Ototoxicity of Divalent Metals

Zebrafish neuromast sensory system: Is it an emerging target to assess environmental pollution impacts?

Environmental pollution poses risks to ecosystems. Among these risks, one finds neurotoxicity and damage to the lateral line structures of fish, such as the neuromast and its hair cells. Zebrafish (Danio rerio) is recommended as model species to be used in ecotoxicological studies and environmental biomonitoring programs aimed at assessing several biomarkers, such as ototoxicity. However, little is known about the history of and knowledge gaps on zebrafish ototoxicity. Thus, the aim of the current study is to review data available in the scientific literature about using zebrafish as animal model to assess neuromast toxicity. It must be done by analyzing the history and publication category, world production, experimental design, developmental stages, chemical classes, neuromasts and hair cell visualization methods, and zebrafish strains. Based on the results, number, survival and fluorescence intensity of neuromasts, and their hair cells, were the parameters oftentimes used to assess ototoxicity in zebrafish. The wild AB strain was the most used one, and it was followed by Tübingen and transgenic strains with GFP markers. DASPEI was the fluorescent dye most often applied as method to visualize neuromasts, and it was followed by Yo-Pro-1 and GFP transgenic lines. Antibiotics, antitumorals, metals, nanoparticles and plant extracts were the most frequent classes of chemicals used in the analyzed studies. Overall, pollutants can harm zebrafish's mechanosensory system, as well as affect their behavior and survival. Results have shown that zebrafish is a suitable model system to assess ototoxicity induced by environmental pollution.

Occupational Lead Exposure Ototoxicity Evaluated With Distortion-Product Otoacoustic Emissions

OBJECTIVES

To evaluate the extent of hearing loss among pottery workers in Mexico exposed to lead.

DESIGN

The authors conducted a cross-sectional study including 315 adult pottery workers. Auditory function was evaluated by air conduction pure-tone audiometry (pure-tone average) and distortion-product otoacoustic emission (DPOAE) levels (amplitude and signal-to-noise ratio). Lead exposure was assessed with a single blood sample test and classified as low, medium, and high according to blood lead tertiles. Logistic regression models were calculated for the association between blood lead levels, pure-tone average, and DPOAE records.

RESULTS

Median (25th-75th) blood lead levels were 14 μg/dL (7.5-22.6 μg/dL). The audiometric pattern and DPOAE records were similar across blood lead levels groups in all frequencies, and no statistically significant differences were found. Adjusted logistic regression models showed no increase in the odds for hearing thresholds >25 dB (HL) and DPOAE absence associated with blood lead levels, and no dose-response pattern was observed ( p > 0.05).

CONCLUSIONS

Given the results from this cross-sectional study, no association was found between blood lead levels and hearing loss assessed with DPOAE. Future longitudinal work should consider chronic lead exposure estimates among underrepresented populations, which can potentially inform safer work practices to minimize the risk of ototoxicity.

Independent and Combined Associations of Blood Manganese, Cadmium and Lead Exposures with the Systemic Immune-Inflammation Index in Adults

Manganese (Mn), cadmium (Cd) and lead (Pb) have toxic effects on the immune system. However, their independent and combined effects on immune-inflammation responses are unclear. In recent years, the systemic immune-inflammation index (SII) has been developed as an integrated and novel inflammatory indicator. A retrospective cross-sectional study of 2174 adults ≥20 years old from the National Health and Nutrition Examination Survey (NHANES) 2015-2016 was conducted. Generalized linear models were used to evaluate the independent and combined associations of SII with blood Mn, Cd and Pb levels. As continuous variables, both blood Cd and Mn showed dose-dependent relationships with the SII before and after adjusting for all potential confounding factors. Metal concentrations were then converted into categorical variables. Compared with the adults in the lowest Cd or Mn tertile, those in the highest tertile had higher risks of elevated SII. Furthermore, co-exposure to Mn and Cd also showed a positive relationship with the SII after adjusting for all confounding factors. However, the single effect of Pb exposure and the joint effect of Pb and other metal exposures on the SII were not observed. This study provides important epidemiological evidence of the associations of SII with single and co-exposure effects of blood Mn, Cd, and Pb.

