Effects of carbon monoxide on cochlear electrophysiology and blood flow

The Burden of Occupational Noise-Induced Hearing Loss From 1990 to 2019: An Analysis of Global Burden of Disease Data

OBJECTIVES

The relationship between long-term exposure to occupational noise and hearing loss has been extensively documented. We aimed to assess spatial and temporal changes in the burden of occupational noise-induced hearing loss (ONIHL) in 204 countries and territories with varying socio-demographic indexes (SDI) from 1990 to 2019.

DESIGN

Temporal and spatial trends in age-standardized disability-adjusted life year rates (ASDR) for ONIHL were estimated by sex, age, SDI level, country, and geographic region from 1990 to 2019. We used the Joinpoint model to calculate annual average percentage changes to assess such trends and projected trends in ASDR for ONIHL globally and across different income regions from 2020 to 2044 using an age-period-cohort model. We fitted the relationship between ASDR and SDI, ASDR and healthcare access and quality index, respectively.

RESULTS

Overall, the global burden of ONIHL has decreased since 1990, especially in middle and lower SDI regions. In 2019, the global ASDR for ONIHL was 84.23 (95% confidence interval: 57.46 to 120.52) per 100,000 population. From 1990 to 2019, the global ASDR for ONIHL decreased by 1.72% (annual average percentage change = -0.05, 95% confidence interval: -0.07 to -0.03). Our projections showed a decreasing trend in the global ONIHL burden until 2044. ASDR and SDI ( R = -0.8, p < 0.05), ASDR and healthcare access and quality index ( R = -0.75, p < 0.05) showed significant negative correlations.

CONCLUSIONS

The global ONIHL burden has decreased over the past three decades, especially in regions with middle and lower SDI levels. However, the global ONIHL burden still remained severe in 2019, notably among males, the middle-aged and elderly, and regions with lower SDI levels.

Clinical impact of hyperbaric oxygen therapy combined with steroid treatment for sudden sensorineural hearing loss: A case-control study

Objectives

The aim of present study was to evaluate the clinical efficacy of hyperbaric oxygen therapy (HBOT) as a primary therapy combined with standard systemic corticosteroid treatment for sudden sensorineural hearing loss (SSNHL) compared to treatment without the use of HBOT (non-HBOT) through clinical data and advanced analytical approaches.

Study Design

Case-control study.

Methods

Conducted across three Japanese medical centers involving 298 SSNHL patients diagnosed between 2020 and 2023. Inclusion criteria encompassed first onset and treatment, WHO grade 3 or 4 initial hearing impairment, receipt of systemic corticosteroid therapy within 14 days of symptom onset, and initiation of HBOT within the same timeframe for the case group. The primary outcome measure was the difference in hearing improvement (mean hearing level in decibels, dB) between the two groups, assessed by pure-tone audiometry at baseline and 3 months post-treatment, using the inverse probability of treatment weighting (IPTW) method adjusted for covariate differences.

Results

The study included 67 patients in the HBOT group and 68 in the non-HBOT group. The HBOT group exhibited significantly greater hearing improvement (IPTW-adjusted difference: 7.6 dB, 95% CI 0.4-14.7; p = 0.038). Patients without vertigo in the HBOT group demonstrated substantial hearing improvement (11.5 dB, 95% CI 2.3-20.6; p = 0.014), whereas those with vertigo showed no significant improvement (-1.8 dB, 95% CI -11.8-8.3; p = 0.729). The HBOT group also had a significantly higher association with complete recovery (IPTW-adjusted odds ratio: 2.57, 95% CI 1.13-5.85; p = 0.025).

Conclusion

In SSHNL, HBOT combination therapy yielded slightly but significantly improved hearing outcomes compared to non-HBOT treatment.

Level of Evidence

4.

The Association between Prenatal Nicotine Exposure and Offspring's Hearing Impairment

OBJECTIVES

The objective of this study is to evaluate whether there is an association between in-utero exposure to nicotine and subsequent hearing dysfunction.

MATERIALS AND METHODS

Secondary analysis of a multicenter randomized trial to prevent congenital cytomegalovirus (CMV) infection among gravidas with primary CMV infection was conducted. Monthly intravenous immunoglobulin hyperimmune globulin therapy did not influence the rate of congenital CMV. Dyads with missing urine, fetal or neonatal demise, infants diagnosed with a major congenital anomaly, congenital CMV infection, or with evidence of middle ear dysfunction were excluded. The primary outcome was neonatal hearing impairment in one or more ears defined as abnormal distortion product otoacoustic emissions (DPOAEs; 1 to 8 kHz) that were measured within 42 days of birth. DPOAEs were interpreted using optimized frequency-specific level criteria. Cotinine was measured via enzyme-linked immunosorbent assay kits in maternal urine collected at enrollment and in the third trimester (mean gestational age 16.0 and 36.7 weeks, respectively). Blinded personnel ran samples in duplicates. Maternal urine cotinine >5 ng/mL at either time point was defined as in-utero exposure to nicotine. Multivariable logistic regression included variables associated with the primary outcome and with the exposure (p < 0.05) in univariate analysis.

RESULTS

Of 399 enrolled patients in the original trial, 150 were included in this analysis, of whom 46 (31%) were exposed to nicotine. The primary outcome occurred in 18 (12%) newborns and was higher in nicotine-exposed infants compared with those nonexposed (15.2 vs. 10.6%, odds ratio [OR] 1.52, 95% confidence interval [CI] 0.55-4.20), but the difference was not significantly different (adjusted odds ratio [aOR] = 1.0, 95% CI 0.30-3.31). This association was similar when exposure was stratified as heavy (>100 ng/mL, aOR 0.72, 95% CI 0.15-3.51) or mild (5-100 ng/mL, aOR 1.28, 95% CI 0.33-4.95). There was no association between nicotine exposure and frequency-specific DPOAE amplitude.

CONCLUSION

In a cohort of parturients with primary CMV infection, nicotine exposure was not associated with offspring hearing dysfunction assessed with DPOAEs.

