Monitoring of functional changes after transient ischemia in gerbil cochlea

Impact of Low Skeletal Muscle Mass and Obesity on Hearing Loss in Asymptomatic Individuals: A Population-Based Study

The relationship between low muscle mass (LMM) with obesity and hearing loss has been poorly studied. We aimed to investigate the association of LMM and obesity on hearing loss in the general population. A total of 265,792 adults who underwent a hearing test and body composition analyses were included. Pre-sarcopenia was defined as having an appendicular muscle mass index <5.7 kg/m2 for women and <7.0 kg/m2 for men, and obesity as a body mass index ≥25 kg/m2, while pre-sarcopenic obesity was defined as the co-presence of LMM and obesity. Participants were divided into four groups according to the presence of pre-sarcopenia and/or obesity. The prevalence of hearing loss was 1.8% in the control, 2.5% in the pre-sarcopenia alone, 3.0% in the obesity alone, and 6.2% in the pre-sarcopenic obesity group (p < 0.001). Hearing Thresholds were the highest in the pre-sarcopenic obesity group compared with the other three groups. In multivariable-adjusted models, the risk of hearing loss was the highest in the pre-sarcopenic obesity group (odds ratio: 1.30 [95% confidence interval: 1.10−1.56]), followed by the obesity alone (1.20 [1.12−1.28]) and pre-sarcopenia alone (1.19 [1.06−1.34]) group compared with the control group (p < 0.001). Pre-sarcopenic obesity was independently associated with a higher prevalence of hearing loss, supporting pre-sarcopenic obesity itself as a risk for the decline in hearing function.

Selection Criteria Optimal for Recovery of Inner Ear Tissues From Deceased Organ Donors

OBJECTIVE

To identify optimal conditions for recovering viable inner ear tissues from deceased organ donors.

SETTING

Tertiary recovery hospitals and Donor Network West Organ Recovery Center.

INTERVENTIONS

Recovering bilateral inner ear tissues and immunohistological analysis.

MAIN OUTCOME MEASURES

Immunohistochemical analysis of utricles from human organ donors after brain death (DBD) or donors after cardiac death (DCD).

RESULTS

Vestibular tissues from 21 organ donors (39 ears) were recovered. Of these, 18 donors (33 utricles) were examined by immunofluorescence. The sensory epithelium was present in seven utricles (two from DBD and five from DCD). Relative to DBD utricles, DCD organs more commonly displayed dense populations of hair cells and supporting cells. Relative to DBD, DCD had significantly shorter postmortem interval time to tissue recovery (<48 h). Compared to donors with no sensory epithelium, donors with intact and viable sensory epithelium (both DCD and DBD) had significantly shorter lag time to resuscitation prior to hospital admission (6.4 ± 9.2 vs 35.6 ± 23.7 min, respectively) as well as a shorter time between pronouncements of death to organ recovery (22.6 ± 30.4 vs 64.8 ± 22.8 h, respectively).

CONCLUSIONS

Organ donors are a novel resource for bilateral inner ear organs. Selecting tissue donors within defined parameters can optimize the quality of recovered inner ear tissues, thereby facilitating future research investigating sensory and nonsensory cells.

Bilateral Carotid Artery Occlusion and Cochlear Oxidative Stress and Hearing Loss in Rats

Introduction:

This study aimed to evaluate the effects of bilateral carotid artery occlusion on cochlear oxidative stress and hearing status in rats.

Methods:

The rats were divided into two sets. The first set was used for electrophysiological recording (click and 4 kHz tone burst auditory brainstem responses and electrocochleography) on the day before surgery and then on the first, fourth, and seventh days after surgery. Animals of the second set were used for biochemical analysis. The cochlea of animals in the second set was collected on the first, fourth, and seventh days after carotids occlusion for biochemical analysis. For the control groups, no carotids occlusion was done. For ischemia induction, both common carotid arteries were occluded for 20 minutes.

Results:

Electrophysiological analysis showed that burst auditory brainstem thresholds significantly elevated after common carotid arteries occlusion on the first, fourth, and seventh days after surgery with abnormal electrocochleography results at 75%, 70%, and 85% on the first, fourth, and seventh days after surgery, respectively. The electrophysiological finding confirmed by biochemical results that showed malondialdehyde and nitric oxide levels increased and superoxide dismutase and catalase activities decreased after occlusion in cochlea tissue.

Conclusion:

This study showed that bilateral common carotid artery occlusion increases cochlear oxidative stress and induces hearing loss in rats.

