Correlation between serum lipid and prognosis of idiopathic sudden sensorineural hearing loss: a prospective cohort study

The role of serun lipid, cytokine production in sudden sensorineural hearing loss

Sudden sensorineural hearing loss (SSNHL) has serious harm to human hearing health, where blood lipid and inflammatory levels may play a key role in it. Thus, the purpose of this investigation was to assess the connection between inflammatory and lipid variables and SSNHL. Patients diagnosed with SSNHL had an analysis of serum lipid parameters, such as total cholesterol (TC), triglycerides, HDL-C, LDL-C, apolipoprotein A (apo A), apolipoprotein B (apo B), and lipoprotein A (Lp(a)), as well as inflammatory factors like TNF-α and IL-10. After that, risk factor analysis was carried out utilizing univariate, multivariate regression, and LASSO retrospective modeling. In all, 72 SSNHL patients and 67 healthy control individuals were involved. The LDL/HDL, total cholesterol, ApoB, LP(a), IL-10, TNF-α, and IFN-γ considerably higher in the SSNHL group than in the healthy control group, however, nervonic acid and coenzyme Q were decreased significantly in SSNHL than Control group. The multivariate logistic regression model's analysis using multifactorial retrospective modeling revealed significant changes in LDL, LDL/HDL, IL-10, and TNF-α. In addition, in the LASSO regression model, the model demonstrated high discrimination, as evidenced by the C-index for the cohort's prediction nomogram, which was 0.998 (95% CI, 0.154-1.115) and confirmed to be 0.925 following bootstrapping validation. Finally, IL-10 and LDL/HDL were the main risk factors in SSNHL. LDL/HDL and IL-10 may be closely related to SSNHL's progress and should be evaluated promptly before treating patients with SSNHL.

Prognostic Significance of Early Posttreatment Audiometry in Idiopathic Sudden Sensorineural Hearing Loss

OBJECTIVE

This study aimed to assess the prognostic value of early posttreatment initiation audiometry findings in patients with idiopathic sudden sensorineural hearing loss (ISSHL).

STUDY DESIGN

A retrospective cohort study.

SETTING

The study was conducted at a single tertiary medical center.

METHODS

A review was conducted on 299 ISSHL patients treated between 2007 and 2023. Patients underwent audiometry on Days 2 to 3 and 5 to 7 posttreatment initiation. Data on demographics, medical history, audiometry results, and prognosis were collected. Prognosis was categorized based on hearing recovery at 1 year postdiagnosis.

RESULTS

Older age, worse initial speech reception threshold (SRT), ischemic heart disease, cerebrovascular accidents/transient ischemic attacks, hypertension, and diabetes were associated with a poorer prognosis. Posttreatment initiation audiometry findings on Days 2 to 3 and 5 to 7 significantly correlated with prognosis. Patients with a ≥30% improvement had substantial or complete recovery in over 80% of cases, while those with <10% had <30% recovery. Multivariate analysis identified a significant improvement on days 5 to 7 as an independent predictor of complete recovery (odds ratio = 4.25 [95% confidence interval 1.96-9.23], P = .0002).

CONCLUSION

Posttreatment initiation audiometry findings, particularly on Days 5 to 7, hold significant prognostic value in ISSHL patients. A substantial improvement during this timeframe is strongly associated with favorable outcomes. These findings emphasize the potential of posttreatment initiation audiometry as a valuable tool for clinicians in counseling patients with ISSHL.

Associations of Blood Lipids with the Risk and Prognosis of Sudden Sensorineural Hearing Loss: A Meta-analysis

Patients with sudden sensorineural hearing loss (SSNHL) may lose their hearing. The relationship between SSNHL and total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) levels is still unclear. The association of TC, TG, HDL-C, and LDL-C levels with the risk and prognosis of SSNHL was explored in this study. After searching for literature in different databases, 13 researches were used to summarize the risk and prognosis of SSNHL associations with TC, TG, HDL-C, and LDL-C using meta-analysis. Total cholesterol had a significant association with the risk of SSNHL (95% CI, 1.34-2.91). Adjustment for confounding factors and grouping criteria of TG were all significant sources of heterogeneity. One of the significant sources of heterogeneity in the LDL-C subgroup analyses was an adjustment for confounders. Sensitivity analysis revealed a robust association between TC and the risk of SSNHL. There was a significant publication bias in the association between TC and SSNHL prognosis High TC level is a risk factor for SSNHL.