Developmental cadmium exposure disrupts zebrafish vestibular calcium channels interfering with otolith formation and inner ear function

Dizziness or balance problems are estimated to affect approximately 3.3 million children aged three to 17 years. These disorders develop from a breakdown in the balance control system and can be caused by anything that affects the inner ear or the brain, including exposure to environmental toxicants. One potential environmental toxicant linked to balance disorders is cadmium, an extremely toxic metal that occurs naturally in the earth's crust and is released as a byproduct of industrial processes. Cadmium is associated with balance and vestibular dysfunction in adults exposed occupationally, but little is known about the developmental effects of low-concentration cadmium exposure. Our findings indicate that zebrafish exposed to 10-60 parts per billion (ppb) cadmium from four hours post-fertilization (hpf) to seven days post-fertilization (dpf) exhibit abnormal behaviors, including pronounced increases in auditory sensitivity and circling behavior, both of which are linked to reductions in otolith growth and are rescued by the addition of calcium to the media. Pharmacological intervention shows that agonist-induced activation of the P2X calcium ion channel in the presence of cadmium restores otolith size. In conclusion, cadmium-induced ototoxicity is linked to vestibular-based behavioral abnormalities and auditory sensitivity following developmental exposure, and calcium ion channel function is associated with these defects.

Combined effects of multiple metals on hearing loss: A Bayesian kernel machine regression approach

BACKGROUND

Exposure to heavy elements is considered one of the risks of hearing loss. The combined effect of multiple metals on different hearing losses has not been extensively studied.

METHODS

This study deals with the association between different metals (Ba, Cd, Co, Cs, Mo, Pb, Mn, Sn, Sb, Tl, W) and hearing loss in the 2013-2018 National Health and Nutrition Examination Survey (NHANES). Associations were estimated by a generalized linear regression model (GLM) adjusting for age, gender, race/ethnicity, educational level, marital status, drinking status, hypertension, diabetes, smoking status, noise exposure, body mass index, and income-poverty ratio. The joint effects of mixed exposure were assessed by weighted quantile sum (WQS) model and Bayesian kernel machine regression (BKMR). The effect of multiple metals on speech-frequency hearing loss (SFHL) and high-frequency hearing loss (HFHL) was further calculated. The odds ratio (OR) indicates the risk of every metal exposure.

RESULTS

A total of 2205 adult participants were enrolled from NHANES. Cd, Pb, Sb and Sn had significant effects on total hearing loss. The WQS model found that mixed exposure to heavy elements was significantly positively associated with total hearing loss (OR: 1.136; 95% CI: 1.031, 1.253) after adjusting for various covariates. The ORs of mixed exposure in the SFHL and HFHL groups were 1.066 (95% CI: 0.994, 1.143) and 1.102 (95% CI: 1.013, 1.199), respectively. BKMR found a significant positive association between multiple metals and hearing loss. The results showed that there may be potential interactions between Cd, Pb and other metals.

CONCLUSIONS

Multiple metals have joint effects on hearing loss in the United States. The findings provide practitioners with important scientific evidence for possible interventions.

Metformin attenuates cadmium-induced degeneration of spiral ganglion neuron via restoring autophagic flux in primary culture

Cadmium (Cd), a common environmental and occupational toxicant, is an important risk factor for hearing loss. After exposure, Cd accumulates in the inner ear and induces spiral ganglion neuron (SGN) degeneration; however, the underlying mechanisms are poorly understood. Dysfunctional autophagy has been implicated in many neurodegenerative diseases, including Cd-induced neurotoxicity. Metformin has been validated to confer not only anti-hyperglycaemic but also neuroprotective effects. However, the relationship between autophagy dysfunction, SGN degeneration, and the effect of metformin on Cd-induced SGN neurotoxicity has not yet been established. In this study, we demonstrate that metformin notably attenuates Cd-evoked SGN degeneration by restoring impaired autophagy flux, as evidenced by the suppression of Cd-induced elevation of autophagy markers microtubule-associated protein 1A/1B-light chain 3-II (LC3-II) and autophagy substrate protein p62 in degenerated SGN. Blockage of autophagy flux by chloroquine abolished metformin-induced neuroprotection against Cd-induced neurotoxicity in SGN. The results of this study reveal that autophagy dysfunction is an important component of Cd-induced SGN degeneration, and metformin may be a potential protective agent for attenuating SGN degeneration following Cd exposure.

Association of plasma lead, cadmium and selenium levels with hearing loss in adults: National Health and Nutrition Examination Survey (NHANES) 2011-2012

To determine the association between hearing loss and environmental Pb, Cd and Se exposure, a total of 1503 American adults from National Health and Nutrition Examination Survey (NHANES) (2011-2012) were assessed. The average of four audiometric frequencies (0·5, 1, 2 and 4 kHz) was used to identify speech-frequency hearing loss (SFHL), while the average of 3 audiometric frequencies (3, 4 and 6 kHz) was used to identify high-frequency hearing loss (HFHL). HFHL adjusted OR determined by comparing the highest and lowest blood Pb and Cd quartiles were 1·98 (95 % CI: 1·27, 3·10) and 1·81 (95 % CI: 1·13, 2·90), respectively. SFHL was significantly associated with blood Cd with the OR = 2·42 for the highest quartile. When further stratified by age, this association appeared to be limited to adults aged 35-52 years. After stratified by gender, except for Pb and Cd, we observed that blood Se showed a dose-dependent association with SFHL in men. In women, only Cd showed a dose-dependent association with speech and high-frequency hearing loss. Hearing loss was positively associated with blood levels of Pb and Cd. Additionally, our study provided novel evidence suggesting that excessive Se supplement would increase SFHL risk in men.

