Round window gentamicin absorption: an in vivo human model

Triamcinolone acetonide can be detected in cerebrospinal fluid after intratympanic injection

Intratympanically applied treatments are of increasing interest to the otologic community to treat sudden sensorineural hearing loss or vestibular disorders but also to deliver gene therapy agents, or biologics to the inner ear. Further diversion from the middle ear and perilymph to blood circulation and cerebrospinal fluid via the cochlear aqueduct are one of the limiting factors and so far not understood well enough. In this study, intratympanically applied triamcinolone acetonide was determined in cerebrospinal fluid. Additionally, perilymph was sampled through the round window membrane as well as at the lateral semicircular canal to determine drug levels. Of the twenty-one included patients, triamcinolone acetonide was quantifiable in cerebrospinal fluid in 43% at very low levels (range 0 ng/ml - 6.2 ng/ml) which did not correlate with perilymph levels. Drug levels at the two different perilymph sampling sites were within a range of 13.5 ng/ml to 1180.0 ng/ml. Results suggest an equal distribution of triamcinolone acetonide to semicircular canals, which might support the use of triamcinolone acetonide as a treatment option for vestibular pathologies such as Menièrés disease. On the other hand, the distribution to cerebrospinal fluid might be limiting current approaches in gene therapy where a central distribution is unwanted.

Endoscopic tympanoplasty type I for tympanic perforations: analysis of prognostic factors

PURPOSE

To assess the anatomical and functional outcomes of endoscopic transcanal tympanoplasty type I for tympanic membrane perforations.

METHODS

Eight hundred thirty-five patients who underwent tympanoplasty between January 2011 and January 2019 were selected. Patients with tympanic membrane perforation treated with a transcanal endoscopic tympanoplasty type 1 and a follow-up period longer than 6 months have been retrospectively reviewed. The presence of cholesteatoma or ossicular chain dysfunctions were considered exclusion criteria. Eighty-one patients were included in the present study population. The main outcome was the rate of overall graft success. Secondary outcomes included hearing results. Prognostic factors related to both the abovementioned outcomes were assessed.

RESULTS

Overall, 66 patients (81.5%) had a successful graft at the last follow-up evaluation. Mean follow-up was 22.1 (range 6-104) months. The anterior quadrants were entailed by the perforation in 62 (76.5%) cases. The overall success rate with cartilage (or cartilage and perichondrium) was 91.2% (p < 0.01). The median preoperative and postoperative ABG were 18.7 (13.4-25.6) and 7.5 (2.5-12.5), respectively, revealing a significant median improvement of 11.2 (p < 0.001). The type of graft and the postoperative tympanic membrane status were significantly associated with the audiologic outcome with p = 0.01 and p = 0.02, respectively.

CONCLUSIONS

Endoscopic tympanoplasty type I is a reliable technique with reasonable anatomic and audiologic results. Tympanic membrane grafting with cartilage (or cartilage and perichondrium) guarantees a higher rate of perforation closure and satisfactory hearing results. Anterior eardrum perforations can be successfully and safely managed with transcanal endoscopic approach avoiding postauricular approach and canalplasty.

Consistent removal of hair cells in vestibular end organs by time-dependent transtympanic administration of gentamicin in guinea pigs

BACKGROUND

Vestibular hair cell loss and its role in balance disorders are not yet completely understood due largely to the lack of precise hair cell damage protocols.

NEW METHOD

Our damage protocol aims to selectively remove type I hair cells in a way that produces consistent and predictable lesions that can be used for reliable inter-animal and inter-group comparison in balance research. This objective is achieved by transtympanic injection of gentamicin on both the round window membrane and oval window over a fixed time period followed by thorough washing.

RESULTS

We achieved nearly total and consistent loss of type I hair cells at 94 % for the crista ampullaris of the lateral semicircular canal (LSC) and 86 % for the utricular macula with negligible loss of type II hair cells at 4% for the crista ampullaris of the LSC and 6% for the utricular macula. While the vestibular function was compromised in the relevant study group, this group had a zero mortality rate with no significant suppression of body weight gain.

COMPARISON WITH EXISTING METHODS

Gentamicin is typically administered via intraperitoneal systemic injection or, more recently, transtympanic injection. The intraperitoneal method is simple, but mortality rate is high. The transtympanic injection method produces ototoxic damage but with inconsistent lesion size. This inconsistency prevents reliable comparisons among animals.

CONCLUSIONS

This protocol employs a transtympanic injection method which selectively targets type I hair cells for removal in the vestibular epithelia in a time-dependent manner, uniformly damages vestibular function, and causes uniform hair cell loss.

Orientation of the round window membrane: A normative study of inner ear anatomical orientation using 2D projections of 3D volumes

INTRODUCTION

Orientation of the Round Window Membrane (RWM) is an important metric to establish if utilized as a potential access for targeted delivery of magnetically guided nanomedicines to the inner ear. Orientation with respect to an internal reference frame (such as the planes defined by the semicircular-canals [SCC]) may provide an internally consistent basis if the basis is orthogonal and consistent (from patient to patient).

MATERIALS AND METHODS

Utilizing a micro computed tomography (CT), 20 temporal bones are scanned for anatomical information. The scanned data sets are loaded into an imaging program to provide volumetric reconstruction and segmentation. Volumetric models of the anatomical relationships between the inner ear SCC and the RWM are utilized to get normative projection angle information and are statistically analyzed.

