Aged Middle Ear

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Loss of hearing with aging is a well known phenomenon called presbycusis. Presbycusis, historically and correctly, is largely attributed to hair cell and primary neurons degeneration from unknown, though probably genetic, causes. This proposal argues that hearing loss in the elderly may also arise from age-related changes in the conductive apparatus of the middle ear. The evidence for age-related changes in the human middle-ear function is equivocal. This is most likely due to limitations in the way conductive hearing loss is assessed rather than anything else. Nevertheless, clear evidence can be found for structural changes in the conductive apparatus of both the human and animal middle ear. The aim of the present research is to develop an animal model that demonstrates the effects of aging on the structural and functional aspects of the middle ear. Such a model would then be employed to detail the consequences of these middle-ear changes on hearing. This aim is realized in a series of experiments hat evaluate structure and function in the young adult and aged middle-ear of the rat. The comparison begins with i careful morphological assessment of the tympanic membrane using light and transmission electron microscopy in these two age groups. Next, immunohistochemistry is employed to identify extracellular matrix proteins associated with the tympanic membrane and differences are sought between the two age groups. Then, laser interferometry is used to measure the tympanic membrane velocity transfer function in the two age groups. This function describes the frequency dependent transmission of sound vibration. Finally, Auditory Bralnstem Reponse techniques are used to ascertain the conductive component to the presbycusis seen in the aged rat. Hypotheses specific to each of these studies predict changes in the older rats reflecting a general deterioration in sound conduction. These experiments may reveal a heretofore unrecognized component of the hearing loss seen in the elderly population.