KEY POINTS

· Nicotine exposure was quantified from maternal urine.. · Nicotine exposure was identified in 30% of the cohort.. · Exposure was not associated with offspring hearing dysfunction..

Long-term exposure to ambient air pollutants and hearing loss in Korean adults

BACKGROUND

Hearing loss is the most prevalent sensory deficit globally, and its prevalence has been growing at a rapid pace, mainly due to aging population. Although there is plentiful evidence that exposure to air pollutants contributes to oxidative stress and free radical formation in inner ear is linked to hearing impairment, there is little awareness of the contribution of air pollution to hearing loss.

OBJECTIVE

To evaluate the associations between long-term exposures to air pollution and hearing loss in Korean adults, while controlling for sociodemographic and clinical factors and occupational, recreational, and firearm noise exposures.

METHODS

We analyzed data from 15,051 adults in the Korean National Health and Nutrition Examination Survey V (2010-2012). Ambient air pollutant concentrations for participant residing in administrative division and local/town units were collected for current-to-3 years prior to the audiometric examination. Pure-tone average (PTA) of hearing thresholds at speech-frequency (0.5, 1, 2, 4kHz) and high-frequency (3, 4, 6kHz) were computed, and hearing loss was defined as PTA >25dB in either ear.

RESULTS

Ambient PM₁₀ was significantly associated with the increased risk of hearing loss at speech-frequency and high-frequency (current year and prior year 3), while CO was significantly associated with the increased risk of hearing loss at speech-frequency (prior year 2 and 3) and high-frequency (prior year 2), when air pollution was assessed at local/town (finer unit). After additional adjustment for regional socioeconomic status and population density (a proxy for traffic noise), NO₂ became significantly associated with hearing loss at speech-frequency. When air pollution was assessed at administrative division (larger unit), PM₁₀ and CO became stronger and SO₂ became significant in hearing loss at speech-frequency.

CONCLUSION

Our results suggest that long-term exposures to environmental PM₁₀, NO₂, CO, and SO₂ at the levels currently observed in the general population may be risk factors affecting hearing loss.

Nonvestibular Dizziness

Dizziness is a common chief complaint with an extensive differential diagnosis that ranges from peripheral, central, to nonvestibular conditions. An understanding of nonvestibular conditions will aid accurate diagnosis and initiation of appropriate management. Thus, the objective of this article is to present an overview of nonvestibular etiologies that may plague a dizzy patient and the recommended treatment options.

Effects of short- and long-term exposure to air pollution and meteorological factors on Meniere's disease

The association between air pollutants and Meniere’s disease has not been explored. The present study investigated the relationship between meteorological factors and air pollutants on Meniere’s disease. Participants, aged ≥ 40 years, of the Korean National Health Insurance Service-Health Screening Cohort were included in this study. The 7725 patients with Meniere’s disease were matched with 30,900 control participants. The moving average meteorological and air pollution data of the previous 7 days, 1 month, 3 months, and 6 months before the onset of Meniere’s disease were compared between the Meniere’s disease and control groups using conditional logistic regression analyses. Additional analyses were conducted according to age, sex, income, and residential area. Temperature range; ambient atmospheric pressure; sunshine duration; and levels of SO₂, NO₂, O₃, CO, and PM₁₀ for 1 month and 6 months were associated with Meniere’s disease. Adjusted ORs (odds ratios with 95% confidence interval [CI]) for 1 and 6 months of O₃ concentration were 1.29 (95% CI 1.23–1.35) and 1.31 (95% CI 1.22–1.42), respectively; that for the 1 and 6 months of CO concentration were 3.34 (95% CI 2.39–4.68) and 4.19 (95% CI 2.79–6.30), respectively. Subgroup analyses indicated a steady relationship of O₃ and CO concentrations with Meniere’s disease. Meteorological factors and air pollutants were associated with the rate of Meniere’s disease. In particular, CO and O₃ concentrations were positively related to the occurrence of Meniere’s disease.

Relationship between cigarette smoking and hearing loss in China: A cross-sectional study in Zhejiang province

INTRODUCTION

Hearing loss and cigarette smoking are major challenges that affect public health in China. Revealing the effect of smoking on hearing loss in the Chinese general population is critical for hearing health protection. We investigated the relationship between smoking status and hearing loss in China, especially in stratified sex and age groups.

METHODS

A cross-sectional study was conducted on 4685 individuals aged 20– 80 years in Zhejiang province from 2016 to 2018, with audiometric testing for hearing loss and a structured questionnaire for collecting smoking status and covariates. Logistic regression was used to estimate the association between smoking and hearing loss.

RESULTS

Cigarette smoking was not significantly associated with hearing loss in females and young males. In middle-aged males, after adjusting for covariates, current smokers and past smokers had a significantly higher prevalence of speech-frequency loss (OR=1.65; 95% CI: 1.17–2.33 and OR=1.88; 95% CI: 1.11–3.17; respectively) and high-frequency hearing loss (OR=2.01; 95% CI: 1.43–2.84 and OR=2.64; 95% CI: 1.50–4.66; respectively). In older males, only past smokers had a significantly higher prevalence of speech-frequency hearing loss than never smokers (OR=2.58; 95% CI: 1.38–4.85). Regarding middle-aged and older current smokers, a dose-dependent relationship between smoking intensity and hearing loss was found. Passive smoking was not significantly associated with an increased hearing loss risk in all the three male groups.

CONCLUSIONS

The relationship between cigarette smoking and hearing loss varied according to gender and age. Therefore, the interventions for smoking need to be tailored according to age in males.

