Turn on the Music: Cochlear Implants

Introduction

“Watch as a girl hears her dad’s voice for the first time.” As little as 60 years ago, these words would have elicited skepticism from even the most prominent otologist and audiologist. Today, many otologists have witnessed a patient hearing for the first time because of a cochlear implant. The emotion felt by a patient after the initial activation of a cochlear implant is pure joy. When activated, acoustic stimulation is converted to an electric signal, carried to the auditory cortex, and stimulation of previously unused neuroconnections occurs and like magic to the patient, they can hear the world around them. The medical, technological, and engineering evolution of cochlear implants has been prodigious.

Timeline of Cochlear Implantation

Not long ago the medical community could offer little to a patient with severe to profound bilateral sensorineural hearing loss. An adult with presbycusis (age-related hearing loss) in the 1950s was prescribed various forms of amplification until the hearing loss was severe enough they depended on lip reading, sign language, and other non-verbal communication. A newborn stricken with congenital cytomegalovirus who developed severe hearing loss would live their entire lives never hearing. In 2013, Graeme Clark, Ingeborg Hochmair, and Blake Wilson, shared the prestigious Lasker-DeBakey Clinical Medical Research Award for their work in the development of the modern cochlear implant. Their contributions, along with others, set the world on a path to make deafness an ailment of the past.

The discovery that electricity could stimulate the cochlear predates the cochlea implant. Benjamin Wilson, inventor of the first electrical capacitor called the Leyden jar, placed the electrodes of his device on the temples of a deaf woman in 1748.¹ A recollection of the experiment states, “there ensued a small explosion. She was much surprised … she felt a small warmth from ear to ear.”² Alessandro Volta, credited with inventing the battery, wrote a paper in 1800 expounding on the “sensible effects” of his battery. “The hearing will be strongly affected by introducing into the ears two probes … it was a kind of jerky crackling as though some paste or tenacious matter was boiling.”² These early observations occurred prior to the scientific community being equipped to discuss a solution for deafness, but they did reveal that electricity could stimulate hearing.

In the 1930s, researchers, Ernest Wever and Charles Bray described the cochlear microphonic in cats.³ Alterations of this potential in the cochlea began to describe how acoustic (sound) energy is converted into electrical energy for transmission to the cochlear nerve. Their understanding of this concept led Gursun and Volokhov, to document how an electrically stimulated cochlea can produce hearing.⁴

A major breakthrough occurred in 1957 in a patient with a history of bilateral extensive cholesteatomas requiring temporal bone resections. The left and right temporal bone resections, performed years apart, left the patient with bilateral profound hearing loss and facial nerve paralysis. Otologist Charles Eyries, was consulted to consider a facial nerve graft, but the remaining nerves were too small for a local graft. However, a chance meeting with Andre Djourno, a physics professor also interested in implanting an electrode for hearing, resulted in the collaboration to perform the facial nerve graft and placing a single electrode in the remaining stump of the right cochlear nerve (which is intimately associated with the facial nerve as it leaves the central nervous system). The patient was able to hear sounds.⁵ Interestingly, the patient’s bilateral cochleas had been removed during the procedure and therefore, this event actually represents the first central nervous system implant for hearing rather than the first true cochlear implant.

In 1961, Georg von Bekesy won a Nobel Prize for his traveling wave theory that described the tonographic mapping of the cochlea with high frequencies represented at the base and low frequencies at the apex (See Figure 1). This same year, arguably the most prominent otologist in history, William House, was reading a newspaper clipping of the Eyries and Djourno surgery. He and neurosurgeon John Doyle performed two implants on patients with bilateral profound hearing loss, inserting the electrode in the cochlea after a hole was drilled adjacent to the round window. These procedures represented the first true cochlear implants.⁶ Initially, details of the implantations were not widely published; it was years before the accounts received notable attention. House and his team detailed their experiments changing the pulse frequency and amplitude of the stimulator that resulted in patients’ ability to hear sounds and improved lip reading.

Figure 1.

Tonographic Mapping of the cochlea.

Source: http://teddysratlab.blogspot.com

Timeline of Cochlear Implantation

1748	Benjamin Wilson stimulates hearing in deaf woman with Leyden jar
1800	Alexander Volta uses his newly invented battery to stimulate hearing
1930s	Ernest Wever and Charles Bray discover the cochlear microphonic, Gursun and Volokhov document how electric can stimulate hearing in the cochlea
1957	Charles Eyries and Andre Djourno performed first implant to restore hearing
1961	Georg von Bekesy wins Nobel Prize for traveling wave theory
1961	William House and John Doyle perform first true cochlear implants
1964	Blair Simmons, John Epley and Robert White first to perform a multichannel cochlear implant; Simmons coins term “cochlear implant”
1967	William House and Jack Urban develop induction coil that fit beneath the skin
1974	Graeme Clark host a telethon in Melbourne to raise money for his cochlear implant research; develops “variable stiffness” electrode
1975	Ervin Hochmair and Ingeborg Desoyer receive funding for cochlear implant research
1976–77	NIH releases Bilger report stating that cochlear implants are safe and beneficial
1984	Led by William House, FDA approves the 3M Corporation’s single channel electrode for adults
1988	Blake Wilson develops continuous interleaved sampling
1989	FDA approves cochlear implant for children over 2
1990	Hochmair’s group founds MED-EL corporation
2000	FDA approves cochlear implant for children over 12 months

