2 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults

of bilateral cochlear implantation in ability.6-9 Binaural is made of a second receiving a poorer SNR. appropriate candidates.4 This article will possible by several sub-phenomena: the An improvement of 3dB on average is review research and presentations of head shadow effect, binaural redundancy, possible when compared to just listening import on the issue of bilateral cochlear and the binaural squelch effect. to the ear receiving the better SNR.13 In implantation in children and adults. effect, the SNR can be about 3dB worse The “head shadow effect” is a physical for a binaural listener and still achieve Binaural Hearing: phenomenon that is the result of the head the same speech understanding as a Subjective Impressions and acting as an acoustic barrier to monaural listener via the squelch effect. Objective Benefits and noise coming from different locations Through these binaural mechanisms in space. The ear furthest from the noise The expected benefits of bilateral (head shadow, redundancy, and source will have a more advantageous cochlear implantation are based on squelch) the binaural listener is able to signal-to-noise ratio (SNR) than the ear known deficits experienced by both understand speech with a more adverse closest to the noise source. This effect 9,14 unilateral cochlear implant users as well SNR than a monaural listener. results in an average of 6.4dB of noise as individuals with unilateral profound attenuation but can be as high as 20dB localization ability is made (single sided deafness- for high frequency speech sounds.10,11 This possible by the central auditory system’s SSD). Subjectively, adult patients with may not seem significant but increasing amazing ability to calculate minute normal hearing who suddenly lose all the SNR by that amount can result in differences in the characteristics of hearing in one ear are a good source substantial improvements in speech sound arriving at each ear. Differences of feedback concerning the benefits of intelligibility, in some listening environments in sound intensity, phase, frequency binaural hearing. It is revealing that when by as much as 50%.9,12 Interestingly, in spectrum, and arrival time are calculated adults who have had normal hearing in patients with unilateral hearing loss, the for each ear to determine the origin of both their entire life suddenly lose head shadow effect still occurs but in sound.9,15-17 The normal human auditory all hearing in one ear, these individuals their case can be a detriment if speech system can distinguish as little as a 1dB are often devastated by the loss. This originates on the opposite side of the difference in sound intensity between is true even though they have better head from their only hearing ear.11 each ear and .1msec difference in arrival hearing in their remaining normal ear time.15,17 Frequency spectrum differences than is currently possible to achieve Binaural redundancy and squelch are are caused by the variable attenuation with a cochlear implant. They invariably two central auditory processes which, of the head shadow effect and the shape report a dramatic decrease of speech when combined, improve on the distinct of the pinna on differenct frequencies. understanding in the presence of even acoustic signals arriving at each ear. The central auditory system, “knowing” relatively mild background noise as well Binaural redundancy can be thought of these spectral effects, can calculate the as marked impairment in the hearing of as the advantage to be derived from origin of sound in both the horizontal sounds originating on their deaf side. listening with two ears, over one, even and vertical planes. These mechanisms Many of these individuals withdraw when the signal received at each ear is are so accurate that the normal hearing from social and occupational arenas the same.13 For example, if speech and binaural listener has an accuracy down to that present challenging acoustic noise is presented from the same source 1 degree for identifying a sound source.17 environments. They state that in front of the listener binaural hearing “hearing” requires more conscious affords a 1 to 2dB advantage in terms These binaural mechanisms greatly and intentional effort resulting in of SNR, compared with the monaural enhance an individuals ability to fatigue by the end of the day. 5 condition. If however the speech and understand speech in quiet as well as noise signals are from spatially separated in the presence of background noise, in Binaural hearing enables optimal sources, the binaural squelch effect some instances by as much as 60% when performance of the auditory system. The comes into play. When speech and noise compared to the monaural condition.9 binaural advantage results in improved are spatially separated each ear receives It is through these mechanisms that speech understanding in quiet and the signal at a different SNR. Binaural the mature auditory system is able to in noise, as well as squelch is demonstrated by the addition process the time and intensity cues of 3 the auditory percept arriving at each ear and binaural squelch effects in noise.20 represents the greatest challenge to for optimal hearing in adverse listening These benefits are indicative of central communication in real world conditions. environments such as noisy restaurants, auditory integration of the two different The fact that bilateral cochlear implants classrooms, and group meetings. modes of stimulation, acoustic and are able to provide a recipient with Individuals with unilateral hearing are, electrical. However, such benefits have significant binaural benefit is now for the most part, unable to utilize these not been demonstrated for patients who well documented by numerous adult binaural mechanisms that are important use two microphones; one over each ear studies.25-33 These