Traditionally, getting a cochlear implant (CI) meant that a patient would sacrifice their remaining hearing. This is no longer the case. Many patients are surprised to learn that they may retain their residual hearing after cochlear implant surgery. There are many reasons why a patient would want their residual hearing to remain intact after cochlear implant surgery.
Keeping hearing after surgery is important because it:
The goal of this article is to help you understand the key considerations around residual hearing. We think that
There are two ways that hearing preservation improves hearing:
Combining a hearing aid and a CI in the same ear is called electroacoustic stimulation (EAS, sometimes called “hybrid hearing” for certain CI models). Patients who are candidates for EAS typically have near normal low frequency hearing. However, their high frequencies are often very poor. Such patients can hear mens voices, but not hear a microwave beep. They can hear without lipreading in quiet, but any amount of background noise makes it impossible to hear.
High levels of low frequency hearing can be augmented with a hearing aid. The high frequency hearing may be improved with a cochlear implant. So combined electric and acoustic hearing, EAS, does better than any other CI arrangement–either (1) a CI alone or (2) a hearing aid in the same or opposite ear. It’s like having the best of both worlds–a hearing aid and a CI in one simplified device to help you hear the world around you.
Let's check out the experience of our patient, "Patient A", who has an audiogram with near normal low frequency residual hearing (gray line). With "soft surgery" techniques, hearing after CI hasn't meaningfully changed (red line).
That's a GREAT thing, because Patient A continues to have very good hearing in the low frequencies. They may use EAS! A hearing aid will give them additional boost in the low frequencies to pick up vowels and vocal tone. A cochlear implant will provide hearing for high frequency consonant sounds.
What a powerful combination! Low frequencies are really important for hearing in noise and for music perception. While many recipients do very well, hearing with a cochlear implant is not the same as natural hearing, especially with low frequency sounds. So preserving low frequency residual hearing is very, very helpful. Patients who use EAS (hearing aid AND cochlear implant) perform dramatically better on hearing in noise and music.
Hearing preservation means that structures inside the cochlea are intact. It is also a marker that the electrode is in the best location to stimulate the cochlea. Let's take a deep dive into the anatomy and discover why!
The CI electrode is designed to go into the "scala tympani" which is one of the chambers inside the cochlea or inner ear. You can think of the cochlea as a long tube that is folded over it self and wound up into a spiral like a snail. Folding the tube over itself creates two tubes, the scala tympani and the scala vestibuli. The small space where each tube meets to form the hearing organ is the scala media. The electrode of a cochlear implant is inserted through the round window into the scala tympani. During insertion, the electrode may cross over to the scala vestibuli and puncture the hearing organ, the scala media. When this happens, the hearing organ is destroyed. So keeping residual hearing suggests that the electrode did not venture off track and remains where it is supposed to be, inside the scala tympani.
The cochlea is a essentially a long tube that is folded over itself. Sound enters the oval window into the scala vestibuli. Sound “turns the corner” at the helicotrema into the “other tube” called the scala tympani. Sound exits the scala tympani via the round window. The electrode of a CI goes through the round window into the scala tympani. Adapted from image of unknown source.
Another view of the three chambers of the cochlea. the organ of hearing is in pink and is sandwiched between the two “tubes” of the cochlea. Used with permission from Dr. Jean Regal University of Minnesota Medical School who adapted the image from this source.
The cochlear implant electrode is in the scala tympani which is the proper location inside cochlea. Note: this is an older electrode array, but great graphic. Courtesy Drs. Patricia Leake & Stephen Rebscher
To get from the scala tympani to the scala vestibuli means that the electrode crossed over into and permanently damaged the organ of hearing. When the electrode is located in the scala vestibuli, hearing will not be as good. Why is this? Good hearing depends on the electrode contacts facing the proper direction to stimulate the nerve elements inside the cochlea. The proper orientation of the electrode contacts is specifically designed for the scala tympani. If the electrode winds up in the scala vestibuli or the balance portion of the inner ear, the electrode is not facing the optimal direction for which it was designed. Hearing will not be as good and balance may be worse.
Images of the cochlea laying flat (A) and in the position that surgeons see it on its side (B). The electrode “popping” out of the scala tympani (red) into the scala vestibuli (blue). In this case, residual hearing is likely completely gone and hearing with a CI is worse. This is likely an older electrode that was not designed for residual hearing preservation. Courtesy of Dr. Brendan O’Connell et al.
New bone formation is another reason why poorly positioned electrodes decrease hearing. Trauma to the cochlea occurs when (1) the electrode is in the wrong chamber and/or (2) soft surgery techniques are not used. Insertional trauma to the inner ear occurs stimulates new bone formation inside the cochlea. New bone between the electrode contacts and the hearing nerve cells reduces the efficiency of cochlear implant stimulation. This impaired signal delivery requires more power is needed to stimulate the nerve. Increased current spreads inside the cochlea and creates “cross talk” between the channels of the electrode.
Remember that the inner ear also provides balance information. Using soft surgery can also improve balance functioning after a cochlear implant.
Cross section of cochlea showing “three tube” arrangement. The electrode is located in the scala tympani. there is no bone formation. Used with permission from Dr. Thomas Roland, NYU Langone Medical Center.
A cochlea showing new bone formation (gray) around the electrode. New bone formation worsens hearing outcome. Courtesy Dr. Floris Heutink
Preserved hearing after a cochlear implant suggests that scarring inside the inner ear is minimal. This may allow patients to use advanced cochlear implant designs and emerging technologies to rebuild the inner ear. Some of the technologies are more advanced cochlear implant electrodes, some of which use light instead of electricity. Other technologies are stem cell and genetic therapies. These therapies may require advanced ways of delivering these to the inner ear. Reducing trauma, scar, and new bone formation inside the delicate cochlea may make it easier to use these technologies.
Be aware, many people delay getting a cochlear implant for better technology. No one knows when these future technologies and therapies might be available. It may or may not occur in your lifetime, so it’s important that you consider cochlear implants now versus later to help improve your quality of life. See our FAQ page for very good reasons why you shouldn't delay surgery for better technology. Instead of putting off surgery, choose a CI center that preserves residual hearing so that you may access new technologies on the horizon.
The prospect that I would lose my residual hearing after a cochlear implant was terrifying. If my CI didn't work well, how would I hear? Because I had “traditional” CI surgery, I have no residual hearing in either ear–I cannot hear the loudest sounds. I no longer hear the deep, rich bass of music. Although I am able to communicate much more easily, I miss my acoustic hearing.
Many patients echo the same concerns and delay CI surgery for years. However, using residual hearing as an excuse not to proceed with CI is a fallacy for most patients. The vast, vast majority will do better with a CI than a hearing aid. It is difficult for most patients to seemingly take a risk without a way to hear if the CI doesn’t work as expected (approximately 1 in 100 of cases).
To reassure my patients about loss of residual hearing, I firmly believe that hearing preservation protocol should be offered to every CI patient. In my and other experienced hands, it is rare to lose residual hearing in the immediate postoperative period. My goal in offering hearing preservation protocol to all patients is to provide a sense of safety and reassurance so that patients feel comfortable proceeding with CI surgery.
Soft surgery results in atraumatic electrode insertion. This preserves hearing no matter how a CI is used–whether with a CI alone or a hearing aid in the same or opposite ear. Atraumatic insertion also improves balance outcome. It may help you adopt new and upcoming technologies for improved hearing.
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