You might have heard of the cochlear implant. It’s an electronic device also referred to as a neuroprosthesis, serving as a bionic replacement for the human ear. These implants have brought an improved sense of hearing to hundreds of thousands around the world.
However, the cochlear implant isn’t the only game in town. The auditory brain stem implant is another device that promises to bring a sense of sound to those without it, albeit by a different route.
While the cochlear implant itself is a highly complicated device, the basic concept behind it is simple. The usual mechanics of the ear, which receive vibrations from the air and turn them into nerve signals, is bypassed entirely. Instead, a small electronic device captures sound with a microphone. The sound is then processed, with a priority on maximising perception of audible speech. This processed sound is used to drive an array of electrodes implanted within the cochlea itself. These electrodes stimulate the auditory nerves in the cochlea, enabling the wearer to perceive sound.
The auditory brain stem implant (ABI) is in many ways similar to the cochlear implant. The basic theory is indeed the same: audio is captured electronically, and then used to stimulate nerves to provide an auditory sense to the brain. Where the ABI differs is that it skips past the cochlea inside the ear entirely. Instead, the ABI stimulates electrodes placed in the cochlear nucleus of the brainstem itself.
The ABI thus has the benefit that it can provide an auditory sense to patients who, for whatever reason, cannot have a cochlear implant fitted to the auditory nerves in the inner ear. Patients with a condition called Neurofibromatosis Type 2 (NF2) were initially the primary group for ABI use. NF2 is a condition that affects the nervous system, and its associated treatment often causes damage to the auditory nerve. Thus, for patients with this condition, an ABI is suitable where a traditional cochlear implant would be impractical. In cases where the auditory nerves in the cochlea may be damaged or destroyed, an ABI may be applicable.
However, the ABI comes with the drawback that it requires a far more complex implantation than a cochlear implant. Surgery involves opening the skull to access the brain stem, which is far more invasive than the simpler procedure required to implant a cochlear device in the inner ear.
Outcomes for patients are by and large not as successful as patients with cochlear implants when it comes to understanding speech either. With a combination of ABI use with lipreading techniques, many patients go on to learn to understand speech, but few can understand speech relying on an ABI alone.
This is largely down to electrode placement. The cochlea itself has a fairly straightforward map of areas that correspond to high and low tones, which can be stimulated in turn by an implant directly. However, when inserting electrodes into the brainstem, it’s harder to map out and stimulate these regions as accurately, and thus an ABI will struggle to deliver as much tonal information to the brain as a cochlear implant would.
The lower performance, more invasive implantation method, and obscure application of the ABI have meant that cochlear implants are far more commonly used in practice. Over 700,000 cochlear implants have been fitted worldwide. However, only a few thousand ABI devices have been implanted at most.
While the results from an ABI may not be up to the standards of a cochlear implant, these bionic devices still have value. For patients that can’t use a cochlear implant at all, an ABI still provides a basic auditory sense that can be useful, particularly when it comes to environmental sounds. Overall, it’s an interesting application of the same technology as the cochlear implant, but dialed in to a unique specific use case.