studies have shown for effective hearing in the real world.9,18 with one cochlear implant and processor that all mechanisms of binaural auditory Even with many years of experience with processing both signals rather than using processing for improved speech unilateral hearing loss central auditory a hearing aid in the non-implanted ear.24 understanding (head shadow, squelch, mechanisms are unable to compensate A bilateral microphone arrangement of and redundancy) can occur in bilateral for one-sided auditory deprivation.9,19 this type was found to have a potentially cochlear implant subjects.26 The head detrimental effect in certain listening shadow effect appears to be the most Adults conditions with regard to sound consistently beneficial mechanism localization and could potentially worsen for adult cochlear implant users. The Lessons from Bilateral Hearing Aids the SNR presented to the cochlear binaural advantage of bilateral vs. and Bimodal Stimulation implant. This supports the premise that unilateral cochlear implants for speech two distinct signals, processed by two discrimination in noise increases with The importance of providing people cochlea are required to achieve binaural more adverse SNR.32 When tested in who have hearing loss with binaural gains. It is apparent from the above more favorable listening conditions (SNR hearing has been recognized for many referenced studies of bimodal benefit that of +10dB) speech scores for Cantonese years with regard to hearing aid (HA) the mature auditory system is capable lexical tones improve an average of fitting practices.6,13,15 It is considered the of making use of whatever good quality 15-20% compared to the unilateral “standard of care” to fit patients with auditory input it has available from either condition.32 However when tested in bilateral symmetrical hearing loss with ear. It should now be considered standard more adverse conditions (SNR -10 to bilateral amplification in order to achieve practice to have all unilateral cochlear -15dB) speech scores are immeasurable binaural gains. Also, it is considered implant recipients utilize a hearing aid on in the unilateral condition, but still standard practice to fit patients with their non- implanted ear if the residual average 80% in the bilateral condition in mild to severe unilateral hearing loss hearing in that ear is capable of providing some patients.32 who have normal hearing in their other a binaural advantage. However, in our ear with a hearing aid in their poorer ear Sound localization ability is greatly 11,13 experience many cochlear implant for the same purpose. The perceived enhanced through bilateral cochlear recipients have poor residual hearing in limitations of sound quality through implants when compared to one implant their non-implanted ear and amplification hearing aids is primarily related to the alone.17,27,28 This is achieved primarily either provides no benefit or even amount and quality of residual hearing through detection of interaural intensity interferes with the hearing of the cochlear available to the individual. differences.31 Most bilaterally implanted implant ear. In such cases the only way to subjects cannot benefit from interaural The benefit of a hearing aid used provide effective binaural hearing is with timing differences with the current opposite a cochlear implant in patients bilateral cochlear implants. arrangement of separately programmed with significant residual hearing in speech processors. their non-implanted ear has also been Bilateral Cochlear Implantation 20-23 studied. Cochlear implant recipients in Adults As is true with unilateral implantation, with preoperative contralateral HA word post- linguistically deafened adults Improved speech discrimination in noise scores > 20% demonstrate the greatest receive the greatest binaural benefit likely represents the most significant bimodal (Contralateral hearing aid + for speech perception in quiet and in benefit of bilateral cochlear implantation, cochlear implant) benefit in terms of noise and sound localization ability since competing background noise improved binaural redundancy in quiet versus their prelinguistically deafened 4 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults 5

peers.27,28 This is to be expected to some sound processors.33 The subjective unilateral hearing loss if the residual extent since, in our center's experience, benefit perceived by the select few hearing can allow for good quality aided although prelinguistically deafened adults prelingual and perilingually deafened benefit that will not interfere with the perceive great benefit for environmental adults implanted at our center is self- normal hearing of the other ear.38,39 In sound awareness and lip-reading cues rated as “high” despite their more both of these instances binaural gains are with their first cochlear implant they modest measurable binaural gains. These seen that are superior to the monaural have more limited open-set speech individuals still overwhelmingly prefer the hearing condition, and optimize the discrimination than their postlingually bilateral to the unilateral condition. child’s academic and social functioning. deafened peers. Bilaterally implanted Bimodal (Cochlear implant + contralateral adults with prelingual onset of severe Children hearing aid) stimulation has also been to profound hearing loss appear to have studied in unilaterally implanted children limited ability to benefit from binaural Effects of Unilateral Hearing Loss who have residual hearing in their mechanisms except the head shadow Studies that analyze the speech-language, non-implanted ear.40,41 Children are effect.27,28 This limitation of central educational, and psychosocial impact of able to merge the different inputs and auditory processes in prelinguistically unilateral profound sensorineural hearing obtain significant binaural gains with this