean PTA loss of 8 dB. Seventeen (71%) patients reported complete or improved vertigo control. One hundred sixteen (59%) of 200 patients with stage 2 through 4 Ménière's disease showed unchanged or improved SDS. All 200 patients showed a mean PTA loss of 11 dB. One hundred forty-eight (74%) patients reported complete or improved vertigo control.

Conclusions: Patients with stage 1 Ménière's disease appear to have similar vertigo control with better hearing preservation than patients with advanced disease when treated with low-dose intratympanic gentamicin (10 mg/mL).

Principles of local drug delivery to the inner ear

As more and more substances have been shown in preclinical studies to be capable of preventing damage to the inner ear from exposure to noise, ototoxic drugs, ischemia, infection, inflammation, mechanical trauma and other insults, it is becoming very important to develop feasible and safe methods for the targeted delivery of drugs to specific regions in the inner ear. Recently developed methods for sampling perilymph from the cochlea have overcome major technical problems that have distorted previous pharmacokinetic studies of the ear. These measurements show that drug distribution in perilymph is dominated by passive diffusion, resulting in large gradients along the cochlea when drugs are applied intratympanically. Therefore, in order to direct drugs to specific regions of the ear, a variety of delivery strategies are required. To target drugs to the basal cochlear turn and vestibular system while minimizing exposure of the apical cochlear turns, single one-shot intratympanic applications are effective. To increase the amount of drug reaching the apical cochlear turns, repeated intratympanic injections or controlled-release drug delivery systems, such as biodegradable biopolymers or catheters and pumps, are more effective. However, if the applied substance does not easily pass through the round window membrane, or if a more widespread distribution of drug in the ear is required, then intralabyrinthine injections of the substance may be required. Intralabyrinthine injection procedures, which are currently in development in animals, have not yet been proven safe enough for human use.

State-of-the-art mechanisms of intracochlear drug delivery

PURPOSE OF REVIEW

Treatment of auditory and vestibular dysfunction has become increasingly dependent on inner ear drug delivery. Recent advances in molecular therapy and nanotechnology have pushed development of alternate delivery methodologies involving both transtympanic and direct intracochlear infusions. This review examines recent developments in the field relevant to both clinical and animal research environments.

RECENT FINDINGS

Transtympanic delivery of gentamicin and corticosteroids for the treatment of Meniere's disease and sudden sensorineural hearing loss continues to be clinically relevant, with understanding of pharmacokinetics becoming more closely studied. Stabilizing matrices placed on the round window membrane for sustained passive delivery of compounds offer more controlled dosing profiles than transtympanic injections. Nanoparticles are capable of traversing the round window membrane and cochlear membranous partitions, and may become useful gene delivery platforms. Cochlear and vestibular hair cell regeneration has been demonstrated by vector delivery to the inner ear, offering promise for future advanced therapies.

SUMMARY

Optimal methods of inner ear drug delivery will depend on toxicity, therapeutic dose range, and characteristics of the agent to be delivered. Advanced therapy development will likely require direct intracochlear delivery with detailed understanding of associated pharmacokinetics.

Evaluating the ototoxicity of topical piperacillin-tazobactam

BACKGROUND AND OBJECTIVE

With the increased use of ototopical ciprofloxacin solution, newly evolved bacterial fluoroquinolone resistance has also become more of a problem. The emergence of ciprofloxacin-resistant Pseudomonas aeruginosa (CRPA) has created a new therapeutic challenge in otology. We evaluated the ototoxicity of topical fortified piperacillin-tazobactam solution by performing experiments in young male albino guinea pigs (weight, 250-300 g each).

MATERIALS AND METHODS

Antimicrobial assay by direct contact test using scanning electron microscope was performed. Twenty guinea pigs (250-300 g) were treated with fortified piperacillin-tazobactam solution. In experimental group 1 (n=10), a gelfoam ball impregnated with 120 microl of fortified piperacillin-tazobactam was implanted on the round window membrane by a posterior approach. In experimental group 2 (n=10), 20 microl of fortified piperacillin-tazobactam was injected into the middle ear cavity via silicone tube with a small hole placed in the superior aspect of the bulla by drilling. The drug application through the tube was performed for 7 consecutive days. In control group (n=5), a gelfoam ball impregnated with 120 microl of gentamicin solution (80 mg/2 ml) was implanted on the round window membrane.

