Cochlear implants and Cochlear implant surgery

Updated: Sep 15

Dr. K. R. Meghanadh

What is cochlea?

The cochlea is an organ in the inner ear that converts mechanical sound signals or energy into electrical signals for the nerves to carry to the brain.

Sound travels in the form of waves. Sound waves fall on the eardrum (the tympanic membrane) and then travel through the ossicular chain to the cochlea.

The ossicular chain consists of 3 bones malleus, incus and stapes. The first bone in the chain, the malleus, is connected to the tympanic membrane. The last bone in the chain is Stapes which ends in the fluid inside the cochlea. These three bones move like a piston to transmit the sound waves to the cochlea.

Cochlea image with other parts of the ear

This piston-like movement by the ossicular chain creates ripples in the cochlear fluid. These waves drag till the end of the cochlea. There are hair cells on the bed of this fluid-filled tube, i.e., hairs attached to a cell on the bed of the cochlear tube. These cells convert soundwaves' mechanical energy to electrical energy. This behaviour is similar to turbines in a dam that move due to flowing water and generate energy. This electrical energy is transmittable to the brain through nerves. The hair cell base has a negatively charged protein, so when the hair moves, the base moves between two proteins. The base transmits this electron from one protein to another and travels to the brain. These electrons are carried by the eighth cranial nerve, i.e., the auditory or cochlear nerve. The auditory or cochlear nerve sends hearing inputs to the brain. the cochlea has a hair cell for every frequency in the human hearing range, i.e., 20 to 20,000 Hz. When a particular frequency sound falls on the tympanic membrane, the corresponding frequency hair cell will be stimulated by the fluid waves in the cochlea, helping the brain to detect the frequencies.

Why does cochlea cause hearing loss?

Why and when should a child get a cochlear implants?

In some children or individuals, the negatively charged protein in the hair cell base in the cochlea responsible for converting sound waves to electrical signals could be missing. The deficiency of this protein could be due to missing or modified genes. So, in these scenarios, the transformation of mechanical sound waves into transmissible electric waves cannot happen, resulting in deafness in both ears. A cochlear implant has the mechanism to overcome this defect.

Why would a person who is not born deaf need a cochlear implant?

The degree of deafness varies. The typical hearing is 15 to 95 decibels. If the hearing loss is less than 75 decibels, we can try a hearing aid to correct it.

We do not have hearing aids that work for more than 75 decibels of hearing loss. In these scenarios, the cochlea cannot receive the signals even when the sound signals are amplified. In such scenarios, cochlear implants are suggested.

What is a cochlear implant?

Functions of cochlear implant

A cochlear implant takes the sound signals through a microphone and converts them into electromagnetic signals. These electromagnetic signals are transmitted to the cochlear implant fitted inside the skin via an electromagnetic coil. This cochlear implant's internal device is called a receiver-stimulator as it receives the signals, codes them into electrical signals, and gives them directly to the cochlear nerve in the cochlea. So, the nerve gets the electric signals instantly. The conversion of mechanical soundwaves to electric signals step is being taken care of by the cochlear implant.

A cochlear implant has two components, one being external and the other component implanted surgically inside the head. These two together replicate the job of the cochlea, eardrum, and middle ear.

Cochlear implant parts

External components

The outer or external component consists of a receiver or a microphone, sound processor, and transmitter. The receiver and sound processor sit behind the ear pinna. The transmitter attaches to the side of the head.

Cochlear implant and cochlear implant surgery

The sound processor converts the sound waves into electrical signals. These electrical signals are sent to the internal device from the external transmitter coil to the internally implanted coil that sits just behind the skin. The signals are transmitted using the radio-frequency transmission.

Internal Components

The internal device has a receiver-stimulator placed just below the skin and on the side of the head. Receiver-stimulator receives the radio-frequency transmission from the external device and encodes these into electrical signals. These signals are given to the cochlear nerve directly through a cable called an electrode array. The thickness of the cable varies from 0.9 mm to 0.4 mm, and this cable contains around 24 fine wires, which are again insulated individually. So, these individually insulated 24 wires of the electrode array are very fragile and need to be handled with extra caution during the surgery. Wrong handling of the cable is one of the most common causes of the failure of cochlear implant surgery. The cable stimulates 24 electrodes responsible for stimulating the 12 points of the nerve in the cochlea. So, the nerve gets the electrons or electrical signals directly, bypassing the hair cells on the cochlear bed.

Cochlear implant surgery

Cochlear implant surgery is a life-changing step, especially for deaf-born babies The steps in the surgery need to be followed with great precision and care. Strict adherence to the Standard operating procedure of a Cochlear implant is important for the implant's longevity and the surgery's success.

Cochlear implant surgery makes a bed for the receiver-stimulator that fixes just below the skin. A circuit or chip and a coil are present in the receiver-stimulator. This is fixed onto the skull, diagonally on top of the ear.

The electrode array from the receiver-stimulator is placed through a groove in the bone that goes towards the cochlea. The terminal area of the electrode array will be placed in the cochlea to stay in close contact with the auditory nerve endings. This process needs to be implemented with great caution. As discussed earlier electrode array is super fragile.

The surgeon’s process determines the cochlear implant’s success or longevity. The more he sticks with the guidelines or SOP, the more longevity.

Are you still deaf if you have gotten cochlear implant surgery?

The answer is not straight yes or no for this question.

You are deaf if you switch off the cochlear implant or remove the external device.

You are not deaf if you switch on the cochlear implant and have the external device on you properly.

Nevertheless, technically, the hearing of a person who gets cochlear implant surgery is not the same as normal hearing.

