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

Mechanisms and Impact of Aminoglycoside-Induced Vestibular Deficits

PURPOSE

Acquired vestibulotoxicity from hospital-prescribed medications such as aminoglycoside antibiotics affects as many as 40,000 people each year in North America. However, there are no current federally approved drugs to prevent or treat the debilitating and permanent loss of vestibular function caused by bactericidal aminoglycoside antibiotics. This review will cover our current understanding of the impact of, and mechanisms underlying, aminoglycoside-induced vestibulotoxicity and highlight the gaps in our knowledge that remain.

CONCLUSIONS

Aminoglycoside-induced vestibular deficits have long-term impacts on patients across the lifespan. Additionally, the prevalence of aminoglycoside-induced vestibulotoxicity appears to be greater than cochleotoxicity. Thus, monitoring for vestibulotoxicity should be independent of auditory monitoring and encompass patients of all ages from young children to older adults before, during, and after aminoglycoside therapy.

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.

Mechanisms of Aminoglycoside- and Cisplatin-Induced Ototoxicity

Purpose This review article summarizes our current understanding of the mechanisms underlying acquired hearing loss from hospital-prescribed medications that affects as many as 1 million people each year in Western Europe and North America. Yet, there are currently no federally approved drugs to prevent or treat the debilitating and permanent hearing loss caused by the life-saving platinum-based anticancer drugs or the bactericidal aminoglycoside antibiotics. Hearing loss has long-term impacts on quality-of-life measures, especially in young children and older adults. This review article also highlights some of the current knowledge gaps regarding iatrogenic causes of hearing loss. Conclusion Further research is urgently needed to further refine clinical practice and better ameliorate iatrogenic drug-induced hearing loss.

Intratympanic gentamicin injection for refractory ménière's disease (MD) has potential effect in preventing contralateral MD occurrence

BACKGROUND

Ménière's disease (MD), characterized by episodic vertigo attacks and fluctuating progressive hearing loss, is treated by low-dose intratympanic gentamicin (ITG) injections. Whether ITG causes hearing loss is controversial, and knowledge about its effects on the contralateral hearing and vestibular function is lacking.

AIMS/OBJECTIVES

We aimed to evaluate the effect of ITG on bilateral auditory and otolith organ function in patients with unilateral refractory MD.

MATERIAL AND METHODS

The data of 30 patients was collected, including history, and pure tone audiometry and vestibular-evoked myogenic potentials (VEMPs) results before and one month after ITG treatment. Changes in vertigo were assessed at a two-year follow-up.

RESULTS

One month after ITG injection, auditory thresholds between 125 Hz and 8 kHz on the injection side remained unchanged but have improved on the contralateral side at 125 Hz, 250 Hz, 1 kHz. The cervical and ocular VEMP solicitation rates on the injection side were lower than before the injection. Two years after treatment, vertigo was improved in 88.5% and complete controlled in 76.7% patients respectively.

CONCLUSIONS AND SIGNIFICANCE

The intractable vertigo of MD can be effectively controlled by ITG injection. This can improve the low and medium frequency hearing level in the contralateral ear, suggesting that it might help prevent contralateral MD occurrence.

Hair cell uptake of gentamicin in the developing mouse utricle

Intratympanic injection of gentamicin has proven to be an effective therapy for intractable vestibular dysfunction. However, most studies to date have focused on the cochlea, so little is known about the distribution and uptake of gentamicin by the counterpart of the auditory system, specifically vestibular hair cells (HCs). Here, with a combination of in vivo and in vitro approaches, we used a gentamicin-Texas Red (GTTR) conjugate to investigate the mechanisms of gentamicin vestibulotoxicity in the developing mammalian utricular HCs. In vivo, GTTR fluorescence was concentrated in the apical cytoplasm and the cellular membrane of neonatal utricular HCs, but scarce in the nucleus of HCs and supporting cells. Quantitative analysis showed the GTTR uptake by striolar HCs was significantly higher than that in the extrastriola. In addition, the GTTR fluorescence intensity in the striola was increased gradually from 1 to 8 days, peaking at 8-9 days postnatally. In vitro, utricle explants were incubated with GTTR and candidate uptake conduits, including mechanotransduction (MET) channels and endocytosis in the HC, were inhibited separately. GTTR uptake by HCs could be inhibited by quinine, a blocker of MET channels, under both normal and stressed conditions. Meanwhile, endocytic inhibition only reduced GTTR uptake in the CoCl₂ hypoxia model. In sum, the maturation of MET channels mediated uptake of GTTR into vestibular HCs. Under stressed conditions, MET channels play a pronounced role, manifested by channel-dependent stress enhanced GTTR permeation, while endocytosis participates in GTTR entry in a more selective manner.