Correlation of Apo B/A1 ratio with hemodynamics and hearing impairment degree in elderly patients with sudden sensorineural hearing loss

Objective

To investigate the correlation of apolipoprotein B/A1 (Apo B/A1) ratio with hemodynamics and degree of hearing impairment in elderly patients with sudden sensorineural hearing loss (SSNHL).

Methods

A total of 82 elderly patients with SSNHL diagnosed and treated in our hospital from July 2019 to September 2022 were retrospectively selected as the research group. The patients were divided into the mild group (22 cases), the moderate group (45 cases), and the severe group (15 cases) according to the degree of hearing impairment. 82 elderly people who underwent physical examination in our hospital during the same period were selected as the control group. The ApoB/A1 ratio and hemodynamic [whole blood low-shear viscosity (LSV), whole blood high-shear viscosity (HSV) and plasma viscosity (PV)] were measured in the two groups. The correlation of ApoB/A1 ratio with hemodynamics and degree of hearing impairment was analyzed. The predictive value of ApoB/A1 ratio and hemodynamics for the severity of SSNHL in elderly patients was analyzed.

Results

Compared with the control group, the ApoB/A1 ratio, and the levels of LSV, HSV and PV were higher in the research group (P < 0.001). The ApoB/A1 ratio and content of LSV, HSV and PV in the moderate group were significantly increased compared with these in the mild group (P < 0.05). Compared with the moderate group, the ApoB/A1 ratio and the levels of LSV, HSV and PV in the severe group were significantly increased (P < 0.05). Pearson correlation analysis showed that ApoB/A1 was positively correlated with LSV, HSV and PV (r = 0.303, 0.312, 0.228, P < 0.01). Logistic regression analysis showed that the ApoB/A1 ratio, LSV, HSV and PV levels were independent risk factors for the degree of hearing impairment in elderly patients with SSNHL (P < 0.05). The area under the curve (AUC) of ApoB/A1, LSV, HSV and PV for predicting the severity of SSNHL in elderly patients was 0.701, 0.817, 0.838, and 0.765, respectively. The AUC of combined prediction was 0.926, the sensitivity was 86.67 %, and the specificity was 90.06 %. The sensitivity and specificity of combined detection were higher than those of single detection.

Conclusion

The contents of ApoB/A1, HSV, LSV and PV were significantly increased in elderly patients with SSNHL, and their levels are significantly related to the degree of hearing impairment. The combined detection has high value in evaluating the severity of the disease.

Traditional and non-traditional lipid parameters as risk factors for sudden sensorineural hearing loss

OBJECTIVE

The purpose was to explore the effects of traditional and non-traditional lipid parameters on Sudden Sensorineural Hearing Loss (SSNHL).

METHODS

The study included 452 patients diagnosed with SSNHL, among whom 206 patients had a level of hearing improvement ≥10 dB after one month of follow-up. A propensity score-matched (2:1) control group was used. Conditional and unconditional logistic regression were used to analyze the risk factors for SSNHL.

RESULTS

Patients with SSNHL had a higher risk of concomitant hypertension and elevated atherosclerogenic lipid levels, with apolipoprotein B and apolipoprotein E identified as independent risk factors for the onset of SSNHL. Additionally, the Lipid Comprehensive Index (LCI) was an independent risk factor for the degree of hearing loss. A positive linear correlation was revealed between triglyceride, non-high-density lipoprotein cholesterol, atherogenic index, Castelli risk index, atherogenic index of plasma, LCI and hearing loss. However, no linear relationship was observed between hearing gain and any lipid parameters. When Total Cholesterol (TC) was in the range of borderline high, the treatment effect was the best. However, the statistical significance disappeared upon adjusting for confounding factors.

CONCLUSION

Patients with SSNHL exhibited markedly dysregulated lipid metabolism. Elevated serum lipid levels may be a causative factor in auditory impairment and can influence the extent of hearing loss. Promptly improving cochlear microcirculation may benefit patients with borderline elevated TC.