Non-invasive intraoperative monitoring of cochlear function by cochlear microphonics during cerebellopontine-angle surgery

In vestibular-schwannoma (VS) surgery, hearing-preservation rate remains low. Besides damage to the cochlear nerve, intraoperative cochlear ischemia is a potential cause of hearing loss. Here, we used non-invasive cochlear microphonic (CM) recordings to detect the cochlear vascular events of VS surgery. Continuous intraoperative CM monitoring, in response to 80-95 dB SPL, 1-kHz tone-bursts, was performed in two samples of patients undergoing retrosigmoid cerebellopontine-angle surgery: one for VS (n = 31) and one for vestibular neurectomy or vasculo-neural conflict causing intractable trigeminal neuralgia, harmless to hearing (n = 19, control group). Preoperative and postoperative hearings were compared as a function of intraoperative CM changes and their chronology. Monitoring was possible throughout except for a few tens of seconds when drilling or suction noises occurred. Four patterns of CM time course were identified, eventless, fluctuating, abrupt or progressive decrease. Only the VS group displayed the last two patterns, mainly during internal-auditory-canal drilling and the ensuing tumor dissection, always with postoperative loss of hearing as an end result. Conversely, eventless and fluctuating CM patterns could be associated with postoperative hearing loss when the cochlear nerve had been reportedly damaged, an event that CM is not meant to detect. Cochlear ischemia is a frequent event in VS surgery that leads to deafness. The findings that CM decrease raised no false alarm, and that CM fluctuations, insignificant in control cases, were easily spotted, suggest that CM intraoperative monitoring is a sensitive tool that could profitably guide VS surgery.

Resistance of Gerbil Auditory Function to Reversible Decrease in Cochlear Blood Flow

The objective was to design in gerbils a model of reversible decrease in cochlear blood flow (CBF) and analyze its influence on cochlear function. In Mongolian gerbils injected with ferromagnetic microbeads, a magnet placed near the porus acusticus allowed CBF to be manipulated. The cochlear microphonic potential (CM) from the basal cochlea was monitored by a round-window electrode. In 13 of the 20 successfully injected gerbils, stable CBF reduction was obtained for 11.5 min on average. The CM was affected only when CBF fell to less than 60% of its baseline, yet remained >40% of its initial level in about 2/3 of such cases. After CBF restoration, CM recovery was fast and usually complete. Reduced CM came with a 35- to 45-dB threshold elevation of neural responses determined by compound action potentials. This method allowing reversible changes of CBF confirms the robustness of cochlear function to decreased CBF. It can be used to study whether a hypovascularized cochlea is abnormally sensitive to stress.

Sarcopenia and Hearing Loss in Older Koreans: Findings from the Korea National Health and Nutrition Examination Survey (KNHANES) 2010

Age-related hearing impairment (ARHI) is becoming a more significant issue as geriatric population increases. Sarcopenia in older people is known to have a diverse health problem in various circumstances in recent studies. We assessed whether the decrease in muscle mass is related to ARHI. We used the 2010 data of the Korea National Health and Nutrition Examination Survey (KNHANES) to examine the associations between sarcopenia and ARHI. A total number of participants was 1,622 including 746 males and 876 females aged 60 years or older. Muscle mass was assessed as an appendicular skeletal muscle mass, and hearing loss was defined as the pure-tone averages (PTA) of test frequencies 0.5, 1, 2, 4 kHz at a threshold of 40 dB or higher in worse hearing side of the ear. Among 1,622 participants, 298 men and 256 women had hearing loss. Appendicular muscle mass (ASM), expressed as kg, was categorized in tertiles. In female population, after adjusting for age, smoking, drinking, amount of exercise, total body fat, education level, income level, and tinnitus, the odds ratio (OR) for hearing loss was 1.57 (95% confidence interval (CI) = 0.92–2.68) in the middle tertile and 1.79 (1.03–3.08) in the lowest tertile, compared with the highest tertile. P for trend in this model was 0.036. Controlling further for hypertension, diabetes mellitus, chronic kidney disease, and three types of noise exposure did not change the association. Larger muscle mass is associated with lower prevalence of hearing loss in elderly Korean females.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Ischemia-reperfusion injury of the cochlea: pharmacological strategies for cochlear protection and implications of glutamate and reactive oxygen species

A large amount of energy produced by active aerobic metabolism is necessary for the cochlea to maintain its function. This makes the cochlea vulnerable to blockade of cochlear blood flow and interruption of the oxygen supply. Although certain forms of human idiopathic sudden sensorineural hearing loss reportedly arise from ischemic injury, the pathological mechanism of cochlear ischemia-reperfusion injury has not been fully elucidated. Recent animal studies have shed light on the mechanisms of cochlear ischemia-reperfusion injury. It will help in the understanding of the pathology of cochlear ischemia-reperfusion injury to classify this injury into ischemic injury and reperfusion injury. Excitotoxicity, mainly observed during the ischemic period, aggravates the injury of primary auditory neurons. On the other hand, oxidative damage induced by hydroxyl radicals and nitric oxide enhances cochlear reperfusion injury. This article briefly summarizes the generation mechanisms of cochlear ischemia-reperfusion injury and potential therapeutic targets that could be developed for the effective management of this injury type.

Hypertensive retinopathy and sensorineural hearing loss

OBJECTIVE

To determine the correlation between hypertensive retinopathy (which is the end-organ damage of the vessels due to chronic hypertension) with sensorineural hearing loss.

