Unilateral cochlear implantation

Unilateral cochlear implants (CIs) have provided significant benefit to adult and pediatric patients with severe-to-profound sensorineural hearing loss. When compared with children with hearing aids, CI recipients have shown significant language and literacy improvements and a much steeper trajectory of receptive and expressive language development. Half or more of pediatric CI recipients obtain language scores within normal for age range. Although both children and adults typically perform well with open-set word and sentence recognition in quiet listening situations, these results can decrease dramatically (20–80 percentage points) in the presence of even moderate levels of background noise. Speech recognition in noise and other performance variables may improve with the addition of a second CI.

Peripheral and central auditory processing

Alteration in auditory processing pathways, measured by cortical and brainstem responses, is required for hearing and speech recognition improvement with bilateral CI. This alteration has been evaluated in multiple studies, and bilateral central auditory processing and development occur from birth to 3 years of age.

Significant differences are recognized in cortical auditory evoked potentials in those implanted before and after 3 years of age. Those children implanted early showed normal wave morphology and P1 latency, whereas those implanted later showed atypical wave morphology and longer P1 latency compared with both normal hearing and children implanted at a younger age. Also, 2 studies revealed that patients who were implanted sequentially (compared with simultaneously) had prolonged auditory brainstem responses and wave latencies in the second implanted ear. Those implanted before 3 years of age showed normalized wave latencies over a shorter period.

Appropriate alterations in auditory processing patterns are necessary for the bilateral CI recipients to have improved speech recognition and hearing outcomes. However, there are limits to auditory pathway plasticity because more significant changes are seen in those implanted before 3 years of age. What remains to be seen, however, is whether unilateral implantation before 3 years of age will suffice for auditory cortical maturation, given that electrical stimulation from the implanted ear transmits neural information to both the contralateral (primary) and ipsilateral (secondary) pathways. Galvin and colleagues showed significant benefit from a second implant obtained with up to 16 years following the first implant for adolescents and/or young adults with congenital sensorineural hearing loss.

At the peripheral level to the auditory brainstem, pediatric recipients of simultaneous CI show a minimal interaural difference in latency between EABRs. Gordon and colleagues studied 3 groups of bilateral pediatric CI recipients:

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  The first group had a delay of more than 2 years between the first and second CIs

  
  
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  The second group had a delay of less than 1 year between the first and second CIs

  
  
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  The third group had no delay between implants.

This study concluded that the group with sequential implantation and long delay (more than 2 years) had a significantly longer latency in neural conduction between the 2 sides. The group with short delay also showed longer latency, but with trends decreasing in the first few months following the second implant. Finally, the group with simultaneous bilateral implants showed no clear difference between the 2 sides.

Another study by Sparreboom and colleagues examined EABR in 30 prelingually deafened children at 6, 12, and 24 months following sequential bilateral implantation. The mean surgical interval time between CI1 and CI2 was 41 months (range 18–86 months). Their data showed no significant interaural differences in EABR wave III at any time points. Also, wave V latencies on the CI2 side were initially prolonged compared with the CI1 side; however, after 12 months, these interaural latency differences were no longer significant.

Based on their results, they suggested that age and time between CI2 and CI1 do not affect auditory brainstem development at the side of the second implant, and auditory brainstem maturation occurred despite the long delay of implantation to the other side.

Peripheral and central auditory processing

Alteration in auditory processing pathways, measured by cortical and brainstem responses, is required for hearing and speech recognition improvement with bilateral CI. This alteration has been evaluated in multiple studies, and bilateral central auditory processing and development occur from birth to 3 years of age.

Significant differences are recognized in cortical auditory evoked potentials in those implanted before and after 3 years of age. Those children implanted early showed normal wave morphology and P1 latency, whereas those implanted later showed atypical wave morphology and longer P1 latency compared with both normal hearing and children implanted at a younger age. Also, 2 studies revealed that patients who were implanted sequentially (compared with simultaneously) had prolonged auditory brainstem responses and wave latencies in the second implanted ear. Those implanted before 3 years of age showed normalized wave latencies over a shorter period.

Appropriate alterations in auditory processing patterns are necessary for the bilateral CI recipients to have improved speech recognition and hearing outcomes. However, there are limits to auditory pathway plasticity because more significant changes are seen in those implanted before 3 years of age. What remains to be seen, however, is whether unilateral implantation before 3 years of age will suffice for auditory cortical maturation, given that electrical stimulation from the implanted ear transmits neural information to both the contralateral (primary) and ipsilateral (secondary) pathways. Galvin and colleagues showed significant benefit from a second implant obtained with up to 16 years following the first implant for adolescents and/or young adults with congenital sensorineural hearing loss.

At the peripheral level to the auditory brainstem, pediatric recipients of simultaneous CI show a minimal interaural difference in latency between EABRs. Gordon and colleagues studied 3 groups of bilateral pediatric CI recipients:

• •
  

  The first group had a delay of more than 2 years between the first and second CIs

  
  
• •
  

  The second group had a delay of less than 1 year between the first and second CIs

  
  
• •
  

  The third group had no delay between implants.

This study concluded that the group with sequential implantation and long delay (more than 2 years) had a significantly longer latency in neural conduction between the 2 sides. The group with short delay also showed longer latency, but with trends decreasing in the first few months following the second implant. Finally, the group with simultaneous bilateral implants showed no clear difference between the 2 sides.

