New research at Johns Hopkins has clearly demonstrated
the ability of cochlear implants in very young animals to
forge normal nerve fibers that transmit sound and to
restore hearing by reversing or preventing damage to the
brain's auditory system.
The findings in cats, published in Science online Dec.
2, help explain why implants are up to 80 percent
successful in restoring hearing in young children born deaf
but rarely effective when implanted in congenitally deaf
adults, the researchers said.
"What we think this study tells parents of deaf
children is that if cochlear implants are being considered,
the earlier they're done, the better," said lead
investigator David Ryugo, a professor of
neuroscience at the
School of Medicine and its
Balance Center. "There is an optimal time window for
implants if they are to avoid permanent rewiring of hearing
stations in the brain and the long-term effects on language
learning that can result."
The Johns Hopkins team, building on years of
experience with cochlear implants in children and adults,
now has more evidence to support its recommendation that
the devices be installed by age 2 or earlier. More than
10,000 children are born deaf each year in the United
States, and an estimated 1.5 million people are believed to
be good candidates for cochlear implants.
Between ages 1 and 2, children's skulls are almost
fully grown, Ryugo said, minimizing complications from
brain surgery and greatly reducing the risk that the
electrical wiring will loosen or pull away from its
attachments under the scalp.
Cochlear implants are tiny devices designed to mimic
the work of a snail-like structure in the inner ear
containing fluid-filled canals and tissues. One of these is
the organ of Corti, which detects pressure impulses and
initiates electrical signals that travel along the inner
ear's auditory nerve to the brain, where the signals are
translated into distinct sounds.
Hearing aids simply amplify sound through an intact
auditory nerve-to-brain system; cochlear implants are much
more complicated. Composed of two parts, the devices
simulate hearing by picking up sound through an external
microphone located behind the ear and outside the scalp and
then transmitting sound as electrical signals across the
skin to an implanted receiver that is directly attached to
In the Science report, Ryugo, graduate student Erika
Kretzmer and John Niparko, a professor of otolaryngology,
report comparisons of brain tissue containing auditory
nerve fibers taken from cats that were born deaf. Three of
the cats underwent implants within months of birth, and
four did not get implants at all.
Both groups of cats were then exposed to three months
of sound stimulation, in which the researchers played music
and let the animals run around the lab, with its various
and everyday background noises. Included with the deaf cats
was a group of three similar cats with normal hearing for
The miniaturized cochlear implants were very similar
to those currently in use in children.
To gauge the animals' hearing development, the deaf
cats--both with and without implants--were subjected to a
unique sound, one for each cat, that measured the cat's
response to cues, such as the sharp clapping of hands or
ringing of a bell, to signify a food reward nearby. Within
a week, implanted kittens responded to their individual
sound cues, rushing to collect their food reward, while
those without implants did not.
Brain tissue analysis later showed that cats with
implants developed synaptic connections--regions between
connecting auditory nerve cells--that closely resembled
those of normal cats. The auditory nerve fibers contained
plentiful supplies of synaptic vesicles, which store the
transmitter chemicals necessary to pass sound signals
between nerve cells, and the specialized nerve membranes
that receive the signal were small and dome-shaped. In the
deaf cats without implants, synaptic vesicles were absent,
and the specialized nerve membranes were large and flat.
Niparko, who has for more than 20 years been studying
the effects of hearing restoration in children, said the
next research goal is to determine what happens between
birth and puberty in the auditory system to diminish the
chances of restoring hearing and language skills over time.
Future experiments will evaluate brain changes that occur
when an animal grows up in an environment that is devoid of
sound, which the scientists believe will guide future
therapies in restoring useful hearing to the deaf.
Funding support for this research was provided by the
National Institutes of Health, Emma Liepmann Endowment Fund
and Advanced Bionics Corp. in Sylmar, Calif., the
manufacturer of the cochlear implant devices used in the
study. None of the researchers involved in the study
received compensation from the manufacturer for
participation in the study.