Bilateral Cochlear Implantation
National Cochlear Implant Users Association
November 2000
Gareth Williams
Consultant ENT Surgeon
University Hospital of Wales, Cardiff
Transcript
Thank you for asking me to talk to you this afternoon
There is a growing body of data and information concerning bilateral CI. This afternoon I will hopefully, - without loosing you in too much technological jargon, - convey to you the current situation as we understand it.
I would like to thank the 3 major CI companies, Cochlear UK, MED-EL and Advanced Bionics, who have provided me with some useful information which I'll refer to this afternoon
I'd like to start by asking all of you a question.
Would you like a second CI? - Yes? No? Don't know?
Many of you will probably have chosen the last option : "don't' know"… I wonder if I can make the decision easier for you??
The main issues in the field of cochlear implantation are fairly few If I was to ask those of you who might be interested in a second implant why you might consider having a second CI, the likely reasons would be:
- The possibility of improved speech perception. This, after all is the main purpose of implantation.
- Secondly, improved sound localisation. In a moment I'll briefly explain the mechanisms of sound localisation and how bilateral CI might (or might not!) improve directional hearing.
- Lastly, and very importantly, the question of quality of life. Can bilateral implantation improve the quality of life? If so at what cost? Might it be possible to predict the potential improvement in quality of life?
If I ask health care workers, professionals and deliverers of Health care what the main issues in bilateral CI are, they would give similar answers, with one additional item - the cost effectiveness. The question of cost is one that many of us would rather not tackle, but we cannot ignore the financial burden of a 2nd CI. The NHS is very interested in cost! The health authorities and other health care providers have a duty to ensure that their money is well spent.
CI have now been around a few years. It was inevitable that sooner or later someone would receive bilateral Cis.
In the early 1990's several small scale studies reported on patients with 2 implants. Some of these reports were on single patients or very small numbers, only 2 or 3 patients.
By the mid 1990s a few patients in the UK had (intentionally) received bilateral CI. I make a distinction between these patients and a few others who have also had bilateral implants where the second side was put in as a backup because the first implant had become infected. By mid 1999 Professor Helms and Dr Muller in Wurzburg, Germany, were reporting their results of bilateral CI. I'll come back to these results shortly
This year, the first UK bilateral study was setup. The study will look at 30 recipients of bilateral implants over the next 12 months or so, and the results will be analysed by Professor Summerfields team at the MRC
We will therefore have to wait some time before we know whether bilateral CI are truly worthwhile. For this reason what I'm now going to say is to some extent speculation. What might we expect from a second CI? To help answer this question I'll look in detail at some analagous situations in the management of deafness
In the field of otology and audiology, second side intervention is not a new concept. We have two ears, and when both of them fail there may be a number of ways to deal with the problem. Many of you will have experienced the other main forms of intervention for deafness before receiving a CI, namely conventional hearing aids and/or surgery. What can be learnt from these forms of intervention when the second ear is being addressed?
Lets look at hearing aids first of all. Are 2 hearing aids better than 1?
The facts are that in individuals with moderate hearing losses, bilateral hearing aid use is generally poor. The two main reasons for this relate to the cost effectiveness of a second aid and the patients preference. Lets consider the maximum possible improvement one could provide using hearing aids, and lets say this represents 100% improvement . For most individuals with a moderate hearing loss in both ears about 80% of the maximum possible improvement is achieved using one aid. (ie a 2nd aid only adds a further 20% of the maximum possible improvement ) Interestingly, this view is echoed by the patients, who, when provided with a second aid, often don't use them. It's not unusual for patient who are offered two aids in this situation to say: "I'm not that deaf am I?" The situation is different for patients with a severe loss. Here the addition of a second aid is cost effective and well accepted by the patients and profession. This is because a single aid can only provide about 60% of the maximum possible improvement (ie the 2nd aid adds a very useful 40% to the maximum possible improvement). These figures are a bit rough and ready, but I have intentionally oversimplified the situation to illustrate the fact that a moderate hearing loss has different requirements to a severe loss. The fact remains that 2 hearing aids are not necessarily significantly better than 1
The other form of second side intervention we are familiar with is middle ear surgery which is used to treat deafness caused by chronic infection, otosclerosis or some forms of congenital deafness. Again we have a fairly clear understanding as to when patients perceive benefit from surgery on the second ear. There are many factors that determine the success of surgery, but these are largely overshadowed by two fundamental facts relating to the hearing improvement.
- Benefit is perceived by patients if the worse hearing ear becomes the better hearing ear
- A technically successful operation is not perceived as very beneficial if, after surgery, the hearing in the operated ear is 15dB worse than in the unoperated (better) ear
These fundamental issues may well apply to bilateral CI provision as well
Professor George Browning from Glasgow, and others, have repeatedly shown that the reported benefit from middle ear surgery depends on the situation before surgery.
If we look at three possible situations:
- A unilateral hearing loss - ie with the opposite (unoperated) ear having basically normal hearing
- Bilateral hearing loss, with the degree of loss similar in both ears
- Bilateral loss with unequal hearing loss
Then these 3 situations can be ranked in order of the perceived benefit from surgery.
