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

Perfil audiométrico e de emissões otoacústicas evocadas por produto de distorção em gestores de trânsito expostos a monóxido de carbono e ruído

OBJETIVO: avaliar o perfil audiométrico e de emissões otoacústicas evocadas por produto de distorção em gestores de trânsito, expostos a monóxido de carbono e ruído, bem como constatar a presença de ambos agentes nos postos de trabalho. MÉTODO:estudo transversal, descritivo, com 37 gestores do trânsito, submetidos a anamnese, meatoscopia, audiometria tonal e emissões otoacústicas, distribuídos em: G1, composto por 18 indivíduos sem histórico de exposição concomitante a monóxido de carbono e ruído; e, G2, formado por 19 trabalhadores expostos simultaneamente aos dois agentes. Para pesquisa da presença dos agentes no ambiente foram utilizadas audiodosímetrias e avaliações de curta duração com medidor instantâneo. As variáveis de anamnese foram analisadas segundo o teste t Student e Mann-Whitney. Para as medidas de otoemissões acústicas e de limiares tonais utilizou-se testes de qui-quadrado (χ2) ou exato de Fisher e dos postos sinalizados de Wilcoxon com significância de 5%. RESULTADOS: foi constatada presença de monóxido de carbono e ruído durante a atividade dos trabalhadores. Não foi observada diferença significante na idade e tempo de função. O G2 obteve média de limiares tonais maior que G1, para orelha direita, em 1KHz (p=0,050) e para orelha esquerda em 3KHz (p=0,016) e 4KHz (p=0,028); e, comparados os limiares tonais alterados G2 apresentou diferença maior em 3KHz na orelha esquerda (p=0,003). Nas emissões otoacústicas, G2 apresentou maior ausência de respostas que G1 em 2.730Hz e 3.218Hz (p=0.016) para orelha direita. CONCLUSÃO: trabalhadores expostos a monóxido de carbono e ruído apresentaram piores resultados audiométricos e nas emissões otoacústicas quando comparado ao grupo de não expostos.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

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.

Age-related hearing loss in the Fischer 344/NHsd rat substrain

Studies of the F344 rat have shown a variety of age-related auditory anatomy and physiology changes. The current study was undertaken to clarify the ARHL in the F344 rat, by examining the auditory pathway of the F344/NHsd substrain that is distributed by Harlan Laboratories for research in the United States. The F344/NHsd rat begins to lose its hearing at about 12 months, and by 24 months, there are 50-60 dB auditory brainstem response threshold shifts at 20 and 40 kHz and 20 dB losses at 5-10 kHz. Distortion product otoacoustic emissions (DPOAE) amplitudes at 1.8-12 kHz stimuli were depressed in the older (18-24 months) rats. Amplitude input-output functions of the compound action potential (CAP) were also depressed across frequency. The endocochlear potential (EP) was 90-100 mV in the 3 month old rats. All but one of the 24 month old rats' EPs were in the +75-85 mV range. Tympanometry revealed no differences in middle ear function between the young and older rats. Collectively, these findings suggest damage to the outer hair cells, but anatomical examination of the outer hair cells revealed a relative lack of cell loss compared to the magnitude of the hearing and DPOAE loss.

[Sensorineural hearing loss as a consequence of carbon monoxide intoxication]

The authors describe the case of acute carbon monoxide intoxication in a 19 year old female patient, which caused, among other symptoms, bilateral sensorineural hearing loss. The symptoms, diagnostic methods and treatment with a particular regard to audiological examination were presented. Pathomechanism of carbon monoxide intoxication was also discussed. Despite the treatment, the effects of therapy have not been satisfactory.

Oxidative stress pathways in the potentiation of noise-induced hearing loss by acrylonitrile

We hypothesize that the disruption of antioxidant defenses is a key mechanism whereby chemical contaminants can potentiate noise-induced hearing loss (NIHL). This hypothesis was tested using acrylonitrile (ACN), a widely used industrial chemical whose metabolism is associated with glutathione (GSH) depletion and cyanide (CN) generation. CN, in turn, can inhibit Cu/Zn superoxide dismutase (SOD). We have shown previously that ACN potentiates NIHL, even with noise exposure approaching permissible occupational levels. However, the relative involvement of GSH depletion and/or CN production in this potentiation is still unknown. In this study, we altered these metabolic pathways pharmacologically in order to further delineate the role of specific antioxidants in the protection of the cochlea. We investigated the effects of sodium thiosulfate (STS), a CN inhibitor, 4-methylpyrazole (4MP), a drug that blocks CN generation by competing with CYP2E1, and l-N-acetylcysteine (l-NAC), a pro-GSH drug, in order to distinguish between GSH depletion and CN production as the mechanism responsible for potentiation of NIHL by ACN. Long-Evans rats were exposed to an octave-band noise (97 dB SPL, 4h/day, 5 days) and ACN (50 mg/kg). Separate pre-treatments with STS (150 mg/kg), 4MP (100 mg/kg) and l-NAC (4 x 400 mg/kg) all dramatically reduced blood CN levels, but only l-NAC significantly protected GSH levels in both the liver and the cochlea. Concurrently, only l-NAC treatment decreased the auditory loss and hair cell loss resulting from ACN + noise, suggesting that GSH is involved in the protection of the cochlea against reactive oxygen species generated by moderate noise levels. On the other hand, CN does not seem to be involved in this potentiation.

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.

Permanent threshold shift caused by acute cochlear mitochondrial dysfunction is primarily mediated by degeneration of the lateral wall of the cochlea

Mitochondrial dysfunction in the cochlea is thought to be an important cause of sensorineural hearing loss. Recently, we have established a novel rat model with acute hearing impairment caused by exposure to the mitochondrial toxin 3-nitropropionic acid (3-NP) to analyze the mechanism of cochlear mitochondrial dysfunction. Both permanent and temporary threshold shifts were observed in this model depending on the amount of 3-NP used to induce hearing impairment. In this study, we demonstrate cochlear morphological changes in the permanent threshold shift model. Marked degeneration was detected in type 2 fibrocytes in the spiral prominence, type 4 fibrocytes in the spiral ligament, marginal cells and intermediate cells in the stria vascularis 3 h after 3-NP administration; these changes were progressive for at least 14 days. Less prominent degeneration was detected in type 1 and type 3 fibrocytes in the spiral ligament. These results indicate that permanent threshold shift caused by acute cochlear mitochondrial dysfunction is primarily mediated by cellular degeneration in the lateral wall of the cochlea, and suggest that therapy of cochlear hearing loss due to acute energy failure may be achieved through protection and regeneration of the cochlear lateral wall.

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.

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.

A narcotic dose of carbon monoxide induces neuronal haeme oxygenase and nitric oxide synthetase in sheep

Twelve Romney ewes were exposed to either 1% carbon monoxide (CO) in air (n=6) or room air alone for 120 min and were killed 15 days later for histological and immunohistochemical examination. This dose of CO was narcotic and induced both haeme oxygenase and nitric oxide synthetase in brain neurons, but not in endothelial cells. The mechanism of the induction is not established here, but cellular theories of CO toxicity will need to be re-examined given these results.