ABSTRACT: Implantable hearing devices for people with hearing loss of varying degrees and causes offer options not available even 2 years ago.
CME ABSTRACT: Readers will be aware of new hearing devices for patients with different types of hearing loss.
Thomas L. Eby, MD, no conflicts of interest
Hearing loss is one of the most prevalent chronic health conditions in the United States, affecting people of all ages, in all segments of the population, and across all socioeconomic levels. More than 28 million Americans suffer from some degree of hearing impairment — approximately 17 in 1000 children aged 18 years and younger and 314 in 1000 people aged 65 years and older, according to the National Institutes of Health.
Because of technological advances in the last 25 years, various implantable hearing devices are now approved for the rehabilitation of conductive, mixed, and sensorineural hearing loss, including cochlear implants, bone-anchored and middle ear hearing aids, and auditory brainstem implants.
Cochlear Implants
THE FIVE MAIN CAUSES OF HEARING LOSS
1. Heredity – at least 100 hereditary syndromes can result in hearing loss
2. Infections, such as bacterial meningitis and rubella
3. Acoustic trauma produced by acute or chronic exposure to loud sounds
4. Prescription drugs, such as streptomycin and tobramycin, and chemotherapeutic agents, such as cisplatin
5. Presbycusis, the hearing loss of old age, is thought to result from repeated acoustic trauma and narrowing of microscopic blood vessels in the inner ear.
National Academy of Sciences. Beyond Discovery™: The Path from Research to Human Benefit Web site. Sound From Silence, The Development of Cochlear Implants. August 1998. Available at: www.beyonddiscovery.org/content/view.article.asp?a=252. Accessed October 14, 2004.
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A cochlear implant involves implantation of an internal receiver behind the ear and a multichannel electrode into the cochlea. An external speech processor then passes electrical stimulation via the electrode to stimulate the auditory nerve. The American Speech-Language-Hearing Association estimates more than 60,000 cochlear implants have been implanted worldwide in the last 2 decades.
"New technology has dramatically opened the door for more adults and children with severe and profound hearing losses to choose this option," says otolaryngologist Thomas L. Eby, MD, who has performed more than 100 cochlear implant operations at UAB.
When the Food and Drug Administration (FDA) first approved a cochlear implant with a single electrode for adults in 1985 and for children in 1990, only those who were almost completely deaf and could only perceive vibrations with a hearing aid qualified. "Children and adults who were not candidates just 2 years ago may be candidates today," he says.
In response to a growing body of research demonstrating that children who receive cochlear implants when they are very young make greater gains in acquiring age-appropriate language skills than children implanted when they are older, FDA recently lowered the age of pediatric candidates from 2 years to 12 months. In children, growth of the skull is anticipated by leaving extra lead wires in the mastoid.
The success of cochlear implant technology also has enabled adults with greater residual hearing and speech recognition to qualify as cochlear implant candidates, Eby notes. According to FDA, adults can now qualify if they have severe-to-profound hearing loss and poorer than 50% speech recognition.
Eby emphasizes the majority of patients feel the cochlear implant improves their quality of life. "Cochlear implants do not restore normal hearing as we know it. Yet, patients are delighted from the first time they hear sound, such as a baby crying or rain hitting the roof. They also feel safer knowing they can detect the wail of a siren or the shriek of a smoke detector."
Implantable Hearing Aids
While potential advantages of implantable hearing aids for patients with residual hearing are less dramatic than cochlear implants for the profoundly deaf, these hearing-assistive devices are an alternative to conventional amplification and can be separated into two general types: bone-anchored devices for conductive hearing loss and middle ear implants for sensorineural hearing loss.
Conductive hearing loss occurs when the auditory nerve is intact but mechanical problems impede sound transmission through the external or middle ear. Common causes of conductive hearing loss include congenital aural atresia, persistent external otitis, persistent draining mastoid cavities, or postoperative sequelae. Because of abnormalities in the ear caused by these conditions, many patients cannot use conventional hearing aids without feedback.
