Disclaimer: The information contained within the Grand Rounds Archive is intended for use by doctors and other health care professionals. These documents were prepared by resident physicians for presentation and discussion at a conference held at Baylor College of Medicine in Houston, Texas. No guarantees are made with respect to accuracy or timeliness of this material. This material should not be used as a basis for treatment decisions, and is not a substitute for professional consultation and/or peer-reviewed medical literature. Update on Hearing Aids Approximately 22 million Americans have some degree of hearing loss. Currently, patients with sensorineural hearing loss represent approximately 90% of hearing aid users. The sales of hearing aids in the United States have grown steadily, from 830,000 units in 1981 to 1,780,000 units in 1992. Behind-the-ear (BTE) aids represent 18.5% of this total, with body and eyeglass aids accounting for only 0.4%; the remaining 81.1% sold are in-the-ear (ITE) aids. This is a distinct change from only ten years ago, when ITE and BTE aids each represented about 50% of the market. Electronic hearing aids consist of a microphone, an amplifier, and a receiver. The microphone transduces sound into an electrical signal and sends it to the amplifier. The amplifier increases the amplitude of the electrical signal, and the signal may be further modified by filters and volume or tone controls. The amplified signal is transmitted to the receiver, where it is transduced into sound. When the amount of amplification is expressed in decibels, it is called the gain of the aid. One complaint of most patients with cochlear loss is a decrease in dynamic range, also called cochlear recruitment. This is because the loudest sounds are still perceived with the same intensity, whereas patients with cochlear loss are unable to hear quiet sounds. Therefore the range between comfortable listening and uncomfortable loudness is much smaller than in normal subjects. Aids must then be able to supply adequate gain to the user without creating sounds which are too loud. Most aids sold today use linear amplifiers, which increase the signal amplitude by a constant amount, regardless of input level. These aids then require some kind of output limiting - such as peak clipping or compression amplification - to differentially amplify soft sounds more than loud sounds. Earmold design is an important factor in aid design. With BTE type aids, a silicone earmold is used to deliver sound to the ear. With ITE type aids, the aid itself fits into the EAC. In both cases, the aid potentially acts as an earplug and creates the so-called "occlusion effect." Completely blocking the canal results in an increased amplification and perception of low frequencies, especially in the sound of the wearer's own voice. One solution to this problem is venting, or making an opening in the earmold or aid, which attenuates (or decreases) the amplification of low frequencies. However, venting may create a problem of its own: feedback. Feedback occurs when a microphone is placed very near a speaker. The microphone picks up the speaker output, which is amplified and sent to the speaker, which is picked up by the microphone and amplified again, etc. The capacity of the amplifier is quickly overloaded, and the high-pitched sound called feedback results. Feedback is an inherent problem with ITE aids because the microphone and speaker are on the same piece of equipment. Venting may further decrease the amount of gain possible before the microphone picks up the speaker output and feedback results. Feedback is a less significant problem in BTE aids because the microphone and speaker are physically separated. There are several types of aids available. Body aids are seldom used today, but have very high gain possible because of the separation of microphone and speaker. Body aids are still used for some patients with profound hearing gloss. Eyeglass aids allow the fitting of CROS and BICROS hearing aids. CROS aids are helpful for patients with unaidable loss in one ear only, and BICROS aids are used for a combination of unaidable loss in one ear and aidable loss in the other. In BTE aids, the controls, microphone and amplifier are in the body, and sound is transduced through an earmold. There are three varieties of ITE aids: the full-concha aid, the half-concha aid, and the canal aid. Typically ITE and BTE aids have 65 to 75 dB of gain, and body aids may have 80 dB or higher gain. The latest technology in hearing aids is the digital aid. In the past, electronic aids have all used analog circuitry, where the sound is transformed into an electrical signal and then further modified. There are natural limitations in the amplifier's ability to amplify the signal without exceeding power limitations or introducing distortion. In digital technology, the electrical waveform is converted into a digital code of a series of ones and zeros. This digital signal is then modified by computer, and transformed back into an electrical signal and sent to the speaker. The advantage of digital processing is that flexibility is much greater. The digital waveform can be transformed in complex ways without loss of precision, and the natural limitations of electronic equipment are not imposed. Specifically, the frequency response can be adjusted more accurately, the output can be limited at any frequency to prevent uncomfortable sound levels, filtering is more accurate, noise is not introduced, and acoustic feedback can be controlled. These modifications result in the more "natural" sound of digital aids, according to many aid users. Although hearing aids are tested and have published specifications, these are obtained by placing the aid in a standardized device called a 2 cm coupler, and measuring performance. Real ear testing involves the placement of a tiny probe in the patient's ear canal. The frequency response of the unaided ear canal is measured, then the aid is placed, and the aided frequency response is measured using different sound inputs. ASSISTIVE LISTENING DEVICES. Many patients have a mild to moderate hearing loss or a narrow band of frequency loss (especially high-frequency loss) that makes hearing aid use difficult. In addition, some patients do not have the manual or mental dexterity to use hearing aids. Assistive listening devices are designed to improve hearing ability in special listening situations - especially if background noise, multiple talkers or large distances are a problem. There are several types of assistive devices.A hard-wired device uses a microphone close to the source which is directly connected to an amplifier and earphone that the patient wears. The AM radio signal has been used as a wireless system to transmit sound from the source to individual receivers. However, AM signal is limited in power and range, and is very sensitive to electrical disturbances and electronic equipment. Wireless FM systems are much more popular because FM provides a clear signal that is almost immune to outside interference. Finally, infrared light has been used as a signal carrier. Infrared light is invisible, but requires an unobstructed view to the receiver, and is interfered with by some other sources of light. These assistive listening devices (ALDs) are all based on the principle of improving signal to noise ratio (SN ratio), which means that the desired sound is amplified with respect to surrounding noise. This is very useful for hearing-impaired persons in lecture halls, school, or church, or for such activities as TV viewing with a large group of people. ALDs may also be used in conjunction with a hearing aid.
Case Presentation A 48-year-old male veteran suffered noise exposure while in the service. He was fitted with a right-side monaural behind-the-ear aid in 1965 while in the military. He has been followed at the Houston Veterans Affairs Medical Center Audiology Clinic since 1972, preferring right ear only hearing aid use. His most recent audiogram shows a stable bilateral moderate to severe sensorineural hearing loss from low to high frequencies. His discrimination was 80% in the right ear, and 70% in the left. Acoustic reflex thresholds were elevated, but consistent with the extent of the sensitivity loss, and tympanometry revealed Type A tympanograms. The patient was recently fitted with bilateral in-the-ear hearing aids, which the patient uses regularly with good results. His real ear testing reveals excellent tuning to the computer-simulated parameters.
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