electrophysiology to quality of life
Klop, W.M.C.
Citation
Klop, W. M. C. (2009, April 8). Exploring the capabilities of modern cochlear implants : from electrophysiology to quality of life. Retrieved from https://hdl.handle.net/1887/13726
Version: Corrected Publisher’s Version
License: Licence agreement concerning inclusion of doctoral thesis in the Institutional Repository of the University of Leiden
Downloaded from: https://hdl.handle.net/1887/13726
Chapter 7
General Discussion
Conclusions and Recommendations
In this thesis, we explored the capabilities of modern cochlear implants. The first goal was to investigate benefits of cochlear implantation in terms of speech perception performance, quality of life, utilities and cost-effectiveness. The second goal was to optimize the information transfer at the electrode-neural interface using electrophysiological techniques.
The benefits achieved with a CI were statistically significant and clinically relevant, not only in traditional candidates (Chapter two), but also in the prelingually deafened adult patients who used to be considered poor CI candidates (Chapter three). Provided that the optimum number of electrodes was determined for each patient individually, clinical application of high-rate speech processing strategies gave evidence of further improvement in speech perception performance especially in noise (Chapter four).
With these clinical data in mind, we explored new features of the modern cochlear implant and used objective techniques to optimize individual implant function.
Analyses of eCAP recordings revealed the existence of a prolonged potential, due to the electrode-tissue interface, which hampered the neural response signal. A new method to deal with the stimulus artifact was introduced which proved to be a more robust way of assessing the eCAP (Chapter five).
In Chapter six, a new eCAP based method investigated the ability of an electrode contact to stimulate nerve fibres in the presence of masking stimuli on both neighbouring electrode contacts. The method demonstrated that specific conditions are required in CI patients to achieve electrode independency in a multi-electrode array. Furthermore, the study evaluated the effectiveness of sequential current steering to stimulate the neural fibres at the centre of the electrode contacts. By increasing electrode separation, the presence of non- stimulated fibres could be demonstrated.
Use of electrophysiological techniques in modern Cochlear Implants As modern cochlear implants have become more sophisticated, the number of signal parameters (number and location of electrodes, pulse characteristics, speech processing strategies, etc) has increased enormously. All these parameters influence speech perception results and therefore need to be investigated:
on the one hand they give insight in the basic mechanisms of cochlear implant function, and on the other hand they may eventually optimize speech processing programming.
One of these parameters, the number of active electrodes, was evaluated using the CIS strategy at a fixed rate (Chapter four). It was concluded that the optimum number of electrodes had to be determined for each patient individually. Figure 7.1 shows the speech reception thresholds (SRT) of this study population. The SRT is defined as the signal-to-noise ratio (SNR) at which 50% of the phonemes is identified correctly. The figure confirms that individual patients achieved maximum performance under different conditions.
Figure 7.1: Speech reception thresholds (SRTs) in 8 CI patients, measured as the signal- to-noise ratio at which 50% of the phonemes in CNC words (NVA word lists) is perceived correctly. The SRT is shown as a function of speech progressing programs (SCLIN vs. high-rate) and number of electrodes, and demonstrates a large inter-individual variability.
Hence, these data suggest that individual parameter selection is obligatory to increase speech perception performance. However, it is physically unattainable, even in a cooperative patient, to evaluate all possible combinations and consequently optimize speech perception. A need exists for direct measures that
may serve as a guideline to optimize CI performance and make it accessible for use in children and prelingually deafened adults.
In an attempt to objectively investigate the independence of electrodes to obtain data on the optimum electrode density, we designed the Apple Core study (Chapter six). Results in this study demonstrated lack of selectivity, especially at a higher stimulation current, and a considerable overlap between neighbouring electrodes in modern cochlear implants. Although the MP5-AC paradigm provided insight in electrode independency, the method was not suitable to indicate the optimal number and location of active electrodes in clinical patients.
However, numerous studies report that eCAP measures can be used to clinically guide speech processor programming. These works compared eCAP thresholds obtained with NRT/NRI to the subjective assessment of the detection threshold per electrode (T-level) and the highest comfortable loudness level per electrode (C-level). A moderate correlation between eCAP threshold and T- and/or C-levels was found.1-5 However, these were group data and individual variation was large.
Hence, the most important clinical application of electrophysiological data concerns the correlation between eCAP threshold and behavioural levels. Other domains of the signal parameters remain largely uninvestigated, although a limited number of objective measurements have been performed on speech processing strategies.
