Aural Rehabilitation Devices
Tang Ho , M.D.
March 30, 2006

Case presentation: The patient is a 77-year-old veteran who presents to the VA Audiology Clinic for evaluation of his bilateral mid- to severe-downsloping sensorineural hearing loss. The patient had sustained significant noise exposure while he was in the military service and had been using bilateral behind-the-ear hearing aids for approximately six years prior to losing his left hearing aid about a couple of months prior to presentation to the clinic. The audiogram shows a stable bilateral sensorineural hearing loss downsloping in configuration. He was subsequently fitted with bilateral behind-the-ear hearing aids, with the Siemens Triano BTE Model, as shown in this photograph. Post-fitting hearing aid evaluation shows satisfactory real ear measurements.

The post-fitting standardized survey, the International Outcome Inventory for Hearing Aids, shows that the patient is hearing significantly better with the hearing aids. This standardized questionnaire attempts to assess how often the patient uses the hearing aids, how satisfied they are generally with the hearing aids, and also the quality of life improvement as a result of the new hearing aid. The patient has achieved high scores on all the questions in this survey.

The definition of a hearing aid is “any device whose function is to amplify acoustic signals to a degree that enables the hearing-impaired individual to use his/her remaining hearing in a useful and efficient manner.” It has been estimated that approximately 28 million Americans today have hearing impairments, and that hearing impairments are the third most prevalent chronic condition affecting older Americans, after arthritis and hypertension. Although some hearing impairment conditions can be treated medically or surgically, for example Ménière’s and otosclerosis, most of these patients will benefit only from amplification. Approximately one-quarter, or about 23%, of the people who could benefit from hearing aids actually wear one, about 5 million. The cost of the hearing aids can range anywhere from $600 to greater than $2,000.

The history of the hearing aid can be traced back to 1588 when Giovanni Battista, or John Baptist Porta, who was a physician scientist, first described the use of wooden hearing aids shaped like animal ears in his book Natural Magick. Francis Bacon, who is familiar to us all, in 1627 A.D., in his book Sylva Sylvarum or a collection of collections, first describes the use of ear trumpets to aid the deaf. He also, in this book, elaborates on the use of speaking tubes to aid the deaf as well.

John Harrison Curtis, in 1803 A.D., was the first to apply the speaking tubes in a practical way to assist the deaf, and he also designed a classical and flexible speaking tube, in 1817, that is still used today. Frederick C. Rhine, in 1802, established the first company that manufactured hearing aids on a commercial basis.

The first pattern in the US for hearing aid was in the form of the acoustic auricle in 1855 by EG Hyde. Richard Rhodes, in 1879, invented and patented an interesting phone conduction hearing fan called Audiophone. Its purpose was for use by hearing impaired ladies in need of fashionable hearing aids during formal occasions; and the device consisted of a thin piece of pliable material that is shaped like a fan which, when in contact with the upper teeth, could actually provide up to about 30 dB of amplification.

Modern hearing aids can be broken down to four main components. The first component is that of microphone, a transducer that converts the sound energy to electrical energy. The electrical energy then is routed to the amplifier where the signal is increased in the amplitude, and then the electrical energy is then routed to the receiver, which converts the electrical signal back to acoustic signal and then goes back to the ear.

Some basic terms in the hearing aids include the use of the term "gain" to refer to the difference in decibels between output level and input levels of the hearing aids. Frequency response refers to the response of a hearing aid showing the amount of gain at each frequency, and this can be adjusted to fit each patient’s hearing loss. Compression refers to the hearing aid's ability to limit the output signal, and the feedback is the phenomenon when the outputs of the hearing aid leaks back into the input, creating what we usually hear as whistling sounds. This can happen when the ear mold of the hearing aid is not properly fitted or is not snug.

The conventional analog hearing aid was the first electrical hearing aid that was introduced onto the market, and it is designed with a particular frequency response based on the audiograms of the patients. It seeks to preserve the relative amplitude frequency and temporal pattern of the input sounds, and it essentially amplifies all sounds, including speech and noise. This can create distortion of the sound signal, and it is the least expensive type of hearing aid available.

The digital hearing aid has gained wide acceptance since its recent introduction. The sound wave in the digital hearing aid is converted to a digital binary code stream. The digital hearing aid has the ability to analyze the sound environment, and change amplification accordingly, depending on the sound environment. As a result, you get an improved signal-to-noise ratio through this selective amplification of the signal of interest and suppression of the background noise.

