This article highlights variables that affect cochlear implant performance, emerging factors warranting consideration, and variables shown not to affect performance. Research on the outcomes following cochlear implantation has identified a wide spectrum of variables known to affect pos0timplantation performance. These variables relate to the device itself as well as individual patient characteristics. Factors believed to affect spiral ganglion cell survival and function have been shown to influence postoperative performance. Binaural hearing affects performance. Social and educational factors also affect postoperative performance. Novel variables capable of affecting performance continue to emerge with increased understanding of auditory pathway development and neural plasticity.
Cochlear implants and postimplantation performance
Cochlear implantation allows most average, postlinguistically deafened pediatric and adult cochlear implant (CI) recipients to achieve meaningful auditory sensation and speech understanding. In the past 25 years, data on the postoperative outcomes following cochlear implantation have identified a wide spectrum of variables known to affect postimplantation performance ( Box 1 ). These variables relate to the device itself, including electrode design, speech processing strategies, and device reliability, as well as individual patient characteristics such as cochleovestibular anatomy, presence of associated disabilities, or the cause of deafness. On a cellular level, factors believed to affect spiral ganglion cell survival and function have been shown to influence postoperative performance, including auditory deprivation, duration of deafness, and age at implantation. Social and educational factors, such as mode of communication, parent/family expectations, postimplantation rehabilitation, and socioeconomic status, are additional variables shown to affect postoperative performance. Novel variables capable of affecting performance, such as auditory training and focused attention, continue to emerge with increased understanding of auditory pathway development and neural plasticity. Despite extensive research examining both adult and pediatric postimplantation outcomes, the considerable variably in postoperative performance remains incompletely understood. Predictions of postimplantation benefit should be individualized and based on comprehensive preoperative assessment, with attention to the complex interplay of the aforementioned patient and device characteristics. Detailed knowledge of these variables not only improves clinician’s predictive accuracy but may also reveal factors that can be manipulated to achieve optimal performance.
- 1.
CI technology
- a.
Processing strategy
- b.
Electrode design
- c.
Device reliability
- a.
- 2.
Neuronal cell physiology and function
- a.
Age at implantation
- b.
Duration of deafness/auditory deprivation
- c.
Auditory neuroplasticity
- d.
Auditory pathway development
- a.
- 3.
Binaural hearing
- 4.
Multiple disabilities
- a.
Autism
- b.
Auditory neuropathy/auditory dyssynchrony
- a.
- 5.
Medical/surgical issues
- a.
Anatomic abnormalities
- b.
Meningitis
- c.
CHARGE (coloboma, central nervous system anomalies, heart defects, atresia of the choanae, retardation of growth and/or development, ear anomalies and/or deafness) syndrome
- a.
- 6.
Preoperative function: hearing level and speech performance
- 7.
Education/rehabilitative environment
- a.
Mode of communication
- b.
Multilingual or bilingual environment
- c.
Education and postimplantation rehabilitation services
- a.
- 8.
Auditory training
- 9.
Social factors
- a.
Socioeconomic status
- b.
Parent/family expectations and motivation
- a.
Items in italics are variables that research suggests do not affect performance.
As initially narrow candidacy criteria have broadened to include many patients affected by sensorineural hearing loss (SNHL), including those at the extremes of age, such as infants and the elderly, patients with multiple handicaps, auditory neuropathy (AN)/auditory dyssynchrony (AD), abnormal cochlear anatomy, residual hearing, and even patients with unilateral deafness, variables affecting performance have become increasingly more complex. However, these interconnected variables can be examined in related groups, making understanding of the increasing list more manageable and less intimidating. This article highlights well-recognized variables known to affect CI performance, newly emerging factors warranting consideration, and certain key variables shown not to affect performance. Overall, the discussion focuses on data published from 2005 onward.