"Bone-anchored hearing aids (BAHAs) use a titanium screw implanted into the skull behind the ear," Eby explains. After this screw becomes integrated — 3 months for adults, 6 months for children — a pedestal and vibrating hearing aid are attached to the screw, usually in an office procedure. The vibrating hearing aid stimulates the cochlea, essentially providing a bridge across the defective area, and sound is once again processed by the inner ear.
"These implanted hearing aids improve hearing sensitivity, permitting speech recognition and detection of quiet sounds," he says, adding that BAHAs avoid the otorrhea associated with the closed earmolds of traditional hearing aids.
Sensorineural hearing loss occurs when the cochlea, eighth cranial nerve, or auditory pathways are damaged, and is most commonly caused by ototoxicity, noise trauma, infection, head injury, tumors, labyrinthine disorders such as Meniere disease, genetic causes, and aging.
"When an individual experiences sensorineural hearing loss but maintains a substantial level of hearing, a number of devices can be effective, including conventional and digital hearing aids," Eby says. "Hearing aids are smaller and less obtrusive than ever before, helping ease their stigma. The progression from conventional analog instruments to digital hearing aids has vastly improved sound quality and allows manufacturers to create hearing aids with enhanced processing features."
The FDA approved the first implantable hearing aid for sensorineural hearing loss in 2000. Current implantable hearing aids combine implanted and external components; an external audio processor in the ear canal or held by a magnet to a screw behind the ear processes acoustic signals. The processed signal is sent to a transducer attached to the ossicular chain either directly or via an implanted wire. Totally implantable devices are now being developed. "These hearing devices are indicated for adults who have a moderate-to-severe sensorineural hearing loss and desire an alternative to an acoustic hearing aid," Eby says.
Auditory Brainstem Implants
Also in 2000, FDA approved a new device to restore hearing in people with neurofibromatosis type 2 (NF2), a disease marked by cranial nerve tumors. People with NF2 must undergo surgery to remove tumors; during the procedure, the auditory nerve is severed to remove growths, resulting in total hearing loss. "Hearing aids and cochlear implants are not beneficial when the auditory nerve has been severed or damaged," Eby says.
Not yet available at UAB, auditory brainstem implants involve placement of a multichannel electrode over the cochlear nucleus area after tumor removal. An external speech processor is individually programmed and provides electrical stimulation, allowing sound perception. Implantation may be performed during removal of an acoustic tumor or in a separate procedure.
The Nucleus 24 Multichannel ABI, manufactured by Cochlear Corporation, was the first device of its kind and the first to receive FDA approval. In US clinical trials, the device was implanted in 90 patients with NF2, aged 12 to 67 years. Of the 60 patients involved in follow up, 82% were able to detect familiar sounds such as doorbells, and 12% could hear well enough to use the telephone.
Eby and colleague Benjamin M. McGrew, MD, hope to offer this surgical option within the next year.
Expanding Opportunities
With continued follow-up research and further advancements in technology, researchers are evaluating new opportunities and additional possible candidates for these procedures.
"The ultimate goal is to find ways to cure hearing loss, and ideally, to prevent it," Eby concludes. "The recent identification of mutations in genes that predispose people to hearing loss was a significant step forward, and experiments with gene therapy are underway to try to prevent hearing loss."
RECOGNIZING HEARING LOSS:
10 QUESTIONS TO ASK PATIENTS
Patients answering yes to three or more of these questions should be referred for further testing.
* Do you have a problem hearing over the telephone?
* Do you have trouble following the conversation when two or more people are talking at the same time?
* Do people complain you turn the television volume up too high?
* Do you have to strain to understand conversation?
* Do you have trouble hearing in a noisy background?
* Do you find yourself asking people to repeat themselves?
* Do many people you talk to seem to mumble or not speak clearly?
* Do you misunderstand what others are saying and respond inappropriately?
* Do you have trouble understanding the speech of women and children?
* Do people get annoyed because you misunderstand what they say?
Better Hearing Institute. Available at: www.betterhearing.org. Accessed October 18, 2004.
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For more information
Dr. Thomas Eby
1.800.UAB.MIST
mist@uabmc.edu
Published in UAB Insight, Winter 2005