2019 Conference on Implantable Auditory Prostheses

14-19 July 2019

Granlibakken, Lake Tahoe

Page 200

T37: EFFECTS OF ELECTRO-ACOUSTIC STIMULATION ON HEARING IN

GUINEA PIGS WITH HYBRID COCHLEAR IMPLANTS

Melissa B. Lawrence, Irina A. Omelchenko, Wenxuan He,

Douglas Fitzpatrick, Michael E. Reiss, Lina A. J. Reiss

Oregon Health and Science University, Portland, OR, USA

University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

The Electro-

Acoustic Stimulation (EAS) or “hybrid” cochlear implant (CI), combines high

-

frequency electric stimulation with residual low frequency acoustic hearing, and can be used

with a hearing aid in the same ear. Hybrid CI users have shown to benefit from this combination,

for example demonstrating greater speech comprehension in background noise in comparison

to traditional CI users. However, 30-55% of hybrid CI users lose residual acoustic hearing days

to months after implantation and activation of the CI, indicating both surgical trauma and electric

stimulation may play a role in this residual hearing loss. Through this study, we aim to identify

the mechanisms of residual acoustic hearing loss related to implantation and activation of hybrid

CIs in an animal model. Additionally, we aim to assess age as a risk factor for increased

residual hearing loss with hybrid CIs.

Two groups of guinea pigs, aged 12-21 months, were implanted with a CI in the left ear. After

four weeks of recovery, the stimulated group (n=3) received 40 hours a week of electro-acoustic

stimulation for a period of 20 weeks. The non-stimulated group (n=5) received no acoustic or

electric stimulation over the same time frame. Auditory brainstem responses (ABRs),

electrically-evoked auditory brainstem responses (EABRs), and cochlear microphonics (CMs)

and compound action potentials (CAPs) were recorded monthly over the course of the

experiment to assess changes in acoustic thresholds, electric hearing, and neural responses.

Following the completion of the experiment, all animals were perfused and cochlear tissue was

harvested for histological evaluation to assess physiological differences between implanted and

control ears, and between stimulated and non-stimulated groups.

Cochlear implantation led to an immediate increase in ABR thresholds in both young and old

animals with thresholds peaking between 3-5 weeks after surgery and then recovering and

stabilizing. The older animals exhibited larger post-operative threshold shifts than younger

animals tested in previous studies, suggesting that older age may increase the amount of

hearing loss caused by surgical trauma. Preliminary findings also suggest that the stimulated

animals had greater delayed low frequency ABR threshold shifts compared to the non-

stimulated group. For all animals pooled together, higher EABR thresholds were positively

correlated with greater high-frequency ABR threshold shifts after surgery. Additionally, EABR

amplitude growth function slopes were negatively correlated with high frequency ABR threshold

shifts. The correlations between acoustic threshold shifts and electric stimulation measures

suggest that surgical trauma may disrupt both acoustic hearing and electric stimulation via the

neural pathway.

This work was funded by a contract with Cochlear and by NIH/NIDCD grant R56 DC016308.