LEVEL OF EVIDENCE: 4

A machine learning screening model for identifying the risk of high-frequency hearing impairment in a general population

BACKGROUND

Hearing impairment (HI) has become a major public health issue in China. Currently, due to the limitations of primary health care, the gold standard for HI diagnosis (pure-tone hearing test) is not suitable for large-scale use in community settings. Therefore, the purpose of this study was to develop a cost-effective HI screening model for the general population using machine learning (ML) methods and data gathered from community-based scenarios, aiming to help improve the hearing-related health outcomes of community residents.

METHODS

This study recruited 3371 community residents from 7 health centres in Zhejiang, China. Sixty-eight indicators derived from questionnaire surveys and routine haematological tests were delivered and used for modelling. Seven commonly used ML models (the naive Bayes (NB), K-nearest neighbours (KNN), support vector machine (SVM), random forest (RF), eXtreme Gradient Boosting (XGBoost), boosting, and least absolute shrinkage and selection operator (LASSO regression)) were adopted and compared to develop the final high-frequency hearing impairment (HFHI) screening model for community residents. The model was constructed with a nomogram to obtain the risk score of the probability of individuals suffering from HFHI. According to the risk score, the population was divided into three risk stratifications (low, medium and high) and the risk factor characteristics of each dimension under different risk stratifications were identified.

RESULTS

Among all the algorithms used, the LASSO-based model achieved the best performance on the validation set by attaining an area under the curve (AUC) of 0.868 (95% confidence interval (CI): 0.847-0.889) and reaching precision, specificity and F-score values all greater than 80%. Five demographic indicators, 7 disease-related features, 5 behavioural factors, 2 environmental exposures, 2 hearing cognitive factors, and 13 blood test indicators were identified in the final screening model. A total of 91.42% (1235/1129) of the subjects in the high-risk group were confirmed to have HI by audiometry, which was 3.99 times greater than that in the low-risk group (22.91%, 301/1314). The high-risk population was mainly characterized as older, low-income and low-educated males, especially those with multiple chronic conditions, noise exposure, poor lifestyle, abnormal blood indices (e.g., red cell distribution width (RDW) and platelet distribution width (PDW)) and liver function indicators (e.g., triglyceride (TG), indirect bilirubin (IBIL), aspartate aminotransferase (AST) and low-density lipoprotein (LDL)). An HFHI nomogram was further generated to improve the operability of the screening model for community applications.

CONCLUSIONS

The HFHI risk screening model developed based on ML algorithms can more accurately identify residents with HFHI by categorizing them into the high-risk groups, which can further help to identify modifiable and immutable risk factors for residents at high risk of HI and promote their personalized HI prevention or intervention.

The value of inflammatory biomarkers in the occurrence and prognosis of sudden sensorineural hearing loss: a meta-analysis

OBJECTIVES

Sudden sensorineural hearing loss (SSNHL) is one of the common emergencies in otorhinolaryngology. Several studies have shown that chronic inflammation is associated with its onset and prognosis. However, the association between some inflammatory biomarkers and SSNHL is still unclear. Therefore, we conducted this meta-analysis to explore the value of inflammatory biomarkers in the occurrence and prognosis of SSNHL.

METHODS

Pubmed, Embase, Cochrane and Web of Science databases were searched comprehensively, the eligible literatures were screened out by formulating the inclusion criteria and exclusion criteria. After extracting sample size, mean and standard deviation, we performed meta-analysis with standardized mean deviation (SMD) and 95% confidence interval (CI) as effect sizes.

RESULTS

A total of 17 articles were included in this meta-analysis, including 2852 subjects, 1423 patients and 1429 healthy controls. The results of meta-analysis showed that the neutrophil-to-lymphocyte ratio (NLR) of the experimental group was significantly higher than the control group (SMD = 1.05, 95% CI 0.87-1.24, P < 0.001), the NLR of the recovery group was significantly lower than the unrecovered group (SMD = 0.68, 95% CI 0.27-1.08, P < 0.05); The platelet-to-lymphocyte ratio (PLR) of the experimental group was significantly higher than the control group (SMD = 0.55, 95% CI 0.34-0.76, P < 0.05), the PLR of the recovery group was significantly lower than the unrecovered group (SMD = 0.44, 95% CI 0.05-0.82, P < 0.05); The C-reactive protein-to-serum albumin ratio (CRP/Alb) of the experimental group was significantly higher than the control group (SMD = 0.39, 95% CI 0.04-0.74, P < 0.05).

