MANAGING HEARING LOSS: New Treatments and Technologies for Better Hearing
MANAGING HEARING LOSS: New Treatments and Technologies for Better Hearing BY JAMIE MORRISON, ASSOC. EDITOR
Bullets whizz overhead. A bomb just exploded 100 yards away. At this moment, his only thought is of survival. But his ears are taking a severe pounding. He's wearing ear plugs but the sound is still deafening. Will his ears ever recover?
For the soldier of the future, the answer will be, "Yes!" Before going into the extreme cacophony of battle, everyone in his platoon will have been given special drugs to protect their delicate inner-ear hair cells. And, even if his ears are damaged, doctors will be able to restore his lost hearing. Recent advances in hearing research are changing this presently fictional scenario into reality sooner than we thought possible. And you won't have to be in the military to benefit from these futuristic technologies.
Millions of Americans work daily in dangerously noisy environments that take a toll on their hearing. Others have experienced hearing loss due to disease or ototoxic drugs (see "Ototoxic Drugs' Dilemma: You Live, Hair Cells Die," p. 39). But for many, the causes of hearing difficulties are not so easy to pinpoint. About a third of Americans between ages 65 and 75 experience presbycusis age related hearing loss with the proportion rising to nearly half of those older than 75. Two to three of every 1,000 babies in the U.S. are born with varying degrees of deafness, due to a number of genetic or pathological factors. And about 4,000 Americans each year have their worlds turned upside down by sudden sensorineural hearing loss partial or total deafness in one or both ears that occurs with no warning and often no discernible cause.
With some 36 million Americans living with hearing loss "the most common sensory defect in developed countries," according to Michael Hildebrand, Ph.D., of the University of Iowa advances in hearing research and technological developments in assistive devices have the potential to make an enormous positive impact on the overall health and quality of life of nearly 10 percent of our nation's population, and hundreds of millions more worldwide.
Restorative and Preventative Therapy
The holy grail of hearing research is restoring hearing loss due to damaged inner ear hair cells. Since more than 80 percent of hearing impairment is caused by the degeneration or loss of hair cells due to loud sound, exposure to ototoxic (ear-damaging) drugs, aging or hereditary genetic defects a breakthrough therapy in hair cell regeneration would have vast public health implications. While damaged human hair cells do not regenerate on their own, many researchers believe that regeneration can be achieved because certain animals possess this ability. Douglas Cotanche, Ph.D., at Boston University, who received several grants from the Deafness Research Foundation (DRF) in the 1980s, says, "Birds are the only other vertebrates besides mammals with a cochlea. But we discovered that when bird hair cells are damaged, they are replaced spontaneously." Fish and reptiles have also demonstrated this facility.
"In contrast, in mammals hair cells are only formed during a brief period in prenatal development," says Matthew W. Kelley, Ph.D., of the National Institute on Deafness and Other Communication Disorders (NIDCD). "Because of this observation, several researchers have examined mammalian embryos to try to fi nd the genes that are 'turned on' in cells as they are forming into hair cells." Researchers uncovered one gene, called "Atoh1," which, when removed from a developing mouse, resulted in no hair cells being developed. Conversely, when Atoh1 was introduced into embryonic mouse cochleae, it caused hair cells to grow. Another approach to regenerating hair cells employs stem cells.
According to Kelley, "one option might be to introduce entirely new cells that behave like young cells that still have the ability to develop into different types of cells." This would involve surgically placing "stem cells within the cochlea in such a way that they would fuse with the remaining cochlear structures and develop and function as hair cells." While embryonic stem cells are perhaps the most widely known stem cells and have involved ethical concerns, many other sources of stem cells show promise, including those from bone marrow, the nervous system, abdominal fat and
even skin.
No hair cell regeneration research has yet been conducted on humans, but Kelley says the progress being made in animal models illustrates "how biomedical researchers are rapidly developing a much more complete understanding of the genetic and cellular processes that will need to be manipulated to initiate a biologically based treatment for restoring lost hearing. While we are still several years from a biologically-based treatment for hearing loss, this is an exciting time for hearing research and the potential for major breakthroughs has never been greater." Indeed, one recent significant stride has come through the research of former DRF grantee Stefan Heller, Ph.D., of Stanford University, who, along with his colleagues, has created the first functional hair cells using mouse stem cells. "Fascinatingly, the cells were working," said Heller, "which is a major step forward."
Such breakthroughs only come when researchers have the funding they need to carry out their research. This is why DRF is committed to raising and allocating the financial resources necessary to enable cutting-edge research to accelerate this effort. Aside from its ongoing efforts in this direction, DRF has spearheaded the ambitious Hearing Restoration Project, an endeavor which aims to raise $50 million over the coming decade to fund the ground-breaking work of the DRF-initiated Hearing Research Consortium. This team of internationally-recognized senior scientists would sychronize private, public and university research in a coordinated, integrated approach to solving the puzzle of regenerating human hair cells.
