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, 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.