There is significant amounts of research into the effectiveness of Cochlear Implants. However research around the use of bilateral Cochlear Implants is quite limited – particularly in children. This is because, until recently, bilateral implantation was quite rare. In New Zealand only 16% of children with Cochlear Implants have bilaterals. Of those the vast majority have received their second Cochlear Implant sequentially.
What we do know from the research is that a single (or unilateral) Cochlear Implant can have a significant positive outcome for a child with profounds or severe hearing loss. However Julia Sarant writes in her 2012 Cochlear Implants in Children: A Review…
A unilateral cochlear implant does not guarantee the development of language, speech production, academic or social skills comparable to those of children with normal hearing. Although there are many children with a unilateral cochlear implant who are able to develop these skills at an age-appropriate rate, there also remain many who show delayed development in these areas, some of whom maintain or increase their delay through to adulthood.
These limitations are likely to be caused by three groups of factors. Firstly for a small but significant number of Cochlear Implant children, hearing issues are but one of an array of medical and developmental issues. Secondly, clinical factors such as the possibility of implanting into the weaker ear can limit the effectiveness of a Cochlear Implant. Thirdly, and perhaps the most important of factors, the lack binaural processing and all that is associated with lacking this ability.
For children with other medical and developmental issues, addressing hearing issues with Cochlear Implants has been shown to make significant quality of life improvements. There is a logic to this – that being that if a child has a number of senses that are compromised, restoring a sense like hearing can bring marginally greater gains because there are fewer other senses available to compensate. For example, giving a person with vision and hearing loss a sense of hearing will likely generate greater gains than giving a person with only hearing loss back their hearing.
The risk of implanting in a weak ear is relatively low. With modern imaging and good pre-implantation audiological testing, the stronger ear can be identified. Likewise issues like malformed cochleas can be identified and appropriate strategies put in place to ensure good outcomes. However there are still likely to be some individuals who could have received better outcomes if their implant was in their other ear.
Binaural processing is the major area where a unilateral Cochlear Implant can not restore a hearing function. Binaural hearing is discussed in detail here. The research around binaural hearing is quite strong, as are the known gains. One research article puts it like this…
Binaural hearing allows listeners with normal hearing to understand speech better in silence and noisy conditions and is an essential requirement for spatial hearing and sound localization. Other benefits of binaural hearing are more natural hearing, reduced listening effort and an improved quality of life
The improved outcomes for children with bilateral Cochlear Implants who can develop binaural hearing are now starting to appear in the research. One paper entitled Bilateral versus unilateral cochlear implantation in young children concluded…
Profoundly deaf bilaterally implanted children are significantly more likely to use vocalisation to communicate, and to use audition when interacting vocally with an adult, compared with unilaterally implanted children.
One of the challenges with this research on bilateral Cochlear Implants, particularly around benefits from binaural hearing, is the impact of simultaneous verses sequential implantation. As noted previously, the vast majority of New Zealand children with bilateral Cochlear Implants have received their second ear sequentially. This pattern, until recently, is repeated onternationally. Research from Tel Aviv University indicates that a large gap between receiving a implants can render binaural processing inactive. The research however does not quantify this gap and only looks at one aspect of binaural processing.
This research does however raise the concern that implanting unilaterally will remove the opportunity for a child to develop a particularly important component of hearing even if the child receives a second Cochlear Implant at a later date.
Research such as this has lead numerous organisations, bodies and countries to conclude that simultaneous bilateral implantation is the most appropriate clinical approach for children with profound or severe bilateral hearing loss.
Position statement on bilateral cochlear implantation – American Academy of Otolaryngology Head and Neck Surgeons
Although unilateral cochlear implantation (CI) generally provides good speech understanding in quiet, and has been highly successful in rehabilitation of deaf adults and children, patients with only one CI frequently report difficulty in every day listening conditions.
Localization of sounds is not possible with only one implant, often creating a safety issue, and hearing in noise is very difficult. Over the past decade, a substantial literature has accumulated demonstrating improved speech intelligibility and sound localization with bilateral CIs. These findings are consistent with the psychoacoustic literature that shows the importance of bilateral hearing for normal hearing people and hearing aid recipients.
The head shadow effect and central effects, based on integration of timing, frequency and level from the two ears, are possible only with binaural hearing and significantly improve speech understanding and sound localization compared to listening with only one CI. The literature is clear that both children and adults perform better with two CIs than with one.
The William House Cochlear Implant Study Group acknowledges the findings reported in the literature and strongly endorses bilateral cochlear implantation in profoundly deaf adults and children. Bilateral cochlear implantation is now considered accepted medical practice.
Second Meeting Consensus on Auditory Implants held in Valencia, Spain in February 2004
Health experts at the Second Meeting Consensus on Auditory Implants held in Valencia, Spain in February 2004 made the following recommendations:
Bilateral cochlear implantation (CI) should be recommended in the following types of patient:
- Those in whom the benefits obtained with one CI is poor.
- In meningitis, that is developing cochlear ossification the implantation should be performed as soon as possible to achieve full insertion.
- Those who want to restore binaural hearing or need it in order to remain in their chosen profession.
- Children with permanent bilateral profound hearing loss. Special attention should be paid to young children who are in their speech and language acquisition periods.
(E. Offeciers (Belgium), C. Morera (Spain), J. Muller (Germany), A. Huarte (Spain), J. Shllop (USA) & L. Cavelle (Spain), International consensus on bilateral cochlear implants and bimodal stimulation, 2004).
NHS – National Institute for Health and Clinical Excellence – Cochlear implants for children and adults with severe to profound deafness 2009 (Reviewed 2011)
1.2 Simultaneous bilateral cochlear implantation is recommended as an option for the following groups of people with severe to profound deafness who do not receive adequate benefit from acoustic hearing aids, as defined in 1.5:
- adults who are blind or who have other disabilities that increase their reliance on auditory stimuli as a primary sensory mechanism for spatial awareness.
Acquisition of cochlear implant systems for bilateral implantation should be at the lowest cost and include currently available discounts on list prices equivalent to 40% or more for the second implant.
1.3 Sequential bilateral cochlear implantation is not recommended as an option for people with severe to profound deafness.
For a list of countries that have now moved to bilateral Cochlear Implantation, please click here.
There are currently a number of large scale longitudinal outcome studies looking at the benefits of bilateral Cochlear Implants in children. One out of Melbourne University is due for publishing in 2014. There are similar studies due slightly earlier out of some US universities. Based on some of the smaller studies and anecdotal evidence these large studies are likely to show significant benefits, particularly for simultaneous bilateral implantation.