Drugs particularly notorious for ototoxicity include the aminoglycoside antibiotics (streptomycin, kanamycin, neomycin, gentamicin, and viomycin), salicylates, the diuretics ethacrynic acid and furosemide, and quinine or chloroquine. Although both the vestibular and auditory portions of the inner ear can be affected, streptomycin, gentamicin, and viomycin are especially toxic to the vestibular system, while neomycin, kanamycin, and amikacin are more toxic to the auditory system.
Salicylates in large doses can cause a reversible hearing loss, tinnitus, and vertigo. The symptoms generally appear with plasma concentrations of about 35 mg/dL. Ethacrynic acid, when administered IV, is associated with permanent hearing loss; coadministered aminoglycoside appears to be an aggravating factor. Furosemide may cause temporary hearing loss. Quinine and chloroquine are associated with both permanent and temporary hearing losses, the former occurring with large doses. Chemotherapeutic agents, particularly nitrogen mustard and cisplatin, are implicated in sensorineural hearing loss.
The elderly, especially those with hearing loss or renal dysfunction, should not be treated with ototoxic drugs unless no suitable alternative exists. In such a case, a baseline and follow-up audiogram should be obtained to document any drug-related changes. Vestibular effects may be insidious, particularly when patients are bedridden. A bedside test of the vestibulo-ocular reflex may help in early detection of vestibular dysfunction.
Although the perilymph of the inner ear tends to concentrate and retain certain ototoxic drugs, regular serum monitoring for therapeutic blood levels helps avoid ototoxicity.

Tumors that affect hearing include paraganglioma and vestibular schwannoma.
PARAGANGL?OMA
Tumors of the middle ear and mastoid are rare. Of these, the most common tumors are paragangliomas, derived from paraganglion tissue (eg, the carotid body). Otherwise known as glomus tumors, these benign lesions generally cause a conductive hearing loss and pulsatile tinnitus. Tumors originating in the middle ear (glomus tympanicum) are clinically evident even when small; those arising in the jugular vein at the mastoid (glomus jugulare) become clinically apparent relatively late and may produce symptoms such as paralysis of cranial nerves IX through XII.
Physical examination may reveal a pulsatile red mass in the middle ear. The integrity of the cranial nerves should be assessed. Audiometric evaluation usually documents a conductive hearing loss; in more widespread tumors, a sensorineural component may also be detected. Because of the multicentricity of these lesions, four-vessel cerebral angiography or magnetic resonance angiography may reveal other, occult growths. Glomus tumors may produce catecholamines, resulting in intermittent hypertension.
Treatment depends on the tumor’s size as well as the patient’s general medical condition. In the healthy patient, small tumors can be removed relatively easily. In the infirm patient, extensive tumors are probably best managed more conservatively. Relatively low-dose radiation therapy and arteriographic embolization are palliative and may be more appropriate in the elderly.
VEST?BULAR SCHWANNOMA
(Acoustic Neuroma)
A benign tumor that develops from the Schwann cells forming the sheaths of the vestibular nerves. Vestibular schwannomas most commonly arise in or immediately medial to the internal auditory canal; with growth, they present as a cerebellopontine angle mass.
Patients usually complain of a unilateral hearing loss accompanied by tinnitus and, occasionally, disequilibrium. Large tumors may also affect cranial nerves V and lower and may produce hydrocephalus. Since physical examination may disclose only a unilateral or asymmetric hearing loss, assessing cranial nerve function is mandatory.
A complete audiogram generally shows an asymmetric sensorineural hearing loss with disproportionately poor speech discrimination scores. The acoustic reflex may be absent or may show abnormal decay, and rollover may be found on performance intensity function testing for phonetically balanced words (see under TESTS OF AUDITORY FUNCTION, below). Auditory brain stem response testing shows abnormalities consistent with a retrocochlear lesion. Radiologic investigation consists of contrast-enhanced CT scan or MRI directed toward the cerebellopontine angle and the internal auditory canals.