Health repercussions of environmental exposure to lead: Methylation perspective

Lead (Pb) exposure has been a major public health concern for a long time now due to its permanent adverse effects on the human body. The process of lead toxicity has still not been fully understood, but recent advances in Omics technology have enabled researchers to evaluate lead-mediated alterations at the epigenome-wide level. DNA methylation is one of the widely studied and well-understood epigenetic modifications. Pb has demonstrated its ability to induce not just acute deleterious health consequences but also alters the epi-genome such that the disease manifestation happens much later in life as supported by Barkers Hypothesis of the developmental origin of health and diseases. Furthermore, these alterations are passed on to the next generation. Based on previous in-vivo, in-vitro, and human studies, this review provides an insight into the role of Pb in the development of several human disorders.

The Role of Manganese, Cadmium, Chromium and Selenium on Subjective Tinnitus

Elevated levels of heavy metals like cadmium (Cd) and manganese (Mn) are known to lead to oxidative damage-related oto-toxicity and decreased levels of chromium (Cr) and selenium (Se) are known to lead to oto-toxicity due to reduced anti-oxidant activity. The aim of the present study was to evaluate serum levels of Cd, Mn, Cr, and Se and their relationship with tinnitus. A total of 48 patients with tinnitus (Group 1) and 40 healthy controls (Group 2) were included in the study. All participants were applied audiology tests. Severity of tinnitus was measured with Tinnitus Severity Index Questionnaire (TSIQ) in group 1. Serum Mn, Cd, Cr, and Se measurements were done by using The Agilent ICP-MS system consisted of a 7700 coupled plasma mass spectrometry (ICP-MS). Serum Cd, Mn, and Cr levels were higher in group 1 and Se level was lower in group 1 than that of group 2. We may conclude that Cd, Mn, Cr, and Se levels could play an important role in etio-pathogenesis of tinnitus, and thereby supplementation or reduction of these elements could be considered as novel therapeutic goals.

Prestin and otolin-1 proteins in the hearing loss of adults chronically exposed to lead

BACKGROUND

Some studies in animal models and humans suggest that exposure to lead is associated with hearing loss. Lead can reach the inner ear through the blood circulation; evidence suggests that lead could accumulate in the inner ear, causing inner ear damage.

AIM

To evaluate prestin and otolin-1 protein levels and their relationship with an increased hearing threshold in participants exposed to lead.

METHODS

We conducted a cross-sectional study with 315 participants from Tlaxcala, Mexico. Blood lead levels (BPb) were evaluated by graphite furnace atomic absorption spectrometry. Serum prestin and otolin-1 were quantified using ELISA. Auditory function at frequencies of 0.125 to 8 kHz was evaluated in a soundproof chamber.

RESULTS

Participants were classified according to BPb: group I (<10 μg/dL) had a median BPb of 6 μg/dL and prestin levels of 11.06 ng/mL. While participants in group II (≥10 μg/dL) had a median of BPb 20.7 μg/dL (p < 0.05) and prestin levels of 0.15 ng/mL (p < 0.001). Participants in both groups showed a normal hearing. Otolin-1 levels were higher for participants with normal hearing and lower for participants with hearing loss in both groups, p > 0.05. Multiple linear regression models predict an average decrease of 0.17 to 0.26 ng/mL in prestin levels per decibel increase for the frequencies evaluated.

CONCLUSIONS

Participants with high BPb showed an increase in hearing threshold, and prestin levels decreased proportionally to the hearing threshold increase. This is the first study to evaluate prestin as a potential biomarker for hearing damage, evaluated by audiometry, in participants with lead exposure.

