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First Implant-Induced Changes in Rostral Brainstem Impair Second Implant Outcomes in Sequential Bilateral Cochlear Implant Children With Long Inter-Implant Delay.

HYPOTHESIS
Long-term unilateral use of a cochlear implant (CI) induces abnormal maturation of the rostral brainstem innervating the contralateral ear.


BACKGROUND
In sequential bilateral CI children with long inter-implant delay, both sides of auditory cortices were dominantly activated by the first CI, but mechanisms of this abnormal development of the auditory system remain unclear.


METHODS
Fifteen sequential bilateral CI children with long delay (mean ± SD, 28.7 ± 12.1 mo) underwent electrically evoked auditory brainstem response (EABR) testing using each of the first and second CI (CI1 and CI2, respectively), immediately after the second implantation. CI1 and CI2 were implanted at age of 23.7 ± 9.6 and 53.1 ± 12.1 months (mean ± SD), respectively. Apical-to-basal difference in electrically evoked interwave III-V latencies (eIII-eV slope) which decreases with implant use was compared between the CI1 and CI2 sides. Their speech perception scores were evaluated 3 years after the second implantation.


RESULTS
eIII-eV slopes evoked by the CI2 showed less mature pattern than those by the CI1. This CI2 versus CI1 difference in eIII-eV slopes, however, reduced as the inter-implant delay was prolonged, suggesting CI1-induced maturation of the rostral brainstem innervating the second ear before the second implantation. The smaller CI2 versus CI1 difference in eIII-eV slopes at the second implantation was correlated to poorer outcomes using the CI2 than the CI1.


CONCLUSIONS
In this population, long-term unilateral CI use induced re-organization of the rostral brainstem innervating the second ear, which affected hearing outcomes using the CI2. Evaluation of eIII-eV slopes at the second implantation may be useful to predict hearing outcomes with CI2.