Treatment of acoustic tumors in the elderly is controversial. Complete surgical excision, as performed in younger patients, is recommended by some surgeons, while others believe that a palliative subtotal resection is wiser. Radiation therapy (eg, with a gamma knife or linear accelerator [LINAC]) may be an alternative in selected cases. The tumor’s size, its associated symptoms, and the patient’s overall medical condition should be considered when deciding on appropriate therapy.

Tumors of the middle ear and mastoid are rare. Of these, the most common tumors are paragangliomas, derived from paraganglion tissue (eg, the carotid body). Otherwise known as glomus tumors, these benign lesions generally cause a conductive hearing loss and pulsatile tinnitus. Tumors originating in the middle ear (glomus tympanicum) are clinically evident even when small; those arising in the jugular vein at the mastoid (glomus jugulare) become clinically apparent relatively late and may produce symptoms such as paralysis of cranial nerves IX through XII.
Physical examination may reveal a pulsatile red mass in the middle ear. The integrity of the cranial nerves should be assessed. Audiometric evaluation usually documents a conductive hearing loss; in more widespread tumors, a sensorineural component may also be detected. Because of the multicentricity of these lesions, four-vessel cerebral angiography or magnetic resonance angiography may reveal other, occult growths. Glomus tumors may produce catecholamines, resulting in intermittent hypertension.
Treatment depends on the tumor’s size as well as the patient’s general medical condition. In the healthy patient, small tumors can be removed relatively easily. In the infirm patient, extensive tumors are probably best managed more conservatively. Relatively low-dose radiation therapy and arteriographic embolization are palliative and may be more appropriate in the elderly.

A benign tumor that develops from the Schwann cells forming the sheaths of the vestibular nerves. Vestibular schwannomas most commonly arise in or immediately medial to the internal auditory canal; with growth, they present as a cerebellopontine angle mass.
Patients usually complain of a unilateral hearing loss accompanied by tinnitus and, occasionally, disequilibrium. Large tumors may also affect cranial nerves V and lower and may produce hydrocephalus. Since physical examination may disclose only a unilateral or asymmetric hearing loss, assessing cranial nerve function is mandatory.
A complete audiogram generally shows an asymmetric sensorineural hearing loss with disproportionately poor speech discrimination scores. The acoustic reflex may be absent or may show abnormal decay, and rollover may be found on performance intensity function testing for phonetically balanced words (see under TESTS OF AUDITORY FUNCTION, below). Auditory brain stem response testing shows abnormalities consistent with a retrocochlear lesion. Radiologic investigation consists of contrast-enhanced CT scan or MRI directed toward the cerebellopontine angle and the internal auditory canals.
Treatment of acoustic tumors in the elderly is controversial. Complete surgical excision, as performed in younger patients, is recommended by some surgeons, while others believe that a palliative subtotal resection is wiser. Radiation therapy (eg, with a gamma knife or linear accelerator [LINAC]) may be an alternative in selected cases. The tumor’s size, its associated symptoms, and the patient’s overall medical condition should be considered when deciding on appropriate therapy.

A patient’s ability to hear should be roughly estimated in a general office setting. In the Weber test, a vibrating 512-Hz tuning fork is placed on the midline of the forehead. Patients with normal hearing perceive the vibratory sound as equally loud in each ear. Patients with conductive hearing loss perceive the vibratory sound as louder on the affected side. In the Rinne test, the stem of a vibrating tuning fork is placed on the mastoid tip (with firm pressure) and then the tines are held just in front of the external auditory canal. Normally, the vibratory sound is perceived as being louder at the external auditory canal (ie, air conduction is greater than bone conduction). The reverse indicates a significant conductive hearing loss.
A whisper test, using a Barany box to mask the contralateral ear, can crudely estimate auditory thresholds, but most clinicians find it too inaccurate to be useful. An audioscope combines both otoscopic visualization and the capacity to check hearing at different frequencies and volumes. Any complaint of hearing loss or any abnormality on office screening tests should be followed up with a complete audiogram.
The complete audiogram (see FIG. 103-3) entails a battery of tests for evaluating pure tone and speech reception thresholds and tympanometry. Thresholds for pure tones at octave or half-octave intervals are obtained for frequencies from 250 to 8000 Hz. Testing is done by both air conduction (using earphones) and bone conduction (placing a vibrotac-tile transducer directly on the mastoid).
