7 – Hearing Aids and Implants

Types of Hearing Aids

Acoustic Hearing Aids

Acoustic hearing aids may be used for people with conductive, sensorineural, or mixed hearing loss. The size, style and features are driven by the patient’s desires as well as their hearing loss. There is an array of sizes with examples below.

Hearing aid types include:



Hearing aids that fit into the ear canal can cause the hearing aid wearer to feel “clogged up.” This clogged up feeling is known as the “occlusion effect” and most hearing aid wearers do not like this feeling. However, if someone wants an “invisible” hearing aid, the completely-in-the-canal type may be the one they desire, because it fits down into the ear canal and is the smallest hearing aid currently available.

behind the ear hearing aids

The hearing aids above are called behind-the-ear (BTE) aids and will have an ear mold that fits into the outer ear and the ear canal, as in the photo below.

behind the ear hearing aid illustration

receiver in the canal (RIC) hearing aids

The two hearing aids above are examples of a “receiver-in-the-canal” (RIC) aid, or one may hear it referred to as an ‘open fit’ hearing aid. Many people today are fit with RIC hearing aids. These hearing aids have a thin tube running from the hearing aid to a piece called a “dome” which sits in the ear canal, as pictured to the right. The dome does not occlude the ear canal, which allows natural sound to enter, as well as amplified sound. The advantage of this type of fitting is a more natural sound that typically amplifies only the frequencies at which a person has hearing loss, such that the wearer perceives increased clarity without increased loudness. The photograph below indicates how this type of hearing aid generally will appear when worn.

behind the ear hearing aid fitting

There are also hearing aids that are made based on a cast taken of a particular ear, called full shell, in-the-canal, completely-in-canal and micro-in-canal as pictured below.

full shell, in the canal hearing aids

And here is how these types of hearing aids look when worn.

in the canal and full shell hearing aid fitting

See more hearing aid information and pictures

Bone Conduction Hearing Aids

While acoustic hearing aids send sound to the ear through the ear canal, bone conduction hearing aids operate in a different way. A bone conduction aid is attached to a metal headband with a vibrating device on one side and a microphone on one or both sides. Instead of sending sound waves through the air, a bone conduction hearing aid takes sound from the air and converts that sound to a vibration that is applied to the skull. When the skull vibrates, the cochlea and the fluids in the cochlea also vibrate, causing the person to hear. From this description one might guess that a bone conduction hearing aid is typically used for conductive hearing losses.

Bone Conduction Hearing Aids

It is the case that now (as of 2012) that bone conduction hearing aids are used infrequently. Most individuals who are candidates for bone conduction hearing aids are also candidates for bone anchored implant hearing aids (BAI, described below), and they opt for the bone anchored aids in many cases. This choice of BAI is typically made based on greater comfort of the bone anchored hearing aid (even though surgery is involved) and reception of better sound.

Candidacy: Acoustic and Bone Conduction Hearing Aids

Who is a candidate for hearing aids? Some individuals with a “mild” hearing loss often report that they do not feel the need for personal amplification. This may mean they feel hearing aids would not be helpful. There are other types of assistive devices that might be beneficial for this individual, but they are probably unaware of those and a referral to an audiologist is warranted. Other individuals with “mild” hearing loss do want personal amplification and can usually also benefit from assistive devices.

Individuals with any degree of conductive hearing loss, losses caused by problems in the outer or middle ear, are hearing aid candidates. For these people sound is muffled and increasing the intensity or amplifying the level on a hearing aid is like turning up the volume control. These individuals report great satisfaction with hearing aid use.

Sensorineural hearing losses, those losses caused primarily by problems in the cochlea, but also problems with the nerve, pose a greater challenge to audiologists and clients to decide whether hearing aids are appropriate. As a general rule, people with a moderate or greater hearing loss will benefit from personal amplification. As with conductive hearing losses, these individuals may benefit from assistive devices plus hearing aids, and clients would be well advised to ask their audiologist about available assistive devices. This suggestion applies to telecoils in particular, as some audiologists do not routinely order telecoils in hearing aids.

