Hearing Aids
The mainstay for hearing rehabilitation in people with sensorineural hearing loss is hearing aids. Hearing aids continue to improve with advances in technology. The end function of all hearing aids is to amplify the intensity of sound to the ear. This increase in intensity is called “gain.” There are limits to how much gain a hearing aid can provide, which partly determines the size of the hearing aid. The bigger the hearing aid, the bigger its processor, and the more gain it will have.
Remember, that how loud a sound has to be for us to hear it is only part of hearing. The other part is how well we comprehend, or discriminate, the things we hear. Hearing aids cannot improve discrimination. In fact, if someone has discrimination hearing loss, a hearing aid may make things worse. It will amplify the distorted sound the person receives.
There are 2 main components to a hearing aid: the microphone and the amplifier. The technological challenge in hearing aids is that the microphone and amplifier are very close together. Anyone who has ever heard the screaming feedback that occurs when a microphone is placed near a speaker sees why this is a challenge. This is called feedback, and the problem of feedback gets worse as the amplification increases.
Modern hearing aids are technological marvels. Older hearing aids used analog circuitry. This limited how adjustable a hearing aid could be. Most hearing loss does not occur evenly across all frequencies. If it did, that would make it much easier to fit a hearing aid. In reality, the amount of amplification needed varies frequency to frequency. Analog aids were not easily “tuned” on a frequency-by-frequency basis.
Most modern hearing aids rely on digital programming. This allows the audiologist to adjust how much amplification is given for each frequency. This is done using a computer interface. Some aids use analog circuitry for amplification, but digital circuitry for programming. The other advance that digital programming allows is feedback cancellation. This helps prevent the “whistling” hearing aids phenomenon. It also allows the hearing aid to have different settings that the wearer can change with the push of a button. These settings can be adjusted to highlight certain sounds (i.e. music) or to filter out sounds (i.e. background noise).
Hearing aids amplify all sounds the same. Our ears actually attenuate certain sounds and highlight others. Some of this comes from the location of our ears and the design of our ear canals and auricles, and some of it comes from the way our hair cells react.
A common complaint from new hearing aid users is that they hear sound that they never used to notice – for example, the tick of the clock or the dishwasher in the background. Modern hearing aids can compensate for some of this. Wearing hearing aids in both ears also helps (if both ears have hearing loss). Binaural (2-ear) hearing amplification makes a significant difference with hearing in noisy environments and in localizing sound. Directional microphones can also help with hearing speech in noisy environments. Hearing aids can be fit with 2 microphones: one that is directional and the other that is universal. The wearer is able to switch between the 2 microphones.
Hearing aids are no small investment. Unfortunately, insurance companies and Medicare do not cover hearing aids, so the investment comes out-of-pocket. The key to getting the correct hearing aid is finding the correct audiologist. The audiologist is the one who will test your hearing, determine what style of hearing aid will work best, make the ear mold impressions, and program the hearing aid. They are the ones you will deal with regularly to adjust both the fit and programming of the aid.
With all the different brands and styles, there are well over a thousand different hearing aids to choose from. Some hearing aid dispensaries (places that sell hearing aids) have financial incentive to sell you a certain hearing aid—and will push you in that direction, whether or not it is the best hearing aid for you. Hearing aid styles are named based on size: “completely in the canal,” “in the canal,” “in the ear,” “behind the ear,” “open ear” (or fit).
Do not handicap the audiologist by insisting on a certain size or style of hearing aid. The type and degree of hearing loss are what determine which hearing aid is best for you. Every style, except for the open fit type, requires an ear mold. One key feature of a good dispensary is being able to return as often as necessary to have the ear mold adjusted so that it fits comfortably.
To fit well, the hearing aid must also be programmed correctly. This often takes several visits. Nearly all dispensaries, and any dispensary should, have a 30 day (at least) return policy. Do not go to a dispensary that does not have this policy. During those 30 days, it is very important that the hearing aid be worn in as many situations as possible.
Record any problems, as well as the situation the problem occurs in. If at the end of the 30 days those problems have not been fixed, or if it seems that the hearing aids do not help, return them.
The open fit type hearing aid is relatively new. It is ideal for high frequency hearing loss. It will not provide much amplification of lower frequencies, so it is not an option for someone with hearing loss across all frequencies. Since there is no ear mold with these hearing aids, they are easy to fit and comfortable to wear.