An important domain to be explored, and a main topic in this thesis, is the rate of stimulation. Modern cochlear implants have the ability to adjust the stimulation rate per channel. Chapter four evaluates the effect of a higher rate strategy (1400 pps/channel) instead of a lower rate (883 pps/ channel) SCLIN strategy in the Clarion CII cochlear implant. Theoretically, a higher rate of stimulation might improve the temporal resolution6 and, as demonstrated in Chapter four, patients can indeed benefit from a high rate strategy, especially in noisy conditions (figure 7.1). Lower stimulation rates, on the other hand, may provide better recognition of weak acoustic envelope information.13 However, in terms of CI patient performance, results in literature do not show a consistent change as a function of stimulation rate.7-12 Recently, a single device study14 was designed to investigate performance and preference at different ACE strategy rates, including low (500pps/channel), moderate (1200pps/channel) and high (3500pps/channel) rates. The study confirmed the view that stimulation rate needs to be optimized
for each subject. This is in line with the conclusion of chapter four in which we demonstrated that the optimum number of electrodes has to be determined for each patient individually.
To explain some of the variability and optimize individual performance in CI-patients, it is imperative to explore characteristics of the auditory nerve with objective measurements. One important parameter to be considered, especially in high rate stimulation strategies, is the refractory period. The Nucleus 24 and Clarion HiRes 90K devices provide the CIS strategy at a stimulus rate higher than 1000pulses/s. Stimulation rates of 1200pps that are routinely used at our center within CI speech coding programs, have an inter-stimulus interval of 833s.
Technically, these stimulus rates are close to the absolute refractory period (ARP) for the human auditory nerve, which is smaller than 500s.15,16 As a result, higher rates might interfere with the refractory properties of the auditory nerve.
This is even more important when one realizes that the ARP varies between nerve fibres17 and that the relative refractory period (e.g. nerve fibres coming out of their refractoriness) is even longer than 500s. To explore the auditory nerve refractory properties in clinical patients, Morsnowski et al.18 investigated refractory recovery functions using the modified forward masking technique in 14 Nucleus 24 recipients. In this extensive clinical study, the authors were able to measure multiple recovery functions at different stimulation sites, estimate an absolute and relative refractory period in CI patients and improve measurement techniques. A comparable important parameter to be explored is, for example, neural adaptation:19 the potential of higher stimulation rates to exhaust the nerve fibres due to reduced depolarization of the sodium channel.20 It is likely that such nerve characteristics have an important influence on the auditory nerve function in response to electrical stimulation at different rates. Hence, it would be interesting to develop a large clinical study in the near future that combines individual auditory nerve properties (ARP, neural adaptation etc.) with speech programming parameters (stimulation rate, number of electrodes, stochastic resonance, etc). Eventually, the objective measurements of an individual’s nerve characteristics might predict the individually optimized speech program.
Speech coding strategies not only differ in stimulation rate, but also in the sequence of the presented stimulus pulses. The CIS speech strategy relies on a
sequential stimulation paradigm. Using non-simultaneously interleaved pulses, CIS was developed to reduce channel interaction. The paired pulsatile sampler strategy (PPS) 21 adds further flexibility to speech processing. This strategy delivers auditory information at twice the speed of CIS by simultaneously stimulating two channels that may improve performance in noise. Nevertheless, perception data22 indicate that sequential stimulation is more appropriate than paired stimulation.
One explanation for the drop in performance score with paired stimulation was the increase in channel interaction and decrease in spatial selectivity. Therefore, if we can objectively quantify the measure of interaction, a more optimized speech coding strategy could be selected for the individual patient. To obtain an estimate of neural excitation spread, telemetric measurements were used by different authors.23-25 The results indicate that neural populations associated with different electrodes overlap and the degree of overlap varies among subjects. Hence, using objective measurements, the Apple Core-study (Chapter six) investigated independency of neighboring electrodes in individuals. Although we were able to interpret some of the clinical findings, the Apple Core method did not result in a new gold standard to determine electrode independency.
