More than 28 million Americans suffer from some degree of hearing loss. Eighty percent of them have irreversible and permanent hearing damage. And a small percentage of people are profoundly deaf. Profoundly deaf individuals typically have damaged or relatively few sensory receptors in the inner ear, called hair cells. Hearing aids or other methods of amplifying sound provide little or no help to these people. The only viable way for them to hear is by using a cochlear implant.
Cochlear implants use electrodes surgically placed in the cochlea, a small organ that coils to about the size of a dime. The electrodes bypass hair cells and directly stimulate auditory nerve fibers which carry signals to the brain where they are perceived as sound. Advances in computer technology and electrode design have yielded implants that let adults and children understand speech without lip reading, listen to music, use conventional telephones and, in general, lead a normal life.
ELECTRODE ARRAY
Engineers at Advanced Bionics, Sylmar, Calif., design and manufacture the Clarion implant, one of the most advanced cochlear implants on the market. Their latest electrode array, dubbed HiFocus, places eight evenly spaced pairs of electrodes in eight different positions in the cochlea. The 0.3-mm platinum-iridium ball electrodes have a smooth side that faces the inner wall of the cochlea. By facing and almost touching the wall, the electrodes are as close as possible to the target of their stimulation, the auditory nerve fibers. This reduces the amount of current needed to elicit hearing and minimizes the amount of current that spreads to neighboring electrodes.
To further minimize inter-electrode interference, dielectric partitions separate each of the 16 electrodes on the array. These pillowlike elevations direct electrical signals toward the cochlear inner wall. They also prevent the metallic electrodes from touching the fragile wall of the cochlea.
Electrodes are mounted on a soft, flexible, prebent substrate. The bend helps the array hug the spiral cochlear wall, while the substrate stops the electrodes from flexing or moving sideways. All these mechanical details keep the electrodes oriented toward the auditory nerve for consistent control over delivery of electric current.
Advanced Bionics also supplies a positioner to help surgeons implant the array. The small, soft device mimics the curve of the cochlea and helps doctors insert the array deep into it. “The array is usually placed about 24 mm into the cochlea, or about one-and-a-half turns. The entire cochlea is about 35 mm long,” says Tom Walsh, product manager for the Clarion who also uses a Clarion implant to hear. “But the array hugs the inside wall, so it travels most the distance into the cochlea. And since the cochlea is fully formed at birth and doesn’t grow any larger, the array can be implanted in very young children and still function years later.”
The positioner is left in the patient after surgery and fills the void where scar tissue may form in reaction to foreign bodies, such as implants. The positioner keeps the cochlea open for implant replacement or removal as new technologies are developed.
HEADPIECES FOR INSIDE AND OUT
The electrode array is attached by tiny wires to an implanted headpiece inserted just beneath the skin in a surgically created depression in the mastoid bone just behind the ear. The implant holds electronics that include eight independent output channels, a send/receive antenna, and self-testing hardware and software. Capacitors connected to each output circuit prevent accidental leakage of dc current which could harm tissue or damage the electrodes. The antenna receives 49-MHz signals through the skin that serve as instructions and channel outputs. The implant transmits to the outer headpiece at a slightly lower frequency on its electronic status. All the electronics, as well as a magnet, are hermetically encased in a ceramic zirconium housing which is biocompatible and transparent to radio waves.
The magnet on the implant aligns with several magnets on the exterior headpiece to hold it in place against the skin. The number and placement of the magnets can be adjusted to make sure they don’t exert too much pressure on the skin over long periods. The exterior headpiece, like the implant, has a receive/transmit antenna for forward and backward telemetry. While forward telemetry naturally includes signals destined for the eight electrode channels, back telemetry reports on the condition of the electrodes, onboard power supplies, and other electronics, as well as the stability of the RF transmissions. The exterior headpiece also has a sensitive microphone to pick up sounds around the patient. Sounds are sent to a sound processor carried on a belt, or in the latest version, a processor miniaturized enough to fit behind the ear.
SOUND PROCESSING
The brain of the cochlear implant is the sound processor, a microprocessor that takes outside sounds, converts them to digital signals, and transforms them into several different types of electrical codes using digital signal processors from Texas Instruments. Signals are converted back to analog before being sent to the eightchannel electrode.
To accommodate different patients, the sound processor and electrode are capable of two basic types of stimulation, simultaneous and sequential. In simultaneous stimulation, all sounds are mapped into a high-resolution digital domain and selectively filtered, amplified, and divided into eight channels according to user preferences.
The eight channels are arranged according to frequency so that high frequencies go to electrodes placed near the mouth or wide end of the cochlea, and low frequencies go to electrodes near the apex or end. The cochlea’s receptors are arranged similarly, with high notes picked up near the wide end and low notes up and around the apex.
Signals are sent at up to 104,000 or 13,000/sec to each electrode, analogous to playing a piano with all 10 fingers at once. “Just like in the natural ear, simultaneous processing stimulates all regions of the cochlea at once along its entire length,” says Walsh. “It lets you hear high frequencies at the same time you hear low frequencies. And even tones that seem like pure low tones have high-frequency components and overtones.”