Another study by Sparreboom and colleagues examined EABR in 30 prelingually deafened children at 6, 12, and 24 months following sequential bilateral implantation. The mean surgical interval time between CI1 and CI2 was 41 months (range 18–86 months). Their data showed no significant interaural differences in EABR wave III at any time points. Also, wave V latencies on the CI2 side were initially prolonged compared with the CI1 side; however, after 12 months, these interaural latency differences were no longer significant.

Based on their results, they suggested that age and time between CI2 and CI1 do not affect auditory brainstem development at the side of the second implant, and auditory brainstem maturation occurred despite the long delay of implantation to the other side.

Binaural summation

Binaural summation is defined as the sensation that a signal is perceptually louder when hearing with 2 ears compared with 1. Binaural summation yields a benefit of approximately 2 dB for listeners with normal hearing. A recently published meta-analysis on bilateral CI including 349 patients with summation data revealed 21% improvement ( P <.0001) in binaural summation in bilateral CI recipients, affording an improvement of 14.6% to 27.4% in speech recognition in noise.

This study also suggests that a bilateral CI recipient would have a 14% improvement in speech recognition in noise compared with a unilateral CI recipient. Several other studies on bilateral CI have shown significant binaural summation effects. Dorman and colleagues pooled data from Litovsky and colleagues, Buss and colleagues, and Koch and colleagues and showed that for 82 adult bilateral CI recipients, the mean binaural summation was significant 11 percentage points for monosyllabic word recognition.

Binaural squelch

Binaural squelch refers to an effect in which an improvement in the signal-to-noise ratio results from a central comparison of time and intensity differences for signals and noise arriving at the 2 ears. Squelch has been shown to offer an improvement in the signal-to-noise ratio of 2 to 5 dB for individuals with normal hearing. Data on binaural squelch in bilateral CI recipients are varied, with some recipients reporting improvements whereas others reporting none. Some studies have demonstrated the presence of binaural squelch with bilateral CIs after an extended period of implant experience including 1 year and 4 years of bilateral electrical stimulation. A meta-analysis of 251 bilateral CI recipients showed significantly greater suppression of noise 8.5% to 22.1% ( P <.0001) compared with unilateral CI recipients. Comparing bilateral CI recipients with those implanted unilaterally and using a hearing aid on the contralateral ear (ie, bimodal hearing), no statistical significance was obtained. These results suggest that the use of a contralateral hearing aid has similar effects regarding binaural squelch as a second CI; however clearly, further research is needed.

Head shadow effect

The attenuation of a sound that occurs when it travels around the listener’s head to the opposite ear is known as the head shadow effect. For normal-hearing listeners, head shadow ranges from 9 to 11 dB. Having a present and equal head shadow across ears is one of the most robust and early benefits of bilateral CI. Multiple studies have reported marked improvements in head shadow effects when comparing single versus bilaterally implanted patients. This is primarily because unilateral CI recipients will have head shadow for just a single ear, provided that noise originates from side of the nonimplanted ear. There is also no statistical difference in head shadow effect in pediatric versus adult bilateral CI patients.

Spatial release from masking

In contrast to the other binaural cues discussed, spatial release from masking describes the phenomenon of improved speech understanding—when listening with 2 ears—for conditions in which the target signal and background noise are spatially separated. Spatial release from masking is thought to be a combination of head shadow and squelch and has been shown to yield a benefit of 3 to 4 dB for both adults and children with bilateral CIs. Given that the evidence surrounding the presence of squelch for bilateral implant recipients is not unequivocal, it is thought that most spatial release from masking observed for individuals with bilateral implants comes from head shadow.

Sound localization

The improved ability of bilateral CI recipients to localize sound in a quiet environment has been previously established. There seems to be a necessary period of binaural hearing adaptation that occurs before recovery of spatial hearing. The first signs of spatial hearing recovery in bilateral CI recipients occur around 3 months after activation, with 1 report of return of spatial hearing as early as 1 month. It has been proposed by many authors that spatial hearing recovery is linked to the amount of auditory experience acquired by the recipient before the onset of deafness.

More recent literature has investigated the increased localization ability in noisy environments. In a study by Mosnier and colleagues, simultaneously implanted patients’ ability to localize sound in a noisy environment was compared with 1 or both implants activated. A statistically significant increased ability in those patients was noted with both implants activated independent of the patient’s speech performance.

Speech recognition in noise

Another proposed advantage of bilateral CI over unilateral CI is improved speech recognition in a variety of different noisy environments. Although not all studies have shown this advantage, the overwhelming majority have shown benefit hearing in noise with the addition of the second CI. However, those with a prolonged period of deafness in the second ear are associated with poorer speech recognition in noise. Similarly, in 2 studies that reported no improvement in speech recognition in noise, the bilateral CI recipients had protracted periods between the time of the first and second implantations. These data suggest that integration of the electrical signals across ears plays a significant role in improved hearing in noise.

Similar to spatial hearing improvement, there also seems to be a lag period for improved hearing in noise with bilateral CI, with performance improvements noted over time. In a recent study by Wolfe and colleagues, there were statistically significant improvements in HINT scores in the bilateral CI condition compared with the unilateral CI condition at 3, 6, and 8 months. There were also statistically significant improvements in comparing individual HINT scores at 3 versus 8 months and 6 versus 8 months.