CONCLUSIONS

The results showed that high NLR, PLR, and CRP/Alb indicated the occurrence of SSNHL, NLR and PLR could predict prognosis of SSNHL.

Cardiovascular Risk Factors in Patients With Sudden Sensorineural Hearing Loss: A Systematic Review and Meta-Analysis

OBJECTIVE

The interruption of vascular supply to the inner ear is one of several proposed etiologies of sudden sensorineural hearing loss (SSNHL). The increased presence of cardiovascular risk factors may predispose patients to SSNHL through this pathway. This systematic review and meta-analysis studies the presence of cardiovascular risk factors in patients diagnosed with SSNHL.

DATA SOURCES

Databases included PubMed/Medline, OVID, EMBASE, Cochrane, and Web of Science.

REVIEW METHODS

Inclusion criteria included studies featuring SSNHL patients presenting with 1+ cardiovascular risk factors. Exclusion criteria included case reports and studies without outcome measures. Two investigators independently reviewed all manuscripts and performed quality assessments using validated tools.

RESULTS

Of 532 identified abstracts, 27 studies met inclusion criteria (19 case-control, 4 cohorts, 4 case series). Of these, 24 underwent meta-analysis encompassing a total of 77,566 patients (22,620 SSNHL patients, 54,946 matched controls). The mean age was 50.43 years. SSNHL patients were more likely to have concomitant diabetes (odds ratio [OR] 1.61 [95% confidence interval [CI]: 1.31, 1.99; p < .00001]) and hypertension (OR 1.5 [95% CI: 1.16, 1.94; p = .002]). An increased standard mean difference of total cholesterol of 11.09 mg/dL (95% CI; 3.51, 18.67; p = .004) was noted in the SSNHL group compared with the controls. No significant differences in smoking, high-density lipoprotein, triglycerides, or body mass index were detected.

CONCLUSION

Patients presenting with SSNHL have a significantly higher risk of concomitant diabetes, hypertension, and higher total cholesterol in comparison to matched controls. This may indicate a higher cardiovascular risk profile in this population. More prospective and matched cohort studies are needed to understand the role of cardiovascular risk factors in SSNHL.

The relationship between serum lipid and sudden sensorineural hearing loss

BACKGROUND

Hyperlipidemia may be part of the important mechanisms for the development of idiopathic sudden sensorineural hearing loss (ISSNHL).

AIMS

So the purpose of this study was to evaluate the relationship between changes in blood lipid levels and ISSNHL.

MATERIALS AND METHODS

We enrolled 90 ISSNHL patients in our hospital using a retrospective study design from 2019.1 to 2021.12. Blood levels of total cholesterol (TC), triglycerides (TG), and low-density lipoprotein (LDL-C). Hearing recovery was analyzed using the chi-square test and one-way analysis of variance (ANONA). Univariate and multifactorial Logstic retrospective analyses to establish the relationship between LDL-C/HDL-C ratio and hearing recovery after adjustment for potential confounding factors.

RESULTS

In our study, 65 (72.2%) patients had their hearing recovered. All group analyses and three group analyses (i.e. Excluding the no-recovery group) found that LDL/HDL was on an upward trend from complete recovery to a slight recovery group and strongly associated with hearing recovery. Univariate and multivariate logistic regression analysis found high levels of LDL and LDL/HDL in the partial hearing recovery group, relative to the full hearing recovery group. Curve fitting intuitively demonstrates the influence of blood lipids on prognosis.

CONCLUSIONS

Our findings suggest that LDL. TC, TC/HDL, and LDL/HDL concentrations may be closely related to the pathogenesis of ISSNHL.

SIGNIFICANCE

Improving the relevant lipid test at the time of admission to the hospital has good clinical significance for improving the prognosis of ISSNHL.

Nomogram for predicting the prognosis of sudden sensorineural hearing loss patients based on clinical characteristics: a retrospective cohort study

Background

Based on the clinical characteristics of patients, a nomogram predicting the prognosis of patients suffering from sudden sensorineural hearing loss (SSNHL) was constructed, which could aid in personalized treatment.

Methods

Data on the clinical characteristics of patients with SSNHL were collected and statistically analyzed. A nomogram for predicting the hearing prognosis of SSNHL patients were then constructed.