The speech reception threshold is defined as the intensity at which the patient can correctly identify 50% of a series of spondees (two-syllable words equally accented, eg, cowboy). The speech reception threshold should be within 10 dB of the pure tone average (average threshold in decibels, at 500, 1000, and 2000 Hz). The speech discrimination score is determined by presenting a list of monosyllables at levels above the speech reception threshold and having the patient repeat them; the score is the percentage correctly identified.
A specialized test of speech discrimination, the performance intensity function for phonetically balanced words, differentiates cochlear from retrocochlear hearing loss. As phonetically balanced words are presented with increased intensity, the speech discrimination score rises and then stabilizes in patients with normal hearing or with a cochlear hearing loss. Patients with retrocochlear hearing loss have an initial rise in the score, followed by a dip (a rollover).
Immittance studies, also called impedance audiometry, consist of tympanometry and acoustic reflex measurements.
Tympanometry (see FlG. 103^4) measures the relative change in acoustic immittance at the plane of the tympanic membrane when air pressure changes are introduced in the external ear canal across a range from high positive to high negative pressure. Normally, the immittance is maximal at atmospheric pressure, where air pressure is about equal on each side of the tympanic membrane. The peak in the tympanogram may shift toward a negative air pressure direction if eustachian tube function is compromised, or the peak may disappear when fluid collects in the middle ear.
The acoustic reflex threshold is the lowest sound intensity (between 500 and 4000 Hz) that will produce a reflex contraction of the stapedius muscle. Reflex decay is an abnormal finding involving a decrease in the original reflex amplitude > 50% over a 10-sec test period. Stapes (acoustic) reflex decay suggests a retrocochlear hearing disorder.
If auditory thresholds are ambiguous, or if the possibility of a ret-rocochlear disorder exists, appropriate testing includes the auditory brain stem response. In this test, recording electrodes are attached to the patient’s earlobes and vertex and are connected to a computer that averages responses. Auditory system activity in response to a sound stimulus (a click) is analyzed by the computer and generally results in the delineation of five sequential waves (see FIG. 103-5). Waves I and II are thought to originate from the peripheral and central auditory nerve, wave III from the cochlear nuclei, and wave IV from activity in the superior olivary nucleus. The lateral lemniscus is thought to give rise to wave V. The intensity of the click needed to elicit the characteristic waveforms indicates the auditory threshold, which is determined for each ear individually. Also important is the time at which each wave appears after the sound stimulus (latency). A delay in onset of the entire sequence of waveforms suggests a conductive hearing loss. In the absence of a conductive hearing loss, a delay in onset of wave V strongly suggests a retrocochlear hearing loss.
Results of special tests to determine central auditory function may be invalidated by a concurrent peripheral hearing loss. In general, these tests evaluate a patient’s ability to synthesize distorted or degraded speech signals divided between the two ears, or to detect a specific message in one ear while a competing message is presented in the other ear. In tests using distorted or degraded messages, a cortical (temporal lobe) lesion may cause poor performance in the ear contralateral to the lesion (the contralateral ear effect). Inability to fuse binaural information into a meaningful message may indicate a brain stem lesion.

Screening for hearing loss is strongly recommended for all elderly persons. Older persons often hide their hearing loss, embarrassed by it and equating it with aging. Those who are not employed and who have few social interactions may remain unaware of mild hearing loss, which places them at risk of injury and further social isolation.
Screening may be conducted at senior centers and can be accomplished in several ways. Questionnaires can augment the traditional review of systems but often must be administered to close social contacts and family members as well as to the patient. Testing is better using standardized instrumentation.
Establishing characteristics of the hearing loss guides the evaluation and helps the physician select the correct treatment. Inquiry should be made into the onset, progression, and severity of the hearing loss. Tinnitus often accompanies presbycusis. Asymmetric, unilateral, or fluctuating hearing losses are not characteristic of presbycusis. Physical examination generally is unrevealing, except when cerumen accumulation is the cause of hearing loss.
