Cochlear Implant

A cochlear implant transforms speech and various other sounds into electrical energy which is then used to stimulate auditory nerve fibers in the inner ear. Sensorineural hearing loss is mainly due to less number of hair cells located all along the basilar membrane of the ear. Hair cell loss or damage may be caused by a genetic mutation, illness such as meningitis, listening to excessively loud noises, and medication with ototoxic chemicals. This implant consists of an external speech processor, headset that sits behind the ear and a surgically implanted receiver/stimulator package with an electrode array.

Cochlear implant has the following parts:

• Microphone: It picks up sound signals from the environment.
• Speech Processor: It filters the sound signals and sends them to the transmitter through a thin cable.
• Transmitter and Receiver/Stimulator: It receives signals from the speech processor, converts them into electric impulses and sends them to the electrodes through an internal cable.
• Electrode Array: It is a group of electrodes that collects the impulses from the stimulator and sends them to different regions of the auditory nerve.

How does a cochlear implant work?

Unlike other hearing aids, cochlear implant does not amplify sounds, it bypasses damaged portions of the ear and directly stimulates the auditory nerve using electrical impulses. Signals generated by the implant are sent to the brain through the auditory nerve, where the brain reads the frequency of the signals and recognizes them as sound. The implant functions by using the tonotopic organization of the basilar membrane of the inner ear. Tonotopic organization or frequency-to-place is the technique in which the ear sorts out different frequencies so that the brain processes that information.

Cochlear Implant Surgery

Before undertaking the surgery, patients should have a detailed evaluation of the implantation by consulting the cochlear implant team comprising of surgeons, audiologists and hearing therapists. This evaluation should consist of different hearing tests, speech and language assessments, hearing aid evaluations and medical examinations. CT and MRI scanning of the ears and routine blood tests should also be performed. After the evaluation, the team should decide whether surgery is the best option for the patient or not. Cochlear implant surgery lasts about two to three hours where the patient is given general anaesthesia before the operation. An electrode array is inserted into the cochlea and the receiver/stimulator is secured to the skull. After the operation, the implant is connected to the processor for testing. An X-ray is done to ensure that the electrode array is placed correctly, and within a week of the surgery the patient can resume work.

Cost and Efficacy

In the United States, the medical cost for the implantation ranges from $45,000 to $105,000 which includes evaluation, surgery, hardware (device), hospitalization and rehabilitation. A cochlear implant is a prosthetic substitute for hearing and is unable to cure deafness or hearing disabilities from its roots. It is helpful in restoring functional comprehension of speech, especially for those people who are functional in spoken language but have lost their hearing ability. Individuals who suffer from deafness and blindness may find that cochlear implants radically improve their daily lives. It will make them independent and provide with more information regarding safety, communication, balance, orientation and mobility. Modern cochlear implants have 24 electrodes to replace the 16,000 delicate hair cells. Hence, the sound quality delivered by the implant is good and users do not have to depend upon lipreading.

Risks and Disadvantages

The cochlear implantation sometimes results in damage of nerve cells within the cochlea, which eventually leads to permanent loss of residual natural hearing of the recipient. This device is incompetent of replicating the quality of sound processed by a natural cochlea. Due to this, some recipients can only recognize the difference between simple sounds while others can clearly understand speech in quiet environments. According to the United States Food and Drug Administration, implant recipients are at a higher risk for meningitis and the skin flaps surrounding cochlear implants also gets mortified. The success rate of this device dominantly depends upon the technology used and condition of the recipient's cochlea.

With proper evaluation of the techniques used, the risks in the implantation can be minimized. Cochlear implant does not restore normal hearing but gives a deaf person useful representation of sounds in the environment and, enables him to understand speech and enjoy a healthy conversation.