Results

A total of 356 patients were included in this study, including 227 and 129 in the recovery group (63.76%) and non-recovery group (36.24%), respectively. Univariable logistic regression demonstrated that age, gender, body mass index (BMI), marital, Audiogram curve, vertigo, hearing loss degree, and time to initial treatment were associated with hearing outcomes. Multivariate logistic models showed that age [odds ratio (OR): 0.479, 95% confidence interval (CI): 0.301-0.748, P<0.001], descending (OR: 0.116, 95% CI: 0.047-0.275, P<0.001) and flat audiogram curves (OR: 0.397, 95% CI: 0.159-0.979, P=0.045), profound hearing loss (OR: 0.047, 95% CI: 0.013-0.152, P<0.001), and treatment initiation after 1 week (8-14 days: OR: 0.047, 95% CI: 0.013-0.152, P<0.001; >14 days: OR: 0.131, 95% CI: 0.039-0.413) were risk factors for the hearing recovery. Logistic regression analyses were conducted to construct the prognostic nomogram. As estimated by the area under the receiver operating characteristic curve (ROC), the model had an accuracy of 0.867 (95% CI: 0.709-0.747). The validation analysis confirmed the high accuracy of the nomogram, and the decision curve showed that the model has potential clinical application value.

Conclusions

This study demonstrated that age, descending and flat audiogram curves, profound hearing loss, and initiating treatment after 1 week of SSNHL onset were independent risk factors associated with a worse hearing recovery prognosis. Using these factors, a nomogram with a high prediction accuracy was developed to predict the hearing recovery rate of SSNHL patients.

Characteristics and prognostic analysis of simultaneous bilateral sudden sensorineural hearing loss

Objective

To evaluate the clinical characteristics of simultaneous bilateral sudden sensorineural hearing loss (Si-BSSNHL) as well as its prognostic factors.

Methods

Patients with Si-BSSNHL who were admitted to the Department of Otology Medicine between December 2018 and December 2021 were enrolled in the case group. Propensity score matching (PSM) for sex and age was used to select the control group, which included people who had unilateral sudden sensorineural hearing loss (USSNHL) during the same time period. Hearing recovery, audiological examinations, vestibular function assessments, laboratory tests, and demographic and clinical manifestations were analyzed for intergroup comparisons. Binary logistic regressions were used for both univariate and multivariate analyses of Si-BSSNHL prognostic factors.

Results

Before PSM, the Si-BSSNHL and USSNHL groups differed significantly (p < 0.05) in terms of time from onset to treatment, initial pure-tone average (PTA), final PTA, hearing gain, audiogram curve type, proportion of tinnitus, high-density lipoprotein level, homocysteine level, and effective rate. After PSM, significant differences were also observed in time from onset to treatment, initial PTA, final PTA, hearing gain, total and indirect bilirubin levels, homocysteine level, and effective rate between the two groups (p < 0.05). There was a significant difference in the classification of therapeutic effects between the two groups (p < 0.001). For prognostic analysis, the audiogram curve type was significantly different between the effective group and the ineffective groups of Si-BSSNHL (p = 0.01), in which the sloping type was an independent risk factor for the prognosis of the right ear in Si-SSNHL (95% confidence interval, 0.006-0.549, p = 0.013).

Conclusion

Patients with Si-BSSNHL had mild deafness, elevated total and indirect bilirubin and homocysteine levels, and poorer prognosis than those with USSNHL. Audiogram curve type was linked to the therapeutic effect of Si-BSSNHL, and the sloping type was an independent risk factor for a poor prognosis in the right ear of Si-SSNHL.

Update on Findings about Sudden Sensorineural Hearing Loss and Insight into Its Pathogenesis

Sudden sensorineural hearing loss (SSNHL) is routinely encountered and is one of the most common emergent diseases in otolaryngology clinics. However, the etiology of SSNHL remains unclear. Due to the inaccessibility of the living human inner ear for biopsy, studies investigating the etiology of SSNHL have been performed by analyzing data obtained from examinations using peripheral blood or imaging. We updated the findings obtained from serological, magnetic resonance imaging, genetic, and viral examinations to reveal the etiology of SSNHL. Regarding viral examination, we focused on sensorineural hearing loss associated with coronavirus disease (COVID-19) because the number of correlated reports has been increasing after the outbreak. The updated findings revealed the following three possible mechanisms underlying the development of SSNHL: thrombosis and resulting vascular obstruction in the cochlea, asymptomatic viral infection and resulting damage to the cochlea, and cochlear inflammation and resulting damage to the cochlea. Thrombosis and viral infection are predominant, and cochlear inflammation can be secondarily induced through viral infection or even thrombosis. The findings about sensorineural hearing loss associated with COVID-19 supported the possibility that asymptomatic viral infection is one of the etiologies of SSNHL, and the virus can infect inner ear tissues and directly damage them.