Even though channel interaction is a possible cause of signal transfer distortion, early research26,27 suggested that additional pitch percepts could be created by stimulating two electrodes at the same time. The use of intermediate percepts between electrode contacts, virtual contacts, allows improvement in the spectral domain through enhanced placement of the excitation area along the tonotopic axis of the cochlea.28 This dual electrode stimulation works with both simultaneous28,30 and sequential31 electrode stimulation and is used in the HiRes120 speech coding strategy.32,33 In these studies however, the number of spectral channels perceivable by patients varied considerably. ECAP measures, for example the Apple Core method, could provide valuable information on the appropriate number of spectral channels in individuals. Information from objective measurements can assist the selection of patients who would benefit most from a
‘HiRes120’ strategy, or a ‘HiRes60’ or ‘HiRes240’ paradigm.
One of the main conclusions from chapter six was that there was a lack of independence between neighboring electrodes at stimulation levels above 600A.
As a result, lower current levels are preferred to obtain specific data on individual electrodes. However, lower stimulation levels result in smaller response signals that are easily disturbed. Thus, optimizing the quality of the response signal is of utmost importance in eCAP measurement. Therefore, we examined the neural response signal and introduced a new method to optimize eCAP recordings (Chapter five). The method allows the use of larger amplification factors, thereby improving the signal quality. Regardless of the advantages of this new method, reliable and clinically applicable measurements at lower stimulus levels can only be provided when the NRT/NRI systems improve and system noise decreases.
Hence, future generations of cochlear implants require improvements, especially in terms of the system noise.
Considerations on measurement tools in selected groups
Technical progress, together with the accumulation of wider experience and improved rehabilitation programs34 permitted the selection of special CI-candidates. Several studies have shown the indisputable benefit of cochlear implantation in patients with borderline indications.35,36 In our studies, at 24 months of CI use, the average CVC word score of the prelingually deafened group is about 14% compared to 55% in the postlingually deafened group. These speech perception scores are in line with, or even exceed those published by others.4, 37 Although results with traditional measurements (e.g. speech perception perfor- mance) are acceptable and easy to compare, outcomes are at least partially related to the specific etiology. For instance, prelingually deafened adults that achieve “improved environmental awareness of sound” with a CI are not easily compared to implanted, postlingually deafened adults who use the telephone again. Therefore, procedures for determining benefit have evolved hand in hand with candidacy criteria. For patients with minimal open set speech understanding, closed set tests might be useful because certain speech features can be combined with lip-reading cues.38 Patients with favorable scores in open set formats might have their performance tested in more difficult circumstances (“speech tracking” and “hearing in noise”) to circumvent ceiling effects. Thus, as cochlear implantation results improve due to the recent advances in CI technology, and criteria for CI widen, the tools to measure the effectiveness have to be adjusted as well.
In Chapter three we measured the effectiveness of cochlear implantation in the prelingually deafened adult population. It was concluded that speech perception and quality of life improves significantly, although individual variability is large.
One of the interesting findings was that patients who did not reach open set speech perception still improved significantly on QoL items. This implies that QoL measurements are indispensable in these specific groups, because these measures provide essential information not detected with traditional speech recognition measurements as already stated by Zwolan et al.39 in 1996.
An additional benefit of QoL measurement in this specific population is the possibility to evaluate economic benefit. It is generally known that resources in health care are scarce. For an expensive intervention such as cochlear implantation, a thorough economic evaluation and an acceptable cost-utility might justify the allocation of resources by policy makers and insurers, especially when it concerns a low-performing group of patients. Despite the small gain in speech perception and high costs, from our small series (Chapter three) we have concluded that implantation in the prelingually deafened adult represents acceptable value for money.40
These figures might even improve when we are able to select the good performers beforehand. A first possible approach was introduced in this thesis by testing the quality of a patient’s own speech production preoperatively. Quality of a patient’s own speech production (QoSP) was identified as a possible predictor of speech perception performance, because it reflects the extent to which patients have been able to hear speech in the past. Moreover, in Chapter three we stated that prelingually deafened patients with appropriate speech production were, in fact, not deafened in the prelingual period. The “true” prelingually deafened adult (e.g., the congenital deafened adult) never experienced speech perception, and therefore lacks viability of the central auditory system. Hence, we advise abandoning the term “prelingually deafened adult” and introducing “the early deafened adult”. In that way, we can focus on the expected performance rather than exclude a patient that is by definition prelingual and thus a potentially poor performer.