In addition to the degree of hearing loss, there are many other considerations for hearing aid candidacy. These considerations include visual acuity, ability to manipulate small objects, ear surgery history, and whether a family/friend support system is in place. By and large these questions apply to the elderly population and not to the working age population.

CROS and BICROS Hearing Aid Fittings

When a patient has hearing loss in only one ear (unilateral hearing loss), or different amounts of hearing loss in each ear, there are a couple of specialized hearing aids to fit them. These setups include Contralateral Routing of the Signal (CROS) and Bilateral Contralateral Routing of the Signal (BICROS) and the basic idea is for a microphone to pick up sound on the side of the head with poorer hearing and send that sound, or route the sound, to the amplification device on the other ear. The CROS fitting is designed for those with hearing loss in one ear and normal hearing in the other ear. A BICROS fitting is for someone with asymmetric hearing loss, one ear has a hearing loss that can be helped with amplification but the opposite ear will not benefit from amplification (that ear is ‘unaidable’). The client will need to experience this type of hearing aid set up to decide whether it benefits them. Sometimes the client will decide they do not benefit and the audiologist will try something different; however, sometimes a client discovers that having access to the sounds on their ‘bad side’ is helpful and will keep the CROS or BICROS hearing aids. See the Unitron hearing aid company website for a demonstration of how CROS and BICROS hearing aids work.

Implantable Hearing Aids

Bone Anchored Implants

A bone anchored implant is a hearing aid with one piece that is implanted, a snap/holder and the device itself. A titanium implant is placed into the skull behind the  auricle and a small piece, the abutment, can be seen on the outside of the head.

Bone Anchored Implants

This surgery is done on most adults under local anesthesia. For adults, after about three months, the snap/holder is placed onto the abutment and the hearing aid will be attached and programmed. This  waiting period is needed to ensure the implanted piece adheres well with the skull, a process known as osseointegration. This process is critical to the successful use of a bone anchored implant because the idea is that the sound entering the device will make the implanted piece, as well as the skull, move as a single unit.

Bone Anchored Implants

Bone anchored implant surgery

University of Maryland Medical Center – bone anchored implants

Candidacy: Bone Anchored Implants

Bone anchored implant hearing aids are appropriate for individuals with conductive hearing loss, some individuals with mixed hearing loss and some people with no hearing in one ear and normal to near normal hearing in the other ear. This last group of people is referred to as having “single-sided deafness.”

Bone Anchored Implants

There are several medical conditions for which bone anchored implants are especially appropriate; typically in these conditions an individual is unable to wear an acoustic hearing aid and are not candidates for surgery on the ear. These conditions include:

  • Treacher Collins Syndrome – the middle ear structures, and sometimes the pinna, do not develop typically.
  • Otosclerosis – the middle ear stapes bone has extra bone growth, preventing its movement against the oval window and preventing the introduction of sound into the inner ear and cochlea.
  • Microtia – the outer ear ( auricle) is smaller than normal and/or may not have developed typically
  • Atresia – the ear canal did not form, thus there is bone or other tissues where most people have open space; surgery may or may not be successful to open the canal

Cochlear Implants

Cochlear implants are hearing aids that are composed of three pieces: a sound processor, an implant/magnet, and an electrode array.

This photo demonstrates how a cochlear implant may appear on a person.

The sound processor is essentially a hearing aid. The difference between a cochlear implant and an acoustic hearing aid is that the person wearing an acoustic hearing aid  has hair cells still functioning in the cochlea and the processed sound must be delivered to the eighth acoustic nerve directly. Because of this need, the delivery system of the cochlear implant is much more complex than an acoustic hearing aid.

Sound enters the microphone of the sound processor, which receives processing as with any hearing aid, and is then sent through the wire above the ear, through the skin, to the implant (#2), then on to the electrode array which has been placed inside the cochlea. The typical hearing process then begins because the signal has been delivered to the acoustic nerve.

The first cochlear implant, developed during the 1960-70s at the House Ear Institute in Los Angeles, California, had one channel. This one channel allowed one piece of information about incoming sound to be delivered to the auditory system. By today’s standards this amount of sound being sent to the wearer is wholly inadequate; however, the first implantees who benefited from the implant were generally quite happy because most of these adults had not received any auditory stimulation for many years.

