Intermittent noise-induced hearing loss and the influence of carbon monoxide

The impact of carbon monoxide inhalation on developing noise-induced hearing loss in guinea pigs

Carbon monoxide (CO) poisoning is one of the most common types of fatal poisonings worldwide. Acute exposure to high levels of CO as well as chronic exposure to low levels of CO and excessive noise can lead to high frequency hearing loss. In this study, twelve guinea pigs were randomly divided into two groups: (1) exposed to noise and (2) exposed to noise plus CO. Auditory brainstem responses (ABRs) were measured prior to the experiment and immediately, 5, 10 and 15 days post exposures. There was a significant difference between the ABR thresholds before and immediately after exposure to noise at frequencies of 4, 8, and 16 kHz and the most threshold shift was observed at 8 kHz. There was also a significant difference between the ABR thresholds before and immediately after exposure to noise and CO at frequencies of 2, 4, 8, and 16 kHz which demonstrated a temporary hearing loss after exposure to noise and CO and the major impact of CO on developing noise induced hearing loss occurred at 8 kHz. No significant difference was observed between the ABR thresholds recorded before conducting the experiments and the ones obtained 5, 10 and 15 days after simultaneous exposure to noise and CO at none of frequencies. Simultaneous exposure to noise and CO contributes to transient hearing loss in guinea pigs with the most evident temporary shift at 8 kHz. The methods were accepted in the Ethics Committee of Iran University of Medical Science (registration No. CTRI/2016/01/017170) on January 18, 2016.

Simultaneous exposure to noise and carbon monoxide increases the risk of Alzheimer's disease: a literature review

Dementia is a syndrome of cognitive and functional decline, commonly occurring in later life as a result of neurodegenerative and cerebrovascular processes beginning earlier in the life course. An excess of free radicals has an essential role in neurodegenerative diseases and aging. This paper aims to review the effects of noise and carbon monoxide as a risk factor in Alzheimer's disease as well as the role of free radicals in the progress of Alzheimer's disease. Articles included in this review were identified through a search of the databases PubMed, Scopus, and Google Scholar using the search terms Alzheimer's disease, dementia, noise, reactive oxygen species, and Carbon Monoxide. The literature search was restricted to the years 1982 to 2020 and articles published in the English language. The metabolism rate of the body is very high when exposed to noise and carbon monoxide; this leads to overproduction of reactive oxygen species and oxidative stress conditions. Oxidative stress has an essential role in the mechanisms concerned in Alzheimer's disease. In addition to the consequences of noise and a chemical substance on the auditory system, they also have non-auditory effects that affect the brain and induced neurodegenerative disease.

The combined effects of occupational exposure to noise and other risk factors - a systematic review

PURPOSE

Noise-induced health effects exacerbate by many other risk factors. This systematic review aims at shedding light on the combined effects of co-exposure to occupational noise and other factors.

MATERIAL AND METHODS

A literature search in Web of Science, Scopus, PubMed, Science Direct, and Google Scholar, with appropriate keywords on combined effects of occupational noise, and co-exposure to noise and other factors, revealed 7928 articles which were screened by two researchers. A total of 775 articles were reviewed in full text. We found 149 articles that were relevant and had sufficient quality for analysis.

RESULTS

We identified 16 risk factors that exacerbate occupational noise-induced health effects. These factors were classified into four groups: chemical (carbon monoxide (CO), solvents, heavy metals, and other chemicals), physical (lighting, heat, vibration, and cold), personal (age, gender, genetics, smoking, medication, contextual diseases) and occupational (workload and shift work). Hearing loss, hypertension, reduced performance, and cardiovascular strains, are the most important risk factors combined effects due to concurrent exposure to noise and other risk factors.

CONCLUSION

Evidences of combined effects of solvents, vibration, heavy metals, CO, smoking, chemicals, aging, heat, and shiftwork were respectively stronger than for other factors. Most of the studies have investigated only the combined effects of risk factors on hearing, and the evidence for non-auditory effects is still limited, and more studies are warranted. Therefore, in the Hearing Conservation Programs, besides noise, aggravating factors of noise effects should also be taken into account.

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.

Towards a general framework for including noise impacts in LCA

PURPOSE

Several damages have been associated with the exposure of human beings to noise. These include auditory effects, i.e., hearing impairment, but also non-auditory physiological ones such as hypertension and ischemic heart disease, or psychological ones such as annoyance, depression, sleep disturbance, limited performance of cognitive tasks or inadequate cognitive development. Noise can also interfere with intended activities, both in daytime and nighttime. ISO 14'040 also indicated the necessity of introducing noise, together with other less developed impact categories, in a complete LCA study, possibly changing the results of many LCA studies already available. The attempts available in the literature focused on the integration of transportation noise in LCA. Although being considered the most frequent source of intrusive impact, transportation noise is not the only type of noise that can have a malign impact on public health. Several other sources of noise such as industrial or occupational need to be taken into account to have a complete consideration of noise into LCA. Major life cycle inventories (LCI) typically do not contain data on noise emissions yet and characterisation factors are not yet clearly defined. The aim of the present paper is to briefly review what is already available in the field and propose a new framework for the consideration of human health impacts of any type of noise that could be of interest in the LCA practice, providing indications for the introduction of noise in LCI and analysing what data is already available and, in the form of a research agenda, what other resources would be needed to reach a complete coverage of the problem.