Effect of noise and ototoxicants on developing standard threshold shifts at a U.S. Air Force depot level maintenance facility

Noise exposure has traditionally been considered the primary risk factor for hearing loss. However, ototoxicants commonly found in occupational settings could affect hearing loss independently, additively, or synergistically when combined with noise exposures. The purpose of this investigation was to determine the combined effect of metal and solvent ototoxicants, continuous noise, and impulse noise on hearing loss. Noise and ototoxicant exposure and pure-tone audiometry results were analyzed for U.S. Air Force personnel (n = 2,372) at a depot-level aircraft maintenance activity at Tinker Air Force Base, Oklahoma. Eight similar exposure groups based on combinations of ototoxicant and noise exposure were created including: (1) Continuous noise (reference group); (2) Continuous noise + Impulse noise; (3) Metal exposures + Continuous noise; (4) Metal exposures + Continuous noise + Impulse noise; (5) Solvent exposure + Continuous noise; (6) Solvent exposures + Continuous noise + Impulse noise; (7) Metal exposure + Solvent exposures + Continuous noise; and (8) Metal exposure + Solvent exposures + Continuous noise + Impulse noise. Hearing loss was assessed at center octave band frequencies of 500-6,000 Hz and using National Institute for Occupational Safety and Health Standard Threshold Shift (STS) criteria. Hearing changes were significantly worse at 2,000 Hz in the Metal exposure + Solvent exposure + Continuous noise group compared to the Continuous noise only reference group (p = 0.023). The Metal exposure + Solvent exposure + Continuous noise group had a significantly greater relative risk (RR) of 2.44; 95% CI [1.24, 4.83] for developing an STS at 2,000 Hz. While not statistically significant, the Solvent exposure + Continuous noise group had a RR of 2.32; 95%CI [1.00, 5.34] for developing an STS at 1,000 Hz. These results indicate that noise exposure may dominate hearing loss at ≥3,000 Hz while combined effects of concomitant exposure to ototoxic substances and noise are only noticeable at ≤2,000 Hz. These results also suggest combined exposures to ototoxicants and noise presents a greater hearing loss risk than just noise.

The ototoxic potential of cobalt from metal-on-metal hip implants: a pilot study on the patient-reported auditory, vestibular, and general neurological outcome

OBJECTIVE

This study aimed to systematically investigate the ototoxic potential of cobalt in patients with a metal-on-metal (MoM) hip implant, using objective auditory and vestibular assessments and a questionnaire. The results of the objective evaluation were published previously, whereas the current study focused on the questionnaire outcome and its relationship to the blood cobalt level. Design and study sample: Twenty patients (33-65 years) with a primary MoM hip implant and 20 non-implanted control subjects, matched for age, gender, and noise exposure, received a questionnaire to evaluate the presence of several hearing and balance symptoms (part 1) and general neurological issues (part 2).

RESULTS

Concerning part 1, the proportion of auditory-related symptoms in general (p = 0.022) and tinnitus (p = 0.047) was significantly higher in the MoM patient group, whereas no group difference was found for hyperacusis, increased listening effort, and decreased speech understanding. Concerning part 2, no significant group differences were detected. Within the MoM patient group, the questionnaire outcome was not significantly different between the low-exposure and high-exposure subgroups according to the blood Co level.

CONCLUSIONS

In line with our previous study, these results potentially imply Co-induced impairment to the auditory system, despite the lack of a clear dose-response relationship.

Environmental Exposures and Hearing Loss

Pollutants that contaminate the natural or built environment adversely affect the health of living organisms. Although exposure to many of them could be avoided or minimized by careful preventive measures, it is impossible to totally avoid exposure to all pollutants. Ototraumatic agents, such as noise, chemicals, and heavy metals, are pervasive pollutants, mostly produced by human activity, and are critical factors in inducing acquired hearing loss. More importantly, exposure to these pollutants often occurs concurrently and, therefore, the synergistic interactions potentiate auditory dysfunction in susceptible individuals. Epidemiological studies have provided compelling data on the incidence of auditory dysfunction after exposure to a number of ototraumatic agents in the environment, while animal studies have offered crucial insights for understanding the underlying molecular mechanisms. Together, they provide a framework for developing effective interventional approaches for mitigating the adverse impacts of environmental or occupational exposure to ototraumatic agents. This article provides a brief overview of the common pollutants that cause hearing loss.

2-Hydroxypropyl-β-cyclodextrin Ototoxicity in Adult Rats: Rapid Onset and Massive Destruction of Both Inner and Outer Hair Cells Above a Critical Dose