Efficacy of the additional effect of hyperbaric oxygen therapy in combination of systemic steroid and prostaglandin E1 for idiopathic sudden sensorineural hearing loss

PURPOSE

The efficacies of hyperbaric oxygen therapy (HBO), systemic steroid, prostaglandin E₁, or the combination of any two modalities have been reported in patients with idiopathic sudden sensorineural hearing loss (ISSNHL). However, little is known about the combined efficacy of HBO, systemic steroid, and prostaglandin E₁ for this disorder. We aimed to investigate the efficacy of HBO combined with systemic steroids and prostaglandin E₁ as triple therapy in patients with ISSNHL.

MATERIALS AND METHODS

We retrospectively evaluated the records of 67 patients with ISSNHL who were treated with systemic steroid and prostaglandin E₁, with (n = 38) or without (n = 29) HBO. The inclusion criteria included a diagnosis of ISSNHL within 14 days of symptom onset, age ≥15 years, treatment according to the protocol, and clinical follow-up of at least 1 month. The patients' hearing levels were evaluated 1 month after hearing loss onset. The primary outcome was hearing improvement on pure tone audiometry. We also evaluated the demographic profiles of patients.

RESULTS

Patients treated with triple therapy showed significantly greater hearing improvement (p < 0.01) than those treated without HBO, despite some differences between the two treatment groups. Multivariate logistic regression analysis revealed a significant positive correlation between pure tone audiometry improvement and hyperbaric oxygen therapy, after adjustment for confounding factors (odds ratio = 7.42; 95% and confidence interval = 2.37-23.3; p = 0.001).

CONCLUSION

HBO with systemic steroid and prostaglandin E₁ administration conferred significant therapeutic benefits for ISSNHL. Therefore, routine use of triple therapy is recommended for patients with ISSNHL.

Association between Smoking and Noise-Induced Hearing Loss: A Meta-Analysis of Observational Studies

The purpose of this study was to synthesize the results of previously published observational studies through meta-analysis to clarify the association between smoking and noise-induced hearing loss (NIHL). We searched several databases as of October 2019. Based on the results of heterogeneity analysis (Q statistic and I² statistic), a fixed effect model (for no heterogeneity; Q test P > 0.1 and I² ≤ 50%) or a random effects model (for heterogeneity) was used to calculate the pooled odds ratios (ORs). We explored the potential dose-response relationship between smoking and NIHL as well. In total, 27 studies involving 30,465 participants were included. Compared with non-smokers, the pooled OR of current smokers was 2.05 (95% Confidence interval (CI): 1.71–2.46), and of former smokers was 1.11 (95% CI: 1.05–1.18). We found a curve linear association between an increasing number of pack-years (packages/day × smoking years) and risk of NIHL. The dose-response meta-analysis suggested that when the number of pack-years was less than fifteen, the risk of NIHL was increasing, and the highest combined OR was 5.25 (95% CI: 2.30–11.96) for pack-years of fifteen. After fifteen pack-years, the pooled OR had a slow decline. Our study indicated that smoking is a risk factor for NIHL. Current smokers have a higher risk than former smokers, and there is a positive dose-response relationship between smoking and NIHL.

Associations of noise kurtosis, genetic variations in NOX3 and lifestyle factors with noise-induced hearing loss

BACKGROUND

Noise-induced hearing loss (NIHL) is a complex disease caused by environmental and genetic risk factors. This study was to explore the association of noise kurtosis, triphosphopyridine nucleotide oxidase 3 (NOX3) and lifestyles with NIHL.

METHODS

This case-control study included 307 patients with NIHL and 307 matched control individuals from Zhejiang province of China. General characteristics, noise exposure data, the exfoliated cells of the oral mucosa, and lifestyle details of individuals were collected. The kompetitive allele specific polymerase chain reaction (KASP) method was used to analyze the genotypes of three single nucleotide polymorphisms (SNPs) of NOX3.

RESULTS

People who exposed to complex noise had a higher risk of NIHL than those exposed to steady noise (adjusted: OR = 1.806, P = 0.002). The GT genotype of additive model and TT + GT genotype of dominant model in NOX3 rs12195525 decreased the risk of NIHL (adjusted: OR = 0.618, P = 0.043; OR = 0.622, P = 0.036). Smoking and exposure to high video volume increased the risk of NIHL (adjusted: OR = 1.486, P = 0.038; OR = 1.611, P = 0.014). Oppositely, regular physical exercise decreased the risk of NIHL (adjusted: OR = 0.598, P = 0.004). A positive interaction was found between complex noise and lifestyles including high video volume exposure and no physical exercise in the additive models (RERI = 1.088, P < 0.001; RERI = 1.054, P = 0.024). A positive interaction was also found between NOX3 rs12195525 GG genotype and lifestyles including smoking and high video volume exposure in the additive models (RERI = 1.042, P = 0.005; RERI = 0.774, P = 0.044).

CONCLUSIONS

Noise temporal structure, NOX3 rs12195525 polymorphism, and the three lifestyles of smoking, video volume, and physical exercise were related to the NIHL. There were the interactions between noise temporal structure and the lifestyle of video volume or physical exercise, as well as between NOX3 and the lifestyle of smoking or video volume. These results provide a theoretical basis for the prevention and genetic testing of NIHL.

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.

Subacute carbon monoxide poisoning presenting as vertigo and fluctuating low frequency hearing loss

It is estimated that up to 25 000 UK residents are exposed to small amounts of carbon monoxide (CO) annually. Symptoms of chronic exposure to CO are vague and non-specific and include dizziness and hearing loss. We describe a case of 38-year-old lady presenting with a 4-month history of vertigo and hearing loss. Initially diagnosed as Meniere’s disease, the patient was investigated and followed up in the clinic. She reported leakage of carbon monoxide from her gas fire identified during a routine safety check. Her symptoms fully resolved after disconnecting the faulty gas fire. A rare cause of fluctuating hearing loss and vertigo is described, and the diagnostic challenges are discussed.