RESULTS

Micro-CT shows low to moderate reliability for reproducibility, intraobserver, and interobserver measurements; in addition, it provides mean values (±SD) for the various measured angles. The combined mean angular values for surface orientation of the RWM, with respect to the SCC basis (quasi-orthogonal spherical coordinate system), was 57.0° ± 20.9°as measured from the line defining the posterior SCC plane in the direction of the line defining the superior SCC plane. An angle of 65.2° ± 19.1° was measured for an angle away from the line defining the horizontal SCC plane.

Permeation Enhancers for Intratympanically-applied Drugs Studied Using Fluorescent Dexamethasone as a Marker

HYPOTHESIS

Entry of locally applied drugs into the inner ear can be enhanced by chemical manipulations.

BACKGROUND

Perilymph drug concentrations achieved by intratympanic applications are well below the applied concentration due to limited entry through the round window (RW) membrane and stapes. Chemical manipulations to increase entry permeability could increase the effectiveness of drug therapy with local applications.

METHODS

Dexamethasone-fluorescein (F-dex) was used as an entry marker. F-dex was applied to the RW niche of guinea pigs as a 20 μL bolus of 1 mM solution. After a 1 hour application, 10 samples of perilymph were collected sequentially from the lateral semicircular canal, allowing F-dex distribution throughout the perilymph to be quantified. Entry was also measured with the applied solution additionally containing dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), saponin, caprate, benzyl alcohol (BA) or poloxamer 407 (P407). Combinations of saponin or BA with P407 were also compared.

RESULTS

In control experiments, F-dex entered the inner ear slowly at both the RW and stapes. The total F-dex recovered in all 10 samples from each animal averaged 2.1 pMoles for controls, 1.71 pMoles for 17% P407, 3.70 pMoles for caprate, 8.04 pMoles for DMSO, 16.32 pMoles for NMP, 31.0 pMoles for saponin, and 67.3 pMoles for 4% BA. Entry with DMSO, NMP, saponin and 4% BA were all significantly higher than the controls (one-way ANOVA).

CONCLUSION

These studies confirm that entry of drugs into the ear can be markedly enhanced with the use of chemical permeation-enhancing agents.

Evaluation of Ototoxicity of an Antifog Agent and the Suspected Underlying Mechanisms: An Animal Study

Use of rigid endoscopes has become widespread in middle ear surgeries, thereby attracting attention to the safety of antifog agents. However, few studies on the ototoxicity of antifog agents have been conducted. The purpose of this study was to evaluate hair cell damage and the underlying mechanisms caused by antifog agents using zebrafish larvae. We exposed zebrafish larvae at 3 days postfertilization to various concentrations of the antifog agent, Ultrastop (0.01, 0.02, 0.04, and 0.08%) for 72 hours. The average number of hair cells within 4 neuromasts of larvae, including supraorbital (SO1 and SO2), otic (O1), and occipital (OC1), in the control group were compared to those in the exposure groups. Significant hair cell loss was observed in the experimental groups compared to that in the control group (P < .01; control: 53.88 ± 4.85, 0.01%: 45.08 ± 11.70, 0.02%: 41.36 ± 12.00, 0.04%: 35.36 ± 16.18, and 0.08%: 15.60 ± 7.53 cells). Concentration-dependent increase in hair cell apoptosis by terminal deoxynucleotidyltransferase (TDT)-mediated dUTP-biotin nick end labeling assay (control: 0.00 ± 0.00, 0.01%: 3.48 ± 2.18, 0.02%: 9.64 ± 5.75, 0.04%: 17.72 ± 6.26, and 0.08%: 14.60 ± 8.18 cells) and decrease in the viability of hair cell mitochondria by 2-(4-[dimethylamino] styryl)-N-ethylpyridinium iodide assay (control: 9.61 ± 1.47, 0.01%: 8.28 ± 2.22, 0.02%: 8.45 ± 2.72, 0.04%: 7.25 ± 2.44, and 0.08%: 6.77 ± 3.26 percentage of total area) were observed. Antifog agent exposure can cause hair cell damage in zebrafish larvae, possibly by induction of mitochondrial damage with subsequent apoptosis of hair cells.

Effects of an enrofloxacin-silver sulfadiazine emulsion in the ears of rabbits with perforated tympanic membranes

OBJECTIVE

To determine whether an enrofloxacin-silver sulfadiazine emulsion (ESS) labeled for treatment of otitis externa in dogs has ototoxic effects in rabbits following myringotomy.

ANIMALS

6 healthy adult New Zealand White rabbits.

PROCEDURES

Rabbits were anesthetized for brainstem auditory-evoked response (BAER) tests on day 0. Myringotomy was performed, and BAER testing was repeated. Saline (0.9% NaCl) solution and ESS were then instilled in the left and right middle ears, respectively, and BAER testing was repeated prior to recovery of rabbits from anesthesia. Application of assigned treatments was continued every 12 hours for 7 days, and rabbits were anesthetized for BAER testing on day 8. Rabbits were euthanized, and samples were collected for histologic (6 ears/treatment) and scanning electron microscopic (1 ear/treatment) examination.

