Surgery for Cochlear Implantation

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Chapter 31 Surgery for Cochlear Implantation

William L. Luxford, Robert D. Cullen

image Videos corresponding to this chapter are available online at www.expertconsult.com.

There have been remarkable advances in cochlear implantation. Although the first attempt to stimulate the auditory system electrically occurred nearly 2 centuries ago, the development of a cochlear prosthesis to restore hearing to patients with sensorineural hearing loss has happened only over the past 5 decades. The early pioneering work of Simmons, Michaelson, and House provided the stimulus to encourage others, including Bonfai, Chouard, Clark, Eddington, and the Hochmairs.1 The initial acceptance of cochlear implants was slow; safety and efficacy were the concerns of the early investigators, and the greatest champions of the implant were the patients themselves. Time and technology have increased the benefits gained by most patients from their cochlear implants. As a result, cochlear implants have become the most successful prosthesis ever used to attempt to restore a sensory deficit.

As of the writing of this chapter, more than 120,000 patients have received cochlear implants worldwide. More than 50,000 patients have received cochlear implants in the United States. In the United States, the Food and Drug Administration (FDA) regulates the manufacturers of cochlear implants. The FDA has approved devices from Advanced Bionics Corporation (Sylmar, CA), Cochlear Corporation (New South Wales, Australia), and MED-EL Corporation (Innsbruck, Austria) for use in adults and children.

Incremental improvements in technology have resulted in the approval of multiple devices over the years. As performance with cochlear implants has improved over the years, the criteria for cochlear implantation have expanded (Table 31-1). In more challenging situations, bilateral cochlear implantation has been shown to provide additional benefit beyond a single cochlear implant, and is now considered an accepted medical practice outside of research protocols.2 Prospective studies continue to examine the benefits of bilateral cochlear implantation in adults and children.

TABLE 31-1 Criteria for Cochlear Implantation

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PATIENT SELECTION

Candidate selection for cochlear implantation has evolved as the devices and patient performance have improved. Generally, adults and children with bilateral severe to profound sensorineural hearing loss, who receive little to no benefit from conventional hearing aids, are in good physical and mental health, and possess the aptitude and motivation to participate meaningfully in the auditory rehabilitation program are potential candidates for cochlear implantation.

MEDICAL EVALUATION

A thorough medical evaluation is necessary to detect problems that may contraindicate surgery or interfere with the patient’s ability to complete postimplantation rehabilitation. Rarely, the cause of hearing loss may be a contraindication to surgery. Aplasia of the cochlea and aplasia of the cochlear nerve are contraindications to surgery because there are no auditory nerve elements to receive stimulus from the implant. A prior history of meningitis with cochlear ossification or fibrosis does not exclude the patient from implantation, but may necessitate modification of the surgical technique.

Two crucial factors influencing auditory performance after cochlear implantation include age of onset of deafness and duration of profound hearing loss. The ideal adult candidate has profound acquired sensorineural hearing loss. A period of auditory experience adequate for development of normal speech, speech perception, and language offers a significant advantage in learning to use the implant. These postlingually deafened patients represent most adults undergoing cochlear implantation. In these patients, there is a significant correlation between duration of profound hearing loss and performance.3 Patients with prolonged auditory deprivation receive similar auditory information as do other implant patients, but are unable to use the information as effectively; this is thought to be due to the loss of central auditory processing. A few adult implant recipients are congenitally or prelingually deafened, with prolonged auditory deprivation and little to no experience with sound. These patients typically have greater difficulty assimilating the new auditory information, and generally have performed less well than patients with some degree of auditory memory.

The presence of auditory memory associated with delayed onset of deafness also correlates with improved speech and language skills in children after cochlear implantation. Congenitally deafened children receive the maximal auditory benefit from cochlear implantation by undergoing cochlear implantation at the youngest age possible. Current FDA guidelines require a child to be at least 1 year old, have profound sensorineural hearing loss, show no benefit from conventional hearing aids, and be free from medical contraindications. In addition to these criteria, the commitment of the family and the child’s educational setting to postimplantation rehabilitation is a crucial determinant of successful implant use.

Audiologic Assessment

The basic testing battery included aided and unaided pure tone and speech detection thresholds, environmental sound recognition, and speech perception tests. Generally, a patient is considered an audiologic candidate for cochlear implantation if the following criteria are met: bilateral severe to profound sensorineural hearing loss, with an unaided three-frequency pure tone average of 70 dB hearing loss or poorer in the better ear, a speech discrimination score of less than 50% in the ear to be implanted, and a speech discrimination score of less than 60% in the best-aided binaural condition.

Audiologic screening in children requires auditory brainstem response testing and otoacoustic emissions testing, in addition to conventional behavioral audiometry. An aggressive trial of appropriately fitted hearing aids and intensive auditory and speech training are integral components of candidacy assessment in children. The global evaluation of cochlear implant candidacy in children is considerably more challenging than in adults, and is best approached by a dedicated team comprising speech and hearing professionals, social workers, psychologists, and educators. The ultimate candidacy of a child is determined not only by a demonstrated physiologic need, but also by the strength of the child’s social and educational background.