MAIN FEATURES

The literature production related to the impacts of noise on human health has been analysed, with considerations of impacts caused by transportation noise as well as occupational and industrial noise. The analysis of the specialist medical literature allowed for a better understanding of how to deal with the epidemiological findings from an LCA perspective and identify areas still missing dose-response relations. A short review of the state-of-science in the field of noise and LCA is presented with an expansion to other contributions in the field subsequent to the comprehensive work by Althaus et al. (2009a; 2009b). Focusing on the analogy between toxicological analysis of pollutants and noise impact evaluation, an alternative approach is suggested, which is oriented to the consideration of any type of noise in LCA and not solely of transportation noise. A multi-step framework is presented as a method for the inclusion of noise impacts on human health in LCA.

RESULTS AND DISCUSSION

A theoretical structural framework for the inclusion of noise impacts in LCA is provided as a basis for future modelling expansions in the field. Rather than evaluating traffic/transportation noise, the method focuses on the consideration of the noise level and its impact on human health, regardless of the source producing the noise in an analogous manner as considered in the fields of toxicology and common noise evaluation practices combined. The resulting framework will constitute the basis for the development of a more detailed mathematical model for the inclusion of noise in LCA. The toxicological background and the experience of the analysis of the release of chemicals in LCA seem to provide sufficient ground for the inclusion of noise in LCA: taken into account the physical differences and the uniqueness of noise as an impact, the procedure applied to the release of chemicals during a product life cycle is key for a valuable inclusion of noise in the LCA logic.

CONCLUSIONS

It is fundamental for the development of research in the field of LCA and noise to consider any type of noise. Further studies are needed to contribute to the inclusion of noise sources and noise impacts in LCA. In this paper, a structure is proposed that will be expanded and adapted in the future and which forms the basic framework for the successive modelling phase.

Effects of cigarette smoking on hearing recovery from noise-induced temporary hearing threshold shifts in mice

HYPOTHESIS

Cigarette smoking may potentiate noise-induced hearing loss.

BACKGROUND

Many epidemiological studies have shown that cigarette smoking is a major risk factor for noise-induced hearing loss.

METHODS

BALB/c mice were exposed to passive smoking for 2 h/d for 2 weeks before exposure to 110-dB sound pressure level white noise for 3 hours once. Hearing was assessed via the auditory brainstem response with tone-burst stimulation and distortion product otoacoustic emissions before and at 1, 3, 5, 7, 14, 21, and 28 days after noise exposure. Oxidative stress and hypoxia were assessed by immunostaining with 8-oxoG and hypoxia-inducible factor 1α, respectively.

RESULTS

Control mice unexposed to both smoking and noise and mice exposed to smoking only showed no shift in hearing threshold. In contrast, mice exposed to noise only or smoking plus noise showed abrupt increases in hearing threshold. In mice exposed to noise only, hearing threshold returned to prenoise levels after 2 weeks. However, in mice exposed to smoking plus noise, the loss of hearing was significantly higher, and hearing threshold did not return to the pre-exposure levels until 4 weeks later. Positive staining with 8-oxoG and hypoxia-inducible factor 1α were observed in the inner ear of the smoking-only and smoking-plus-noise group similar to noise-only mice, whereas no positive staining was observed in control group.

CONCLUSION

These results indicate that cigarette smoking may potentiate the harmful effects of noise on hearing and disturb the recovery mechanism in the cochlea.

A weight of evidence approach for the assessment of the ototoxic potential of industrial chemicals

