Changes in apical cochlear mechanics after cochlear implantation

Project Summary
Sound entering the cochlea induces a longitudinally propagating travelling wave along the cochlear partition
which includes the organ of Corti. The organ of Corti amplifies travelling waves via force production by outer hair
cells. Where this amplification is lost, an array of electrodes called a cochlear implant replaces sound stimulation
with electrical stimulation of the auditory nerve. Improved cochlear implants combine electrical and sound
stimulation in patients with some intact hearing. These combined implants lead to improved performance.
However, approximately half of combined cochlear implant recipients experience a loss of their remaining
hearing months after implantation. This implantation-induced hearing loss reduces speech recognition and
musicality.
Implantation-induced hearing loss may have multiple interacting causes; immune, metabolic, and mechanical.
We hypothesize that cochlear scarring (fibrosis/ossification) induced by implantation disrupts travelling wave
propagation to the site of low frequency hearing. Links between hearing loss and implant-induced scarring are
seen in rodent models, reflecting clinical findings. However, there are no direct measurements of the mechanical
consequences of cochlear implantation for low frequency hearing.
We will combine our expertise with rodent models of cochlear implantation and the use of the latest generation
of imaging interferometry – optical coherence tomography (OCT). In a bid to produce the first data of its kind, we
will use OCT vibrometry to characterize low frequency mechanical function in the cochlear apex of chronically
implanted animals. We will then produce a 3D map of the scarring inside each cochlea using OCT imaging.
Coupled with histology and machine learning powered image analysis, we will compare the extent, location and
type of scarring with organ of Corti gain, tuning, distortion, phase and group delay in each cochlea. The results of
our OCT vibrometry experiments will be interpreted by computer models of cochlear function. Experiments will also be
conducted in acutely implanted models to assess the effect of the cochlear implant upon apical mechanics
prior to scarring. Additionally, we will use a model with noise induced hearing loss prior to implantation to test the
contribution of high frequency outer hair cells to low frequency hearing performance.
Our multidisciplinary team will offer a direct insight into cochlear implant-induced hearing loss and will allow us
to test the scarring hypothesis. This project will guide avenues of research geared towards minimizing or
preventing cochlear implant-induced hearing loss, and lead to improved quality of life for the recipients of
cochlear implants.

Grant Number: 5R21DC020794-03
NIH Institute/Center: NIH
Principal Investigator: George Burwood