Physical Examination

It is important to identify preoperatively any external or middle ear diseases, including perforations of the tympanic membrane, that must be treated before cochlear implantation. For young children, the size of the implant in relation to the size of the child’s skull must be evaluated, and issues involved with skull maturation must be considered. The distance between the cochlear promontory and the mastoid cortex, the approximate sites of the electrode array, and the receiver-stimulator increases about 1.7 cm from birth to adulthood, with one half of the increase occurring during the first 2 years of life.4 The electrodes must be long enough to tolerate the increase in height and width of the skull that occurs with the child’s growth. The accommodation occurs through the gradual straightening of the excess electrode length within the air-containing mastoid cavity.5

Imaging

Preoperative imaging completes the candidacy evaluation process and assists in surgical planning. Choice of imaging modality varies at individual implant centers. High-resolution computed tomography (CT) scanning of the temporal bone and magnetic resonance imaging (MRI) are currently used. CT scanning provides excellent detail of inner ear morphology, the course of the intratemporal facial nerve, and aeration of the mastoid. Limitations of CT include the inability to evaluate retrocochlear pathology and cochlear patency in patients with fibrosis of the cochlea as a precursor to cochlear ossification. The ability to detect an absent cochlear nerve also is limited. MRI addresses these limitations,6 but this imaging modality incurs additional expense and may require general anesthesia in young patients.

Complete agenesis of the cochlea and an abnormal acoustic nerve resulting from congenital malformation, trauma, or surgery are contraindications for cochlear implant placement.7 Cochlear hypoplasia (present in Mondini’s deformity) is not a contraindication for cochlear implantation. Adults and children with incomplete congenital cochlear malformations have received implants successfully.8 Ossification or fibrous occlusion of the cochlea or the round window does not exclude a patient from implantation, but it may influence outcome. Occlusion of the cochlea may lead to partial insertion of the electrode carrier. MRI has become more useful than CT in the evaluation of the membranous inner ear in detecting cochlear fibrosis.

Promontory Evaluation

A few implant teams perform an electric stimulation test at either the promontory or the round window membrane.9 A positive response is a perception of sound on stimulation. Some investigators do not believe that such testing is crucial in the selection of candidates because patients with a negative response, particularly at the promontory, may respond to intracochlear stimulation with an implant. Promontory stimulation may be helpful in cases of suspected cochlear nerve agenesis.

SELECTION OF EAR

Selection of the side for implantation is governed by several factors. The most patent cochlea is typically chosen for implantation. It is generally believed that the ear with the shortest duration of deafness may serve as the best ear for implantation. If the patient uses a hearing aid in only one ear (the side that is perceived as the better hearing ear), implanting the contralateral “worse” ear does not have a negative impact on performance. When no specific factors lead to the choice of one ear over the other, the ear on the side of the dominant hand is chosen to facilitate device manipulation.

If there is no difference acoustically, we place the implant in the better surgical ear, based on CT evaluation. The side with the least ossification or fibrosis within the scala tympani is chosen. Results from vestibular tests should be given the least weight in the selection of the side of cochlear implantation. The ear with the least caloric response should receive the implant. For patients undergoing bilateral cochlear implantation, the need to select one ear over the other is obviated.

SURGICAL TECHNIQUE

The cochlear implant should be implanted only by qualified surgeons specifically trained to perform the procedure. Although specific details of implantation vary based on the device implanted, the general approach to implantation is the same.

The implant is inserted via a transmastoid facial recess approach to the round window and scala tympani. Placement of cochlear implants in children is essentially the same as in adults. By age 1 year, the mastoid antrum and facial recess are adequately developed. In patients with mastoid cavities or absent posterior ear canal, obliteration of the mastoid cavity with blind sac closure of the external auditory canal is preferably done at the time of disease removal. Cochlear implant placement is performed at a second stage approximately 4 to 6 months later.

Surgery is performed with the patient under general anesthesia with the use of continuous intraoperative facial nerve monitoring. Perioperative antibiotics are routinely administered before making the initial incision. Many different incisions have been designed to allow placement of the receiver-stimulator (Fig. 31-1). Generally, the skin flap must be large enough to cover the receiver-stimulator completely. The length of the incision and size of the skin flap have been reduced over the years. By reducing incision and flap size, there is less interruption of the vascular supply to the operative field; this seems to correlate with fewer wound and flap complications. Skin and periosteal incisions should overlap by at least 1 cm; the skin incision should be at least 1 cm anterior to the front edge of the receiver-stimulator (Fig. 31-5).

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FIGURE 31-1 Incision. The shape of the incision may vary from surgeon to surgeon, but it is important to maintain 1 to 2cm from the edge of the implant to the incision. A small postauricular incision (2) is planned close to the postauricular crease (1). A periosteal incision (3) is made 1 cm posterior to the skin incision. By separating these incision lines, device extrusion is less likely to occur. The periosteum is undermined to allow placement of the implant in a tight sub-periosteal pocket. This serves to immobilize the implant tightly against the skull.

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