Once hearing loss is suspected, referral to an otolaryngologist or audiologist is in order. The minimal diagnostic evaluation entails a medical evaluation and a complete audiogram, including pure tone (an electronically produced sound of a single frequency), speech, and tympanometric testing (see TESTS OF AUDITORY FUNCTION, below). Asymmetry in the audiogram or retrocochlear signs should be pursued with an auditory brain stem response test and a CT or MRI scan to rule out an acoustic neuroma or other cerebellopontine angle tumor.
TESTS OF AUDITORY FUNCTION
A patient’s ability to hear should be roughly estimated in a general office setting. In the Weber test, a vibrating 512-Hz tuning fork is placed on the midline of the forehead. Patients with normal hearing perceive the vibratory sound as equally loud in each ear. Patients with conductive hearing loss perceive the vibratory sound as louder on the affected side. In the Rinne test, the stem of a vibrating tuning fork is placed on the mastoid tip (with firm pressure) and then the tines are held just in front of the external auditory canal. Normally, the vibratory sound is perceived as being louder at the external auditory canal (ie, air conduction is greater than bone conduction). The reverse indicates a significant conductive hearing loss.
A whisper test, using a Barany box to mask the contralateral ear, can crudely estimate auditory thresholds, but most clinicians find it too inaccurate to be useful. An audioscope combines both otoscopic visualization and the capacity to check hearing at different frequencies and volumes. Any complaint of hearing loss or any abnormality on office screening tests should be followed up with a complete audiogram.
The complete audiogram (see FIG. 103-3) entails a battery of tests for evaluating pure tone and speech reception thresholds and tympanometry. Thresholds for pure tones at octave or half-octave intervals are obtained for frequencies from 250 to 8000 Hz. Testing is done by both air conduction (using earphones) and bone conduction (placing a vibrotac-tile transducer directly on the mastoid).
The speech reception threshold is defined as the intensity at which the patient can correctly identify 50% of a series of spondees (two-syllable words equally accented, eg, cowboy). The speech reception threshold should be within 10 dB of the pure tone average (average threshold in decibels, at 500, 1000, and 2000 Hz). The speech discrimination score is determined by presenting a list of monosyllables at levels above the speech reception threshold and having the patient repeat them; the score is the percentage correctly identified.
A specialized test of speech discrimination, the performance intensity function for phonetically balanced words, differentiates cochlear from retrocochlear hearing loss. As phonetically balanced words are presented with increased intensity, the speech discrimination score rises and then stabilizes in patients with normal hearing or with a cochlear hearing loss. Patients with retrocochlear hearing loss have an initial rise in the score, followed by a dip (a rollover).
Immittance studies, also called impedance audiometry, consist of tympanometry and acoustic reflex measurements.
Tympanometry (see FlG. 103^4) measures the relative change in acoustic immittance at the plane of the tympanic membrane when air pressure changes are introduced in the external ear canal across a range from high positive to high negative pressure. Normally, the immittance is maximal at atmospheric pressure, where air pressure is about equal on each side of the tympanic membrane. The peak in the tympanogram may shift toward a negative air pressure direction if eustachian tube function is compromised, or the peak may disappear when fluid collects in the middle ear.
The acoustic reflex threshold is the lowest sound intensity (between 500 and 4000 Hz) that will produce a reflex contraction of the stapedius muscle. Reflex decay is an abnormal finding involving a decrease in the original reflex amplitude > 50% over a 10-sec test period. Stapes (acoustic) reflex decay suggests a retrocochlear hearing disorder.
If auditory thresholds are ambiguous, or if the possibility of a ret-rocochlear disorder exists, appropriate testing includes the auditory brain stem response. In this test, recording electrodes are attached to the patient’s earlobes and vertex and are connected to a computer that averages responses. Auditory system activity in response to a sound stimulus (a click) is analyzed by the computer and generally results in the delineation of five sequential waves (see FIG. 103-5). Waves I and II are thought to originate from the peripheral and central auditory nerve, wave III from the cochlear nuclei, and wave IV from activity in the superior olivary nucleus. The lateral lemniscus is thought to give rise to wave V. The intensity of the click needed to elicit the characteristic waveforms indicates the auditory threshold, which is determined for each ear individually. Also important is the time at which each wave appears after the sound stimulus (latency). A delay in onset of the entire sequence of waveforms suggests a conductive hearing loss. In the absence of a conductive hearing loss, a delay in onset of wave V strongly suggests a retrocochlear hearing loss.
