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during childhood. Thus, although substantial progress has been made, many gaps remain with current EHDI programs (White, 2003) and with universal newborn hearing screen- ing in general (Wada et al., 2004).
Although the objective and rapid-test administration prop- erties of evoked OAEs make them an ideal procedure for hearing screening in large numbers of military personnel or industrial workers who are exposed to high levels of envi- ronmental sound, their application to these necessary moni- toring programs has not been widely instituted (e.g., Seixas et al., 2005). There are only a few isolated reports in the lit- erature on using OAE screening to identify hearing prob- lems in military personnel, musicians, or school children. Furthermore, to our knowledge, there is no published work on the worthwhile application of using OAEs to generally screen for hearing impairment in the elderly.
Evaluating Efferent System Function
OAEs have become a convenient tool for examining the in- fluence of the descending efferent auditory nervous system on OHC activity. This system arises from various structures in the central auditory system to provide feedback to the ear through synapses on the OHCs (Guinan, 2006). Thus, the OHCs receive descending input directly from auditory brain- stem structures. Using moderate broadband noise applied to the ear opposite (contralateral) to the test ear, Collet and col- leagues (1990) took advantage of this anatomical arrangement to noninvasively assess the status of efferent system function. The underlying assumption of comparing OAE levels in the presence and absence of the contralateral noise stimulation is that the emitted responses provide the most direct means of observing a major efferent effect on OHC activity that consists of a reduction in the levels of both TEOAEs and DPOAEs of ~0.5-1 dB in the overall emission level.
In recent years, modified stimulus protocols (e.g., extend- ing primary-tone on-times from <100 ms to ~1 s) have been used to test the adaptation properties of DPOAEs, in particular by using both monaurally (Kim et al., 2001) and binaurally (Meinke et al., 2005) applied stimuli. Primary tones applied ipsilateral to the test ear have the advantage of activating a larger portion of the descending cochlear effer- ent innervation to OHCs than does the contralateral-noise approach. Moreover, clearly, the binaural administration of primary tones provides an opportunity to evaluate the func- tional status of the entire efferent system innervating the OHCs of a given test ear.
Certainly, work aimed at uncovering the clinical usefulness of evoked OAEs as part of a test battery for examining auditory efferent system function is only in its beginning stages. Based on the interesting findings to date in select patient popula- tions, the reduction in the normal contralateral noise-induced decrement in DPOAE levels in, for example, aging ears in the presence of normal emissions (Kim et al., 2002) suggests that it is likely that efferent testing using evoked OAEs will eventu- ally become an important part of the assessment of more cen- trally based hearing impairments, such as central auditory- processing disorders. Moreover, based on the notion that an important functional role of the cochlear efferent system is to protect the ear from noise-induced hearing loss, there is a cur- rent interest in relating the robustness of indigenous efferent activity to the ear’s susceptibility to sound overexposure (e.g., Luebke et al., 2014).
Summary: The Future of Clinical OAEs
Clearly, applications of OAEs in the hearing sciences and clinical audiology are varied. Without a doubt, OAEs are use- ful in the research laboratory for evaluating and/or monitor- ing the status of cochlear function in experimental animal models. Moreover, clinically, they contribute to determining the differential diagnosis of cochlear versus more centrally based or retrocochlear disorders. Furthermore, their practi- cal features make them helpful in the hearing screening of newborns worldwide. Additionally, they have proven useful in monitoring the progressive effects of agents such as oto- toxins and loud sounds on cochlear function. In fact, there is accumulating evidence that it is possible to detect such ad- verse effects of drugs or noise or a developing familial hear- ing disorder on OHC function using OAEs before a related hearing loss can be detected by pure-tone audiometry. And OAEs are providing a noninvasive means for assessing the integrity of the cochlear efferent neural pathway. In general, OAEs supply unique information about cochlear function in the presence of hearing problems, and this capability makes them ideal response measures in both the basic- and clini- cal-hearing sciences.
Most certainly, the dedicated and steady accumulation of a scientific knowledge base about the processes that generate OAEs has laid the basis for developing successful clinical ap- plications using this unique response measure of cochlear function. Remarkably, the practical uses of OAEs began to be apparent in less than a decade after their initial discovery. Consequently, the discovery of OAEs is a good example of combining both scientific and clinical research to steadily
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