Proefschrift_vd_Beek

INTRODUCTION Defining stimulation levels in cochlear implant recipients is an essential part of the fitting procedure. It becomes an increasingly time-consuming task for cochlear implant centers due to the increasing number of recipients. Moreover, particularly in children or complicated cases, this process is based on behavioral responses and therefore strongly depends on the audiologist’s experience. It is well known that threshold and maximum levels vary considerably between recipients [Wesarg et al., 2010], and a lot of effort is put into obtaining these levels, mostly using multiple behavioral or objective measurements of the individual recipient. Although the number of cochlear implant recipients has risen dramatically over the years, average mean levels along the array were, to our knowledge, only published for Nucleus (Cochlear, Sydney, N.S.W., Australia) implant recipients [Wesarg et al., 2010], and a widely applicable reference of levels and level profiles is lacking. For the present study, a data set of levels for 151 adult subjects using a CII/HiRes 90K cochlear implant (Advanced Bionics, Sylmar, Calif., USA) has been obtained. Threshold levels (T-levels) and maximum comfort levels (M-levels) were analyzed, looking for ways to generate generally applicable level profiles. Moreover, it was investigated whether this data set could serve as a reference for determining levels during initial and follow-up fittings. This knowledge could especially be helpful in children or other difficult cases not providing proper feedback. The fitting process increased in complexity over the years due to the increasing number of parameters which can be varied, but defining the threshold and maximum levels continues to be its core. Although levels are implemented differently for each cochlear implant manufacturer and different units and names are used, an upper limit for electrical stimulation per active electrode contact is always defined. For readability, all maximum and most comfortable levels will be referred to as M-levels throughout this manuscript. In an attempt to produce a reasonably automated prediction of levels and facilitate the fitting process, many studies have investigated whether objective measures such as evoked stapedius reflex threshold (eSRT), auditory brain stem response (eABR) or compound action potential (eCAP) could predict the Tand M-levels or level profiles [Shallop et al., 1991; Mason et al., 1993; Brown et al., 1994, 1999; Hodges et al., 1999; Brown et al., 2000; Allum et al., 2002; Seyle and Brown, 2002; Smoorenburg et al., 2002; Brown, 2003; Gordon et al., 2004; Cafarelli et al., 2005; Caner et al., 2007; Miller et al., 2008; Alvarez et al., 2010; Botros and Psarros, 2010; Jeon et al., 2010]. The general conclusion of these studies is that objective measures can be indicative of levels, but unfortunately, significant correlations between eSRT, eABR and eCAP measurements and Tand M-levels were shown to be of moderate strength and not appropriate for predictions in individual users. Some studies found a substantial correlation of the level profile with the eCAP profile (r = 0.82) [Smoorenburg et al., 2002], but this could not be confirmed by others [Cafarelli et al., 2005; Abbas et al., 2006]. The eCAP thresholds are routinely above behavioral thresholds but not always below maximum comfort levels [Miller et al., 2008]. Thus, one should be careful not to overstimulate when fitting M-levels on the basis of eCAP measures. Although more difficult to measure than eCAPs, some groups advocate the use of eSRTs in order to avoid overstimulation when determining M-levels

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