House’s original implants were single channel, meaning they lacked the ability to take full advantage of the tonographic mapping of the human cochlea. In 1964, otologist Blair Simmons, John Epley, and engineer Robert White were the first to implant a multichannel electrode, placing electrodes at different locations along the cochlea to stimulate different frequencies. Multiple electrodes allowed the implant to take advantage of the tonographic mapping of the cochlea. Simmons reportedly coined the term “cochlear implant,” using it as the title of his presentation at a workshop on microsurgery of the ear.^4,7

House collaborated with electrical engineer Jack Urban to develop an induction coil that fit beneath the skin in 1967. He implanted three patients between 1969 and 1970 using this technology along with a single channel electrode. Claude Chouard, a colleague of Charles Eyries, developed cochlear implants with multiple electrodes that each had to be inserted through a separate hole made in the cochlea.⁸

In the mid-1970s, cochlear implants were still thought to be dangerous and not worth the risk, despite growing interest and advances in technology. In 1976–1977, the National Institute of Health commissioned a study led by Robert Bilger to independently evaluate all 13 U.S. patients with cochlear implants. They concluded that the procedure was well tolerated and patients had increased scores when tested for lip reading and recognition of environmental sounds.¹ The Bilger Report was instrumental in the future development of the cochlear implant because it provided evidence that the implants were a benefit to patients.

While playing with turban-shells and grass on the beach, Australian Graeme Clark was inspired to create an electrode of variable stiffness for assistance in insertion into the cochlea.⁹ Clark obtained funding from a telethon in Melbourne in 1974 and subsequently implanted his first multi-electrode cochlear implant in 1978. The “variable stiffness” of Clark’s electrode permitted deeper insertion into the spiraling cochlea (see Figure 2). This allowed for stimulation of a wider range of the sensory organ and took advantage of the known frequency mapping described earlier. Clark’s cochlear implant became the first commercially successful multi-channel electrode with the Cochlear/Nucleus cooperation.

Figure 2.

Schematic of electrode in scala tympani of cochlea.

Source: http://med.stanford.edu/ohns/earinstitute/otology-neurotology/resources/cochlear_implant.html

In Vienna in 1975, Ervin Hochmair and Ingeborg Desoyer received funding for cochlear implant research. They teamed with the 3M Corporation to market William House’s single channel broadband analog implant. This was the first implant to gain FDA approval in 1984. Although Clark’s multi-channel electrode eventually became standard of care, the single channel electrode created by House was the first commercially successful cochlear implant. In 1990, the Hochmair group formed MED-EL, an industry leader in implantable hearing solutions. Ingeborg Hochmair (formerly Ingeborg Desoyer) left her academic career to head the privately held MED-EL.⁴

Similar to most technology of the time, the physical hardware (cochlear implant), preceded the software required to optimize its function. In 1988, a Duke University engineer, Blake Wilson, developed continuous interleaved sampling, a method of mapping the wide dynamic range of environmental sound into the smaller range of electrically evoked hearing. His algorithm exponentially improved hearing quality with cochlear implants and is the basis for speech processors in modern cochlear implants.

The programming of cochlear implants requires more commitment from a patient and family than the surgery itself. The implant will be activated around 3 weeks following surgery by the audiologist. A “map” will be made using that minimal electrical stimulus a patient can perceive and the level of stimulus required for comfortable hearing. This represents the dynamic range of the cochlear implant. From this point, much of the software used in hearing aids can be applied to cochlear implants. Modern users have multiple settings based on their environment (noisy, outdoors, quiet, etc) with the implant itself able to switch between settings on its’ own. Patient’s must continue to follow up routinely to have the programming optimized.

The rapid progress of cochlear implant technology may indicate endless possibilities for the future treatment of the deaf. The FDA approved cochlear implants for adults in 1984 and children over 2 years of age in 1989, lowered to children age 12 months in 2000. For the past several years, there has been a trend for bilateral cochlear implantation. This gives deaf children binaural hearing and improves both localization of sound and speech perception. Because of new designs doctors can take advantage of electroacoustic simulation which allows patients to use the cochlear implant and any residual hearing simultaneously. The use of cochlear implantation for single sided deafness is also being researched.

Engineers are also developing a way to use light to stimulate the cochlea instead of electricity. This increases the specificity that the cochlear nerve can be stimulated, resulting in a more dynamic sound perception.¹⁰ Scientists developed auditory brainstem implants for patients who cannot receive a cochlear implant, such as patients born without a cochlea. It is difficult to imagine a future where hearing is not an option for any patient.

Without experiencing deafness oneself, comprehension of the totality hearing loss has on one’s life is difficult, but the benefit of a cochlear implant is profound in a patient’s life. Cochlear implantation in adults who lose their hearing is shown to correlate directly with increased income, possibly slow or even improve dementia in the elderly and improve the quality in all areas of life measurements.^11,12 Children with cochlear implants have demonstrated improved cognitive function, social communication and motor skills and increased hearing and language performance comparable to normal hearing children.^13,14

At the end of 2012, approximately 324,000 people had received cochlear implants worldwide thus hundreds of thousands of people have been impacted with technology that did not exist in any form 60 years ago.¹⁰ Today, a child born without hearing can be implanted young enough to develop excellent speech and oral communication. An adult who loses his hearing in his 80s can have his hearing restored. Great minds like Clark, Hochmair and Wilson have literally turned on the music.

Biography

Zachary W. Bear, MD, (left), is a Resident, Department of Otolaryngology at Saint Louis University. Anthony A. Mikulec, MD, MBA, (right), is an Associate Professor, Department of Otolaryngology, Chief of Division of Otology at Saint Louis University.

Contact: tonymikulec@gmail.com