Auditory Brainstem Implantation
Gregory P. Lekovic, MD, PhD, JD
L. Fernando Gonzalez, MD
Mark J. Syms, MD*
C. Phillip Daspit, MD*
Randall W. Porter, MD
Division of Neurological Surgery and *Section of Neurotology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona
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Auditory brainstem implants (ABIs) can restore meaningful hearing to patients deafened by neurofibromatosis type 2. These devices are similar to cochlear implants on which their design is based. Current multichannel ABI technology affords a level of performance in aiding speech comprehension similar to that offered by single-channel cochlear implants. ABIs differ from cochlear implants in that the brainstem implants stimulate the cochlear nuclei directly, bypassing the cochlear nerve. Multichannel ABIs can be implanted at the time of first or second side surgery for tumors involving the auditory nerve. Most surgeons favor a translabyrinthine approach for ABI placement, because the stimulating leads need to be implanted within the lateral recess of the fourth ventricle. At this location, the ventral cochlear nucleus can be stimulated along its lateral to medial axis. Intraoperative monitoring of brainstem auditory evoked potentials is essential and helps confirm adequate positioning of the device. After programming, about 85% of patients experience auditory percepts. Only a relatively rare, but significant, minority of patients is able to attain sound-only open-set speech comprehension with their ABI. Most experience a significant improvement in open-set speech comprehension when used as an adjunct to lipreading. Speech comprehension continues to increase as long as 8 years after device implantation. More than 90% of recipients use their ABI daily. Most are satisfied with their decision to obtain an ABI and would recommend the device to others.
Key Words: acoustic neuroma, auditory brainstem implantation, auditory prosthesis, neurofibromatosis type 2, vestibular schwannoma
Since 1979 auditory brainstem implants (ABIs) have provided prosthetic hearing for patients deafened by neurofibromatosis-2 (NF-2). The first ABI was a comparatively primitive, single-channel device that aided users by increasing awareness of environmental sounds and lipreading. Contemporary multichannel ABIs afford a level of performance comparable to that achieved with single-channel cochlear implants, that is, the ability to discriminate environmental sounds, marked improvement in lipreading, and limited openset speech comprehension. Some ABI users can even communicate on the telephone. This article reviews the development of ABIs, emphasizing relevant neuroanatomical and physiological principles, intraoperative technique, related surgical anatomy, clinical outcomes, and future directions for research.
History of ABIs
At the time of this writing, more than 400 ABIs have been placed worldwide. The first ABI implant was a simple ball electrode implanted into the parenchyma of the cochlear nucleus at the House Ear Institute in 1979. This electrode migrated from its implantation site, and the design was soon modified into a pair of platinum plates with a synthetic mesh backing. Subsequent iterations of ABI design culminated in the development of multichannel implants composed of 8 to 21 platinum discs embedded in plastic with a synthetic mesh backing, depending on the manufacturer or model.
In 1994 a clinical trial was begun in the United States to evaluate the safety and efficacy of multichannel ABIs. This trial was concluded in 2000 and culminated in the Food and Drug Administration (FDA) approving the Nucleus 24 Contour (Cochlear Corporation, Englewood, CO) and Nucleus 22 (Cochlear Corporation, Englewood, CO) for implantation via the translabyrinthine approach for the treatment of deafness related to NF-2. Currently, the Nucleus 24 is the most widely implanted ABI in the world, including North America, Australia, and the European Union. Other available ABIs include the MXM Digisonic (Laboratories MXM, Valaurious Cedex, France), Med-EL (Med EL Corporation, Research Triangle Park, NC), and Clarion (Advanced Bionics, Sylmar, CA). So far no evidence suggests that the efficacy of the various multichannel ABIs differs.
Figure 1. (A) The auditory brainstem implant consists of a radio receiver-stimulator that is implanted in the temporal bone. A ground electrode is inserted under the temporalis muscle, and the multichannel brainstem implant paddle is inserted into the lateral recess of the fourth ventricle. (B) Sound is picked up at the pinna by a microphone that sends the signal to a speech-processor/digitizer. The latter sends the signal to the brainstem implant receiver through the transmitter coil. Photographs courtesy of Cochlear Ltd.
Design of ABIs
All ABIs share certain features regardless of specifications (Fig. 1). The basic electrode itself is placed within the lateral recess of the fourth ventricle and connected via a cable implanted into a groove in the temporal bone to a receiver-stimulator. The latter is seated in the bone of the skull behind the helix of the ear above the canthomeatal line. A second component consists of microphone, speech processor, and radiotransmitter coil that detects sound, converts it to a digital signal, and transmits it to the receiver, respectively. The transmitter attaches to the receiver-stimulator by a magnet. Once an ABI is implanted, monopolar cauterization is strictly forbidden from use for fear of damaging the electrode, brain, or both. Recently, the magnet has been replaced with a nonmagnetic plug at surgery, and the transmitter coil is fixed to the scalp with adhesive. These changes permit MR imaging from the outset.
ABIs are not activated until a patient recovers from surgery. Because of concerns about the potential activation of brainstem structures, many centers prefer that the electrode be activated in a monitored setting. After successful ABI implantation, medical and audiological follow-up is performed at least every 3 months for the first year and then annually thereafter.
Indications for ABIs
The FDA has approved the ABI for patients with NF-2 who are 12 years or older who have reasonable expectations and motivation. The degree of hearing loss is not a deciding factor (i.e., there is no audiological cut-off for ABI placement). Other factors also distinguish optimal ABI candidates: good overall health, acceptable vision (because the ABI may aid most with lipreading), high motivation, excellent family support, acceptable anatomical status, and an interest in spoken communication. Because the ABI creates an 'unnatural' quality of sound, many users are disappointed with it at first. High motivation and excellent family support are important to ensure that users do not drop out of the ABI program.