METHODS

Pure tone hearing threshold of 56 hypertensive patients were compared with 56 normal age and sex matched control. Comparisons of pure tone hearing threshold are made among different group (grade) of hypertensive retinopathy patients and control (non-hypertensive patients).

RESULTS

The mean hearing thresholds were higher (worse) in all the frequencies on both sides in the hypertensive study group compared with normal subjects. However it was found to be statistically significant when tested using independent sample test (p < 0.05) on right ear at 2,000 Hz, 4,000 Hz and 8,000 Hz only. The mean hearing threshold is higher in all frequencies in the presence of retinopathy compared to control. However, the difference was found to be only statistically significant in the mean hearing threshold between grade I and control at 4,000 Hz and 8,000 Hz on both sides and at 1,000 Hz and 2,000 Hz on right ear. Hypertensive patient with grade I retinopathy had higher pure tone hearing thresholds at 4,000 Hz and 8,000 Hz compared to hypertensive without retinopathy and normal control.

CONCLUSIONS

Hypertensive retinopathy appears to be associated with high frequency sensor neural hearing loss.

Preventing hearing damage using topical dexamethasone during reversible cochlear ischemia: an animal model

HYPOTHESIS

Local application of dexamethasone to the round window (RW) niche prevents cochlear damage caused by local reversible ischemia.

BACKGROUND

Cochlear ischemia induced by internal auditory artery (IAA) compression/stretching is thought to cause postoperative sensory hearing loss after attempted hearing preservation removal of acoustic neuroma tumors. Dexamethasone administered to the RW niche traveling through the membrane to the cochlear fluids may prevent ischemic damage.

MATERIALS AND METHODS

Ten young albino rabbits were used for this study. Ischemic episodes were induced by compressing the IAA. Laser Doppler cochlear blood flow was measured using a probe positioned at the RW niche. Transtympanic electrocochleography was measured at 4, 8, and 12 kHz. In 5 test ears, dexamethasone was administered topically at the RW for approximately 50 minutes before the IAA compressions, whereas in 5 control ears, saline was applied in the same way. Each ear underwent one 10-minute IAA compression with a 60-minute postischemic period of transtympanic electrocochleography monitoring.

RESULTS

In both control- and dexamethasone-treated ears, ischemic episodes measured by Laser Doppler cochlear blood flow were comparable. Fifty minutes after IAA decompression, in dexamethasone-pretreated ears, cochlear microphonic and compound action potential amplitudes at all test frequencies were 10 to 15% less reduced than those in control ears. Compound action potential latencies in dexamethasone-pretreated ears resulted in shorter latency delay than in control ears.

CONCLUSION

The RW seems to be an efficacious route for the administration of dexamethasone into the inner ear. Dexamethasone showed a protective effect on cochlear function after local ischemia. Transtympanic electrocochleography was found to be a sufficient and effective tool in monitoring hearing.

Experimental otoacoustic emission and auditory brainstem response changes by stellate ganglion blockage in rat

INTRODUCTION

To investigate the effect of stellate ganglion (SG) block on hearing in rats.

MATERIALS AND METHODS

Sixteen male adult rats were randomly divided into 2 equal groups. Both groups underwent preblock auditory brainstem responses (ABRs) in response to tone bursts at 4, 6, and 8 kHz and otoacoustic emissions in response to distortion products as a function of f2 frequency at 1, 2, 4, and 6 kHz. Local anesthetic (0.2 mL of 2% prilokain) was administered to the left SG of the study group by posterior cervical percutaneous approach for cervical sympathetic blockage. In the control group, 0.2 mL of physiological saline was injected to the left SG. Postblock hearing evaluations were made after 15 minutes of injections.

RESULTS

Both Dp-gram and I/O function records suggested that whereas hearing thresholds were not affected in lower frequencies after SG blockage, it tended to increase at higher frequencies. In ABR records, waves I and II showed marked latency shift across all frequencies. The interpeak latency of waves I and II was shortened after blockage. Saline injection did not show any significant ABR or distortion-product otoacoustic emission threshold shift across frequencies at 60, 70, 80, and 90 dB sound pressure level.

CONCLUSION

Our data demonstrate that SG block improved the hearing parameters in rats with normal cochlear blood flow. To recommend SG blockage as a treatment option in the vascular pathologies of cochlea, further investigation should assess the efficiency of ganglion blockage in hearing parameters of rats with impaired cochlear blood flow.

