Growth Factors May Boost CI Function
Editor: Scientists have discovered that injecting growth factors into
an animal's ears before cochlear implant (CI) surgery increases CI
performance. The story is from Newsday (http://www.newsday.com/news/health/ny-dstop2621104mar12.story)
via bhNEWS.
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Doses of growth factors may boost function of implants
By Robert Cooke STAFF WRITER
March 12, 2002
MODERN HEARING DEVICES called cochlear implants may work far better
if growth factors are used first to reawaken damaged nerves, scientists
report.
Cochlear implants - hearing devices that directly stimulate the
auditory nerve - are now widely used to restore at least crude hearing
in people who are not helped by even the best hearing aids. The implant
device's performance is sometimes hampered, however, because nerves
serving the ear have begun to fail after years of not being used.
Trying to see if resurrecting nerve function is possible,
neuroscientist Josef Miller and his colleagues implanted tiny pumps into
the ears of deafened animals. The pumps released constant low doses of
nerve growth factors for a month before the animals were given cochlear
implants. The treatment, tried only in laboratory animals, succeeded.
Use of the nerve growth factors not only increased the survival of
neurons in the animals' auditory nerve, Miller and his colleagues
reported, but it "significantly enhanced the functional
responsiveness" of the animals' hearing once they were set up with
cochlear implant devices.
"The animal work looks terrific, and it's supported by a variety
of other studies" that suggest it's likely to work when applied to
humans, Miller said in an interview. "Human auditory nerve cells
are sensitive to these growth factors," so they should respond
properly, he added.
The slowdown or loss of nerve function occurs because as sensory
cells in the ear die, they stop producing growth factors that the
auditory nerve cells need to stay healthy. "But it's not like
muscle atrophy from lack of use," Miller explained. "With the
death of cells, there is a lack of the survival factors that the cells
and the central nervous system normally provide," And, given time,
the auditory nerve cells begin to die away.
Hearing impairment - including deafness - "is the most frequent
disability in industrialized countries," the research team wrote in
the Proceedings of the National Academy of Sciences. Loud machinery,
blaring music, drug use and infectious diseases all contribute to what
can be considered a modern epidemic of hearing loss, approaching
deafness. Hearing ability also wanes with age as the number of so-called
hair cells in the cochlea decreases.
More than 40,000 cochlear implants are now in use worldwide. Although
the battery-powered implants do not completely restore normal hearing,
they do allow deaf people to recognize numerous sounds. Some patients
can even understand much of what is said to them.
"They're doing remarkably well," Miller said. "The
majority of them can talk" and hear on the telephone because their
cochlear implants work so well.
Maurice Miller, professor of audiology at New York University, said
that Josef Miller's results are very important. "It's on the
threshold of a breakthrough in the treatment of sensory deafness in
humans. He's an outstanding scientist, and this is quality stuff."
(The two Millers are not related.)
In a report, Josef Miller and coworkers from Sweden and Japan
predicted their results will "have great clinical importance for
the treatment of deaf patients with cochlear implants." Josef
Miller and one colleague, Richard Altschuler, are both at the Kresge
Hearing Research Institute at the University of Michigan.
The cochlea, a snail-shaped organ in the inner ear, is filled with
fluid and lined with hair cells. The task of these highly specialized
cells is to convert the mechanical vibrations of sound into electrical
impulses that are sent to the brain.
As the hair cells die off - either gradually or suddenly - the nerves
carrying electric impulses are left unemployed. Also, Josef Miller said,
because growth factors released by the hair cells and other cells are
missing, the auditory nerve cells begin to deteriorate.
The technology for delivering small, constant doses of the growth
factors already exists. What might pose problems, Josef Miller said, is
the danger of side effects. When the same growth factors - called
neurotrophic factors - are used for other treatments, the side effects
"are enormous. Most trials have been discontinued because of the
side effects," which include nervousness, sleeplessness, digestive
disorders and impotence. The growth factors were being tested against
neurological maladies, including Parkinson's disease and Lou Gehrig's
disease.
Josef Miller and his colleagues hope that side effects can be
avoided, or minimized, by confining the growth factors to the inner ear.
"We're in a potentially unique situation, because it [the cochlea]
is so encased in bone, and there are very few channels for it to get
into other parts of the body," he explained.
If the research progresses as well as hoped, "we have a goal of
being able to begin in two years" with tests in humans, Josef
Miller added.
The growth factor treatment would not be useful for people who wear
conventional hearing aids, which work by sending amplified sound into
the ear to stimulate any hair cells that remain. Cochlear implants are
aimed at persons who have lost almost all of their hair cells and need
devices that bypass the hair cells and stimulate the nerves directly.
Tiny electronic probes are inserted into the cochlea, where they
interact with the auditory nerve. By this method, even if the hair cells
have all died, signals can still enter the brain.
After almost 40 years of research and development, the cochlear
implant "now provides significant speech understanding, with a
score for everyday sentence understanding of about 80 percent, without
lip reading, in the majority of patients," the research team wrote.
How the Implant Works
1. Microphone picks up sound and converts it to electrical energy
that is amplified and filtered by a small processor that can be tucked
into clothing or worn on a belt.
2. Amplified signal sent to external transmitter that converts it to
magnetic energy.
3. Magnetic signal sent to internal receiver implanted under skin.
4. Signal travels from internal receiver to inner ear on a platinum
electrode.
5. Current flowing between active electrode in inner ear and nearby
ground electrode stimulates nerve fibers in a way the brain interprets
as sound.
