Adult Bilateral Cochlear Implantation - technical
Sequential bilateral cochlear implantation: The placement of a cochlear implant in each ear where the individual surgeries are separated by a given time period.
Simultaneous bilateral cochlear implantation: The placement of a cochlear implant in each ear where the devices are placed bilaterally during the same operation.
Since the introduction of cochlear implants (CI) over 25 years ago, tens of thousands of patients worldwide with bilateral, severe to profound sensorineural hearing loss (SNHL) have undergone cochlear implantation for hearing rehabilitation. According to the U.S. Food and Drug Administration (FDA), as of December 2010, approximately 219,000 people across the world have received CIs. In the United States alone, roughly 42,600 adults have undergone cochlear implantation.
In the infancy of CI technology, candidacy for surgery was strict, and only adults with profound, bilateral sensorineural deafness were considered. Furthermore, monaural implantation was the rule. Unilateral CI users were able to achieve benefits such as improved sound and speech perception, normal or near-normal auditory and verbal language abilities, and tinnitus suppression, to name a few. Over the next decades, with the continued improvement of speech-coding strategies and modernization of hardware and software design, speech perception achieved by profoundly deaf patients with a CI approached that of normal hearing listeners. As a result of these successes and the refinement of CI surgery, audiologic candidacy criteria broadened to include patients with measurable amounts of residual hearing and word recognition, various etiologies for hearing loss, prolonged periods of profound deafness, and elderly patients with associated comorbidities.
Despite these tremendous successes with monaural CI, many of these patients continued to suffer disabilities associated with unilateral deafness such as the inability to localize sound and difficulties with speech comprehension in noisy listening environments when compared to peers with binaural stimulation. As the literature expanded supporting the benefits of binaural hearing, selected patients with residual hearing in the non-implanted ear were fit with traditional amplification devices, and using bimodal stimulation were able to recover some of the advantages seen in normal binaural listeners. For many CI patients, however, the non-implanted ear derived no significant benefit from amplification due to profound deafness and bimodal stimulation was not possible. For this growing subset of patients, bilateral cochlear implant (BCI) surgery was an important new era in hearing rehabilitation.
Since the first report of BCI in 1988, BCI users have demonstrated significantly improved speech understanding in quiet and in noise, improved sound localization, subjective reports of reduced social isolation, reduced subjective perceptions of hearing disability, and a trend toward reduced emotional distress compared to their unilaterally implanted peers (Peters et al. 2010). Based on these positive results and similar data in the literature, BCI is now considered “accepted medical practice” and has been endorsed by medical and professional societies from around the world (Balkany et al. 2008).
Advantages of Bilateral Cochlear Implantation
Sound localization in the horizontal plane
Sound does not have a physical dimension in space, and therefore, localization of sound and improved speech recognition requires binaural processing by the human auditory system utilizing cues such as interaural time differences (ITD) and interaural latency differences (ILD). This type of binaural benefit is seen in normal hearing listeners and individuals using bilateral hearing aids (HA) and more recently has also been reported in bilateral cochlear implant (BCI) recipients. ITD is the difference in arrival time of a sound between the ear closest to the source and the ear furthest away from the source and is useful only at low frequencies (<800 Hz). Conversely, ILD is the difference in loudness level (amplitude) between the two ears based on the location of a sound source closer to one ear, and is useful only in the higher frequencies (>1,000–2,000 Hz). Both ILD and ITD can be used to provide binaural information in the midrange frequencies, and this is referred to as the duplex theory of sound localization. While most single-sided CI users localize sound at or near a chance level, recent data suggests that BCI users have improved discrimination of ILD and ITD and are therefore more proficient at sound localization in the horizontal plane (van Hoesel and Tyler 2003).
