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Biomedical subjects

D Henderson

Publications and source records attributed to D Henderson.

At least 91 records · Page 5Linked to original sources
PubMed1998

Acquisition of noise-induced hearing loss by railway workers.

OBJECTIVE: The rate of growth of hearing loss was examined in a population of workers exposed to noise. Of special interest was the course of loss in the first 10 yr of work, for which international standards provide little information. Based on the International Standards Organization (ISO) 1999 compilation, it was hypothesized that the proportional growth of hearing loss would be constant in log years. DESIGN: Of 5000 workers compensated for hearing loss by a railroad, 500 were selected at random for analysis. For these workers, 2660 audiograms were available. The audiograms were distributed unsystematically over work careers, and for most workers were sparsely distributed. Thresholds were age corrected using ISO 1999 Annex B and averaged as a function of years on the job for two partitions, 0.5, 1, 2, and 3 kHz and 2 and 4 kHz. The average curves were compared with projections based on the ISO 1999 data as suggested by Dobie (1993a). A subset of workers with substantial hearing loss when hired was examined to evaluate the effect of pre-existing hearing loss. RESULTS: Hearing was worse at hire for this sample of workers than suggested by ISO 1999 Annex B for an unscreened sample of the general population at comparable ages. As a result, the early hearing loss was underpredicted by the ISO 1999 projection. Except at the earliest years of employment, the rate of growth toward the final hearing loss for the mean of the sample of workers was approximated by the projection from ISO 1999. The ISO 1999 projection is based on growth of the percentage of loss at the final audiogram obtained each year. In subgroups of workers, i.e., for the lower quartile and for the upper quartile of the sample, the projection based on ISO 1999 was similarly effective. Workers hired with prior loss showed growth of hearing loss that was flatter than that of either the mean of all workers or the ISO 1999 projection, but the growth curve for these workers approximated the average curve shifted to later years to bring the initial loss into correspondence with the predicted loss. CONCLUSIONS: For a group of workers exposed to a noisy environment, growth of hearing loss can be projected fairly well based on ISO 1999, without knowledge of the details of spectrum or level of noise exposure. This suggests that it would be possible to allocate the percentage of a known total hearing loss acquired in a group of workers on a year-by-year basis, given knowledge of work history for a claimant population in the absence of detailed knowledge of the exposure or detailed sequential audiometric data.

Adult↗
PubMed1998

[The technique of chronic infusion of drugs into the cochlea by an osmotic pump].

OBJECTIVE: To introduce a method of chronic infusion of drugs into the cochlea using a osmotic pump. METHODS: A stainless steel tube was surgically implanted into the basal turn of the chinchilla cochlea and connected to the pump. To assess the reliability of the procedure, and the pump, Hank's solution or L-buthionine-[S,R]-sulfoximine (BSO) was infused into the cochlea by this pump and inferior colliculus (IC) potential and hair cell morphology were examined. RESULTS: In three Hank's solution infused cochleas, the thresholds of IC potential were kept in the normal level and no hair cell loss was found 14 days after the beginning of the infusion, suggested that the pump implantation and Hank's solution infusion had no significant effects on the cochlear functions. In contrast, threshold shifts of IC potentials and hair cell loss were found in two BSO infused animals, suggested that the infused solution had distributed into the whole cochlea. CONCLUSION: This study indicates that the osmotic pump can be used in chronic delivery of experimental drugs into the cochlea.

Animals↗
PubMed1998

Confocal microscopic analysis of the chinchilla organ of Corti following exposure to high-level impact noise.

To better understand the nature of mechanical changes following exposure to high-level impact noise, normal cochleas and cochleas from chinchillas exposed to either 125 or 131 dB SPL noise were stained with phalloidin for F-actin and examined using confocal microscopy. As seen in previous experiments, 125 dB exposures produced much more variable results than 131 dB exposures. Some cochleas were relatively unscathed by the exposure, whereas others showed damage to outer hair cells (OHCs) immediately after the exposure that included gross distortions of cell bodies and reduced F-actin in cuticular plates. Twenty-four hours later, there was also disorientation of actin filaments in supporting cells. After 30 days, Deiters cells were disarrayed and cups were separated from OHC neural poles. Exposure to noise at a level of 131 dB SPL produced less variable results than 125 dB exposure, and damage was generally more widespread and severe.

