Cochlear implant

A cochlear implant is a device that is used to treat profoundly deaf people who are not helped by hearing aids. Unlike a hearing aid, which amplifies sounds, a cochlear implant converts sounds into electrical signals that are relayed to the cochlear nerve deep in the inner ear.

The implant consists of tiny electrodes that are surgically implanted in the cochlea, and a receiver that is embedded in the skull just behind and above the ear. A microphone, sound processor, and transmitter are worn externally.

A cochlear implant does not restore normal hearing, but it enables patterns of sound to be detected. Combined with lip-reading, it may enable speech to be understood.

Sounds picked up by the microphone are converted into electronic signals by the sound processor and relayed to the external transmitter, which sends them through the skin to the receiver. The waves then travel along the wire to the electrodes in the cochlea, where sound is normally received.

Cochlear Implants (CIs) in detail - technical

Definition

A cochlear implant is a surgically implantable device that helps restore hearing in patients with severe-to-profound hearing loss, unresponsive to amplification by conventional hearing aids. Cochlear implants are electronic devices designed to detect mechanical sound energy and convert it into electrical signals that can be delivered to the cochlear nerve, bypassing the damaged hair cells of the cochlea. These electrical signals are processed by an external speech processor and sent via a radio-frequency interface into an array of electrodes implanted surgically within the cochlea. The implant system preserves the tonotopic map of the cochlea as in nature and hence the auditory brain perceives these electrical impulses as sound (Picture 1).

cochlear implant picture

Picture 1: cochlear implant

History of Implants

Djourno and Eyries published the first description of cochlear implants in 1957 (Waltzmann et al. 2000). In 1961, House used a single channel cochlear implant and in 1984, Clark developed the popular multichannel implant. FDA approval for CI was obtained in 1985. The first pediatric cochlear implantation was done in the USA in1987. Today, a spectrum of implants is available along with improved speech processing strategies. Rapid technological advancements in bioengineering and implant manufacturing methods have led to miniaturization of the device, with high quality refinement in sound signals, providing better “hearing in noise” and music appreciation among cochlear implantees.

Components of a Cochlear Implant

The implant has external components consisting of a microphone which receives and transduces sound into an electrical wave form, a speech processor which divides the signals into components for each of the electrodes, and a transmitting coil which sends the signals across the scalp to the internal components. The internal components include a receiver-stimulator, which receives the signals from the transmitting coil and sends it to the electrode array, which is implanted in the scala tympani of the cochlea. Speech processors are currently available as body-worn and ear-level speech processors (Picture 2).

Picture of cochlear implant internal and external components

Picture 2: cochlear implant internal and external components

Indications

Bilateral profound cochlear hearing loss, unresponsive to amplification by the most powerful hearing aids, is the prime indication for a cochlear implant. All children below the age of 6 years who have congenital or acquired profound hearing loss and who will not benefit from conventional hearing aids and all adults who have lost hearing after acquisition of language are ideal candidates.

The only true prerequisite is an intact auditory nerve. Postlingual candidates do extremely well with an implant and in pre- and perilingual candidates, an important factor influencing candidacy is neural plasticity and the emphasis is now on implantation as early as possible to maximize speech understanding and perception. In very young children language acquisition is easier, hence the need for early implantation.

Owing to the loss of neural plasticity in older prelingually deaf people, the response to implantation may not be optimal and extensive preoperative counseling regarding realistic expectations is crucial. Today, the indications have expanded to include candidates with lowfrequency residual hearing and those with severe hearing loss. These expanded indications for implantation are related to age, additional handicaps, residual hearing, and special etiologies of deafness.

The minimum age for implantation in children has come down and children as young as 6 months of age have been implanted. Because the cochlea is full size at birth, there is no anatomic difficulty with electrode insertion in very young children. Medical and radiological criteria have expanded to include significant cochlear abnormalities including additional handicaps, as in syndromic deafness.

The recent trend is toward bilateral simultaneous or sequential implantation, which provides immense benefits of binaural hearing. Cochlear aplasia, absence of auditory nerves, retrocochlear causes of deafness, central deafness, presence of external or middle ear infections, and coexistent severe medical illness are contraindications for cochlear implantation.

Preoperative Assessment

Prior to implantation a basic workup including hematological, chest X-ray, ECG, TORCH screen need to be performed. An audiologic assessment is the primary means of determining implant candidacy. Audiological and electrophysiologic investigations include pure tone or behavioral audiometry and impedance audiometry, otoacoustic emissions (OAEs), brainstem evoked response audiometry (BERA), auditory steady-state response (ASSR), aided audiometry, and a hearing aid benefit evaluation. Promontory stimulation testing can be done in older children and adults to assess the response of the cochlea to electrical stimulation.

