What Is Residual Hearing?

Everyone knows what “hearing” is, and “hearing loss” is pretty self-explanatory. But there’s another word that is often used when talking about hearing whose meaning isn’t so clear, and that word is “residual hearing.” So, what is residual hearing? Why does it matter?

If someone has a sensorineural hearing loss, it means that they can’t hear sounds of a certain frequency that are below a certain volume. The extent of hearing loss is different for everybody, and having “hearing loss” doesn’t necessarily mean that someone can’t hear any sounds.

This is where residual hearing comes in.

How Does Residual Hearing Occur?

As we discussed earlier, hearing is a process where approximately 20,000 tiny hair cells in the cochlea convert the mechanical vibrations of sound waves into electrical nerve pulses that the brain understands as sound. Sensorineural hearing loss, one of the most common types of hearing loss, occurs when these hair cells are missing or damaged. However, because there are so many of these hair cells it’s possible that even with a profound hearing loss some of them will not be missing or damaged.

Residual hearing occurs when some of these hair cells are not missing or damaged. This means that they can still respond to certain sound frequencies, and that they are still connected to the nerve cells that lead to the brain. The most common type of residual hearing is low-frequency residual hearing. It happens when someone loses their ability to hear some or all high-frequency sounds but can still hear low-frequency sounds. This is often experienced by older adults, and you might be familiar with it if your grandfather or grandmother is unable to hear sounds like birds chirping but does hear sounds like car engines or some male voices.

Why Does Residual Hearing Matter?

Residual hearing is important for everybody with a hearing loss, whether or not they have received a cochlear implant, because it can represent hearing potential.

For individuals who haven’t received a cochlear implant, it’s the residual hearing they rely on to hear all sounds. If they have low-frequency residual hearing, then it’s the low-frequency sounds that they’ll hear best. Hearing aids, for example, take advantage of residual hearing by amplifying the sound waves sent to the cochlea so that the remaining hair cells can hear the sounds more easily.

And residual hearing is also important for individuals who have a cochlear implant. For these individuals, this means preserving residual hearing so that it still exists after the cochlear implant’s electrode array is inserted.

At first this might be surprising, because cochlear implants are designed to replicate the sense of hearing even in individuals who have absolutely no residual hearing. So, why would someone want to keep their residual hearing if a cochlear implant works even without residual hearing?

It’s because any existing residual hearing supports the sounds that are stimulated by the cochlear implant, which means that a recipient can have the best possible hearing. Cochlear implants are amazing technologies, and are the only technology that replicates a human sense, but however residual hearing is still better than artificial electrical stimulation.

Even more, if someone has enough residual hearing they might be a candidate for Electric Acoustic Stimulation, or EAS for short. EAS is a type of hearing implant designed for individuals with low-frequency residual hearing because it provides both electric stimulation for the high-frequency sounds as well as acoustic amplification for the low frequency sounds, and it can provide even better hearing performance than a cochlear implant alone^{Usami et al. (2014) Hearing preservation and clinical outcome of 32 consecutive electric acoustic stimulation (EAS) surgeries. Acta Oto-Laryngologica, 134(7), 717-27.[1]}.

If someone has preserved residual hearing, that means we believe that their cochlea could better take advantage of any therapies or technologies that are developed in the future.

How Is Residual Hearing Preserved?

There are two main aspects to what we call preserving residual hearing, or what we call Structure Preservation. The first has to do with the design of electrode arrays, and the second is about the surgeon’s technique. Here we’ll focus on the electrode arrays, but it’s important to note that the surgical technique is essential.

Since our very first electrode array was developed, we’ve dedicated ourselves to preserving residual hearing. We know that electrode arrays designed to be soft and flexible are the best at preserving the 20,000 delicate hair cells within the cochlea, and that’s why all of our electrode arrays feature unique wave-shaped wires and a flexible design. This way they can be gently inserted into the cochlea, helping to preserve the hair cells and therefore any residual hearing.

If you’d like to learn more about what an electrode array designed for structure preservation looks like, check out our blog post that focuses on the soft and flexible design of MED-EL electrode arrays.