Cochlear implants to help regrow auditory nerves

Modern medical technology often achieves stunning results when it works with the body to promote the body’s own healing process. The potential of cochlear implants to restore hearing through gene therapy is an exciting example of this beneficial marriage between science and nature.

Cochlear implants do an amazing job of restoring hearing for many people but they have their limitations. Cochlear implants work well for speech but don’t cope so well with music for example. The success of cochlear implants also depends on other factors including age when hearing loss was sustained, the development of speech prior to hearing loss, and even visual processing abilities.

But recent exciting discoveries have opened up new possibilities to both improve cochlear implant function and restore hearing through gene therapy at the same time.

Cochlear implants mimic natural hearing

Cochlear implants effectively function as an artificial ear to re-establish hearing function.

When our hearing suffers injury the hair cells responsible for transmitting sounds to the neurons die back and, at the same time, the neurons also atrophy increasing the gap between the hair cells and the neurons. Thus the inability to capture the sounds in the first place is further compounded by the difficulty of transferring those signals to the neurons in the auditory nerves.

A cochlear implant attempts to replace the lost functionality of the hair cells with its own conversion of sound vibrations into electronic signals that are passed onto the neurons via electrodes. The gap between the damaged hair cells and neurons requires the implant electrodes to deliver a relatively high voltage but the higher voltage means that less precise pitch information is conveyed to the neurons.

So, the problem of getting that signal across the gap to the neurons remains a real sticking point in the fight to restore hearing. Research[1] conducted by scientists at the University of New South Wales, Australia has shown that it is possible to improve this connectivity by using the implant electrodes to enhance the action of nerve regenerating DNA neurotrophins whilst reducing the voltage required.

Regrowth of hearing neurons is possible

The concept of nurturing cell regeneration through gene therapy has been explored in a number of applications for some years now but the challenges of regenerating nerve cells are much greater than those of regenerating skin tissue for example.

The research team injected a gene that encourages neuron growth into the auditory nerve cells and then attached a cochlear implant. In a process called electroporation an electrical stimulus passes through the cochlear electrodes to open up pores in the cells allowing the nerve growth stimulating DNA to infiltrate the cells.

The resulting growth of neuron fibres led to a significant improvement in hearing in the test subjects. Although the team used guinea pigs for the tests the results are seen as highly encouraging due to the fact that guinea pig ear canals have a close similarity to human ears.

My cochlear implant is so good I’ve thrown it away

No, you can’t throw your cochlear implant away just yet for a number of aspects still need to be developed and some problems ironed out.

One problem that will stop you chucking your implant for ever is that ongoing electrical stimulation from the implant’s electrodes is necessary to keep the new nerve fibres vitalised. Without the stimulation from either the implant’s electrodes or from normal healthy hair cells the regenerated neuron tissue will begin to die back again.

And one scientist, Stephen O’Leary noted[2] that the beneficial effect of the gene therapy only lasts a few weeks before the regenerated nerve tissue will begin to atrophy. Successful gene therapy requires repeated exposure to the neurotrophin gene. The other aspect that needs further work is reducing the amount of voltage required to open up the cell pores allowing the neurotrophin in to achieve the optimal results.

However, the potential benefits of this research are really promising: increased sensitivity to pitch for future cochlear implants (better sound processing in noisy environments and listening to music) and the possibility of an effective gene therapy delivery system to encourage nerve regrowth.

A number of research teams are exploring different ways of delivering gene therapy into the required area. And it is hoped that a simple procedure to deliver the DNA followed by some electrical stimulation will have your ears like new but that still lies some way in the future.

There may come a day when you can throw away your cochlea implant forever but just not yet.

[1]  J.L. Pinyon et al., “Close-field electroporation gene delivery using the cochlear implant electrode array enhances the bionic ear,” Science Translational Medicine, 6: 233ra54, 2014.


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