deafened patients supports the concept loss in children can be useful in predicting arrangement. The degree of benefit the of a “critical period” of neural plasticity the potential benefits of bilateral cochlear hearing aid provides is related to the in early childhood which will be detailed implantation in this age group. Bess, amount of residual hearing in the non- further in the discussion on pediatric Tharpe, and Gibler (1986) demonstrated implanted ear and the child’s experience bilateral implantation. that the more adverse the listening with the bimodal condition. The use of a The favorable results of bilateral situation (greater amounts of background hearing aid on the non-implanted ear may implantation in adults are consistent noise), the greater the difference in be very important in children during the with the subjective experiences of these speech recognition ability between “critical period” of auditory integration patients. Patient satisfaction and benefit children with normal hearing in both ears if future use of that ear is ever desired. 34 questionnaires overwhelmingly favor and children with unilateral hearing loss. In cases of profound hearing loss in the bilateral implantation in both prelingually These children experience far greater non-implanted ear, early bilateral cochlear and postlingually deafened adults.27,33 difficulty in school and are 10 times implantation may be the only way to more likely to fail a grade or to require achieve binaural hearing and to develop In studies for which experienced bilateral educational resource assistance. They viable bilateral auditory pathways. implant users were required to go are twice as likely to exhibit behavioral without the use of one implant for a difficulties in the classroom as their Bilateral Cochlear Implantation period of time, their great frustration binaurally hearing peers. More recent in Children with monaural hearing was similar reviews of the conflicting literature on this to that described by normal hearing topic have drawn similar conclusions.35 As mentioned in the introduction, there individuals who experience sudden This information lends support for an are currently at least 1600 bilaterally 27 sensorineural hearing loss in one ear. approach to provide binaural hearing to implanted children globally. Numerous Once they had experienced binaural children whenever possible. scientific presentations have been given hearing through two cochlear implants, at internationally significant scientific they found the impairments of monaural Hearing Aids and Bimodal meetings over the past four years hearing to be unsatisfactory. It is likely Stimulation in Children that describe the benefits bilaterally that our current measures of binaural implanted children receive and the benefit are still somewhat inadequate As in adults, bilateral hearing aid fitting active long term evaluations that are in their ability to clearly quantify what has been the standard of care for in process.42-58 Of greatest importance bilaterally implanted patients describe children with bilateral hearing loss for is the absence of any pattern of 36,37 as the “three dimensional” aspects of many years. It is also recommended adverse outcomes or complications sound they enjoy when wearing bilateral for children with moderate to severe from the various centers involved with 4 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults 5

bilateral implantation in children in the that may be underestimated in such a their hearing and speech outcome.61-63 references sited here. The more than controlled study. What is needed, as Children who do not receive their first seven year history of children who will be discussed below, are measures of cochlear implant prior to 6 years of age have had bilateral cochlear implants is central auditory development that can have significantly poorer communication encouraging.44 Children tolerate bilateral be reliably correlated with outcomes. abilities than their peers who do.62-64 simultaneous surgery well and no long- The central auditory system requires A measurable correlate of the critical term vestibular complications have been sound input in the first few years of life period is now available in the form of reported to date.44 if effective central auditory development cortical auditory evoked potentials The greatest difficulty when studying is to take place, lest irreversible changes (CAEP). These potentials reflect EEG bilaterally implanted young children is in the brain occur. This window of activity in response to sound stimulation. the complexity of measuring the various opportunity is known as the “critical” The most commonly measured P1 aspects of binaural benefit in this age or “sensitive” period and appears to wave of the CAEP is thought to reflect group and comparing their outcomes be maximal in the first 3.5 years of synaptic propagation through the to unilaterally implanted children.49,50 life, remains open in some (but not all) thalamo-cortical portions of the central The young age and developing until approximately age 7 years and auditory pathways. The P1 latency is language of these children make speech completely closes by 12 years of age.59,60 considered an index of maturation discrimination in noise and sound Because every year of opportunity in in these areas. Children who receive localization testing very challenging. the critical period that is lost is very their first cochlear implant prior to 3.5 Comparing language outcomes between difficult if not years of age unilaterally and bilaterally implanted impossible have their P1 children requires several years of to recover, Children who receive their first latencies come detailed monitoring. Testing protocols children implant after age 7 years never achieve into the range have been developed for evaluating are now normal P1 latencies, correlating with for normal sound localization ability