High-resolution CT scans of the temporal bones are done to plan the surgical route for implantation, identify the vital structures like the facial nerve and promontory and also to rule out any evidence of middle ear disease/mastoiditis. Magnetic resonance imaging is the gold standard investigation for the assessment of cochlear anatomy and the vestibulocochlear bundle. It reveals anomalies like Mondini’s and Michel’s aplasia, labyrinthitis ossificans, or absent eighth nerve. Both CT and MRI scans complement each other in providing us complete details of the intricate temporal bone anatomy. Although few centers prefer doing only CT scans prior to implantation (due to nonavailability of MRI and financial implications associated with it), while most implant referral centers worldwide prefer to perform both CT and MRI scans prior to implantation.

Rapid advances in genetics and molecular biology are revolutionizing our understanding of congenital deafness and genetic counseling should play an important part in prevention. Hence, a genetic specialist’s opinion is sought in patients with syndromic etiology of deafness. Children need to get evaluated by a child psychologist for assessment of mental functions and IQ, prior to implantation and an ophthalmologist needs to perform a fundus examination to rule out associated visual impairment as seen in Usher’s syndrome. In children, preimplant meningococcal vaccination is carried out. Preoperative habilitation is important before surgery. Counseling patients and parents prior to implantation to develop realistic expectations of the likely outcome is vital. Hence, candidates and parents need to meet and interact with other cochlear implantees, to have a perspective on the procedure and its outcome.

Cochlear Implantation Surgery

The goal of cochlear implant surgery is to insert the entire electrode array into the scala tympani with as little damage as possible to the structure of the inner ear. The success of cochlear implantation depends on scrupulous attention to technique at all the various steps of the procedure. Implantation is performed with strict aseptic precautions and is done under general anesthesia. Surgery is essentially the same in children and adults because the anatomic structures are of adult configuration at birth. However, in very young children, there is a slightly increased risk of facial palsy and hypovolemic shock due to blood loss.

The steps of surgery are as follows, usually an extended postauricular incision is made to expose the mastoid cortex. The incision should be made more than 1 cm away from the location of the coil of the implant. Themastoid is drilled out to expose themastoid antrum. Saucerization of the cavity is not done. A posterior tympanotomy is performed, the facial recess is opened and the promontory and round window niche are exposed, without exposing the facial nerve. A well for receiver-stimulator is fashioned in the skull behind the mastoid cavity using a template as a guide and a groove is made to connect it to the mastoid cavity. Tie-down holes are placed on either side of the well for securing the implant. A cochleostomy is performed at the basal turn of the cochlea which is opened anterior to the round window (RW) to straighten the axis of introduction of the electrode. The electrode array is inserted atraumatically into the scala tympani using a clawshaped instrument. Alternatively, a round window insertion may be performed after drilling out the anterior lip of the RW niche and adequately exposing the secondary tympanicmembrane. Once the electrodes are inserted, diathermy should not be used. Fixation of the device and electrode array and wound closure is done.

Electrophysiological testing – impedance telemetry, neural response telemetry, and electrically evoked stapedial reflex thresholds are performed intraoperatively to confirm the optimal performance of the implant in situ. This assures the implant team that the device is functioning and that the patient is receiving an auditory stimulus and responding appropriately.

Complications of Cochlear Implantation

Major complications include facial palsy, implant exposure due to flap loss and wound infection. Other complications include facial nerve stimulation, device failure, deterioration of hearing, tinnitus, temporary balance problems, numbness of scalp, loss of taste, electrode/ device extrusion, CSF leak, and meningitis.

Switch-On and Mapping of Cochlear Implant

The switch-on and speech processor tuning is done 3 weeks after surgery. Programming the cochlear implant to suit the implant-aided auditory thresholds of the cochlear implantee is termed as “mapping.” Mapping is done at periodic intervals till a stable map is achieved. Frequent mapping sessions are required and prolonged and intensive (re)habilitation after implantation is essential. Habilitation aims at improving receptive language skills and expressive skills. The habilitation program is started out based on baseline skills of the patient and periodic assessments of outcome needs to be done in terms of environmental sound, open-set, closed-set speech, speech discrimination, and telephonic conversation. The recommended period for auditory verbal habilitation is 1 year.

Measuring Level of Performance

The outcomes of cochlear implantation are measured using the Category of Auditory Performance (CAP) and Speech Intelligibility Rating (SIR) scores, described by O’Donoghue et al. 1999 (O’Donoghue et al. 1999). The extent of the auditory perception in terms of utility of auditory mechanisms to pursue day-to-day tasks from awareness of environmental sounds to making telephonic conversations and the ability to discriminate and understand speech with or without lip reading are assessed through CAP scores. SIR scores measure the outcome of cochlear implantation with respect to speech, measuring the intelligibility of speech, and the quality, which might be recognizable by the listener. The overall outcome needs to be categorized accordingly, taking into account the number of months taken to achieve the necessary CAP and SIR scores.

