A Comparison of Electrical Stimulation Levels Across Ears for Children With Sequential Bilateral Cochlear Implants
Authors: Galvin Karyn L. , Abdi Roghayeh , Dowell Richard C. , Nayagam Bryony 
Published: February 2019

Objectives:
To compare threshold and comfortable levels between a first and second cochlear implant (CI) for children, and to consider if the degree of difference between CIs was related to the age at bilateral implantation or the time between implants. A secondary objective was to examine the changes in levels over time for each CI.

Design:
Fifty-seven participants were selected from the 146 children and young adults who received a first Nucleus CI as a child, and received a second implant at the Royal Victorian Eye and Ear Hospital between September 2003 and December 2011. Exclusion criteria included an older implant type, incomplete array insertion, incomplete data available, and a pulse width higher than the default. Using measurements from clinical sessions, the threshold levels, comfortable levels, and dynamic range of electrical stimulation were compared at three electrode array regions and at the “initial” (first 10 weeks), 2-year, and 5-year postoperative time points. The T-ratio and C-ratio for each array region and each time point were calculated by dividing each mean (n = 3 electrodes) level for the second implant by that for the first implant.

Results:
The T-ratio was generally not significantly different to one, indicating no differences in threshold levels between the second and first implants; however, threshold levels were lower for the second implant in the apical region at the initial time point, and there was a significant difference in threshold levels in the apical region for children with a Contour Advance array for the second implant and an older-style array (i.e., Contour) for the first implant. For each implant individually, there were no significant changes in threshold levels across time. The C-ratio was significantly <1 at all electrode array regions at all time points, indicating lower comfortable levels for the second implant. The difference between implants was greater for children with variable array type (i.e., a Contour Advance array for the second implant and an older-style Contour or Straight array for the first implant). There was a significant increase in the C-ratio between the initial and 2-year time points, driven by an increase in comfortable levels for the second implant over this time period. A longer time between implants was associated with a narrower dynamic range, due to lower comfortable levels, for the second implant.

Conclusions:
For this sequentially implanted group, threshold levels were similar between implants, with some differences in cases with a newer array type for the second implant. Comfortable levels were lower for the second implant; although this difference decreased between the initial and 2-year postoperative time points, it was still evident at 5 years postoperative. A longer time between implants was associated with a narrower dynamic range. These findings are likely to apply to children using other brands of implant. Knowing what to expect in terms of programming children with a second implant will help clinicians to recognize and respond to unexpected outcomes. The work raises important questions to be addressed in future research regarding the implications of the programming outcomes for actual listening performance.