Oxidative Stress: A Unifying Mechanism for Cell Damage Induced by Noise, (Water-Pipe) Smoking, and Emotional Stress-Therapeutic Strategies Targeting Redox Imbalance

SIGNIFICANCE

Modern technologies have eased our lives but these conveniences can impact our lifestyles in destructive ways. Noise pollution, mental stresses, and smoking (as a stress-relieving solution) are some environmental hazards that affect our well-being and healthcare budgets. Scrutinizing their pathophysiology could lead to solutions to reduce their harmful effects. Recent Advances: Oxidative stress plays an important role in initiating local and systemic inflammation after noise pollution, mental stress, and smoking. Lipid peroxidation and release of lysolipid by-products, disturbance in activation and function of nuclear factor erythroid 2-related factor 2 (Nrf2), induction of stress hormones and their secondary effects on intracellular kinases, and dysregulation of intracellular Ca²⁺ can all potentially trigger other vicious cycles. Recent clinical data suggest that boosting the antioxidant system through nonpharmacological measures, for example, lifestyle changes that include exercise have benefits that cannot easily be achieved with pharmacological interventions alone.

CRITICAL ISSUES

Indiscriminate manipulation of the cellular redox network could lead to a new series of ailments. An ideal approach requires meticulous scrutiny of redox balance mechanisms for individual pathologies so as to create new treatment strategies that target key pathways while minimizing side effects.

FUTURE DIRECTIONS

Extrapolating our understanding of redox balance to other debilitating conditions such as diabetes and the metabolic syndrome could potentially lead to devising a unifying therapeutic strategy. Antioxid. Redox Signal. 28, 741-759.

Treatment of Acute Carbon Monoxide Poisoning with Extracorporeal Membrane Trioxygenation

Objective

To treat acute carbon monoxide poisoning (ACOP) with extracorporeal membrane trioxygenation (ECMO3), and to determine the efficacy and safety of ECMO3.

Method

Thirty-two New Zealand white rabbits were divided randomly into four groups including ECMO3 group (G1-ECMO3) oxygen treatment group (G2–FIO2), untreated ACOP group (G3-ACOP), and control group (G4-control). Rabbits in the first three groups were intraperitoneally injected with 99.99% CO at a dosage of 200 ml/kg, and those in the control group were treated with placebo. The dynamic changes in carboxyhemoglobin (COHb) concentration, blood oxygen saturation (SO2) level, base excess of blood (BE-B) as well as the vital signs of the rabbits were monitored.

Results

All the experimental rabbits had significantly higher levels of COHb (p = 0.000<0.05) than those in the control group after 30 min of CO injection with poisoning reactions. The respiration and heart rate of the rabbits in the ECMO3 group and FIO2 group were recovered to a level close to those of the rabbits in the control group by the end of the treatment, and they were significantly lower than those in the ACOP group (p = 0.000, <0.05). The COHb levels of rabbits in the G1–ECMO3 group were significantly lower than those in the G2-FIO2 and the G3-ACOP groups (F = 42.799, p = 0.000), and were similar to those in the control group. After 0.5 h of treatment, the SO2 and BE-B levels of rabbits in the G1-ECMO3 and the G2-FIO2 groups were higher than those in the G3-ACOP group (p<0.05, p = 0.000<0.05).

Conclusions

ECMO3 treatment effectively lowered the COHb levels, increased SO2 levels, and cured acid poisoning, making it a safe and promising ACOP treatment strategy.

A novel effective chemical hemin for the treatment of acute carbon monoxide poisoning in mice

There is no effective drug for the therapy of acute carbon monoxide (CO) poisoning. The purpose of the present study was to investigate the potential preventive and therapeutic effects of hemin on an animal model of acute CO poisoning and to provide a potential therapeutic candidate drug. A total of 80 Kunming mice were randomly divided into four groups, namely the air control, acute CO poisoning, hemin-treatment + CO and hemin-pretreatment + CO groups (n=20 each). Furthermore, the mortality rate of mice, blood carboxyhaemoglobin (HbCO) concentration and serum malondialdehyde (MDA) concentration were measured, and pathological changes of the hippocampal area were determined using histochemical staining. The mice with acute CO poisoning had a 50% mortality rate at 1 h, with an increase in blood HbCO, serum MDA levels and pathological impairments of the hippocampus. Furthermore, the mortality rate, blood HbCO and serum MDA levels of mice with pretreatment and treatment of hemin were decreased. Additionally, the pathological changes of the hippocampal area were improved in the hemin-treatment and hemin-pretreatment groups compared with the mice treated with CO. These results suggest that hemin is a novel effective chemical for the prevention and treatment of acute CO poisoning in mice. Therefore, the present study provides a novel method and experimental basis for the application of hemin in treating patients with acute CO poisoning.

The combined effect of cigarette smoking and occupational noise exposure on hearing loss: evidence from the Dongfeng-Tongji Cohort Study

Combined effect of cigarette smoking and occupational noise exposure on hearing loss has rarely been evaluated among Chinese population, especially among females. This cross-sectional study was conducted in 11196 participants of Dongfeng-Tongji cohort study. Smoking status was self-reported through questionnaire and occupational noise exposure was evaluated through workplace noise level and/or the job titles. Hearing loss was defined as a pure-tone mean of 25 dB or higher at 0.5, 1, 2, and 4 kHz in both ears. Compared with participants without occupational noise exposure, the risk of hearing loss was significantly higher for noise exposure duration ≥20 (OR = 1.45, 95%CI = 1.28–1.65). The association was particularly evident among individuals who were males (OR = 1.74, 95%CI = 1.45–2.08) and aged ≥ 70 (OR = 1.74, 95%CI = 1.30–2.33). Similarly, the risks increased with the increasing of pack-years in males and all age groups except for those aged <60. As to the combined effect, the hearing loss risk was highest for noise exposure duration ≥20 and pack-years ≥25 (OR = 2.41, 95%CI = 1.78–3.28), especially among males (OR = 2.42, 95%CI = 1.74–3.37) and those aged ≥70 (OR = 2.76, 95%CI = 1.36–5.60). Smoking may be an independent risk factor for hearing loss. And it may synergistically affect hearing when combined with occupational noise exposure, especially among males and older participants.

