The Change in Electrical Stimulation Levels During 24 Months Postimplantation for a Large Cohort of Adults Using the Nucleus® Cochlear Implant
Authors: Gajadeera Emalka A. , Galvin Karyn L. , Dowell Richard C. , Busby Peter A. 
Published: February 2017

Objectives:
To examine electrical stimulation data over 24 months postimplantation in adult implant users. The first aim was to calculate mean T and C levels for seven time points, for four cochlear segments, and two array types. The second aim was to (a) analyze the degree of change in each of the T and C levels as a function of dynamic range for six consecutive time point comparisons, for the four segments, and (b) to determine the proportion of participants with an acceptable degree of change. The third aim was to examine relationships between demographic factors and degree of change.

Design:
T levels, C levels, and dynamic ranges were extracted for 680 adults using Nucleus implants for the following postimplant time points: 2-, 3-, 6-, 9-, 12-, 18-, and 24-month. For each time point, mean levels were calculated for the four segments. The degree of change in each of the levels was analyzed for six consecutive time point comparisons. The criterion for an acceptable degree of change was ≤20% of DR.

Results:
Mean T level was significantly lower for the 2-month time point compared with all time points after the 3-month time point. Mean C level was significantly lower for the 2- and 3-month time points compared with all other time points. Mean T level was significantly lower for the apical compared with all other segments and for the lower-basal compared with the upper-basal segment. Mean C level was significantly different across all four segments. Mean C level for the basal segments was 4 CLs higher for the perimodiolar array compared with the straight array. No significant differences were evident for the mean degree of change between consecutive time point comparisons. For all segments, approximately 65 to 75% of the participants showed an average acceptable degree of change in levels from the 3- to 6-month comparison. The mean degree of change in T levels was significantly greater for the basal segments compared with all other segments. The mean degree of change in levels was significantly greater for the otosclerosis group compared with all other groups, and for the prelingual onset of deafness group compared with the postlingual group.

Conclusion:
Given the very large cohort, this study provides evidence for the mean levels and the degree of change in these levels that should be expected for four segments in the first 24 months postimplantation for adults using Nucleus implants. The mean T and C levels were consistent after the 3- and 6-month time points postimplant, respectively. The degree of change was variable between individuals. For each segment, however, a large percentage of participants showed an average change of ≤20% in each of the T and C levels from the 3- to 6-month comparison. Given the large degree of change in levels for some groups, the results provide strong evidence in favor of frequent monitoring of levels in the first 24 months postimplantation for patients with otosclerosis, prelingual onset of deafness, and those who exhibit >20% change in levels after 3 months postimplantation.