Implant technology
Since the introduction of cochlear implantation, advances in hardware, software, and speech processing technology have directly affected performance, successively improving postimplantation speech understanding with each significant technologic advance. Of these, the most significant factor affecting performance thus far is processing strategy. In time, changes in rate of electrode stimulation, mode (sequential or simultaneous, analog or pulsatile), number of channels, waveform or speech feature extraction of the auditory signal have led to increasingly advanced and sophisticated paradigms for electrode stimulation.
Speech Perception
Upgrades to spectral peak (SPEAK) from multipeak (MPEAK), and from SPEAK to continuous interleaved sampling (CIS) saw concomitant improvement in postoperative speech understanding. Multifactorial data obtained following each transition support the direct impact of superior processing strategy on outcome. Research on novel processing strategies continues to support this trend, especially regarding the newer goals of improved word understanding in noise and music appreciation. Use of virtual channels (VCs) in speech processing software has been recently incorporated in HiRes Fidelity 120 (HiRes 120), the commercial processing strategy developed by Advanced Bionics, LLC (Sylmar, CA) and available with the HiRes 90K CI. By varying the proportion of current delivered to each electrode of an electrode pair, current steering can increase the number of spectral channels beyond the number of physical electrodes provided by the CI array. In the HiRes 90K device, 16 active electrodes (and 15 electrode pairs) are used to create 8 additional stimulation sites, resulting in up to 120 virtual channels.
Tonal Language Perception and Music Appreciation
Recent research by Firszt and colleagues (2009) showed significant clinical benefit in both speech perception and music appreciation with the HiRes 120 current steering technology compared with traditional processing strategies. Subjective ratings of music pleasantness, sound quality, and instrument distinctness were also higher with the HiRes120 speech processing. Improved pitch resolution afforded by VC may be especially clinically relevant for speech perception of tonal languages, such as Mandarin or Cantonese, in which pitch may be the only auditory clue signaling a difference in word meaning. Recent research by Chang and colleagues (2009) suggested that, compared with HiRes, HiRes 120 offered significant benefit to Mandarin-speaking pediatric CI recipients in areas of tone discrimination and speech perception. Additional research on native language is necessary, specifically the use of tonal language, as an independent variable affecting performance.
Neural Elements and Residual Hearing
Since the introduction of multi-electrode CI, advances in electrode design have focused on the development of smaller, less traumatic arrays that allow preservation of residual hearing and more selective activation of neural elements. Specific neuronal subpopulation stimulation depends on several factors, including the geometric arrangement of the electrodes, individual cochlear anatomy, and the proximity of the implanted array to the auditory neurons. Introduction of modiolar-hugging electrodes, such as used by the Nucleus CI512 (Sydney, Australia) by Cochlear, are designed to self-coil during or after insertion to reside close to the spiral ganglion cells in the modiolus. Perimodiolar arrays are intended to reduce the distance, and thereby the energy required, to stimulate neuronal cells as well as reduce insertion trauma. Unlike processing strategy modifications, introduction of modiolar-hugging electrodes has not been shown to definitively affect performance, although additional research is this area is ongoing. In contrast, ability to preserve residual hearing is intimately related to electrode configuration and size, as well as other factors such as atraumatic cochleostomy and insertion techniques. Preservation and use of residual hearing, as afforded by advances in all aspects of implant technology, including electrode design, seems to affect performance. This benefit has been attributed to improved frequency resolution and pitch discrimination afforded by preserved low-frequency residual hearing. The Nucleus Hybrid L24 is a 16-mm long 22-contact electrode array designed to preserve residual low-frequency hearing in the cochlear apex while providing a greater number of electrodes to the high-frequency and midfrequency regions. Lenarz and colleagues (2006, 2009) reported initial and longer-term preservation of residual hearing with the Hybrid L24 using a round-window insertion approach. Patients with preserved residual hearing show improved speech understanding in noise and enhanced spatial hearing. Similar results have been shown in the ongoing Nucleus Hybrid L24 Food and Drug Administration (FDA) clinical trial in the United States, which uses an atraumatic cochleostomy technique.