Animals↗
PubMed1998

Duration and peak level as co-factors in hearing loss from exposure to impact noise.

The Equal Energy Hypothesis (EEH) is based on the assumption that hearing loss from a given exposure is proportional to the total energy of the exposure. A corollary of this assumption is the power of the exposure and the duration of the exposure are interchangeable. Studies of impulse and impact noise show that hearing loss does not follow the prediction of the EEH (Ward, 1986; Danielson et al., 1991). Thus, the following experiments were designed to assess the relative importance of the duration and peak level of impact noise in the production of hearing loss. Monaural chinchillas served as subjects. Their quiet thresholds were estimated before and after exposures using the evoked potential recorded from a chronic electrode in the inferior colliculus. The four groups of the "intensity" series were exposed for 7.5 hours to 200 msec impacts at 1 per second at levels of 113, 119, 125, and 131 dB peak equivalent SPL. The three groups of the "duration" series were exposed to the 125 dB impacts for either 1.9, 7.5, or 30 hours. With each dB increase in the "duration" series, there was approximately 1.7 dB of increase in hearing loss. For each dB increase in peak level above 125 dB, there was an average of 6.6 dB increase in hearing loss. The 125 dB exposure is just below the "critical level" where the mode of cochlear damage shifts to mechanical failure. The results are discussed in terms of "critical" level and implication for noise standards.

Animals↗
PubMed1998

Effects on noise exposure on the auditory functions.

Noise induced hearing loss is accompanied by a wide variety of hearing deficits, including threshold and suprathreshold deficits, which often prevent effective rehabilitation even when hearing aids are used. A study of the neural mechanisms responsible for deficits associated with sensorineural loss is based on neurophysiological and anatomical research. In this paper, we intend to review the basic physiological characteristics of the peripheral auditory system.

Animals↗
PubMed1998

Temporary and permanent threshold shift: an overview.

Temporary threshold shift (TTS) as well as permanent threshold shift (PTS) represent the most common hearing effect of acute and chronic high level acoustic stimulation. TTS is typically related to the traumatizing stimulus spectrum and to the exposure level and duration. The stapedial acoustic reflex as well as the repetition rate of the exposure may influence TTS. PTS is related to a number of factors including exposure duration, subject's age, exposure to other ototoxic factors, presence of impulse noise components, etc. PTS is usually accompanied with alterations of several cochlear functions leading to altered speech discrimination mostly in presence of background noise. The main characteristics of both TTS and PTS are reviewed and discussed.

Acoustic Stimulation↗
PubMed1997

Characteristics of glutathione-capped ZnS nanocrystallites.

The titration of increasing equivalents of inorganic sulfide into preformed Zn-glutathione led to the appearance of UV/VIS spectral features attributable to ZnS nanocrystallites. Glutathione-ZnS complexes upon irradiation caused reduction of methylviologen confirming their semiconductor properties. Size-fractionation of glutathione-ZnS samples on a gel filtration column showed the formation of a range of complexes whose spectral properties were correlated with the sulfide content. The stoichiometry of Zn:glutathione increased from 1:2 to a maximum of about 7:1 as the sulfide/Zn ratios increase from 0 to approximately 1.0 in Zn-glutathione complex indicating up to 14-fold increase in the Zn-binding capacity of glutathione upon sulfide incorporation. Spectral characteristics of GSH-capped ZnS nanocrystallites were significantly influenced by pH and by the stoichiometry of Zn, sulfide and glutathione in the complex. Samples containing least glutathione and highest sulfide showed maximal luminescence at pH 6, whereas those with higher glutathione and lower sulfide content showed maximal luminescence at pH 11.

