An auditory brainstem implant (ABI) is a surgically implanted device that provides a sensation of sound to a person who has severe hearing loss due to damage to the inner ear (cochlea) and auditory nerve.
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The ABI was first developed for people with neurofibromatosis type 2 (NF2) who had lost their hearing due to damage to the auditory nerve. NF2 is a rare genetic disorder in which tumors form along the auditory nerve. These tumors are called acoustic neuromas (also known as vestibular schwannomas). Growth of the tumors and surgery to remove them and/or treatment with radiation can forever damage the auditory nerves, resulting in deafness in both ears. In most cases, these patients cannot be helped with hearing aids or cochlear implants.
Since its initial use in patients with NF2, others with severe hearing loss have found benefit from an ABI. These infants, children, and adults include:
. Those who are born with no functioning auditory nerve in either ear (aplasia) or improperly developed auditory nerve (hypoplasia).
. Those who are deaf because of an abnormally shaped inner ear (cochlea), incomplete development of the inner ear (cochlear hypoplasia), absence of inner ear structures (cochlear aplasia), overgrowth of bone in the inner ear (cochlear otosclerosis), or improper bone development in the inner ear (cochlear ossification).
. Those who have injured or fractured an auditory nerve caused by temporal bone (skull area above the ear) fractures on both sides of the head.
. Those with other causes of severe deafness who cannot benefit from a hearing aid or a cochlear implant.
First, it’s important to know that the implant does not restore normal hearing. However, an auditory brainstem implant can:
. Improve sound awareness.
. Improve the ability to detect and tell apart sounds (for example, female vs male voice; child vs adult voice).
. Improve the ability to identify environmental sounds (for example, dog barking vs phone ringing).
. Improve face-to-face communication by improving lip reading ability.
When examining results by different patient groups, patients with NF2 showed improved closed-set word recognition. (Closed-set speech recognition means understanding the meaning of a sound or word or being able to tell the difference in sounds or words among a limited number of options within the patient’s sight.) Patients who did not have NF2 as the cause of their deafness generally had better hearing outcomes than those who had NF2. Patients without NF2 achieved open-set, sound-only sentence recognition. (Open-set speech recognition means understanding the meaning of a sound or word or being able to tell the difference in sounds or words when there is an unlimited number of possibilities and/or without being in a person’s sight.) Children, with or without NF2, achieved environmental sound awareness and language development, including simple words and sentences. The hearing abilities in children continued to improve in the years after the device was implanted.
It is important to note that results vary widely. Researchers are still exploring possible reasons, which include the cause of the deafness, surgical approach, surgeon expertise, device design, signal processing, device programming issues, and other factors. Before a decision is made to proceed with surgery, expected outcomes need to be carefully discussed with each patient and/or family.
The implant consists of two separate parts. One part, worn on top of the ear, is called the processor. This part contains a microphone that picks up the surrounding sounds. The processor converts sounds to an electrical signal and sends it to the internal implant. The internal implant consists of a receiver, which is implanted just under the skin on the side of the head, and ends in an electronic “paddle.” The paddle is positioned on the surface of the brainstem in an area called the cochlear nucleus complex. The electrodes on the paddle activate different neuron types in the brainstem, creating sound and pitch sensations.
The auditory brainstem implant uses technology similar to that used in a cochlear implant. The main difference is where and what the electrodes stimulate. With the cochlear implant, the array of electrodes stimulates the auditory nerve in the cochlea. The ABI’s array of electrodes are placed directly on the brainstem. They bypasses the cochlea and the auditory nerve entirely.
First, a complete medical history is gathered. Imaging studies are done to examine the condition of the cochlea, auditory nerve, and the surrounding structures. These tests include a CT scan and an MRI. The MRI helps view the soft tissue anatomy such as the auditory nerve. Behavioral testing is also used to determine hearing ability. A communication evaluation is done to examine language development and communication style. Electrophysiologic testing is used to evaluate the extent of hearing loss and its location. A neuropsychological evaluation examines a person’s level of brain function (intellect, learning and memory, language development, motor abilities, organization/planning skills, concentration).
Complications include cerebrospinal fluid leaks, facial nerve palsies (loss of facial movement due to nerve damage), wound infection, meningitis, incomplete tumor removal, and implant failure (failure to provide useful auditory sensations or movement/displacement of the electrodes). The rate of serious complications is low, especially when the surgery is performed in a medical facility with an experienced staff. There can also be minor complications that are easily controlled and usually completely heal.
ABI implantation is a complex surgery which is performed by a team that typically includes a neurosurgeon, anesthesiologist, and neurotologist. Additionally an electrophysiologist and audiologist will perform extensive testing during the procedure to confirm the function and help fine tune the placement of the implant. An ABI is most often implanted during surgery to remove an acoustic neuroma in patients with NF2 whose hearing is not expected to be preserved after surgery. It can also be performed as a separate surgery in some cases such as when prior surgery or radiation therapy has already been done and no hearing is present, or in the cases of inner ear malformations, trauma, and other causes of deafness that are not candidates for cochlear implant surgery.
The typical hospital stay ranges from 2 to 4 days. The stay is longer for patients with NF2 since these patients have had a tumor removed as well. Device programming begins in the operating room during implant placement. ABI device activation usually takes place 4 to 6 weeks after surgery. Device programming continues to be adjusted over the course of several days, then monthly during the first year after the implant.
Although the design and function of the implants are similar, these devices are different. The cochlear implant is used in individuals with cochlear damage but who still have a functioning auditory nerve. A cochlear implant works by bypassing the damaged areas of the inner ear. It uses an array of electrodes to stimulate the auditory nerve directly. The auditory nerve sends signals generated by the implant to the brain, which recognizes the signals as sound. The auditory brainstem implant bypasses the inner ear and the auditory nerve. It uses an array of electrodes to stimulate the hearing pathways on the brainstem directly.
Cochlear implant surgery is an inner ear surgery. Auditory brainstem implant surgery is brain surgery and is much more complex.