Hearing Loss Considerations

A primary consideration in successful hearing aid use is the nature, degree, and configuration of hearing sensitivity loss. Although no firm criteria exist about how much hearing loss is too little or too much to preclude successful hearing aid use, some general rules guide prognosis.

In an effort to provide a realistic view of the characteristics of hearing aid users, we evaluated data from 1860 ears of 1049 consecutive patients who were fitted with hearing aids at the Henry Ford Hospital during 2007. Patients ranged in age from 1 to 101 years old. The distribution of ages is shown in Figure 162-1. Of all patients, 81% were at least 60 years old, and 61% were 70 years or older. These percentages are typical of the hearing aid population in general. The prevalence of hearing loss is progressively greater with age, as is the use of hearing aids. Hearing aids are predominantly fitted on ears with sensorineural (88%) or mixed (11%) hearing loss. In this sample, less than 1% of hearing aid devices were fitted on ears with pure conductive hearing loss.

Figure 162-1. Distribution of hearing aid patients by age.

Among people with hearing loss, there are significantly more individuals with mild hearing loss than severe or profound loss. Among this group of hearing aid users, most patients who choose to use hearing aids have hearing loss that is at least moderate in degree. The proportion of patients who have hearing loss and use hearing aids increases substantially with increasing severity of hearing loss. Stated the other way, the penetration of hearing aids into the potential market is low for mild hearing loss and considerably greater as hearing loss increases.

Slope of the audiometric configuration is another factor that influences hearing aid usage. Most patients have flat or sloping hearing loss—that is, hearing sensitivity that is similar across frequencies, or that is better in the low frequencies than the high frequencies. For some audiometric configurations, it is very challenging to provide appropriate amplification. One difficult configuration is a high-frequency precipitous loss, where hearing sensitivity is normal through 500 Hz and decreases dramatically at higher frequencies. There may even be dead regions in the cochlea, where hair cell loss is so complete that no transduction occurs. Depending on the frequency at which the loss begins and the slope of the loss, this type of hearing loss can be very difficult to fit effectively. The other extreme is the rising audiogram, a relatively unusual audiometric configuration in which a hearing loss occurs at the low frequencies, but not the high frequencies. Although this type of loss seldom causes enough of a communication problem to warrant hearing aid use, when it does, certain aspects of the fitting can be troublesome, and the prognosis for successful fitting is limited. In the present sample, less than 2% of ears had rising audiograms.

Although degree of loss is associated with hearing aid use, the audiogram offers only one piece of information needed to determine candidacy. Most patients with minimal to mild hearing loss do not pursue hearing aids, although some do. This fact indicates that the presence of hearing loss alone may be insufficient to compel a patient to hearing aid use, but that a mild degree of hearing loss does not contraindicate hearing aid use. Even patients with minimal hearing losses can wear mild gain hearing aids with benefit. Generally, if the hearing impairment is enough to cause a problem in communication, the patient is a candidate for hearing aids.

Some patients have too much hearing loss for hearing aid use. Severe or profound loss can limit the usefulness of even the most powerful hearing aids. In many cases of profound hearing loss, a hearing aid can provide only environmental awareness or some rudimentary perception of speech. Many patients do not consider this benefit to be valuable enough to warrant the use of hearing aids. In these cases, cochlear implantation is often the most beneficial treatment strategy.

Another audiometric consideration in hearing aid use is suprathreshold speech recognition ability. In most patients, speech recognition is commensurate with degree of hearing loss and is simply a reflection of audibility of spoken speech. In these patients, amplification of the sound that is missing can provide significant enhancement of speech recognition. In other patients, speech recognition is poorer than expected for a given hearing loss. For example, cochlear hearing loss secondary to endolymphatic hydrops can cause substantial distortion of sound, resulting in very poor speech recognition ability. If it is sufficiently poor, hearing aid amplification may contribute to audibility but will be unsatisfactory overall.

