Deafness and implants
Deafness can have many causes. The whole hearing process starts from the ear, all the way in through the ear canal, to a series of structures inside the bone of our heads. There is a small organ in the shape of a snail, called the cochlea, that receives the sound vibrations and translates them to electrical impulses. The cochlear nerve transmits these impulses to our brain, where they are processed and transformed into our perception of sound.
All stages of this process can be completely or partially damaged, causing different levels of deafness in one or both ears. Deafness can be cured or treated depending on the place of the lesion. In cases where the neurological components of hearing aren't damaged, non-neurological components can be by-passed with a device that is implanted through surgery.
A cochlear implant is one of these devices, and there are several surgical options for placing it.
How does a cochlear implant work?
This implant has both internal and external parts and can be in one or both ears. It consists on a circular microphone that is attatched to the outside of the head, usually behind the ear. This microphone is connected to a processor placed inside the skull, also close to the ear. The microphone detects high frequencies of sound and transmits them to the processor.
These signals are then sent through a small cable to electrodes placed inside the cochlea. Electrodes stimmulate the cochlear nerve just as the cochlea itself would do in response to the vibrations of sound. This way, the signals get to the brain and the perception of sound can take place, even if the hearing system itself is damaged.
Cochlear implants can be unilateral or bilateral, and placed at any age. There are some risks to the surgery, including damaging the ear or head, causing tinitus - the perception of a constant high-pitched sound in one or both ears - and infections. In some cases, it takes a few months to the cochlear nerve to adapt to the electrodes, and during that period of time the patient may experience inner ear related problems like dizziness or balance difficulties.
When to proceed with the surgery
A cochlear implant works in both people born deaf and with acquired deafness. As long as the nervous segment of the hearing system is intact, a cochlear implant can increase the perception of sound.
In the last decade many studies have been testing different cases of deaf people with and without cochlear implants, in order to answer the question: at what age is it best to make the implant? There are several factors that influence this decision.
Research shows that placing a cochlear implant short after the moment of hearing loss has some advantages. The brain is naturally plastic and it can adapt to changes in the environment and our own body. In the case of hearing, oral language is heavily based on our capacity to hear and understand phonems. We use different areas of our brain for understanding oral language - auditive cortex - and sign language - visual cortex. When a person is deafened and it switches to sign language for everyday communication, brain cells from the auditive cortex start adapting to a new visual function, in order to increase language performance. The sooner a cochlear implant is placed, the less auditive cortex will swithc to visual functions, thus increasing visual performance at the expense of hearing performance.
Research also shows how cochlear implants can affect prelingually deafened children or babies born deaf. The brain can adapt to the implant too, but in this case the language is not the focus of the concern. Instead, scientists have discovered that there is a critic point at three years and a half of age. Children implanted before that age show the best results at hearing tests.
Before proceeding with the surgery, it is important to determine whether or not the patient has any natural hearing left. Some implants may increase high-pithced hearing, but damage preserved low-pitch hearing. In other words, an implant can help improve partial deafness in one or both ears, but at the same time affect negatively natural hearing. In order to preserve natural hearing, the surgery must be carried out with extreme care. The electrodes are implanted directly inside the cochlea, where the cochlear nerve starts. Surgery may damage or cut all or some of the ramifications of this nerve, and even affect the cochlea itself, which is a very small and delicate structure inside the bone of our heads. Before surgery, an exhaustive evaluation of the patient's natural hearing in both ears must be carried out.