Detailed article about hearing aids.

Definitions

To begin with some definitions:

1. Sensorineural Hearing Loss: Hearing loss due to cochlear hair cell or auditory nerve dysfunction in the inner ear or in the neural structure from ear to brain.
2. Conductive Hearing Loss: Sound conduction to the inner ear is diminished or nonoccurring due to a dysfunction in the external and/or the middle ear.
3. Mixed Hearing Loss: The presence of both sensorineural and conductive hearing loss.
4. Single-Sided Deafness: The presence of severe to profound hearing loss in one ear.
5. Monaural/Binaural Hearing Aid: One hearing aid (monaural). Two hearing aids (binaural).
6. Analog Hearing Aid: A type of hearing aid that utilizes analog audio processing to provides amplification by representing the voltage of sound signals as a continuous set of values. This amplification system creates an electrical signal that is analogous to the input acoustic signal in frequency, intensity, and temporal patterns.
7. Digitally Programmable Analog Hearing Aid: Contains an analog sound path that is modified by digital control circuits rather than an integrated circuit. The digital control circuits are programmed by an external device and do not require the hearing aid to possess any fitter controls allowing for large numbers of controls. The digital controller also contains more than one memory setting so that the user can access different programs than adjustamplification levels according to the environmental setting surrounding the user.
8. Digital Hearing Aid: Contains an analog-to-digital converter, which changes sound to a series of numbers. These numbers are then manipulated before being converted back to an analog signal. By processing numbers, more complex and fine processing of sound to specific frequencies can be accomplished through the integrated circuit. This circuit is also smaller and therefore uses less power than the analog processing method.

Introduction

The treatment for hearing loss has several approaches depending on the location of the lesion and the degree of deficit across frequencies based on audiologic evaluation. Lesions in the external and middle ear result in conductive hearing loss, which is treated with medical or surgical treatment. Hearing loss due to inner ear or eighth cranial nerve lesions result in a sensorineural deficit treated through amplifying the signal. The decision to provide patients with amplification is based on the degree of hearing loss and the individual’s self-perceived communication difficulty. For some individuals, a mild hearing loss has a severe impact on their ability to function, and for others, a moderate-to-severe hearing loss has little impact on their perceived day-to-day function. For pediatric patients, the American Academy of Audiology Pediatric Amplification Guidelines indicates that amplification with hearing instruments should be considered for a child who demonstrates a hearing loss, including sensorineural, conductive, or mixed hearing losses of any degree (Palmer 2009). The amplification of sound through the conversion of speech is made possible due to the hearing aid’s ability to convert a speech signal into an electrical impulse directed and amplified toward the inner ear.

Individuals have suffered with hearing loss since the evolution of man. The earliest recorded forms of hearing aids as a method of sound amplification came in the form of large, horn-shaped, unpowered, ear trumpets used to direct sound into the ear of a hearing-impaired person in the late seventeenth century. Beginning in the eighteenth century, boneconducting aids, unlike the former air conducting hearing aids, were noted in literature that described sound passing through a solid wooden object that the listener placed against their teeth. Bone conduction allowed sound to transmit to the inner ear via the vibration of the teeth and skull. By the 1920s, hearing aids began using an external power source to operate a microphone. The microphone converted the sound to an electrical signal that was transmitted to a carbon body, used to amplify the current to be released via speaker into the external ear canal. Vacuum tube hearing aids replaced the carbon body in the 1950s increasing the efficiency of amplification, yet units were still large. The development of the transistor paved the way for one piece, present-day analog hearing aids that can be placed behind the ear or deep within the external auditory canal. These analog hearing aids refer to the mechanism where the electrical voltage is similar to the sound pressure level entering the microphone.

The fundamental structure of a hearing aid is composed of a microphone, amplifier, and receiver. The microphone receives the acoustic signal (Fig. 1) and converts it to an electric or binary signal, depending on analog or digital technology, respectively. This signal is then passed through the amplifier that intensifies the signal and then converts it back into an acoustic signal through the receiver and is funneled to the eardrum.

