Hearing Loss Associated with Old Age
Introduction
Hearing loss is attributable to many causes, some are naturally-occurring, some as a result of external factors such as noise and ototoxic drugs. Age-related hearing loss occurs in 35% of people between the ages of 65 and 75, and 50 % for those aged 75 and older. (Scholtz, Kammen-Jolly and Felder, 2001) This paper discusses patterns of normal hearing loss and the hearing disorder called presbycusis.
Functions of the healthy ear
The healthy ear can detect frequencies or pitch between 20 to 20,000 Hz, although sounds between 500 to 4,000 Hz are the most typical sounds such as normal speech. Loudness is measured in decibels (dB), the softest sound normally heard given a value of 0 dB and 85 dB considered the safety limit for prolonged periods of exposure. Exposure of more than 8 hours a day to sounds of 90 dB i.e. lawnmowers would eventually lead to hearing loss.
The sounds are first captured by the outer ear or auricle, passes through the ear canal, causing the ear to vibrate. The vibrations are picked up by the middle ear, amplified and sent to the inner ear causing the fluid to move the sensory hairs in the cochlea to move. This movement is sent by the auditory, or cochlear nerves to the brain to be interpreted as sounds.
Each of these structures perform an important part in the way the sense of hearing works, but in decoding the sounds into any sort of meaning occurs in the inner ear, composed of the auditory nerve, the cochlea and the vestibular nerve. Sound is inherently mechanical in nature, so the sensitivity of the inner ear to the resultant vibrations determines how well a person will hear.
Types of hearing loss
There are three types of hearing loss: sensorineural, conductive and central. Sensorineural hearing loss is the most common type, accounting for 90% of all cases of hearing loss and about 23% of people over 65 years of age. It indicates some cochlear or vestibular nerve deficit. Sound gets to the inner ear but is not processed normally. Conductive hearing loss, on the other hand, is when sounds fail to reach the inner ear because of some blockage such as cerumen or ear wax, fluid in the middle ear or damage to the small bones, called ossicles, of the middle ear. The condition is often more treatable than sensorineural hearing loss and occurs in 8% of the population with hearing loss. Central hearing loss is the most infrequent type of hearing loss, presenting as inconsistent auditory reactions, responding to environmental sounds while deaf to loud noises. Research suggests that central hearing loss is due to bilateral brainstem lesions.
Age-related hearing loss, or presbycusis, is of the sensorineural type of hearing loss. It is the most common form of hearing loss among the elderly characterized as symmetric, gradual, bilateral inability to hear high-pitched sounds.
Hearing loss is classified according to the hearing threshold detected in a patient based on audiometric tests. If the threshold is between 0-25 dB, there is no hearing loss. Between 26-40 dB is considered mild hearing loss, 41-55 dB is moderate, 56-70 dB moderate to severe, 71-90 dB as severe and over 90 dB as profound. (Gacek and Schuknecht, 1969)
In the elderly, hearing loss is noticed when a significant number of sensory hairs (in sensory presbycusis) or neurons (in neural presbycusis) of the cochlea die as part of the aging process, typically after age 60.
Causes of presbycusis
Incidence of presbycusis varies greatly from one society to the next, but hearing loss associated with old age appears to be a general pattern. There are no racial nor gender patterns to presbycusis. No common physical abnormalities had been observed from subjects suffering from presbycusis.
The exact pathology of presbycusis has yet to be definitively pinpointed but four sites in the cochlea that lead to presbycusis is described and characterized. Sensory presbycusis is the atrophy and loss of cochlear hair cells. The atrophy begins at middle age and slowly progresses over time. Neural presbycusis is a deficit in cochlear or auditory nerves, also gradually over time. When 90% of the neurons are destroyed, hearing loss becomes apparent. Metabolic presbycusis is the atrophy of the stria vascularis, which controls the metabolism of the cochlean. This atrophy occurs between the age of 30 and 60 years of age and progresses gradually. Both neural and metabolic presbycusis is thought to be genetic in nature. (Christensen, Frederiksen and Hoffman, 2001). Mechanical presbycusis involves the thickening of the basilar membrane of the cochlea. (Gacek and Schuknecht, 1969)
Presbycusis normally occurs in two or more of the sites described, rarely isolated to just one site. Studies indicate genetic abnormalities may be the primary cause of the disorder and may be triggered by mitochondrial DNA deletions. It is further suggested that these abnormalities lead to changes in the anatomy of the inner ear. (Dai, Yang and Jiang, 2004)
Some factors not associated with drugs and ototoxic chemicals are being investigated as contributing to the development presbycusis in the elderly. One is arteriosclerosis, which restricts the flow of oxygen to the cochlea, which may damage the inner ear due to excess of oxygen metabolites and free radicals. Diabetes may accelerate arteriosclerosis, so diet and metabolism may have an indirect effect on presbycusis. It is also indicated that a diet that decreases the production of oxygen metabolites (i.e. antioxidant supplements) may have a positive effect. Stress and genetics are also indicated. (Seidman, 2000)
Figure 1: Audiogram showing typical hearing loss pattern with presbycusis at age 60
Source: http://www.merck.com/mrkshared/mmg/figures/128f1.jsp
Figure 2: Audiogram showing hearing loss by age bracket
Source: http://www.neuroanatomy.wisc.edu/coursebook/audclinical04.pdf
In figure 2 above, there is a progressive loss of range, with mild hearing loss at age 30, increasing steadily by 5 dB jumps per decade and recording severe to profound high-frequency hearing loss by subjects over the age of 70.
