5 Cochlear Implantation Cochlear implantation is a technique that fundamentally changed the therapeutic possibilities for patients with a profound to severe congenital or acquired sensorineural hearing loss. With the insertion of an electrode into the cochlea (the cochlear implant, CI), the cochlear nerve is stimulated electrically instead of acoustically. This leads to a different kind of sound perception. CI patients therefore have to learn to listen to and interpret these new sounds and will need extensive rehabilitation with a speech-language therapist and preferably a training partner at home. Many cochlear implant patients will only reach their fullest performances after months to years. The procedure for selecting the right candidates for cochlear implantation is a multifactorial process usually implemented in a multidisciplinary team consisting of • ENT surgeon (surgical feasibility and medical history) • Audiometrist (audiometric indication) • Speech-language therapist (speech intelligibility rate, lip-reading skills, learning ability) • Psychologist (learning ability, resistance to changes) or remedial teacher (developmental possibilities of a child, contact skills, need for communication) • Social worker (social network, training partner, home situation) Each discipline takes different factors into account, all of which may influence the decision to implant and the successful outcome of cochlear implantation. The outcomes of cochlear implant surgery remain variable, depending on many patient-related factors and some implant-related factors. It is therefore necessary to counsel candidates about the variability of outcomes, incorporating the experience of the different disciplines in the advice provided. In candidates with a possibly less favorable outcome—such as, following meningitis, those with severe inner ear malformation, or congenitally deafened children with cerebral damage (cytomegalovirus)—it is wise to counsel about the expected less favorable outcome. Patients and parents need to have enough understanding of the advantages and disadvantages to be able to choose or reject implantation. Being transparent about the expected results will also prevent expectations running too high. Counseling can describe the following levels of outcome: I, signal function (awareness of sounds); II, support of lip-reading skills; III, open set speech understanding. Only some of the patients in this last category can use the telephone for longer conversations. In general we can identify three groups of candidates: 1. Postlingually deafened children and adults 2. Prelingually deafened children 3. Early pre- and perilingually deafened adults and adolescents Postlingually deafened patients with acquired hearing loss are good CI candidates, as long as the cochlear nerve has kept its integrity and the cochlea is accessible. Infants with congenital (prelingual) hearing loss are often good CI candidates with very good linguistic development after implantation. Their outcome depends on the age of implantation, the etiology of the hearing loss, and on their additional disabilities and cognitive status. It is important for this group to be implanted at an early age, because the development of their auditory pathways and concomitant speech-language development will depend on the input received from the cochlear implant. Currently, the preferred age of implantation is around 6 to 10 months. Early deafened adults and adolescents are also possible CI candidates, although the results in this group are very variable. The outcome is related to the amount of development of the auditory pathways in earlier years and its concomitant speech-language development. Better hearing at a younger age for a longer period of time or auditory stimulation of residual hearing using hearing aids for a prolonged period of time will lead to better outcome with the cochlear implant. Careful selection of the individuals in this group of candidates is therefore necessary and their speech intelligibility score, language development, type of education, and lip-reading skills are factors to be taken into account. The speech therapist plays an essential role in this decision-making process. The etiology of the patient’s congenital or acquired sensorineural hearing loss can influence the work-up and timing of implantation. In adults the most common causes are presbycusis, otosclerosis, Meniere disease, autosomal dominant mid- or late-onset deafness, and hearing loss caused by infection or surgery of the middle ear or mastoid. Less common but well-known causes of severe hearing loss or acute deafness are bacterial meningitis, autoimmune inner ear diseases, and skull base lesions, including fractures of the otic capsule. In children with congenital hearing loss the most common cause is genetic hearing loss, with more than 60% being autosomal recessive in origin and around 30% syndromic.1 Other important causes of hearing loss in children are congenital or acquired cytomegalovirus infection, malformations of the inner ear, aplasia or hypoplasia of the cochlear nerve, and bacterial meningitis. Some of these indications need a different work-up and decision-making process, as well as a different surgical technique. These entities are discussed in separate chapters in the book (see Chapters 12 to 17). In this chapter we discuss standard cochlear implantation in adults. Special surgical approaches and their indications, such as electroacoustic stimulation, subtotal petrosectomy approach combined with cochlear implantation, and implantation in cochlear ossification, are presented in Chapters 9 to 11. The indications mentioned below are not rigid limits but criteria that many implant teams use. Each patient is assessed individually and the decision is based on many factors. Audiometric evaluation is one of them. The basic audiometric indication is: • Severe to profound bilateral sensorineural hearing loss with little or insufficient benefit from conventional hearing aids. • In adults with postlingual hearing loss: – Speech audiometry tested with hearing aids in a free field