Head shadow effect
The head shadow effect produces the most robust binaural advantage in the hearing-in-noise condition ranging from 4 to 7 dB, and can attenuate high frequency sounds by as much as 20 dB while offering 2–6 dB attenuation in the lower frequencies (Laszig et al. 2006). When a listener is presented with sound, the sound arriving at the ear further from the source is attenuated relative to sound arriving at the ear nearer the source, with the head producing an “acoustic shadow.” Utilizing both ears can provide separation of the acoustic signal from the noise source creating an improved signal to noise ratio (SNR) in the protected ear. This phenomenon allows the listener to selectively attend to the ear with better SNR and improve speech intelligibility in noise. While the head shadow effect is not a direct function of central auditory processing (CAP), it is the physical consequence of sound diffraction around an object and benefits for hearing-in-noise are immediately perceived following BCI (Gantz et al. 2002; van Hoesel and Tyler 2003; Murphy and O’Donoghue 2007).
In contrast to the head shadow effect, which relies purely on the physical separation of signal and noise to localize a sound, binaural squelch is a CAP effect by which both ears combine to create a signal at the auditory cortex different than could be achieved from either single ear alone. When competing noise is spatially separated from the target signal on the horizontal plane, each ear receives different inputs (often referred to as the “cocktail party effect”). Adding an ear with a poorer SNR allows the auditory system the opportunity to compare timing, amplitude, and spectral differences between the two signals to provide better speech understanding in noise. The benefit obtained when comparing the better unilateral condition to the bilateral condition is reported as squelch (also known as binaural unmasking) (Basura et al. 2009). The binaural advantage from squelch is limited, with estimations of improvement on the order of 3 dB. However, squelch does not appear to be an immediate effect following BCI. Instead, the effects require months or years of binaural listening before the advantages are realized, perhaps indicating a necessary central neural reorganization for bilateral input following years of deafness in adults receiving BCI.
Similar to squelch, binaural summation is also a result of complex CAP which occurs in the binaural listening condition but results in a more modest benefit (1.5–3 dB) than squelch or the head shadow effect. When both ears are presented with a similar signal, the perceived sound is louder than that presented to either single ear alone. The doubling of perceptual loudness, called binaural summation (or binaural redundancy), is also accompanied by increased sensitivity to differences in intensity and frequency, leading to improvements in speech intelligibility in both quiet and, more importantly, in noisy conditions. Binaural summation has been shown to offer BCI recipients improved speech understanding in noise when compared to single CI users (Litovsky et al. 2006).
Speech perception in quiet
Following the initial introduction of cochlear implant technology, for many years post-implantation, objective performance testing for subjects with either unilateral or bilateral CI relied on speech testing in quiet. A number of studies in the literature clearly proved that patients performed better on these measures using bilateral electrical stimulation (i.e., BCI) when compared to unilateral CI. Many of these trials also showed that performance continued to increase over time with the bilateral condition outperforming either unilateral ear alone (Laszig et al. 2006; Litovsky et al. 2006; Tyler et al. 2007; Buss et al. 2008). These results appear to be durable and replicated over the lifetime of the CI. Speech perception in noise. As speech processor technology improved, asymptotic performance results in speech in quiet testing conditions were noted in unilateral CI and BCI users alike. In addition, patients’ demands increased, prompting postoperative performance outcomes to be judged by the user’s ability to hear in noisy environments rather than in quiet. Adding a second CI enables a listener to take advantage of an ear with a better SNR, resulting in improved speech perception in noise as a result of the head shadow effect (Gantz et al. 2002; van Hoesel and Tyler 2003). This finding is especially important in the hearing rehabilitation of patients frequently exposed to environments with a substantial background noise floor that cannot be easily altered to aid the monaural listener. Adding an ear with a poorer SNR also allows binaural squelch to provide better speech understanding in noise, although the advantages of squelch are much less than those from head shadow or binaural summation effects (Litovsky et al. 2006). Buss et al. (2008) demonstrated that binaural squelch benefits in BCI patients continued to increase over time from 6 months to 12 months following simultaneous bilateral CI surgery. While results uniformly show improvements for speech perception in noise for BCI users, individual results tend to vary, suggesting the possible influence of specific patient variables such as hearing loss characteristics and duration, age, residual hearing levels, and speech-coding strategies.