A different approach to select potentially good performers and to predict the viability of the auditory cortex could be the use of functional imaging (fMRI);
visualizing activation of the central auditory system in hearing-impaired. The fMRI
provides high resolution, noninvasive reports of neural activity detected through a blood-oxygen level dependent signal.41,42 This new ability to directly observe brain function opens new opportunities to advance our understanding of brain organization and neurological status. A drawback of the fMRI is that, due to the magnetic capacities, its use is limited to the pre-implant situation. Another new and interesting development is the application of FDG-PET. FDG-PET is a functional imaging modality that enables the in vivo determination of tissue metabolism and pathofysiology. Giraud and Lee43 correlated pre-operative distinct brain organization patterns with post-operative speech perception in congenitally deaf children. They concluded that assessing brain organization immediately before cochlear implantation can efficiently predict subsequent speech outcome.
The value of the abovementioned options as predictors of speech performance in the early-deafened adult group is, as yet, unestablished. Based on our own preliminary results, we have started a prospective study on predicting factors, including the quality of a patient’s own speech production, in the early deafened adult population.
Considerations on quality of life measurement
Cochlear implants are, above all, communication tools. Communication is a dynamic process associated with physical, psychological and social functioning44, ultimately influencing well-being. The negative consequences of adult-onset hearing loss are not only limited to auditory impairment, but also involve activity limitations and participation restrictions.45 When hearing is, at least partially, restored with a cochlear implant, HRQoL considerations are of central importance in evaluating treatment and monitoring effects over time. While speech perception tests, in all their variety, can be used to test improvement in spoken word recognition, HRQoL assessments result in a comprehensive picture of the benefits and costs that accrue from cochlear implantation.
While there is general agreement on the potential value of quality of life measures in cochlear implantation, in practice it can be a burden for patients and doctors, because it is potentially time consuming, sometimes difficult to understand and not in the immediate interest of the patient. Based on our findings and on other studies, the strengths and shortcomings of different QoL instruments used
in this thesis will be discussed and a selection for practical use in CI research recommended.
To elicit health state valuations, the VAS and TTO were used. Irrespective of whether the TTO is considered the most valid method (Chapter 1), it is important that the method is feasible in daily practice. In our experience, explaining the TTO to deaf patients is time consuming. Further, guidance of the patient in a face- to-face setting is mandatory to complete the test. On the other hand, although valuing the health state on a visual analogue scale (VAS) is easily understood, it does not include a tradeoff. Also, it has been shown that respondents were incapable of specifying why the mark was placed at that precise point.46 On the basis of our own findings, we will not recommend the VAS and TTO in future studies on HRQoL in the deafened population.
Another feasible approach is to construct valuations based on the general population, such as those used with the Health Utility Index. Naturally, evaluation will be affected by whose preferences are used and this could result in an under- or over estimation, depending on the perspective. From a practical perspective, the HUI2 provides a description of health status including the sub-domains hearing and speech (important in CI). Indeed, the HUI2 showed large improvements, specifically on hearing-specific QoL-domains, was easy to use and allowed for cost-effectiveness calculations in our studies (Chapters 2 and 3).
In the current quality of life studies we preferred the HUI2 to its successor HUI3 as it allowed us to compare our results with the Nijmegen series.47,48 However, in a recently published study comparing HUI2, HUI3 and EQ5D49, the HUI3 proved to be more responsive and therefore more appropriate for evaluating HRQoL in a population with hearing complaints, and is therefore the generic instrument of choice in this population.
Our study demonstrated that cochlear implants have a large and significant impact on HRQoL in the postlingually deafened adult population. Moreover, it describes the first economic evaluation of cochlear implantation performed in the Netherlands. At the Leiden University Medical Center, cochlear implantation in the postlingually deafened population proved to be a cost-effective procedure.
In contrast to other studies50, 51 we did not find a significant positive correlation between gain in HUI and gain in speech perception in our series (data not published). One explanation may be the complicated nature of HRQoL as an
outcome measure. On the other hand, from the early-deafened adults in Chapter three, we know that, despite a marginal auditory perception, a significant gain in HRQoL can be achieved. Apparently, the patient’s subjective perception of well-being outweighs the gain in speech perception as measured with traditional audiological measures.
