ELECTRONIC
IMPLANTABLE HEARING DEVICES
A B I [Auditory Brainstem Implants]
Remarkable progress has been
made in prosthetic electrical stimulation of the auditory system. Cochlear
implants can provide high levels of auditory function.
Unfortunately, cochlear
implantation will not benefit a relatively small but significant group of
deafened individuals-those with neurofibromatosis type 2 (NF-2). Designed to
stimulate auditory neurons in the cochlear nucleus complex, the
auditory brainstem implant (ABI) has proven
helpful for these patients.
Clinical trials with the
device have successfully concluded and worldwide approximately 150 recipients
have received implants. The device has been shown to provide information about
the sound environment and has demonstrated improvement in communication ability
over lip-reading alone in most recipients. In a few users, some open-set speech
recognition has been observed.
The process of patient
selection, implantation, device activation, and follow- up differs significantly
from that of cochlear implantation.
HARDWARE DESIGN
The multichannel ABI
manufactured by Cochlear Corporation (Englewood, CO) consists of the same main
elements as a cochlear implant:
an implanted electrode array,
associated cabling, and a receiver / stimulator electronics package.
Externally, there is a
transmitter coil, microphone pickup, associated cabling, and a speech processor.
In the clinical trials,
patients received an eight- electrode implant and used the Spectra sound
processor (Cochlear Corporation, Englewood, CO). A 21-electrode array has
recently replaced the eight-electrode version, and the Sprint speech processor
has replaced the Spectra.
Highly important for ABI
function are a design that allows the activation of any pair of electrodes on
the array and the ability to rank order electrode channels with respect to any
electrode-specific pitch sensations experienced by the recipient.
Because of its intracochlear
electrode array placement, cochlear implantation generally does not require
extensive modifications to the normal electrode ordering.
Variations in cochlear nucleus
anatomy, placement of the electrode array, and proximity of the electrodes to
nonauditory and auditory structures require careful evaluation of responses to
fit the ABI properly.
Fitting is a dynamic process
that also benefits from periodic reassessments to accommodate slight but
significant changes that normally occur over time.
IMPLANTATION
Implantation of the ABI has
been primarily undertaken as part of surgery to remove eighth nerve tumors in
patients with NF-2;
However, implantation may now
occur in individuals who have already had both vestibular schwannomas removed.
A translabyrinthine approach
is used which provides an optimal exposure for tumor removal as well as access
to the foramen of Luschka, the opening into the lateral recess of the fourth
ventricle.
The ABI electrode array is
placed within this opening .
Monitoring of intraoperatively
evoked electrical potentials assists in identifying the optimal target for
placement of the array.
The array is positioned to
span the surface of the ventral and dorsal cochlear nuclei. It is secured by a
tamponade of polytetrafluoroethylene (Teflon) felt.
Approximately 4 to 8 weeks
after implantation, the device is activated for the first time. Subsequent
follow-up includes regular assessment of responses to stimulation and
appropriate reprogramming of the sound processor. Learning to use and benefit
from the ABI is a gradual process that requires some measure of determination
and motivation.
Initial responses to sound
vary, but most patients have reported extensive use and benefit from their
implants on a daily basis, with improvements in performance occurring over many
years.