RESULTS

Most hearing thresholds (11/12 ears) were subjectively increased after myringotomy, with BAER measurements ranging from 30 to 85 dB in both ears. All day 8 hearing thresholds exceeded baseline (premyringotomy) values; results ranged from 30 to 85 dB and 80 to > 95 dB (the upper test limit) in saline solution-treated and ESS-treated ears, respectively. All ESS-treated ears had heterophilic otitis externa, epithelial hyperplasia of the external ear canal, various degrees of mucoperiosteal edema, and periosteal new bone formation on histologic examination. Scanning electron microscopy revealed that most outer hair cells in the ESS-treated ear lacked stereocilia or were absent.

CONCLUSIONS AND CLINICAL RELEVANCE

Results supported that ESS has ototoxic effects in the middle ear of rabbits. Further research is needed to confirm these findings. Myringotomized laboratory rabbits may be useful to study ototoxicity of drugs used in human medicine.

Long-term Vertigo Control and Vestibular Function After Low-dose On-demand Transtympanic Gentamicin for Refractory Menière's Disease

OBJECTIVE

To describe the long-term clinical vertigo control along with measured lateral canal vestibular function in patients with unilateral refractory Menière's disease (MD) treated with gentamicin transtympanic injections (TTI).

STUDY DESIGN

Retrospective analytic study.

SETTING

Tertiary referral center.

PATIENTS

Thirty-eight patients treated by TTI for medically refractory unilateral MD, defined by the 1995 AAO-HNS criteria, between May 2006 and December 2012.

INTERVENTION(S)

One-year course of treatment with gentamicin TTI following a low dose on-demand protocol. TTI were repeated in new courses of treatment when MD recurrence occurred.

MAIN OUTCOME MEASURE(S)

AAO-HNS class of control, caloric tests (CalT), recurrence rate.

RESULTS

After an average clinical follow-up of 71 months, all patients entered a class of control A (78%) or B (22%), with an average of 2.3 TTI received. The mean maximal obtained deficit was 88.5%, and the mean long-term deficit was 85.5%. Ten (26%) patients had disease recurrence requiring a new course of treatment. A value of the first CalT in the 3 months following the first TTI strictly higher than 78% was significantly associated with disease control and the absence of symptom recurrence (p≤0.01). In the "recurrence" group, four patients had a significantly lower mean value of all CalT performed after the first TTI when compared with other patients (p≤0.001), indicating gentamicin resistance CONCLUSION:: Achieving a sustainable vestibular deficit on caloric testing is key for MD symptom control after gentamicin TTI. Gentamicin resistance must be diagnosed early to adapt therapeutic strategies.

Otic drug delivery systems: formulation principles and recent developments

Disorders of the ear severely impact the quality of life of millions of people, but the treatment of these disorders is an ongoing, but often overlooked challenge particularly in terms of formulation design and product development. The prevalence of ear disorders has spurred significant efforts to develop new therapeutic agents, but perhaps less innovation has been applied to new drug delivery systems to improve the efficacy of ear disease treatments. This review provides a brief overview of physiology, major diseases, and current therapies used via the otic route of administration. The primary focuses are on the various administration routes and their formulation principles. The article also presents recent advances in otic drug deliveries as well as potential limitations. Otic drug delivery technology will likely evolve in the next decade and more efficient or specific treatments for ear disease will arise from the development of less invasive drug delivery methods, safe and highly controlled drug delivery systems, and biotechnology targeting therapies.

Intratympanic (IT) Therapies for Menière's Disease: Some Consensus Among the Confusion

Purpose of Review

Aminoglycosides and corticosteroids are commonly used to treat Menière's disease. Intratympanic (IT) administration of these medications allows high inner ear concentrations without significant adverse systemic effects. As a direct result, IT therapy has grown in popularity. Recent studies have compared patient outcomes between IT aminoglycosides and corticosteroids. This review summarizes these findings.

Recent Findings

Trials comparing IT corticosteroids to IT placebo or oral therapy have had conflicting results. Most recently, Lambert et al. investigated the effect of IT dexamethasone in a sustained-release formulation compared to placebo. Their findings demonstrated improvement in some secondary measures of vertigo with the sustained-release formulation.IT gentamicin is known to be effective in controlling vertigo in Menière's disease. In a recent study from 2016, Patel et al compared IT gentamicin and IT methylprednisolone in a double-blind, randomized controlled trial and identified no significant differences between the two in vertigo control.

Summary

IT injections of aminoglycosides and corticosteroids can improve vertigo control. Hearing and vestibular loss however may result with IT aminoglycosides. Corticosteroids demonstrate limited hearing loss but may not have the same efficacy in controlling vertigo. Further investigation in the etiology of Menière's disease is needed to tailor the proposed treatment to suit the disease mechanism.

Gentamicin Applied to the Oval Window Suppresses Vestibular Function in Guinea Pigs

Intratympanic gentamicin therapy is widely used clinically to treat the debilitating symptoms of Ménière's disease. Cochleotoxicity is an undesirable potential side effect of the treatment and the risk of hearing loss increases proportionately with gentamicin concentration in the cochlea. It has recently been shown that gentamicin is readily absorbed through the oval window in guinea pigs. The present study uses quantitative functional measures of vestibular and cochlea function to investigate the efficacy of treating the vestibule by applying a small volume of gentamicin onto the stapes footplate in guinea pigs. Vestibular and cochlea function were assessed by recording short latency vestibular evoked potentials in response to linear head acceleration and changes in hearing threshold, respectively, 1 and 2 weeks following treatment. Histopathology was analyzed in the crista ampullaris of the posterior semi-circular canal and utricular macula in the vestibule, and in the basal and second turns of the cochlea. In animals receiving gentamicin on the stapes footplate, vestibular responses were significantly suppressed by 72.7 % 2 weeks after treatment with no significant loss of hearing. This suggests that the vestibule can be treated directly by applying gentamicin onto the stapes footplate.