setting at 65–75 dB sound pressure level with a score of ≤ 50%. • In congenitally deaf children or (young) children with prelingual hearing loss: – Auditory brainstem response audiometry (ABR) with no responses or responses at 80 dB or more. – Free field test with hearing aids after trial of at least 3 months, with responses only above 60 dB or no responses at all (up to 100–120 dB stimulation). • In early deafened adults and adolescents: – Speech audiometry tested with hearing aids in a free field setting at 65–75 dB sound pressure level with a score of ≤ 50% + additional evaluation of speech intelligibility and lip-reading skills. • Cochlear nerve aplasia, confirmed by appropriate testing • Cochlear aplasia, Michel deformity • Deafness of central origin • Cochlear nerve hypoplasia • Bilateral cochlear ossification, confirmed on MRI and not accessible via drill-out procedure • Psychiatric pathology The general medical and psychological condition of the patient has to be in a state that allows elective surgery. The patient needs to be able to participate in speech-language therapy. A social support system is important around the time of implantation. • For the technique of electroacoustic stimulation, see Chapter 9. • For the technique of subtotal petrosectomy combined with cochlear implantation (see Chapter 10): – Chronic otitis media/cholesteatoma (see Chapters 10 and 14) – Presence of a radical cavity (see Chapters 10 and 14) – Cochlear ossification/obliteration (see Chapters 10 and 11) – Inner ear malformations (see Chapters 10 and 15) – Fracture of the labyrinth (see Chapters 10 and 17) – Unfavorable anatomical conditions (see Chapter 10) – Revision surgery (see Chapter 10) • For techniques in cochlear ossification (see Chapter 11): – Meningitis (see Chapters 11 and 12) – Otosclerosis (see Chapters 11 and 13) – Otomastoiditis/labyrinthitis (see Chapters 11 and 14) – Trauma (see Chapters 11 and 17) • For technique in selected skull base lesions (see Chapters 10, 16, and 17) The surgery requires proper positioning for mastoidectomy, hair shaving, and facial nerve monitoring. The use of antibiotics, the required instruments, monitoring, and other perioperative management are discussed in Chapter 4. Surgical Steps in Cochlear Implantation • Skin incision • Musculoperiosteal layer • Mastoidectomy • Posterior tympanotomy • Exposure of the round window • Drilling out of the receiver well • Fixation of the implant • Insertion of the cochlear electrode • Electrophysiologic testing • Fixation of the electrode • Closure Different skin incisions are used for cochlear implant surgery, the choice depending largely on the preference of the surgeon. The rationale behind most skin incisions is to prevent extrusion of the array and to create adequate coverage of the implant. It is important that enough space for the speech processor behind the auricle is available, without creating pressure points on the skin lateral to the subcutaneous receiver-stimulator (being stuck between speech processor and receiver-stimulator). For this reason the receiver-stimulator should not be positioned too far anteriorly. Preferably the skin incision is situated anterior-inferior to the location of the receiver-stimulator and not crossing it, protecting the receiver-stimulator maximally by covering with skin and temporal muscle. Fig. 5.1 Four types of skin incisions. When there has been previous surgery, effort must be made to use the previous (scarred) incision, because vascularization of small islands of skin created by several incisions close together can become problematic, with skin necrosis and resulting defects. Four types of skin incisions are frequently used (see Fig. 5.1): • Normal retroauricular incision • Wide C shaped retroauricular incision • Lazy S incision • Minimal access incision When previous surgery has been performed, it is best to use the same normal retroauricular incision. When a new incision needs to be made, the templates that the implant companies provide can be used to ensure that the incision line is situated just behind the position of the speech processor. The wide C shaped retroauricular incision is especially useful in well-aerated mastoids with a lot of temporal bone drilling or in surgery where more access might be necessary, as in subtotal petrosectomy approach. However, the use of this type of incision creates a wound and a scar over the location of the receiver-stimulator; this is not a favorable situation. Using the lazy S incision in adults gives proper coverage of the implant and also good access for drilling out the receiver well. In children care must be taken to avoid the mastoid tip when using this incision, because of the more lateral position of the facial nerve at the mastoid tip. Minimal access incisions can be used, but severe stretching of the skin for introduction of the implant or getting access to the middle ear and cochlea creates more problematic scarring of the skin. In minimal access surgery two incisions are sometimes used: one for getting access to the mastoid and middle ear, the other for placement of the receiver-stimulator. This creates extra risks of skin flap–related problems and infections. It is necessary to create a musculoperiosteal flap or periosteal flaps that adequately cover the cochlear implant and electrode arrays postoperatively. Some surgeons create alternating flaps in relation to the skin incision, so as to get the best coverage and protection of the implant.2 Care must be taken not to create too large a pocket underneath the temporal muscle, or between temporal muscle, the periosteal flaps, and the skin: this can lead to a void filled with hematoma and to risk of infection. The mastoidectomy can be limited when no pathology is present. An internal rim of cortical bone can be left along the cavity to hold the electrode array. Visualization of the horizontal, tympanic part of the facial nerve, the short process of the incus, the lateral semicircular canal, and sometimes also the digastric ridge is helpful for assessment of the position of the vertical, mastoidal segment of