Pediatric Residual Hearing
Limited data exist on the preservation of residual hearing in the pediatric population. Skarzynski and Lorens (2010) reported their results of cochlear implantation on partial deafness in 15 children. Rates of speech perception in quiet improved from 34% before surgery to 67% after surgery and from 7% to 47% for hearing in noise. Although promising, additional research on hearing preservation in the pediatric population, including long-term follow-up, is necessary.
Intraoperative Technology
Additional advances in implant technology include the capability for intraoperative objective, electrophysiologic measurements of the electrical compound action potential. Termed neural response telemetry (NRT) in the Nucleus implant by Cochlear (Sydney, Australia), neural response imaging (NRI) in CIs by Advanced Bionics, LLC (Symlar, CA, USA), and auditory nerve response telemetry (ART) for MED-EL (Innsbruck, Austria), these objective measures are used during surgery to assess the response of a patient’s auditory system to electrical stimulation immediately following intracochlear insertion of the CI electrode. At the time of surgery, intraoperative NRT provides valuable information regarding the electrical output of the implant, the response of the auditory system to electrical stimulation, and preliminary device programming data; however, recent data suggest that it is not a valuable predictor of postoperative performance. Furthermore, the absence of NRT does not necessarily indicate a lack of stimulation and, in itself, is not an important variable in postoperative outcome.
Device Reliability
A review of implant technology and its effect on performance is not complete without a discussion of CI device reliability. Although device failure remains an important concern for both patients and clinicians, increased reliability has been achieved in all devices over time. Although uncommon, implant failure requires device removal and reimplantation. The most common cause is a hard failure with sudden and complete loss of function. Evidence by Carlson and colleagues (2010) and Zeitler and colleagues (2009) suggests that deactivation of 3 or 5 electrodes, respectively, can suggest impending device failure and warrant close monitoring and/or device removal. Overall, evidence to date suggests that reimplantation does not affect performance. Using careful surgical technique, revision surgery can lead to preservation or improvement in preoperative performance in most pediatric and adult patients.
Implant technology
Since the introduction of cochlear implantation, advances in hardware, software, and speech processing technology have directly affected performance, successively improving postimplantation speech understanding with each significant technologic advance. Of these, the most significant factor affecting performance thus far is processing strategy. In time, changes in rate of electrode stimulation, mode (sequential or simultaneous, analog or pulsatile), number of channels, waveform or speech feature extraction of the auditory signal have led to increasingly advanced and sophisticated paradigms for electrode stimulation.
Speech Perception
Upgrades to spectral peak (SPEAK) from multipeak (MPEAK), and from SPEAK to continuous interleaved sampling (CIS) saw concomitant improvement in postoperative speech understanding. Multifactorial data obtained following each transition support the direct impact of superior processing strategy on outcome. Research on novel processing strategies continues to support this trend, especially regarding the newer goals of improved word understanding in noise and music appreciation. Use of virtual channels (VCs) in speech processing software has been recently incorporated in HiRes Fidelity 120 (HiRes 120), the commercial processing strategy developed by Advanced Bionics, LLC (Sylmar, CA) and available with the HiRes 90K CI. By varying the proportion of current delivered to each electrode of an electrode pair, current steering can increase the number of spectral channels beyond the number of physical electrodes provided by the CI array. In the HiRes 90K device, 16 active electrodes (and 15 electrode pairs) are used to create 8 additional stimulation sites, resulting in up to 120 virtual channels.
Tonal Language Perception and Music Appreciation
Recent research by Firszt and colleagues (2009) showed significant clinical benefit in both speech perception and music appreciation with the HiRes 120 current steering technology compared with traditional processing strategies. Subjective ratings of music pleasantness, sound quality, and instrument distinctness were also higher with the HiRes120 speech processing. Improved pitch resolution afforded by VC may be especially clinically relevant for speech perception of tonal languages, such as Mandarin or Cantonese, in which pitch may be the only auditory clue signaling a difference in word meaning. Recent research by Chang and colleagues (2009) suggested that, compared with HiRes, HiRes 120 offered significant benefit to Mandarin-speaking pediatric CI recipients in areas of tone discrimination and speech perception. Additional research on native language is necessary, specifically the use of tonal language, as an independent variable affecting performance.