Inner ear lesion and the differential roles of hypoxia and hypercarbia in triggering active movements: Potential implication for the Sudden Infant Death Syndrome

Infants that succumb to Sudden Infant Death Syndrome (SIDS) have been identified with inner ear dysfunction (IED) at birth and on autopsy. We previously investigated whether IED could play a mechanistic role in SIDS. We discovered that animals with IED displayed significant suppression of movement arousal to a hypoxic-hypercarbic gas mixture under light anesthesia. In the current study we investigated the role of each gas in triggering movements and the response to hypercarbia during natural sleep without anesthesia. Seventeen-day-old CD-1 mice received intra-tympanic gentamicin (IT-Gent) injections to precipitate IED. The movement response to hypercarbia, hypoxia and hypoxia-hypercarbia was compared to controls under light anesthesia. Hypercarbia did not stimulate vigorous movements in any animals under either sleep condition. Hypoxia triggered vigorous movements in controls (p<0.05) and a decreased response in IT-Gent animals under light anesthesia. This contrasted with combined hypoxia-hypercarbia, in which IT-Gent animals displaced significantly suppressed movements compared to controls (p<0.05). Our findings portray that a degree of intact inner ear function is necessary for instigating the movement response. Additionally, hypoxia is the trigger for the movement response while carbon dioxide (CO2) suppresses it. The finding that carbon dioxide did not stimulate movement during natural sleep is an important finding. This contrasts with other studies that have identified hypercarbia as an arousal stimulus with EEG. Further studies are warranted to evaluate the precise role of the inner ear in the movement response and potential association with SIDS. The early detection of IED in SIDS predisposed cases could be invaluable.

Cochlear changes in serous labyrinthitis associated with silent otitis media: A human temporal bone study

PURPOSE

To determine histopathological findings in the cochlea of human temporal bones with serous labyrinthitis.

MATERIALS AND METHODS

We compared human temporal bones with serous labyrinthitis (20 cases) associated with silent otitis media and without serous labyrinthitis (20 cases) to study location of serous labyrinthitis, the degree of endolymphatic hydrops, number of spiral ganglion cells and hair cells, loss of fibrocytes in the spiral ligament, and areas of the spiral ligament and stria vascularis.

RESULTS

The serous labyrinthitis caused significant loss of outer hair cells in the lower basal (P=0.006), upper basal (P=0.005), and lower middle (P=0.011) cochlear turns, and significant increase in the degree of endolymphatic hydrops than the control group (P=0.036). No significant difference was found in the loss of inner hair cells, in the number of spiral ganglion cells and fibrocytes in the spiral ligament, and in areas of the stria vascularis and spiral ligament (P>0.05).

CONCLUSIONS

Serous labyrinthitis resulted in significant loss of outer hair cells and significant increase in the degree of endolymphatic hydrops.

Why are ototopical aminoglycosides still first-line therapy for chronic suppurative otitis media? A systematic review and discussion of aminoglycosides versus quinolones

Objective:

This systematic review aimed to establish that quinolones are as effective as aminoglycosides when used to treat chronic suppurative otitis media.

Method:

The review included good quality, randomised, controlled trials on human subjects, published in English, that compared topical aminoglycosides with topical quinolones for the treatment of chronic suppurative otitis media.

Results:

Nine trials met the criteria. Two studies showed a higher clinical cure rate in the quinolone group (93 per centvs71 per cent,p= 0.04, and 76 per centvs52 per cent,p= 0.009). Four studies showed no statistically significant difference in clinical outcome. A significant difference in microbiological clearance in favour of quinolones was shown in two studies (88 per centvs30 per cent,p< 0.001, and 88 per centvs30 per cent,p< 0.001).

Conclusion:

Topical quinolones do not carry the same risk of ototoxicity as aminoglycosides. Furthermore, they are equal or more effective in treating chronic suppurative otitis media and when used as prophylaxis post-myringotomy. Topical quinolones should be considered a first-line treatment for these patients.

Inner ear insult ablates the arousal response to hypoxia and hypercarbia

INTRODUCTION

Sudden Infant Death Syndrome (SIDS) remains the leading cause of infant mortality in Western societies. A prior study identified an association between hearing suppression on the newborn hearing test and subsequent death from SIDS. This is the first finding of an abnormality in SIDS cases prior to death. A following study identified that inner ear dysfunction precipitates a marked suppression of the hypercapnic ventilatory response (HCVR). Failure of arousal has been proposed to be a key component in SIDS. The objective of the present study was to assess whether inner ear dysfunction not only weakens the hypercapnic response, but also plays a role in suppressing the arousal response to suffocating gas mixtures.

METHODS

Wild-type mice (n=28) received intra-tympanic gentamicin (IT-Gent) injections bilaterally or unilaterally to precipitate inner ear hair cell dysfunction. Three control groups (n=22) received intra-tympanic saline (IT-Saline) bilaterally or unilaterally (right or left), or intra-peritoneal gentamicin (IP-Gent). The body movement arousal responses to severe hypoxia-hypercarbia combined (5% CO2 in nitrogen) were tested under light anesthesia 8 days following the administration of gentamicin or saline.

