Cochlear Implants: Perspectives from an Otologist

Address Correspondence to:
Eric W. Sargent, MD
The Michigan Ear Institute
30055 Northwestern Hwy, #101
Farmington Hills, MI 48334

FAX: (248) 865-6161
Telephone: (248) 865-4444
Email: sargente@mac.com

Introduction:

The cochlear implant is an electronic prosthesis that stimulates cells of the auditory spiral ganglion to provide a sense of sound to persons with hearing impairment (Figure 1).
While Cochlear Implants (CIs) are often thought of as the "last resort" for patients with severe or profound hearing loss, current FDA guidelines allow implantation in patients who get some benefit from conventional hearing aids.

The impressive results obtained with CIs over the last few years have ignited consumer and professional interest in this device.

While patient-specific outcomes for individual patients cannot be predicted, we do know that the range of results is highly variable and depends on a number of physical and psychosocial factors (see below). The trend towards improved performance with increasingly sophisticated CI hardware and software has caused the indications and applications for cochlear implantation to be dramatically expanded.

Although cochlear implants were originally touted as an aid to speech reading for profoundly hearing impaired patients, it is now common to find the speech perception of
implanted patients exceeding the performance of conventional "hearing aid-aided" patients.

CI Demographics:

The number of patients worldwide who have received cochlear implants exceeds 50,000. Approximately 6,000 units are implanted across the world annually, and the trend continues to increase.

Although no one really knows the exact number of people in the USA who have severe and profound hearing loss, Blanchfield, Feldman and Dunbar (1999) used three data sets to estimate the probable quantities of people within these two hearing loss categories. Their estimates indicate a probable range of between 464,000 to 738,000 people within the USA who have severe or profound hearing loss.

CI Outcome Issues:
Four factors are of primary importance in determining the outcome of CI patients: 1) Age at onset of deafness, 2) Duration of deafness prior to implantation, 3) Progression of hearing loss, and, 4) Educational setting.

Generally speaking, earlier implantation and a lesser duration of hearing loss favor more rapid development of oral communication ability. Progressive hearing loss, as opposed to immediate profound hearing loss, allows for development of speech-reading skills and also favors post-implant performance. Placement in school settings that stress oral (versus signed) communication is important for the best outcome of implantation. However, many variables remain unknown since about 50% of the variance in post-implant performance cannot be predicted from these factors. While the strongest predictors of performance include are duration of deafness and age at implantation (Gantz, 1988), these factors account for only part of the variance in cochlear implant performance.

Children and CIs:
Childhood Deafness:

Congenital or acquired severe to profound loss sustained prior to the development of language is estimated to occur between 0.5/1000 to 4/1000 births. The most common cause of childhood deafness is genetic hearing loss, accounting for approximately one-third to one-half of all cases. A large number of "genetic" hearing loss cases are single gene mutations. Approximately one-quarter to one-third of all cases of childhood deafness are due to 'non-genetic' or environmental causes. Lastly, approximately one-quarter to one-third of all cases are sporadic, for which the etiology is not determined. A large proportion of 'sporadic' cases will likely prove to be genetic in origin.

Meningitis:

Meningitis causes about 9 percent of childhood deafness and can make implantation difficult (see labyrinthitis ossificans, below). Of the organisms that commonly cause meningitis (from most common to least common: H. influenzae, S. pneumoniae, and N. meningitis), the organism with the highest incidence of hearing loss is S. pneumoniae (31%). Factors that will likely reduce the frequency of deafness caused by meningitis include the increased use of immunization against H. influenzae and the acute administration of dexamethasone in meningitis, which reduces the incidence of post-meningitic moderate or profound hearing loss from 15.5 to 3.3%.

Candidacy Issues for Children:

Audiologic criteria for pediatric patients follow guidelines similar to adults. The pure tone average should equal or exceed 90 dB (HL). The child should be receptive to wearing a hearing aid before being implanted since all CIs require wearing an external processor. A period of appropriate hearing aid use to determine the development of aided communication ability is the critical criterion for determining candidacy in young children.

After determining that audiologic criteria have been met, parental expectations and attitudes should be carefully assessed. Unrealistic expectations can frustrate and negatively impact the efforts of the child, the parents and the implant team. When counseling patients and their families, realistic expectations, the probable need for long-term auditory rehabilitation, the variable outcomes of implantation and the limitations of implantation should be stressed.