and speech implanted the observed poorer speech and hearing understanding in noise.57,58 A pattern with their language outcomes for this age group. peers within of clearly positive benefit has emerged first device as 6 months from these various studies with regard young as 12 of implant to objective test measures as well as months of age with the expectation of an use.60,61 However, children who receive parent questionnaires.48,54 excellent speech and language outcome. their first implant after age 7 years never Just as with unilateral implantation, the achieve normal P1 latencies, correlating Since most children with cochlear age a child receives their second implant with the observed poorer speech and implants receive intensive auditory-verbal has a significant effect on the amount language outcomes for this age group. therapy (especially those participating in of benefit received and even a child’s controlled studies) the comparison of The same sensitive period and time willingness to use the second device.57 language outcomes between unilaterally course for normalization of the CAEP This is a consistent pattern in the 40 and bilaterally implanted children may is now known to exist for the second children we have bilaterally implanted, all underestimate the real world benefit implanted ear in bilaterally implanted of whom are high performing unilateral of bilateral implantation for children. It children. Even early implantation users and who ranged in age from 12 is possible, if not likely, that bilaterally and long term CI use in one ear is months to 15 years at the time of their implanted children may be able to inadequate in preserving the plasticity second implant. This is in keeping with acquire speech and language more of the auditory pathways that serve the concept of a “critical” or “sensitive” passively as an unconscious part of the opposite ear. 65 In the children period of neural plasticity and central every day life than unilaterally implanted sequentially implanted at our center auditory integration.59,61 It is now well children. This could result in a cost who have been tested to date, the known that the younger a child receives savings by reducing the frequency trajectory of P1 latency change of their first cochlear implant, the better and duration of post implant therapy the second implanted ear was similar 6 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults 7

to the trajectories of children who their second ear whose aided CAEP fails of patients. Long-term vestibular received their first cochlear implant at to show a time course of development, complaints are rare in both adult the same age at which the test subject cochlear implantation of that ear is the and pediatric populations. Anecdotal received their second implant. In other only way to achieve optimal central experience at our center suggests that words, there was limited benefit to binaural development. vestibular complaints are no greater in the central auditory system bilaterally implanted patients than in serving the second ear of the These findings call into question those after unilateral implantation. early implantation of the first the wisdom of "saving" one ear for Nonetheless, consideration of the ear. This is strong evidence that future technological developments. vestibular function and balance history a sensitive period or “window” of patients is prudent prior to both of opportunity exists for children unilateral and bilateral implantation. These findings call into question to acquire effective binaural integration the wisdom of “saving” one ear of a from their second ear. Based on Guidelines for Bilateral pediatric cochlear implant recipient for preliminary CAEP data, reported Cochlear Implantation future technological developments. If outcomes for sequentially implanted a unilaterally implanted child’s second children, and what is known about Based on published data and our center's ear does not receive auditory input outcomes with unilateral implantation experience, reasonable guidelines can during the critical period of cortical the window of opportunity for be described for bilateral implantation development, later technology, no children to maximally develop central in both children and adults. It is notable matter how advanced, will only provide binaural mechanisms from second ear that factors supporting a good outcome input to a cortex incapable of receiving implantation would seem to be highest parallel the candidacy for unilateral it. The benefit of such technology is under 3.5 years of age, intermediate implantation. In these patients, the likely to be arrested at that stage in an potential up to 7 years of age, and available data supports the theory individual’s life. If there are no effective minimal potential over age 12 years, that the benefits of bilateral cochlear connections within cortical layers and to despite being high performers with implantation significantly outweigh the higher-order auditory and language their first implant. 57,59,65 the risks and can be considered an cortex, even hair cell regeneration will appropriate treatment. It should not be It is reasonable to assume, and consistent be of limited benefit after the critical- automatically assumed that patients who with observations at our center, that the sensitive period has elapsed, whether or fail to meet these guidelines are ineligible use of a hearing aid in a child’s non- not the individual is an effective unilateral or that they will not benefit from implanted second ear will enable some cochlear implant user. bilateral implantation. Further research maturation of the auditory pathways is needed to define other categories of for that ear, enabling easier transition Complications of Bilateral candidacy and benefit in patients with to a cochlear implant later on. This will Cochlear Implantation more complex histories. Such patients