Just as important as restoring lost hearing is the ability to prevent hearing loss in the first place. When it comes to noise-induced hearing loss, there's good reason for hope. Richard D. Kopke, M.D., of the Hough Ear Institute in Oklahoma City, has studied the protective effects of N-acetylcystine, or NAC, an antioxidant compound approved by the Food and Drug Administration (FDA) some 25 years ago for treating liver damage caused by overuse of acetaminophen (e.g., Tylenol). A preliminary study of 650
military personnel in 2004 found that NAC seemed to reduce the incidence of hearing loss by 25 percent. Kopke says that NAC "neutralizes the toxins, it helps the injured tissue to repair itself and it prevents some of the injured cells from dying. The result is a substantial lessening of permanent deafness than the noise would have normally caused."
A more comprehensive evaluation of the sorts of treatments Kopke has initiated has been launched, but the results are yet to be released. And, while some experts question the effectiveness of compounds such as NAC in regard to protecting hearing, the overall thrust of Kopke's research seems to be bolstered by the work of other scientists, such as Jochen Schacht, Ph.D., and his colleagues at the Kresge Hearing Research Institute at the University of Michigan. Schacht has found promising results both from animal experiments and human clinical trials and his research suggests that antioxidants, and even aspirin, can reverse hearing damage caused by ototoxic drugs. Agents such as resveratrol, found in red wine, along with vitamin E and d-methionine, have been found to limit hearing loss in those taking drugs that are known to damage hearing. Furthermore, Jianxin Bao, Ph.D., of Washington University in St. Louis, has found that two FDA-approved anticonvulsant medications used to treat epilepsy prevented noiseinduced hearing loss in mice when administered either just before or just after exposure to loud sounds.
While Kopke, Schacht and Bao employ substances already approved by the FDA, scientists developing new drugs got a boost with the February 2010 announcement by the Department of Health and Human Services that the National Institutes of Health and FDA are developing new collaborative efforts and policies aimed at speeding the delivery of safe and effective products and treatments to market. While we anticipate biological and pharmaceutical- based solutions to hearing loss, we are not left, without present remedy in the form of effective medicine and technology.
Technology Answers
Digital hearing aids, which have been on the market for 23 years, continue to innovate and improve. State-of-the-art hearing aids use technology to abate the tinny or hollow sound of hearing one's own voice amplifi ed (known as occlusion),as well as the annoying squeal of feedback, background noise and other formerly problematic byproducts of amplifi cation. Receiver-in-the-ear hearing aids put the receiver, or speaker, inside the ear canal, which reduces both feedback and occlusion. The microphone, amplifier and battery remain behind the ear, connected via a nearly invisible wire.
Major manufacturers have developed a number of ways to eliminate feedback. Siemens Hearing Instruments, for example, employs FeedbackStopper™ which "marks" the amplified sound transmitted into the ear so that, if it returns to the microphone to be re-transmitted as feedback, the instrument immediately recognizes the marked sound and stops it.
Hearing aid users have named directional microphones as the innovation that has made the greatest impact on their satisfaction with amplifi cation. Additional microphones in the hearing aid receiver can be manually or automatically engaged to pick up sound from the side or back, while simultaneously reducing sound coming in from other directions. Now standard in many digital hearing instruments, manufacturers are advancing this concept in new proprietary technologies. Oticon's Spatial Sound technology enables a pair of their Epoq hearing instruments to communicate with each other wirelessly to preserve the location and direction of sounds, so the wearer can discriminate left from right, and up from down and always know where to look.
Smart hearing instruments have taken the guesswork out of adjusting aids, too. In the past, hearing healthcare professionals made adjustments on the basis of patients' self-reports, but new datalogging allows the hearing aid to record or log information about what sorts of sounds the user is encountering day in and day out. With this information about specifi c listening requirements, hearing healthcare professionals are better equipped to accurately program the hearing aid. Similar to datalogging, data learning enables the hearing aid itself to analyze sound information by searching for patterns. The device then "learns" how its user responds to various listening environments and adjusts itself to provide the optimum settings in different contexts.
Furthermore, wireless connectivity is turning hearing aids into complete listening systems. Some instruments access remote FM input, some automatically switch on when the user enters an induction loop fi eld, and some connect via Bluetooth to neck-worn or handheld devices that input and synchronize sound signals from devices such as cell phones, computers and MP3 players.
Implants: High-Tech Meets Medicine
In the works since the late 1800s, research on using electrical stimulation inside the ear to restore and improve hearing took a giant leap forward in the 1970s with the work of William House, M.D., which was funded in part by DRF. Together with the engineering genius of Jack Urban, the first cochlear implant (CI) was success-fully developed. Since then, thousands of people have received CIs and the technology continues to improve and diversify, resulting in implant technology for use even in the middle ear.