Simultaneous stimulation usually uses bipolar coupling of the electrodes, sending current between the electrode pairs. This type of coupling reduces current spread, so when all electrodes are activated at once, there’s less channel interaction. “Channel interaction makes most people lose the distinctiveness of sounds,” explains Walsh.
In general, adults and children using simultaneous stimulation do better in terms of speech comprehension and phone usage, and subjectively, they report getting more information, including background noises with simultaneous stimulation. Other factors that might affect their perceptions include how long a person has been deaf, how viable their nerves are, and what stimulation levels they need to hear.
In sequential stimulation, digitally remapped signals are sent to electrodes one at a time, more like playing the piano with only one finger and hitting notes extremely fast. In this mode, the electrodes put out 6,500 signals/sec. That translates to 813 signals/sec at each electrode pair, a rate high enough to convince the brain that several tones are being “heard” at once.
Sequential stimulation tends to work better with monopolar coupling between the electrodes. In this method, signals are sent from one electrode in the pair to a ground electrode on the implanted headpiece. This sends current through a larger area than bipolar coupling and often creates too much crosstalk for simultaneous stimulation. However, some patients need higher levels of stimulation to get nerve fibers to react, and monopolar provides higher current levels. “Sequential stimulation and monopolar coupling lets you stimulate nerves that may be less viable or hard to activate without getting too much channel interaction,” notes Walsh. “In my case, for example, I couldn’t use the simultaneous strategy because it wouldn’t give me any range in loudness levels and I couldn’t perceive the signal being presented.”
The process can also generate a user-modified hybrid of sequential and simultaneous stimulation. Based on user inputs, the processor simultaneously excites some channels while sequentially exciting others. Under this processing strategy, the processor sends a maximum of 53,000 biphasic signals/sec.
The processor, as well as the internal electrodes and electronics are powered by a custom rechargeable lithium battery that cost about $50. It lasts 12 hr, and most people use two per day. The device can also use standard AA batteries.
The sound processor can be programmed to mix and match different types of coupling and stimulation using a Windows-based PC program. It also lets clinicians set upper and lower thresholds for specific frequencies, alter loudness levels, and even change the order of electrode pairs if they find one is out of synch i.e., it doesn’t follow the tonal map of the typical cochlea. Together, the clinician and patient construct three maps or programs that can be optimized for different situations such as listening to music, taking notes in class, or talking on the phone in an office. “And most people I know who use cochlear implants have a ‘party program’ with settings that make it easy to hear conversations in a noisy background.” says Walsh.
Users always have access to the buttons that select programs, adjust volume, and set sensitivity. This lets users adapt the implant to suit their individual needs and abilities. And unlike other cochlear implants that use speech processors, Advanced Bionics approach is to process all sounds and give users control over the output. This gives them the flexibility to tailor the input so that their brain, the ultimate sound processor, can make sense of it.
Patients have had great success with the Clarion implant. And it helps both those who have been deaf all their lives and those who had hearing for some part of their lives but later lost it. Tom Walsh, for example, was profoundly deaf all his life and had worn ineffective hearing aids for 28 years. Since his implant in 1995, he can work in or out of the office, including on the phone, without a second thought.
Total cost of the Clarion system, including the evaluation, operation, therapy, hardware and tune-ups, is about $35,000 to $45,000, depending on where a patient lives. According to Advanced Bionics, most insurance companies typically cover these costs.
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FROM DREAM TO REALITY Other scientists tried variations on this “experiment” over the next 50 years, but by the mid-1800s the idea of using electrical stimulation as a therapy for deafness was thought to be a dead end. In the 1930’s, however, research teams in the U.S. and the U.S.S.R. were once again studying the effects of electrical stimulation on hearing. They discovered they could elicit hearing sensations in deaf patients by electrically stimulating the middle ear. But neither team came up with any practical applications due to insurmountable technical difficulties. By the late 1950s, French scientists reported the first successful electrical stimulation of hearing nerves by inserting an electrode in a deaf subject’s inner ear. The patient perceived the rhythm of speech and said the stimulation helped in lip reading. Tremendous energy went into the study and development of cochlear implants in the 1960s, and by 1970 the first widespread clinical tests were underway. These first-generation implants were single-channel devices that sent coded information to a single electrode site in the inner ear. They gave patients speech and sound awareness and enhanced lip reading, but generally did not provide auditory-only speech recognition. Multichannel devices introduced in the 1980s stimulated auditory nerve fibers at several electrode sites along the cochlea. They could also stimulate all the electrodes at once, one at a time, and sequentially. These types of devices let deaf patients hear and understand speech without lip reading. The FDA approved use of cochlear implants for adults in 1980 and for two-year-old children in 1990. Currently, children as young as 18 months can receive the implant. About 12,000 people in the U.S. and 22,000 people worldwide have cochlear implants. |