HopkinsMagWords
This is yet another version - incorporating outside sources now - of the piece I'd done on Dr. David Ryugo and his work with cochlear implants at the Med School. Hopkins Magazine asked me to write this for their February issue. So... here it is! Ryugo once more:
*side note: I had wanted to call this piece, "The Sound in the Furry: Cochlear Implants Preserve Auditory Nerve Structure in Deaf Cats"... but I'm not so sure what Faulkner would think. wink, wink.*
--Cochlear Implants Restore Auditory Nerve in Deaf Cats--
Some children born deaf may never hear again, but not if David Ryugo’s cats have anything to do with it. With the help of a rare collection of deaf felines, he and fellow scientists at the Johns Hopkins Center for Hearing and Balance have discovered why medicine’s most advanced hearing restoration devices – cochlear implants – benefit only 80% of the deaf children who have them.
Ryugo, a Ph.D. and professor of otolaryngology at Hopkins Medical School, used cochlear implants to electrically stimulate the nerves responsible for hearing in young, deaf cats. He did this over a three month period, and his results, published in Science on December 2nd, point to a link between introduced nerve activity, and the structure – abnormal or not – of the auditory nerve ending. Moreover, his work explains something that scientists haven’t understood for the last 4 decades: how cochlear implants actually work. “It has just been assumed that the auditory system functions normally when an implant is inserted,” Ryugo explained. “The reasoning has been something like this: if a car runs out of gas, it stops. Put gas in, and the car runs again.”
His work has filled in the gaps, though, illustrating exactly how the “gas” – or cochlear implants – jumpstart the hearing process. In his cats, all of which regained hearing after electrical stimulation, cochlear implants functioned for one reason: they preserved the normal structure at the end of the auditory nerve. Nerve endings permit communication between neurons in the auditory pathway and must be intact to send sound, as electrical waves, to the brain. When intact, they exhibit distinct structural characteristics which allow for the release and capture of chemicals called neurotransmitters. This process – occurring at the end of a healthy auditory nerve - permits sound to reach the brain. In other words, it permits hearing.
In a deaf ear, nerve endings are abnormal; they cannot convert sound vibrations into the electrical impulses that hit the auditory nerve, causing the brain to register sound. Like the deaf cats in Ryugo’s study, deaf children have inner ear damage which prevents them from generating electrical signals. Thus, as has been assumed, the cochlear implant generates them instead.
Now though, Ryugo’s research suggests that in this process, implants do one more thing: they preserve the auditory nerve ending. Nerve endings exhibit a certain plasticity, and as such, Ryugo says that implants are capable of preserving their structure (and thereby restoring hearing), if and only if they are inserted in a timely fashion-- before the nerve ending withers away.
Before evaluating the nerve endings of his deaf cats, inserted with implants, Ryugo made sure that they could hear. “The cats exhibited certain behavior responses that alerted us to the fact that they were responding to sounds in their environment,” Ryugo said. He had trained each implanted cat that a particular sound -- a rhythmic cadence, finger-snapping, or hand clap, for example -- signaled a special food treat. Different sounds signaled different treats (tuna, roast beef, sardines) to the different cats, he explained, “and voila! They would come to these sounds the same way a hearing cat comes when you shake its food box. We knew they could hear.”
Graduate student Erika Kretzmer compared nerve tissue from the inner ears of three groups: deaf cats with implants, deaf cats without them, and normal hearing cats. She discovered that deaf cats that had received cochlear implant stimulation actually maintained the nerve connections critical to hearing. “There wasn’t a significant difference between nerve endings of normal hearing cats and implanted cats,” Ryugo said.
In the deaf cats without implants, however, nerve connections were withered. “Their nerve endings were disrupted,” explained Ryugo. Instead of being characteristically branched and elaborate, “they were stubby and truncated, like trees growing on the edge of a windy cliff.” This image also describes nerve structure in deaf children.
For many reasons – including an appreciation of Deaf culture - not every parent of a deaf child considers cochlear implantation. But for those who do, understanding the trade-off between waiting and acting is important. Each year, 3 out of 1000 children in the United States are born unable to hear. The success of implantation in any of these children, Ryugo says, depends upon the progression of abnormalities at the auditory nerve ending. If children born deaf are left untreated for too long, the ends of their nerves may start to wither. (Ryugo observed this in his cats.) Eventually, this withering abnormality may become irreversible, meaning that implants will be powerless to preserve the normal nerve ending.
By illustrating how cochlear implants impact physical abnormalities in cats, Ryugo’s research is helping to define the “window of opportunity” for cochlear implantation in humans, who probably benefit in the same way; the auditory systems of both species are nearly identical, and “the cochlear implants used to stimulate our cats were the same technology that was developed for deaf children,” Ryugo said,”only smaller.” Ultimately, this work will give insight to doctors debating how long they can wait to perform risky implant surgery.
