Chapter 2 The Epidemiology of Hearing Impairment in Elderly People Chapter 2 The Epidemiology of Hearing Impairment in Elderly People TYPES AND CAUSES OF HEARING LOSS AMONG ELDERLY PEOPLE Hearing impairment can be partial or complete. sensorineural hearing impairment are listed in It can be unilateral (one ear) or bilateral (both ears), table 1. temporary or permanent, stable or progressive. A central processing disorder is a hearing im- Types of hearing impairment include conductive, pairment that influences complex aspects of hear- sensorineural, mixed, and central processing dis- ing, such as understanding speech. The hearing orders. These types are based on the site of struc- tural damage or blockage (see figure 1). Conduc- Table 1 .—Causes of Conductive and Sensorineural tive hearing impairment involves the outer and/or Hearing Impairments middle ear. Sensorineural impairment involves Causes of conductive hearing impairments: damage to the inner ear, the cochlea, and/or the External blockage: buildup of wax or presence of a foreign fibers of the eighth cranial nerve. A mixed hear- object in the ear. ing impairment includes both conductive and sen- Perforated eardrum.’ a hole or tear in the eardrum that can occur as a result of injury, sudden pressure change, or in- sorineural components. Causes of conductive and fection. Genetic and congenital abnormalities.’ malfunction and/or malformation of the outer and/or middle ear that can oc- Figure 1 .—Structure of the Ear cur in connection with hereditary disease or as a result of illness or injury before or at the time of birth. Otitis media.’ middle ear infection with fluid accumulation. Otosclerosis: hereditary disease process resulting in over- Malleus growth of a small bone in the middle ear which interferes with sound conduction. Causes of sensorineural hearing impairment: Prenatal and birth-related causes: infections such as rubel- I la contracted by expectant mothers, drugs taken during pregnancy, or difficult labor and delivery. Hereditary causes: a variety of disorders that damage the cochlea or higher nerve centers and are usually present at birth; the gradual loss of hair cells in the cochlea that begins as young as the twenties and thirties in some in- dividuals and may be caused by heredity. Viral and bacteria/ infections: infections such as mumps, spi- nal meningitis, and encephalitis. Trauma: a severe blow to the head, an accident, or a stroke or brain hemorrhage that affect the ear, nerve pathways, and auditory brain centers. Tumors: tumors called acoustic neuromas that invade the tube eighth nerve. Noise: exposure to loud sounds that irreparably damage the Middle I hair cells. Outer Ear Ear Inner Ear Cardiovascular conditions: hypertension, heart disease, or other vascular problems that alter blood flow to the inner ear. In the healthy ear, sound waves gathered by the outer ear are trans- Ototoxic aspirin, some antibiotics, diuretics, and cer- mitted through the eardrum and three small bones in the middle ear— drugs: the malleus, incus, and stapes—and into the inner ear. There sound tain powerful anticancer drugs that damage the hair cells is converted from vibrations into electrical impulses by tiny sensory or other vital parts of the inner ear. receptors, called hair cells, in the cochlea. The electrical impulses Meniere’s disease: a disorder characterized by fluctuating pass through the eighth cranial nerve Into the auditory centers of the hearing loss, dizziness, and tinnitus; possible causes in- brain. clude aIlergy, hypothyroidism, diabetes, and syphilis. SOURCE Vlckl Friedman, Washington University Medical Center at St. Louis SOURCE: NINCDS, 1982 (124), 11 12 impaired person may hear the words but not make impairment could be identified and possibly any sense out of them. Some words are difficult treated (73). For example, wax buildup in the outer to interpret, almost as if the person were listen- ear frequently causes hearing impairment in ing to a foreign language. This kind of hearing im- elderly people. If presbycusis is diagnosed, the real pairment can be caused by disorders of the audi- problem—ear wax—might be missed and go un- tory pathways in the brain. With aging, the speed treated. of nerve impulses may slow or the brain may lose The term presbycusis can be confusing because the ability to interpret words that come at a rapid it is used to describe three situations: 1) treatable pace. conditions caused by disease, 2) conditions caused Tinnitus is a condition that often accompanies by disease for which no treatment is known, and hearing loss. It is a ringing, buzzing, or hissing in 3) conditions believed to result from normal aging, the ears or head that can be continuous or inter- For health care professionals, elderly people, and mittent. The causes of tinnitus are not well un- their families, a diagnosis of presbycusis may mis- derstood but can include obstructions in the outer takenly imply that nothing can be done for the ear, perforation of the eardrum, middle ear infec- patient. Greater attention to isolating the cause of tion, repeated exposure to loud