Results of special tests to determine central auditory function may be invalidated by a concurrent peripheral hearing loss. In general, these tests evaluate a patient’s ability to synthesize distorted or degraded speech signals divided between the two ears, or to detect a specific message in one ear while a competing message is presented in the other ear. In tests using distorted or degraded messages, a cortical (temporal lobe) lesion may cause poor performance in the ear contralateral to the lesion (the contralateral ear effect). Inability to fuse binaural information into a meaningful message may indicate a brain stem lesion.

Amplification is the best rehabilitative strategy if the hearing loss cannot be treated medically or surgically. A sensorineural hearing loss does not preclude benefit from a hearing aid. Likewise, audiometric configuration (pattern of loss across frequencies), decreased speech discrimination, and the presence of recruitment (an abnormally rapid rise in perceived loudness with increased signal level) do not exclude the possibility of successful hearing aid use.
A person with a sharply sloping pattern on audiometric tracings, severely decreased speech discrimination ability, or greatly reduced dynamic range (difference between the threshold of sensitivity and the threshold of discomfort) may have difficulty adapting to amplification devices. However, no single finding in the history, physical examination, or audiometric evaluation can accurately predict how well a patient with presbycusis will be rehabilitated with amplification. Factors that contribute to successful accommodation to amplification include the patient’s desire to communicate (socially and vocationally), expectations and motivation, manual dexterity, and audiometric characteristics. Audiologists experienced in interacting with elderly persons and in dealing with physical and psychologic limitations are most likely to succeed in arranging appropriate amplification.
AMPLIFICATION
Certain guidelines help when communicating with any hearing-impaired person. Communication is most effective when competing environmental sound is absent or minimal. The speaker must first ensure that his face is well illuminated and that the listener is attentive. Optimally, the speaker should be about 3 ft from the listener’s better or aided ear, lips and facial expression should be visible, and speech should be slow and clear. Shouting is not necessary and may worsen the patient’s ability to discriminate. If the person misunderstands a statement, it should not be repeated word for word; instead, the original statement should be paraphrased.
Assistive Listening Devices
These devices help hearing-impaired persons overcome problems using the telephone, television, or radio and communicating in small or large groups. Portable and nonportable amplifiers boost telephone speaker output, and special devices signal an incoming call with either a louder ring or a flashing light. Other devices can sufficiently amplify television and radio signals for the hearing-impaired person while family members listen at normal volume levels. Telecaptioning may benefit those with good vision whose residual hearing is not sufficient to benefit from amplified sound signals.
For small-group communication (eg, in card games), relatively inexpensive devices that have a portable microphone, amplifier, and headset can be used. For large-group communication (eg, in a concert hall or church), many public facilities have group amplification systems such as infrared transmission, and the person can borrow a special portable receiver.
Hearing Aids
Amplification with hearing aids can help persons with conductive or sensorineural hearing losses. Although hearing aids vary in size and power, they share certain features. A microphone receives the sound, transforms it into electrical energy, and sends this signal to an amplifier that increases its energy. How much the incoming signal is amplified, as manifested in the output of the hearing aid speaker, is known as gain. The earmold channels the hearing aid output into the ear canal and affects the acoustic characteristics of the delivered signal.
The hearing aid evaluation matches the patient’s auditory thresholds across the frequency band and speech discrimination ability with the type of hearing aid recommended for those characteristics. Some patients with relatively normal pure-tone thresholds demonstrate very poor speech discrimination and are less likely to benefit from a hearing aid. Some patients with hearing loss and recruitment will require a hearing aid with input automatic gain control and output limiters, technologic advances that prevent pain from overamplification.
When the type of hearing aid is selected, the patient should be taught how to adjust the device and how to keep it clean and functional. Important factors in determining whether a patient will be a successful hearing aid user include motivation, the need to communicate, and appropriate expectations for what can be achieved with the hearing aid. The social stigma as well as the expense of the hearing aid may be difficult to overcome.
