The auditory brainstem implant (ABI), which was conceived by William F. House at the House Ear Institute in Los Angeles, California in the 1970s, is a device that provides auditory sensation by directly stimulating the cochlear nucleus of the brainstem. By bypassing the cochlea and the cochlear nerve, it has become an invaluable resource for hearing rehabilitation in patients with neurofibromatosis type 2. Additional clinical advances have expanded the use of ABIs to pediatric patients, and additional research may establish other clinical applications.
1 The History and Development of Auditory Brainstem Implants
1.1 Early Work in Stimulation of the Brainstem
The first report of stimulation of the human brainstem was published in 1964. In this study, Simmons et al stimulated the inferior colliculus, but this yielded no sound perception or awareness. 1 Several years elapsed before any significant advances were made. At the House Ear Institute (HEI) in Los Angeles, California, William F. House had begun surgical placement of cochlear implants in the late 1960s. He realized that auditory rehabilitation in patients with no auditory nerves, such as patients with neurofibromatosis type 2, remained unaddressed.
House began to design a device to stimulate the cochlear nucleus of the brainstem directly. He enlisted the assistance of Jean Moore, a neuroanatomist at HEI. Moore mapped out the target area for brainstem implantation. Based on his prior experience with the development of the cochlear implant, House designed an initial device with a two-ball-electrode configuration. This communicated percutaneously with an external receiver.
On May 24, 1979, House and William E. Hitselberger operated on a 51-year-old female with a vestibular schwannoma in her only-hearing ear. Following tumor resection, they placed the first auditory brainstem implant (ABI) with the electrode residing next to the cochlear nucleus. This was percutaneously coupled with a modified body-worn Bosch hearing aid. This provided the patient with sound awareness, but by 1980, the patient developed a sensation of “twitching” in the ipsilateral leg. The electrode was deemed to have migrated. The patient continued to use the ABI and external processor until her death in her late 80s.
In conjunction with Douglas McCreery of the Huntington Medical Research Institute (HMRI), House designed a new electrode with a Dacron mesh backing, to which an increasing number of electrodes was later added (Fig. 1.1). This mesh was designed to provide increased stability of the electrode after implantation. In 1981, the original subject underwent a second surgery. The prior electrode was removed, and the new Dacron mesh electrode was placed on the surface of the brainstem. This ABI allowed once again for auditory sensation, and was paired several years later with a House-Sigma single-channel cochlear implant processor. The patient continues to use the ABI and external processor to this day. 2 A seminal paper by Edgerton et al later summarized the preliminary efforts and physiology of direct stimulation of the cochlear nucleus. 3
1.2 Auditory Brainstem Response and Advances in Device Manufacturing
In 1982, Michael D. Waring, an audiologist working with HEI, recorded the first electrically evoked auditory brainstem response from the first ABI recipient previously discussed. This provided additional encouragement, and HEI began to construct and evaluate electrode designs for future ABI devices. J. Phil Mobley and Franco Portillo, engineers with HEI, supervised the installation of fabrication facilities at HEI. In 1984, the first implantation of an ABI device fabricated at HEI was performed.
In 1985, three patients underwent ABI placement. Broken wires and electrode migration were both encountered, but nonetheless the implants showed promise. Technical details were described in a subsequent publication by McElveen et al, which confirmed the feasibility of using the ABI for auditory stimulation. 4 In 1986, HEI received investigational device exemption status from the Food and Drug Administration (FDA) to pursue the ABI program.
Another advance came in 1987, when Portillo suggested the use of braided wires rather than single-stranded wires. Braided wires were more flexible and less prone to breakage. Derald E. Brackmann, a neurotologist at HEI, worked with Portillo to improve the reliability and biocompatibility of the percutaneous plug. The same year, Eisenberg et al published the first audiological perspective on the ABI, confirming that ABI recipients experienced tone perception and significant auditory discrimination exceeding what is expected due to chance. 5