Sex-Related Cochlear Impairment in Cigarette Smokers

Background

A number of studies have documented the influence of cigarette smoking on hearing. However, the association between sex and hearing impairment in smokers as measured by otoacoustic emissions (OAEs) has not been clearly established. The aim of this study was to analyze sex-specific effects of smoking on hearing via conventional and ultra-high-frequency pure tone audiometry (PTA), and OAEs, specifically spontaneous OAEs, click-evoked OAEs, and distortion-product OAEs.

Material/Methods

The study included 84 healthy volunteers aged 25–45 years (mean 34), among them 46 women (25 non-smokers and 21 smokers) and 38 men (16 non-smokers and 22 smokers). The protocol of the study included otoscopic examination, tympanometry, low-, moderate-, and ultra-high-frequency PTA, evaluation of spontaneous click-evoked (CEAOEs) and distortion-product otoacoustic emissions (DPOAEs), assessment of the DP-grams for 2f₁-f₂ (f₁ from 977 to5 164 Hz), and input/output function at L₂ primary tone level of 40–70 dB SPL.

Results

Smokers and non-smokers did not differ significantly in terms of their hearing thresholds assessed with tone audiometry. Male smokers presented with significantly lower levels of CEAOEs and DPOAEs than both male non-smokers and female smokers.

Conclusions

Smoking does not modulate a hearing threshold determined with PTA at low, moderate, and ultra-high frequencies, but causes a significant decrease in OAE levels. This effect was observed only in males, which implies that they are more susceptible to smoking-induced hearing impairment. Sex-specific differences in otoacoustic emissions level may reflect influences of genetic, hormonal, behavioral, and/or environmental factors.

Concurrent effect of noise exposure and smoking on extended high-frequency pure-tone thresholds

OBJECTIVE

Concurrent effect of noise and smoking on hearing loss is a recent concern. In this study, the concurrent effect of noise and smoking on hearing loss in conventional frequencies and frequencies higher than 8 kHz was assessed.

DESIGN

This was a cross-sectional study on workers exposed to noise who were divided into two groups: smokers and non-smokers. Hearing thresholds were assessed by conventional audiometry, and HFA. Data were analysed using non-parametric tests and Student's t-test.

STUDY SAMPLE

There were 212 workers.

RESULTS

Ninety-seven subjects were smokers and 115 individuals were non-smokers. All subjects were exposed to 92.1 ± 2.4 dBA (Leq8h). The highest threshold in conventional and high-frequency audiometry was observed at 6 kHz and 16 kHz, respectively. Hearing threshold at frequencies above 1 kHz was significantly higher in the smokers than non-smokers. There was no correlation between hearing thresholds and pack-years of smoking.

CONCLUSIONS

Concurrent exposure to noise and smoking may be associated with more hearing loss than exposure to noise alone in the conventional and high frequencies. However, other differences between smokers and non-smokers may explain these differences as well.

Cigarette smoking during pregnancy regulates the expression of specific nicotinic acetylcholine receptor (nAChR) subunits in the human placenta

Smoking during pregnancy is associated with low birth weight, premature delivery, and neonatal morbidity and mortality. Nicotine, a major pathogenic compound of cigarette smoke, binds to the nicotinic acetylcholine receptors (nAChRs). A total of 16 nAChR subunits have been identified in mammals (9 α, 4 β, and 1 δ, γ and ε subunits). The effect of cigarette smoking on the expression of these subunits in the placenta has not yet been determined, thus constituting the aim of this study. Using RT-qPCR and western blotting, this study investigated all 16 mammalian nAChR subunits in the normal healthy human placenta, and compared mRNA and protein expressions in the placentas from smokers (n = 8) to controls (n = 8). Our data show that all 16 subunit mRNAs are expressed in the normal, non-diseased human placenta and that the expression of α2, α3, α4, α9, β2 and β4 subunits is greater than the other subunits. For mRNA, cigarette smoke exposure was associated with increased expression of the α9 subunit, and decreased expression of the δ subunit. At the protein level, expression of both α9 and δ was increased. Thus, cigarette smoking in pregnancy is sufficient to regulate nAChR subunits in the placenta, specifically α9 and δ subunits, and could contribute to the adverse effects of vasoconstriction and decreased re-epithelialisation (α9), and increased calcification and apoptosis (δ), seen in the placentas of smoking women.

Carbon monoxide inhalation increases microparticles causing vascular and CNS dysfunction

We hypothesized that circulating microparticles (MPs) play a role in pro-inflammatory effects associated with carbon monoxide (CO) inhalation. Mice exposed for 1h to 100 ppm CO or more exhibit increases in circulating MPs derived from a variety of vascular cells as well as neutrophil activation. Tissue injury was quantified as 2000 kDa dextran leakage from vessels and as neutrophil sequestration in the brain and skeletal muscle; and central nervous system nerve dysfunction was documented as broadening of the neurohypophysial action potential (AP). Indices of injury occurred following exposures to 1000 ppm for 1h or to 1000 ppm for 40 min followed by 3000 ppm for 20 min. MPs were implicated in causing injuries because infusing the surfactant MP lytic agent, polyethylene glycol telomere B (PEGtB) abrogated elevations in MPs, vascular leak, neutrophil sequestration and AP prolongation. These manifestations of tissue injury also did not occur in mice lacking myeloperoxidase. Vascular leakage and AP prolongation were produced in naïve mice infused with MPs that had been obtained from CO poisoned mice, but this did not occur with MPs obtained from control mice. We conclude that CO poisoning triggers elevations of MPs that activate neutrophils which subsequently cause tissue injuries.

Hearing Loss due to Carbon Monoxide Poisoning

Carbon monoxide poisoning is one of the rare causes of hearing loss which may cause reversible or irreversible, unilateral or bilateral hearing loss after acute or chronic exposure. In this report, we present a case of bilateral sensorineural hearing loss in a secondary smelting workshop worker after an acute exposure to carbon monoxide. This complication was diagnosed by pure-tone audiometry and confirmed by transient evoked otoacoustic emissions. Hearing loss has not improved after 3 months of followup.