Transmission of the signal, style, monaural versus binaural amplification, circuitry, signal processing and output, and program options dictate the type of hearing aid that is used by the individual. The sound signal can be delivered through bone conduction, for individuals suffering from middle and outer ear disease, or by air conduction when the deficit concerns only the inner ear. Bone conduction hearing aids, where surgical/ medical treatment is not warranted or is declined by the individual, can come in the form of an external bone vibrator or as an implantable device, as seen in the bone-anchored hearing aid (BAHA). Transcranial contralateral routing through bone can also be of use in transmitting sound from the contralateral nonfunctioning ear to the functioning ear. Air conducting devices, utilized by the majority of patients, rely on air conduction in the outer and middle ear. Several air conduction models are present and selection of a specific style is based on the size and shape of the ear canal, power requirements, and features needed to compensate and address hearing loss, and the aesthetic needs of the patient. Styles include behind-the-ear, in-the-ear, in-the-canal, and completely-in-the-canal. Regardless the method of conduction, a choice between monaural and binaural compensation must be made. Monaural external hearing aid is best utilized when hearing losses are minimal, or unilateral hearing deficits are significant. Binaural amplification however, should be utilized in individuals with hearing loss in both ears. Binaural hearing aids elicit a significant advantage to noise suppression and enhancement in the signal-to-noise ratio (Kim and Barrs 2006).

Implantable devices are becoming more widely utilized to overcome compliance issues dealing with conventional external hearing aids. The conventional aid has had complaints of stigma, acoustic feedback, occlusion effect, difficult manipulation controls, and being easily lost or damaged. Regardless of the type of hearing aid, implantable or not, all aids require a fitting and verification process (Counter 2008).

Initially, an audiologic examination and a discussion of the individual’s communication needs are undertaken. The optimal method of delivering a sound signal (air/bone conduction, programmability of the aid, monaural/binaural, bandwidth) is selected while still upholding the patient’s preference in style and level of compliance. After the final decision is made on the type of hearing aid, an ear impression is made. The quality of the impression that captures the external auditory meatus and outer ear dictates the physical fit and comfort of the hearing aid. Once received, verification of hearing aid output that is in accordance to design parameters is assessed through electroacoustic function analysis done in a test chamber. The verification of output parameters leads to further customization as the hearing aid is programmed to the patient’s audiometric needs and expectations. Decisions to be made while programming include memory options, feedback reduction, and gain responses. It is after programming that the physical fit of the device is assessed. If later the hearing aid does not meet the acoustic expectations of the user, the digital sound processing technology of modern hearing aids allows for programming to fit a wide degree of hearing loss. The fitting process is uniform for most patients; however, certain circumstances surround the fitting of infants and the elderly (Stach and Ramachandran 2010).

Pediatric Hearing Aid Fitting

The fitting of an aid for a child or infant is challenging for several reasons. The smaller external ear anatomy and canal results in higher sound pressures, while the resonance qualities of the child are different from the adult making some frequencies appear louder than others (Stach and Ramachandran 2010). This has given rise to pediatric guidelines that are different from those of adults. With growth, anatomy and sound characteristics/environments change and must be regularly accounted for in hearing aid adjustment, selection, and fitting.

Aside from physical variances from adults, the child’s hearing deficit is not as well characterized, which makes prescribing gain difficult. Provided the difficulties, it is crucial to ensure rehabilitation early in life as neural connections in the brain that allow speech to be understood are formed based on the signals they receive from the cochlea (Moodie 2004). In order tomaximize expressive language ability, it is best if both cochleas are sending signals to the auditory cortex; for this reason, binaural hearing aids are essential unless there is a contraindicating concern or severe unilateral hearing loss.

The style of choice is a behind-the-ear hearing aid to allow for growing ears and a less costly replacement of only the ear mold rather than the entire aid. In addition to the economic practicality, BTE aids allow for assistive listening devices such as FM or other wireless systems to be utilized. This allows for wireless transmission from an external microphone to the receiver found in the ear, aiding in the ability to discriminate a sound signal from noise, found especially useful in the classroom. Difficulties in fitting continue as individuals become older (Palmer 2009).

Hearing Aid Fitting in the Elderly

As patients age, changes in neuronal activity and the central nervous system sometimes affect auditory senses. These neural changes compound earlier hearing deficits. If auditory nervous system function is extremely affected, these elderly patients do not appear to benefit as much from hearing aids as those adults with similar hearing deficits. These findings point to complementary components and assistive listening devices, in addition to hearing aids, for improved signal-to-noise ratio in the fitting guidelines for the elderly. Components include directional microphones, noise-reduction processing, and remote microphones. A diminished neural system can also cause cognitive deterioration resulting in the inability to correctly position the aid. In addition, manifestations of old age may result in difficulty while manipulating the hearing aid due to arthritis and a myriad of other complications (Kricos 2006).