RESULTS

After injections, the bilateral and unilateral IT-Gent-treated animals behaved similarly to controls, however the HCVR as well as the arousal movements in response to severe hypoxia-hypercarbia were suppressed in IT-Gent-treated animals compared to control animals (P<0.05). Thus the HCVR was significantly decreased in the bilateral (n=9) and unilateral IT-Gent-treated mice (n=19) compared to bilateral (n=7) and unilateral IT-Saline (n=9) control groups (p<0.05). Arousal movements were suppressed in the bilateral IT-Gent group (n=9) compared to bilateral IT-Saline controls (n=7, P<0.0001) and in the unilateral IT-Gent group (n=19) compared to unilateral IT-Saline controls (n=10, P<0.0001).

DISCUSSION

The findings support the theory that inner ear dysfunction could be relevant in the pathophysiology of SIDS. The inner ear appears to play a key role in arousal from suffocating gas mixtures that has not been previously identified.

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.

A regulated delivery system for inner ear drug application

OBJECTIVE

We have recently developed a novel inner ear drug delivery system using chitosan glycerophosphate (CGP) hydrogel loaded with drugs commonly used for treatment of inner ear diseases, significantly improving the drugs' sustained delivery. The goal of this study is to evaluate the effectiveness of chitosanase as a "switch off" mechanism for this drug delivery system when side effects and potential ototoxicities appear during treatment. To evaluate this effect, we tested gentamicin (GENT) in the inner ear following CGP delivery with/without regulation.

METHODS

Purified chitosanase was obtained and used for regulating the CGP delivery system. In vitro studies were performed to evaluate the effect of the interaction between chitosanase and CGP-hydrogel loaded with GENT or Texas Red-labeled GENT (GTTR). In vivo studies were performed using our mouse model to investigate the regulatory effect of chitosanase application on the delivery of GENT to the inner ear. To assess the potential drug rerouting regulatory effect of chitosanase the GTTR fluorescence intensity was evaluated at the round window niche (RWN) and the Eustachian tube (ET). To further characterize this regulatory effect, GENT concentration in the perilymph of the inner ear was analyzed by chromatographic tandem mass spectrometry (LC-MS/MS), and the uptake in the inner ear cells was measured using fluorescence microscopy following CGP delivery with/without chitosanase application.

RESULTS

The chitosanase effectively digested the CGP-hydrogel, quickly releasing GENT and GTTR from the system in vitro. When reacted with GENT alone chitosanase did not produce any reducing sugars and did not affect GENT's antimicrobial activity. In vivo GTTR was effectively rerouted from the RWN to the ET, limiting its uptake in inner ear hair cells. Concurrent with these findings, GENT concentration in the inner ear perilymph was significantly decreased after chitosanase application.

CONCLUSION

Our study findings suggest that, for the first time, sustained and controlled inner ear drug delivery can be successfully regulated enhancing its translation potential for clinical application. The use of chitosanase to digest the CGP-hydrogel results in the rerouting of the loaded drug away from the RWN, effectively downregulating its delivery to the inner ear. This important modification to our drug delivery system has the ability to deliver therapy to the inner ear until desired effect is achieved and to stop this process when side effects or treatment-related ototoxicities start to occur, providing a novel and salient approach for safe and effective delivery to the inner ear.

Ototoxic destruction by co-administration of kanamycin and ethacrynic acid in rats

It is well known that ethacrynic acid (EA) can potentiate the ototoxicity of aminoglycoside antibiotics (AmAn) such as kanamycin (KM), if they were applied at the same time. Currently, to create the model of EA-KM-induced cochlear lesion in rats, adult rats received a single injection of EA (75 mg/kg, intravenous injection), or followed immediately by KM (500 mg/kg, intramuscular injection). The hearing function was assessed by auditory brainstem response (ABR) measurement in response to click and/or tone bursts at 4, 8, 12, 16, 20, 24, and 32 kHz. The static microcirculation status in the stria vascularis after a single EA injection was evaluated with eosin staining. The pathological changes in cochlear and vestibular hair cells were also quantified after co-administration of EA and KM. After a single EA injection, blood flow in vessels supplying the stria vascularis rapidly diminished. However, the blood supply to the cochlear lateral wall partially recovered 5 h after EA treatment. Threshold changes in ABR were basically parallel to the microcirculation changes in stria vascularis after single EA treatment. Importantly, disposable co-administration of EA and KM resulted in a permanent hearing loss and severe damage to the cochlear hair cells, but spared the vestibular hair cells. Since the cochlear lateral wall is the important part of the blood-cochlea barrier, EA-induced anoxic damage to the epithelium of stria vascularis may enhance the entry of KM to the cochlea. Thus, experimental animal model of selective cochlear damage with normal vestibular systems can be reliably created through co-administration of EA and KM.