Neural Elements and Residual Hearing
Since the introduction of multi-electrode CI, advances in electrode design have focused on the development of smaller, less traumatic arrays that allow preservation of residual hearing and more selective activation of neural elements. Specific neuronal subpopulation stimulation depends on several factors, including the geometric arrangement of the electrodes, individual cochlear anatomy, and the proximity of the implanted array to the auditory neurons. Introduction of modiolar-hugging electrodes, such as used by the Nucleus CI512 (Sydney, Australia) by Cochlear, are designed to self-coil during or after insertion to reside close to the spiral ganglion cells in the modiolus. Perimodiolar arrays are intended to reduce the distance, and thereby the energy required, to stimulate neuronal cells as well as reduce insertion trauma. Unlike processing strategy modifications, introduction of modiolar-hugging electrodes has not been shown to definitively affect performance, although additional research is this area is ongoing. In contrast, ability to preserve residual hearing is intimately related to electrode configuration and size, as well as other factors such as atraumatic cochleostomy and insertion techniques. Preservation and use of residual hearing, as afforded by advances in all aspects of implant technology, including electrode design, seems to affect performance. This benefit has been attributed to improved frequency resolution and pitch discrimination afforded by preserved low-frequency residual hearing. The Nucleus Hybrid L24 is a 16-mm long 22-contact electrode array designed to preserve residual low-frequency hearing in the cochlear apex while providing a greater number of electrodes to the high-frequency and midfrequency regions. Lenarz and colleagues (2006, 2009) reported initial and longer-term preservation of residual hearing with the Hybrid L24 using a round-window insertion approach. Patients with preserved residual hearing show improved speech understanding in noise and enhanced spatial hearing. Similar results have been shown in the ongoing Nucleus Hybrid L24 Food and Drug Administration (FDA) clinical trial in the United States, which uses an atraumatic cochleostomy technique.
Pediatric Residual Hearing
Limited data exist on the preservation of residual hearing in the pediatric population. Skarzynski and Lorens (2010) reported their results of cochlear implantation on partial deafness in 15 children. Rates of speech perception in quiet improved from 34% before surgery to 67% after surgery and from 7% to 47% for hearing in noise. Although promising, additional research on hearing preservation in the pediatric population, including long-term follow-up, is necessary.
Intraoperative Technology
Additional advances in implant technology include the capability for intraoperative objective, electrophysiologic measurements of the electrical compound action potential. Termed neural response telemetry (NRT) in the Nucleus implant by Cochlear (Sydney, Australia), neural response imaging (NRI) in CIs by Advanced Bionics, LLC (Symlar, CA, USA), and auditory nerve response telemetry (ART) for MED-EL (Innsbruck, Austria), these objective measures are used during surgery to assess the response of a patient’s auditory system to electrical stimulation immediately following intracochlear insertion of the CI electrode. At the time of surgery, intraoperative NRT provides valuable information regarding the electrical output of the implant, the response of the auditory system to electrical stimulation, and preliminary device programming data; however, recent data suggest that it is not a valuable predictor of postoperative performance. Furthermore, the absence of NRT does not necessarily indicate a lack of stimulation and, in itself, is not an important variable in postoperative outcome.
Device Reliability
A review of implant technology and its effect on performance is not complete without a discussion of CI device reliability. Although device failure remains an important concern for both patients and clinicians, increased reliability has been achieved in all devices over time. Although uncommon, implant failure requires device removal and reimplantation. The most common cause is a hard failure with sudden and complete loss of function. Evidence by Carlson and colleagues (2010) and Zeitler and colleagues (2009) suggests that deactivation of 3 or 5 electrodes, respectively, can suggest impending device failure and warrant close monitoring and/or device removal. Overall, evidence to date suggests that reimplantation does not affect performance. Using careful surgical technique, revision surgery can lead to preservation or improvement in preoperative performance in most pediatric and adult patients.
