Proefschrift_vd

44 | Chapter 3

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

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