Complex level alterations of the 2f (1)-f (2) distortion product due to hypoxia in the guinea pig

It is controversially discussed inasmuch acute hearing disorders might originate from impaired cochlear circulation. Hypoxia-specific alterations of inner ear parameters measurable in patients with acute sensorineural hearing loss would therefore be of great interest. Aim of this study was to characterize hypoxia-related alterations of the 2f (1)-f (2) distortion product. Nine guinea pigs were anaesthetized by i.m. administration of Midazolam, Medetomidin and Fentanyl. For introduction of hypoxia, the spontaneously breathing animals were offered a gas mixture of N(2)O and O(2) containing either 21 or 12-13% O(2). Distortion product otoacoustic emissions (DPOAEs) were continuously monitored at f (2) = 16 kHz; f (2)/f (1) = 1, 2; DP-definition = 2f (1)-f (2); L (1) = 65 dB and L (2) = 55 dB, while inhaled oxygen was switched from 21 to 12-13% and back. Oxygen saturation (SaO(2)) was continuously monitored. Data from an hypoxic interval were only used for further data processing if DPOAE levels were stable before and after hypoxia. Six hypoxic intervals in five animals fulfilled the stability criterion. During the hypoxic interval with the highest measured SaO(2) (75%), no alterations of DPOAE levels were observed. During the remaining five hypoxic intervals, when SaO(2) ranged between 57 and 70%, DPOAE levels were on average lower with an increased standard deviation compared to mean pre-hypoxic levels. Mean decrease correlated with the decrease of SaO(2 )(r = 0.90, P = 0.014). Alterations followed a characteristic time course-when hypoxia was started, DPOAE levels exhibited a short increase before they decreased and remarkably destabilized. After re-oxygenation DPOAE levels showed a pronounced level decrease, while SaO(2) already had recovered to pre-hypoxic values. After reaching a minimum, DPOAE levels slowly recovered to pre-hypoxic values. The decrease of DPOAE levels during hypoxia and the post-hypoxic level alterations have similarly been described by other authors before, while the distinct destabilization and transiently increased DPOAE levels have not been explicitly mentioned. A micromechanical mechanism that might explain a transient level increase and the post-hypoxic DPOAE level changes is discussed.

[Otoacoustic emissions in clinical and surgical practice]

OBJECTIVES

Otoacoustic emissions (OAEs), discovered in 1978, have a well-established cochlear origin. They strongly depend on the outer hair cells and are widely used in experimental research as a means for testing cochlear function. However, outside screening, OAEs are only rarely used in clinical practice. The objective of this paper was to show their vast clinical utility.

MATERIAL AND METHODS

First, a review of the biophysical and physiological knowledge on OAEs is provided, concerning transient OAEs as well as distortion-product OAEs, recalling the origin and the meanings of these acoustic signals. Several clinical situations are then presented, and the corresponding OAE alterations are explained, such as hearing screening in neonates, diagnosis of hearing impairment with particularities related to the age of the patient, situations critical to the cochlea such as ototoxic treatments, and surgical procedures to the cerebellopontine angle.

RESULTS

OAEs appear to be a powerful tool in clinical practice, particularly in hearing screening and diagnosis of deafness. They can also be used to monitor hearing function during cerebellopontine angle tumor resection.

CONCLUSION

OAEs are still rarely used as a diagnostic tool by clinicians despite their clinical value, which should make them a primary choice.

In vivo imaging of mammalian cochlear blood flow using fluorescence microendoscopy

AIMS

We sought to develop techniques for visualizing cochlear blood flow in live mammalian subjects using fluorescence microendoscopy.

BACKGROUND

Inner ear microcirculation appears to be intimately involved in cochlear function. Blood velocity measurements suggest that intense sounds can alter cochlear blood flow. Disruption of cochlear blood flow may be a significant cause of hearing impairment, including sudden sensorineural hearing loss. However, inability to image cochlear blood flow in a nondestructive manner has limited investigation of the role of inner ear microcirculation in hearing function. Present techniques for imaging cochlear microcirculation using intravital light microscopy involve extensive perturbations to cochlear structure, precluding application in human patients. The few previous endoscopy studies of the cochlea have suffered from optical resolution insufficient for visualizing cochlear microvasculature. Fluorescence microendoscopy is an emerging minimally invasive imaging modality that provides micron-scale resolution in tissues inaccessible to light microscopy. In this article, we describe the use of fluorescence microendoscopy in live guinea pigs to image capillary blood flow and movements of individual red blood cells within the basal turn of the cochlea.

METHODS

We anesthetized eight adult guinea pigs and accessed the inner ear through the mastoid bulla. After intravenous injection of fluorescein dye, we made a limited cochleostomy and introduced a compound doublet gradient refractive index endoscope probe 1 mm in diameter into the inner ear. We then imaged cochlear blood flow within individual vessels in an epifluorescence configuration using one-photon fluorescence microendoscopy.

RESULTS

We observed single red blood cells passing through individual capillaries in several cochlear structures, including the round window membrane, spiral ligament, osseous spiral lamina, and basilar membrane. Blood flow velocities within inner ear capillaries varied widely, with observed speeds reaching up to approximately 500 microm/s.

CONCLUSION

Fluorescence microendoscopy permits visualization of cochlear microcirculation with micron-scale optical resolution and determination of blood flow velocities through analysis of video sequences.