Auditory processing disorders in young children and in aging adults can also reduce the benefit from conventional hearing aid amplification. It is not unusual for geriatric patients who were previously successful hearing aid users to experience increasingly less success as their central auditory nervous systems change with age.³ The problem is seldom extreme enough to preclude hearing aid use, but these patients may benefit more from assistive technology to complement hearing aid use.⁴ Prognosis for hearing aid benefit in these patients is guarded.

Communication Disorder and Motivation

The impact of a hearing loss can vary considerably among patients and is an important factor in hearing aid candidacy. Some patients with slow onset presbycusis, of even moderate degree, perceive no difficulty with communication. Such patients effectively use communication strategies such as speech reading and manipulation of the environment to minimize the impact of hearing loss. The prognosis for successful hearing aid use is limited by the patient’s lack of recognition of the hearing loss. Other patients, who have considerably less hearing loss but greater communication demands or less success with coping strategies, might perceive communication difficulties and be anxious to quantify the hearing loss for the purpose of obtaining hearing instruments.

Patient motivation is another key factor in predicting success with hearing aids. A patient who is internally motivated to hear better is an excellent candidate for successful hearing aid use. The breadth of amplification options for such a patient is substantial. In contrast, an unwilling patient who gives in to the requests of a spouse or other family members to seek hearing aid amplification often finds numerous reasons why hearing aid amplification is unsatisfactory.

Candidacy for hearing aid amplification is fairly straightforward. If a patient has a sensorineural or other nontreatable hearing loss that is causing a communication disorder, the patient is a candidate for amplification. Even when a hearing impairment is mild, if it is causing difficulty with communication, and the patient is motivated to do something about it, the patient is a candidate for hearing aid amplification and is likely to benefit from its use.

Otologic and Other Factors

Other factors can influence hearing aid candidacy, although they are more likely to influence the selection of type and style of device than to preclude its use. Although a progressive or fluctuating hearing loss does not rule out conventional hearing aid use, the selected hearing instrument must be sufficiently flexible to allow for alterations in programming to accommodate the patient’s changing hearing. Patients with microtia or other anomalies may have ear canals or pinnae that prevent fitting with a conventional custom hearing instrument or the ear mold used with a behind-the-ear (BTE) hearing aid. More commonly, the size of the ear canal of a patient sometimes limits the use of certain in-the-ear (ITE) hearing aids. Other physical and medical limitations can make conventional hearing aid use difficult. Occasionally, a patient with hearing loss has external otitis or ear drainage that cannot be controlled medically. Placing a hearing aid in such an ear can be a constant source of problems. Certain rare pain disorders may also limit access to the ear canal.

Physical and cognitive limitations may influence hearing aid candidacy. Physical limitations are primarily limitations pertaining to dexterity issues from arthritis and other conditions that affect a patient’s ability to manipulate certain styles of hearing aids.^5,6 Limited cognitive ability can be a barrier to successful hearing aid use when a patient has difficulty remembering how to operate a hearing aid and its components.

Evolution of Hearing Aid Technology

Hearing aid technology has advanced rapidly. Modern hearing aids use DSP for high-fidelity sound reproduction. They have adaptive directionality, feedback control, noise reduction, and low battery drain. They are readily programmable, providing substantial flexibility for precise fitting. Modern hearing aids also have advanced amplification strategies that reduce distortion and enhance listening comfort.⁷

One early step in the progress of technology was component miniaturization. Microphones, amplifier circuits, loudspeakers, and batteries all have been reduced in size. This miniaturization has allowed sophisticated signal processing circuits to be fitted into a completely-in-the-canal (CIC) style of hearing aid. It has also allowed for more technology components, such as wireless receivers, to be incorporated into smaller sized devices worn over or in the ear.

Programmability of hearing aids was a major step in the advancement of hearing aid technology. This programmability provided flexible control of hearing aid characteristics. In the recent past, a hearing aid circuit had to be matched carefully to a patient’s audiogram at the time of hearing aid selection. Now hearing aids are available that can be programmed over a wide fitting range so that the matching is done at the time of hearing aid fitting, rather than at the time of selection. Programmability also provides multiple memories for a single hearing aid, allowing the potential for use of different response parameters for different listening situations.