There is accumulating epidemiological evidence that exposure to some solvents, metals, asphyxiants and other substances in humans is associated with an increased risk of acquiring hearing loss. Furthermore, simultaneous and successive exposure to certain chemicals along with noise can increase the susceptibility to noise-induced hearing loss. There are no regulations that require hearing monitoring of workers who are employed at locations in which occupational exposure to potentially ototoxic chemicals occurs in the absence of noise exposure. This project was undertaken to develop a toxicological database allowing the identification of possible ototoxic substances present in the work environment alone or in combination with noise exposure. Critical toxicological data were compiled for chemical substances included in the Quebec occupational health regulation. The data were evaluated only for noise exposure levels that can be encountered in the workplace and for realistic exposure concentrations up to the short-term exposure limit or ceiling value (CV) or 5 times the 8-h time-weighted average occupational exposure limit (TWA OEL) for human data and up to 100 times the 8-h TWA OEL or CV for animal studies. In total, 224 studies (in 150 articles of which 44 evaluated the combined exposure to noise and a chemical) covering 29 substances were evaluated using a weight of evidence approach. For the majority of cases where potential ototoxicity was previously proposed, there is a paucity of toxicological data in the primary literature. Human and animal studies indicate that lead, styrene, toluene and trichloroethylene are ototoxic and ethyl benzene, n-hexane and p-xylene are possibly ototoxic at concentrations that are relevant to the occupational setting. Carbon monoxide appears to exacerbate noise-induced hearing dysfunction. Toluene interacts with noise to induce more severe hearing losses than the noise alone.

Relation between outer hair cell loss and hearing loss in rats exposed to styrene

The relationship between outer hair cell (OHC) loss and cochlear sensitivity is still unclear, because in many animal models there exist surviving but dysfunctional OHCs and also injured/dead inner hair cells (IHC). Styrene is an ototoxic agent, which targets and destroys OHCs starting from the third row to the second and first rows depending on the exposure level. The remaining cells may be less affected. In this experiment, rats were exposed to styrene by gavage at different doses (200-800 mg/kg/day) for varying periods (5 days/week for 3-12 weeks). An interesting finding was that the cochlear sensitivity was not affected in a few rats with all OHCs in the third row being destroyed by styrene. A further loss of OHCs was usually accompanied with a linear input/output (I/O) function of cochlear compound action potentials (CAP), indicating the loss of cochlear amplification. However, normal CAP amplitudes at the highest stimulation level of 90 dB SPL were often observed when all OHCs were destroyed, indicating normal function of the remaining IHCs. The OHC-loss/hearing-loss relation appeared to be a sigmoid-type function. Initially, styrene-induced OHC losses (<33%) did not result in a significant threshold shift. Then CAP threshold shift increased dramatically with OHC loss from 33% to 66%. Then, CAP threshold changed less with OHC loss. The data suggest a tri-modal relationship between OHC loss and cochlear amplification. That is, under the condition that all surviving OHCs are ideally functioning, the cochlear amplifier is not affected until 33% of OHCs are absent, then the gain of the amplifier decreases proportionally with the OHC loss, and at last the amplifier may fail completely when more than 67% of OHCs are lost.

Functional and morphological effects of fotemustine on the auditory system of the rat

OBJECTIVE

This study aimed to elucidate the potential inner-ear effects of fotemustine, a chemotherapeutic agent which crosses the blood-brain barrier and is used in the treatment of primary and metastatic brain tumours and metastatic melanoma.

METHODS

This study utilised distortion product otoacoustic emissions and transmission electron microscopy in order to conduct electrophysiological and morphological assessments, using a rat experimental model. Twelve ears of six male rats were examined two months following intraperitoneal slow infusion of fotemustine (100 mg/m2 or 7.4 mg/kg). Pre- and post-treatment measurements were compared. Finally, electron microscopy was performed on three rat temporal bones.

RESULTS

After infusion of fotemustine, distortion product otoacoustic emissions revealed a significant reduction in signal-to-noise ratios only at 3600 Hz (from 11.95 +/- 7.52 to -0.26 +/- 9.45 dB) and at 3961 Hz (from 18.09 +/- 7.49 to 6.74 +/- 12.11 dB) (referenced to 2f1 - f2). Transmission electron microscopy of the temporal bone revealed ultrastructural changes in the outer hair cells, stria vascularis and cochlear ganglion at the cochlear basal turn. The ganglion cell perikarya were unaffected.

CONCLUSIONS

Fotemustine was administered via intraperitoneal slow infusion in a rat experimental model. Twelve ears of six survivors, from 10 rats, were evaluated at the second month. Fotemustine was determined to have a potential for ototoxicity at 3600 and 3961 Hz. Three randomly chosen rats underwent electron microscopy for morphological analysis. Morphological effects in the cochlear basal turn were observed. Oedematous intracytoplasmic spaces and perivascular areas of the stria vascularis, as well as distorted chromatin content, were detected, thereby suggesting potential ototoxic effects for this agent. Further experimental and clinical studies are required in order to determine whether the effect seen in this pilot study is reversible, and to analyse effects in humans.