Contemporary cochlear implants, by contrast, may have 22 or 24 channels and more channels allow more information from the incoming sound to be transmitted to the wearer. However, when cochlear implants are turned on and tuned, it is possible that a few channels will not be usable. This loss of some channels may result from a particular electrode causing  discomfort when it is turned on and so will simply be deactivated.  It is also true that the loss of some channels does not compromise the wearer’s ability to benefit from the cochlear implant.

The processing that cochlear implants perform is quite different from acoustic, bone conduction and bone implantable hearing aids. The reason for this difference lies in the fact that the three types of acoustic hearing aids mentioned send sound to the cochlea for the auditory system to   encode the sound. Because the electrode array of a cochlear implant has replaced the hair cells within the cochlea, the processing must include some of the encoding work that would typically be done by the hair cells in the cochlea.  The processing strategies of early cochlear implants included picking out important pieces of the speech signal to deliver to the eighth nerve. These early cochlear implants were regarded as quite good at conveying speech information but less good at conveying other sounds such as music. Current cochlear implant technology has improved a lot and most users report satisfaction in both speech understanding and listening to music.

Some of the same advances seen in acoustic hearing aids are available with cochlear implants. For example, the sound processor, the part of the cochlear implant that resembles a behind the ear hearing aid, may have dual microphones. Dual microphones allow the device to focus on the sound directly in front of the wearer and reduce noise from behind, increasing the person’s ability to better distinguish speech in a noisy background. Additional examples include a telecoil that automatically recognizes electromagnetic telephones, devices to use cellular telephones, cords to connect to an iPod, etc., and are also Bluetooth friendly. See an explanation for the various devices that can be used with the MED-EL cochlear implant.

Many individuals who opt for cochlear implants choose to have both ears implanted, although the surgery is generally performed on only one ear at a time. Other individuals choose to wear a hearing aid on one ear with a cochlear implant on the opposite ear. Research is ongoing at the moment to determine whether one of these situations may produce better results than the other, however, as with most issues in personal amplification, the opinion of the wearer is often the deciding factor.

Candidacy: Cochlear Implants

Who is a candidate for cochlear implants? One of the companies that make cochlear implants, Cochlear Americas, poses the following questions to adults on their website:

  • Do you have to ask people to repeat themselves during one-on-one conversations, even in a quiet room?
  • Do you need captions to understand television programs?
  • Do you depend on lip reading to understand a conversation?
  • Do you find yourself exhausted at the end of the day because communication requires such a high degree of concentration?
  • Do you have a hard time keeping up at work? Do you find communication at work to be difficult and to interfere with carrying out responsibilities of your job?
  • Do you have trouble hearing on the phone?

MED-EL, another company that produces cochlear implants, says that MED-EL cochlear implants, are being implanted successfully in over 90 countries worldwide in both prelingually and postlingually deafened children and adults. While it is not possible to predict exact benefits for each cochlear implant user, older children and adults with previous speech and language development generally perform better with a cochlear implant than children who have not yet developed speech and language.

Cochlear implants are designed for:

  • Adults with severe to profound sensorineural hearing loss in both ears, however, a long period of profound hearing loss may limit the benefits of a cochlear implant
  • Individuals who receive little or no benefit from hearing aids
  • Individuals with access to education and (re)habilitation follow-up programs

Cochlear Implant Graph-use

The audiogram above shows graphically how much hearing loss a person should have to be considered a candidate for a cochlear implant by the U.S. Food and Drug Administration (FDA). The gray area on the graph represents the degree of hearing loss required when cochlear implants were first introduced in the 1970s and for some time beyond that. As research was performed, the FDA ‘relaxed’ the standard such that individuals with somewhat less hearing loss were allowed to be implanted. Today individuals must have at least severe hearing loss above 1000 Hz but may have moderate hearing loss in the low frequencies.

A second criterion for cochlear implant candidacy is that people do not receive adequate benefit with acoustic (‘regular’) hearing aids. Such benefit is shown in the clinic with having people try to repeat back individual words. This listening-only ask is difficult because individual words provide no language context to help the person figure out what is being said. Often, people with very low scores on this test are candidates for cochlear implants.

See more information from the FDA regarding cochlear implants