Bimodal and Elektro-Acoustic Stimulation (EAS) (Part 5)

In order to compensate for a hearing loss there are several technical possibilities how to amplify and/or modify the physical signals in order for the acoustic nerves and in the end the auditory cortex to be able to receive, translate and interpret those signals.

Bone Conduction Hearing System

If the reason for a hearing loss lies exclusively in the outer and/or middle ear it is possible to circumvent the source of the hearing loss by using the mastoid to conduct the soundwaves into the inner ear. To do this a bone conduction hearing system is required, be it a  bone anchored or a implanted vibration system  or be it an external bone conductor. The latter can be worn on binoculars or with headbands etc. wich apply enough pressure for it to work.

Air Conduction Hearing System

The cause for a sensorineural hearing loss is normally to be found within the cochlea. Located within the organ of Corti many thousand of inner and outer hair cells (cilia) translate the physical signal into electric potentials which are then transferred onto the acoustic nerve fibers. If the hearing cells are partially or completely damaged no signals are transferred to the corresponding nerve fibers.

Typically this results in a hearing loss which varies regarding frequencies and volume. That kind of hearing loss can be compensated with hearing systems from behind the ear, receiver in canal or in the ear hearing aids.

Cochlear Implant

If the cochlear damage is such that hardly any signals are translated and transferred to the corresponding nerve e fibers there is still a possibility to insert a probe with electrodes into the cochlea and to stimulate the nerve fibers directly with electric signals. This is how  cochlear implant works.

Now there are several forms how those systems mentioned above can be combined.

Bimodal Stimulation

The term bimodal stimulation is normally used to describe a combination of electric stimulation e.g. a cochlear implant on one side and a acoustic transmitting hearing system on the other side. As by today it is known that this combination of technologies works very well and I can confirm that as a fact. I am wearing a CI on my right and a hearing instrument on my left side.

So what is electro-acoustic stimulation (EAS)?

Today there are various cochlear implants that allow for electric stimulation via probe as well as acoustic stimulation via a receiver. It is in fact a hybrid with the combination of two technologies in one instrument.

As the cochlea similar to a piano allocates the frequencies form very high (up to 20'000 Hz) at its base (basal) until very low frequencies (16 Hz) at the very tip of the cochlea (apical) with all other frequencies lined up in between, those two technologies work very nicely together. 

A probe of a cochlear implant can roughly be inserted one and a quarter rounds, which by definition amplifies the higher sounds. In my case the lowest frequency I can detect via my cochlear implant is around 600 Hz.

In contrast to this, common hearing instruments are perfectly equipped to amplify lower sounds. This provides a perfect combination together with the high frequency stimulation via the probe, provided there is still usable low frequency hearing.

It is obvious from my audiogram that I'm actually not a candidate for a electro-acoustic stimulation. But I was wondering if I could try to do a transcranial CROS via the EAS part of my cochlear implant. The hypothesis would be the following. The lower the frequency the harder it gets to verify from which direction a signal is coming. Knowing this it is possibly not that important which one of my ears provide me with the low frequency information. For directional hearing we use manly the upper frequencies around and above 2000 Hz.

Transcranial CROS

CROS stands for  Contra-Lateral Routing Of Signals, meaning to lead the signal over from the weaker audiological side in order to receive it on the audiological better side. For a transcranial CROS we use the mastoid and the skull to conduct the signal provided the high vibrations aren't perceived uncomfortable or even painful. this would then be seen as a contra indication.

For a bimodal and electro-acoustic stimulation you'll need the instruments seen above:

For a bimodal stimulation a cochlear implant is needed (in my case on the right hand side) and on the other side a hearing instrument (in my case on the left hand side). For a electro-acoustic stimulation the CI needs to enable electric stimulation via the probe as well as acoustic stimulation. In my case the CI in question is the Advanced Bionics Naída CI M90 with the EAS-earhook (instead of the T-Mic).

Once I had the EAS-earhook fitted to my CI I used the NoahLink Wireless to connect my CI to the software. As I was wearing the devices myself I could instantaneously realize all the changes applied in the software. I performed a AudiogramDirect with the acoustic part of my CI in order to calculate a target gain for it. 

I was positively surprised with the sound quality I received via the CI because of the acoustic enhancement via the EAS-earhook. There was definitely more low frequency sounds which made the hearing experience much more natural via the CI. So even I, with no usable low frequency hearing on my right ear, had a sensation of low frequency sounds. The funny thing is that I don't realize that the low frequency sounds are actually perceived by my good left ear due to the transcranial CROS. I'm even more confused as I tend to realize instantaneously when I start to overhear with my better ear during an audiogram for example.   

I'm wearing my cochlear implant with the electro-acoustic earhook for a while now and I can definitely say that I have a fuller and more natural sound due to the electro-acoustic stimulation. The speech-understanding has remained approximately the same but the quality of sound has changed dramatically for the better.