noise, trauma, and a person’s hearing loss can help ensure prompt some medications, and effective treatment in some cases. In general, however, more research is needed to describe the Presbycusis is the term most often used to de- underlying pathologies that cause presbycusis and scribe hearing impairment in elderly people. The to differentiate between treatable and untreatable word presbycusis means “old hearing. ” It is usu- causes of these conditions. ally defined as a sensorineural loss caused by changes in the inner ear, but some experts include To develop a more complete understanding of middle ear changes associated with aging in the hearing loss associated with aging, we must im- definition (124) and others emphasize the impact prove our knowledge about the basic mechanisms of changes in the eighth cranial nerve and audi- of hearing in the normal ear. While the normal tory brain center (54). functioning of the outer and middle ear is rela- tively well understood, the structural and biochem- The diagnosis of presbycusis is used for hear- ical mechanisms of the inner ear and auditory brain ing impairment associated with a variety of sen- centers are less well understood. The cochlea, a sory, neural, metabolic, mechanical, and vascu- pea-sized organ with more than a million moving lar changes seen in elderly people. Yet little is parts, is one of the most complex mechanical struc- known about the underlying causes of presbycu- tures in the human body. Only within the past dec- sis and the term is sometimes used when no spe- ade have methods been developed to study this cific cause of the condition can be identified. tiny structure. Research on the molecular struc- Conditions that are frequently diagnosed as pres- ture and mechanical properties of hair cells in the bycusis include gradual loss of hair cells in the coch- cochlea and the biochemical processes by which lea and fibrous changes in the small blood vessels 1 sound vibrations are converted to neural impulses that supply the cochlea. Some researchers believe has significantly increased our understanding of that these changes are caused primarily by envi- the basic mechanisms of hearing (53). But little is ronmental factors and disease while others believe known about how the damaged ear processes they are primarily a result of normal aging (125). sound (88). However, not all elderly individuals are affected by presbycusis, and some people over 90 retain Lack of communication among researchers in acute hearing. this field has been a problem but recent research developments have stimulated increased inter- The diagnosis of presbycusis is sometimes given action. A national conference on auditory bio- mistakenly when a specific cause of the hearing chemistry held in 1984 brought many prominent researchers together for the first time and par- IThe cochlea is dependent on a single artery for blood supply, making hearing very susceptible to damage as a result of cardiovas- ticipants hope that this process will be repeated cular disease [73), regularly (30). 13 MEASURING HEARING IMPAIRMENT Hearing impairment is measured by two meth- of impairment as significant. Reported prevalence ods: interviews and audiometric tests. Definitions also varies depending on whether the interview of the levels of impairment reflect these two meth- or audiometric testing method is used. Prevalence ods. The interviewr method relies on self -reported figures based on interviews tend to underestimate hearing loss and the resulting statistics are pre- the frequency of hearing impairment because sented in categories such as: “no trouble hearing, ” many people, particularly elderly people, are not ‘(can hear words spoken in a normal voice, ” ‘(can aware of their hearing loss or may deny or mini- hear words shouted across a room,” and “at best mize its severity in an interview. Some experts be- can hear words shouted in ear” (119). Audiomet- lieve that audiometric testing provides more ac- ric tests measure an individual’s response to sounds curate measurement but prevalence figures based of varying intensity or loudness, and level of im- on both methods are widely cited in the literature pairment is reported in terms of the weakest sound (10). the individual can hear. Intensity of sound is meas- z Four types of audiometric tests are commonly ured in decibels; one scale used to define level used to measure hearing loss: 1) pure tone air con- of impairment is illustrated in table 2. duction, 2) bone conduction, 3) speech reception, The level of hearing impairment that is consid- and 4) speech discrimination. Pure tone air and ered significant varies for different surveys, re- bone conduction tests measure hearing loss at spe- search projects, and clinical applications. Hearing cific frequency levels. Hearing loss varies greatly specialists and researchers continue to debate the according to frequency, especially in elderly peo- most appropriate level to call significant for spe- ple, and information about hearing deficits at spe- cific applications.3 cific frequencies is important for diagnosis, treat- ment, and research.