Proefschrift_vd_Beek

In contrast with previous reports, another study did not show significant differences in T-levels between patients with the Nucleus Contour and the straight array (Hughes, 2003).

Better frequency selectivity is, in addition to lowered threshold stimulation levels, thought to be associated with improved speech perception. Different methods have been used to obtain estimates of the spatial selectivity, as the longitudinal spread of excitation along the tonotopic cochlea is of utmost importance for the spectral percepts of the patients. Psychophysical studies indicated that patients are able to exploit the tonotopic organization of the cochlea and a correlation was found between electrode discrimination and speech perception (Busby, Tong, & Clark, 1993). However, psychophysical measures of spatial selectivity failed to correlate with the distance of the electrode array to the modiolus (Cohen et al., 2001). Different approaches are needed to measure spatial selectivity without the drawbacks of subjective tests. An important role inmeasuring spatial selectivity may arise for the telemetry systems of the contemporary cochlear implants (neural response imaging/telemetry, NRI/ NRT, of Clarion and Nucleus cochlear implants respectively). These systems can measure both the intracochlear potential during current injection as well as the small biological potentials generated by the auditory nerve. Although spatial selectivity measurements using eCAP are still under development, recent data point out that a closer proximity of the electrode contacts to the modiolus is associated with a narrower excitation pattern (Cohen et al., 2003; Hughes, 2003). Recently, an impedance model has been developed, which can be used to study the spatial distribution of the injected current (Vanpoucke, Zarowski, Casselman, Frijns, & Peeters, 2004). This impedance model is based on objective measurements obtained with Electrical Field Imaging (EFI) of the Clarion cochlear implant. Initial clinical evaluations of the Clarion HiFocus 1 (Frijns, Briaire, de Laat, & Grote, 2002) and Nucleus Contour (Tykocinski et al., 2001) showed excellent speech understanding. After implantation with the Nucleus Contour, a large variation in the degree of coiling across subjects could be observed. This variation in coiling is presumably surgeon and patient dependent and showed no significant effect on thresholds or speech perception (Marrinan et al., 2004). A recent study showed that the perimodiolar designed Nucleus Contour electrode contributed to improved speech understanding compared to its straight predecessor (Bacciu et al., 2005). In 2002, the manufacturer of the Clarion HiFocus with a separate positioner system (Advanced Bionics Corp., Sylmar, CA) withdrew its system from the market. The decision to withdraw the positioner was made after the Food and Drug Administration reported meningitis cases associated with cochlear implantation (http://www.fda.gov/cdrh/safety/ cochlear.html). More research to reveal the causes of the meningitis of cochlear implant patients followed and recommendations concerning the prophylaxis and treatment were published (Cohen, Roland, Jr., & Marrinan, 2004; Lefrancois & Moran, 2003; Nadol, Jr. & Eddington, 2004; Reefhuis et al., 2003). Afterward, the array was inserted without positioner, as a one-component electrode, after a hypothesis was postulated suggesting that space between the positioner and the electrode could act as a possible

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