RESULTS

The fortified piperacillin-tazobactam 20 microl treated CRPAs showed destruction of their cell membrane in antimicrobial assay by direct contact test. No significant difference in the mean auditory brainstem response (ABR) thresholds before and after drug administration was found for the experimental group. However, significant elevation of the mean ABR thresholds was found for the control group. In experimental group, scanning electron microscopy showed almost normal sterociliary arrangements and surface structure on the inner and outer hair cells. However, significant destruction of outer hair cells was identified in control group.

CONCLUSION

This present study's data suggests that fortified piperacillin-tazobactam solution can be effectively used for topically treating CRPA otorrhea in those patients who suffer with chronic suppurative otitis media.

Interpretation and use of caloric testing

Caloric testing is an otoneurologic evaluation of the status of the vestibular-ocular reflex; it allows an evaluation of each labyrinth separately. The main aspects on the use and interpretation of caloric testing are reviewed.

Method

A systematic review of papers published in the past one hundred years on caloric testing was undertaken. The inclusion criteria were: cross-sectional, longitudinal, original articles, reviews and meta-analyses. Reviews of patient charts, case reports and editorials were excluded. The key-words were: caloric testing, nystagmus, vestibular system, directional preponderance, labyrinth predominance, monothermal caloric test, ice water caloric testing, Bell´s phenomenon. The databases were: COCHRAINE, MEDLINE, LILACS, CAPES.

Results

Ninety-three of 818 abstracts fulfilled the inclusion criteria. After reading these articles, 55 were selected for this study. These papers discussed the basics of caloric testing, the types of stimulation, the interpretation of results, variables, artifacts, and the monothermal and ice water caloric test.

Final comments

Caloric testing reference values may vary according to each unit; the the cutoff point is defined based on local studies. Attention to the technique is essential to maximize test sensitivity.

Interpretação e utilidade da prova calórica

A prova calórica é o teste da avaliação otoneurológica que verifica a integridade do reflexo vestíbulo-ocular e possibilita avaliar cada labirinto separadamente. Os principais aspectos relacionados à realização, interpretação e utilidade da prova calórica foram revistos. MÉTODOS: Realizou-se revisão sistemática sobre as publicações ocorridas nos últimos cem anos sobre o assunto. Incluíram-se artigos originais transversais e longitudinais, de revisão e meta-análise. Excluíram-se revisões de papeleta, relatos de caso e editoriais. Os descritores utilizados foram: prova calórica, nistagmo, sistema vestibular, preponderância direcional, predomínio labiríntico, teste calórico monotermal, teste calórico com água gelada, fenômeno de Bell. Pesquisou-se as bases de dados COCHRAINE, MEDLINE, LILACS, CAPES. RESULTADOS: De 818 resumos de artigos, selecionou-se inicialmente 93 que preenchiam os critérios de inclusão. A leitura dos artigos resultou na seleção final de 55. Na análise dos artigos, enfatizou-se na discussão fundamentos da prova calórica, tipos de estimulação utilizados, prova calórica monotermal e com água gelada, questões relacionadas à interpretação dos resultados, variáveis e artefatos. COMENTÁRIOS FINAIS: os valores de referência utilizados na prova calórica podem variar de serviço para serviço, com ponto de corte definido a partir de estudos locais. Semiotécnica cuidadosa é fundamental para elevar a sensibilidade do exame.

Influence of topical application of basic fibroblast growth factor upon inner ear

BACKGROUND

Basic fibroblast growth factor (b-FGF) has recently been shown to have a positive effect on the treatment of postoperative mastoid cavity problem, but its ototoxicity has not been investigated.

OBJECTIVE

To investigate the effect of b-FGF on the inner ear of guinea pigs.