For a time, the most widely used hearing aid was the behind-the-ear or postauricular version. The bulk of the device is hooked above and posterior to the pinna; it is connected to the earmold by flexible tubing. Improvements in technology have made it possible to assemble all the required components into a shell small enough to be inserted in the ear; these in-the-ear aids have become popular, in part because they are so unobtrusive. Generally, the smaller the device, the less powerful it is. More important, inserting and adjusting these small devices may be difficult for some elderly persons. Eyeglass-mounted aids have diminished in popularity, although they remain useful for some.
The contralateral routing of signals (CROS) aid is for those whose hearing is totally lost in one ear and relatively normal in the other. A microphone directs sound from the poorer-hearing to the better-hearing ear. If the better-hearing ear is also impaired, the signal from the poorer-hearing ear can be amplified; this type of device is called a BiCROS aid. Originally, the two devices were connected by cords, but newer models can communicate via FM signals.
A body aid, the most powerful type available (ie, producing the highest gain), may be concealed in a pocket or worn with a body strap and is connected by wires to the earpiece, which is connected to the earmold. The elderly, particularly those with impaired fine motor skills, may find it easier to manage the controls of a body aid. Alternatives include geriatric molds, designed for easier handling, and remote controls similar to a credit card to facilitate adjusting settings.
Hearing aids have various modifications, according to individual need. The T switch enables telephone communication through the hearing aid. Circuitry modifications include automatic gain control, automatic signal processing, and multiple signal processing. Automatic gain control automatically adjusts loud or soft sounds to about the same volume. Automatic signal processing and multiple signal processing attempt to improve the signal (human speech)-to-noise ratio and thus ameliorate hearing ability.
In contrast to air-conduction aids, which require an earmold, the bone-conduction aid is placed in direct contact with the head, usually over the mastoid, with a headset. Bone-conduction aids may be appropriate when an in-the-ear aid is contraindicated, as in persistent, uncontrollable otorrhea or canal atresia.
Cochlear Implants
The cochlear implant is approved for profoundly deaf adults who derive no benefit from the most powerful hearing aid. A microphone picks up the incoming sound and sends it to a speech processor; the processor modifies the signal and transmits it to external circuitry, which then relays the information to the receiver in the implanted device. From the internal circuitry, one or more electrodes implanted in the cochlea stimulate the remaining neural elements of hearing.
Implantation generally requires mastoid surgery and brief hospitalization. The device is expensive, and insurance coverage varies. Although it serves a limited population, the cochlear implant can make a tremendous difference in the life of a totally deaf and otherwise isolated person.
Vibrotactile Devices
Vibrotactile devices are alternative modes of rehabilitating profoundly deaf persons; vibrators placed either on the wrist or sternum or around the waist transform speech and environmental sounds into vibrations that can be perceived on the skin. With appropriate training, patients can learn to identify and localize sounds and can use the vibrotactile information to communicate better.
AUDITORY REHABILITATION
The goal of rehabilitation is to achieve the best hearing possible by using a combination of amplification, speech reading, and auditory training.
Speech Reading
The term speech reading has replaced lip reading because linguistic information is obtained not only by watching the speaker’s lips but also by following facial expressions and gestures. Such visual cues may be an adjunct or an alternative to hearing aids. Decreasing visual acuity and poor short-term memory may militate against mastering speech reading.
Auditory Training
Auditory training is often combined with an amplification device to maximize benefit. Auditory training can help patients discriminate between distinctly differing sounds with hearing alone, eventually enabling them to develop schemes for making fine distinctions, particularly between similar speech sounds. In essence, such training makes the patient more aware of subtle auditory clues.
With auditory training, as with any aspect of auditory rehabilitation, having interested family members accompany the geriatric patient is helpful. Family members not only can provide encouragement and support but also may prompt the older patient when short-term memory fails.

