Zwaardemaker is credited with documenting the reduction of high-frequency hearing associated with increased age in 1891. His interest, however, involved determining the highest frequency heard at various ages rather than acuity at a given frequency. Bunch in 1929 documented not only the aging pattern - presbycusis - but also the fact that females tended to retain hearing acuity more than age-matched males. Worlds Fair reports as well as the National Health Survey of 1960-62 helped to further our understanding of changing hearing thresholds associated with aging.
Many researchers have examined the prevalence of hearing loss in older populations. The Senate Committee on Aging estimated in 1968, that 30-50% of the population over age 65 had hearing loss significant enough to be restricting in nature. Subsequently, the American National Standards Institute in 1969 provided composite audiograms that take age and gender into consideration. Adults are now surviving longer, but with longevity comes an increase in the incidence of communication disorders, such as hearing loss. With an increasing population of hearing impaired older adults, we need to better understand the specific structures responsible for hearing loss as well as the anatomic and physiologic changes associated with the aging process as it impacts hearing.
The Outer Ear:
The pinnas role in audition is believed to be localization of high frequency sounds by funneling them towards the external auditory canal. In addition, the pinna helps distinguish between sounds originating from in front of, or behind a person. Changes in the pinna have been noted for decades, these include; elongation due to loss of elasticity and resilience in the epithelial layers, scaliness, dryness, rough appearance, increase of hair growth and pigmentation spots. None of these changes significantly affect hearing, although a more flaccid pinna could contribute to a collapsing ear canal during audiometric testing under headphones. While cerumen production has not been shown to change with age in terms of consistency or rate of production, a flaccid pinna could lead to increased retention of the cerumen produced.
The Pinna and Aging:
While a flaccid pinna does not compromise hearing, per se, it can compromise the habilitation process. When fitting amplification, feedback and retention issues can arise related to a flaccid pinna. Using silicone impression materials with medium viscosity should help insure better impressions with snugger fitting hearing aids for patients with soft, flaccid pinnae.
Additionally, as previously mentioned, a flaccid pinna can lead to collapsing ear canals during audiometric testing. Research indicates that up to 41% of all patients over age 80 years, or 1/3 of all patients over the age of 65, exhibit collapsing canals during audiometric testing. However, routine use of insert earphones or modifying earphone test procedures should effectively manage the problem.
The Tympanic Membrane:
The tympanic membrane is believed to help in the coding of both frequency and intensity information by altering its vibratory pattern. Thinning, rigidity, translucence and loss of mobility has been described in the aging patient and could affect the ears ability to code and transmit auditory information.
The Middle Ear System:
The middle ear systems main function is to conduct sound from the outer ear to the oval window in an amazingly efficient manner. Due to surface area differences (a.k.a. 'areal ratio') between the tympanic membrane and the footplate of the stapes located in the oval window, as well as the increased force due to the longer length of the manubrium of the malleus compared to the long process of the incus (a.k.a. 'lever action') we expect a 27 dB increase in intensity through this highly efficient information transfer system.
Both intensity and frequency are coded via vibratory patterns of the ossicular chain. Atrophy, ossification of the ossicular chain, calcification of the auricular cartilage and increased stiffness have been described with aging as well as degeneration or atrophy of ossicular ligaments and tensor tympanic/stapedius muscles. Documentation of changes in the ability of the eustachian tube to function with aging have been mixed. Some authors feel that a greater degree of eustachian tube dysfunction is present in older adults while others report no change.
While increases in the prevalence of middle ear disorders in older adults has not been noted, the older adults middle ear comprises the history of an entire lifespan. The aftermath of otosclerosis, cholesteatoma, otitis media or otologic and acoustic trauma experienced during the lifespan will be present during the older years.
Middle ear disorders create conductive hearing losses which require attention when ordering circuit and circuit options for hearing aids. Factors such as middle ear drainage, perforation, enlarged mastoid cavities, require special fitting considerations. Increased gain, increased maximum output and increased 'reserve gain' are common when fitting ears with conductive disorders.
The Inner Ear & Auditory Nerve
After the ossicular chain has transferred the acoustic stimulus to the footplate of the stapes, the stapes transfers this motion via the oval window to the fluid-filled cochlea. Traveling from the basal end to the apex, this traveling wave allows for shearing action of the cilia of the hair cells which generates electrical impulses that are transmitted via the auditory nerve to the cortex. Frequency information and periodicity appear to be coded through the site of maximum displacement, while intensity information is apparently coded via discharge rate. The outer hair cells allow fine tuning by altering the tectorial membranes movement.