Neuronal cell physiology and function: age at implantation, duration of deafness/auditory deprivation, auditory plasticity, auditory pathway development
These grouped variables relate to the survival, physiology, and function of spiral ganglion cells and the effects of the lack of auditory input over time. Positive effects of chronic electrical stimulation on spiral ganglion cells have been well shown in animal models. However, a critical period for auditory pathway development in humans has not been definitively established, although research in this area is ongoing and is described later. Effects of deprivation and plasticity on performance can be interpreted via their related clinical correlates: age at implantation and duration of deafness. Widespread newborn hearing screening has led to an increase in early diagnosis and greater opportunities for early intervention, including children younger than 1 year. Cochlear implantation in children less than age 12 months has shown both short-term and long-term safety and efficacy. Performance testing in very young children presents unique challenges because most tests are language based and not appropriate for children less than 1 year old. However, using methodology designed for age-appropriate abilities, a growing body of literature supports improved auditory and linguistic outcomes in children implanted before 12 months of age.
Using the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) to assess speech perception, Waltzman and Roland (2005) and Roland and colleagues (2009) suggested that implantation before age 1 year may allow deaf children to reach their full hearing potential, which may approach that of normal-hearing peers in some cases. Colletti and colleagues (2005) used the Category of Auditory Performance (CAP; a global measure of auditory receptive abilities) in their study of 10 children less than 12 months old. They found that outcomes in children less than 1 year old exceeded those of children implanted later. In an initial report involving 6 CI recipients less than 1 year old, Holt and Svirsky (2008) did not find a difference between children implanted before 12 months of age and those implanted between 1 and 2 years old. However, recent data by these investigators including CI recipients less than 1 year old (N = 35) suggest a significant advantage in areas of speech perception compared with later-implanted groups. Tajudeen and colleagues (2010) found a benefit of early implantation when comparing children of the same age, but not when comparing children the same time after implantation. For this reason, they postulated that the sensitive period for word identification likely extends to at least 3 years old.
Evidence for a sensitive period for language development within the first 2 years of life is accumulating. Connor and colleagues (2006) and Miyamoto and colleagues (2008) provided data for improved speech perception and oral linguistic skills in children implanted before their second birthday compared with children implanted when older than 2 years of age. As mentioned earlier, postimplantation outcome assessment in children less than 1 year old requires unique methodology geared toward age-appropriate abilities. Using the Rosetti Infant-Toddler Language Scale (RI-TLS), Dettman and colleagues (2007) examined communication abilities of 19 children implanted before 12 months of age. These children achieved rates of both receptive and expressive language growth comparable with their normally hearing peers and significantly greater than rates achieved by children implanted between 12 and 24 months of age. Nott and colleagues (2009) compared lexical acquisition in CI recipients and normal-hearing children and found that those implanted earlier, before 12 months of age, were closest to their hearing peers in time to acquisition of their first and 100th word. Unlike Dettman and colleagues (2007) Holt and Svirsky (2008) found improved receptive language skills in children implanted before 1 year of age, but negligible differences in expressive ability between CI recipients less than 1 year old and those implanted between 12 and 24 months of age. In a prospective study of pediatric CI recipients, Niparko and colleagues (2010) showed improved rates of both speech and language performance in children implanted before 18 months of age, most of which paralleled the performance of normal-hearing controls. Although evidence regarding critical periods for auditory and linguistic development continues to emerge, the variables of age at implantation and length of deafness have a clear impact on CI performance in children.
In adolescent CI recipients, data suggest that performance is also affected by age at implantation and length of deafness. A study of 45 prelingually deafened adolescents with a mean age at implantation of 13.5 years (range 11–18 years) found age at implantation, duration of deafness, and preoperative hearing threshold to affect speech perception outcomes. All patients showed significant improvement from preoperative scores.