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.

Instrumental head impulse test changes after intratympanic gentamicin for unilateral definite Ménière's disease: A systematic review and meta-analysis

OBJECTIVE

To estimate how much could intratympanic gentamicin (ITG) interfere with the vestibular-ocular reflex (VOR) parameters on instrumental head impulse test (HIT), either with scleral search coil or video head impulse test and, eventually, foresee the control of vertigo crisis in unilateral intractable Ménière's disease (MD).

METHODS

A literature search was conducted in PubMed, Scopus, Web of Science and Cochrane search engines. The search terms used were "vestibular ocular reflex", "head impulse test", "gentamicin," and "Meniere's disease". Limitations included text availability to be full text, species to be humans and language to be English. All study types were included. 89 articles were screened identifying four eligible studies were identified. Studies were included after consensus of the authors. Meta-analysis was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Data was analysed using Review Manager software.

RESULTS

Instrumental HIT, after ITG for MD, demonstrated, in the treated ear, a decreased gain in the horizontal, posterior and superior semicircular canals (SCC), of 0.36 (0.26; 0.47; 95% CI), 0.35 (0.22; 0.48; 95% CI) and 0.28 (0.21; 0.35; 95% CI), respectively. Gain asymmetry increases between the treated and non-treated ear of 23.78 (7.22; 40.35; 95% CI), 32.01 (12.27; 51.76; 95% CI) and 17.49 (9.99; 24.99; 95% CI), were similarly detected in the horizontal, posterior and superior SCC, respectively. Significantly smaller gain values after the first treatment were observed for a single injection group versus multiple injection group in the horizontal (p=0.002) and superior SCCs (p=0.016).

CONCLUSIONS

Instrumental HIT is effective in evaluating the SCC function after ITG for intractable unilateral MD. VOR gain changes in the direction of the treated ear in the three SCC have been clearly registered. An increased reduction of the VOR gain in the horizontal and anterior SCC also seemed to foresee the control of vertigo crisis. Still, after meta-analysis, the small number of patients' data available did not allow to define a treatment end-point value. This review also indicated that further and better-designed studies are warranted.

The Degeneration of the Vestibular Efferent Neurons After Intratympanic Gentamicin Administration

Intratympanic gentamicin (ITG) has been used to treat refractory Ménière's disease. Disequilibrium after ITG was still a challenge for some patients, and the underlying mechanism is poorly understood. Our previous study demonstrated that gentamicin distributed in the bilateral vestibular efferent neurons (VEN) after ITG; however, does it lead to VEN damage and cause further disequilibrium in patients following ITG? In this study, we observed severe damaged gentamicin-positive neurons of VEN and severe fractured myelin layer plates around neural fibers when viewed under transmission electron microscopy at day 3 after ITG. At day 30, neurons of VEN presented with relatively normal structures. Compared with the control group, the total number of choline acetyltransferase (CHAT) immunolabeling neurons in bilateral VEN showed a significant decrease both at day 3 and day 30. However, there was no significant difference in the total number of CHAT immunolabeling neurons between day 3 and day 30. It indicates that gentamicin is not only retrogradely transported into bilateral VEN, but also results in the degeneration of VEN after ITG. These findings may be related to patients' disequilibrium symptom after ITG. Furthermore, we speculate that VEN may play a role in vestibular compensation.

End-point indicators of low-dose intra-tympanic gentamicin in management of Ménière's disease

CONCLUSIONS

One-shot, low-dose intra-tympanic gentamicin (ITG) treatment was effective and safe for Ménière's disease (MD) patients. Head thrust test (HTT) and vestibular evoked myogenic potentials (VEMPs) test could be used as endpoint indicators for vertigo control in MD patients.

OBJECTIVES

The present study is to explore end-point indicators of ITG injection in MD.

METHODS

Patients with MD were reviewed from June 2012 to March 2014. Single-shot ITG at a concentration of 30 mg/ml was administered to patients. The sensitivity and specificity of HTT and VEMPs for vertigo control were measured.

RESULTS

All 37 patients with a median follow-up of 26 months were included. Of those 37 patients, 24 patients (64.9%) obtained class A vertigo control and seven patients (18.9%) obtained class B vertigo control. Only six patients had class C control (16.2%). The sensitivity and specificity of HTT for vertigo control were 74.2% and 50.0%. Meanwhile, the sensitivity and specificity of VEMPs threshold were 83.9% and 33.3%. When combined HTT and VEMPs, sensitivity and specificity were 93.5% and 66.7%. Based on the four-tone average thresholds at 0.5, 1, 2, 3 kHz, 78.4% patients had no significant change in PTA and 16.2% patients experienced significant improvement.