2-Hydroxypropyl-β-cyclodextrin (HPβCD), a cholesterol chelator, is being used to treat diseases associated with abnormal cholesterol metabolism such as Niemann-Pick C1 (NPC1). However, the high doses of HPβCD needed to slow disease progression may cause hearing loss. Previous studies in mice have suggested that HPβCD ototoxicity results from selective outer hair cell (OHC) damage. However, it is unclear if HPβCD causes the same type of damage or is more or less toxic to other species such as rats, which are widely used in toxicity research. To address these issues, rats were given a subcutaneous injection of HPβCD between 500 and 4000 mg/kg. Distortion product otoacoustic emissions (DPOAE), the cochlear summating potential (SP), and compound action potential (CAP) were used to assess cochlear function followed by quantitative analysis of OHC and inner hair cell (IHC) loss. The 3000- and 4000-mg/kg doses abolished DPOAE and greatly reduced SP and CAP amplitudes. These functional deficits were associated with nearly complete loss of OHC as well as ~ 80% IHC loss over the basal two thirds of the cochlea. The 2000-mg/kg dose abolished DPOAE and significantly reduced SP and CAP amplitudes at the high frequencies. These deficits were linked to OHC and IHC losses in the high-frequency region of the cochlea. Little or no damage occurred with 500 or 1000 mg/kg of HPβCD. The HPβCD-induced functional and structural deficits in rats occurred suddenly, involved damage to both IHC and OHC, and were more severe than those reported in mice.

Environmental exposure to lead and cadmium and hearing loss in Chinese adults: A case-control study

Hearing loss is the second most common nonfatal problem affecting the Chinese population. Historical studies have suggested an association between exposure to heavy metals, such as cadmium and lead, and hearing loss. Few studies have investigated this relationship in the general population in China. We conducted a case-control study with 1008 pairs of participants from a cross-sectional epidemiological survey conducted in Zhejiang Province. A self-designed questionnaire was adopted to collect information on demographics, chronic diseases, lifestyles and environmental noise. Pure-tone averages of hearing thresholds at frequencies of 0.5, 1, 2, and 4 kHz were computed. Blood lead and cadmium levels were analyzed with an atomic absorption spectrometer. After adjusting for all other potential confounding factors, compared with the lowest blood cadmium quartile (0.00-0.53 μg/L), blood cadmium quartile 2 (0.54-0.92 μg/L), quartile 3 (0.93-1.62 μg/L) and quartile 4 (1.63-57.81 μg/L) exhibited significantly elevated risks for hearing loss, with odds ratios of 1.932 (95% CI: 1.356-2.751), 2.036 (95% CI: 1.423-2.914) and 1.495 (95% CI: 1.048-2.133), respectively (P-trend<0.001). However, an association of lead with hearing loss was not found. Young age (less than 60 years), male sex and current smoking were associated with increased blood cadmium concentration. Additionally, a positive association between blood cadmium and lead concentrations was found. Therefore, we conclude that exposure to environmental cadmium may be a risk factor for hearing loss among the general population in China.

Hearing loss risk and DNA methylation signatures in preschool children following lead and cadmium exposure from an electronic waste recycling area

Experimental studies have uncovered chemical exposure-induced ototoxicity, but population-based hearing risk assessment especially for early-life exposure to heavy metals and relevant biological mechanism remains unclear. We aimed to measure lead (Pb) and cadmium (Cd) levels, blood DNA methylations of Rb1, CASP8 and MeCP2 and hearing in 116 preschool children 3- to 7-years of age from an e-waste and a reference area, and to evaluate the association of exposures with hearing loss potentially affected by epigenetic modifications. A higher median Pb level but not Cd was found in the exposed group than the reference group. Average hearing thresholds in either ear of the exposed children were higher. Higher promoter methylation levels at cg02978827 and position +14, and lower at position +4 of Rb1 were found in the exposed group. Pb was positively correlated with chewing pencil habit while negatively correlated with washing hands before dinner. Slightly negative trends of promoter methylations in Rb1 and CASP8, while a strong positive trend of MeCP2 promoter methylation, were found along with increasing Pb and Cd levels. Logistic analyses showed the adjusted OR of Pb for hearing loss in the left ear and both ears was 1.46 (95% CI: 1.12, 1.91) and 1.40 (95% CI: 1.06, 1.84), respectively. Our results show an elevated Pb level, altered promoter DNA methylations and hearing ability in children of e-waste areas, suggesting that epigenetic changes of specific genes involves in the development of the auditory system during early exposure to environmental chemicals.

The role of zinc chelate of hydroxy analogue of methionine in cadmium toxicity: effects on cadmium absorption on intestinal health in piglets

