It is an electronic device that allows patients with severe to profound sensory neural hearing loss to hear. With the ability to hear, patients can communicate better and lead routine lives.
A cochlear implant stimulates the auditory nerve endings, called spiral ganglia. It allows it to bypass the defective cochlea, allowing the patient to hear.
Components of a Cochlear Implant
A cochlear implant consists of two components:
- Internal Component- This is implanted surgically behind the ear and the wire (electrode) is inserted into the cochlea. The internal component of the implant comprises a receiver and a stimulator with an electrode array.
- External Component- This consists of the external processor and transmitter. The speech processor is worn behind the ear like a hearing aid.
Sound is picked up by the microphone and processed by the processor. The processor analyses the sounds and converts them into electric signals. The electrical signals are sent to the transmitting coil. The coil then sends the signals to the implanted receiver via radiofrequency. The receiver then decodes the signal and transmits it to the electrode array. The electrode array then stimulates the spiral ganglion and auditory nerve. Once the auditory nerve gets stimulated, it sends these electrical signals to the brain. The brain then interprets it as sound.
Candidacy for Cochlear Implantation
Both children and adults can be implantees. However, there are a few criteria that define a patient’s candidacy for cochlear implantations:
- Bilateral severe to profound sensory neural hearing loss.
- Bilateral severe to profound sensory neural hearing loss.
- Patients who do not get any benefit from hearing aids
- Patients must be fit for surgery
- Ample social and familial support through the habilitation process
- Adequate cognitive function to be able to use the device
Candidates are classified into two groups, i.e. pre-lingual or post-lingual. It depends on whether they were deaf at birth or lost their sense of hearing later on in life.
In the case of pre-lingual hearing loss, early intervention is critical for auditory stimulus. The lack of this auditory stimulus in the early developmental phase of a child causes a degeneration of the central auditory pathways. It limits the benefit in terms of speech and language development after cochlear implantation.
Importantly, moderate to severe hearing loss patients do not need cochlear implantation. For these patients, hearing aids suffice. Only in cases of severe to profound hearing loss would this be suggested.
Clinical Evaluation for Cochlear Implantation
A thorough evaluation of the patient is crucial in the selection process of candidates for cochlear implantation. The main aim is to determine the fitness and audiological suitability for implantation. It helps the surgeon counsel the family about the expected outcomes of the surgery.
A medical evaluation is vital to confirm a patient’s fitness for general anaesthesia. It is done through a detailed history along with a physical examination. The surgeon would also advise a CT Scan and MRI. It allows them to see the internal structure and identify any abnormalities that may complicate the surgery.
Audiological evaluations include:
- Pure Tone Audiogram (PTA)
- Tympanometry (Impedance audiometry)
- Otoacoustic Emissions (OAE)
- Brainstem Evoked Response Audiometry (BERA)
- Auditory Steady-state Response (ASSR).
Cochlear implantation patients must undergo a hearing aid trial and evaluation to determine their candidacy. These tests include aided speech perception, aided free-field sound detection thresholds and discrimination scores.
A speech and language evaluation is required initially to assess the child’s communicative status as well as to determine any developmental language or articulation disorders. It would help form a baseline for future evaluations post-implantation and determine the patient’s progress. Overall, this would help to adjust the programming of the patient’s device during habilitation.
At times, a psychological evaluation is advised when there are concerns regarding the mental function or cognitive status of the patient. This is also a crucial step in identifying children with disabilities other than just hearing loss. This evaluation helps to further counsel parents and families about expectations after cochlear implantation.