Posture systematically alters ear-canal reflectance and DPOAE properties

Picker Engineering Program, Smith College, 51 College Lane, Northampton, MA 01063, USA.
Hearing research (Impact Factor: 2.97). 03/2010; 263(1-2):43-51. DOI: 10.1016/j.heares.2010.03.003
Source: PubMed


Several studies have demonstrated that the auditory system is sensitive to changes in posture, presumably through changes in intracranial pressure (ICP) that in turn alter the intracochlear pressure, which affects the stiffness of the middle-ear system. This observation has led to efforts to develop an ear-canal based noninvasive diagnostic measure for monitoring ICP, which is currently monitored invasively via access through the skull or spine. Here, we demonstrate the effects of postural changes, and presumably ICP changes, on distortion product otoacoustic emissions (DPOAE) magnitude, DPOAE angle, and power reflectance. Measurements were made on 12 normal-hearing subjects in two postural positions: upright at 90 degrees and tilted at -45 degrees to the horizontal. Measurements on each subject were repeated five times across five separate measurement sessions. All three measures showed significant changes (p<0.001) between upright and tilted for frequencies between 500 and 2000 Hz, and DPOAE angle changes were significant at all measured frequencies (500-4000 Hz). Intra-subject variability, assessed via standard deviations for each subject's multiple measurements, were generally smaller in the upright position relative to the tilted position.

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    • "Büki et al. (2009) stated that the magnitudes of OAEs exhibit only a moderate sensitivity to intralabyrinthine pressure changes and indirectly to ICP changes. Recently, Voss et al. (2010) compared the magnitude of DPOAE level alterations induced by changes of postural position with related data published by Büki et al. (2000) and described a considerable discrepancy . In the present study, an approach is presented that might clarify the poor correlation of DPOAE level alterations and absolute ICP values. "
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    ABSTRACT: Objectives: Minor conductive hearing loss (CHL) can compromise the evaluation of the inner ear function with distortion product otoacoustic emissions (DPOAE). Elevation of the intracranial pressure (ICP) is also expected to alter the middle ear (ME) sound transmission. An impaired ME sound transmission leads to an attenuation of the DPOAE primary tone levels (L1 and L2) during forward transmission and of DPOAE levels (LDP) during backward transmission. The effect of the attenuation of L1 and L2 is complex and might have unexpected effects on DPOAE levels. In this work, CHL- and ICP-induced alterations of LDP versus L1 growth functions (constant L2 and increasing L1) are investigated. The first aim of this study is to explain how alterations of ME sound transmission can affect DPOAEs and to characterize the dependency of DPOAE level alterations on L2,L1 combinations and on the individual shape of LDP versus L1 growth functions. The second aim is to identify analogies between CHL- and ICP-induced alterations of DPOAEs and to discuss implications for a potential noninvasive ICP monitoring. Design: This study focuses on general theoretical considerations, supported by selected experimental observations in different species and simulations. DPOAEs were measured in guinea pigs before and after induction of a CHL (1 ear) and during increased ICP (1 ear), and in humans during changes of the postural position to alter the ICP (4 ears). Results: In both CHL and elevated ICP, DPOAE levels are not only reduced, but LDP versus L1 growth functions exhibit a shift to higher L1. The absolute DPOAE level alterations strongly depend on the L2,L1 combinations and the individual shape of the LDP versus L1 growth functions. In steeper LDP versus L1 growth functions, the DPOAE level alterations exhibit a larger variation. DPOAE levels can even increase. The largest variation can be found in ears with a nonmonotonic DPOAE growth behavior. An example of a guinea pig ear is presented with a nonmonotonic DPOAE growth behavior and a CHL of 4 dB, where the DPOAE level alterations varied between -32 and +9 dB depending on L1. Conclusions: The data enable a more comprehensive view of DPOAE level alterations during CHL and elevated ICP. The observations also explain the problem that DPOAE and ICP alterations do not correlate linearly. An evaluation of the shift of the LDP versus L1 growth function along the L1 axis provides a potential tool to improve both the assessment of the inner ear function in the presence of a CHL and noninvasive ICP monitoring with DPOAEs.
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    ABSTRACT: Spontaneous otoacoustic emissions can be detected as peaks in the Fourier spectrum of a microphone signal recorded from the ear canal. The height, center frequency, and spectral width of SOAE peaks changed when a static pressure was applied to the ear canal. Most commonly, with either increasing or decreasing static pressure, the frequency increased, the amplitude decreased, and the width increased. These changes are believed to result from changes in the middle ear properties. Specifically, reduced middle ear transmission is assumed to attenuate the amplitude of emissions. We reconsidered this explanation by investigating the relation between peak height and width. We showed that the spectral width of SOAE peaks is approximately proportional to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ 1/\sqrt {{{\hbox{peak}}\;{\hbox{height}}}} $$\end{document}. This is consistent with a (Rayleigh) oscillator model in which broadening of the SOAE peak is caused by broadband intra-cochlear noise, which is assumed to be independent of static ear canal pressure. The relation between emission peak height and width implicates that the intra-cochlear oscillation amplitude attentuates relative to the intra-cochlear noise level when a static ear canal pressure is applied. Apparently, ear canal static pressure directly affects the active mechanics in the inner ear.
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    ABSTRACT: Deviant middle ear pressure has a negative effect on the forward and backward transmission of stimulus and emissions through the middle ear. Resolving this deviant middle ear pressure is expected to lead to better middle ear transmission and, as a result of this, stronger otoacoustic emissions, which are better detectable. We investigated the effect of compensation o a deviant tympanic peak pressure on click-evoked otoacoustic emissions (CEOAEs). Second, we compared patient data to model predictions made by Zwislocki's middle ear model. University Medical Center. Fifty-nine children aged between 0.5 and 9 years (mean, 4.4 yr). Hearing investigations including CEOAE measurements at ambient and at compensated tympanic peak pressure (TPP). CEOAEs at ambient and compensated TPP. Compensation of TPP resulted in higher emission amplitudes below 2 kHz (increase of 8-11 dB). In addition, the compensated measurement showed an increased phase lag (up to one-fourth cycle). For ears with mild deviations of TPP, Zwislocki's model could describe these changes. Pressure compensation was well described by a compliance increase of the tympanic membrane, the malleus, and the incus. Compensating the ear canal pressure for negative tympanic peak pressure increased CEOAE amplitudes below 2 kHz and increased the phase lag. These changes can be predicted from an increase of the compliance of the tympanic membrane, incus, and malleus, as a consequence of the pressure compensation.
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