Certain guidelines help when communicating with any hearing-impaired person. Communication is most effective when competing environmental sound is absent or minimal. The speaker must first ensure that his face is well illuminated and that the listener is attentive. Optimally, the speaker should be about 3 ft from the listener’s better or aided ear, lips and facial expression should be visible, and speech should be slow and clear. Shouting is not necessary and may worsen the patient’s ability to discriminate. If the person misunderstands a statement, it should not be repeated word for word; instead, the original statement should be paraphrased.

These devices help hearing-impaired persons overcome problems using the telephone, television, or radio and communicating in small or large groups. Portable and nonportable amplifiers boost telephone speaker output, and special devices signal an incoming call with either a louder ring or a flashing light. Other devices can sufficiently amplify television and radio signals for the hearing-impaired person while family members listen at normal volume levels. Telecaptioning may benefit those with good vision whose residual hearing is not sufficient to benefit from amplified sound signals.
For small-group communication (eg, in card games), relatively inexpensive devices that have a portable microphone, amplifier, and headset can be used. For large-group communication (eg, in a concert hall or church), many public facilities have group amplification systems such as infrared transmission, and the person can borrow a special portable receiver.

Amplification with hearing aids can help persons with conductive or sensorineural hearing losses. Although hearing aids vary in size and power, they share certain features. A microphone receives the sound, transforms it into electrical energy, and sends this signal to an amplifier that increases its energy. How much the incoming signal is amplified, as manifested in the output of the hearing aid speaker, is known as gain. The earmold channels the hearing aid output into the ear canal and affects the acoustic characteristics of the delivered signal.
The hearing aid evaluation matches the patient’s auditory thresholds across the frequency band and speech discrimination ability with the type of hearing aid recommended for those characteristics. Some patients with relatively normal pure-tone thresholds demonstrate very poor speech discrimination and are less likely to benefit from a hearing aid. Some patients with hearing loss and recruitment will require a hearing aid with input automatic gain control and output limiters, technologic advances that prevent pain from overamplification.
When the type of hearing aid is selected, the patient should be taught how to adjust the device and how to keep it clean and functional. Important factors in determining whether a patient will be a successful hearing aid user include motivation, the need to communicate, and appropriate expectations for what can be achieved with the hearing aid. The social stigma as well as the expense of the hearing aid may be difficult to overcome.
For a time, the most widely used hearing aid was the behind-the-ear or postauricular version. The bulk of the device is hooked above and posterior to the pinna; it is connected to the earmold by flexible tubing. Improvements in technology have made it possible to assemble all the required components into a shell small enough to be inserted in the ear; these in-the-ear aids have become popular, in part because they are so unobtrusive. Generally, the smaller the device, the less powerful it is. More important, inserting and adjusting these small devices may be difficult for some elderly persons. Eyeglass-mounted aids have diminished in popularity, although they remain useful for some.
The contralateral routing of signals (CROS) aid is for those whose hearing is totally lost in one ear and relatively normal in the other. A microphone directs sound from the poorer-hearing to the better-hearing ear. If the better-hearing ear is also impaired, the signal from the poorer-hearing ear can be amplified; this type of device is called a BiCROS aid. Originally, the two devices were connected by cords, but newer models can communicate via FM signals.
A body aid, the most powerful type available (ie, producing the highest gain), may be concealed in a pocket or worn with a body strap and is connected by wires to the earpiece, which is connected to the earmold. The elderly, particularly those with impaired fine motor skills, may find it easier to manage the controls of a body aid. Alternatives include geriatric molds, designed for easier handling, and remote controls similar to a credit card to facilitate adjusting settings.
Hearing aids have various modifications, according to individual need. The T switch enables telephone communication through the hearing aid. Circuitry modifications include automatic gain control, automatic signal processing, and multiple signal processing. Automatic gain control automatically adjusts loud or soft sounds to about the same volume. Automatic signal processing and multiple signal processing attempt to improve the signal (human speech)-to-noise ratio and thus ameliorate hearing ability.
In contrast to air-conduction aids, which require an earmold, the bone-conduction aid is placed in direct contact with the head, usually over the mastoid, with a headset. Bone-conduction aids may be appropriate when an in-the-ear aid is contraindicated, as in persistent, uncontrollable otorrhea or canal atresia.