occur, however, only to a degree related must have all factors carefully considered Unilateral cochlear implantation is to the amount of residual hearing. on an individual basis. Until implantation of the second ear now a common procedure with few takes place, hearing aid use in that ear complications. With regard to bilateral Simultaneous Implantation: It has should be strongly encouraged if any cochlear implantation, the effect on been our experience that this should auditory benefit can be obtained from the vestibular system has been the be reserved for only the most ideal doing so. This will likely maintain some complication of greatest concern. The candidates and we have attempted degree of central auditory preparedness effect of unilateral and bilateral cochlear to develop guidelines to assist in for later implantation of that ear. The implantation on the vestibular system the decision between simultaneous 67-70 effectiveness of a hearing aid for such has been studied in detail. Vestibular and sequential implantation for our purposes can be monitored with CAEP.66 symptoms are often present for a few patients. Simultaneous surgery allows In children with profound hearing loss in days but quickly abate in the majority a more seamless adjustment to the 6 January 2006 Rationale for Bilateral Cochlear Implantation in Children and Adults 7

two devices, particularly in young implantation and the continued use of in the first implanted ear should raise children. If a child or adult has factors a hearing aid on the ear with the most significant concern, which must be that raise concern, a sequential favorable responses. There should be addressed on an individual basis. approach should be considered. normal cochlear anatomy, no other Children should be good-to-excellent neurological or medical comorbid In our center’s guidelines, adults unilateral implant users and their factors, and strong, conscientious should have a history of adult onset families have shown themselves parental involvement. hearing loss (beginning in late teenage conscientious in therapy. The years or after) to assure full binaural Sequential Implantation: The sequential age at which a second implant is cortical development. They should approach allows the patient to continue contemplated matters a great deal have the most favorable hearing loss hearing aid use in the non-implanted unless consistent hearing aid use has history; that is, a short duration of ear for later determination of bimodal continued in that ear. In the absence profound hearing loss of less than 15 benefit before committing the second of consistent contralateral hearing aid years or the continued consistent use ear to implantation. It also allows the use, we consider children less than of amplification in both ears up to the center to ascertain the response to the eight years of age to be the most time of implantation. There should be first implant and the conscientiousness of ideal candidates. We are reluctant no prior history of vestibular disorders the patient and/or parents in follow-up. to implant the second ear of children or recurrent vestibular symptoms such over 12 years of age who did not Adults are considered for sequential as Meniere’s disease. Aided CNC word use a hearing aid in the second ear bilateral implantation based on the scores should be < 30% in each ear with at least until six years of age. With successful use of the first implant and normal cochlear anatomy in both ears. If continued hearing aid use, children the degree of bimodal benefit derived one ear has word scores >30% unilateral of any age can be considered good from an optimally fitted hearing aid in implantation should be considered first candidates. For this reason, we strongly the second ear. The second ear should to allow testing in the bimodal condition. encourage continued hearing aid use have a favorable hearing loss history Sequential bilateral implantation can in our unilaterally implanted children. (useful hearing either with or without always be considered. Audiometric assessment of bimodal a hearing aid into the teenage years). benefit is used in older children but Children should be 12 to 36 months Implantation of that ear is likely to be can be difficult in younger children of age with profound hearing loss advantageous if testing in the bimodal because of language limitations. CAEP in both ears and no notable aided condition fails to increase word scores by on the aided ear can be used to benefit. Preferably aided CAEP can 10% or sentence scores by 20% in both determine if the hearing aid input is be performed prior to implantation quiet and in noise (SNR +10dB) when sufficient for central auditory system to ascertain if any signs of cortical compared to the implant only condition. development. If so, hearing aid use is development are present with either A history of prior vestibular disorders continued in lieu of implantation. ear that would prompt unilateral or the absence of vestibular function

Conclusion: Provision of binaural hearing should be considered the standard of care for hearing-impaired patients whenever it can be provided without significant risks. In severe to profoundly hearing impaired individuals, this can only be provided with bilateral cochlear implantation when hearing aids are inadequate. In carefully selected candidates, the benefits derived are significant, the surgical procedures well tolerated, and negative effects infrequent in both children and adults. Future research will hopefully help define the roll of bilateral cochlear implantation in patients with less favorable historical factors and challenging co-morbid conditions as well as improving binaural gains with newer processing strategies and more advanced devices.

Author Contact Information: Dr. B. Robert Peters President, Dallas Hearing Foundation • Director, Dallas Otolaryngology CI Program email: [email protected]