Prevalence of Noise Induced Hearing Loss in Indian Air Force Personnel

BACKGROUND

Noise exposure is an inherent part of duty of the aircrew and maintenance staff in the Indian Air Force (IAF). An audiometric survey of IAF personnel was carried out to analyze the prevalence and severity of noise induced hearing loss (NIHL).

METHODS

An audiometric study of 1000 IAF personnel of various trades, age and exposure groups were carried out at an operational base. Age related correction of 5 dB per decade was applied for personnel above SO years of age. Personal factors like smoking, blood group and use of ear defenders were analysed for their effect on the prevalence of NIHL.

RESULT

The study revealed an overall incidence of NIHL of 22.9 % in the personnel with a higher prevalence among the technical trades (26.18 %) as compared to non technical trades (12.5 %). There was a significant increase in prevalence of hearing loss with duration of exposure with 50.8% of personnel above 30 years of exposure having hearing loss. A higher incidence of hearing loss was observed in personnel of blood group O and smokers whereas use of ear defenders was protective.

CONCLUSION

The study reveals a significantly increased prevalence of NIHL among the technical trades of the IAF personnel. The deleterious effects of smoking and protective effect of regular use of ear defenders are emphasized.

Characterization of noise and carbon monoxide exposures among professional firefighters in British Columbia

OBJECTIVES

To characterize exposures to noise and carbon monoxide (CO) among firefighters in British Columbia, Canada.

METHODS

Subjects were recruited from 13 fire halls across three municipalities in Metro Vancouver. Personal full-shift noise and CO samples were collected using datalogging noise dosimeters and CO monitors on both day and night shifts. Determinants of exposure (DoE) information were recorded by trained research staff and hygienists through direct observation during the measurement period.

RESULTS

In total, 113 noise and 156 CO samples were collected from 45 male firefighters, aged 41.0 ± 7.2 years with 14.2 ± 9.0 years of experience. Mean L(eq) and peak noise levels were 81.1 ± 4.8 dBA and 137.1 ± 5.2 dB, respectively; 45% of samples exceeded occupational limits. Noise levels were significantly greater on day shifts, among firefighters in non-supervisory jobs, for those working on engine and rescue trucks, by number of emergency calls they attended and in particular for motor vehicle accident (MVA) and building alarms calls, if subjects worked near or used fire equipment, or if they participated in active firefighting training activities. Full-shift and peak CO levels were 1.0 ppm [geometric mean (GM) = 0.7, geometric standard deviation (GSD) = 1.8] and 42.9 ppm (GM = 9.95, GSD = 5.6), respectively; 1% of CO samples exceeded occupational limits. Both full-shift and peak CO levels were significantly correlated by number of MVAs and building alarms calls.

CONCLUSIONS

Our results show that firefighters may be at an increased risk of exposure to high noise levels, but CO exposures were lower than anticipated. Additional exposure studies are needed to confirm our results and to better understand the DoE to noise and CO among this occupational group.

Analysis of auditory measures in normal hearing young male adult cigarette smokers using multiple variable selection methods with predictive validation assessments

Studies have shown that cigarette smoking is a risk factor for hearing loss; however, no information is available on auditory preclinical indicators in young chronic cigarette smokers. Cigarette smoking involves exposure to many harmful chemicals including carbon monoxide (CO). In this study, the CO level in 16 young normal hearing male chronic smokers was measured with a CO monitor, and was used as the outcome measure. Subjects were administered a battery of audiological tests that included behavioral and electrophysiologic measures. The goal was to investigate which auditory test measures can be used as potential predictors of the outcome measure. Using ordinary least squares estimation procedures with best-subsets selection and bootstrapped stepwise variable selection procedures, an optimal predictive multiple linear regression model was selected. Results of this approach indicated that auditory brainstem response peak V amplitudes and distortion product otoacoustic emissions had the highest predictive value and accounted for most of the variability.

Subjective audiological tests and transient evoked otoacoustic emissions in patients with rheumatoid arthritis: analysis of the factors affecting hearing levels

We investigated hearing functions in patients with rheumatoid arthritis (RA) using audiological tests and transient evoked otoacoustic emissions (TEOAEs). The study group consisted of 20 adult patients with RA (7 males, 13 females); 20 adult healthy subjects without RA (7 males, 13 females) were recruited as controls. All patients were evaluated by pure tone audiometry, high frequency audiometry, tympanometry and TEOAEs. There were no statistical differences between the study and control groups with respect to the pure tone and high frequency audiometries. TEOAE results of 1.0-2.0 kHz % and of 1.5 and 3.0 kHz amplitude values were significantly lower, and ipsilateral stapes reflex threshold value at 1.0 kHz was significantly higher in the study group when compared to respective values in the control group. In elderly patients and those with longer disease duration, RA nodules and higher methotrexate cumulative doses, hearing thresholds increased and TEOAE values decreased. In active stage of the disease, hearing thresholds diminished and in higher Brinkman Index values, TEOAE values decreased. Compliance values decreased in patients with higher Ritchie Articular Index, C-reactive protein, erythrocyte sedimentation rate and platelet counts, and longer disease duration. Sensorineural hearing loss is generally observed in patients with RA, and this condition may be detected by TEOAEs in an early period of the disease. Inflammation during the active stage of the disease and the subsequent fibrosis may cause conductive hearing loss of varying degrees. In those patients detected as having initiation of TEOAE decrease, vasodilator treatment and antioxidant drugs may be useful in protecting the inner ear.

Influence of smoking on developing cochlea. Does smoking during pregnancy affect the amplitudes of transient evoked otoacoustic emissions in newborns?

OBJECTIVE

Maternal tobacco smoking has negative effects on fetal growth. The influence of smoking during pregnancy on the developing cochlea has not been estimated, although smoking has been positively associated with hearing loss in adults. The objective of this study was to determine the effects of maternal smoking on transient evoked otoacoustic emissions (TEOAEs) of healthy neonates.