"There is an optimal time window for inserting implants,” Ryugo said. Doctors are sometimes hesitant to do it at young stages, though, because once implants are inserted, a patient loses all changes of regaining hearing on their own. Futhermore, “it is always difficult to know the age at which a child is strong enough to endure the surgical process,” Ryugo explained. “But 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.”
The study provides more evidence to support the current recommendation of many hearing specialists that cochlear implants be installed by age 2. “In Europe,” Ryugo said, “they’re even putting implants in kids at 12 months now.” The chairman of the Neurobiology and Anatomy department t the University of Utah School of Medicine, Thomas Parks, confirms the significance of Ryugo’s work as a call for advanced implantation. “This study provides additional strong evidence that early intervention with cochlear implants in children is essential. It prevents deterioration of neuronal circuits that are thought to be vital for both normal speech perception and sound localization, perhaps the two most serious problems for youth with severe hearing loss.”
According to Dr. Robert Shepherd, director of The Bionic Ear Institute in Melbourne, Australia, “the work of Dr. Ryugo and his colleagues is very significant because it shows for the first time, that when neural activity is re-introduced to the auditory nerve via a cochlear implant, changes at nerve ending structures vital to the auditory pathway can be at least partially reversed.” Ryugo’s work has important implications for understanding neural function in the pathological sense, and, as Shepherd explained, “it also suggests that the central auditory pathway is capable of plastic change after an implant’s in there.”
Shepherd, who is familiar with Ryugo’s work, recalled earlier studies in which the Hopkins otolaryngologist showed that nerve ending structures undergo change following deafness. “It was hypothesized that these changes were a result of a lack of neural activity in the deafened auditory nerve,” Shepherd said, “and this new work supports that hypothesis, too.”
Ryugo’s research is also interesting because, until now, there’s never been a good group of animal models to use for studying the effects of cochlear implants in the deaf. “We’re the only ones in the country with a colony of congenitally deaf cats,” Ryugo explained. The cochlear implants were unique, too; Advanced Bionics specially miniaturized the implants just for his study.
In the future, Ryugo wants to further define the “window of opportunity” for cochlear implantation by using his cats to perform time-course studies which will elucidate specific stages of development in the auditory nerve ending. “This could help doctors really pinpoint how much leeway they have, when thinking about surgery in children,” he said. Already, though, his work has sounded a call which is resonating – loud and true -- among hearing specialists worldwide: when it comes to implants in deaf children, the younger the better. And though until today, the window of opportunity for cochlear implantation has been unknown, and as such, closed for some 20% of deaf children, Ryugo’s work ensures that someday, physicians armed with knowledge of the auditory nerve ending will never again have to shut this window on deaf ears.
.MGW.
*side note: I had wanted to call this piece, "The Sound in the Furry: Cochlear Implants Preserve Auditory Nerve Structure in Deaf Cats"... but I'm not so sure what Faulkner would think. wink, wink.*
--Cochlear Implants Restore Auditory Nerve in Deaf Cats--
Some children born deaf may never hear again, but not if David Ryugo’s cats have anything to do with it. With the help of a rare collection of deaf felines, he and fellow scientists at the Johns Hopkins Center for Hearing and Balance have discovered why medicine’s most advanced hearing restoration devices – cochlear implants – benefit only 80% of the deaf children who have them.
Ryugo, a Ph.D. and professor of otolaryngology at Hopkins Medical School, used cochlear implants to electrically stimulate the nerves responsible for hearing in young, deaf cats. He did this over a three month period, and his results, published in Science on December 2nd, point to a link between introduced nerve activity, and the structure – abnormal or not – of the auditory nerve ending. Moreover, his work explains something that scientists haven’t understood for the last 4 decades: how cochlear implants actually work. “It has just been assumed that the auditory system functions normally when an implant is inserted,” Ryugo explained. “The reasoning has been something like this: if a car runs out of gas, it stops. Put gas in, and the car runs again.”
His work has filled in the gaps, though, illustrating exactly how the “gas” – or cochlear implants – jumpstart the hearing process. In his cats, all of which regained hearing after electrical stimulation, cochlear implants functioned for one reason: they preserved the normal structure at the end of the auditory nerve. Nerve endings permit communication between neurons in the auditory pathway and must be intact to send sound, as electrical waves, to the brain. When intact, they exhibit distinct structural characteristics which allow for the release and capture of chemicals called neurotransmitters. This process – occurring at the end of a healthy auditory nerve - permits sound to reach the brain. In other words, it permits hearing.
In a deaf ear, nerve endings are abnormal; they cannot convert sound vibrations into the electrical impulses that hit the auditory nerve, causing the brain to register sound. Like the deaf cats in Ryugo’s study, deaf children have inner ear damage which prevents them from generating electrical signals. Thus, as has been assumed, the cochlear implant generates them instead.
Now though, Ryugo’s research suggests that in this process, implants do one more thing: they preserve the auditory nerve ending. Nerve endings exhibit a certain plasticity, and as such, Ryugo says that implants are capable of preserving their structure (and thereby restoring hearing), if and only if they are inserted in a timely fashion-- before the nerve ending withers away.