Digital Hearing Aid Technology

Technological advances have greatly improved the usability of conventional air conducting aids. The most notable change in hearing aids has been the transition from the analog aid to the most recently developed digital signal processing technology. Analog aids produce electrical voltage from the microphone that is similar to the sound pressure that is received. Amplification in this method is not always ideal for individuals with decreased dynamic range resulting in the need for greater or lesser amplification and feedback.

Digital aids are programmable and provide preset programs to allow automatic changes in the output of the hearing aid depending on the acoustic situation. This is made possible through the converter that is placed within the microphone that modifies the original analog signal into one that is based on a series of numbers. This digital signal is then filtered for distortion and processed at specific frequency bands by means of a preset algorithm. The amplified binary information is then converted and filtered into sound energy prior to entering the external auditory canal. Through the filtration processes, the digital hearing aid is able to reduce the acoustic distortion. The ability to include a preset programmable algorithm, which controls the amplification of specific frequency channels, compensates for a decreased dynamic range in those individuals with hearing loss. The decreased range is due to an increase in the threshold of audibility while the upper limit of discomfort stays the same.

Current digital hearing aids also attempt to remove noise from the speech signal by recognizing noise as a constant frequency and speech as a fluctuating one. In removing noise, speech is brought out of the background increasing the signal-to-noise ratio. Other breakthroughs have been made to boost the signal-tonoise ratio through advances in microphone modes. These advances have led to hearing aids with dual microphones rather than the single omnidirectional microphone. The dual microphones act to focus one microphone at a point anterior to the individual with greater sensitivity and the other microphone, pointing posteriorly, is used to acoustically delay and cancel the background noise. This allows a specific direction to preferentially become amplified while the other direction is preferentially suppressed producing an improved signal-to-noise ratio. With further automation providing high-fidelity amplification, hearing aid technology has made tremendous strides since the day of the ear trumpet.

Hearing Aid Styles

Behind-the-Ear Hearing Aids

Current Behind-the-Ear (BTE) hearing aids have evolved from the classic hearing aid models. They consist of a case that is situated behind the auricle and includes all three components of the hearing aid: microphone, amplifier, and receiver. A hollow tube, which passes over the pinna and connects to a customfitted ear mold inside ear canal, transmits the amplified sound. The tube and mold materials and shape can change the intensity and frequency gain. This enables providers to design customized hearing aids. Subcategories of BTEs include open-canal fittings and receiver-in-the-canal (RIC). The first one uses a thinner tube and a standard mold that does not occlude the canal completely. This model is suited for high-frequency hearing loss since it allows lowfrequency sounds to pass the open ear canal. In RICs, the receiver is separate from microphone and it is connected to the main behind the ear case with a thin wire. This allows lower acoustic feedback and a smaller BTE case.

BTEs are relatively larger than other styles (Fig. 2) and accommodate larger batteries that allow longer wearing time. User buttons to control volume or program options are located on the backside. Approximately 31% of hearing aid users wear BTE styles (Natalizia et al. 2010).

In-the-Ear (ITE) ITE hearing aids contain the same components as BTEs, but they sit entirely in the concha and external auditory canal. They are custom-made based on the patient’s anatomy. The customization makes these instruments inappropriate for young children and infants due to frequent changes in ear size and geometry. ITEs are relatively smaller than BTEs (Fig. 2) but are still able to utilize large batteries and are equipped with a directional microphone. The acoustic feedback is higher than BTEs but still reasonable. ITEs are the most popular hearing aids with about 38% of patients using them (Natalizia et al. 2010).

In-the-Canal (ITC)

While its name implies that this system lies in the canal, this hearing aid is actually a smaller version of the in-the-ear hearing aid that fills a smaller part of the concha not extending beyond the tragus. This style of hearing aid proves to be difficult to insert and remove from the ear canal (Fig. 2); however, its basic appeal is cosmetic and improves the communication abilities of those patients with marginal hearing loss. The placement of the aid gives the user the advantage of pinna acoustic effects and increases gain in the high frequencies due to the location of the microphone being at the entrance of the ear canal (Bailey et al. 2006).