Outcomes of Cochlear Implantation

The success of a CI program is directly dependent on its ability to address the issue of patient expectations and balance it with the outcomes. A multidisciplinary approach is required involving the otolaryngologist, audiologist, speech therapist, auditory verbal habilitationist, child psychologist, and pediatrician. The patients and their family must also be highly motivated for the implant. Variables affecting the outcome of CI in children are the duration and etiology of deafness, age at onset of deafness, preimplant amplification history, communication mode, age at implantation, type of speech processor used, and duration of implant usage. In very young children, language acquisition is easier and hence the need for early implantation.

Owing to the loss of neural plasticity in older prelingual deaf people, the response to implantation may not be optimal and extensive preoperative counseling regarding realistic expectations is vital. Factors influencing the overall outcomes are the transparency of the program, expertise of the team, patient motivation, family support, and facilities for rehabilitation.

Difficult Scenarios in Cochlear Implantation

With increasing experience in cochlear implantation, the indications for implant surgery have widened to include cochlear anomalies, syndromic associations, and individuals with multiple handicaps. Implantation has been shown to be beneficial in these widening groups of children, who were previously not considered to be implant candidates. However, the surgeon must anticipate greater challenges in these more complicated children during both implantation and the subsequent habilitation.

Cochlear Implantation in Labyrinthitis Ossificans

Ossification of the cochlea mostly occurs as a postmeningitic sequelae. Other pathologies that may predispose to ossification include otosclerosis, chronic otitis media, ototoxicity, autoimmunity, and trauma. This remains one of the significant surgical challenges for the otologist. It is diagnosed with a CT/MRI scan. On confirmation of an obstructed basal turn, the proximal turn is drilled with a microdrill to a depth of 6–8 mm until an open lumen is discovered and the electrode array is inserted. In total ossification, a complete drill-out of the basal turn is required and the implanted array is seated in a trough that surrounds the modiolus. A double-array implant may be used with some electrodes into the basal turn and others into the second turn.

Cochlear Implantation in Mondini’s Deformity/ Large Vestibular Aqueduct Syndrome

CSF leak during cochleostomy has to be sealed. A variety of techniques may be used to help control the flow of CSF including firm plugging of the cochleostomy using soft tissue coupled with reducing the flow of CSF by lumbar drainage and IV mannitol drip, if necessary. Such leaks may also be encountered in cases of enlarged vestibular aqueduct.

Auditory Neuropathy/Auditory dys-synchrony Spectrum Disorder (AN/ADSD)

Normal outer hair cell (OHC) function and dys-synchronous neural responses characterize this disorder. Patients will show a normal OAE with absent BERA waveforms, which is pathognomonic of this condition. Cochlear Implants are a viablemanagement option for patientswith AN/ADSD and are beneficial in bypassing the de-synchronous neural network, but the outcomes may be suboptimal or guarded and the family needs to be counseled carefully.

Cochlear Implantation in Individuals with Multiple Handicaps

Early diagnosis and rehabilitation of deafness and additional handicaps is crucial. An implant helps in the rehabilitation of deafness and other handicaps as well. However, patient selection criteria must be stringent. The decision to pursue implantation should be fully informed. Evaluation, surgical intervention, and postimplantation management of these patients can be challenging. Long-term intensive rehabilitation is essential after implantation.

Minimally Invasive Cochlear Implantation

Due to improvements in cochlear implant technology, smaller and more powerful implantable cochlear implants have evolved which has enabled smaller external incisions, smaller skin flaps, shortened surgical time, and faster healing. Current techniques in cochleostomy (peephole cochleostomy) and round window electrode insertion (soft insertion) have resulted in preservation of residual hearing. Unfortunately, peephole cochleostomy, sometimes called soft surgery, has occasionally led to misinsertion of electrodes into the scala media and scala vestibuli because of lack of proper visualization and control.

Perimodiolar and Mid-scalar Cochlear Implantation

These implants are assumed to have a slightly enhanced speech perception. After the electrodes are inserted into the cochlea, the stylet is withdrawn and the electrodes come into a perimodiolar/mid-scalar position. The electrode – neural interface seems to be minimal in this position and hence clarity of auditory inputs is much better.

Bilateral Cochlear Implantation

Bilateral CI has significant benefits which include improved speech perception in noisy environments and improved sound localization (Tyler et al. 2003). Patients receive elimination of head-shadow effect and obtain significant benefits from summation effects (improvement in hearing threshold from redundant information presented to each ear) and squelch effects (improvement in hearing threshold from brainstem processing of interaural time and intensity differences).

Speech perception in noise with bilateral implantation is significantly better than unilateral implantation and continues to improve even up to 24 months after implantation. Thus, patients may be able to neurally integrate inputs from both implants to enhance speech perception over time. Further improvements in performance are to be expected with a greater understanding of the neural pathways and integration of binaural hearing along with improvements in implant programming and technology.

The Future of Cochlear Implantation

Auditory neural prosthesis is highly successful in restoring normal hearing to profoundly deaf individuals. Cochlear implant surgery and technology continue to evolve. In the future, fully implanted devices like the TIKI prototype (Totally Implantable Cochlear Implant), improved speech coding strategies, cochlear hair cell, and nerve growth factors used in conjunction with an implant may be available.