METHODS

This study was undertaken as part of neonatal screening for hearing impairment and involved both ears of 200 newborns. Newborns whose mothers reported smoking during pregnancy (n=200 ears) were compared to a control group of newborns (n=200 ears), whose mothers were non-smokers. Exposure to tobacco was characterized as low (<5 cigarettes per day, n=88 ears), moderate (5< or =cigarettes per day<10, n=76) or high (> or =10 cigarettes per day, n=36).

RESULTS

In exposed neonates, TEOAEs mean response (across frequency) and mean amplitude at 4000Hz was significantly lower than in non-exposed neonates. Comparisons between exposed newborns' subgroups revealed no significant differences. However, by comparing each subgroup to control group, we found statistically significant decreases of TEOAEs amplitudes at 4000Hz for all three groups. Mean TEOAEs responses of highly exposed newborns were also significantly lower in comparison to our control group.

CONCLUSION

In utero, exposure to tobacco smoking seems to have a small impact on outer hair cells. These effects seem to be equally true for all exposed newborns, regardless of the degree of exposure. Further studies are needed in order to establish a potential negative effect of maternal smoking on the neonate's hearing acuity.

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.

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.

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.

Preventing internal auditory artery vasospasm using topical papaverine: an animal study

BACKGROUND

Internal auditory artery (IAA) spasm is thought to be one of the causes of postoperative sensory hearing loss after attempted hearing preservation removal of an acoustic neuroma. The use of topical papaverine, a nonspecific vasodilator, to prevent vascular insufficiency to the inner ear and to improve hearing outcomes has been suggested but not proven.

MATERIALS AND METHODS

Vasospasm was mechanically induced by compressing the IAA in the control ears of six rabbits after application of topical saline. The subsequent reduction of cochlear blood flow (CBF) was measured using a laser-Doppler (LD) flow-monitoring technique. Functional loss of cochlear activity was verified with distortion product otoacoustic emissions (DPOAE). The contralateral experimental ears were treated with the topical application of papaverine directly to the IAA and cochleovestibular nerve complex. CBF and DPOAE were compared between the control and papaverine treated ears for 3-minute and 5-minute IAA compressions.

RESULTS

Every control ear demonstrated some degree of postcompression IAA vasospasm (i.e., reduced CBF) and reduction of DPOAE. Nearly complete recovery of CBF and DPOAE to baseline was observed in all of the papaverine treated ears.

CONCLUSIONS

An animal model of IAA vasospasm was described. Mechanically induced vasospasm of the IAA was prevented by the topical application of papaverine. These findings have clinical implications for surgical procedures involving the internal auditory canal/cerebellopontine angle such as acoustic neuroma removal.

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.

Transient cochlear ischemia causes delayed cell death in the organ of Corti: an experimental study in gerbils

To elucidate whether ischemia-reperfusion can cause delayed cell death in the cochlea, the effects of transient cochlear ischemia on hearing and on neuronal structures in the cochlea were studied in Mongolian gerbils. Ischemia was induced by bilaterally occluding the vertebral arteries for 5 minutes in gerbils, which lack posterior cerebral communicating arteries. In gerbils, the labyrinthine arteries are fed solely by the vertebral arteries. Occlusion of the vertebral arteries caused a remarkable increase in the threshold of compound action potentials (CAPs), which recovered over the following day. However, 7 days after the onset of reperfusion, the threshold began to increase again. Morphologic changes in the hair cell stereocilia were revealed by electron microscopy. The number of nuclear collapses was counted in cells stained for DNA and F-actin to evaluate the degree of cell death in the organ of Corti. Changes in spiral ganglion cell (SGC) neuron number were detected, whether or not progressive neuronal death occurred in the SGC. These studies showed that sporadic fusion of hair cells and the disappearance of hair cell stereocilia did not begin until 4 days after ischemia. On subsequent days, the loss of hair cells, especially inner hair cells (IHCs), and the degeneration of SGC neurons became apparent. Ten days after ischemia, the mean percentage cell loss of IHCs was 6.4% in the basal turn, 6.4% in the second turn, and 0.8% in the apical turn, respectively, and the number of SGC neurons had decreased to 89% of preischemic status. These results indicate that transient ischemia causes delayed hearing loss and cell death in the cochlea by day 7 after ischemia.

Combined effect of smoking and occupational exposure to noise on hearing loss in steel factory workers

BACKGROUND

Evidence has accumulated concerning the adverse effects of smoking on hearing acuity, but it is not clear whether smoking modifies the association between exposure to noise and hearing loss.

AIMS

To examine the synergistic effect of these variables on hearing.

METHODS

Data used were derived from periodic health examinations for 4624 steel company workers in Japan and included audiometry testing and information on smoking habits. Occupational exposure to noise was determined based on company records. Logistic regression was used to examine the dose-response association between smoking and hearing loss. The Cochran-Mantel-Haenszel method was used to calculate the prevalence rate ratio (PRR) of hearing loss for each combination of smoking and noise exposure factors, taking non-smokers not exposed to occupational noise as a reference. The interaction between smoking and noise exposure was assessed using a synergistic index, which equals 1 when the joint effect is additive.

RESULTS

Smoking was associated with increased odds of having high frequency hearing loss in a dose-response manner. The PRR for high frequency hearing loss among smokers exposed to occupational noise was 2.56 (95% CI 2.12 to 3.07), while the PRR for smokers not exposed to noise was 1.57 (95% CI 1.31 to 1.89) and the PRR for non-smokers exposed to noise was 1.77 (95% CI 1.36 to 2.30). The synergistic index was 1.16. Smoking was not associated with low frequency hearing loss.

CONCLUSIONS

Smoking may be a risk factor for high frequency hearing loss, and its combined effect on hearing with exposure to occupational noise is additive.

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.