Cadmium (Cd) is a toxic heavy metal that poses a threat to the health of humans and animals. It can cause serious damage to the small intestine, which is the main absorption site of Cd and the primary target organ after oral administration. Our previous study found that zinc chelate of hydroxy analogue of methionine (Zn-HMTB), a new type of feed additive, decreased Cd accumulation in the liver and kidneys. The aim of this study was to investigate the effect of Zn-HMTB on Cd absorption and Cd-induced toxicity in the small intestine of piglets. Twenty-four piglets (Landrace × Large White, 13.22 ± 0.58 kg BW) were randomly divided into four dietary treatment groups: basal diet, and diets containing 30 mg/kg Cd from CdCl2 and 0, 100 or 200 mg/kg Zn from Zn-HMTB. The experiment lasted 27 days. The feed intake and final BW of each piglet were recorded at the end of the experiment. Gastrointestinal (GI) tract tissue and samples of liver, kidney, spleen, heart, lung and longissimus muscle tissue and faeces were collected. The concentrations of Cd and metal trace elements in the GI tract and organs were analysed, as was the relative messenger RNA (mRNA) expression of inflammatory cytokines and metal element transporters in the small intestine, and epithelial apoptosis in the small intestine. The results showed that, compared with Cd-treated piglets, piglets in the Zn-HMTB and Cd cotreatment groups had less Cd deposition in the stomach, ileum, caecum, colon, liver, kidneys, spleen, lungs, heart and muscles (P < 0.05), and lower Cd concentrations in faeces (P < 0.05), suggesting that Zn-HMTB increased Cd absorption and the excretion of Cd in other forms (possibly urine). Zinc chelate of hydroxy analogue of methionine increased Zn deposition in the jejunum and the relative mRNA expression of divalent metal transporters 1 and zinc transporter 5 in the duodenum (P < 0.05), indicating that Zn-HMTB may promote the absorption and transportation of Cd and Zn together by upregulating metal element transporters. Competition between Zn and Cd may be responsible for accelerating Cd excretion. Furthermore, Zn-HMTB reduced Cd-induced apoptosis of enterocytes and inflammatory stimuli in the small intestine, suggesting that Zn-HMTB reduced Cd-induced toxicity to the small intestine. These results suggest that Zn-HMTB can be helpful in decreasing Cd accumulation in the GI tract and organs of piglets and relieving Cd-induced toxicity to the small intestine but cannot reduce the absorption of Cd.

Association between cadmium exposure and hearing impairment: a population-based study in Korean adults

Background

The present study aimed to evaluate the clinical association between cadmium exposure and hearing impairment among the Korean population.

Methods

This retrospective cross-sectional study used the data obtained from the Korean National Health and Nutrition Examination Survey were used for our study. Finally, 3,228 participants were included in our study, which were then divided into quartiles based on their blood cadmium levels: first quartile (1Q), second quartile (2Q), third quartile (3Q), and fourth quartile (4Q) groups. The hearing thresholds were measured using an automatic audiometer at 0.5, 1, 2, 3, 4, and 6 kHz. Hearing loss (HL) was defined as >25 dB average hearing threshold (AHT).

Results

All the groups had 807 participants each. The area under the receiver operating characteristic curves of cadmium level for HL were 0.634 (95% confidence interval [CI], 0.621–0.646). The participants in the 4Q group had higher Low/Mid-Freq, High-Freq, and AHT values than those in the other groups in the multivariate analysis after adjusting for confounding factors. The logistic regression showed that the OR for HL per 1 µg/L increase in cadmium was 1.25 (95% CI, 1.09–1.44; p=0.002) on the multivariate analysis. Moreover, the multivariate logistic regression analyses revealed that the participants in the 4Q group exhibited a 1.59-, 1.38-, and 1.41-fold higher odds for HL than those in the 1Q, 2Q, and 3Q groups, respectively.

Conclusion

High cadmium level quartile was associated with increased hearing thresholds and HL among the Korean adult population.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ₁₀, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.

Environmental ototoxicants, a potential new class of chemical stressors

Hearing loss is an injury that can develop over time, and people may not even be aware of it until it becomes a severe disability. Ototoxicants are substances that may damage the inner ear by either affecting the structures in the ear itself or by affecting the nervous system. We have examined the possibility that ototoxicants may present a health hazard in association with environmental exposures, adding to existing knowledge of their proven hazards under medical therapeutic conditions or occupational activities. In addition to the already described human environmental ototoxicants, mainly organochlorines such as polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethylene (DDE), hexachlorocyclohexane (HCH) and hexachlorobenzene (HCB), we have examined the ubiquitous chemical stressors phthalates, bisphenol A/S/F/, PFCs, flame retardants (FRs) and cadmium for potential ototoxic properties, both as single substances or as chemical mixtures. Our literature review confirmed that these chemicals may disturb thyroid hormones homeostasis, activate aryl hydrocarbon receptor (AhR), and induce oxidative stress, which in turn may initiate a chain of events resulting in impairment of cochlea and hearing loss. With regard to auditory plasticity, diagnostics of a mixture of effects of ototoxicants, potential interactions of chemical and physical agents with effects on hearing, parallel deterioration of hearing due to chemical exposures and ageing, metabolic diseases or obesity, even using specific methods as brainstem auditory evoked potentials (BAEP) or otoacoustic emissions (OAEs) registration, may be difficult, and establishment of concentration-response relationships problematic. This paper suggests the establishment of a class of environmental oxotoxicants next to the established classes of occupational and drug ototoxicants. This will help to properly manage risks associated with human exposure to chemical stressors with ototoxic properties and adequate regulatory measures.