STUDY DESIGN AND SETTING

In groups A (n = 10) and B (n =11), b-FGF was applied into the left external auditory canal and the middle ear, respectively. The right ear served as a control. At 1 week after b-FGF administration, the endocochlear DC potential (EP) was measured, and morphology of the cochleae was examined by scanning electron microscopy.

RESULTS

In group A, the EP values of experimental ears and controls were 90.0 +/- 8.4 (mV, mean +/- SD) and 89.4 +/- 4.3 (P > 0.05). In group B, the values were 86.5 +/- 11.4 and 87.5 +/- 6.1, respectively, (P > 0.05). Morphological findings showed no damage on the cochlear.

CONCLUSION

Application of b-FGF to the external and middle ears does not seem to have an apparent risk of ototoxicity.

Concentration gradient along the scala tympani after local application of gentamicin to the round window membrane

OBJECTIVES

The distribution of gentamicin along the fluid spaces of the cochlea after local applications has never previously been demonstrated. Computer simulations have predicted that significant basal-apical concentration gradients might be expected, and histologic studies indicate that hair cell damage is greater at the base than at the apex after local gentamicin application. In the present study, gradients of gentamicin along the cochlea were measured.

METHODS

A recently developed method of sampling perilymph from the cochlear apex of guinea pigs was used in which the samples represent fluid originating from different regions along the scala tympani. Gentamicin concentration was determined in sequential apical samples that were taken after up to 3 hours of local application to the round window niche.

RESULTS

Substantial gradients of gentamicin along the length of the scala tympani were demonstrated and quantified, averaging more than 4,000 times greater concentration at the base compared with the apex at the time of sampling. Peak concentrations and gradients for gentamicin varied considerably between animals, likely resulting from variations in round window membrane permeability and rates of perilymph flow.

CONCLUSIONS

The large gradients for gentamicin demonstrated here in guinea pigs account for how it is possible to suppress vestibular function in some patients with a local application of gentamicin without damaging auditory function. Variations in round window membrane permeability and in perilymph flow could account for why hearing losses are observed in some patients.

Ultrastructural analysis of aminoglycoside-induced hair cell death in the zebrafish lateral line reveals an early mitochondrial response

Loss of the mechanosensory hair cells in the auditory and vestibular organs leads to hearing and balance deficits. To investigate initial, in vivo events in aminoglycoside-induced hair cell damage, we examined hair cells from the lateral line of the zebrafish, Danio rerio. The mechanosensory lateral line is located externally on the animal and therefore allows direct manipulation and observation of hair cells. Labeling with vital dyes revealed a rapid response of hair cells to the aminoglycoside neomycin. Similarly, ultrastructural analysis revealed structural alteration among hair cells within 15 minutes of neomycin exposure. Animals exposed to a low, 25-microM concentration of neomycin exhibited hair cells with swollen mitochondria, but little other damage. Animals treated with higher concentrations of neomycin (50-200 microM) had more severe and heterogeneous cellular changes, as well as fewer hair cells. Both necrotic-like and apoptotic-like cellular damage were observed. Quantitation of the types of alterations observed indicated that mitochondrial defects appear earlier and more predominantly than other structural alterations. In vivo monitoring demonstrated that mitochondrial potential decreased following neomycin treatment. These results indicate that perturbation of the mitochondrion is an early, central event in aminoglycoside-induced damage.

Effect of Intratympanic Application of Aminoglycosides on Click-Evoked Myogenic Potentials in Guinea Pigs

OBJECTIVES

Although numerous studies have identified damage to the cochlear and vestibular end organs as the primary site of aminoglycoside-induced ototoxicity, the effect on the saccule remains poorly understood, possibly due to lack of monitoring saccular function in experimental animals. Therefore, this study applied three kinds of aminoglycosides into the tympanic space of guinea pigs to examine their toxic impact on the saccule by way of click-evoked myogenic potential test coupled with morphologic assessment.