Configurations of hearing loss associated with presbycusis
Presbycusis is defined as the gradual, symmetric and bilateral hearing loss associated with high-pitched sounds. It occurs in both ears with the same degree of hearing loss, and occurs in a progressive manner, getting worse over time. There is good hearing for low and middle frequency sounds. Sensory presbycusis is characterized by a sudden loss of high-tone signals, while loss of word discrimination is indicative of neural presbycusis. Flat, threshold patterns indicate damage to the stria vascularis, indicating metabolic prebycusis. For audiometry showing a gradual decrease in linear distribution with no relation to observable pathologic changes, this is identified as mechanical or cochlear conductive presbycusis.
Indications of presbycusis
Elderly people are at risk for hearing loss and should be tested yearly. Presbycusis affect a person’s ability to distinguish high-frequency sounds so that while the volume of the sound is sufficient, there is difficulty in distinguishing the sound heard, such as individual words in conversations. English for example, uses many consonants, which are high in frequency; thus a person with presbycusis hears mumbling instead of the clear enunciation of consonant sounds such as f, s and t. Vowel sounds are clearer because they have lower frequencies. Music is also perceived to be less dynamic, less crisp. Male speakers are more easily understood than female speakers and a roaring, hissing or ringing sound may occur. There may also be a sensation of fullness in the ear, sometimes accompanied by headaches but not necessarily.
People with presbycusis often avoid social situations because they have difficulty in following conversations. Hearing loss typically affect the work environment, and many hide the symptoms because it is associated with old age. It is also difficult to use the telephone, or to answer the doorbell, for those with severe to profound degree of presbycusis. It is essential that tests be carried out to determine the type and nature of the hearing loss, especially when signs are detected early on, because the impact of some types of presbycusis can be alleviated if not eliminated.
Tests for presbycusis
An adult is screened for hearing loss first through medical history. Prior episodes of vertigo, ear infection, ear pain, disorientation in the dark, abnormal facial weakness or development of an abnormal sense of taste would indicate the need for further otologic examination. Initially, a questionnaire such as the Hearing Handicap Inventory for the Elderly—Screening Version could be used followed by a series of physical and audiologic tests.
The physical test is an investigation of the outer ear for any obstruction such as ear wax, infections or malformations. Perforations, otitis media or cholesteatoma in the tympanic membrane, may also be discovered.
Tuning fork tests are also used to determine whether the hearing loss is sensorineural or conductive in nature. The Weiner test determines if the hearing loss is unilateral or bilateral. The tuning fork is placed at the midline of the head and the patient is asked to indicate in which ear the sound is heard loudest. The Rinne test determines whether the hearing loss is due to an obstruction by bypassing the middle ear. The tuning fork is placed at the mastoid for bone conduction and then near the pinna for air conduction. The patient is then asked to indicate if bone or air conduction is better.
Audiologic tests may include the measuring of pure-tone thresholds, speech reception and discrimination, acoustic reflex testing and tympanometry using an audiometer. Pure-tone threshold is measured by the delivery of sounds of specific frequencies at varying volumes and the patient identifies the volume at which the specific tone is heard. Speech audiometry measures speech reception threshold and word recognition levels by using word lists with specific tonal qualities and intensities. The acoustic reflex is a protective response of the tympanic membrane to loud sounds, and is tested by the measurement of the intensity of the tympanic membrane to stimuli. Tympanometry tests the flow of sound through the middle ear by placing a probe in the ear canal to record the compliance of the middle ear to pressure. Results of these tests will determine if further testing required. (Bhatt, Liberman and Nadol, 2001)
Presbycusis is indicated if the tests reveal that the hearing loss is sensorimotor in nature, bilateral, symmetric and gradual.
Treatments for presbycusis
There is no known cure for presbycusis, but research has been extensive towards this aim, given the significant population suffering from the disorder. In the meantime, however, some measures may be taken to lessen the impact of presbycusis on the well-being of the stricken patient.
The most common tool for alleviating hearing loss is the hearing aid which amplifies sounds. There are many types of hearing aids, and the selection and fitting of such devices must be closely coordinated with a specialist. The most expensive is not always the best choice. Presbycusis, which is characterized by the impairment of hearing high frequency sounds, mere amplification will not help. The hearing aid must be programmed to specifically target those missing or muted sounds and must also be binaural (in both ears) as presbycusis is symmetric as well. Hearing aids best function when there is minimal background noise. Even the best devices, however, cannot duplicate normal hearing.
Coping strategies are enumerated to be used on its own or as an augmentation to amplification devices, depending on the severity of the disorder and the situation of the patient. Alerting systems in the home and community that uses lights or tactile means to indicate the door bell or fire alarm may easily be set up. Lip reading coupled with cued speech would significantly assist in speech discrimination. However, some difficulty may be experienced by some people who have poor eyesight as well. The same is true for sign language.
Aural rehabilitation is also recommended for those with presbycusis considering amplification devices. Because the disorder characteristically occurs gradually, the patient has normally grown accustomed to the sounds filtered by the loss of hearing, and may detract from the benefits expected of the augmentation. Since rehabilitation normally occurs in a social setting at regular periods, it may also prove to be beneficial in more than the aural sense.
Summary
Presbycusis is a disorder that affects many elderly people, and since it is suspected to be genetically-linked, it would be to the general public’s interest to know as much about the disorder as possible. The rehabilitation of people already suffering from presbycusis is important, but perhaps the research and development of treatments or strategies to minimize or prevent presbycusis should be the priority.
References
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