Biomedical Engineering Reference
In-Depth Information
6.10
AUDITORY BRAINSTEM IMPLANTS
Patients with neurofibromatosis develop tumors that, when removed, result in deafness
and also damage to the auditory nerve. In these cases, the only way to restore some hearing
is to use an auditory brain stem implant (ABI). This was first attempted in 1979 when an
array was placed over patients' ventral and dorsal cochlear nuclei by William House and
William Hitselberger.
Modern ABIs are similar in design and function to multichannel cochlear implants
except for the stimulating arrays. This is possible because the neurons in the cochlear
nucleus maintain a frequency sensitivity that maps to their spatial distribution. However,
array placement of the electrode array is more challenging than it is with cochlear implants.
First, the procedure is far more risky than placing implants in the visual cortex to try
to restore some vision. Damage to the visual cortex affects only sight, but at the brain stem
level every neuron destroyed could damage some other important function. It has taken
15 years of background research, experiments with animals, and surgery on cadavers to
convince surgeons that the procedure could be undertaken reasonably safely.
The cochlear nucleus complex is part of the floor of the lateral process of the fourth
ventricle, and it is partially obscured by the cerebellar peduncles, making access difficult.
The ventral cochlear nucleus (VCN) consists of a rostral area of spherical cells, a caudal
region containing octopus cells, and a central region containing a heterogenous mix of
globular, multipolar, and small cells. The dorsal cochlear nucleus (DCN) lies within the
lateral recess of the fourth ventricle, whereas the VCN extends to the foramen of Luschka.
Once the cochlear nucleus is exposed, the electrode array position must be optimized as
shown in Figure 6-41 by making use of electrically evoked auditory brainstem responses
obtained by stimulating the nucleus and recording from electrodes placed on the scalp. In
addition to facial nerve monitoring, the lower cranial nerves are also monitored to detect
any nonauditory sensations. Penetrating ABIs can be positioned in a similar manner.
To date, more than 500 patients have had ABIs implanted, but the results have not
been as positive as those for cochlear implants. ABIs enable the person to hear, but usually
not well enough to understand speech because the implant cannot separately stimulate
different groups of nerves corresponding to distinct frequency ranges.
FIGURE 6-41
Auditory brainstem
implant. (a) Sketch
of the cochlear
nucleus. (b) Some
surface electrode
alignment issues.
