The Impact of a Cochlear Implant Electrode Array on the Middle Ear Transfer Function

Objectives: As a treatment for partial deafness with residual hearing in
the lower frequency range, the combined acoustic and electric stimulation
of the cochlea has become widespread. Acoustic stimulation is
provided by a hearing aid’s airborne sound and the electric stimulation
by a cochlear implant electrode array, which may be inserted through the
round window or a cochleostomy. To take advantage of that concept, it is
essential to preserve residual hearing after surgery. Therefore, the intracochlear
electrode array should not compromise the middle ear vibration
transmission. This study investigates the influence of different electrode
types and insertion paths on the middle ear transfer function and the
inner ear fluid dynamics.
Design: Sound-induced oval and round window net volume velocities
were calculated from vibration measurements with laser vibrometers
on six nonfixated human temporal bones. After baseline measurements
in the “natural” condition, a cochleostomy was drilled and closed with
connective tissue. Then, four different electrode arrays were inserted
through the cochleostomy. Afterwards, they were inserted through the
round window while the cochleostomy was patched again with connective
tissue.
Results: After having drilled a cochleostomy and electrode insertion,
no systematic trends in the changes of oval and round window volume
velocities were observed. Nearly all changes of middle ear transfer functions,
as well as oval and round window volume velocity ratios, were
statistically insignificant.
Conclusions: Intracochlear electrode arrays do not significantly
increase cochlear input impedance immediately after insertion. Any
changes that may occur seem to be independent of electrode array type
and insertion path.