Completely-in-the-Canal (CIC)

The development and marketing of this type of aid was in response to the public’s continual demand for a hearing aid that could not be detected when placed in the canal. Unlike the ITC hearing aids, the completely-in-the-canal (CIC) aid fits entirely within the ear canal, with no part protruding into the concha (Fig. 2). However, this comes with discomfort, as well as problems with inserting and removing the hearing aid. In order to obtain maximum benefits from CIC aids, certain fitting criteria must be met, including the distance of the lateral end of the hearing aid from the meatal opening, and the distance of the medial end of the hearing aid from the tympanic membrane. Its deep insertion takes full advantage of pinna and concha effects, boosting real gain in the high frequencies, and reduces occlusion effects (Bailey et al. 2006). This also eliminates feedback from telephone use as experienced by the more protruding ITE and ITC hearing aids; however, it also precludes the use of a directional microphone.

As body styles continue to decrease in size, so do their batteries. This comes with problems in replacing the battery and aid as greater dexterity becomes more important. Lyric (InSound Medical Inc., Newark, CA) is a continuous wear hearing aid developed to solve battery insertion and insertion/removal issues with CIC hearing aids. The Lyric aid remains 24 h a day inserted into the bony part of the external auditory meatus for up to 4 months at a time. The fitting process is also designed without the need for ear canal impressions.

Partial and Complete Middle Ear Implant

Middle ear implants are aids for individuals with pure sensorineural and some mixed hearing loss. The implants are available in two categories: partially or totally implantable using either piezoelectric or electromagnetic systems. One of the strongest reasons for a middle ear implant over conventional amplification methods, used for sensorineural hearing loss, appears to be improved cosmetics (Chasin et al. 2002). In the partially implantable aid, the processor and microphone are held to the scalp and transmit a signal to the output transducer that converts an electrical signal into a vibration to the ossicular chain. These partial implants, where parts are found outside the middle ear, are driven by electromagnetic and electromechanical principles to signal the output transducer. Totally implantable devices have the system’s microphone and transducer placed beneath the skin. Electromagnetic devices, working best at lower frequencies, are low mass and do not have a significant impact on the vibration of the middle ear as they are not rigidly attached (Spindel 2002). Piezoelectric crystal principles are optimal for high-frequency signals because when coupled to transducer and ossicle, the rigidity of the device will dampen the vibration of the middle ear (Spindel 2002).

The first FDA-approved partially implantable electromagnetic device is the Vibrant Soundbridge® device (MED-EL GmbH, Innsbruck, Austria), available since 2000. The Vibrant device (VSB) mechanically causes the ossicles to vibrate in the middle ear by directly driving the incus. The amplitude of vibrations can be adjusted for optimal hearing compensation. This semi-implantable device (Fig. 3) has an external microphone, Audio Processor (AP), and an internal Vibrating Ossicular Replacement Prosthesis (VSOP). The electromagnetic prosthesis is composed of a Floating Mass Transducer™that is attached to the incus and its magnet aligns with the vibration of the stapes. The typical user must change the battery of the processor once a week.

An example of a totally implantable piezoelectric device is the Envoy Esteem (Envoy Medical, St. Paul, MN). This technology takes advantage of the acoustic features of the external ear as the eardrum is used as the microphone device. A piezoelectric transducer is placed on the body of the incus (Fig. 4) that sends an electric current to the processor when mechanically stimulated by ossicular chain vibrations. The electrical signal is processed, amplified, and sent back to the transducer. The transducer converts the electrical stimulus to a mechanical stimulus to vibrate the stapes by a piezoelectric driver. In order to implant the processor, a portion of the incus must be surgically removed. The completely implantable device does not have any external features and has a battery life of 3–5 years that can be replaced during an outpatient procedure. (Haynes et al. 2009).

Bone Conduction Hearing Aids

It has been known for a long time that the cochlea is capable of hearing through bone conduction. Bone conduction hearing occurs when vibrations of sound are transmitted to the cochlea via vibrations of the skull. Bone conduction hearing occurs via: (1) sound radiated into the external auditory canal, (2) inertia ofmiddle ear ossicles, (3) cochlear fluid inertia, (4) cochlear wall compression, and (5) transmission of pressure via the cerebrospinal fluid. The cochlear fluid inertia appears to be themost important factor (Stenfelt and Goode 2005). Hearing aids can stimulate the cochlea using bone conduction. Bone conduction hearing aids are generally indicated in cases where air conduction hearing aids cannot be used. These conditions include patients who suffer from chronic otitis externa or otitis media, or congenital atresia of the ear canal. Bone conduction hearing aids include a microphone, sound processor and bone oscillator. The hearing device has to be held tightly against the skull for efficient conduction of vibrations to the skull (Fig. 5). The bone conduction hearing device can be held to the skull with a headband or other means such as glasses, etc.