At the other end of the age spectrum, cochlear implantation in elderly patients has raised issues of age-related degeneration of both peripheral and central auditory systems as well as overall cognitive deterioration and decreased neural plasticity associated with aging. Successful cochlear implantation requires an intact and functional auditory processing pathway, from spiral ganglion cells to the auditory cortex. Research suggests that multiple areas along this pathway may be affected by the aging process, thereby influencing CI outcomes in the elderly. Although histologic studies show that older individuals have lower spiral ganglion cell counts than younger individuals, the relationship between absolute number of remaining spiral ganglion cells and speech performance with a CI in the elderly is complex. Multiple studies show significant improvement in speech perception scores following implantation in elderly patients. Budenz and colleagues (2010) showed that, although younger patients out-performed elderly subjects, between-group differences correlated with duration of deafness rather than age. For quality-of-life outcomes, multiple studies support improvements in self-esteem, independence, and functional status, including a return to part-time or full-time employment, following cochlear implantation. Overall, evidence supports significant and widespread benefit of CIs in the elderly population and sheds light on the complex relationship between duration of auditory deprivation, age at implantation, neuronal plasticity, and outcome.
Binaural hearing
Although a full discussion of the benefits of binaural hearing is beyond the scope of this article, improved performance is seen in bilateral CI recipients and bimodal users (individuals with a unilateral CI and contralateral hearing aid) compared with patients with a unilateral CI. In the last decade, research has shown improved accuracy in sound localization, spatial acuity, and speech understanding, especially in challenging listening environments, in patients with bilateral CIs. These results have been shown for both simultaneous and sequential implantation in both children and adult recipients. Recent data for sound localization and spatial hearing following bilateral implantation suggest that these skills develop during the early stages of binaural hearing and improve over time with increasing experience. In sequentially implanted children, localization abilities were greatest in children who received their first implant before age 2 years and those who attended a mainstream school (vs a school for the deaf). In general, sound localization skills correlate positively with speech intelligibility in noise in that patients who are better able to localize a sound source perform better on tests of speech perception. In a specific study of speech perception in noise, Dunn and colleagues (2010) found that recipients of bilateral CIs showed significantly better performance than matched patients with unilateral CIs. In addition to objective measures, subjective data also support the binaural advantage with both adult bilateral recipients and parents of children with bilateral CIs reporting improved communication in daily life, including complex listening situations and spatial hearing. Although multifactorial, the most significant determinant of postoperative speech performance following sequential bilateral implantation is performance after the first implantation. Data suggest that auditory perception benefits in children with sequentially implanted bilateral CIs are attained over time and, although related to age at initial implantation and chronologic age, seem unaffected by the length of time between implantations.
Data comparing speech perception and localization skills of bimodal patients with bilateral CI recipients are less clear. Although some studies suggest that a greater binaural advantage is attained with a second CI compared with a hearing aid, others fail to show significant differences between the 2 groups. Overall, the bimodal literature shows (1) a binaural advantage compared with unilateral CI, and (2) support for central integration of electric and acoustic hearing. Evidence for effective cortical integration of differing binaural auditory stimuli is also provided by Budenz and colleagues (2009). In their study, benefits of bilateral implantation were unaffected by differing and/or newer technology in the second implanted ear, suggesting successful central or cortical integration of differing peripheral auditory input.
Performance data on simultaneously implanted patients parallel those of sequential bilateral CI recipients. Improved word recognition and sound localization abilities show stability in the long term with the greatest gains shown in the 12 months following implantation. In adult simultaneous bilateral patients followed over time, localization abilities continued to improve up to 6 years following CI, suggesting that certain binaural advantages may develop in a longer time period. Whether using bimodal technology or bilateral CIs, evidence to date clearly shows a performance advantage with binaural hearing.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
