THE AGING EAR
The normal process of aging affects all parts of the ear, but the greatest clinical impact is on cochlear and vestibular function. Presbycusis, the loss of hearing associated with aging, is the most common type of auditory dysfunction and is thought to be due to a series of insults over time including age-related degeneration, noise exposure, and diseases of the ear. It is greatly affected by genetic background, diet, and systemic disease. Vestibular
symptoms are present in over half of elderly persons. Because balance depends on input from the ears, eyes, and peripheral sensory systems, all of which degenerate over time, impaired function in any of these systems contributes to vestibular complaints.
The pinna is commonly involved in actinic disorders, especially basal and squamous cell carcinoma. Sun protection and frequent inspection are important. The external auditory canal suffers a decrease in cerumen production due to degeneration of cerumen glands and a reduction in total number. This may lead to a drier cerumen that is less protective of the underlying skin and results in a higher incidence of impaction and infection.
The skin also undergoes atrophy, which results in itching, fragility, and subsequently to self-induced lacerations. The use of topical emollients has been recommended for difficult cases.
Mild middle ear histologic changes may be caused by inflammatory effects of degenerative arthritis on arthrodial joints (incudomalleolar and incudostapedial). Histopathologic correlates may include hyalinization or calcification of the joint capsules and calcification of the articular cartilage.Little or no conductive hearing loss has been associated with these conditions.
Presbycusis, auditory dysfunction associated with the aging process, is a generic term used to include several forms of degeneration of hearing. This very common disorder may have a devastating effect on the older individual by reducing their ability to communicate, jeopardizing autonomy and limiting opportunities of being an active member of society. With the growth of the aged population, presbycusis has become a great challenge to the
otologist. The secondary effects of sensory deprivation include changes in perception, changes in personality (especially introversion), social disengagement, as well as socioeconomic considerations.
Prevalence of presbycusis
Accurate estimates of the prevalence of presbycusis in particular, and sensorineural hearing loss in general are not available. However, survey data demonstrate that the estimated number of hearing impaired people in the United States is 20,732,000 or 8.8% of all Americans.9 The tendency for hearing impaired people is to under-report their hearing loss, and therefore the numbers may be higher.
The specific causes of presbycusis are speculative at this time, but probably represent a combination of the effects of years of function, exposure to noise, chemicals, and so forth, and genetically programmed biologic degeneration.
There is direct evidence of gradual loss of cells of most organ systems that accompanies the aging process. This decrease in normal weight, cell count, and other cellular components is found in all aging organisms. Remaining cells may be larger and total body mass is only slightly changed during aging due to increase in adipose tissue.
Physiologic changes include decrease in oxygen use, reduced circulation, intracellular accumulation of lipofuscin and other substances, and extracellular deposition of cholesterol lipids.
Aging also may be seen as a degenerating genetic cellular system. Genetic damage may be a result of random deterioration, as well as programmed deterioration. In the former, somatic cell genes undergo inactivation of deoxyribonucleic acid (DNA) at an irregular rate that accumulates throughout life. In the latter, there is selective activation and repression of genes during ontogeny of the cell.
It is clear that morphologic changes in human beings, as well as animal models, regularly demonstrate age-related loss of inner and outer hair cells and supporting cells primarily from the basal turns of the cochlea. Outer hair cells decrease more than inner hair cells. Age-related loss of eighth nerve fibers has been reported to be as high as 20% in old rats.23 Age-related changes may occur as high as the superior olivary complex in the brainstem.
Nixon, in 196256 and Glorig and Davis17 showed high frequency conductive hearing losses attributed to stiffness and laxity of the joints in the aging middle ear. They also proposed the concept of an inner ear conductive hearing loss due to stiffness of the cochlear partition.
Gacek and Schuknecht15 defined four histopathologic types of presbycusis: (1) sensory, characterized by hair cell loss, (2) neural, associated with loss of spiral ganglion cells and axons, (3) metabolic, characterized by strial atrophy, and (4) mechanical or conductive.
The audiometric findings in this type of presbycusis include an abrupt, steep, and high frequency sensorineural high loss with slow symmetrical bilateral progression beginning usually in middle age. Pathologic lesions are limited to the first few millimeters of the basal turn of the cochlea. There is flattening and atrophy of the organ of Corti due to loss of hair cells and supporting cells. There is also an accumulation of lipofuscin, the aging pigment.
Audiometric findings include gradual hearing loss with a moderate slope towards the high frequencies, but disproportionately severe decrease in speech discrimination. This difficulty with speech discrimination makes hearing loss refractory to amplification in many cases. Atrophy of the spiral ganglion and nerves of the osseous spiral lamina occur mainly in the basal turn of the cochlea. The organ of Corti is largely intact, as opposed to
Strial presbycusis (metabolic presbycusis)
The hearing loss associated with strial presbycusis is flat sensory loss beginning in the third through sixth decades and progressing slowly. Speech discrimination is generally good and no recruitment is present. This condition is often familial. Patients do well with amplification. The characteristic pathologic findings are that atrophy of the stria vascularis is either patchy in the basal and apical turns, or diffuse. The organ of Corti and spiral ganglion cells are usually unaffected.
Inner ear conductive presbycusis
Both inner ear conductive presbycusis and atrophy of the spiral ligament cause bilateral symmetrical sensorineural loss with a upward slope towards the high frequency and preserved speech discrimination. No anatomical correlates to conductive sensorineural hearing loss are known, but it is hypothesized that the functional loss is due to stiffness of the basilar membrane, which correlates with its anatomical shape. The histopathologic pattern of atrophy of the spiral ligament includes different degrees of pathologic changes progressive through the patientís life. It is most noticeable in the apical turn and least in the basal turn. Cystic degeneration may cause detachment of the organ of Corti from the lateral cochlear wall resulting in hearing loss.
