Intracochlear Position of Cochlear Implants Determined Using CT Scanning versus Fitting Levels: Higher Threshold Levels at Basal Turn

Abstract

Objectives: In this study, the effects of the intracochlear position of cochlear implants on the clinical fitting levels were analyzed. Design: A total of 130 adult subjects who used a CII/HiRes 90K cochlear implant with a HiFocus 1/1J electrode were included in the study. The insertion angle and the distance to the modiolus of each electrode contact were determined using high-resolution CT scanning. The threshold levels (T-levels) and maximum comfort levels (M-levels) at 1 year of follow-up were determined. The degree of speech perception of the subjects was evaluated during routine clinical follow-up. Results: The depths of insertion of all the electrode contacts were determined. The distance to the modiolus was significantly smaller at the basal and apical cochlear parts compared with that at the middle of the cochlea (p < 0.05). The T-levels increased toward the basal end of the cochlea (3.4 dB). Additionally, the M-levels, which were fitted in our clinic using a standard profile, also increased toward the basal end, although with a lower amplitude (1.3 dB). Accordingly, the dynamic range decreased toward the basal end (2.1 dB). No correlation was found between the distance to the modiolus and the T-level or the M-level. Furthermore, the correlation between the insertion depth and stimulation levels was not affected by the duration of deafness, age at implantation or the time since implantation. Additionally, the T-levels showed a significant correlation with the speech perception scores (p < 0.05). Conclusions: The stimulation levels of the cochlear implants were affected by the intracochlear position of the electrode contacts, which were determined using postoperative CT scanning. Interestingly, these levels depended on the insertion depth, whereas the distance to the modiolus did not affect the stimulation levels. The T-levels increased toward the basal end of the cochlea. The level profiles were independent of the overall stimulation levels and were not affected by the biographical data of the patients, such as the duration of deafness, age at implantation or time since implantation. Further research is required to elucidate how fitting using level profiles with an increase toward the basal end of the cochlea benefits speech perception. Future investigations may elucidate an explanation for the effects of the intracochlear electrode position on the stimulation levels and might facilitate future improvements in electrode design.

Towards a Complete In Silico Assessment of the Outcome of Cochlear Implantation Surgery

“…cu [44] . This value, known as eCAP threshold, depends on the region of the cochlea where the stimulating electrode is located, and increases towards the base up to approximately 350 cu [27,44].…”

“…However, the perimodiolar approach uses an electrode design that may cause severe internal trauma due to the penetration of the array and translocation to scala vestibuli, while having the same efficacy as lateral electrodes in terms of eCAP measurements [28]. There is still controversy about whether the electrode-modiolus distance has an influence on the CI outcome [20,24,26,27,29,45].…”

“…Dynamic range, place pitch or spread of excitation on the auditory nerves have been a focus of interest due to their direct relation to the CI performance [6,13,17,20,23]. ANF response according to the intra-cochlear electrode position has also been investigated in several studies, in which different and often contradictory conclusions were reported [24][25][26][27][28][29]. To reduce the spread of excitation and obtain a better pitch discrimination, an electrode position closer to the modiolus has been reported to decrease the threshold in which the first response on the ANF is obtained [24,26].…”

“…To reduce the spread of excitation and obtain a better pitch discrimination, an electrode position closer to the modiolus has been reported to decrease the threshold in which the first response on the ANF is obtained [24,26]. On the other hand, some authors observed that the distance between the electrode and the modiolus did not affect, or affected minimally, this neural response threshold, or that its relation could not be clearly determined [27,30]. Generally, a better speech perception of CI patients is obtained when the electrode is completely placed on the scala tympani [28,29].…”

Electrical Stimulation of the Auditory System

“…However, the variability is such that the same SGC count can show variations of between 30% and 75% for CNC words, or the same CNC word score can be associated with 3000 or 18,000 SGCs. Examining the threshold current for detecting electrical stimulation in a group of 130 lateral wall electrode array users [36] showed significant differences between four groups: the increase in group mean threshold being associated with a reduction in monosyllabic word score. This works suggests that a higher SGC population (lower electrical threshold) is associated with better speech understanding.…”