La maladie de Fabry : l'atteinte vasculaire multi-organe pourrait également intéresser la cochlée

PURPOSE

Fabry disease is an inborn error of metabolism due to a deficient activity of the lysosomal enzyme alpha-galactosidase A. The enzyme defect leads to the systemic accumulation of neutral glycosphingolipids in tissues, mainly in the vascular endothelium.

STRONG POINT

The aim of this paper is to present a review of the auditory manifestations in Fabry disease, and to discuss hypothesis on the vascular origin of deafness.

PERSPECTIVES

Sensorineural hearing loss in Fabry disease could be the first documented vascular pathology of the inner ear.

A model of real time monitoring of the cochlear function during an induced local ischemia

The aim of this study was to investigate the utility of distortion product otoacoustic emissions (DPOAEs) in intraoperative monitoring (IM) of cochlear ischemic episodes in animals during internal auditory artery (IAA) compression. The IAA was exposed using the posterior fossa approach and then compressed for 3 and 5 min intervals to effect ischemia. DPOAE amplitudes and phases were measured at 4, 8, and 12 kHz geometric mean frequency (GMF). In each monitored ear, laser-Doppler cochlear blood flow (CBF) was measured. All IAA compressions resulted in rapid decrease of DPOAE amplitude and CBF, with simultaneous DPOAE phase increase. DPOAE phase changes were found to increase consistently within several seconds of IAA compression, while corresponding DPOAE amplitudes changed more slowly, with up to 30-40 s delays. Following IAA release, DPOAEs at 12 kHz GMF were characterized by longer delays in returning to baseline than those measured at lower frequencies. In some cases, CBF did not return to baseline. In this animal model, DPOAEs were found to be sensitive measures of cochlear function during transient cochlear ischemic episodes, suggesting the utility of DPOAE monitoring of auditory function during surgery of cerebello-pontine angle tumors.

The association between cardiovascular disease and cochlear function in older adults

The purpose of this research was to evaluate the relation between self-reported cardiovascular disease (CVD) and cochlear function in older adults. The Epidemiology of Hearing Loss Study (EHLS) is an ongoing population-based study of hearing loss and its risk factors in Beaver Dam, Wisconsin. As part of the EHLS questionnaire, participants were asked about their cardiovascular medical history. CVD history was determined from questions regarding history of angina, myocardial infarction (MI), and stroke. Questions about the use of antihypertensive medication and blood pressure measurements determined the presence or absence of hypertension. Among the audiologic measures completed were distortion product otoacoustic emissions (DPOAEs). Cochlear function was measured using DPOAEs and participants were categorized as having (a) cochlear impairment, (b) possible cochlear impairment, or (c) no cochlear impairment. There were 1,501 participants with complete CVD and DPOAE data from the 1998-2000 examination phase. Women with a self-reported history of MI were twice as likely (age-adjusted odds ratio [OR] = 2.00, 95% confidence interval [CI] = 1.15-3.46) to have cochlear impairment than women without a history of MI. This association was not significant in men (age-adjusted OR = 0.98, 95% CI = 0.61-1.58). Additionally, no other CVD variables were associated with cochlear impairment. This study provides data on a possible sex-specific association between CVD and DPOAEs in older adults.

Frequency specificity of distortion-product otoacoustic emissions produced by high-level tones despite inefficient cochlear electromechanical feedback

Distortion product otoacoustic emissions (DPOAEs) are thought to stem from the outer hair cells (OHCs) around the normally narrow place tuned to the primary tone stimuli. They are thus said to be frequency-specific: their local absence should accurately pinpoint local OHC damage. Yet the influence of impaired tuning on DPOAE frequency specificity is poorly documented. Mice with local damage to OHCs were examined. Their DPOAEs were frequency-specific in that audiometric notches were accurately tracked. The same cochleae were further impaired by ischemia or furosemide injection inducing strial dysfunction with flat loss of sensitivity and tuning, while the preexisting pattern of damaged OHCs remained unaltered. Despite the loss of cochlear activity, DPOAEs produced by high-level (> or =70 dB SPL) primaries remained large in about the same interval where they had been initially normal, i.e., that with nondamaged OHCs, albeit with a slight frequency shift, of -1.1 kHz on average. Thus, the ability of DPOAEs to map structurally intact OHCs cannot be a mere consequence of cochlear tuning as it largely persists in its absence. The key element for this correct mapping is likely part of intact OHC structures (e.g., stereocilia bundles) and must have some tuning of its own.