Do cochlear implants restore normal hearing?

Although a patient with a cochlear implant can hear and the person might be able to differentiate the sounds like an average person, cochlear implant hearing is different. Usually, a person can listen to 20,000 types of sounds, but a cochlear implant patient can listen to only 12 to 24 types of sounds. Let us explain this clearly in the next paragraph.

The cochlear implant machine cannot individually stimulate the 20,000 nerves( part of the auditory nerve) that exist on the bed of the cochlea. Instead, the cochlear implant stimulates a bunch of nerves together. It divides the frequencies into 12 groups, so the sound differs from normal hearing. Thankfully, our brain can still understand these sounds perfectly, enabling the patient to talk like a normal hearing person.

How often do cochlear implants need to be replaced?

How often are revision surgeries required?

The cochlear implant manufacturing companies give ten years replacement guarantee on the receiver-stimulator planted inside the head conditionally, as per the author. For accidental damages, the guarantee does not apply. If there is a manufacturing defect or some wear and tear kind of defect, the cochlear receiver-stimulator should be replaced as per Dr K. R. Meghanadh ’s knowledge.

Nevertheless, Dr K. R. Meghanadh has done 600 cochlear implant surgeries in the last 20 years, and almost all the cochlear implants are still working correctly. Out of the 600 surgeries, only one patient required revision surgery after 12 years. The reason for this exception is unknown, guesses are there could be a minor fall or accident damaging the receiver-stimulator. If there was a manufacturing defect or faulty surgical procedure, it should have stopped working earlier and not waited for over a decade. In other cases, people who got surgery, even before the particular patient, haven't reached out for revision surgery. So, the longevity of these implants is unknown but should work for a lifetime in most cases.

Dr K. R. Meghanadh added that the currently available components' quality is very good, irrespective of the brand. Although he says he cannot give or prove that cochlear implants can work lifelong, he believes they can and time will prove it.

What is the success rate of a Cochlear implant surgery?

To answer this question, we need to define what success is in cochlear implant surgeries.

There could be two ways of defining this success.

1. The patient can hear all sounds after surgery

2. A deaf or hard of hearing patient can not only hear but also talk after surgery.

Regarding the first point, i.e., the hearing part success rate is nearly 100% for any experienced surgeon who follows SOPs strictly and does the procedure with patience and utmost care.

Regarding the second point, this depends not on the type of implant or variety or the surgeon but on the timing of surgery. A child below one year will be able to hear and talk normally. The ideal time to get surgery for a child would be nine months as it would give wonderful results. As age increases, learning language and differentiating sounds can become tougher, and the quality of speech and perception of the hearing will come down.

Table Description: Speech clarity achieved in deaf born babies the timing of cochlear implant surgery

Implantation in both ears at the same time is called simultaneous bilateral cochlear implants, which can give the best results.

What if there is a deviation from the standard operating procedure(SOP) or the process is not done with utmost care?

  • Implant rejection

  • Infection

  • Soft failure The computerized scanning and check of the implant will be normal, but the patient’s hearing is unstable and keeps going on and off. This occurs when the electrode placement is not in the bone groove, and the electrode was handled harshly.

  • Hard failure The computerized scanning and check of the implant will be faulty. This could be a manufacturing defect or lack of adherence to the SOPs.

Are there any patients who regret cochlear implants? Non-users of cochlear implants

Almost 600 patients observed had normal hearing, except for one case. The patient chose not to hear and moved in with the conventional deaf community, who mastered sign language under the spouse’s influence. Such cochlear implant patients are called non-users. Some individuals who lived longer are habituated to the non-hearing world and do not want to come into the hearing world and be part of a deaf non-talking community. These people have accepted that they are deaf and proudly use sign language to communicate. Trying to enter or adjust to the hearing world might make them feel odd, different, or complex for us to understand. They might not be able to gel well with us and feel at home and better accepted in those communities. It is a matter of choice and an individual’s happiness that must be respected.

Is a cochlear implant wrong? Or can it go wrong? What can the complications be?

Cochlear implants are not harmful when SOPs are followed properly during the surgery. The cochlear implants have not caused neurological problems in the 600 patients author has operated on.

1 or 2 per cent of implants worldwide had complications like infections or non-infectious rejections, which are a part of the procedure.“However, we have not seen such complications in 600 surgeries. These complications can occur mainly if the surgeon violates operating procedures, SOP, or protocols. It depends on how strictly a surgeon follows the given guidelines.

Refer to the “What if there is a deviation from the standard operating procedure(SOP) or the process is not done with utmost care?” section for more details.

Cochlear implants have some metals, so an MRI scan could be impossible. Nevertheless, now we have the latest cochlear implants with the advanced technology of MRI-compatible rotatable magnets.

So, look out for Cochlear implants with rotatable magnets.

Who can get Cochlear implant surgery?

Anyone with more than 75 decibels of hearing loss or not showing enough improvement with the best available hearing aid can go for Cochlear implants.

A person should not have hearing loss for more than six months due to neural plasticity. In neural plasticity, the brain can assign the jobless nerves after six months to different tasks, and it is difficult to change back the function of the nerves when the implants are fitted. Therapy could be required to get the nerves back on track, and the results are unpredictable.

How much does Cochlear Implants surgery cost?

As the author is from India, he has given details about the surgeries cost in India.

Cochlear implant surgery can cost from 9 Lakh rupees to 35 Lakh rupees. He has given all the details in below article. The approximate costs are also mentioned in USD.

Article: How much do cochlear implants and surgery cost in India?

Dr K. R. Meghanadh

49 views0 comments

Related Posts

See All