Radiographic Issues in Children:

In pediatric or young adult patients with progressive hearing loss, neurofibromatosis II should be excluded with a Magnetic Resonance Imaging (MRI) scan before proceeding with implantation. Although Computed Tomography (CT) has been the primary tool for evaluating cochlear patency, occasional CT scans erroneously suggest a patent scala tympani that at surgery proves to be obstructed with bone or fibrous tissue. MRI offers the ability to better examine the fluid spaces of the cochlea and is increasingly the primary modality for imaging the cochlea.


Figure 1. Labyrinthitis ossificans. The cochlea on the left shows obliteration by bone after meningitis. The scala tympani of the cochlea on the right was patent and the patient underwent successful implantation with complete electrode insertion.


Figure 1 shows the CT of a child deafened by meningitis whose left cochlea is ossified. In this child's case, successful implantation of the patent right cochlea was accomplished. For patients at risk of labyrinthitis ossificans, implantation at the time early ossification or fibrosis is identified may be indicated. Implant teams may follow patients newly deafened by meningitis with serial imaging and implant at the first sign of replacement of the scala tympani with fibrous tissue or bone. Otherwise, implantation in cases of post-meningitic deafness is usually recommended after a six-month period to allow for the possible recovery of aidable hearing in at least one ear.

Minimal Age Requirements and Other Issues:

While 12 months is currently the FDA age limit on implantation for the Cochlear Corporation device and 18 months for the Advanced Bionics Corporation device, there may be factors that cause the implant team to proceed before that time. In particular, a child deafened by meningitis may develop labyrinthitis ossificans (filling the cochear duct - usually the scala tympani starting near the round window - with bone) that may necessitate special techniques to implant and may render the result of implantation sub-optimal.

Adults and CIs:

The patient's general health should be considered before implantation. Poor general health is rarely a contraindication to implantation. There is no established upper age limit for implantation.

Hearing Aids:

As with children, adult candidacy can only be determined by performing tests for speech recognition under best-aided conditions. If patients present to the cochlear implant team with inadequate or malfunctioning hearing aids, a trial period with appropriate hearing aids should be undertaken before determining CI candidacy.

Certainly there are patients who were appropriately fit with hearing aids many years ago, who are no longer wearing appropriate hearing aids due to further progression of their hearing loss. Additionally, there are patients who are unable to get maximal benefit from their hearing aids due to old, poor fitting, leaking or inappropriate earmolds, hearing aids needing repair, sub-optimal hearing aid selection and also due to underpowered hearing aids. Hearing aids available in 2002 are more powerful and much better technically, than those available previously. In brief, hearing aids need to be maximized in order to appropriately test the "best-aided" conditions.

Counseling and Aural Rehabilitation:

After determining that audiologic criteria have been met, expectations and attitudes should be carefully assessed. As was mentioned above, unrealistic expectations can frustrate and negatively impact the outcome of cochlear implantation. When counseling patients and their families, realistic expectations, the probable need for long-term auditory rehabilitation, variable outcomes and limitations of implantation should be stressed.

For example, for some patients the implant may give only improved awareness of environmental sounds and improved speech reading ability. Others will be 'transparent' users. That is, they will be able to converse with hearing individuals with little evidence of hearing loss, they'll use a telephone and perform well in difficult hearing environments. While increasing numbers of implant users perform in the latter group, most occupy an area between the two extremes. Cautious, conservative counseling with realistic expectations and peer group referral is typically recommended.

Radiographic Imaging in Adults:
A
s in children, imaging of the cochleo-vestibular structures prior to implantation is mandatory in adults. The CT or MRI may detect cochlear malformations or ossification of the cochlea that mandate a change in the selection of which ear to implant, or the technique used.

Meningitis in Adults:

Adults deafened by meningitis are treated in a manner similar to children. Because of the fairly high rate of recovery of hearing in at least one ear following meningitis, an observation period of at least 6 months should be allowed prior to cochlear implantation. As in children, identification of incipient obliteration of the cochlea by fibrous tissue or bone may cause implantation to be recommended early. Thus, serial imaging may be indicated for surveillance.

Socio-economic challenges:

Although cochlear implantation can present a number of surgical challenges, the greatest impediment to the use of this technology is worsening financial support for surgery, post-operative programming, and audiologic rehabilitation from the government and 3^rd party payers.

Additionally, the number of patients receiving CIs represents only a small fraction of the individuals who could benefit from implantation, but are denied access to a CI due to a failure to recognize and refer appropriate individuals and importantly, due to a lack of financial resources.

Surgical Challenges:

Although generally a straightforward operative procedure, a variety of acquired and congenital conditions can complicate placement of a CI electrode.