There have been multiple studies done to compare digital and analog devices. Wood and others, in 2004, found that overall, digital aid users had better speech recognition performance, greater satisfaction, and less aversiveness to sound. However, there was no difference found in quality of life measures. Parving and others, in 2001, conducted an audit of 32,000 Danish hearing aid users with three different types of hearing aids: the analog, programmable analog, and digital hearing aids. They found no significant difference in any of the benefit parameters. They did, interestingly, find that people who used the analog hearing aids had a slightly greater satisfaction with their hearing aids. This finding was thought to be attributed to some of the service issues that were encountered by the digital hearing aid users at that time.

How does a hearing aid actually process the sound signal? The traditional technology was that of a peak clipping circuitry where the sound amplifies linearly up to a predetermined level and then, after that point, as the term implies, the amplification flattens out. So, regardless of how much louder the input signal gets, the output remains the same. As a result, you can cause distortion of the sound signal and reduce the quality of the speech that is perceived by the hearing aid user.

The modern circuitry uses a technology called compression circuitry, of which there are various types. The most common types are compression limiter and wide dynamic range compressor power circuitry. This automatically reduces the instrument gain by a fixed ratio when the input or output signals exceed a predetermined level at an earlier point. As a result, you get a better amplification of crucial speech components that are needed for speech understanding; however, the downside of these compression circuitries is that sometimes the alteration in the signal can be apparent to the user.

A study that compared different hearing aids circuits has shown that all three circuits provide significant benefits in both quiet and noisy environments. It did, however, show that compression hearing aids tend to generally perform slightly superior than the peak clipping circuit in the overall listening experience for the user and also has higher word recognition scores, although the differences are small. The preference rankings are also significantly higher for compression limiter circuitry compared to the other types of circuitry. The caveat of this study, however, is that it has a limited applicability in patients with mild hearing loss since all the subjects in this study had a severe hearing loss.

As with other electronics, the trend in hearing aids has been to make them smaller. Small hearing aids can sometime produce a more natural sound because the pinna and the external auditory canal acoustic properties are preserved. Also, they have a higher cosmetic appeal and the benefit of reduced wind noise interference, as well as ease for telephone use. However, the small hearing aids also have less electronic flexibility and have fewer user controls and available acoustic modifications. Also, they tend to have more problem with feedback as well and also tend to be more expensive. Maintenance issues can also be a problem for smaller hearing aids; they tend to break more easily and are also more easily lost.

Body hearing aid was the first wearable hearing aid that was introduced. These consisted of a case that is worn on the body that contains the microphone, the amplifier, the battery, as well as the control switch. The receiver comes from the case and goes to the earlobes. There are few in use today secondary to the size and the position of the microphone. However, it is still available for a select population and still used in some parts of the world today.

Behind the ear, or the BTE aids, as the patient in this case has, contains all major components in the body of the hearing aid behind the auricle. It is the most common aid that was dispensed during the 1960s to the 1980s. It is easily serviced and has a sufficient space for fitting of digital circuitry in the hearing aids. The important part about the BTE hearing aid is that is it is very flexible and is appropriate for all types and degrees of hearing loss.

The in-the-ear, or the ITE aids, have a faceplate that contains the microphone opening and control switches; and the shells are made from ear mold impressions that are customized for each individual patient. The full shell of the hearing aid fits the entire concha and is the largest of the ITE aids, and can address a more severe type of hearing loss. The half-shelled ITE aid fills only the concha cavum. It is more cosmetically appealing, but is used to address a mild to moderate hearing loss and can have more feedback problems when compared to the full-shell type.

The in-the-canal, or ITC aids, fit within the concha and outer half of the ear canal, and the advantage of this hearing aid is that it can provide a higher gain at higher frequencies because of complete insertion of the hearing aid. It also preserves the acoustic resonance of the concha itself. The completely-in-the-canal aid, or CIC aid, is the smallest hearing aid available. The hearing aid fits entirely with in the canal; it is also known as a peritympanic hearing aid because of its close proximity to tympanic membrane. It is appropriate for mild hearing loss and cosmetically pleasing to the patients. It also has the benefit of reduced occlusion effect, which is the plugged-up sound that some users can get when using the hearing aids.

