and balance experts at Johns Hopkins report successful
testing in animals of an
electrical device that partly restores a damaged or
impaired sense of balance.
Though human testing of the so-called multichannel
vestibular prosthesis remains a few years
away, the scientists say that such a device, which is
partially implanted in the inner ear, could aid the
30,000 Americans the experts' estimates show are coping
with profound loss of inner ear balance.
These people often suffer from unsteadiness, disequilibrium
or wobbly vision. Problems with vestibular
sensation can be inherited at birth or result from use of
antibiotics, chemotherapy drugs, Meniere's
disease, viral infection, stroke or head trauma.
The study, done in chinchillas because their inner ear
function is well studied, "is proof of
concept that we can restore three-dimensional sensation of
head movement with a multichannel
vestibular prosthesis," said Charles C. Della Santina,
director of the Vestibular Neuroengineering
Laboratory at Johns Hopkins.
"While everyone knows about the five senses of sight,
smell, taste, touch and hearing, few
people think about a possible sixth sense — the
sensation of head orientation and movement — until
system fails," said Della Santina, who has been working on
this prosthesis since 2002.
In its report in the June edition of the journal
I.E.E.E. Transactions on Biomedical Engineering,
the Johns Hopkins team showed that a matchbox-size
prototype device, weighing less than three
ounces, effectively mimics the workings of the inner ear's
three semicircular canals by sensing head
rotation and transmitting that information to the brain.
Adapting the design of cochlear implants, which
restore hearing through electrical stimulation
of the cochlear nerve, researchers constructed a circuit
that could measure and transmit 3-D balance
information to the brain through multiple electrodes
connected to the vestibular nerve.
The device, which researchers started testing more
than a year ago, consists of a head-
mounted battery-operated box containing the sensors, which
are positioned outside the head so that
the sensors are parallel to the animal's actual
semicircular canals, where head rotation is normally
sensed. The sensors are connected to a microprocessor and
up to eight electrodes surgically implanted
in the inner ear and separately connected to nerve endings.
Each electrode can act as one information
Della Santina says that people disabled by loss of
vestibular sensation often feel chronically off
balance and lose the ability to keep the eyes steadily
pointed at an object when they move their head,
"seeing the world like the wobbly image on a shaky handheld
According to Della Santina, an assistant professor of
otolaryngology — head and neck surgery and
biomedical engineering at the School of Medicine, this
is the first implantable device made with
multiple sensors and channels of processing that can
measure and encode head rotation in all
Each of the three sensors, he notes, can measure the
speed of head rotation about one of three
axes, or directional planes.
Della Santina says that previous implants developed
elsewhere were limited to one functioning
sensor and electrode and one plane or axis of rotation,
"when, in reality, we move in multiple
Every measurement in the balance device is processed
in the implanted central microprocessor
unit, using computer software developed by Della Santina
and his team.
Once processed, the information is used to tailor
timing of brief electronic pulses through the
electrodes implanted near the three branches of the
vestibular nerve that respond to changes in head
rotation. These branches normally carry signals from the
inner ear's three semicircular canals.
In the chinchilla tests, pulses lasting less than a
millisecond were delivered with timing patterns
that mimicked normal nerve activity.
Della Santina and his colleagues first caused
imbalance in chinchillas by treating them with a
high dose of gentamicin, an antibiotic known to wipe out
the tiny hairlike projections on cells in the
inner ear canals that are normally key to sensory balance
function. Treated animals displayed unsteady
walking and wobbly eye movements commonly seen in people
with impaired balance. Precise
measurements of eye movements, using a technique of video
tracking adapted by researchers, were
made during a fixed set of head movements. Results
confirmed profound loss of normal eye-stabilizing
The animals were then fitted with the vestibular
prosthesis, with sensors oriented parallel to
the semicircular canals they replaced. Post-activation eye
testing in three chinchillas showed that the
animals partially regained their vision-stabilizing
Researchers say that many hurdles remain before a
human device will be available. Efforts are
under way to reduce electrical interference to other nerve
branches and to refine the timing patterns
of electrical stimulation to make them more like normal. In
addition, the scientists plan to work on
making the device smaller and hermetically sealed so that
it can fit completely inside the head
beneath the skin.
The resulting vestibular implant, they say, could
ultimately serve as a safety net, for example,
for people who need high-dose gentamicin therapy to combat
severe abdominal infections after bowel
"People with profoundly impaired balance need better
treatment options," Della Santina said.
"Many cope through rehabilitation exercises or by
restricting their activities, but the chronic
disequilibrium and blurry vision can be disabling."
The study was funded by the National Institute on
Deafness and Other Communication
Disorders, a member of the National Institutes of Health;
and by the American Otological Society.
Patents are pending on several aspects of the
Other researchers involved in this study were Americo
Migliaccio and Amit Patel.