Practical Advantages of Bilateral Cochlear Implantation
There are several other advantages of BCI that are not related to binaural hearing effects. First of all, BCI guarantees that the better hearing ear is implanted. While speech perception testing scores, length of deafness, etiology of deafness, and macroscopic anatomy may be identical between the two ears, it is possible that microscopic factors such as neuronal integrity or hair cell survival between the two ears differ explaining, at least in part, why one ear may demonstrate improved performance with electrical stimulation compared to the contralateral ear. This makes predicting the better ear prior to implantation difficult and a process potentially fraught with errors. Furthermore, if each ear provides separate encoding of different speech cues and auditory information, then the synergistic effect of BCI may support better fine structure analysis of speech and result in better performance than a single cochlear implant can provide.
While the reliability of the newest generation cochlear implant devices has continued to improve, occasionally, there is a device malfunction either related to hard failure (hardware malfunction demonstrated on either in-vivo or ex-vivo integrity testing) or soft failure (declining performance, aversive symptoms, or intermittent function associated with normal device integrity testing and device imaging) of the internal receiver-stimulator. Additionally, external speech processor batteries can lose charge and external hardware can become nonfunctional rendering the CI temporarily inactive. Individuals with BCI are much less likely to be “off line” while the affected device is nonfunctional, and in the cases where the internal device must be replaced, having one functional device allows greater flexibility while considering the appropriate timing of cochlear implantation revision surgery.
The cochlear implant literature unequivocally demonstrates the binaural listening benefits discussed above in BCI patients. However, very few of these studies investigate whether these improvements in hearing lead to a significant improvement in quality of life (QOL) in BCI patients, or if the improvements are substantial enough to justify BCI from a cost-effectiveness standpoint. Recently, there has been an emphasis on QOL outcome measures, and it is clear that there is a measurable QOL advantage and increased subjective benefits seen in patients with BCI when compared to unilateral CI patients. While incremental QOL benefits may be greater in unilateral CI patients when compared with BCI patients, subjective benefits in BCI patients include improvements in depression and loneliness, emotional well-being, cognition, ease of communication, and pain (Peters et al. 2010). Furthermore, some patients showed equal gains in QOL after the second implant when compared with the first implant, and even approached normal QOL ratings following BCI (Bichey and Miyamoto 2008). Further analysis of these QOL improvements together with cost-utility measures showed not only that BCI can make a significant improvement in QOL thereby justifying the cost of the second-sided CI procedure, but also that the incremental improvement in QOL after a second CI is large enough to significantly improve the cost-utility of the second CI compared with the actual cost of the implant itself (Bichey and Miyamoto 2008).
Sequential Versus Simultaneous Cochlear Implant Surgery
Bilateral cochlear implants can either be placed simultaneously (simultaneous bilateral CI) or sequentially (sequential bilateral CI) (Fig. 1). The question of optimal timing for BCI continues to be debated in the literature, and in a recent survey sent to 35 leading CI centers around the world, Peters et al. (2010) reported that 76% of all adult BCI surgeries are performed sequentially, and only in children less than 3 years old did simultaneous BCI predominate (58%).
There are no studies directly comparing postoperative objective outcomes between patients undergoing simultaneous versus sequential BCI, but several studies have indirectly addressed the issue. While the literature clearly supports early implantation in prelingually deafened children in order to take advantage of cortical plasticity, for post-lingually deafened adults, the issue of prolonged auditory deprivation has lesser influence upon outcomes and the results following cochlear implantation are somewhat more variable. In some studies, sequential implantation with long delays between ears has resulted in poor second ear performance and has limited the degree of bilateral benefit that can be obtained by these users. Conversely, other studies have shown that the time between the first and second CI does not affect performance in the second implanted ear regardless of the time between implants (as many as 15 years in some subjects) (Zeitler et al. 2008). However, one clear benefit to simultaneous BCI or sequential BCI with a short inter-implant interval is that it ensures the devices will be matched and that bilateral stimulation of the auditory system will occur (Basura et al. 2009).
One argument for simultaneous BCI involves postsurgical auditory rehabilitation programs. Currently, no consensus exists in the literature concerning the optimal rehabilitation techniques to use for the second ear in BCI patients. In patients undergoing sequential BCI, many centers encourage complete auditory deprivation of the dominant, firstimplanted ear for an extended period of time following second-sided surgery. This presents significant practical challenges for those dependent on optimized hearing in day-to-day activities (i.e., professional duties, child care, etc.). Such an approach can lead to low compliance rates with the rehabilitation protocol and possible compromise of the objective results in the second side.