DESIGN

Albino guinea pigs were treated with saline, gentamicin, tobramycin, or amikacin, with 10 animals assigned to each group. Each compound was injected directly overlying but not through the round window membrane on the left ear, with the right ear serving as a control. One week after injection, each animal underwent auditory brain stem response, caloric test, and click-evoked myogenic potential test. Animals were then killed for morphologic assessment through the use of light and electron microscopic examinations.

RESULTS

The animals treated with saline, gentamicin, tobramycin, or amikacin exhibited abnormal auditory brain stem response in 0%, 30%, 100%, and 30% of cases; abnormal caloric responses were found in 0%, 100%, 40%, and 40% of cases; absent click-evoked myogenic potentials were found in 0%, 100%, 30%, and 40% of cases, respectively. Gentamicin and other groups differed significantly in abnormal rates of caloric responses and click-evoked myogenic potentials. Morphologic study of the gentamicin-treated animals confirmed that the absence of click-evoked myogenic potential originated from the lesion in the saccular macula.

CONCLUSIONS

Gentamicin represents the dominant susceptibility of aminoglycoside-induced vestibulotoxicity for eliminating both semicircular canal and saccular functions. This study further confirms the findings of human studies in which the caloric and vestibular evoked myogenic potentials responses were monitored to assess the abolition of vestibular function in patients treated with intratympanic gentamicin injection.

Pattern of cochlear damage caused by short-term kanamycin application using the round window microcatheter method

CONCLUSION

Short-term local application of kanamycin by the microcatheter method can control the region and severity of drug effect on the cochlea by changing the concentration and administration time.

OBJECTIVES

To investigate the effect of short-term round window administration of kanamycin by the microcatheter method on cochlear stereociliary bundle loss in a guinea pig model.

MATERIALS AND METHODS

Two concentrations and two time periods were used for drug administration to the inner ear. In groups of six animals, a total dose of 172.5 mg/ml or 345 mg/ml of kanamycin was instilled at the rate of 0.1 ml/h into the round window niche over either a 1 h or 2 h period by the microcatheter method. One group of six animals received a saline infusion as a control. The animals were sacrificed after 4 days and stereociliary bundle loss was observed by SEM. Functional changes were evaluated by auditory brainstem response.

RESULTS

With regard to the severity of the damage, higher drug concentrations were associated with more severe effects. The severity of damage was the same when the same total dose was used; however, it was found that when the dose was administered over a longer period, the damage region was wider. The functional changes also corresponded with the morphological changes.

Demonstration of a longitudinal concentration gradient along scala tympani by sequential sampling of perilymph from the cochlear apex

Local applications of drugs to the inner ear are increasingly being used to treat patients' inner ear disorders. Knowledge of the pharmacokinetics of drugs in the inner ear fluids is essential for a scientific basis for such treatments. When auditory function is of primary interest, the drug's kinetics in scala tympani (ST) must be established. Measurement of drug levels in ST is technically difficult because of the known contamination of perilymph samples taken from the basal cochlear turn with cerebrospinal fluid (CSF). Recently, we reported a technique in which perilymph was sampled from the cochlear apex to minimize the influence of CSF contamination (J. Neurosci. Methods, doi: 10.1016/j.jneumeth.2005.10.008 ). This technique has now been extended by taking smaller fluid samples sequentially from the cochlear apex, which can be used to quantify drug gradients along ST. The sampling and analysis methods were evaluated using an ionic marker, trimethylphenylammonium (TMPA), that was applied to the round window membrane. After loading perilymph with TMPA, 10 1-muL samples were taken from the cochlear apex. The TMPA content of the samples was consistent with the first sample containing perilymph from apical regions and the fourth or fifth sample containing perilymph from the basal turn. TMPA concentration decreased in subsequent samples, as they increasingly contained CSF that had passed through ST. Sample concentration curves were interpreted quantitatively by simulation of the experiment with a finite element model and by an automated curve-fitting method by which the apical-basal gradient was estimated. The study demonstrates that sequential apical sampling provides drug gradient data for ST perilymph while avoiding the major distortions of sample composition associated with basal turn sampling. The method can be used for any substance for which a sensitive assay is available and is therefore of high relevance for the development of preclinical and clinical strategies for local drug delivery to the inner ear.