SoundBite Hearing System

In an effort to maintain cosmetic appeal and avoid the need for surgical procedures for a bone conduction device, the SoundBite Hearing System (Sonitus Medical, San Mateo, CA) has been developed. A behind-the-ear (BTE) microphone unit and removable in-the-mouth (ITM) hearing device are wirelessly connected to provide treatment for single-sided deafness or conductive hearing loss. The microphone is placed within the ear canal of the impaired ear and connected to a BTE digital signal processor that wirelessly transmits the signal to the ITM device. The ITM device is safely mounted (Fig. 6) to the molars of the maxilla (Murray et al. 2011) and delivers mechanical vibrations to both cochleas through the teeth once the wireless stimulus is received. This allows for simultaneous activation of both cochleas in a unilateral conductive defect and quickly transmits the sound signal to the contralateral cochlea in the case of unilateral hearing loss. Hearing Aid, Fig. 5 Conventional Contralateral Routing of Signal (CROS)-bone conduction hearing aid

Bone-Anchored Hearing Aid (BAHA)

The BAHA device has two components: (1) a titanium abutment that is attached to the skull during a surgical procedure and (2) the external processor (Fig. 7). The processor contains the microphone, digital signal processing, and the bone oscillator. The oscillator’s vibrations are transmitted to the skull via the surgically implanted screw. The BAHA device overcomes some of the cosmetic and functional problems of a traditional bone conduction hearing aid. First, the mechanical vibrations that are transmitted to the skull are more efficiently transmitted through an abutment which is attached via a titanium screw in the skull. This eliminates some of the loss of signal and distortion that occurs from transmitting vibrations through the skin. Second, without a headband, the device has the advantage of being less cosmetically visible. The BAHA device can be used for unilateral deafness. A BAHA on one side of the head transmits vibrations to both cochleas with minimal attenuation; therefore, bilateral BAHA can not only provide bilateral output, but more beneficial input from two microphones (Dun et al. 2011).

Unilateral Deafness Devices

Single-sided sensorineural hearing loss is often treated with amplification to the side of the hearing defect. When this deficit becomes extremely significant or the individual is completely deaf in one ear, transmission of sound to the contralateral cochlea is the standard of care. A conventional Contralateral Routing of Signal (CROS) aid employs a microphone in or behind the ear of deficit. The microphone then transfers the signal via wire or FM transmission to a BTE or ITE processor to the more functional ear. The CROS arrangement leaves much to be desired in terms of cosmetics, comfort, head shadow, and satisfaction (Hol et al. 2005). Bone conducting devices such as the BAHA contralateral routing system and SoundBite device, explained above, transmit sound vibration to the skull for delivery to the better functioning cochlea. The development of this treatment for bi-and unilateral hearing loss also overcomes some aesthetic and functional features. Another nonsurgical bone conducting device is the TransEar (Ear Technology Corp., Johnson City, TN). This hearing aid employs mechanical vibrations to the bony skull for contralateral routing of sound from within the ear canal of the deficient ear. The noninvasive design requires the creation of auditory canal impressions for the case that will house an oscillatory device. This device will make contact with the bony portion of the ear canal; the initial site of vibratory conduction. The TransEar processor is a BTE unit that also contains the microphone (Fig. 8).

Radio-Frequency Transmission/FM

Transmission As a type of assistive listening device, this technology can be either coupled to a hearing aid or used alone in order to aid a person’s ability to hear and discriminate speech and environmental sounds. FM systems transmit auditory signals using radio waves. The device consists of an external microphone and transmitter, which is placed near the speaker. The microphone will obtain audio signal and send it to a receiver worn by the listener or those listeners wearing hearing aids with the receiver integrated into the aid. The delivery of the signal by the transmitter is done through frequency modulated (FM) signal to the receiver. The FM transmitter system may also be coupled directly to the output of a television, radio, or tape recorder, rather than external microphone (Bailey et al. 2006). Where FM transmission is used in combination with a hearing aid, the FM system provides the greatest signal-tonoise reduction enhancement relative to other auditory assistive devices (Stach and Ramachandran 2010). The large reduction in noise is due to the proximity of the speaker’s mouth and the microphone, found inches away, and is sent directly to a listener’s ear, thereby eliminating the impact of noise and distance, making it the ideal solution for the patient with difficulty hearing in noise. The FM transmitter device is best used for students in noisy classrooms or for the elderly in a restaurant, lecture, or church where external noises and distance from the speaker can cause distortion or interference with the speech signal.

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