Bilateral cochlear implant is of special consideration in patients with post-meningitic deafness, and the timing of BCI after meningitis remains controversial. Proponents for simultaneous BCI at the time of the initial surgical intervention believe this approach guarantees capture of the better hearing ear while at the same time preserves binaural hearing. Furthermore, the opportunity for BCI following meningitis may be lost with ongoing cochlear ossification in the non-implanted ear following unilateral CI. Proponents of simultaneous BCI also argue that spontaneous or late recovery of hearing is anecdotal, and that early electrical stimulation of the spiral ganglion neurons may increase neural survival and prevent neuronal degeneration.
On the other hand, there have been reports of late, spontaneous recovery of hearing loss following meningitic deafness and imaging of the bony and soft tissue anatomy of the labyrinth has become incredibly precise. Therefore, with less than severe hearing loss and normal cochlear anatomy on computed tomography (CT) and/or magnetic resonance imaging (MRI), some propose a more conservative “watch-and-wait” approach with frequent imaging of the temporal bones (using both CT and MRI), regularly scheduled comprehensive audiological testing, and use of traditional amplification. In this case, unilateral CI in the poorer hearing ear or the ear with more severe ossification can be done early and contralateral bimodal stimulation used. Sequential BCI can then be performed with changes in hearing or worsening cochlear anatomic compromise.
There are also some medical and surgical considerations in deciding between sequential versus simultaneous BCI. Simultaneous BCI surgeries result in longer operative times, increased anesthetic risks, and increased blood loss when compared with unilateral CI surgery. However, surgical tolerability to BCI has been found to be equivalent in both sequential and simultaneous BCI surgery (Basura et al. 2009). For young, healthy patients, these factors may not have significant effect on surgical outcomes, but with the rising age of CI recipients, each patient must be considered individually if bilateral CI surgery is being considered. Additionally, using the traditional transmastoid, facial recess approach to the middle ear, the facial nerve is placed at risk during exposure of the basal turn of the cochlea. Despite the use of continuous intraoperative monitoring of the facial nerve, simultaneous BCI poses a small but theoretical risk of bilateral iatrogenic facial nerve paresis/paralysis that would be devastating to a patient. Therefore, many CI surgeons have adopted an “adaptive approach” in cases of simultaneous BCI, in which the decision to perform a second CI may be aborted intraoperatively with signs of anesthetic compromise, significant blood loss, or other complications such as facial nerve injury or cerebrospinal fluid leak/ perilymphatic gusher.
Disadvantages of Bilateral Implantation
Despite the literature objectively and subjectively supporting BCI in adults, there are some disadvantages one must consider before deciding to proceed with second-sided CI surgery. As discussed above, there are some medical and surgical disadvantages to operating on both ears either sequentially or simultaneously. Second, placement of the intracochlear electrode array may damage remaining neural structures and/or distort the normal anatomy precluding use of the ear in the future. Therefore, many patients are interested in adopting a “wait-and-see approach” for the contralateral ear with rationales including “saving” the second ear for hair cell regeneration, anticipation of the development of microbiological rehabilitation such as stem cell transplantation, and/or a desire to take advantage of future advances in CI technology. While these options remain experimental and investigative at this point, it is unclear whether implementation of these therapies will require a naı¨ve cochlea or some amount of residual hearing, both of which are compromised with CI.
Bilateral CI users likely benefit from coding strategies that are developed with the intention of providing detailed interaural cues. However, currently, BCI patients use two separate signal processors, one controlling each ear, which function independently from each other in terms of automatic gain control circuitry. Furthermore, these strategies may not replicate timing and intensity cues faithfully enough to permit optimal binaural hearing due to the unsynchronized processor outputs. Therefore, BCI patients may be unable to preserve differences in interaural levels accurately and will fail to detect coherent fine structure information at the two ears. Tyler et al. (2007) showed that patients with BCI with different electrodes, insertion depths, processing algorithms, channel numbers, and/ or pulse widths were able to obtain binaural benefits despite these differences between devices. It is unclear if these results are comparable to those obtained in patients with two identical devices implanted bilaterally. While there have been only a small number of studies in the literature to date, there is some hope that these disadvantages of BCI may be improved with the development of a single speech processor that is capable of controlling bilateral electrode arrays.
