Definition– Cochlear implants are surgically placed electrical device that receive sound and transmit the resulting electrical signals to electrodes implanted in the cochlea of the ear. The signals stimulate cochlea, allowing patient to hear. It is also known as Bionic ear.
- Child above 12 months and below 7 years are pre-lingually deaf children. At birth the cochlea is fully formed but the auditory pathway is not. Auditory pathway is dependent on stimulation for its maturation and this stimulation is vital to acquisition of speech and language skill as well as amount of cognitive development.
- Post lingual deaf no age limit.
- Degree of deafness- Profound SNHL > 90 dB with poor discrimination in both ears with cochlear nerve.
- In those who do not benefit from a hearing aid, at least 3 to 6 months of use.
- Absence of contraindications like cochlear aplasia or absent cochlear nerves are absolute contraindications to cochlear implantation.
- Severe or profound hearing loss with PTA of 70 dB or greater hearing loss.
- Little or no benefit from hearing aids.
- Aided scores on open-set sentence test of less than 50%.
- No evidence of central auditory lesions or lack of an auditory nerve.
- No medical or radiological contraindications for surgery.
- Electrical stimulation– It is complete electric stimulation when there is no residual hearing in both ears.
- Electroacoustic stimulation (hybrid implants)- Here lower frequencies are stimulated acoustically via hearing aid while higher frequencies are stimulated electrically via cochlear implant.
- Bimodal stimulation– Here one ear uses implant while other uses a high gain hearing aid on another ear.
- Microphone: This picks up the sound.
- Speech processor:
- Transmitter coil: It is transcutaneous and transmits signals into the implanted receiver coil without need of any wires.
- Receiver coil: It is an implanted coil into the bone of the skull behind the ear. The signals from the transmitter are decoded and relayed to the electrodes. The electrodes are placed in the cochlea to stimulate the VIIIth nerve.
- Electrode Array:➣ It consists of electrodes and electrode carrier.
➣ Electrode carrier is the wire which extends from the receiver to the electrodes.
➣ Electrodes are of 2 types: Extracochlear electrodes and Intracochlear electrodes.
➣ Insertion depth of Cochlear Implant electrodes
- The mean length of human being cochlea is 33– 36 mm.
- The implants don’t reach to the apical tip, it may reach up to 25 mm which corresponds to a tonotopical frequency of 400 Hz.
- Advanced bionics Hi Res Sylmar
- Nucleus Cochlear Implant System
- Clarion Cochlear Implant Systems
- MED-EL Cochlear Implant
- Incision may be C-shaped, inverted U, inverted J shaped.
- The flap is elevated, it includes periosteum of the mastoid, temporalis fascia, and temporalis muscle. Flap thickness should not be greater than 6mm.
- It is made for the placement of stimulator.
- More superior placement is done in small children in the area temporal squama.
- In adults implant is placed on occipital portion of temporal bone.
- In children stimulator placed over exposed dura.
- Channel is formed over the bone to pass the electrode lead.
- It is performed after creating the site for well.
- The mastoidectomy cavity should not be saucerized as edges help to retain the electrode leads.
- Facial recess is identified and widely opened.
- Care should be taken of the anomalous facial nerve or absent facial nerve.
- The most inferior part facial recess is important for visualization of round window niche.
- Round window niche is clearly seen after opening the facial recess.
- Cochleostomy is created inferior to inferior attachment of round window membrane.
- The size of cochleostomy varies between 0.8 mm to 1.2mm in diameter.
v) Insertion of electrode array
- When device is brought into operative field the monopolar cautery is to be removed.
- The electrode array is inserted into the cochleostomy.
- The tip of the electrode array should be directed inferiorly so that it will slide along the lateral wall of the scala tympani.
- The stimulator is fixed to skull with sutures.
- Drill holes are made above and below the receptacle site and sutures are passed through them. It can cause perforation and CSF leak in children.
- Alternatively, a strip of material is placed over the stimulator secured with miniplates. Nonabsorbable material like gortex or absorbable material like alloderm can be used.
Middle Cranial Fossa approach
- It is done in post lingually deafened adult.
- Individuals who have open canal wall down mastoidectomy cavities.
- It is non-mastoidectomy technique.
- Done through end-aural route for cochleostomy.
- Trans canal tunnel drilled in the posterior canal wall.
- It has faster healing and earlier fitting of the processor.
- Chances of facial nerve trauma is less.
- Out of box failure- Due to factory defects or during surgical manipulation.
- Extracochlear implantation can occur when hypotympanic cells are mistaken for scala tympani.
- The electrode array may get migrated after correct placement.
- Most common cause of displaced electrode is movement of electrodes array after drill out procedure.
- Extrusion or exposure of the device.
- Displacement of device.
- Late device failure:➣ Usually due to internal device failure, due to trauma or spontaneously. ➣ In this case the external component is first replaced, sometimes that solves the problem. ➣ Fine cuts (1mm-2mm) CT of temporal bone done to look for the position of stimulator and electrodes.
- Development of speech perception with training in implant listeners.
- Children with implants need the implant system to be working well, and it should be worn consistently in good listening conditions when good communication opportunities are available.
- All external parts should be kept in good functioning order and consult an audiologist who specializes in CI on a regularly scheduled basis.