the facial nerve. These landmarks are also essential for drilling the posterior tympanotomy. The chorda tympani can be explored more inferiorly, leading to the chordal crest, which can be removed for better access. The posterior tympanotomy is then created; it has the shape of a triangle. Widening the posterior tympanotomy improves the visibility of the cochlea and round window niche and improves the working space, and it also increases the illumination by the microscope. Some surgeons prefer to identify the facial nerve before drilling the posterior tympanotomy. Inexperienced CI surgeons tend to drill not medially enough (the facial nerve is then not properly skeletonized) when starting the posterior tympanotomy: during drilling anteriorly and laterally care must be taken not to damage the tympanic annulus, which can be found just lateral to the chorda tympani (see Chapter 2, Fig. 2.12). The buttress can be left for protection of the short process of the incus, and this will give better fixation of the cochlear electrode if fixation within the posterior tympanotomy is used. Variations of the normal anatomy make the round window niche more or less visible through the posterior tympanotomy (Fig. 5.2a–c). Even a superior extension of the pneumatization of the hypotympanum can mask the real position of the niche (Fig. 5.2d). The round window can usually only be seen in a restricted view, covered by a bony overhang superiorly and a bony projection inferiorly creating the round window niche. Frequently, a pseudomembrane is present, a mucosal layer covering the niche. The overhang and inferior bony projection need to be removed to visualize the complete round window. In particular, the superior part needs to be removed to enable one to reach the scala tympani at the right axial line for insertion. This would be from a posterior-superior toward anterior-inferior position. See Fig. 5.3–Fig. 5.5. The color of the true round window is dark gray. The pseudo-membrane has a whitish aspect. In cases with fibrosis or ossification of the cochlea a white aspect of the round window can also be seen. For some implants a receiver-stimulator well has to be drilled out; templates of the implants are used, provided by the supplier. For others only a pocket needs to be created or the implant can be fixated with screws. See Fig. 5.6–Fig. 5.9. In children and adults with a thin cortical bone of the skull, drilling of the receiver well with removal of the most external circle of the tabula interna creates an island of bone that can be pushed medially. This offers the possibility of producing an adequate fit even in a shallow cortical bone. Fixation of the implant is advisable; MRI examination may be necessary at a later time and fixation of the implant is mandatory for this. Two small bony canals, superior and inferior of the receiver well, are created for fixation of the implant using a nonresorbable tie-down suture. In children the skull can be too thin for these bony canals, but tight suturing of the temporal muscle over the implant is necessary. Care must be taken not to place the fixating suture over the fragile electrodes, which could result in damage to the electrode. Damage to the dura or the emissary vein must also be avoided as a subperiosteal or even subdural hematoma can occur.3 See Fig. 5.10–Fig. 5.13. Some implants can be fixated by drilling a canal from the receiver well to the mastoid cavity for the electrode arrays to run through. This fixates the implant and also protects the most vulnerable part of the electrode, just where it exits the receiver-stimulator. Mobilization of the skin and temporal muscle to create access for the receiver-stimulator gives opportunity for developing a hematoma, with subsequent risk of infection. For this reason the skin and muscle should be mobilized as little as possible, preferably not further than a tight pocket, and a firm bandage for 24 to 48 hours is mandatory. Just before the actual insertion of the electrode array into the cochlea, the implant is placed in its receiver pocket or well and fixated. The insertion of the electrode into the cochlea is usually performed in the widest and easiest accessible scala, the scala tympani. The scala tympani approach can be performed through the round window, with or without widening of the entrance, or through a separate bony cochleostomy created inferiorly to the round window. The round window approach gives safe and easy access to the scala tympani, with a minimum amount of drilling on the cochlea, good chances of saving the residual hearing, and the smallest likelihood of vestibular complaints postoperatively.4 The landmarks of the round window are discussed in Chapter 2. The round window membrane is opened using a small hooked instrument as atraumatically as possible. The direction of the hooked instrument should be anterior-inferiorly. Care must be taken not to open the superior region, because the basilar membrane is situated immediately behind the round window. The crista fenestrae, on the inferior inside border of the round window, also called the “hook” region, forms the narrowest anatomical point to the scala tympani. It may represent a sharp obstacle during the insertion of the electrode array, with the possibility of damaging the array. In some patients this needs to be opened up more to gain proper access to the scala tympani (“margin cochleostomy”). Fig. 5.2a–d
5.1 Cochlear Implant Candidates
5.2 Etiology of Hearing Loss
5.3 Current Indications for Cochlear Implantation
5.3.1 Contraindications for Cochlear Implantation
Contraindications
Relative contraindications
5.3.2 Special Indications, Differential Work-up, and Special Surgical Techniques
5.4 Surgical Steps
5.4.1 Skin Incision
5.4.2 Musculoperiosteal Layer
5.4.3 Mastoidectomy and Posterior Tympanotomy
5.4.4 Exposure of the Round Window
5.4.5 Drill-out of the Receiver Well
5.4.6 Fixation of the Implant
5.4.7 Cochleostomy
a–c Different degrees of visibility of the round window niche.
d Superior extension of hypotympanic pneumatization.
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