Factors Associated With High Blood Lead Levels in a Sample of 100 Children in Tehran

Background:

Lead is considered a neurotoxic agent. We aimed to evaluate the blood lead level (BLL) in young population and determine probable risk factors of lead exposure in Iran.

Methods:

In a cross-sectional study, a total of 100 children were entered and their BLLs were checked.

Results:

In all, 25 and 8 patients had BLLs above 5 and 10 µg/dL, respectively. There was a significant univariate correlation between BLL and place of living, water pipe type, using dairy products, and stature in both cut-offs of 5 and 10 µg/dL. Binary regression analysis showed that pipe type was associated with high BLLs at cut-offs of 5 and 10 µg/dL, respectively. Also, there was an association between 50th percentile of stature for age and cut-off of 5 µg/dL.

Conclusions:

Higher BLLs may be seen in short stature pediatric population. Polyvinyl chloride (PVC) and polypropylene water pipes may even cause more release of lead and result in higher absorption of this metal in the pediatric population.

I’m Wearing My Hearing Protection – Am I Still At Risk for Hearing Loss? Lurking Ototoxins in the Military Environment

Objective

Information is summarized from the overall body of published literature regarding ototoxic chemicals encountered outside of clinical exposures, largely in occupational settings. While summarizing the most common non-pharmaceutical ototoxins, this review provides clinically relevant information and recommendations such that hearing health professionals may adopt a more comprehensive and appropriate diagnostic case history, test battery, documentation scheme, and education delivery.

Methods

Solvents, metals, and asphyxiants literature was reviewed using PubMed, national and international agency websites, and communications with known ototoxicity experts.

Results

Initial intentions to summarize the existing programs for occupational ototoxicity monitoring fell short when it was discovered that such programs have not yet formalized across the major oversight agencies in the United States. Instead, recommended guidance documents and fact sheets, which highlight existing occupational exposure limits and suggest monitoring and education are discussed.

Conclusions

While evidence in humans is limited, potentially ototoxic substances are worthy of improved surveillance and further research to understand their ototoxic mechanisms, effects, and possible mitigation strategies. A triad approach of monitoring, protecting, and educating is recommended for effective prevention of hearing loss: the Department of Defense Hearing Center of Excellence’s Comprehensive Hearing Health Program model employs such an approach.

Influence of In Vitro Electrical Stimulation on Survival of Spiral Ganglion Neurons

Patients scheduled for cochlear implantation often retain residual hearing in the low frequencies. Unfortunately, some patients lose their residual hearing following implantation and the reasons for this are not well understood. Evidence suggests that electrotoxicity could be one of the factors responsible for this late adverse effect. Therefore, the aim of this study was to investigate the survival of spiral ganglion neurons (SGN) subjected to in vitro electrical stimulation (ES). A stimulation setup was developed to provide defined electrical fields at given points of the chamber. SGN isolated from Sprague Dawley rats (P3-4) were dissociated and cultured in the chamber for 24 h prior to biphasic, pulsed electrical field exposure for another 24 h. The current varied in the range of 0 to 2 mA and the pulse width from 10 to 400 μs. Neurite growth and survival were evaluated with respect to the charge density at the position of the cells. Non-exposed SGN cultures served as control. Charge densities below 2.2 μC·cm^-2·phase^-1 appeared to have no effect on SGN survival and neurite outgrowth. Charge densities above 4.9 μC·cm^-2·phase^-1 were detrimental to almost all cells in culture. After fitting results to a sigmoidal dose response curve, a LD₅₀ of 2.9 μC·cm^-2·phase^-1 was calculated. This screening regarding survival and outgrowth of SGN provides parameters that could be used to further investigate the effect of ES on SGN and to develop possible protection strategies, which could potentially rescue residual hearing in the implanted patients.

Auditory and Histopathological Effects of Topical Mercurochrome Treatment in Rats with Tympanic Membrane Perforation

OBJECTIVES

Topical treatment is first choice in the treatment of uncomplicated chronic otitis media. It was intended to assess auditory and histopathological safety of ototopical use of mercurochrome solution in rats with induced tympanic membrane perforation.