Physiopathological significance of distortion-product otoacoustic emissions at 2f1-f2 produced by high- versus low-level stimuli

Distortion product otoacoustic emissions emitted by the cochlea at 2f1-f2 in response to pairs of pure tones at f1 and f2 (DPOAE) form a class of otoacoustic emissions and as such, are viewed as a reliable tool for screening outer hair cell (OHC) dysfunctions on a pass/fail basis. However, the persistence of residual DPOAEs from impaired cochleae at high stimulus levels has suggested that above 60-70 dB SPL, instead of reflecting "active" cochlear motion, DPOAEs might represent another "passive" modality: they would thus become unsuitable for analyzing cochlear function. The present work reports the consequences on high- vs low-level DPOAEs of three types of cochlear impairments involving OHCs: progressive OHC degeneration of genetic origin in CD1 mice, complete cochlear ischemia in gerbils, and furosemide injection vs ischemia-reperfusion in gerbils. An alternative to the "active-passive" model was used wherein regardless of stimulus level, cubic DPOAEs are produced by N (probably OHC-borne) nonlinear elements driven by input I and modulated by a function F3 of their operating point o; thus, DPOAE proportional to NI3F3(o). When OHCs degenerated, thereby implying a decrease of N, DPOAE levels also decreased regardless of the stimulus level up to 80 dB SPL, in line with the previous formula but at variance with the prediction of the active-passive concept. Instead of affecting N, the other two experiments impaired the efficiency of the cochlear feedback loop as a result of its electrical drive being decreased by strial dysfunction. As it is well accepted that the impaired basilar-membrane motion, although greatly reduced at low levels, tends to catch up with a normal one at higher levels, it was assumed the same was true with I so that DPOAE levels had to be, and indeed were little affected at high levels while plummeting at low levels, without any need for invoking two modalities for DPOAE generation. Finally, comparisons of furosemide vs ischemia effects revealed additional influences on DPOAEs, possibly accounted for by function F3(o). These results lead to the proposal that although high-level DPOAEs are expected to be poor audiometric indicators, they seem well adapted to assessing the functional integrity of nonlinear elements in OHCs, i.e., presumably their mechanoelectrical transduction channels.

Long-term observations on the reversibility of cochlear dysfunction after transient ischemia

To examine the reversibility of functional damage to the cochlea after transient ischemia, cochlear ischemia of 0-60 min was induced in 34 albino guinea pigs. Thresholds of auditory brainstem response (ABR) were then followed for 5 days after ischemia. Although the ABR threshold returned to almost the pre-ischemic value after 15 min ischemia, ischemia of 30 and 60 min duration induced irreversible dysfunction. Aminoguanidine, an inducible NO synthase (iNOS) inhibitor, significantly ameliorated the post-ischemic cochlear dysfunction induced by 60 min ischemia. Morphological findings of the hair cells were consistent with these functional results. These results indicate that ischemia of 30 min or longer induces irreversible damage to the cochlea and that iNOS plays injury-producing roles in this type of injury.

Does endogenous or exogenous adenosine facilitate the functional recovery of the cochlea after ischemia?

The present study was undertaken to determine whether adenosine attenuates cochlear dysfunction induced by transient ischemia. Adenosine or erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor, was administered by perilymphatic perfusion to albino guinea pigs that were subjected to cochlear ischemic episodes of 30-minute duration. The threshold shift of the compound action potential (CAP) from the preischemic value was significantly reduced in the animals perfused with EHNA 1 hour after the onset of reperfusion. However, perfusion of adenosine at concentrations of 100 micromol/L to 10 mmol/L did not reduce the postischemic CAP threshold shift by either 1 hour or 4 hours after the onset of reperfusion. These results suggest that the elevation of the adenosine concentration did not exert a protective effect on the cochlear ischemia-reperfusion injury, and that the protective action of EHNA is unrelated to elevating the adenosine concentration.

[Idiopathic sudden deafness]

PURPOSE

Sudden idiopathic deafness is a sensorineural hearing loss with no recognized causes at the time of onset. The impairment site is usually localized in the cochlea, but some cases of retrocochlear lesions (e.g., cerebellopontine angle tumors, degenerative neural diseases, neuraxial ischemic lesions) can induce sensorineural deafness. The medical management of patients presenting with sudden deafness aims at detecting a causal mechanism, and at administering emergency therapeutic drugs. The diagnosis of idiopathic sudden deafness can be definitely made when no causes are found. Usually, the impairing mechanism involves the cochlea. The pathophysiology of this sensorineural alteration is still unknown. It is most likely that several mechanisms are associated together, their common point being an impairment to the feedback loop of the organ of Corti.

CURRENT KNOWLEDGE AND KEY POINTS

It is very likely that reactivation of neurotropic viruses and/or cochlear ischemia are frequent etiologies. Whatever the cause, the treatment is to be administered urgently, and consists of a high-dose corticotherapy at the least. Other treatments have never really proven to be effective. It is secondarily checked that no retrocochlear pathological processes, such as a cerebellopontine angle tumor, is present, in particular in young people.

FUTURE PROSPECTS AND PROJECTS

One of the current objectives is to determine when cochlear ischemia is involved, in a mini-invasive manner, such as with laser Doppler flowmetry, so that the treatment can be optimized. From a therapeutic point of view, early acoustic protection has been proven to be effective in cases of cochlear ischemia in small laboratory animals. Its efficacy in case of sudden deafness, non-exclusive of other causes than ischemia, is being assessed in a multicentric project.