Hearing aids have progressed since the late 1990s from analog to DSP. In analog hearing aids, acoustic signals followed an analog path that was under analog control, wherein an acoustic signal was converted by a microphone into electric energy in a continuously variable manner. In DSP hearing aids, acoustic signals are converted from analog to digital and back again, with digital control over various amplification parameters. DSP eliminated many of the barriers faced in trying to design analog circuits to fit in a small hearing aid and run on low-powered batteries. Along with flexibility inherent in enhanced programmability, modern devices provide more precise and flexible frequency shaping, more sophisticated compression algorithms, better acoustic feedback reduction, and enhanced noise reduction algorithms.^8,9 The degree of sophistication of signal processing that is used in modern hearing aids seems limited only by the conceptual framework of how hearing aid amplification should work.

The most recent technologic advance in hearing aids is the growing opportunity of wireless connectivity, permitting communication from one hearing aid to another and from hearing aids to other electronic devices and signal sources.¹⁰

Hearing Instrument Components

A hearing aid is an amplification device that consists of three basic components: a microphone, an amplifier, and a receiver. The power source for the amplifier is a battery. Most hearing aids also have some external controls, such as a program button or volume control on the hearing aid, or a remote control. A schematic of the basic components is shown in Figure 162-2.

Figure 162-2. The components of a hearing aid.

The most common input transducer in a hearing aid is a microphone, which converts acoustic energy into electric energy. As a sound source vibrates, it creates pressure waves of expanding and compressing air molecules. The membrane of the microphone vibrates in response to these pressure changes, creating an electric energy flow that corresponds to the amplitude, frequency, and phase of the acoustic signal.

Sound can also be delivered to the amplifier of a hearing aid via direct audio input, wherein a sound source reaches the hearing aid directly via a wire connector or “boot” that connects to a BTE hearing aid. One of the most common alternative input transducers is the telecoil, or t-coil. A telecoil allows the hearing aid to receive electromagnetic signals directly, bypassing the hearing aid microphone. The telecoil is often used with the telephone to reduce background noise, and to minimize the potential for feedback that occurs when placing a device near the microphone. A telecoil can be activated by a manual control on the hearing aid, by remote control, or automatically when the hearing aid senses an electromagnetic field.¹¹

The telecoil can also be used to provide input for remote-microphone sources. The signal from the remote microphone is transmitted in some form to a loop of wire, which transmits the signal electromagnetically to the telecoil of the hearing aid.¹¹ This type of technology is often used in a classroom situation, to allow the signal delivered from the remote-microphone worn by a teacher to be directly input to the hearing aid worn by the student.¹² This technology minimizes the degradation of the auditory signal caused by background noise and distance.

A hearing aid may have some other form of wireless transducer, such as a frequency-modulated (FM) receiver or Bluetooth or other modern wireless connectivity. An FM receiver can be built into a hearing aid or attached as a boot on a BTE hearing aid. The FM receiver acts like an FM radio, receiving signals from a transmitter and directing them to the amplifier of the hearing aid.¹³ Increasingly, modern hearing aids are equipped with other wireless connectivity solutions, so that the hearing aid amplifier can receive signals directly from mobile phones, computers, or personal music players.¹⁰

The function of a hearing aid is to amplify sound. This amplification is accomplished through the power amplifier of the hearing aid. The amplifier adds gain to the level of the electric signal that is delivered to the hearing aid’s transducer. The amplifier can differentially enhance higher or lower frequency and higher or lower intensity sounds. It also has a limiting function so that it does not deliver excessive energy to the ear.

The output transducer of a hearing aid is its receiver, or loudspeaker. The loudspeaker receives an amplified electric signal from the hearing aid amplifier and converts it back into acoustic energy. Hearing aid receivers have a broad, flat frequency response to reproduce accurately the signals being processed by the hearing aid amplifier.

