Describe cochlear implantation.

Principle and Selection

• A cochlear implant is a device that bypasses the damaged hair cells and stimulates the spiral ganglion and cochlear nerve directly with electrical impulses.
• It replaces the transduction function of the cochlea, and is therefore quite unlike a hearing aid.
• Components — external: a microphone, a speech processor which converts sound into a coded signal, and a transmitting coil held over the skin by a magnet. internal: a receiver-stimulator implanted in the temporal bone, and an electrode array inserted into the scala tympani through the round window or a cochleostomy

Surgery, Rehabilitation and Outcomes

• Surgery — through a postauricular incision, a cortical mastoidectomy and posterior tympanotomy (facial recess approach) give access to the round window; a well is drilled for the receiver-stimulator; and the electrode is inserted with soft-surgery technique to preserve any residual hearing
• Complications — facial nerve injury, the most feared, which is why monitoring is used; chorda tympani injury; wound infection and flap necrosis or device extrusion; CSF gusher at cochleostomy in a malformed cochlea; meningitis, hence the vaccination; vertigo and loss of residual hearing; device failure requiring re-implantation; and facial nerve stimulation, which is commoner in otosclerosis and malformed cochleae because current spreads through demineralised bone
• Switch-ON is at 3 to 4 weeks, once the wound has healed, followed by repeated mapping of the processor over months as the patient reports thresholds and comfort levels
• Rehabilitation is what determines the outcome — intensive auditory training and speech and language therapy over months to years, with educational support. The operation is the beginning of the treatment, not the end of it
• Outcomes — most users achieve open-set speech understanding and can use a telephone; children implanted early develop age-appropriate speech and language and are educated in mainstream schools; but music appreciation and hearing in noise remain limited.
• Bilateral implantation improves localisation and hearing in noise, and is increasingly standard in children
• Precautions for the user — the external processor is removed for MRI (and some devices require magnet removal); avoidance of static electricity and contact sport head injury; and prompt attention to any middle ear infection
• In INDIA — cost is the principal barrier, and state and national schemes such as ADIP fund implantation for children; but the shortage is of rehabilitation services rather than of surgeons, and an implant placed without follow-up therapy is a waste of a scarce resource
• An implant replaces transduction, an aid only amplifies.