Efeitos ototóxicos da exposição ao monóxido de carbono: uma revisão

TEMA: efeitos ototóxicos da exposição ao monóxido de carbono: uma Revisão. OBJETIVO: analisar a literatura sobre audição e o monóxido de carbono (CO). MÉTODO: descrever as propriedades físicas, a absorção, a distribuição e o metabolismo do CO, bem como a sua origem, sua produção, suas fontes e os seus limites internacionais de exposição ocupacional. Foram discutidos vários estudos sobre os efeitos do CO no sistema auditivo animal e humano. Finalmente, foram identificados os principais setores onde podemos encontrar a exposição combinada ruído e CO e descrevermos os mecanismos básicos de ação do CO que poderão potencializar a perda auditiva induzida por ruído. CONCLUSÃO: a revisão de literatura indicou que: 1. A poluição atmosférica, o fumo passivo, a exposição ocupacional, e o tabagismo ativo, são exemplos de fontes de exposição ao CO. 2. A ação tóxica principal do CO resulta em anoxia provocada pela conversão da oxihemoglobina em carboxihemoglobina. 3. Os estudos animais sobre a exposição combinada ao ruído e ao CO foram realizados em sua grande maioria com ratos e o conjunto destes estudos demonstraram os efeitos da exposição aguda e simultanêa ao CO e ao ruído. 4. Os estudos relatando a nocividade da exposição ao CO sobre o sistema auditivo humano foram realizados, na grande maioria, seguidos de uma exposição aguda ao CO. A exposição ao ruído não foi relatada ou controlada como um fator relacionado com os problemas auditivos observados. A evidência existente até o momento indica a necessidade do desenvolvimento de pesquisas sobre os efeitos auditivos da exposição a CO, com e sem exposição ao ruído.

Oxidative stress: a potential basis for potentiation of noise-induced hearing loss

In the past two decades, researchers have determined that a broad range of environmental and occupational contaminants can interact with noise to enhance noise-induced hearing loss. This manuscript focuses upon the hypothesis that chemicals that promote oxidative stress might increase the risk of noise-induced hearing loss. Evidence is presented that confirms the role of oxidative stress in the production of hearing loss by both carbon monoxide and by acrylonitrile when noise is present at the time of chemical exposure.

Argon protects hypoxia-, cisplatin- and gentamycin-exposed hair cells in the newborn rat’s organ of Corti

During the last few years, an important protective effect of the noble gas xenon against neuronal hypoxic damage was observed. However, argon (Ar), a gas from the same chemical group, but less expensive and without anesthetic effect at normobaric pressure, has not been studied in terms of possible biological effects on cell protection. Ar was tested for its ability to protect organotypic cultures of the organ of Corti from 3-5 day old rats against hypoxia, cisplatin, and gentamycin toxicity. Cultures were exposed to nitrogen hypoxia (5% CO2, 95% N2), Ar hypoxia (5% CO2, 95% Ar) or normoxia for 30 h. Ar protected the hair cells from hypoxia-induced damage by about 25%. Ar-oxygen (O2) mixtures (21% O2, 5% CO2, 74% Ar) had no effect on the hair cell survival. Cisplatin (7.5-25 microM) and gentamycin (5-40 microM) exposed in medium under air damaged the hair cells in a dose-dependent manner. The exposure of cisplatin- and gentamycin-treated cultures to the Ar-O2 atmosphere significantly reduced the hair cell damage by up to 25%. This protective effect of Ar might provide a new protective approach against ototoxic processes.

Moderate variations of mood/emotional states related to alterations in cochlear otoacoustic emissions and tinnitus onset in young normal hearing subjects exposed to gun impulse noise

This study was designed to test whether under impulse noise exposure mood and emotional states could play a role in the onset of tinnitus and/or could modify cochlear sensitivity objectively measured with distortion product otoacoustic emissions (DPOAEs). The experimental design consisted in a short follow-up study of 54 young military subjects (20+/-2 years old), psychologically normal, with normal hearing, during two consecutive days of target practice rounds. Data collection included an abbreviated version of the profile of mood states (POMSs) inventory [Profile of Mood States, Educational and Industrial Testing Service, San Diego, 1971], questionnaires on tinnitus perception (previous history and after shooting) and DPOAEs measurements before and after shooting. Higher scores of tension-anxiety were found in subjects having previous history of tinnitus. Association between tinnitus previous history and tinnitus after shooting was found significant. Perception of tinnitus after target practice rounds was associated with significantly lower DPOAEs at 3 kHz. The most tense-anxious subjects were found to have DPOAEs decreases of 3.35+/-6 dB at 3 kHz after shooting. This study clearly shows that, in young healthy population, psychologically normal and with normal hearing, moderate variations in mood and emotional states were related to tinnitus onset and DPOAEs alterations. It is possible that stronger variations in mood and/or emotional condition would increase risks of tinnitus and alterations of cochlear sensitivity.