Imaging, either using CT or MRI, is always performed prior to implantation to evaluate the cochleo-vestibular apparatus and internal auditory canals. As in Figure 1, it can reveal the absence or abnormal caliber of the internal auditory canal and/or cochlear dysplasia. This may alter the choice of the implanted ear or raise other issues.

The patient in Figure 2, for example, has an abnormal internal auditory canal on the left that suggests the absence of a VIIIth nerve. The cochlear dysplasia on the right is not a contraindication to implantation of that ear, since satisfactory post-implant performance in dysplastic cochleas has been reported.


Figure 2. Cochlear malformations. The neural foramen on the right is absent. The right arrow indicates a rudimentary vestibule. On the left is a severe cochlear malformation (Thick arrow). The small arrow indicates the internal auditory canal.

However, the surgeon should be prepared for a cerebro-spinal fluid leak due to an incomplete partition between the cochlea and internal auditory canal. The patient and family should be informed of the potential risk of cerebrospinal fluid leak and meningitis and the variable performance of patients with dysplastic cochleas who undergo implantation.
Ossification of the cochlea, shown in Figure 3, is seen following meningitis.


Figure 3. Cochlear implant electrode passing through the facial recess to the scala tympani.

Because the process obliterates the lumen of the scala tympani and, less commonly, the scala vestibuli with fibrous or bony tissue, electrode insertion may be incomplete or impossible. For these situations, a variety of strategies have been devised to place electrodes in proximity to the spiral ganglion.

In some cases, more extensive drilling of the cochlea may reveal a patent scala tympani distal to an obstructed basal turn. Not uncommonly, further drilling exposes a patent scala vestibuli. This can be recognized by its smaller lumen. While placement of the electrode in the scala vestibuli affords some benefit, it is a fairly poor second to the scala tympani since the electrical contacts will be some distance from the cell bodies of the spiral ganglion.

Patients with partial or complete cochlear obstruction may benefit from dual-array electrodes available from 2 implant companies, specifically Cochlear and Med-El. These electrodes divide the electrodes between 2 leads. One lead is inserted into the cochlea near the round window and the other into the cochlea anterior to the stapes. Dividing the array potentially places more stimulating electrodes in proximity to the surviving neural elements of the cochlea.

Off-Label Implantation:

Patients lying outside current guidelines who, in the judgment of an implant center, would benefit from implantation, are common. While the FDA reviews and sets criteria for the use of implants as a medical device, patients can be implanted outside guidelines - so-called 'off-label' implantation. However, there are important ethical, legal and economic considerations, as follows:

First, use of a medical device off-label, especially implants that involve risk to the patient, needs to be carefully discussed with the patient during preoperative counseling. If patients do not meet current FDA criteria for the device, they need to be informed and the alternatives reviewed. Realistic expectations and potential complications need to be reviewed. If the device is legally marketed in the USA, "off-label" use of a device is considered to be the practice of medicine and is a decision between the physician and the patient (or patient's family in the case of pediatrics). The manufacturer of the device may not promote the device for off-label use.

Second, besides the obvious medical and ethical implications, off-label implantation may have significance to 3^rd party payers. For example, if a device is used off-label, Medicare and other 3^rd party payers may refuse payment.

Conclusions:

Cochlear implantation is an exciting technological advance in the treatment of the hearing impaired. However, rapid expansion of clinical indications for cochlear implantation has come with increased complexity. This is apparent in the last column of Table 1 where adult and pediatric indications are fragmented and criteria approved by the USA FDA differ between competing manufacturers.



Audiologists skilled in the administration and interpretation of tests, particularly in children, are needed to properly determine cochlear implant candidacy.

While cochlear malformations and cochlear obliteration by meningitis may present surgical challenges, they are not by themselves contraindications to implantation.

References:

Blanchfield, B.B., Feldman, J.J., and Dunbar J.L: The Severely to profoundly Hearing Impaired Population in the United States: Prevelance and Demographics in https://www.bionicear.com/

Cohen NL, Waltzman SB, Fisher SG, et al. A prospective, randomized study of cochlear implants. NEJM 1993; 328:233-7.

Gantz BJ, Tyler RS. Knutson JF, et al. Evaluation of five different cochlear implant designs: audiologic assessment and predictors of performance. Laryngoscope 1988; 98: 1100-06.

Gantz BJ, Woodworth GG, Abbas PJ, Knutson JF, Tyler RS. Multivariate predictors of audiological success with multichannel cochlear implants. Ann Otol Rhinol Laryngol 1993; 102:909-16.

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