The hearing specific QoL was evaluated using the NCIQ. This instrument was highly responsive to the interventions for which it was designed, and showed large, significant improvements in all sub-domains (Chapter two). Recently, the NCIQ has been used in at least two centres52 in the Netherlands, including ours, as well as in Germany53 and the United States.54 This improves comparability and strengthens test results. Experience has shown that the questionnaires are easy to use due to their response mode (items are formulated as statements with five answering categories). Additionally, even if single questions are not applicable, or missing, sub-domains scores can be computed. A drawback of the NCIQ is that it is not designed to calculate health state valuations.
In conclusion, we recommend the NCIQ and HUI3 as appropriate tools to measure HRQoL for routine clinical use as well as for research in CI patients. Both provide feasible measurements that are easy to obtain. Moreover, both instruments are complementary and can be used in clinical trials; additionally the HUI3 enables cost-effectiveness analysis. A disadvantage of these systems is that they may lack the sensitivity to detect clinically important differencesbetween (relatively) closely related outcomes as discussed in Chapter two. The concept of the minimal clinically important difference (MID), introduced in 1989 by Jaeschke55, was defined as “the smallest difference in score in the domain of interest which patients perceive as beneficial and which would mandate, in the absence of troubling side effects and excessive costs, a change in the patient’s management”. We have shown that the effect of cochlear implantation exceeds the MID in different quality of life items for the majority of postlingually deafened adult patients. Demonstrating change by the MID is potentially important evidence in patient reported outcomes, especially when incorporated in clinical trials. The key element in these calculations, however, is the determination of the MID. In the absence of further situation specific knowledge, we used a distribution based (“statistical”) method to calculate the MID. However, the current opinion is that the MID estimation should be based on multiple, anchor based approaches.56,57
These multiple approaches, such as patient-based and clinical anchors, should result in a narrow range of MID values, which could then provide a basis when interpreting clinical trial results, and help clinicians, patients and health authorities to understand the effects of an intervention.58 A clinical study to determine a patient based estimation of the MID, using the HUI3 and NCIQ, is currently underway.
Conclusions and recommendations
In this thesis we explored the capabilities of modern cochlear implants and evaluated their benefits.
The following conclusions can be drawn from these studies:
Cochlear implantation benefits speech perception and health related quality
—
of life in the postlingually deafened adult population. Improvements are not only statistically significant, but also clinically relevant as determined by the minimal clinical important difference and effect size (Chapter two).
Cochlear Implantation is cost-effective in the postlingually deafened adult
—
population (Chapter two).
Implanted, early-deafened adults can reach open-speech perception in
—
selected cases. Despite a generally small improvement in speech recognition, a significant increase in quality of life can be achieved (Chapter three).
A higher quality of a patient’s own speech production is associated with
—
better speech perception performance in the implanted early-deafened adult (Chapter three).
High-rate speech processing strategies can improve speech recognition in
—
individual patients, provided that the number of electrodes is optimized per patient (Chapter four).
Applying a compensation circuit at the input stage of the amplifier to reduce
—
the effect of residual charge appears to be beneficial in eCAP recordings as it is a more robust way of assessing the eCAP and enhances the clinical applicability (Chapter five).
The MP5-AC paradigm provides insight concerning the independence of
—
electrodes and the efficacy of current steering in individuals (Chapter six).
In modern multichannel devices, electrode independency generally commences
—
at larger (2-3) electrode separation. Electrode independency only occurs at stimulus levels smaller than 600A (Chapter six).
Analyzing the results of a cochlear implant program is essential to optimize clinical care. Traditional audiological techniques and HRQoL assessment confirm that clinical results are steadily improving. For further progress, we have to concentrate on optimizing individual CI-patient performance using objective
measurements. This requires further characterization of the auditory nerve properties, investigating electrical interaction and improving selective stimulation of remaining nerve fibers in the deafened cochlea. The modern cochlear implant systems, equipped with objective tools such as NRI/NRT, allow us to investigate these items in clinical patients.
Currently, research focuses on improving speech processing strategies and/
or hardware function (e.g. electrodes). Manufactures attempt to improve the device function and minimize insertion trauma by reducing the dimension of the individual electrodes and increasing their number. However, a cochlear implant will never compete with a healthy cochlea. Hence, alternative approaches to improve speech perception performance are underway, such as the application of stem cell technology to regain hair cell function, as well as the introduction of neurotrophic factors to stimulate nerve fibre growth. Apart from this purely biological approach, we might focus on a psychological approach, including the restoration of plasticity of the brain after years without audiological input.
Eventually, if this biological-psychological approach is feasible, results for the hearing-impaired population might improve even further.
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