Promotion of noise-induced hearing loss by chemical contaminants

Recent studies have underscored the ability of a wide range of chemical agents to potentate noise-induced hearing loss. Given the ubiquitous nature of noise exposure particularly in many work settings, the high rate of noise-induced hearing loss, the limited degree to which auditory function can recover following damage to the inner ear, and the disparate chemical structures that appear capable of impairing hearing, this issue appears to have great public health significance. A compendium of chemicals known to potentiate noise induced hearing loss is presented along with a hypothesis that might explain at least one basis for potentiation of noise-induced hearing loss by certain chemical toxicants. The use of benchmark dose analysis to undertake a risk assessment for promotion of noise-induced hearing loss by both carbon monoxide and hydrogen cyanide is described.

Acrylonitrile potentiates noise-induced hearing loss in rat

Acrylonitrile, one of the 50 most commonly produced industrial chemicals, has recently been identified as a promoter of noise-induced hearing loss (NIHL). This agent has the potential to produce oxidative stress through multiple pathways. We hypothesize that acrylonitrile potentiates NIHL as a consequence of oxidative stress. The objectives of this study were to characterize acrylonitrile exposure conditions that promote permanent NIHL in rats and determine the ability of this nitrile to produce auditory dysfunction by itself. Additionally, we sought to determine whether a spin-trap agent that can form adducts with ROS would protect against the effects of acrylonitrile. Acrylonitrile administration produced significant elevation in NIHL detected as a loss in compound action potential sensitivity. The effect was particularly robust for high-frequency tones and particularly when acrylonitrile and noise were given on repeated occasions. Acrylonitrile by itself did not disrupt threshold sensitivity. Administration of the spin-trap agent phenyl- N- tert-butylnitrone (PBN), given to rats prior to acrylonitrile and noise, did block the elevation of NIHL by acrylonitrile. However, PBN at the dose and time interval given was ineffective in protecting auditory function in subjects exposed to noise alone. The results suggest that oxidative stress may play a role in the promotion of NIHL by acrylonitrile.

Mild carbon monoxide exposure diminishes selectively the integrity of the cochlea of the developing rat

Rat pups were chronically exposed to carbon monoxide (CO) concentrations (12 or 25 ppm) in air starting at day 8, through 22 days of age, to examine the changes in the peripheral auditory system. Gastrostomy-reared rat pups, with or without CO exposure, were used and compared with mother-reared pups. The organ of Corti and the neurons of the spiral ganglion were analyzed for their morphology by using immunochemical and histological techniques. The inner and outer hair cells in the organ of Corti of animals exposed to 12 and 25 ppm CO were not different from the controls. However, at 25 ppm CO exposure, the nerve terminals innervating the inner hair cells were swollen. The somata of neurons in the spiral ganglion showed mild changes in the cytoplasm, and signs of mild vacuolization were observed in myelin covering their central processes. Synaptophysin, a marker for synaptic vesicles, and choline acetyltransferase, a marker for cholinergic terminals, showed no difference in immunoreactivity in CO exposed animals at 12 and at 25 ppm when compared with their age-matched controls. Also, Na(+)K(+) ATPase immunoreactivity patterns were normal compared with controls. Three enzymes were significantly reduced at the 25 ppm CO exposure: Cytochrome oxidase, NADH-TR, and calcium ATPase were decreased in both the organ of Corti and the neurons of the spiral ganglion, and decreased immunostaining for the neurofilament and myelin basic proteins was found. We conclude that components of the cochlea are selectively affected by mild chronic CO exposure during development.

Mild carbon monoxide exposure impairs the developing auditory system of the rat

The object of this study was to determine if chronic exposure to mild concentrations of CO in air caused changes in the integrity of the inferior colliculus during the most active period of synaptogenesis/auditory development. We examined all subregions of the inferior colliculus (IC) of rats by immunocytochemical approaches after pups were exposed chronically to CO concentrations of, 0, 12.5, 25, and 50 ppm in air starting at Day 8 through 20-22 days of age. Mother-reared pups were compared to the gastrostomy-reared pups with or without CO exposure for basal neural activity, using c-Fos immunoreactivity as a marker. Half the rats were examined at 27 days of age, 5 days after the end of CO exposure, and the other half were examined 50 days later at 75-77 days of age. In the central nucleus of the IC, the number of cells expressing a basal level of c-Fos was decreased significantly in the CO-exposed animals when compared to controls; however, there was little or no difference in the number of cells expressing c-Fos in the other subregions of the IC. We conclude that the central nucleus of the inferior colliculus is affected selectively by mild CO exposure (0.0012% in air) and that this reduction in neuronal activity persists into adulthood.