The primary hearing loss associated with aging occurs within the inner ear and the auditory nerve. Many areas have been identified as being vulnerable to the aging process, including; atrophy of support cells within the organ of corti, degeneration of hair cells within the cochlea, destruction of first-order neurons in the cochlea, loss of neurons throughout the auditory pathway, degeneration or thickening of the capillary walls of the stria vascularis, degeneration of the spiral ganglion and shrinkage of the spiral ligament. So many changes have been noted within the cochlea and auditory nerve, that one author divides presbycusis into six distinct classifications: Sensory, neural, strial, cochlear conductive, mixed and indeterminate.
Beyond audiometric acuity, performance on speech audiometry tasks is reduced secondary to aging. Maximum word recognition scores for older adults are lower than for younger adults and older adults require increased signal-to-noise ratios to understand speech, even when hearing loss is taken into consideration. Age related changes in speech audiometry provides a potential barrier to successful use of amplification.
Changes within the cochlea and auditory nerve. Secondary to aging, often result in the condition we term recruitment. Recruitment, regardless of the type or etiology has an impact upon hearing aid selection. Hearing aid amplification can certainly aggravate recruitment problems, often requiring compression circuits to fit the reduced dynamic range of the patient.
The Efferent System:
The olivocochlear bundle is comprised of neurons originating in the superior olivary complex which innervate hair cells within the cochlea. A medial group, comprised of large, myelinated, crossed neurons innervate the outer hair cells directly while a lateral group, comprised of smaller, unmyelinated, uncrossed neurons innervate the inner hair cells by way of synapses on afferent fibers. These fibers are thought to; alter membrane potentials, improve the detection of signals in noise, offer protection from noise, help control the mechanical state of the cochlea and play a role in attention. The efferent system plays a critical role in audition, and it too is affected and its efficiency is decreased through the aging process. Virtually any damage to the auditory system manifests itself in the functioning of the outer hair cells to which the innervation of the efferent system is directed. This is particularly ominous for adult males who will have lost the vast majority of their outer hair cells in the high frequency region by the age of 65 years or so.
The Auditory Cortex:
The auditory pathway from the cochlea to the cortex is complicated. The tonotopic organization (or frequency specific organization) of the cochlea and auditory nerve continues into the cochlear nucleus, the superior olive, the lateral lemniscus, the inferior colliculus and the medial geniculate body. These higher order fibers have complex firing patterns and receive both ipsilateral and contralateral stimulation. These areas are sensitive to interaural timing and intensity differences and are dependent upon such things as state of alertness.
'Normal aging' takes a toll on the central auditory system. A significant loss of neurons within the cortex begins early in the life span and continues throughout ones life. A startling 50% decrease in neurons in some areas of the cortex has been reported in older adults. Brain weight (a.k.a. 'mass') decreases with aging too. Accumulation of lipids in the cytoplasm and decrease in cerebral blood flow has been noted in older adults.
Functionally, we find that older adults do not perform as well when evaluated via central auditory processing tasks such as; time compressed speech, binaural fusion, binaural separation and Performance Intensity-Phonetically Balanced (PI-PB) assessments. Electrophysiologic testing results have been mixed: Some studies indicate that auditory brainstem response results for older adults are more consistent with disorders such as multiple sclerosis while other studies demonstrate that early auditory brainstem response results are the same for older and younger adults.
Intrinsic and Extrinsic Etiology:
When assessing agings impact upon habilitation, it is important to have a general idea as to the etiology of the hearing loss. Additionally, decreased auditory function associated with aging may be thought of as characterized by intrinsic and extrinsic factors.
Intrinsic factors include; decreased functioning of the vascular system, biologic aging, genetic predisposition and a positive familial history with respect to hearing loss. Extrinsic variables includes; diet, noise exposure, bacterial and viral infections, metabolic disorders, vitamin deficiencies, allergies, pregnancy, ototoxic drugs, and disease. Additionally, a secondary category of extrinsic variables might include emotional triggers such as strain, fright, grief and frustration.
Summary and Conclusion:
Aging impacts the entire auditory system; the pinna, cerumen in the external auditory canal, the tympanic membrane, the cochlea, the auditory nerve, the cortex and the final perception referred to as 'hearing'. Through a better understanding of changes to auditory anatomy and physiology secondary to aging, we can better serve our patients. While this paper has focused on aging in older adults, it is important to note that degeneration within the cochlea possibly begins at or before birth. Agings impact upon the auditory system is inescapable and affects all of us -- regardless of age.
An earlier version of this paper was published in the July-August, 1998 edition of Audecibel, the official journal of the International Hearing Society. We are grateful to IHS for allowing us to publish this updated, rewritten and re-edited version of that paper.
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