Origin of cubic difference tones generated by high-intensity stimuli: effect of ischemia and auditory fatigue on the gerbil cochlea

Cubic difference tone (CDT) otoacoustic emissions are thought to arise from the feedback loop allowing outer hair cells to enhance the sensitivity and tuning of the organ of Corti. The existence of residual CDTs during complete cochlear ischemia is therefore disturbing. That stimulus intensities must exceed 50-60 dB SPL for residual CDTs to be recorded and for level notches to be present in CDT growth functions is often cited as evidence for a two-component, "active/passive" model: one component, the residual one, would originate from a passive, hardly vulnerable mechanism and thus be unsuitable for hearing screening purposes. This model was probed in gerbil ears after complete interruption of the cochlear blood flow. Cochlear potentials and CDTs were controlled simultaneously through continuous monitoring of CDT level and phase for 50 and 60 dB SPL stimuli and group-delay measurements. After a clear initial decay, CDT levels elicited at 60 dB SPL plateaued for several minutes at about 20 dB below initial level, and when early level notches were observed, CDT phase changes remained minor. The CDT group delays decreased by less than 30%. Later CDT level notches were associated with sharp phase reversals but the similarity between CDT characteristics before and after a notch was hardly consistent with a two-component interpretation. When mild sound overexposure (pure tone, 90-95 dB SPL, 15-30 min) had been performed prior to ischemia, little or no ischemic CDT came from the frequency bands where auditory fatigue had been detected (within 1 kHz), irrespective of the stimulus intensity. It suggests that instead of being passive, residual ischemic CDTs were vulnerable and produced according to a near-normal tonotopy by the same mechanisms that were sensitive to auditory fatigue. All the results lined up with a simple feedback model of cochlear function assuming a single CDT source related to mechano-electrical transduction in outer hair cells. More parsimonious than a two-component model, it posits that although early stages of ischemia dramatically impair the overall performance of the cochlea, the nonlinear mechanical stages responsible for the existence of CDTs keep working albeit at higher intensities.

Protection of outer hair cells from reperfusion injury by an iron chelator and a nitric oxide synthase inhibitor in the guinea pig cochlea

To examine whether an active process of the cochlea was injured by ischemia-reperfusion, time courses of distortion-product otoacoustic emissions (DPOAEs) were examined before, during and after 30 min cochlear ischemia using albino guinea pigs. DPOAEs decreased to the minimum level when the animals were subjected to ischemia. When the cochlea was recirculated, DPOAEs initially recovered with time until 20 min after the onset of reperfusion. However, thereafter the amplitude of DPOAEs gradually decreased toward the noise level. Administration of deferoxamine (an iron chelator) or N-nitro-L-arginine (a nitric oxide synthase inhibitor) ameliorated this decrease of DPOAEs during reperfusion and significantly increased the DPOAE amplitudes 60 min after the onset of reperfusion as compared with those in non-treated animals. These results suggest that cochlear reperfusion as well as ischemia injured the active process of the cochlea and that free radicals and nitric oxide play important roles in this injury.

A model of cochlear function assessment during reversible ischemia in the Mongolian gerbil

An in vivo model in the Mongolian gerbil is used to assess the auditory changes during reversible cochlear ischemia. This model allows the recordings of cochlear potentials (microphonics, summating potential and compound action potential) and otoacoustic emissions (cubic difference tones, CDTs), together with laser Doppler cochlear blood flow (CBF) measurements, over reversible cochlear ischemia. Ischemia is achieved by compression of the eighth nerve complex at the porus of the internal acoustical meatus, whereas the compression release allows the reperfusion to occur. While CBF monitoring permits to objectively determine the degree of ischemia and reperfusion, the combined analysis of cochlear potentials and CDTs makes it possible to point out the preferential site of main functional damage within the cochlea, i.e., outer hair cells (OHCs), inner hair cells, or ganglia cells. At least three ischemia-reperfusion sequences can be used on one side, and sometimes both ears can serve for experiments. This model could be applicable for testing drugs with alleged protective effects against cochlear ischemia and/or reperfusion, in treated vs. non-treated animals.

Vulnerability of the gerbil cochlea to sound exposure during reversible ischemia

Cochlear ischemia induces a sensorineural hearing loss, in part through a fast functional impairment of outer hair cellls. Assuming that the cochlea is rendered fragile during ischemia and reperfusion and that stimulation itself can jeopardize its functional recovery, we used a model of reversible selective cochlear ischemia in Mongolian gerbils to establish what type of sound exposure can be deleterious during and immediately after reversible ischemia. Several groups of gerbils were used, with different ischemia durations and levels of sound exposure. Control groups were only exposed to tones at 80 and 90 dB SPL during 30 min, while other groups underwent complete and fully reversible blockage of the labyrinthine artery, during 5.5 or 8 min, and were exposed to 60 or 80 dB SPL tones during 30 min. The amount of ischemia and reperfusion was measured by means of laser Doppler velocimetry, whereas outer hair cells' function was continuously monitored through distortion-product otoacoustic emissions (DPOAEs). The losses of DPOAE levels after 8 min transient ischemia and 60 dB SPL exposure were as large as those induced by 80 dB SPL exposures combined with 5.5 min ischemia, or 90 dB SPL exposures without ischemia, with a maximum loss around 25-30 dB, half an octave above the stimulus frequency. These results give evidence for an extremely high cochlear vulnerability to low-level sound exposure when associated with reversible ischemia. This vulnerability may have important clinical consequences in patients with cochlear circulatory disturbances.