Susceptibility of the hair cells of the newborn rat cochlea to hypoxia and ischemia

Hypoxia and ischemia are thought to be important pathogenetic factors in bringing about hearing loss. In order to study the effect of these determinants on the loss of inner and outer hair cells (IHCs/OHCs), we used an in vitro hypoxia and ischemia model of the newborn rat cochlea. The specimens of the organ of Corti were exposed either to hypoxia (10-20 mm Hg) or to normoxic glucose deprivation or to both (ischemia) in artificial perilymph for different exposure periods. The number of IHCs and OHCs was counted and the hair cell loss was compared to controls. Normoxic aglycemia did not cause significant hair cell loss as compared to controls. Hypoxia and ischemia led to hair cell loss in a dose-dependent manner, with the loss in the ischemia groups found to be markedly higher than that in the hypoxia groups. Hypoxia resulted in a mean loss of 8% OHC and of 14% IHC after an 8-h exposure. Ischemia increased the loss to 19% OHC and 39% IHC after the same exposure period of 8 h. Our findings suggest that IHCs are more susceptible to hypoxia/ischemia than OHCs.

Chemical exposure as a risk factor for hearing loss

In 2002, the National Institute for Occupational Safety and Health and the National Hearing Conservation Association cosponsored the "Best Practices Workshop: Combined Effects of Chemicals and Noise on Hearing." This article summarizes the main results of the Workshop. Its goals were to review the knowledge of chemical ototoxicity and to stimulate participant discussion on how to address this risk. Speakers provided an overview of the effects of chemicals on the auditory system (http://www.cdc.gov/niosh/noise/noiseandchem/noiseandchem.html). Research priorities were discussed in concurrent working group sessions. The Workshop concluded with a panel of the groups' facilitators reporting on these sessions. The following key issues were identified: rationale and proposal of a list of priority chemicals; valid procedures for exposure (animal studies), exposure assessment, and audiological testing; need for mechanistic research and a Response Level; recommendations for preventive actions; and information dissemination.

The relationship between noise-induced hearing loss and hair cell loss in rats

Noise-induced hearing loss (NIHL) and hair cell loss are known to show only a moderate correlation. One reason for this is that NIHL may reflect not only the sum of dead hair cells, but also the sum of impaired but still living hair cells. This report compares hair cell loss in different cochlear regions in rats with noise-induced compound action potential (CAP) threshold elevation at corresponding frequencies. CAP threshold elevation and hair cell loss were determined 4 weeks after noise exposure. In the apical turn (<35% from the apex) there was no hair cell loss even when a 60 dB CAP threshold elevation was induced. In the region of 40-60% from the apex in the middle turn, significant hair cell loss was not observed until CAP threshold elevation exceeded about 40-50 dB. This critical level decreased towards the basal turn. In the basal turn, outer hair cell (OHC) loss was observed in almost all of the noise-exposed rats, even in some cases without detectable NIHL, but inner hair cell (IHC) loss was still not observed until 50 dB threshold elevation. In the region of 75-90% from the apex related to the highest frequencies tested in this study (30-40 kHz), a linear NIHL/OHC loss relationship was observed. The results of this paper suggest that the high frequency hair cells in rat cochlea may die relatively rapidly after injury, leading to a linear relation between NIHL and hair cell loss, but that the low frequency hair cells may survive without auditory function.

Effect of hypoxia on noise-induced auditory impairment

Chemical asphyxiants have been shown to potentiate noise-induced hearing loss (NIHL) in our previous reports. However, it is still unclear whether hypoxia interacts with noise to potentiate NIHL. This is an important issue, since many people at risk of exposure to noise may also suffer from hypoxia. In the present study, rats were exposed to 14.1-kHz octave-band noise at 105, 110, and 115 dB SPL and hypoxic air (10% O(2) and 18% O(2)) for 4 h. Cochlear compound action potential threshold loss and hair cell loss were determined 4 weeks after the exposure. Though the hypoxia did not cause hearing loss and hair cell loss, the combined exposure to noise and hypoxic air caused more hearing loss and hair cell loss than the noise alone. The NIHL potentiation by hypoxia appeared to increase linearly with the hypoxia level. 10% O(2) exposure significantly potentiated NIHL and 18% O(2) exposure also showed an evident NIHL potentiation. This suggests that people suffering from hypoxia may have increased risk to noise trauma.

Free radical generation in the cochlea during combined exposure to noise and carbon monoxide: an electrophysiological and an EPR study

Ototoxicity following combined exposure to noise and carbon monoxide (CO) is known to result in more severe permanent threshold shifts than exposure to noise alone. We have previously demonstrated that such potentiation of noise-induced auditory impairment by CO can be prevented by the administration of a nitrone spin-trapping agent. Although such protection implicates injury via free radical pathways, drug-induced protection does not provide direct evidence for the presence of free radicals in the cochlea. The objective of this study was to demonstrate the actual presence of nitrone spin adducts in the cochlea following simultaneous exposure to noise and CO. Using electrophysiological end-points, the protective effects of the nitrone spin-trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) were assessed following combined exposure of adult male Long Evans hooded rats to noise and CO. In addition, an ex-vivo evaluation of POBN spin adducts was done by electron paramagnetic resonance spectroscopy (EPR). The noise used was octave band noise with center frequency 13.6 kHz at 100 dB(Lin) for a duration of 2 h. The level of CO used was 1200 ppm. Electrophysiological results demonstrate that POBN protects against combined exposure to noise plus CO. The EPR study demonstrates POBN spin adducts in the cochleae of animals exposed to noise plus CO. Therefore, this study provides evidence to the hypothesis that ototoxicity due to noise plus CO exposure is mediated via free radicals.