The challenge with standard hearing aids in someone with high frequency hearing loss is that as soon as you put an ear mold into the ear, you occlude the normally functioning low frequency part of their hearing. This is called the “occlusal effect.” To overcome this with traditional hearing aids, a vent (or opening) was built into the hearing aid. This lets the low frequency sound pass through. Only a limited amount of venting was possible because it promoted feedback. Open fit hearing aids have overcome this.
It takes time and effort to adjust to hearing aids. It is important to remember that they will never be perfect, but that is not the goal. Hearing aids are meant to improve your ability to understand and participate in communication. Take time to adjust to sound as it is processed through the hearing aid. Practice with them, and over time the investment will pay off.
Cochlear Implant
At some point, hearing loss becomes so severe that just amplifying the sound will no longer provide any communication benefit. People in this category may benefit from a cochlear implant. A cochlear implant will only work if there is a cochlea and a hearing nerve present. The cochlear implant skips the malfunctioning inner ear hair cells and stimulates the nerve directly.
Implantation requires surgery. An incision is made behind the ear, and a drill is used to remove bone from the behind the ear so that the cochlea is accessible. A small opening is made in the cochlea, and a wire is inserted into the cochlea. There are 24 electrodes along the end of the wire. The wire is attached to a processor. Part of the surgery is making a shallow well in the skull behind the ear where the processor is secured.
Once it heals, the patient wears a receiver on their ear which looks like a hearing aid. There is a wire that extends from the receiver that leads to a disc. The disc has a magnet and an induction coil in it, which keeps it securely centered over the processor (which is under the skin).
The receiver picks up the sound waves and converts them to an electrical signal. That signal is passed though the skin into the internal processor, which in turn transmits it down the wire to the electrodes. The electrodes are frequency-specific, and they directly stimulate the cochlear nerve. The nerve then transmits the signal to the brain.
Cochlear implants do a great job allowing people who can not hear to communicate. Currently, the criteria for a cochlear implant require severe to profound hearing loss in both ears. Also, if someone has been without hearing for several years, the brain loses its ability to interpret the sounds it hears, and as a result the patient will get almost no benefit from the implant.
In addition, the candidate is given a test where sentences are spoken with background noise and they must repeat them. This test is given with their hearing aids in. If they are able to repeat better than 40% of the sentences correctly, they are not considered a good candidate because their hearing is too good. Over time, these standards are becoming less stringent as we recognize that more and more people would benefit from a cochlear implant.
However, the surgery to place the implant is the easiest part of receiving a cochlear implant. It takes an extended period (years) of aural rehabilitation (a type of speech therapy) and programming to get the maximum benefit. For children, this means that the parents and family must be willing to commit to the therapy. If someone had hearing and then lost it they have an easier time interpreting the sound they receive from the implant. Someone who has never had hearing has to be taught what the new sounds they are hearing mean. If they have never had hearing, it helps if they have been taught to speech read as part of their communication plan. The whole process is more complicated in children.
Auditory Brainstem Implant
It is rare, but some people do not have a cochlea and/or a hearing (cochlear) nerve. Sometimes, this is the result of surgery in patients that have tumors on both of the hearing nerves. Again it is rare, but some people are born without an inner ear and/or nerve. In these people, a cochlear implant is not an option. The only option for hearing in this group is an auditory brainstem implant. A brainstem implant provides some environmental sound awareness but without lip reading does not give speech comprehension.
Once the electrical impulse has left the cochlea in a normally functioning ear, it is transmitted via the nerve to various “relay stations” in the brain. The first of these relay stations is in the brainstem, in a location called the cochlear nucleus. The auditory brainstem implant is placed on the surface of the brainstem directly over the cochlear nucleus. The electrodes are imbedded in a silicone “paddle.” The paddle is placed against the brainstem and Teflon fibers are packed around it to hold it in place.
The wire then travels up to a processor that looks just like the processor used in a cochlear implant. This processor is secured in a well in the skull. The patient also wears an ear-level receiver, just like the cochlear implant. In this case, the electrical signal travels down the wire and into the electrodes that stimulate the cochlear nucleus. The nucleus then relays the signal on to the other parts of the brain for interpretation.