Those with a unilateral loss perceive less benefit than those with a symmetrical or equal loss, who in turn perceive less benefit than those with unequal hearing loss.
It is interesting to speculate where the potential recipient of a second CI would lie in this ranking order. Assuming that the first implant is working well they may be considered as having effectively a unilateral loss, (down the bottom of the scale). On the other hand if they get little benefit from the CI they could be considered as having a bilateral unequal loss (at the top of the scale)
This is speculation though. Nevertheless it is likely that the lessons we have learnt from bilateral hearing aid users, and patients having bilateral surgery will have some relevance in CI patients. Having laid the foundation and talked generally about second side intervention, what about the specific issues of bilateral CI.
The main issues pertaining to CIs as a 'medical intervention' are:
- Safety
- Effectiveness
- Cost
- Cost effectiveness
Lets look at Safety first of all
In theory, 2 CI doubles the risk of complications. In practice though this is unlikely
Concomitant surgery reduces the risks from a second anaesthetic.
Patient related risks will exclude some individuals from having second side surgery. Serious complications are therefore still likely to be uncommon.
What about effectiveness?
The effectiveness of bilateral CI can be assessed by measuring speech perception, sound localisation and quality of life issues. So, what are the potential benefits of bilateral cochlear implants for speech perception? The next 4 slides attempt to answer this question.
Firstly, 2 CI provide the bilateral benefit of being able to listen using the ear with the better signal-to-noise ratio. By that I mean that when speech and noise come from different locations the so called signal-to-noise ratio of the two ears differ, usually because of the head shadow effect.
This graph (courtesy of cochlear UK) shows the results from one subject at the University of Iowa for open set sentence scores. The signal to noise ratio was 10dB, and the signal (or speech) was nearest the right ear. As you would expect the scores for just the left ear alone are poorer than for just the right ear alone because of the advantage of having the signal nearer the right. With both implants in use the scores are slightly better than the right alone.
The bilateral benefit is the ability to use the 'best' ear for the circumstance. The 'best' ear will change with changing circumstances. Put simply, an individual with bilateral CIs will always have their implant next to the speaker
Secondly, 2 CI provide a binaural advantage - which is slightly different to a 'bilateral advantage'. Let me explain:
Binaural advantage is the ability to combine the speech signals from both ears. In normal hearing individuals there is only a very small advantage from the additive effect of having the same signal in both ears, - stress the term 'same'.
There is an advantage from having a signal which differs slightly in both ears. In reality of course the signal reaching the ears is slightly different, mainly because there is a slight delay in the signal reaching one ear compared to the other. The auditory system makes use of this tiny difference and can use it to improve speech perception compared to monaural listening. Interestingly, bilateral CI recipients who have tried using both implants from the same processor have noticed very little improvement compared to using just one of the implants on its own.
Binaural hearing is best measured by having the signal and noise come from the same place. This graph is another star subject from the University of Iowa. This particular individual demonstrated an improvement in scores when using both implants, - but be aware this is unusual in bilateral CI users. It seems that only a small number of bilateral CI users have shown this binaural advantage. This may be because the binaural advantage depends on timing differences between the ears and bilateral implant subjects typically have poor perception of timing differences
Here is another graph of a set of patients with bilateral implants from the University of Wurzburg, and I am indebted to Dr Muller who provided me with this information.
There is clearly a wide variation, with some patients gaining little benefit with either one or two implants (1), and others clearly benefiting from bilateral implantation (3, 5 & 7)
In general then, it does appear that bilateral CI improves speech perception in some patients
Moving on from speech perception, the other question is the possible benefit of bilateral implantation for sound localisation.
Sound localisation is the ability to locate and identify the person speaking, or the location of a sound in the environment. Without binaural hearing this is very difficult, and noise and reverberation make the task even more difficult. Normal listeners can detect directional differences of only 1-2 degrees in the horizontal plane. The mechanisms by which sound localisation is achieved are the interaural time delay and the interaural intensity differences.
Interaural time delay is the slight difference in time of speech reaching one ear and then the other. Normal ears can detect differences of 10 microseconds, CI users can detect differences of between 50 - 1000 microseconds. Interaural intensity differences is the loudness difference between the sound reaching both ears. A difference of 0.5 - 1 dB can be detected by normal ears, but CI users can detect much smaller differences, as low as 0.2dB!!, - here CI recipients have a clear advantage over the normal ear! Which is more important? Some CI users with poor inter-aural time delay detection have very good localisation skills which implies that interaural intensity differences are the more important cues used for localisation.
There have been very few comprehensive studies of sound localisation with bilateral CI subjects in rigorous test environments.
Such a test uses an array of loud speakers in an anechoic chamber. For example, this particular subject could detect an angular difference of 18 degrees with an interaural time delay of 400us (remember most CI have ITD of 50 - 1000us, so this is sort of average), and an interaural loudness difference of 0.8 dB (remember that many CI users have lower, or better, ILD).
The best achieved by bilateral CI users is about 10 degrees.