Chromatography, Gel↗
PubMed1997

Adsorption of a Hard Sphere Fluid in a Disordered Polymerized Matrix: Application of the Replica Ornstein-Zernike Equations

A model of hard spheres adsorbed in disordered porous media is studied using the associative replica Ornstein-Zernike (ROZ) equations. Extending previous studies of adsorption in a hard sphere matrices, we investigate a polymerized matrix. We consider an associating fluid of hard spheres with two intracore attractive sites per particle; consequently chains consisting of overlapping hard spheres can be formed due to the chemical association. This is the generalization of the model with sites on the surface of Wertheim that has been studied in the bulk by Chang and Sandler. The matrix structure is obtained in the polymer Percus-Yevick approximation. We solve the ROZ equations in the associative hypernetted chain approximation. The pair distribution functions, the fluid compressibility, the equation of state and chemical potential of the adsorbed fluid are obtained and discussed. It is shown that the adsorption of a hard sphere fluid in a matrix at given density, but consisting of longer chains of overlapping hard spheres, is higher than the adsorption of this fluid in a hard sphere matrix.

Journal Article↗
PubMed1997

An Expression for the Dispersion Force between Colloidal Particles

A result for the integral of R-12 over two spheres of radii a and b is given. This result, together with Hamaker's result for the integral of R-6, leads to an expression for the integral of the Lennard-Jones potential over two spheres that is suitable for calculations involving colloidal particles.

Journal Article↗
PubMed1997

Low-frequency 'conditioning' provides long-term protection from noise-induced threshold shifts in chinchillas.

Studies have shown that loss of auditory sensitivity caused by exposure to high-level acoustic stimuli can be significantly reduced by pre-exposing the subject to moderate-level acoustic stimuli. Although the protective effects of such 'conditioning' exposures have been well documented, very little is known about the persistence of conditioning-induced protection, or about the biological mechanisms underlying it. In the present study, the persistence of conditioning-induced protection was examined in chinchillas by imposing either a 30- or 60-day recovery period between conditioning (10 days of exposure to 0.5 kHz noise at 90 or 95 dB, 6 h/day) and high-level (0.5 kHz noise at 106 dB for 48 h) exposures. Comparisons of threshold shifts between conditioned animals and control animals exposed only to high-level noise indicated that conditioning provided significant protection from noise-induced threshold shifts for at least 2 months. Conditioned animals sustained outer hair cell losses similar to controls, ranging from 15 to 30% in the apical half of the cochlea. The results suggest that low-frequency conditioning can trigger long-lasting changes in cochlear homeostasis rather than temporary changes in physiology or reductions in susceptibility to hair cell loss in chinchillas.

Acoustic Stimulation↗
PubMed1997

The role of the cochlear efferent system in acquired resistance to noise-induced hearing loss.

Previous work has shown that the cochlear efferent system may play a role in protecting the ear from noise-induced temporary threshold shifts (TTS) following exposures to a single tone or series of moderate-level noises ('toughening'). However, whether the olivocochlear bundle (OCB) is important in decreasing noise-induced permanent threshold shifts (PTS) remains an open question. The importance of the OCB in decreasing the ear's susceptibility to noise, as reflected by 2f1-f2 distortion product otoacoustic emissions, was assessed by sectioning both the ipsilateral and contralateral divisions of the efferent system and exposing chinchillas while awake to an octave band noise (4 kHz) at a low level (85 dB SPL) for 10 days (6 h/day) and then at a high level (95 dB SPL) for 48 h. Complete de-efferentation was verified by cochlear acetylcholinesterase staining. The ears that were de-efferent showed substantially more TTS, greater PTS and larger cochlear lesions of outer hair cells. The results suggest that the efferent system may influence the ear's ability to develop resistance to noise trauma.

Animals↗
PubMed1997

Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity.

The present study was conducted to examine the re-establishment of IHC/VIII nerve synapses following kainic acid (KA) excitotoxicity and to discern if the re-organized afferent could render not only a normal auditory threshold but also a normal supra-threshold function. KA (60 mM) applied to the intact round window membrane in chinchilla destroyed postsynaptic endings of the auditory nerve, depressed the input-output (I/O) functions of auditory evoked potentials (EVP) and produced an average loss of sensitivity of over 80 dB at 4, 8, and 16 kHz, with less substantial losses (40-60 dB) at lower frequencies. However, there was no significant difference in 2f1-f2 distortion-product otoacoustic emissions (DPOAE) before and after the application of KA. The nerve endings went through a sequence of swelling, degeneration and recovery over a 3-5 day period at higher frequency. Auditory sensitivity and supra-threshold response returned accordingly. In contrast, complete recovery at lower frequencies (1 and 2 kHz) required more than 5 days. The results provide strong evidence that (1) excitotoxically damaged cochlear afferent neurons can recover and render both a normal EVP threshold and EVP I/O function and (2) afferent innervation to IHCs is not necessary for DPOAE generation.