Potentiation of noise-induced hearing loss by amikacin in guinea pigs

Noise and aminoglycosides initially attack cochlear outer hair cells (OHCs). Distortion product otoacoustic emissions (DPOAEs) are used for the early diagnosis of damage to OHCs. The effects of sub-damaging doses of amikacin, an aminoglycoside antibiotic agent, on noise-induced hearing loss (NIHL) were examined in guinea pigs. Animals were grouped by gender and exposed to broadband noise at 105 dB SPL for 12 h and/or injected i.m. with either amikacin (100 mg/kg/day) or saline for 10 days. Auditory brainstem response (ABR) thresholds, along with DPOAE amplitudes, were measured serially before and after noise exposure. DPOAE amplitudes decreased and ABR thresholds elevated immediately after noise exposure and then gradually recovered. At all frequencies, the emission amplitudes recovered completely to pre-exposure baseline values by 4 days after noise exposure. There was no effect of amikacin on either the ABR threshold or DPOAE amplitudes, in animals treated with amikacin only. However, amikacin significantly prolonged the effect of noise exposure on DPOAE amplitude but not on the noise-induced temporary threshold shift (TTS) of the ABR. In animals treated with a combination of noise and amikacin, significant changes in DPOAE amplitudes were still observed at 4 weeks after cessation of noise exposure. No gender difference in the responses to noise and/or amikacin could be demonstrated. The present findings indicate that even sub-damaging dosages of amikacin might impair recovery from NIHL in guinea pigs.

Acute effect of nicotine on non-smokers: I. OAEs and ABRs

This paper is the first in a series of three investigating the role of cholinergic mechanisms in the auditory system by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on aggregate responses within the auditory pathway. In a single-blind procedure, auditory responses were obtained from 20 normal-hearing, non-smokers (10 male) under two conditions (nicotine, placebo). After the drug session, plasma tests revealed a subject's nicotine concentration. The effects of nicotine on early, exogenous responses of the auditory system (otoacoustic emissions and auditory brainstem potentials) are described in this first paper. Results indicated that transdermal administration of nicotine to non-smokers does not significantly affect cochlear activity but does acutely affect the neural transmission of acoustic information. Overall, otoacoustic emissions were unaffected by transdermal nicotine while wave I of the auditory brainstem response was significantly increased in latency and decreased in amplitude.

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.

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.

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

Intermittent noise causes less hearing loss than continuous noise of equal intensity. The reduction in damage observed with intermittent noise may be explained by the fact that the auditory system has time to recover between the noise phases. Simultaneous carbon monoxide (CO) exposure produces greater noise-induced hearing loss than does noise alone (Chen and Fechter, 1999). In the present study, intermittent noise (octave-band with a center frequency of 13.6 kHz, 100 dB) of a 2 h total duration but with a different duty cycle (% of noise during exposure) was used. The intermittent exposure that had a shorter noise duty cycle induced a less permanent threshold shift (PTS) than those that had a longer noise duty cycle (or less rest periods). This relation between the loss in compound action potential (CAP) sensitivity and the noise duty cycle (or rest period) was abolished by the presence of CO. The cochlear microphonic (CM) amplitude revealed similar results to those seen using the CAP. While intermittent noise that had a short noise duty cycle did not cause hair cell loss by itself, the combined exposure to noise and CO (1200 ppm) caused remarkable OHC loss in the basal turn.

Elevation of reactive oxygen species following ischemia-reperfusion in mouse cochlea observed in vivo

An in vivo method for assessment of changes in hydroxyl radical levels in cochlear perilymphatic spaces is described and applied to cochlear ischemia-reperfusion in the mouse. Cochlear blood flow was reversibly reduced by compression of the anterior inferior cerebellar arterial network. Changes in the production of hydroxyl radicals, used as an index of tissue production of reactive oxygen species (ROS), were determined by measuring the conversion of salicylate to 2,3-dihydroxybenzoic acid. Low levels of salicylate (0.1 mM) in artificial perilymph were applied by perfusion of the cochlea using a round window entry and apical exit. Perfusate was collected and analyzed by high-performance liquid chromatography. Forty minutes of partial ischemia led to a > 10-fold average increase over baseline in the concentration of hydroxyl radical, which increase persisted for at least 40-80 min following reperfusion. Our observations support previous results obtained using less direct methods, indicating that cochlear ischemia-reperfusion and related damage is associated with elevated ROS levels. Development of an in vivo method for assessing changes in cochlear ROS in mice will facilitate the study of the relation between deafness genes, vulnerability to insults and dynamics of cellular processes that produce and regulate ROS.

Potentiation of octave-band noise induced auditory impairment by carbon monoxide

In previous studies from our lab, broadband noise induced hearing loss has been found to be potentiated by simultaneous carbon monoxide (CO) exposure. In the present study, octave-band noise induced auditory impairment was studied with the presence of CO at levels of 1500, 1200, 700, 500 and 300 ppm and zero (noise alone). Four octave-band noises (1.2-2.4, 2.4-4.8, 4.8-9.6 and 9.6-19.2 kHz) were used. Experimental subjects (rats) were grouped for the exposure (8 h) to each noise, CO and their combinations. The compound action potential (CAP) and cochlear microphonics (CM) were recorded 4 weeks after the exposure. The noise induced elevation of the CAP threshold and the CM iso-amplitude curve were potentiated by the simultaneous CO exposure when the CO level reached 500 ppm or higher. CO exposure alone had no effect on CAP or CM. The CO potentiation can occur in any frequency region depending on the noise band. The combined exposure can also induce threshold shifts in some cases in which both the noise and the CO alone did not cause threshold shifts. The size of the potentiation shown by CAP and CM was similar, indicating a possible origin of the CO potentiation from the damage to the outer hair cells. Interestingly, the hearing loss induced by noise alone gradually recovered (partially), but the hearing loss caused by the combined exposure did not. The potentiation may be due to the reduction of the cell's ability to repair noise induced damage by CO.