Marker entry into vestibular perilymph via the stapes following applications to the round window niche of guinea pigs

It has been widely believed that drug entry from the middle ear into perilymph occurs primarily via the round window (RW) membrane. Entry into scala vestibuli (SV) was thought to be dominated by local, inter-scala communication between scala tympani (ST) and SV through permeable tissues such as the spiral ligament. In the present study, the distribution of the ionic marker trimethylphenylammonium (TMPA) was compared following intracochlear injections or applications to the RW niche, with or without occlusion of the RW membrane or stapes area. Perilymph TMPA concentrations were monitored either in real time with TMPA-selective microelectrodes sealed into ST and SV, or by the collection of sequential perilymph samples from the lateral semi-circular canal. Local inter-scala communication of TMPA was confirmed by measuring SV and ST concentrations following direct injections into perilymph of ST. Application of TMPA to the RW niche also showed a predominant entry into ST, with distribution to SV presumed to occur secondarily. When the RW membrane was occluded by a silicone plug, RW niche irrigation produced higher concentrations in SV compared to ST, confirming direct TMPA entry into the vestibule in the region of the stapes. The proportion of TMPA entering by the two routes was quantified by perilymph sampling from the lateral semi-circular canal. The TMPA levels of initial samples (originating from the vestibule) were markedly lower when the stapes area was occluded with silicone. These data were interpreted using a simulation program that incorporates all the major fluid and tissue compartments of the cochlea and vestibular systems. From this analysis it was estimated that 65% of total TMPA entered through the RW membrane and 35% entered the vestibule directly in the vicinity of the stapes. Direct entry of drugs into the vestibule is relevant to inner ear fluid pharmacokinetics and to the growing field of intratympanic drug delivery.

Colchicine prolongs patency of myringotomy in an animal model

OBJECTIVE

Chronic otitis media with effusion (COME) is a frequently observed condition in childhood. The most common and effective surgical therapy for COME is myringotomy with insertion of a ventilation tube (VT). Our aims were to investigate the combined effect of myringotomy and the topical application of Colchicine solution to the external ear canal for the prolongation of patency in the treatment of patients with COME and to evaluate the ototoxicity of Colchicine applied directly to the middle ear.

METHODS

A prospective study on 47 ears in 26 fat sand rats was fashioned. In the first phase, solutions of different concentrations of Colchicine were applied to the middle ear cavity in order to determine the drug's ototoxicity, assessing inner ear function with ABR. In the second phase myringotomy was performed and a non ototoxic concentration of Colchicine applied to 12 external ear canals, while saline was applied to nine.

RESULTS

In the first phase, Colchicine concentration of 0.1% and higher applied to the middle ear cavity caused an ABR threshold elevation. In the second phase, the mean closure time after 0.01% Colchicine application was prolonged to >2.14 weeks (P<0.05).

CONCLUSION

Colchicine has a proven potential for prolongation of myringotomy patency when applied as a solution to the external ear as a 0.01% solution. Further investigations are required to validate these results in humans and to study the potential effect of repetitive Colchicine application on the duration of myringotomy patency for the treatment of COME.

Inner ear insult suppresses the respiratory response to carbon dioxide

Compensated respiratory acidosis has been observed in a significant number of patients with active vestibular disease. We therefore hypothesized that the inner ear may play an unrecognized integral role in respiratory control. To test this premise, we investigated whether mice with induced inner ear injury demonstrated any alteration in their respiratory response to inhaled carbon dioxide (CO(2)). Experimental mice and control mice were included in two separate experiments. Intra-tympanic gentamycin injections were administered to induce inner ear damage in experimental animals. Hearing loss and vestibular dysfunction were tested 1-week after injections to confirm presence of inner ear insult, following which the animal's respiratory response to inhalation of 8% CO(2) was examined. Mice with inner ear injury (n=60) displayed a significantly diminished hypercapnic ventilatory response (HCVR). This contrasted with the normal HCVR seen in control mice that had not undergone tympanic injections (n=30), controls that received tympanic injections with saline (n=5), and controls that had gentamicin administered systemically (n=5). In response to inspired CO(2), the mean respiratory frequency of control mice increased by an average of 50% over their baseline values for both parts of the experiment. In contrast, the ear-damaged experimental group mean values increased by only three breaths per minute (bpm) (2%) in the first experiment and by 28 bpm (11%) in the second experiment. Inner ear damage significantly reduces the respiratory response to CO(2) inhalation. In addition to the established role of the inner ear organ in hearing and balance, this alludes to an unidentified function of the inner ear and its interconnecting neuronal pathways in respiratory regulation. This finding may offer valuable new clues for disease states with abnormal respiratory control where inner ear dysfunction may be present.

Middle ear exploration in patients with Ménière's disease who have failed outpatient intratympanic gentamicin therapy

OBJECTIVE

Treatment of medically refractory Ménière's disease (MD) with intratympanic (IT) gentamicin has now become a standard therapy. This procedure is effective in controlling vertigo attacks, but approximately 10% of patients do not have an adequate response. The objective of the current study was to evaluate the option of middle ear exploration with direct application of gentamicin to the round window (MEE-G) in patients with persistent MD after transtympanic injection of gentamicin.

STUDY DESIGN

A retrospective chart review of 191 patients with MD treated with IT gentamicin revealed 16 who failed to have symptomatic relief after transtympanic injection. Options discussed with these patients included labyrinthectomy, vestibular nerve section, and MEE-G.

SETTING

Tertiary referral center.

PATIENTS

Eight patients opted for MEE-G.

INTERVENTION

Patients were taken to the operating room for MEE-G. After removal of the round window obstruction, gentamicin-soaked pledgets were placed for at least 30 minutes.

MAIN OUTCOME MEASURES

Control of MD-related vertigo and need for additional therapy.