Animals↗
PubMed1997

The influence of the cochlear efferent system on chronic acoustic trauma.

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals while awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell (OHC) function was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and cochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchanged in de-efferented ears as compared with efferented control and sham-operated ears. Following noise exposure, the ears that were de-efferented showed significantly more depression of DPOAE input/output functions and greater decrement of CM amplitude. The differences between de-efferented and efferent-innervated ears were evident across all the frequencies. The cochlear lesions of the OHCs reflected by traditional cytocochleograms, however, were minimal in both efferented and de-efferented ears. The results indicate that cochlear de-efferentation decreases the CM in chinchilla and increases the ear's susceptibility to noise-induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencies.

Animals↗
PubMed1997

Changes in F-actin labeling in the outer hair cell and the Deiters cell in the chinchilla cochlea following noise exposure.

It has been found that 'conditioning' noise exposures can render the inner ear more resistant to traumatic noise exposures. To explore the possible mechanisms underlying this phenomenon, filamentous actin (F-actin), labeled by rhodamine-phalloidin, was examined in the chinchilla cochlea using confocal fluorescence microscopy. The conditioning noise was 0.5 kHz octave band noise (OBN) at 90 dB SPL for 6 h/day and the high-level noise was the same noise but at 105 dB SPL for 4 h. A variety of pathological changes were found in the chinchilla cochlea after exposure to noise. Subjects exposed to conditioning noise (1 day or 10 days) and only high-level noise showed an increase in F-actin labeling than unexposed controls. By contrast, subjects who had 5 days quiet after the 10-day conditioning exposure exhibited a decrease in F-actin labeling. Interestingly, subjects exposed to high-level noise with prior 10-day conditioning exposure also showed a decrease in F-actin labeling in the cuticular plate and the stereocilia. The F-actin decreases in the stereocilia and the cuticular plates may decrease the mechanical rigidity of the organ of Corti. A more pliable organ of Corti may have reduced the possibility of fracture or ripping of cell junctions during the motion of the basilar membrane induced by acoustic overstimulation.

Acoustic Stimulation↗
PubMed1997

Age-related decline of auditory function in the chinchilla (Chinchilla laniger).

The aim of this study was to examine the functional consequences of aging in the chinchilla, a rodent with a relatively long life span and a range of hearing similar to that of humans. Subjects were 21 chinchillas aged 10-15 years, and 23 young controls. Thresholds were determined from auditory evoked potentials (EVPs), and outer hair cell (OHC) functioning was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAEs). Six cochleas from 11-12-year-old animals were examined for hair cell loss and gross strial pathology. The results show that the chinchilla exhibits a small but significant decline of auditory sensitivity and OHC functioning between 3 and 15 years of age, with high-frequency losses exceeding and growing more rapidly than low-frequency losses. Compared to rodents with shorter life spans, the chinchilla has a rate of loss that is more similar to that of humans, which could make it a valuable model for understanding the etiology of human presbycusis.

Acoustic Stimulation↗
PubMed1997

Suprathreshold measures of auditory function in the aging chinchilla.

The purpose of this study was to examine the relationship between aging and suprathreshold auditory function in the chinchilla, a rodent with a relatively long life span and a range of hearing similar to that of humans. Subjects were six 11-12-year-old chinchillas and six young controls. Tests of auditory function utilized evoked potential (EVP) recordings from electrodes implanted in the inferior colliculus and measurements of 2f1-f2 distortion product otoacoustic emissions (DPOAEs). Tests of frequency resolution (EVP tuning curves for a 2 kHz probe tone), temporal resolution (EVP forward masking functions using a 2 kHz probe tone), cochlear non-linearity (remote masking), and a DPOAE 'tone challenge', in which DPOAEs at six frequencies were monitored before and after each of five 1-min exposures to a 2 kHz tone at 80 dB SPL, were performed. The results indicate that aged chinchillas can exhibit deficits in suprathreshold tests of auditory function even when the test stimulus is in a region of relatively normal-hearing sensitivity. The results support the notion that aging, independent of hearing loss, can influence suprathreshold auditory function.

Aging↗