Contralateral routing of signal, or CROS aids, are useful for individuals with single-sided deafness and minimal hearing loss or normal hearing on the contralateral ear. The microphone transmitter is mounted on a deaf ear and the receiver of the hearing aid is mounted on the hearing ear. Transmission can be done either via a wire behind the neck, or wireless through radiofrequency, and BICROS is the variation where there is amplification on both sides.

Some interesting hearing aids statistics include 2.2 million hearing aid units sold in 2005, showing a growth of 3% over the previous year and 36% growth in hearing aids sales during the past 10 years. The VA is the largest single purchaser of hearing aids in the United States, accounting for 14% of all hearing aids purchased in the US. In Texas, 99,000 hearing aids were sold in 2004. Of all the hearing aids on the market today, 90% are digital hearing aids.

A breakdown of the types of hearing aids in the US shows that 38% are ITE models and this is the most popular model. Behind-the-ear hearing aids are the second most popular at 31%. The CIC and ITC models account for the rest of the hearing aids that are sold today.

Who is the ideal candidate for hearing aids? Obviously, audiological consideration is important, and the type of loss and degree of hearing loss are important to consider. Also, dynamic range of the individual is important to consider as well; and this is defined as the target area for amplification. The dynamic range is defined as the range between the hearing threshold and the point at which the stimulus can become uncomfortably loud. It is often much reduced in a patient with sensorineural hearing loss because there is an upward shift of the lower limit of the dynamic range secondary to the elevated threshold of the hearing loss. Also, the upper limit can be shifted down secondary to the recruitment phenomenon, resulting in a narrow dynamic range and making it more difficult to fit appropriate hearing aids. Age is an important consideration as well, because hearing impairment can delay or prevent development of the communication system in a child; therefore, adversely affecting learning and psychosocial growth. Early intervention of hearing deficits is mandated by law. The BTE model is the most frequently employed in children because the growth of a child requires frequent change of the ear mold. With the BTE model, you can keep the hearing aid and replace only the ear mold.

Neurotological consideration needs to be taken into account as well. The anatomy of the auricle, the external auditory canal, and the TMJ should be assessed. For example, a BAHA may be more appropriate in someone with an anatomic deformity such as atresia. Middle ear pathology, such as otitis media, needs to be evaluated and treated prior to hearing aid evaluation. In someone with fluctuating hearing loss, such as a patient with Ménière's, may require both pharmacotherapy as well as auditory rehabilitation. Finally, last, but not least, physical and psychosocial characteristics, such as manual dexterity and cognition, are important considerations particularly in the elderly population. Motivation is another important factor. Is the patient at all in denial of his/her hearing loss? Social support and expectations of the patient about the hearing aids are among other considerations.

There have been some interesting studies done on the profile of the hearing aid seekers. In a study, done in 2005, the authors found that hearing aid seekers can generally be characterized as people who are more pragmatic and routine oriented. They tend to feel more personally empowered in dealing with the challenges, and they tend to utilize the social support system less often than those people who do not want hearing aids. There have also been comparison studies between those hearing aid users in private practice versus that of public health systems. In this study, the VA was the sample used for a public health sector. They found that VA patients tend to report higher expectations and tend to have a more severe hearing deficit at the time of hearing aid evaluation. They also found that VA patients also reported more favorable hearing aid outcomes than their private sector counterparts.

The process of a hearing aid fitting refers to the actual process of matching the frequency response of the hearing aids to the outputs and the gain requirements of the user. Various fitting protocols exist. The half-gain rule was the oldest and the first one that came out, where the gain of hearing aids were simply cut in half of the measurable amount of hearing loss. The NAL, which stands for the National Acoustic Laboratory of Australia, is probably the most popular hearing aid fitting protocol being used today. It is loosely based on the half-gain rule and it attempts to equalize loudness across all speech frequencies.

Ear mold shaping is an important part of the hearing aid process because it helps to direct the sound flow to the ear and enhance the sound quality and reduce the feedback phenomenon. The steps for proper ear mold fitting include cleaning the external auditory canal with complete TM visualization, a protective otoblock is then placed next to the TM and finally, atraumatic injection of properly prepared ear mold material. The proper fitting of hearing aids by trained professionals rarely results in complications. However, when the complications do occur, it is important to recognize them. Some of the unusual hearing aid fitting complications that require surgical intervention are investigated in this study by Cohen and others, in which they found entrapped ear mold caps behind the TM in one patient. TM perforation with ear mold materials in middle ear space was found in two patients, and impacted ear mold material in canal wall down cavity was found in three patients. These patients all presented with fairly nonspecific otologic complaints; however, they can be all managed successfully with a standard otologic surgical technique.

