Auditory Brainstem Implants

The first report of direct stimulation of human auditory cortex is attributed to Penfield in the 1950s, which was done under local anesthesia. In 1964 Simmons and his group described their experience with electrical excitation of the cochlear nerve and inferior colliculus. William House implanted the first auditory brainstem implant (ABI) with singlechannel electrode array in 1979 after removal of an acoustic tumor.15 Since 1992, advanced multichannel ABIs have been implanted.

At the present ABI has a narrow set of indications related to bilateral retrocochlear deafness. The leading cause is loss of bilateral auditory nerve function after vestibular schwannomas removal in neurofi-bromatosis type 2 (NF-2) followed by other rare bilateral auditory nerve pathologies. Approximately 90% of NF-2 patients exhibit bilateral acoustic neuromas.16 In the United States patients undergoing ABI are under strict protocols including a comprehensive battery of psychophysical and speech perception tests.

In deciding on a patient's candidacy for cochlear implantation versus ABI, the surgeon must have a clear distinction between cochlear and retrocochlear deafness. Pre- and postoperative topodiagnosis of deafness after bilateral tumor resection in NF-2 may be crucial in determining hearing rehabilitation strategies. Transtympanic extracochlear monopolar electrical stimulation on the promontory or round window in conjunction with transtympanic electro-cochleography is a well-accepted method of distinguishing cochlear from retrocochlear deafness. It has been postulated that in patients with acoustic neuromas, cochlear deafness, either before or after tumor resection, is of vascular etiology when the auditory nerves are preserved. Only very large acoustic tumors are thought to affect auditory nerve

Tympanic Membrane Implant
FIGURE 29-7 Auditory brainstem implant receiver/ stimulator (bottom), and auditory brainstem implant speech processor, postauricular microphone, and transmitter coil. (Courtesy of Cochlear Corp., Englewood, CO.)
Auditory Brainstem Implant

directly. Therefore, preserving the integrity of auditory nerve is a major surgical consideration so that cochlear implantation can be performed in the future. If surgical preservation of auditory nerve cannot be accomplished, however, subsequent hearing rehabilitation with ABI should be considered. Topodiagnostic studies are of great value in these situations.

Thorough knowledge of anatomic landmarks is essential in the localization of cochlear nucleus. Finding the lateral recess depends on identifying a 5- by 6-mm triangle formed by the exiting nerves of VII, VIII, and IX as well as the foramen of Luschka.15 The design of Nucleus 22 and the more current model, Nucleus 24, ABIs takes into account the structural nuances of the lateral recess and the surface of cochlear nucleus (Fig. 29-7). As essential as anatomic landmarks are, the optimal placement of ABI electrode array largely depends on intraoperative monitoring via evoked auditory brainstem

responses.

In terms of surgical approaches there are certainly multitude routes to the internal auditory canal (IAC) such as translabyrinthine, transcochlear, middle fossa, retrolabyrinthine, retrosigmoid, and suboccipital approaches. Translabyrinthine approach was originally advocated by House and Hitselberger et al15 as the most direct access to the foramen of Luschka (Fig. 29-8). The retrosigmoid approach in combination with the transmastoid approach appears to offer several advantages in situations where either cochlear nerve preservation or simultaneous ABI implantation is performed.18 Significant features of this route are the ease of intraoperative monitoring of cochlear nerve action potentials, excellent visualization of the lateral recess of the fourth ventricle, and greater room to manipulate skull base endoscopes as means of improving three-dimensional orientation.

From 1992 to 1999, 58 patients were implanted with ABIs at 18 centers in Europe and Asia.19 Most patients were noted to use their ABI daily. Some reported perception of different sounds and frequencies, enhancement of lip-reading ability, and even the use of telephones. Patients in multiple electrode array studies demonstrated different pitch perception and the use of tonotopic organization of cochlear nucleus complex. Similar results were also reported by the House Institute in Los Angeles.20

Surgical application of ABIs and postoperative rehabilitation have demonstrated effectiveness and safety in helping patients improve their communicative skills, acoustic orientation, and overall quality of life.

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