Measuring the cochlear blood flow and distortion-product otoacoustic emissions during reversible cochlear ischemia: a rabbit model

Impairment to the cochlear blood flow likely induces many types of sensorineural hearing loss. Models using several small laboratory animals have been described in the literature that permit the simultaneous monitoring of the cochlear blood flow with laser-Doppler flowmetry and cochlear function using evoked responses. However, these models have not permitted a direct application of the resulting knowledge to the human condition, primarily due to differences in the translucence of the otic capsule between species. In the present study, to approximate conditions relevant to the human patient, the rabbit was utilized to develop a procedure in which laser-Doppler flowmetry could be used to measure the cochlear blood flow in an animal with an opaque otic capsule. At the same time, the cochlear function was monitored non-invasively using distortion-product otoacoustic emissions. In this manner, a laser-Doppler probe was positioned in the round window niche and the cochlear function measured using distortion-product otoacoustic emissions during a systematic series of ischemic episodes. Cochlear ischemia was produced by deliberately compressing the eighth nerve complex at the porus of the internal acoustic meatus, for periods lasting from 1-3 min, while cochlear blood flow and distortion-product otoacoustic emission measures were obtained simultaneously before, during and following the occlusion. Results demonstrated that the cochlear blood flow sharply decreased within 1 s after compression onset, whereas distortion-product otoacoustic emissions showed obstruction-induced changes after a delay of several seconds, provided that the blood flow decreased, at least 40%. Similarly, upon release of the compression, the cochlear blood flow began to recover within 1 s, whereas the recovery of the corresponding distortion-product otoacoustic emissions was slightly delayed. Although not apparent in the distortion-product otoacoustic emission recovery time course, the cochlear blood flow consistently overshot its initial baseline value during the recovery process. Thus, although cochlear ischemia produced changes in the distortion-product otoacoustic emission activity that generally followed the resulting alterations in the cochlear blood flow, the detailed relationship between the two measures was complex.

Comparison of the auditory-evoked brainstem response wave I to distortion-product otoacoustic emissions resulting from changes to inner ear blood flow

OBJECTIVE/HYPOTHESIS

Examine and compare in detail the time courses of the auditory brainstem response (ABR) wave I amplitude and latency to the distortion-product otoacoustic emission (DPOAE) amplitude and phase measured in the rabbit model following deliberate obstruction of cochlear blood flow (CBF).

METHODS

Using a posterior fossa craniotomy in five rabbits, the internal auditory artery (IAA) was compressed with a probe. ABR and otoacoustic emission were continuously monitored before, during, and after the compressions.

RESULTS

ABR wave I amplitudes demonstrated measurable decreases at a mean of 28.3 s after IAA compression, whereas DPOAE amplitudes decreased after a mean of 14.8 s. Wave I latencies began to increase at a mean of 18.3 s after occlusion, while DPOAE phase measures changed after a mean of only 4.8 s following IAA compression. The time-course patterns were similar for the amplitudes of both ABR wave I and DPOAE.

CONCLUSIONS

ABR wave I amplitude follows a similar, though delayed (by approximately 10 s) time-course pattern to that of the DPOAE following IAA compression. The implication of these findings for intraoperative auditory monitoring is that changes in many currently employed measures will lag actual surgically induced alterations in CBF by at least 20 to 30 s.

The effect of mannitol upon cochlear dysfunction induced by transient local anoxia

Transient local anoxia of the cochlea was induced by pressing the labyrinthine artery, and compound action potential (CAP) or endocochlear potential (EP) was measured before and after transient local anoxia ranging from 5 to 60 min using 106 albino guinea pigs. The complete interruption of the cochlear blood flow by this procedure and its full restoration after releasing the pressure on the artery was confirmed by a laser-Doppler flowmeter. The anoxia of less than 10 min induced no post-anoxic cochlear dysfunction, whereas the anoxia of a longer duration induced an irreversible dysfunction of the cochlea. It was evident that the post-anoxic recovery of the CAP threshold was worse as the anoxia period was prolonged, and CAP was almost completely abolished after 60-min anoxia. In animals which were administered mannitol intravenously just after the restoration of the cochlear blood circulation, the recovery of the CAP threshold was significantly better than that in the control animals, when the animals were subjected to local anoxia of 15- to 30-min duration. No beneficial effect, however, was observed in the 60-min anoxia group. In conclusion, local anoxia of 10 min or longer caused cochlear dysfunction, which was partially but significantly alleviated by mannitol.