RESULTS

At the time of MEE-G, all 8 patients were found to have adhesions, bone dust blocking the round window, or a thickened round window membrane. In 6 of these patients, vertigo symptoms due to MD either resolved with no further therapy (4 patients) or with subsequent IT gentamicin injections in clinic (2 patients). The remaining 2 patients underwent a vestibular nerve section, which resolved MD symptoms in each case.

CONCLUSION

Anatomic barriers to the round window membrane may be a significant cause of IT gentamicin failure, and MEE-G can be considered before ablative therapy in this subset of patients with Ménière's disease.

Aminoglycoside ototoxicity

PURPOSE OF REVIEW

To summarize mechanisms of ototoxicity associated with aminoglycoside antibiotics and discuss possible protective strategies.

RECENT FINDINGS

Studies in the past 15 years have demonstrated that aminoglycoside ototoxicity is mediated by an apoptotic form of cell death which employs caspase-dependent pathways. Reactive oxygen species have been demonstrated in the sensory epithelium after aminoglycoside administration and methods of blocking reactive oxygen species in the cochlea have been attempted, but not found to be uniformly effective in protecting against cell loss or threshold shift. Aspirin has recently been studied in a human chemoprevention trial in China, and while data suggest the possibility of protection, there was a significant increase in gastrointestinal bleeds associated with aspirin use.

SUMMARY

There are currently no recommendations for pretreatment or posttreatment therapies to attenuate ototoxicity associated with aminoglycoside antibiotics. Our understanding of the mechanisms of ototoxicity has improved and apoptotic pathways are clearly responsible for hair cell demise. Further studies are necessary before significant improvement in hearing outcome can be expected after use of ototoxic antibiotics.

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.

Ototoxicity of Ototopical Drops—An Update

Various topical preparations are widely used for treating external otitis, acute otitis media with perforation, post-tympanostomy tube otorrhea, and chronic suppurative otitis media. Solutions and suspensions that enter the middle ear through a perforated tympanic membrane can reach the inner ear by crossing through the round window membrane. The safety of various components of otic solutions used in this way, particularly aminoglycosides, corticosteroids, solvents, and preservatives, has been questioned in light of their ototoxic potential. The authors' objective is to review the literature discussing the safety of commonly used classes of otic preparations in settings in which the constituents can access the middle and inner ear in high concentrations and discuss their clinical experience with ototoxicity and experience with medical-legal issues.

Topical antibiotic ear drops: are they safe?

A discharging ear is a very common condition for the general practitioner and the ENT surgeon. Oral and intravenous antibiotics have potential complications, are costly, and exhibit increasing resistance. This study explores the advantages and disadvantages of all common otic preparations and compares effectiveness, safety, cost and complication rates. In chronic otitis media (chronic discharging ears), topical antibiotics seem to be the treatment of choice by comparison with oral or intravenous antibiotics. Cultures and antibiograms do not correspond directly to clinical efficacy, as laboratory determination of resistance does not take into account the high concentration of antibiotics in local preparations. It is safer to use quinolone drops as a first-line treatment, but it is still possible to use short courses of other drops if quinolones are either unavailable or contraindicated (e.g. allergy), or when bacteria are resistant to them. However, in such a situation, a round window membrane involved in an established inflammatory process and therefore less permeable to the passage of topical preparations is the preferred setting, as ototoxicity is a potential complication, especially in the case of gentamicin, in which case patients should be warned accordingly.

Local inner-ear drug delivery and pharmacokinetics

Several drugs that are applied directly to the inner ear are in widespread clinical use for the treatment of inner-ear disorders. Many new substances and drug delivery systems specific to the inner ear are under development and in some cases are being evaluated in animal experiments and in clinical studies. However, the pharmacokinetics of drugs in the inner ear is not well defined and the field is plagued by technical problems in obtaining pure samples of the inner-ear fluids for analysis. Nevertheless, a basic understanding of the mechanisms of drug dispersal in the inner ear has emerged, which facilitates the design and interpretation of future pharmacokinetic studies.

Intratympanic gentamicin treatment after endolymphatic sac surgery

CONCLUSIONS

Interval treatment with up to three intratympanic gentamicin injections once weekly effectively controlled vertigo while preserving hearing in patients with Ménière's disease and recurrent or resistant vertigo after saccotomy.

OBJECTIVES

Recurrent or resistant incapacitating vertigo may occur after endolymphatic sac surgery (saccotomy) in patients with Ménière's disease. In these patients, revision saccotomy, vestibular nerve section or labyrinthectomy are the established treatment options. We advocate a once-weekly application of intratympanic gentamicin (12 mg) as an effective alternative in this group of patients.

MATERIAL AND METHODS

Five patients (age range 39-65 years) with definite Ménière's disease according to the 1995 American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) criteria and incapacitating vertigo underwent gentamicin treatment after saccotomy. Control of vertigo and hearing preservation were the aims of treatment. The follow-up period ranged from 26 to 59 months. History and pure-tone audiometry were used to assess vertigo control and hearing, respectively. The frequency of vertigo in the 6-month period before gentamicin treatment ranged between 0.5 and four definitive episodes per month. Hearing stage (AAO-HNS criteria) before gentamicin treatment ranged between 2 and 4. Pre- and post-treatment pure-tone hearing thresholds at 0.5, 1, 2 and 3 kHz were compared by means of the Mann-Whitney U-test.

RESULTS

Complete vertigo control (class A; AAO-HNS) and hearing preservation at 0.5, 1, 2 and 3 kHz were achieved.