Thromboelastography Predicting the Prognosis of Sudden Sensorineural Hearing Loss

OBJECTIVES

To explore the association between thromboelastography and the clinical features as well as the prognosis of sudden sensorineural hearing loss (SSNHL).

DESIGN

Single-centre, retrospective study.

SETTING

A hospital in China PARTICIPANTS: In total, 133 patients presenting with SSNHL within 2 weeks before the study, who did not receive treatment.

MAIN OUTCOME MEASURES

The patients' medical history was collected. Audiological, imaging, and haematological examinations were performed before treatment. Patients with abnormal thromboelastography were re-examined 1 week after treatment, and efficacy was evaluated 1 month after treatment. The chi-squared test and binary logistic regression analysis were used to analyse the association between factors, such as vertigo, degree and type of hearing loss, vestibular function, inner ear MRI results, thromboelastography, and efficacy of treatment.

RESULTS

Thromboelastography was correlated with prognosis (P=0.049) and degree (P=0.030) and type of hearing loss (P=0.013) in patients with SSNHL. The R (P=0.002) and angle values (P=0.010) correlated with prognosis. The MA (P=0.022) and G values (P=0.020) correlated with the degree of hearing loss. The R (P=0.033) value correlated with inner ear MRI results, and the ΔG (P=0.010) value correlated with fibrinogen levels. ΔThromboelastography (P=0.032) was correlated with the prognosis of patients with abnormal thromboelastography results. Logistic regression analysis showed that thromboelastography correlated with prognosis (P=0.013), and Δthromboelastography correlated with the prognosis of patients with abnormal thromboelastography results (P=0.013) and vertigo (P=0.016).

CONCLUSION

Thromboelastography is an independent risk factor affecting the prognosis of SSNHL with the R and angle values playing a major role.

LDLR expression in the cochlea suggests a role in endolymph homeostasis and cochlear amplification

There is now growing evidence that hypercholesterolemia and high serum levels of low-density lipoproteins (LDL) predispose to sensorineural hearing loss. Circulating LDL-cholesterol is delivered to peripheral tissues via LDL receptor (LDLR) -mediated endocytosis. Recently, it has been shown that LDLR gene polymorphisms are associated with higher susceptibility to sudden deafness. These findings suggested that we should investigate the expression of LDLR from the postnatal maturation of the mouse cochlea until adulthood. In the cochlea of newborn mice, we observed that LDLR is mostly expressed in the lateral wall of the cochlea, especially in a band of cells directly facing the cochlear duct. Moreover, LDLR is expressed in the inner and outer hair cells, as well as in the adjacent greater epithelial ridge. In early postnatal stages, LDLR is expressed in the marginal cells of the immature stria vascularis, in the root cells of the spiral ligament, and in the adjacent outer sulcus cells. At the same time, LDLR begins to be expressed in the pillar cells of the immature organ of Corti. From the onset of hearing, LDLR is expressed in the marginal cells of the stria vascularis, in the outer sulcus cells, and in the capillaries of the adjacent spiral ligament. In the organ of Corti, LDLR is expressed in outer pillar cells and Deiters' cells, i.e. in the non-sensory supporting cells that directly surround the outer hair cells. These cells are believed to provide a mechanical coupling with the outer hair cells to modulate electromotility and cochlear amplification. In the stria vascularis of three-month-old mice, LDLR is further expressed in both marginal and intermediate cells. Overall, our results suggest that LDLR is mostly present in cochlear cells that are involved in endolymph homeostasis and cochlear amplification. Further functional studies will be needed to unravel how LDLR regulates extracellular and intracellular levels of cholesterol and lipoproteins in the cochlea, and how it could influence cochlear homeostasis.