There has been a debate as to whether or not it is advantageous for binaural amplification. The advantage of binaural amplification include improved speech understanding in noise, for the same reason that listening with two ears is better than one, and also one has a better ability to localize sound when we have two hearing aids in place. In the study that compared the sound localization ability of normal hearing, bilaterally aided, monaurally aided, and non-aided subjects, they found that a normal-hearing individual and bilaterally-aided listeners had a high degree of symmetry in judgment of the perceived angle versus those who are monaurally fitted. The literature indicates that also, over time, the unaided ear of a monaurally fitted individual can lose functional capacity in a phenomenon called auditory deprivation, and there is a worsened speech discrimination score and PB scores in the unaided ear of the monaurally fitted patient over time.

The disadvantages of bilateral amplification include financial costs and also self-image. There tends to be more negative association with aging and disparity of hearing loss. At least to the patients who have two hearing aids, they are more likely to report feeling that the hearing aids are noticeable to others. Binaural interference is a phenomenon that has been observed in a small portion of elderly that have poor speech discrimination when listening binaurally versus monaurally, and it has been reported to be as high as 10% in certain study populations. This is thought to be related to deterioration in the fusion of input from two ears, therefore causing interaural asymmetry. There are various hypotheses as to why this occurs. Some of these hypotheses include monaural deprivation, changing turning properties of one cochlea relative to another secondary to the loss of hair cell function, and also a decreased efficiency of interhemispheric transfer of auditory information through corpus callosum secondary to aging. Also, disadvantages of binaural amplification can include increased wind noise and the need for management of additional hearing aids.

Some of the hearing aid advances that are coming out today include the use of a directional microphone, which is actually an old technology that was introduced in the US back in 1970s but did not get popularized until recently. The directional microphone reduces the sound from the rear compared to the front, and as a result, improves the signal-to-noise ratio and improves understanding the speech in the presence of noise. There has been a systematic review of the clinical evidence available that shows that there is a significant difference but a mild audiologic advantage with the use of directional microphones.

On a social basis, there has been introduction of low cost, solar powered hearing aids. Two hundred fifty million people worldwide are hearing impaired, and two-thirds of these people are located in developing countries. The World Health Organization has sponsored the production of a solar-powered body hearing aid that is very cheap to produce: it costs about $60 to $100. Initial studies have shown that a subjective satisfaction from these hearing aids is comparable to that of modern hearing aids.

The semi-implantable middle ear hearing device, the vibrant sound bridge system, has gained FDA approval. This device has an external sound processor and amplifier and has an internal vibrating ossicular prosthesis. However, this device never quite gained a wide popularity. A totally implantable middle ear hearing device, the Envoy system, has just recently passed the FDA phase I trial. In this device, which is shown here, we have the sensor that is surgically implanted and patched to the incus, and the sound is processed by the sound processor here, and then it is transmitted back to the driver of the device which is surgically attached to the stapes. The advantage of this hearing aid is that it has being reported to reproduce a more natural sound, has no occlusion effect or feedback, and is more cosmetic appealing for the patients because it is totally implanted. However, there are further refinements needed because the battery life is short for these hearing aids. There has been an unacceptably high device failure rate in the phase I trial. Also, in order to implant the device, this requires separation of the IS joint as well.

Case Presentation:

A 77-year-old male veteran presents to the Houston Veterans Affairs Medical Center Audiology Clinic for evaluation of bilateral downsloping mild to severe sensorineural hearing loss and hearing aid replacement. He had sustained loud noise exposure without hearing protection while in the military service. He has been using bilateral behind-the-ear hearing aids for nearly 6 years but had recently lost the left hearing aid.

On evaluation in the clinic he was found to have stable mild to severe bilateral downsloping sensorineural hearing loss. PTA is 37/48 dB for the right ear and 37/43 dB for the left ear. PB score is 80% at 80 dB for the right ear and 96% at 80 dB for the left ear.

Patient was refitted with bilateral behind-the-ear hearing aids (Siemens Triano ® BTE). Hearing aid evaluation demonstrated satisfactory hearing aid response and real ear measurements.

Follow-up evaluation with International Outcome Inventory – Hearing Aids (IOI-HA) survey showed patient satisfaction with the new hearing aids.

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