Envoy Esteem (www.envoymedical.com) is an example of a device that can be surgically implanted behind the ear and connected to the middle ear in adults with stable, bilateral sensorineural moderate to severe hearing loss. It is invisible from the outside and the only maintenance required is outpatient surgery every five to nine years to replace the battery. Esteem uses a person's own eardrum as its microphone, picking up the vibrations received by the eardrum and converting them into electrical impulses. It then processes the sound to optimize it for the person's particular needs and mechanically stimulates the stapes, or "stirrup," in the middle ear, which then transmits the improved sound into the cochlea.
MED-EL manufactures the VIBRANT SOUNDBRIDGE® an implantable middle ear prosthetic intended to treat moderate sensorineural hearing loss. The company is currently researching the potential of this device to treat conductive and mixed hearing loss. The advantage of middle-ear implants is amplified hearing free of the care of hearing aids, occlusion or outer ear irritation.
For people who are severely to profoundly deaf, the CI has truly lived up to its nickname the bionic ear. By implanting an electrode array in the inner ear (cochlea) and connecting it to an external audio processor, the deaf can hear.
This can be a tremendous help to children who would otherwise battle to learn. A recent study led by John K. Niparko, M.D., of Johns Hopkins University in Baltimore, noted an increased ability to acquire spoken language in children who received CIs before they were fi ve years old. The results, published in the Journal of the American Medical Association, found that "the use of cochlear implants in young children was associated with better spoken language learning than would be predicted from their preimplantation scores."
And if bionic hearing in one ear makes such an impact, imagine what it can do in both ears. Prelimenary research indicates that the investment of cost and effort to receive two (bilateral) implants may pay dividends in signfi cantly better hearing.
And yet there remain unsolved mysteries in converting sound to electrical impulse. Cochlear implants supply limited information on pitch, which creates certain challenges when it comes to enjoying music. Implant manufacturers have given great attention to the matter and according to Ward R. Drennan, Ph.D., of the University of Washington, are making progress. "An Advanced Bionics deviceuses current-steering, a process by which the electrical current is balanced between electrodes, creating 'virtual channels.' It has been well documented that current-steering can create an increased number of pitch perceptions with a single pair of electrodes." People with MED-EL implants, which include their Fine Structure Processing, have reported more enjoyment of music. And Cochlear is also developing technological and training approaches to increase music enjoyment.
Another promising avenue for enhancing the CI's ability to supply sound discrimination and musical enjoyment is being explored by Bruce J. Gantz, M.D., and Christopher W. Turner, Ph.D., of the University of Iowa. For those who have residual hearing prior to implantation of the cochlear device, smaller electrodes can successfully be used in order to preserve a person's existing hearing. This "hybrid" approach allows a person to hear certain frequencies of sound via their own ears, while hearing other frequencies, to which they had become deaf, via the CI. Hybrid hearing is thus far proving to allow for better sound discrimination in noisy contexts, as well as improved music enjoyment when compared to hearing via the CI alone. (See "Preserving Residual Acoustic Hearing with Combined Acoustic and Electric Hearing" in the Spring 2010 archives at www.hearinghealthmag.com.)
Tinnitus
With hearing loss often comes another malady phantom noise. Roughly 25 million Americans have experienced tinnitus in which they hear ringing, roaring, hissing or clicking sounds, even though no such sounds exist externally. "Some cases are so severe that it interferes with a person's daily activities," says the NIDCD. "People with severe cases of tinnitus may fi nd it diffi cult to hear, work or even sleep."
Former DRF grantee Michael Burger, Ph.D., of Lehigh University in Pennsylvania, is one researcher whose work could lead to treatments that alleviate tinnitus. His focus is the cochlear nucleus, the fi rst central synapse in the brain, in which an excess of excitation combined with a lack of inhibition of neural activity is believed by many researchers to be a cause of the phantom noise.
While Burger and others, such as DRF grantee Adrin Rodrguez-Contreras, Ph.D., of the City University of New York (see "In the Absence of Sound," p. 46), explore potential tinnitus solutions, a currently available treatment is already solving problems for some who struggle with the ailment.
"Neuromonics Tinnitus Treatment includes relaxation music as part of the treatment sound which is more pleasant to listen to than the noise provided by maskers or amplified everyday sounds when using hearing aids," according to the dcvice's developer, Paul B. Davis, Ph.D., a hearing and speech therapist in Davie, Fla. Neuromonics modifies the acoustic properties of ordinary music for each person's particular hearing loss profile. With time, users report being less aware of their tinnitus and less disturbed by it. Davis claims that "95 percent of patients reported improvement in tinnitus disturbance of at least 40 percent after six months" while researchers at the House Ear Institute in Los Angeles concluded that "the Neuromonics device appears to be useful as a means of significantly reducing the effects of tinnitus on an individual's daily life." Additional tinnitus treatments also show promise for helping ameliorate phantom noise. (See "Showstoppers! On the Red Carpet in San Diego" on p. 12.)
With the advances taking place in hearing research and technological development, the future looks bright for those facing hearing challenges. And it will no doubt sound even better than it looks.