Before evaluating the nerve endings of his deaf cats, inserted with implants, Ryugo made sure that they could hear. “The cats exhibited certain behavior responses that alerted us to the fact that they were responding to sounds in their environment,” Ryugo said. He had trained each implanted cat that a particular sound -- a rhythmic cadence, finger-snapping, or hand clap, for example -- signaled a special food treat. Different sounds signaled different treats (tuna, roast beef, sardines) to the different cats, he explained, “and voila! They would come to these sounds the same way a hearing cat comes when you shake its food box. We knew they could hear.”
Graduate student Erika Kretzmer compared nerve tissue from the inner ears of three groups: deaf cats with implants, deaf cats without them, and normal hearing cats. She discovered that deaf cats that had received cochlear implant stimulation actually maintained the nerve connections critical to hearing. “There wasn’t a significant difference between nerve endings of normal hearing cats and implanted cats,” Ryugo said.
In the deaf cats without implants, however, nerve connections were withered. “Their nerve endings were disrupted,” explained Ryugo. Instead of being characteristically branched and elaborate, “they were stubby and truncated, like trees growing on the edge of a windy cliff.” This image also describes nerve structure in deaf children.
For many reasons – including an appreciation of Deaf culture - not every parent of a deaf child considers cochlear implantation. But for those who do, understanding the trade-off between waiting and acting is important. Each year, 3 out of 1000 children in the United States are born unable to hear. The success of implantation in any of these children, Ryugo says, depends upon the progression of abnormalities at the auditory nerve ending. If children born deaf are left untreated for too long, the ends of their nerves may start to wither. (Ryugo observed this in his cats.) Eventually, this withering abnormality may become irreversible, meaning that implants will be powerless to preserve the normal nerve ending.
By illustrating how cochlear implants impact physical abnormalities in cats, Ryugo’s research is helping to define the “window of opportunity” for cochlear implantation in humans, who probably benefit in the same way; the auditory systems of both species are nearly identical, and “the cochlear implants used to stimulate our cats were the same technology that was developed for deaf children,” Ryugo said,”only smaller.” Ultimately, this work will give insight to doctors debating how long they can wait to perform risky implant surgery.
"There is an optimal time window for inserting implants,” Ryugo said. Doctors are sometimes hesitant to do it at young stages, though, because once implants are inserted, a patient loses all changes of regaining hearing on their own. Futhermore, “it is always difficult to know the age at which a child is strong enough to endure the surgical process,” Ryugo explained. “But 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.”
The study provides more evidence to support the current recommendation of many hearing specialists that cochlear implants be installed by age 2. “In Europe,” Ryugo said, “they’re even putting implants in kids at 12 months now.” The chairman of the Neurobiology and Anatomy department t the University of Utah School of Medicine, Thomas Parks, confirms the significance of Ryugo’s work as a call for advanced implantation. “This study provides additional strong evidence that early intervention with cochlear implants in children is essential. It prevents deterioration of neuronal circuits that are thought to be vital for both normal speech perception and sound localization, perhaps the two most serious problems for youth with severe hearing loss.”
According to Dr. Robert Shepherd, director of The Bionic Ear Institute in Melbourne, Australia, “the work of Dr. Ryugo and his colleagues is very significant because it shows for the first time, that when neural activity is re-introduced to the auditory nerve via a cochlear implant, changes at nerve ending structures vital to the auditory pathway can be at least partially reversed.” Ryugo’s work has important implications for understanding neural function in the pathological sense, and, as Shepherd explained, “it also suggests that the central auditory pathway is capable of plastic change after an implant’s in there.”
Shepherd, who is familiar with Ryugo’s work, recalled earlier studies in which the Hopkins otolaryngologist showed that nerve ending structures undergo change following deafness. “It was hypothesized that these changes were a result of a lack of neural activity in the deafened auditory nerve,” Shepherd said, “and this new work supports that hypothesis, too.”
Ryugo’s research is also interesting because, until now, there’s never been a good group of animal models to use for studying the effects of cochlear implants in the deaf. “We’re the only ones in the country with a colony of congenitally deaf cats,” Ryugo explained. The cochlear implants were unique, too; Advanced Bionics specially miniaturized the implants just for his study.
In the future, Ryugo wants to further define the “window of opportunity” for cochlear implantation by using his cats to perform time-course studies which will elucidate specific stages of development in the auditory nerve ending. “This could help doctors really pinpoint how much leeway they have, when thinking about surgery in children,” he said. Already, though, his work has sounded a call which is resonating – loud and true -- among hearing specialists worldwide: when it comes to implants in deaf children, the younger the better. And though until today, the window of opportunity for cochlear implantation has been unknown, and as such, closed for some 20% of deaf children, Ryugo’s work ensures that someday, physicians armed with knowledge of the auditory nerve ending will never again have to shut this window on deaf ears.
.MGW.
posted by Meagan G at
1:36 PM
<< Home