Zeitverlauf der telemetrisch gemessenen, elektrisch evozierten Summenaktionspotentiale bei 101 Cochlea-Implantat Versorgungen

Document Type
Doctoral Thesis
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Jaeger, Doris

The aim With the invention of cochlear implants medicine achieved a significant step towards the renewal of a sense organ. For no other sense organ it has been able to find an at least approximate correlate. However, with the new achievements new problems came. The desire for the renewal of an auditory impression the natural sense organ leads us to the limits of technical ability. Since the introduction of neural response telemetry (NRT) for nucleus 24 cochlear implant (CI24), clinicians and researchers have investigated the feasibility of using the electrically evoked compound action potential (ECAP (Abbas, 2004)) threshold to objectively predict psycho- physical measurements that are used in the programming of the speech processor. There are number of studies that have examined characteristics of the ECAP measured at the time of surgery and postoperatively, but not proved the changes over time. There are still variations in ECAP thresholds with respect to time periods. The aim of this study was to show how the ECAP thresholds change over the first year after implantation and to estimate if this variability could be used to predict ECAP thresholds and may influence ECAP based fitting methods in the future. Design A total of 206 CI implantations of the University Hospital of Erlangen-Nuremberg were analyzed. This involved 137 patients with Freedom implants and 69 patients with CI512 implants. The data we received from Auto- NRT measurements on 9 electrodes, the electrodes that were measured are: 22, 19, 16, 13, 11, 8, 6, 3, and 1, listed from apical to basal. For 101 implantations (72 Nucleus Freedom and CI512 29) the three following conditions where needed and available: 1) Intraoperative Auto NRT data for pre- and post-conditioning present, 2) One or more datasets during the first fit period of 3 to 5 days present and 3) One or more datasets during routine checkups in the periods 6 – 12 months, 1 – 2 years and 2 - 4 years after first fit present. The software we used intraoperatively is named Custom Sound EP© postoperative Custom Sound© or Custom Sound EP©. To stimulate we used the software own automatically stimulation to measure the auto- NRT (compare chapter 17.3). In addition, we used a stimulus artifact reduction method (compare chapter 14.4), which is already
implemented in the software for auto NRT measurement and conducted additional conditioning (compare chapter 17.2). The average threshold of the remaining 72 Nucleus Freedom™ and 29 Nucleus CI512 subjects where calculated with the scaled ECAP threshold profile methods of Botros (Botros, 2010) and shifted back to the group mean average for comparing the average profiles between the different time periods. Besides the relative change of threshold profiles where determined. In addition the relative change for all threshold, averaged for all electrodes where calculated for each time period. Results As in Botros et al. (Botros, 2010) described the characteristic curve of the average NRT thresholds where shown. The curve increases linear towards the basal electrodes with a characteristic drop over the electrodes 3 to 5 which is interrupting a linear increase of the profiles is shown. The physiological position of the cochlear implant could be a possible explanation for this drop in this area. As observed already in previous studies, we could also show a typical decrease of the threshold profiles in the period intraoperatively- First Fit. Compared with the studies carried out by Gordon et al. (2004) a drop to 3-9 CL, Spivak et al. (2011) 10-20 CL and the group led by Mueller-Deile et al. (2005) a decrease by 2 +/- recorded 12 CL, we were able to show a decrease by 10-20 CL (current level ). In the course of, or during routine tests, the threshold profile changed, especially in the high and the low frequency ranges (E22/E1). The largest decrease of threshold profiles especially at low frequencies was detected. An explanation could be the consideration that postoperatively a tissue proliferation takes place such as scarring after surgery at the cochlear. Because of this tissue growth the distance for transmitting signal increases (Abbas, 1999). A reason for the mainly apical and basal change has not yet been named, but it is believed the reason for this in the anatomical position of the implant in the cochlear (distance/ proximity to the nerve to be excited), as well as the degeneration of the nerve (Abbas, 1999).
There are also studies on the impedance changes during the wearing time, which can influence the changes in the ECAP profiles. However, the form in which this happens and how this can be quantified is not yet fully understood. The relative change of the thresholds of intraoperative to postoperative averages 8CL (FF minus post- OP). When considering the increase in NRT levels at the time an average growth of NRT thresholds is almost linearly by 1.9 CL / year show (from FF to 2-4 Y”). The highly significant decrease of NRT thresholds between intraoperative and postoperative was detected in 83% of patients in detail. The significant increase in mean NRT thresholds in the course of routine investigations could be observed in detail in 56% of patients. In relation to the different types of implant, no significant difference could be observed. Conclusions There is a typical drop of the threshold profiles by 10-20 CL (current level) in the period intraoperatively to First Fit. The fact that during the tests the threshold profile, especially in the high and the low frequency ranges (E22/E1) changes, with the largest drop in the low frequencies, was detected. The relative change of the thresholds of intraoperative to postoperative was in average 8CL (First Fit minus postoperative). That means there is a linear increase in average NRT thresholds for measurements in the periods of 6 months to 4 years after implantation. When considering the rise of the NRT levels based on the time characteristic , we have shown an average increase of a NRT thresholds nearly linearly to 1.9 CL / year (from FF to 2-4” Y). These statements show a trend of the development of NRT profiles over time. This evaluation of the measurements can not only be used to adapt but also for the predictability of changes during the period of wearing. Due to individual subject as well as the extensive influence factors like the position of the electrode, it prevents a reliable estimation of ECAP based threshold profiles from intraoperative NRT thresholds. Still, a manual adjustment is required.

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