MATERIALS AND METHODS

The study was conducted on 21 female Wistar-Albino rats which were randomly assigned into 3 groups. In all rats, perforation was performed at right tympanic membrane. Distortion product otoacoustic emissions (DPOAEs) measurements were performed at frequencies of 2000, 3000 and 4000 Hz (with L1/L2: 70 /70 dB at 2f1-f2 frequency; f2/f1 ratio: 1:22) before recovery from anesthesia and signal-to-noise ratio (SNR) were recorded. Normal saline, 2% mercurochrome and gentamicin were given to group 1, 2 and 3 twice daily over a week, respectively. Rats were sacrificed after DPOAE measurements on day 14. Right temporal bone specimens were examined under light microscope after processing.

RESULTS

Based on DPOAE results, there was no significant difference among groups before treatment. On day 14, significant differences were found in DPOAE measurements at 3000 and 4000 Hz, and in mean SNR values in 2% mercurochrome and gentamicin groups when compared to normal saline group while no significant difference was detected at 2000 Hz among groups. In addition, significant degeneration was detected in Corti organs, spiral ganglions and stria vascularis in groups 2 and 3.

CONCLUSION

In this study, it was observed that mercurochrome use in external otitis and otitis media with tympanic membrane perforation could cause ototoxicity and concluded that the solution should be used cautiously.

Hearing loss in children with e-waste lead and cadmium exposure

Environmental chemical exposure can cause neurotoxicity and has been recently linked to hearing loss in general population, but data are limited in early life exposure to lead (Pb) and cadmium (Cd) especially for children. We aimed to evaluate the association of their exposure with pediatric hearing ability. Blood Pb and urinary Cd were collected form 234 preschool children in 3-7years of age from an electronic waste (e-waste) recycling area and a reference area matched in Shantou of southern China. Pure-tone air conduction (PTA) was used to test child hearing thresholds at frequencies of 0.25, 0.5, 1, 2, 4 and 8kHz. A PTA≥25dB was defined as hearing loss. A higher median blood Pb level was found in the exposed group (4.94±0.20 vs 3.85±1.81μg/dL, p<0.001), while no significance was found for creatinine-adjusted Cd. Compared with the reference group, the exposed group had a higher prevalence of hearing loss (28.8% vs 13.6%, p<0.001). The PTA in the left, right and both ears, and hearing thresholds at average low and high frequency, and single frequency of 0.5, 1 and 2kHz were all increased in the exposed group. Positive correlations of child age and nail biting habit with Pb, and negative correlations of parent education level and child washing hands before dinner with Pb and Cd exposure were observed. Logistic regression analyses showed the adjusted OR of hearing loss for Pb exposure was 1.24 (95% CI: 1.029, 1.486). Our data suggest that early childhood exposure to Pb may be an important risk factor for hearing loss, and the developmental auditory system might be affected in e-waste polluted areas.

Ototoxicity: The Radical Drum Beat and Rhythm of Cochlear Hair Cell Life and Death

The function and structure of the auditory information processing system establishes a unique sensory environment for the "perfect storm." The battle between life and death pits the cascade of an apoptotic storm, programmed cell death cascades, against simple cell death (necrosis) pathways. Live or die, the free radical biology of oxygen and hydroxylation, and the destruction of transition metal migration through the mechanical gate sensory processes of the hair cell lead to direct access to the cytoplasm, cytoplasmic reticulum, and mitochondria of the inner workings of the hair cells. These lead to subsequent interactions with nuclear DNA resulting in permanent hearing loss. The yin and yang of pharmaceutical product development is to document what kills, why it kills, and how do we mitigate it. This review highlights the processes of cell death within the cochlea.

Hearing loss, lead (Pb) exposure, and noise: a sound approach to ototoxicity exploration

To determine the state of the research on ototoxic properties of Pb, evaluate possible synergistic effects with concurrent noise exposure, and identify opportunities to improve future research, we performed a review of the peer-reviewed literature to identify studies examining auditory damage due to Pb over the past 50 years. Thirty-eight studies (14 animal and 24 human) were reviewed. Of these, 24 suggested potential ototoxicity due to Pb exposure, while 14 found no evidence of ototoxicity. More animal studies are needed, especially those investigating Pb exposure levels that are occupationally and environmentally relevant to humans. Further investigations into potential interactions of Pb in the auditory system with other hazards and compounds that elicit ototoxicity are also needed in animal models. To better assess the effects of Pb exposure on the human auditory system and the possibility of a synergism with noise, future epidemiological studies need to carefully consider and address four main areas of uncertainty: (1) hearing examination and quantification of hearing loss, (2) Pb exposure evaluation, (3) noise exposure evaluation, and (4) the personal characteristics of those exposed. Two potentially confounding factors, protective factors and mixtures of ototoxicants, also warrant further exploration.