Electroacoustic Characteristics

The acoustic response characteristics of hearing aids are described in terms of frequency gain, input-output, and output limiting. Gain is the amount of energy that is added to the input signal. The amount of gain varies as a function of frequency to accommodate varying hearing loss configurations. The relationship of gain as a function of frequency is the frequency gain response of a hearing aid. The frequency gain response represents the gain produced by a hearing aid to a specified intensity level of a signal presented across the frequency range. It reflects the difference between the intensity level of the output of the hearing aid and the intensity level of the input. Most methods used for prescribing a hearing aid are based on providing a specified amount of gain at a given frequency, based on a patient’s pure-tone audiogram.

The amount of gain also varies as a function of intensity level. The input-output characteristic of a hearing aid describes this relationship. The input-output function can be linear or nonlinear.

With linear amplification, the same amount of amplification, or gain, is applied to an input signal regardless of the intensity level of the signal so that low-intensity sounds are amplified to the same extent as high-intensity sounds. Figure 162-3 shows an example of a linear input-output function. In this example, for every dB increase in the input, there is a corresponding dB increase in the output. Fitting of linear hearing aids was a standard approach in the early years of hearing aids and remains applicable for conductive hearing loss and some mild sensorineural hearing losses. A problem with this type of amplification is that it does not address the nonlinearity of loudness growth that occurs with sensorineural hearing impairment. Many patients with sensorineural hearing loss do not hear low-intensity sounds, but can hear high-intensity sounds normally or even excessively loud owing to recruitment phenomena. Because a linear device amplifies soft and loud sounds identically, if a low-intensity sound is made loud enough to be audible, a high-intensity sound is likely to be too loud for the listener.¹⁴

Figure 162-3. Relationship of sound input to output in linear and nonlinear hearing aid circuits. For the linear circuit, gain remains at a constant 20 dB, regardless of input level. For the nonlinear circuit, amount of gain changes as a function of input level. SPL, sound pressure level.

In nonlinear amplification, the relationship between input and output is not constant, so that low-intensity sounds are amplified to a greater extent than high-intensity sounds; this is shown schematically in Figure 162-3. The overall effect is to make low-intensity sounds audible; moderate sounds comfortable; and high-intensity sounds loud, but not too loud.

Nonlinear amplification is designed to address two problems with sensorineural hearing loss: reduced dynamic range, and loudness growth that is different from that of a normal ear. Dynamic range in this case is a term used to describe the decibel difference between the level of a person’s threshold of hearing sensitivity and the level that causes discomfort. In an individual with normal hearing, this range is 0 to about 100 dB hearing level (HL). In a patient with sensorineural hearing loss, the range is reduced. A patient with a 50-dB hearing loss and a discomfort level of 100 dB has a dynamic range of only 50 dB. Compression circuitry is used to boost the gain of low-intensity sounds so that they are audible, but to limit the gain of high-intensity sounds so that they are not uncomfortable. Figure 162-4 illustrates the difference between linear output and dynamic-range compression as it relates to an ear with sensorineural hearing loss and nonlinear loudness growth.

Figure 162-4. The difference between linear and nonlinear amplification in an ear with sensorineural hearing loss and nonlinear loudness growth. SPL, sound pressure level.

(From Stach B. Clinical Audiology: An Introduction. 2nd ed. Clifton Park, NY: Delmar Cengage Learning: 2010, 583.)

Nonlinear amplification is achieved with compression circuitry. Compression is a term that is used to describe how the amplification of a signal is changed as a function of its intensity. Compression techniques are used to limit the maximum output of a hearing aid and to provide nonlinear amplification across a wide range of inputs.^14,15

Numerous nonlinear compression strategies have been developed to address the dynamic-range issue, and they vary in approach and complexity. Some strategies are designed to provide compression over a portion of the dynamic range. Partial dynamic-range compression typically provides linear amplification for low input signals and some level of compression when the input reaches a certain level. Other strategies are designed to provide compression over a wider portion of the dynamic range. Wide dynamic-range compression has as its basis the enhanced amplification of quiet sounds and reduced amplification of loud sounds, and is designed to package speech into a listener’s residual dynamic range. For most systems, dynamic-range compression can be altered in multiple frequency bands. In this way, if a patient’s dynamic range is reduced in one frequency range and nearly normal in another, the compression can be tailored to the frequency band where it is needed, and the other band can act as more of a linear amplifier.^14–16