Otologic Effects of Topical Mitomycin C: Phase I-Evaluation of Ototoxicity

HYPOTHESIS

To determine ototoxicity of topical mitomycin C when placed in the middle ear at varying concentrations.

BACKGROUND

Despite meticulous surgical technique and diligent postoperative care, some patients develop excessive scar and granulation tissue in the middle ear or mastoid cavity. Poor wound healing may result in infection, tympanic membrane perforation, or conductive hearing loss, which may necessitate further surgery. Use of topical mitomycin C in the ear may be beneficial in reducing scar and granulation tissue formation. This phase of the study was developed to determine the safety of topical mitomycin C in the rat model relative to ototoxicity.

METHODS

Twelve Sprague-Dawley rats were evaluated with auditory brainstem response testing before and after treatments. Topical mitomycin C was injected in the middle ear of the right ear of eight animals. Varying concentrations of 0.125 to 0.5 mg/ml were used. Saline was injected in the left ear of each animal to serve as a control. Four separate animals were evaluated with placement of topical mitomycin C on Gelfoam into the middle ear. In two animals, Gelfoam was placed in the middle ear for 1 minute and then removed. In two animals, Gelfoam was placed in the middle ear and left in place. Auditory brainstem response testing was performed at 4 weeks and at 8 weeks.

RESULTS

Using a high concentration of mitomycin C (>0.25 mg/ml) resulted in ototoxicity, with an increase in the auditory brainstem response threshold at 4 weeks and at 8 weeks. At low concentrations (<0.20 mg/ml), no change in auditory brainstem response threshold was noted. Animals treated with Gelfoam soaked in mitomycin C showed no change in auditory brainstem response threshold.

CONCLUSION

The results of this study indicate that topical mitomycin C on Gelfoam applied in the middle ear appears safe when low concentrations are used, even in the rat, which has a higher susceptibility to gentamycin toxicity than humans. Higher concentrations may lead to ototoxicity based on changes in Wave V on auditory brainstem response. This treatment may prove to be an important option for patients suffering from chronic granulation tissue or scar tissue in the external or middle ear.

The biology of intratympanic drug administration and pharmacodynamics of round window drug absorption

The application of therapeutic agents to the round window holds great promise in the treatment of inner ear disease. The primary benefit of this route of administration is the ability to achieve high inner ear concentrations of drugs without systemic side effects. Recent research has elucidated the anatomy and physiology of the round window and provided important information on the inner ear pharmacokinetics and the pharmacodynamics of drugs administered intratympanically. Although amino-glycosides and steroids have been most thoroughly studied, many other classes of pharmaceuticals, including otoprotective agents, other antibiotics,and topical anesthetics, have therapeutic potential in the inner ear and will probably be the subject of future studies. The authors believe that direct delivery approaches other than through the round window, perhaps with slow or sustained release formulations, may hold the key to the future treatment of inner ear disorders.

Round window membrane and labyrinthine pathological changes: an overview

The round window membrane is considered the most likely pathway from the middle to the inner ear. Various substances placed in the middle ear have been seen to pass through the round window membrane. Once toxic substances or inflammatory mediators such as cytokines and nitric oxide enter the inner ear, various inner ear sequelae such as labyrinthitis, endolymphatic hydrops, sensorineural hearing loss or more insidious diseases can occur.

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.

Transtympanic perfusion: indications and limitations

PURPOSE OF REVIEW

To review the most recent literature regarding the application of transtympanic inner ear perfusion in the treatment of inner ear disorders including Meniere disease, sudden sensorineural hearing loss, and autoimmune inner ear disease.

RECENT FINDINGS

The use of gentamicin perfusion in the management of Meniere disease with intractable vertigo has been demonstrated to have a very high rate of success, and is much less invasive than alternative surgical procedures such as vestibular nerve section or labyrinthectomy. The technique for achieving the highest rate of success while still minimizing the risk of cochleotoxicity continues to be investigated. Sustained delivery techniques such as the Silverstein MicroWick appear to achieve the best pharmacokinetic profile within the inner ear fluids. The end point of treatment does not necessarily require complete vestibular ablation to cure the patient, and shorter courses of treatment may help to reduce the risk of hearing loss. Cochlear Meniere disease can be treated with dexamethasone 4 mg/cc perfusion of the inner ear, which may improve the hearing, tinnitus, and pressure in the ear. Sudden sensorineural hearing loss has been managed with transtympanic steroid delivery, and this appears to be beneficial for some patients who have failed to respond to oral steroids, or have medical contraindications to systemic steroids.

SUMMARY

Inner ear perfusion via transtympanic delivery is an emerging technique in the management of inner ear disease. Improved results are expected over time as research in this area answers questions about dosage and delivery techniques, as well as identifying new applications and pharmaceuticals.

Imaging of the guinea pig cochlea following round window gadolinium application

Precise, non-invasive determination of the aetiology and site of pathology of inner ear disorders is difficult. The aim of this study was to describe an alternative method for inner ear visualization, based on local application of the paramagnetic contrast agent gadolinium. Using a 4.7 T MRI scanner, high contrast images of all four cochlear turns were obtained 3.5 h after placing gadolinium on the round window membrane. Gadolinium cleared from the cochlea within 96 h. Auditory brainstem response measurements performed on a separate group of animals showed no significant threshold shifts after the application, indicating that gadolinium is non-toxic to the guinea pig cochlea.