Disruption of cochlear potentials by chemical asphyxiants. Cyanide and carbon monoxide

While ischemia, hypoxic hypoxia, and carbon monoxide (CO) have received extensive study designed to characterize mechanisms by which they disrupt cochlear function, little data are available concerning cyanide's potential to disrupt auditory function. In this study, disruption of the compound action potential (CAP) and endocochlear potential (EP) by cyanide and CO was compared in rats treated with potassium cyanide (KCN) (7 mg/kg ip), saline, CO (35 ml/kg ip), and air. Acute KCN administration significantly suppressed CAP and EP transiently. The effect was seen initially on EP with CAP impairment occurring a few minutes later. Acute CO injection also suppressed the CAP significantly, but the effect was far smaller, occurred later in time, and lasted longer than the effect of KCN. The effect of CO on EP was equivocal. There was a good correspondence between blood cyanide levels and impairment of cochlear function; carboxyhemoglobin (HbCO) levels were elevated during the period when cochlear function was impaired, but recovery of cochlear function preceded the return of normal oxyhemoglobin. Both KCN and CO had somewhat preferential effects on high-frequency tones. Repeated cyanide administration caused a persistent CAP threshold elevation despite the rapid recovery of EP and CAP observed following acute KCN administration. The data suggest that acute KCN administration has a prominent disruptive effect at the stria vascularis presumably by disrupting the electron transport chain in this metabolically active structure. The principal target for acute CO ototoxicity in the cochlea is probably not the stria vascularis.

NMDA receptor blockage protects against permanent noise-induced hearing loss but not its potentiation by carbon monoxide

While a clear role has been proposed for glutamate as a putative neurotransmitter at the inner hair cell type I spiral ganglion cell synapse, the possible role of excessive glutamate release in cochlear impairment and of NMDA receptors in such a process is uncertain. The present study compares the protective effects of (+)-MK-801, an NMDA receptor antagonist, and the relatively inactive isomer (-)-MK-801 against permanent noise-induced hearing loss (NIHL). The study also asks whether (+)-MK-801 can protect against the NIHL potentiation by carbon monoxide (CO). Rats (n = 6) were exposed to 100-dB, 13.6-kHz octave-band noise for 2 h after receiving injection of (+)-MK-801 hydrogen maleate (1 mg/kg), (-)-MK-801 hydrogen maleate (1 mg/kg), or saline. Other groups of animals were exposed to the combination of noise and CO (1200 ppm) after receiving (+)-MK-801 or saline. Additional subjects received (+)-MK-801, saline or CO exposure alone. Compound action potential (CAP) threshold sensitivities were compared 4 weeks after the exposures. The results show significant protection by (+)-MK-801 against the permanent CAP threshold elevation induced by noise alone, but no protective effect of (-)-MK-801. (+)-MK-801 produced limited protection against threshold shifts induced by the combination of noise and CO. Outer hair cell (OHC) loss was not protected by (+)-MK-801 administration. The data suggest that NMDA receptor stimulation may play a role in NIHL resulting from fairly mild noise exposure. The data do not support a role for NMDA receptor stimulation in the potentiation of NIHL that results from simultaneous exposure to CO and noise.

Increased noise severity limits potentiation of noise induced hearing loss by carbon monoxide

This study evaluates the influence of noise intensity and duration on auditory dysfunction due to simultaneous exposure to noise and carbon monoxide (CO). Previous studies have demonstrated that CO potentiates noise induced hearing loss (NIHL). It is not known whether auditory dysfunction due to combined exposure parallels impairment due to noise alone. Based on the 5 dB exchange rate between noise intensity and exposure doubling time, equivalent noise exposure conditions were used. Long Evans hooded rats were divided into groups that received noise alone (95, 100 and 105 dB SPL), and noise plus CO (1200 ppm), for durations of 4, 2 and 1 h, respectively. Controls were exposed to air or CO alone. Thresholds were evaluated 4 weeks later using an electrophysiological endpoint, the compound action potential threshold. Results demonstrate that the 5 dB exchange rate is not conserved under the conditions and subjects used. Moreover, dysfunction due to combined exposure did not parallel dysfunction due to noise alone. Further, although an increase in exposure duration results in increased auditory dysfunction, no further potentiation of NIHL by CO is observed. This suggests that at increasing noise severity, dysfunction due to combined exposure is limited by impairment due to noise alone.