Fluorescent aminoglycosides reveal intracellular trafficking routes in mechanosensory hair cells

Aminoglycosides (AGs) are broad-spectrum antibiotics that are associated with kidney damage, balance disorders, and permanent hearing loss. This damage occurs primarily by killing of proximal tubule kidney cells and mechanosensory hair cells, though the mechanisms underlying cell death are not clear. Imaging molecules of interest in living cells can elucidate how molecules enter cells, traverse intracellular compartments, and interact with sites of activity. Here, we have imaged fluorescently labeled AGs in live zebrafish mechanosensory hair cells. We determined that AGs enter hair cells via both nonendocytic and endocytic pathways. Both routes deliver AGs from the extracellular space to lysosomes, and structural differences between AGs alter the efficiency of this delivery. AGs with slower delivery to lysosomes were immediately toxic to hair cells, and impeding lysosome delivery increased AG-induced death. Therefore, pro-death cascades induced at early time points of AG exposure do not appear to derive from the lysosome. Our findings help clarify how AGs induce hair cell death and reveal properties that predict toxicity. Establishing signatures for AG toxicity may enable more efficient evaluation of AG treatment paradigms and structural modifications to reduce hair cell damage. Further, this work demonstrates how following fluorescently labeled drugs at high resolution in living cells can reveal important details about how drugs of interest behave.

Uptake of fluorescent gentamicin by peripheral vestibular cells after systemic administration

Objective

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

Results

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

Conclusions

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

Gentamicin-induced structural damage of human and artificial (biomimetic) otoconia

CONCLUSIONS

Gentamicin causes irreversible structural damage of human and artificial otoconia by progressive dissolution of calcite. The inner architecture of otoconia is strongly affected by degradation scenarios during gentamicin exposure. Artificial otoconia can be used as a model system mimicking the chemical attacks for detailed investigations.

OBJECTIVES

To investigate the chemical interactions of gentamicin with natural calcite and human and artificial otoconia under in vivo conditions.

METHODS

Pure calcite crystals and artificial and human otoconia were exposed to gentamicin injection solutions at various concentrations. Morphological changes were observed in time steps by the use of environmental scanning electron microscopy (ESEM).

RESULTS

Dissolution of pure calcite crystals results in the formation of well oriented nanoshoots indicating an irreversible chemical reaction with gentamicin. Human and artificial otoconia reveal irreversible structural changes of their surface areas as well as of their inner structure, resulting in characteristic changes at different gentamicin concentrations. Minor changes are first observed by surface alterations and dissolution of calcite in the belly region. Major changes result in further reduction of the belly area reaching the center of symmetry. Finally, a complete dissolution of the branches takes place. Artificial otoconia provide detailed insight into surface alterations.

Intratympanic gentamicin for Meniere's disease: short- and long-term follow-up of two regimens of treatment

OBJECTIVES

(1) To compare the results of the 2 regimens of treatment at 2-year follow-up and (2) to evaluate the need and the efficacy of retreatment after the recurrence of vertigo attacks in a longer period of follow-up (using the Kaplan-Meier method of analysis).

STUDY DESIGN

Retrospective chart review.

SETTING

Tertiary referral center.

SUBJECTS AND METHODS

We analyzed 77 patients treated with intratympanic gentamicin (ITG). Thirty-five patients were treated with high-dose (HD) ITG (in total 6 injections, twice a day, repeated every 3 days) and 42 with low-dose (LD) ITG (1-2 injections). The results of treatment were evaluated in terms of functional level scale, control of vertigo, and hearing impairment.

RESULTS

At 2-year follow-up, a similar percentage of vertigo control was obtained in the 2 groups; the incidence of hearing loss and posttreatment disequilibrium was significantly higher in patients treated with HD-ITG. The long-term follow-up showed a control of vertigo attacks with a single round of treatment in 71.4% of patients treated with HD-ITG and in 55% of those treated with LD-ITG. With repeated rounds, an effective control of vertigo could be achieved in 88.5% using a HD-ITG protocol and 97.7% using a LD-ITG protocol.

CONCLUSIONS

LD-ITG allows obtaining good results in term of vertigo attacks associated with a limited occurrence of side effects. The long-term follow-up showed that LD-ITG needed repeated rounds more frequently than the HD-protocol. HD-ITG ran less risk of needing repeated rounds, but retreatment was ineffective in 40% of the cases requiring surgical therapy.