Circulatory disorders have long been proposed as the cause of hearing loss in aging persons. However, there is insufficient histopathologic evidence of this etiology for confirmation. The relationship between high frequency sensorineural hearing loss and the degree of cerebral atherosclerosis has been used to support this theory. Unfortunately, both may be independent but age related. Atherosclerotic disease of renal vessels and inner ear
vessels has also been related to age. Johnson and Hawkins30 in 1959 demonstrated the progressive involution of the human cochlear vasculature from the fetus and newborn through the aged. They noted that during the first decade of life, the radiating arterioles and outer spiral vessels in the basal coil attain adult size. Devascularization of capillaries and arterioles was subsequently found in the spiral ligament associated with aging. They found a similarity between the degeneration of inner ear vessels with analogous changes in the retina due to microangiopathy20 and demonstrated that plugging of vascular canals by bony tissue is a generalized phenomenon related to aging. They felt that plugging of vascular canals was one of the major causes of presbycusis.
Diabetic angiopathy is thought to be a specific entity. In this disorder, disseminated proliferation and hypertrophy of the intimal endothelium of arterials, capillaries, and venules occurs, causing significant narrowing of the lumen. There also is precipitation of lipids and other substances in the vascular wall. In addition, arteriolosclerosis is thought to be more common and more extensive in patients with diabetes. However, clinical audiologic and pathologic studies of the correlation between diabetes and presbycusis are conflicting and contradictory. The great variability of incidence of hearing loss in diabetics is thought to be due to poor design in previously performed studies. In well-controlled studies, no correlation between sensorineural hearing loss and diabetes was found.
Noise is thought to be a common cause of presbycusis. It is clear that a direct correlation exists between noise-induced inner ear damage and the frequency, intensity, and duration of noise exposure. However, some may effectively argue that noise exposure causes hearing loss at any age, and is not true presbycusis.
Noise-induced hearing loss may arise from mechanical damage, metabolic exhaustion, or vascular changes. Mechanical damage is seen in cochleas exposed to high intensity or impulse noise of short duration. There may be detachment of the organ of Corti from the basal membrane. Metabolic exhaustion is characterized by changes of intracellular ultrastructure indicating depletion of enzymes and metabolites in overstimulated sensory cells.
Noise has clearly been shown to cause ischemic changes of the inner ear. Capillaries below the basilar membrane have been noted histologically to undergo spasmodic changes. In addition, edema of endothelial cells impairs blood flow to the spiral ligament and stria vascularis. Sludging and aggregation of erythrocytes with increased blood viscosity secondary to decreased capillary flow also occurs.
In Rosenís studies of Finnish patients on long-term controlled diets, reduction of saturated fat resulted in significant lowering of serum cholesterol and improvement in auditory threshold testing.66 Subsequent studies demonstrated that hypolipoproteinemia is the correlate to Rosenís clinical epidemiologic studies and is related to presbycusis as well as obesity, atherosclerosis, and coronary artery disease.
Diagnosis of genetic sensorineural hearing loss, especially when of adult onset and without external abnormalities, is extremely difficult, but should be considered. The diagnosis of these disorders relies primarily on the history given by the patient and on the audiometric configuration that is characteristic. The typical hearing loss is a basin-shaped curve with good discrimination and no recruitment. Over the years, this pattern may change to a gradual or abrupt slope. Pathologically, the most prominent feature in this disorder is atrophy of the stria vascularis, which is parallel to Schuknechtís strial atrophy category.
Hearing and dementias
Recent studies of the cochlea in temporal bones from patients with confirmed Alzheimerís disease showed lack of degeneration in the cochlea, which is typical of Alzheimerís patients. This finding is distinguished from findings in the peripheral olfactory and visual systems, which show the typical neurofibrillary tangles and neuritic plaques.
Conversely, a possible relationship between central auditory dysfunction was found in the Framingham follow-up study of 1662 subjects. However, that study is weakened by the absence of objective testing in competing message tests.
Unfortunately, the term presbycusis is somewhat vague, and specific signs and symptoms should be identified to clarify its definition. Classic presbycusis includes bilaterally symmetrical sensorineural hearing loss in the absence of other etiologies. Although dietary measures over the long term may be effective in reducing the progression of hearing loss in certain aging patients, further data are necessary before clinical acceptance of this treatment modality. Amplification remains the mainstay of treatment for presbycusis.
Cochlear implantation in the elderly
Severe to profound sensorineural deafness in the elderly is most often due to an underlying pathologic process such as Meniereís disease or otosclerosis in combination with presbycusis. The latter does not produce this degree of hearing impairment on its own. One of the authors of this chapter performed cochlear implantation on patients up to 85 years of age with good results. Mean audiologic test scores increase significantly following implantation and are comparable to results achieved by younger recipients. In addition, the isolation of aging and loss of known
companionship (e.g., spouse and close friends) is greatly compounded by the social effects of severe to profound hearing loss. Elderly cochlear implant recipients are among the most satisfied due to their renewed ability to communicate, socialize, and in most cases use the telephone. The latter is especially important for confidence, in that they will be able to obtain emergency help if needed. Complications have not been experienced, but may be expected to be somewhat higher in elderly patients with intercurrent cardiac or respiratory disorders.