Protective effects of phenyl-N-tert-butylnitrone on the potentiation of noise-induced hearing loss by carbon monoxide

Free radical injury has been implicated in cochlear damage resulting from exposure to high-intensity noise and due to carbon monoxide (CO) hypoxia. Although exposure to noise plus CO is common in occupational settings and noise-induced hearing loss (NIHL) is enhanced in the presence of CO, potential mechanisms resulting in auditory impairment have not been studied. This study evaluates protective effects of the free radical scavenger phenyl-N-tert-butylnitrone (PBN) against potentiation of NIHL by CO. Three PBN administration protocols have been evaluated in subjects exposed to noise plus CO or noise alone. Long Evans hooded rats were exposed to octave band noise at 100 dB(Lin), center frequency (cf) = 13.6 kHz for a duration of 2 h. The level of CO used was 1200 ppm. Endpoints used to detect permanent auditory impairment were compound action potential (CAP) threshold and 1 microV root mean square (RMS) cochlear microphonic (CM). Testing was done 4 weeks following exposure. PBN administration prior to and following simultaneous exposure provided significant protection against auditory impairment in subjects receiving noise plus CO. Partial protection was observed in the protocols where PBN was injected following noise plus CO exposure. PBN administration appeared to reduce auditory impairment in animals exposed to noise alone, but the difference was not found to be statistically significant. Protective effects of PBN following simultaneous exposure to noise plus CO suggest that free radicals may be generated during combined exposure.

Low-level toluene disrupts auditory function in guinea pigs

Toluene appears to have adverse effects on the human auditory system, but it is difficult to estimate its potency since it is commonly present in the workplace in combination with noise exposure; workplace noise exposures are often highly variable. Studies designed to assess toluene ototoxicity specifically have been limited to high-dose studies in a single laboratory animal model, the rat. Here permanent hearing loss has been observed at concentrations of 1000 ppm toluene and greater after inhalation exposure for 5 days, 6 h/day. The OSHA threshold limit value for toluene is only 100 ppm. The current study focuses on the onset of toluene ototoxicity acutely in the guinea pig and in adducing a mechanism of effect. In this study, evidence is presented for the impairment of auditory function by toluene in the guinea pig, at a concentration substantially lower than that used for studying permanent impairment in the rat. The impaired function was correlated with reduced energy metabolism in outer hair cells. Assessment of auditory function was made using distortion product otoacoustic emissions (DPOAE) with subsequent measurement of succinate dehydrogenase (SDH) staining density in hair cells using surface preparations. Temporary disruption of auditory function in guinea pigs is seen in subjects exposed to 250, 500, and 1000 ppm toluene for 8 h/day, 5 day/week for 1 and 4 weeks. Concentrations as low as 250 ppm toluene were able to disrupt auditory function acutely in the guinea pig, and 500 and 1000 ppm toluene produced greater acute dysfunction. SDH staining suggests that reduced enzyme activity in the midfrequency region of the cochlea occurs acutely following toluene exposure. Although the auditory dysfunction progressed between 1 and 4 weeks of exposure, a permanent loss did not develop for these subjects and hair cell death was not seen. The current study identifies early evidence of auditory system impairment in the guinea pig at low toluene concentration and evidence for impairment of energy production in hair cells. While even a transient auditory impairment has implications for workplace safety, additional study on the transition from such acute effects to permanent impairment is essential.

Succinate dehydrogenase (SDH) activity in hair cells: a correlate for permanent threshold elevations

Hair cell loss is often used as a histological correlate of hearing loss. However, the histological and the physiological data are not always well correlated. This paper investigates the use of succinate dehydrogenase (SDH) activity in the hair cells as a marker of cellular dysfunction and so the loss of auditory sensitivity. In our previous studies, potentiation of noise-induced auditory threshold elevation by carbon monoxide (CO) was observed [Chen and Fechter, 1999; Chen et al., 1999]. However, its histological basis is still unclear. In this study, rats were exposed to 100-dB octave-band noise (center frequency=13.6 kHz, 2 h) or to the combination of the noise and CO (1200 ppm). Threshold elevation of compound action potential (CAP) and cochlear histological changes were assessed 4 weeks after exposure. The noise alone caused CAP threshold elevations with little if any or without hair cell loss. However, the SDH activity in the hair cells decreased after the exposure. The SDH reduction, especially in the inner hair cells, was well related to the loss of auditory sensitivity. The combined exposure to noise and CO caused more severe CAP threshold elevation and SDH activity reduction than did the noise alone and it also caused significant outer hair cell loss. However, across all the test frequencies, neither the hair cell loss nor the SDH reduction alone had good correlation to the reduction of the auditory sensitivity. Under this situation, CAP threshold elevation seemed to follow OHC loss at high frequencies and to follow SDH reductions in the IHCs at low frequencies, where no hair cell loss occurred.