There's no need to tell you that this is a vast improvement on the ability of unilateral CI users
Having looked at two of the possible improvements from bilateral CI, - speech perception and sound localisation, let me move on and mention some of the possible problems with bilateral Cis.
The biggest and most obvious difference between one CI and 2 CI is that with 2, - different stimuli to both ears need to be fused by the brain into a unitary image. Could this be a problem. Obviously normal hearing individuals can do this, but only because the different signals reaching the two ears are only marginally different. In fact, as I've already explained, this difference is important in binaural hearing, and can improve speech perception.
In patients with bilateral CI it is possible that in certain circumstances the degree of difference between the two signals will be so great that the brain might not be able to fuse them into a unitary image.
Such conditions may be as a result of anatomical differences between the ears, or possibly large differences in the duration of deafness between the two ear.
Also, there may be factors relating to the implants, such as stimulation rate, update rate and coding strategies which may be important. Much research is needed to identify these factors.
To illustrate this point imagine a perfect situation of bilateral implantation, with the implant fully inserted in both ears.
When the sound signal reaches the processor an electrical stimulus is sent to the relevant electrode. Remember, an electrical stimulus in different parts of the cochlea is heard by the brain as a different pitch, so an electrical signal at the one end of the cochlea is heard as a high frequency pitch and a signal at the other end is heard as a low pitch.
In this example then, both ears hear the same pitch, - corresponding to the pitch of the original sound signal because in both ears the same part of the cochlea is stimulated (yellow dot)
Now imagine an individual with 2 implants in which the right hand implant is NOT fully inserted. The electrical stimulus will be at a slightly different part of the cochlea, and the brain might hear a different sound on the right compared to the left!
Finally, the most important question to address is the quality of life.
Can someones quality of life improve by having two implants compared to one.
Also, we professionals want to know whether our intervention is worthwhile in the context of general medical care. More specific, how might bilateral CI compare to say bilateral hip replacement, or bilateral cataract surgery?
We can make these comparisons using quality of life measurements
I will now introduce you to a new term - 'qualys'
Qualys is a term used to refer to quality of life measurement.
The way this is done is by measuring something called the Health Utility Index. This scores the quality of life from full health, represented by 1, to no health represented by 0.
A sudden life threatening event can be represented graphically like this:
Imagine an intervention occurs to prevent death, but that the quality of life is effectively halved, - to 0.5 on the scale. If the individual lives for another 25 years, the intervention has achieved an equivalent gain in quality adjusted life years of 12.5 years.
Next, imagine that after the initial life saving intervention, some form of additional intervention occurs that improves the quality of life. This sequence of events can be represented like this. It is possible to measure the effect of this secondary intervention, and so compare it with other forms of intervention.
How might this approach apply to CI?
The Health Utility Index for profound deafness tells us that total deafness is equivalent to a loss of 0.34 on this scale.
Also, we know that a unilateral CI is equivalent to a gain of 0.13
Where will a second CI appear on this scale?. If the second implant is to be as effective as the first it must improve the quality of life index to 0.92. This is equivalent to an individual with a moderate hearing loss, - ie a loss measured at 50 - 70 dB. What does this mean,- well my voice now measures on average 45 - 60 dB, ie an individual with a moderate hearing loss should be able to understand what I say without too much difficulty
The cost effectiveness of any intervention relates the cost to the qualys gained. We know the cost of CI and as such the cost utility of unilateral CI has been measured. The same will now need to be done with bilateral implants.
It is clear from what Iíve said that bilateral CIs offers potential benefits which have yet to be fully understood and quantified.
These are some of the possible disadvantages which I have also touched upon.
Should one ear be reserved for future developments, - such as hair cell regeneration within the cochlea? These sorts of intervention are just starting to be studied in the laboratories and are likely to be long way off and may never happen.
Iíve also pointed out that there are situations where 2 implants may cause interference centrally in the brain and reduce the effectiveness of either implant.
There are also the funding and ethical issues
An important question revolves around the ethical dilemma of providing bilateral implants in a health service with finite funding
Will bilateral implantees be recipients of an additional ìqualyî at the expense of potential implantees who would have had a larger ìqualyî from one implant.
If a health authority will fund 10 CI per year, should we implant 10 patients unilaterally or 5 patients bilaterally?
Second side interventions in other forms of deafness seem to suggest that the benefit from the second side intervention is rarely as dramatic as the first side intervention
In conclusion
- It is likely that bilateral CI will improve performance, especially in difficult listening environments.
- It is likely that bilateral CI will improve sound localisation.
- Similar types of improvements may be possible with technological improvements in CI hardware
- Using the analogy of other second side interventions for deafness (hearing aids and surgery) it is likely that the perceived benefit from the 2nd implant will be significantly less than from the first implant.
- The quality of life improvement from bilateral CI has yet to be determined, as has the cost effectiveness.
- Bilateral CI studies are progressing, and although the jury are still out, the fact of the matter is that bilateral CI are with us, and whether they stay or not largely depends on what you and your fellow users tell us.
I wonder…… has my talk helped or hindered.
Have you changed your minds?
Thank you for listening.