Auditory brainstem implant
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An auditory brainstem implant (ABI) is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf, due to retrocochlear hearing impairment (due to illness or injury damaging the cochlea or auditory nerve, and so precluding the use of a cochlear implant).
The auditory brainstem implant uses technology similar to that of the cochlear implant, but instead of electrical stimulation being used to stimulate the cochlea, it is used to stimulate the brainstem of the recipient.
Only about a thousand five hundred recipients have been implanted with an auditory brainstem implant, due to the nature of the surgery required to implant the device (as it requires brain surgery to implant the device).
In the United States ABIs were previously only approved for adults (18 & over) and only for patients with neurofibromatosis type II (NF2). In January 2013, the US FDA approved a clinical trial of auditory brainstem implants for children. In Europe, ABIs have been used in children and adults, and in patients with NF2 as well as other auditory complications, such as auditory nerve aplasia and cochlea ossification.
The ABI was originally developed at the House Ear Institute in 1979 for NF2 patients who lost their VIIIn function bilaterally following surgery to remove vestibular schwannomas (VS). The ABI has provided therapeutic benefit for NF2 patients in terms of sound awareness, identification of some environmental sounds and improved performance over lipreading alone when communicating face-to-face  However, speech understanding without visual cues (commonly called “open-set” speech recognition) was generally poor.
Recently Colletti and colleagues observed high levels of open set speech recognition without visual cues in ABI patients who had lost their VIIIn from causes other than NF2. Many of these non-tumor ABI patients were able to achieve better than 50% recognition of sentences presented in quiet and could even achieve conversational use of the telephone. This result suggested that the ABI device and electrode placement in the lateral recess of the IV ventricle was capable of providing good speech recognition. The limited speech understanding observed in NF2 ABI patients was thus thought to be due to negative factors related to NF2. However, new results in NF2 ABI patients have also showed excellent speech recognition,suggesting that the cause of improved speech understanding was not related to NF2 alone. Prior to surgery, some NF2 patients have normal hearing and speech recognition in the tumor ear even with a large tumor, suggesting that the presence of the tumor itself does not necessarily cause an auditory deficit. The loss of hearing and speech recognition in most NF2 patients may be due to the tumor interfering with the blood supply to the cochlea, VIIIn, and/or cochlear nucleus. In addition, physical compression by the tumor of the nerve and brainstem might impair neural excitability. The recently observed improvement in NF2 ABI speech recognition may be due to differences in patient etiology, surgical technique, damage to the brainstem before and during tumor removal, or electrode placement, device design, signal processing, or other factors.
The auditory brainstem electrodes were first implanted in humans in 1979 at the House Ear Institute, CA, USA. This original ABI consisted of two ball electrodes which were implanted near the surface of the cochlear nucleus. A change from a percutaneous connection to a wireless transcutaneous connection, and from ball electrodes to flat electrodes were the only changes to the implant until 1991, where 25 people had received the ABI.
In the US in 1992 an eight electrode implant was developed by Cochlear Limited, the House Ear Institute and Huntington Medical Research Institute.An electrode array with 21 electrodes developed by Cochlear Limited was developed for the European market at the same time. The processor for both the eight and 20 electrode implants used Nucleus 22 ABI (Cochlear Limited) external speech processors. Since 1999 a 21 electrode array implant has been used with the Nucleus 24 ABI (Cochlear Limited) speech processor.
A 12 electrode array implant with a speech processor based on the C40+ cochlear implant (Med-El) The first Combi 40+ ABI implantation was performed in 1997 by Prof. Behr at the University of Wurzburg, Germany. The device reached its CE mark in 2003. A 16 electrode array implant with the Clarion-1.2 cochlear implant (Advanced Bionics) have also been developed.
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