An amplifier also contains circuitry that limits its output. It is important that the output level of a hearing aid be limited to some maximum because high-intensity sounds can be damaging to the ear and uncomfortable to the listener. The classic method of limiting output, known as peak clipping, does not allow the peaks of signals to exceed a certain level. This method commonly causes distortion for high-intensity inputs. Compression limiting, the current standard method of output limiting, allows the amplifier to become nonlinear as input signals reach a predetermined level, so that the amount of gain is diminished significantly near the maximum output level.^14,15

Other Hearing Aid Features

Today’s hearing aids include many features that make an important contribution to successful hearing aid fitting: directionality, noise reduction, feedback reduction, program management, automatic adaptivity, and data logging. During normal hearing, sound pressure waves reach the tympanic membrane after the acoustic signal has been modified by the resonance characteristics of the pinna, concha, and ear canal. These changes to the acoustic signal greatly aid the localization of a sound source in space, and contribute to the ability to hear in the presence of background noise.¹⁷ With hearing aid use, sound is received by the microphone, which is removed from the location of the tympanic membrane. The signal from the microphone does not include the spatial cues provided by the ear, reducing the ability to hear in background noise.

One important strategy for restoring some of these spatial cues is the use of directional microphones. A directional microphone is one that is designed to be more sensitive to sounds emanating from one direction than another. In most listening situations, the signal of interest is in front of the listener, and background noise is separated spatially, often from behind the listener. A directional microphone is designed to enhance the signal from the front and reduce the signal from behind or from wherever else it might be detected.

In its simplest form, a microphone is equally sensitive to sounds coming from all directions, or is omnidirectional in nature. A microphone can be made to be more directional by using two ports, one on the front and one on the back of the hearing aid. Each port leads to the opposite side of the diaphragm of the microphone. The rear port of the microphone is filtered in a way that causes a phase shift, or time delay, between sounds reaching each side of the microphone. In this way, sounds emanating from behind the listener are delayed so that they arrive at the back of the diaphragm simultaneously with their arrival at the front of the diaphragm. Equal sound pressure levels at both sides of the diaphragm essentially cancel those from behind because the diaphragm cannot move. The directional response pattern can be manipulated by changing the spacing of the ports or the time delay or both.

The modern version of the directionality feature uses two or more directional microphones in an array within the hearing aid. The directionality is created by subtracting the sound at the rear microphone from that at the front. The time delay is manipulated acoustically and electronically based on the number of microphones, the physical nature of the array, and the filters used to optimize the directionality pattern.

Most hearing aids can be changed from an omnidirectional to a directional setting. Typically, omnidirectional microphones are used in quiet environments.¹⁸ Directional microphones are beneficial in background noise so that the microphone sensitivity is focused in a desired direction. In some instruments, the directionality feature is activated manually by the patient. In other instruments, it can be activated automatically and adaptively when the hearing aid senses background noise, changing the amount of directionality based on the extent of the noise.¹⁹

Noise reduction circuitry is available in most modern hearing aids to reduce unwanted background noise in an effort to improve patient comfort and speech recognition. The sophistication of noise reduction strategies has advanced dramatically,²⁰ and a thorough description is beyond the scope of this chapter. A simple example might serve to illustrate how the problem can be approached. The frequency and intensity of ongoing speech is highly variable and of short duration, whereas some background noise is relatively constant in nature. A DSP hearing aid can easily detect the difference in time constant and reduce the gain of the hearing aid in the frequency band of constant noise. Modern strategies use increasingly sophisticated algorithms to improve comfort and audibility in noise.

Acoustic feedback occurs when the amplified sound emanating from a receiver is directed back into the microphone of the same amplifying system. Physical separation of the microphone and receiver is the most effective way to reduce feedback. Another feedback reduction mechanism is feedback suppression circuitry. With feedback suppression, the hearing aid recognizes the occurrence of feedback based on frequency, intensity, and temporal characteristics. It reduces amplification in the offending frequency range to reduce feedback or uses phase cancellation of the feedback signal to eliminate audible feedback.^21,22

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