Next-Generation Hearing Technology
Cochlear implants have been life-changing for many people with severe hearing loss. But cochlear implants have a number of limitations, and they don’t work for all types of hearing loss, for example when the cochlear nerve isn’t functional, as can be the case for patients with Neurofibromatosis type 2, cochlear ossification, certain types of head trauma, and other conditions. Auditory brainstem implants could offer an alternative to people for whom cochlear implants aren’t an option.
Cochlear implants use electrodes to directly stimulate the cochlear nerve, which provides auditory signals to the brain. The brain then interprets these signals. Auditory brainstem implants, on the other hand, bypass the cochlear nerve, transmitting signals directly to the brainstem.
And if someone has one type of hearing loss in one ear and a different type in the other? In a very small number of experimental cases, such as that of Jessica Toews, an ABI and a CI have been deployed together.
In 2009, Toews developed Neurofibromatosis type 2, which resulted in a tumor growing in her left ear. Doctors removed the tumor, but Toews lost hearing in that ear in a way that couldn’t be helped by a cochlear implant. Two years later, doctors treated a tumor in her right ear with medication and radiation. It shrank the tumor enough that they could implant a cochlear implant. But Toews still couldn’t hear out of her left ear. In 2017, Dr. Ravi M. Sami implanted an ABI and a newly-FDA-approved sound processor, the CochlearTM Nucleus 7, in Toews’s left ear, making her one of two patients in the United States to have both types of implants. The combination of the ABI and the sound processor allows Toews to not only hear, but also to stream phone calls, video, music, and entertainment.
Not every ABI will provide streaming entertainment on demand, but a 2020 study found that an auditory brainstem implant can improve a patient’s speech perception with or without lip reading, improve sound recognition, and improve a patient’s overall quality of life.
Latest Development
An auditory brainstem implant system consists of the implant itself, which is applied to the brainstem, and an external sound processor, which is placed behind the ear. The processor has a microphone, a processing unit, and a coil. The microphone detects sounds from the environment. The processor then translates those sounds into digital signals and sends them to the coil. Then the coil transmits the digital signals through the patient’s skin to the implant.
Once the signals reach the implant, they go to a paddle-shaped electrode array, which has been placed on the cochlear nucleus of the brainstem. In this way, the ABI unit bypasses the cochlear nerve, sending the digital sound signals directly to the brain.
Current auditory brainstem implants have a number of limitations. One such limitation arises from the rigid materials from which ABIs are constructed. The brainstem is curved, and the rigidity of the materials mean that the implant may not conform well to the surface. Air gaps can form between the ABI and the brainstem, which can lead to excessive current spread. As a result, patients may experience unintended nerve activation, which can lead to side effects such as facial twitching or dizziness. To reduce the effects of unwanted nerve activation, doctors often turn off a majority of the electrodes, which reduces the quality of the perceived sound, rendering sounds vague and speech less intelligible.
A Soft Auditory Brainstem Implant
Researchers at the Laboratory for Soft Bioelectronic Interfaces at EPFL in Switzerland are developing a next-generation auditory brainstem implant made from softer, more flexible materials. Their device, which uses micrometer scale platinum electrodes embedded in silicone, is just a fraction of a millimeter thick, and is highly flexible. It’s hoped that this thinness and flexibility will improve contact with the brainstem, and reduce unintended nerve stimulation and its subsequent side effects.
Alix Trouillet, a former postdoctoral researcher at EPFL and co-first author of the study, says, “If the array naturally follows the brainstem’s curved anatomy, we can lower stimulation thresholds and maintain more active electrodes for high-resolution hearing.”
The new, more flexible materials also make the implant reconfigurable to fit different individuals’ brainstems.
“The design freedom of microlithography is enormous,” says Trouillet. The researcher envisions a variety of configurations and layouts that will enable finer frequency-specific tuning. The current flexible ABI design, for example, has 11 electrodes. But future designs could house many more than that.
But before researchers could test the new ABI on macaques, they had to first figure out what the animals were actually hearing.
Emilie Revol, co-first author on the project and a former PhD student at EPFL said, “Half the challenge is coming up with a viable implant, the other half is teaching an animal to show us, behaviorally, what it actually hears.”
Toward that end, the researchers ran extensive behavioral tests on macaques with normal hearing, to measure how well they could distinguish electrical stimulation patterns. Revol trained the macaques with normal hearing to press and release a lever to indicate whether consecutive tones were the same or different.
After that, the researchers introduced electrical stimulation from the soft ABI in a step by step manner, blending that stimulation with normal tones, so that the test animals could bridge the difference between acoustic and prosthetic hearing.
“Ultimately,” Revol said, “the goal was then to see if the animal could detect small shifts from one electrode pair to another when only stimulating the soft ABI. Our results suggest that the animal treated these pulses almost the same way it treated real sounds.”
Animal testing also appears to confirm the new, flexible unit’s increased comfort and decreased side effects. The researchers reported that the test animals showed no signs of discomfort or facial twitching, and they pressed the lever to trigger stimulation over and over.
“If the prosthetic input had been unpleasant,” Revol says, “it probably would have stopped.”
Researchers reported that the implants stayed in place in test animals for several months, with no measurable electrode migration, which can be an issue with standard ABIs.
The Next Steps
Despite these promising developments, the road to market for the soft ABI will involve additional regulatory and research steps, including human testing. In addition, each and every material within the implant must demonstrate robust, long-term reliability.
Nonetheless, for patients with hearing loss types that are incompatible with cochlear implants, these developments are hopeful.