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 say.
"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," says David Ryugo, Ph.D., the lead investigator in the study. "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," adds Ryugo, a professor of otolaryngology and neuroscience at The Johns Hopkins University School of Medicine and its Hearing and Balance Center.
The Hopkins team, building on years of experience with cochlear implants in children and adults, now has more evidence to support their 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 notes, minimizing complications from brain surgery and greatly reducing the risk that the electrical wiring will loosen or pull away from their 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.
Unlike hearing aids, which 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 the brain.
In the Science report, Ryugo, with graduate student Erika Kretzmer, B.S., and Hopkins professor of otolaryngology John Niparko, M.D., 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 further comparison.
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 in 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 regions, called synaptic connections, 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, says 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, the 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.
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