There are continuing debates regarding the impact of CI on vestibular function and more specifically the possibility of bilateral vestibular deafferentation with BCI. Anecdotally, a small group of patients suffer temporary, short-term vertigo following cochlear implantation perhaps due to a serous labyrinthitis similar to that seen in the postoperative period following stapedectomy. Postoperative vertigo seems to be more common in the adult and elderly population, perhaps due to preexisting vestibular hypofunction in this cohort. However, estimation for the risk of bilateral vestibular compromise following BCI is probably less than 10% in adults, and more likely in the range of less than 1% (Basura et al. 2009). In patients with a history of vestibular compromise in the contralateral ear such as endolymphatic hydrops, vestibular nerve surgery, or labyrinthitis/vestibular neuritis, there may be a role for preoperative vestibular testing in order to identify possible subclinical vestibular hypofunction in the second, unaffected ear prior to BCI. However, lack of consistent correlation between vestibular testing and postoperative symptoms of vertigo has made vestibular testing optional in the preoperative BCI algorithm.
Bilateral cochlear implantation was introduced several decades ago, but in a majority of the early patients undergoing BCI, the indications were technical complications or failure of their first implant rather than the rehabilitation of their second ear. It was not until the late 1980s or early 1990s that interest in BCI for the primary purpose of restoring binaural hearing to profoundly deafened adults became popular.
In the last two decades, clinical research evaluating objective speech outcomes, industry dollars dedicated to product research and development, basic science investigations into hearing preservation and restoration, and technology advancements have drivenworldwide acceptance of BCI. The twenty-first century has ushered in an exponential rise in the number of adult BCI patients who desire not only restoration of sound awareness and moderate perceptual improvements, but results that rival those experienced by their normal hearing peers.
A major concern with current processing strategies is that they may limit the potential for binaural CI outcomes, as these strategies may not accurately replicate normal binaural hearing. Current research has been focusing not only on matching electrode pairs postoperatively, but also exploiting these differences in order to improve signal detection. There is also ongoing research using other mechanisms such as frequency table matching, patient-driven frequency table selection, and high-definition imaging algorithms that may be used to create more subjective and objective perceptual symmetry and synergism between bilateral implant devices.
One question regarding BCI in adults that has demanded recent attention is the decision of whether to implant pre-lingually deafened adults unilaterally or even bilaterally. While many of these patients have retained residual speech perception abilities, and may develop rudimentary or in some cases moderate speech intelligibility, late secondary plasticity in pre-lingually deafened adults appears unable to achieve what is lost from chronic auditory deprivation during the critical period of speech and language development. Anecdotally, there are cases in which these patients obtain modest but important benefits in hearing from BCI. Future research should strive to identify those pre-lingually deafened adults who may benefit the most from BCI and perhaps predict those patients who may benefit from central nervous system reorganization and plasticity despite years of deprivation. Additionally, changes in processor strategies may assist these patients obtain results more similar to those patients with post-lingual deafness.
With the emphasis inmodern healthcare on evidencebased medicine and cost-effective medicine in hopes of improving access, improving quality, and improving outcomes, there has been a movement toward the creation of clinical guidelines for the allocation of various treatments andmedical devices. This trend has also been evident in determining CI candidacy, and specifically BCI, as various professional societies and study groups have proposed algorithms for BCI candidacy. While these guidelines could feasibly assist the practitioner in many cases to determine ideal BCI candidates, it is not practical for clinical guidelines to be comprehensive enough to cover all possible clinical scenarios for the placement of either simultaneous or sequential BCI. The decision to proceed with BCI in each individual patient must be considered independently as these decisions are often straightforward, but at other times can be complex and require the CI surgeon to utilize sound judgment, experience, and even intuition. As more experience with BCI in the adult population is acquired, as research results materialize, and as long-term outcomes become available, the hope is to better predict those patients who will benefit significantly from BCI.
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