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6 Lekovic ABI-Website Copyright © 2004, Barrow Neurological Institute Auditory Brainstem Implantation ince 1979 auditory brainstem im- Gregory P. Lekovic, MD, PhD, JD Splants (ABIs) have provided pros- L. Fernando Gonzalez, MD thetic hearing for patients deafened by † Mark J. Syms, MD neurofibromatosis-2 (NF-2). The first † C. Phillip Daspit, MD ABI was a comparatively primitive, sin- Randall W. Porter, MD gle-channel device that aided users by increasing awareness of environmental Auditory brainstem implants (ABIs) can restore meaningful hearing to patients sounds and lipreading. Contemporary deafened by neurofibromatosis type 2. These devices are similar to cochlear multichannel ABIs afford a level of per- implants on which their design is based. Current multichannel ABI technology formance comparable to that achieved affords a level of performance in aiding speech comprehension similar to that of- with single-channel cochlear implants, fered by single-channel cochlear implants. ABIs differ from cochlear implants in that is, the ability to discriminate envi- that the brainstem implants stimulate the cochlear nuclei directly, bypassing the ronmental sounds, marked improve- cochlear nerve. Multichannel ABIs can be implanted at the time of first or second ment in lipreading, and limited open- side surgery for tumors involving the auditory nerve. Most surgeons favor a trans- set speech comprehension. Some ABI labyrinthine approach for ABI placement, because the stimulating leads need to users can even communicate on the be implanted within the lateral recess of the fourth ventricle. At this location, the telephone. This article reviews the de- ventral cochlear nucleus can be stimulated along its lateral to medial axis. Intra- velopment of ABIs, emphasizing rele- operative monitoring of brainstem auditory evoked potentials is essential and vant neuroanatomical and physiological helps confirm adequate positioning of the device. After programming, about 85% principles, intraoperative technique, re- of patients experience auditory percepts. Only a relatively rare, but significant, mi- lated surgical anatomy, clinical out- nority of patients is able to attain sound-only open-set speech comprehension with comes, and future directions for re- their ABI. Most experience a significant improvement in open-set speech com- search. prehension when used as an adjunct to lipreading. Speech comprehension con- tinues 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 History of ABIs ABI and would recommend the device to others. At the time of this writing, more than 400 ABIs have been placed world- Key Words: acoustic neuroma, auditory brainstem implantation, auditory wide.17 The first ABI implant was a sim- prosthesis, neurofibromatosis type 2, vestibular schwannoma ple ball electrode implanted into the pa- renchyma of the cochlear nucleus at the House Ear Institute in 1979. This elec- trode migrated from its implantation site, and the design was soon modified into a pair of platinum plates with a syn- thetic mesh backing. Subsequent iter- ations of ABI design culminated in the development of multichannel implants composed of 8 to 21 platinum discs em- bedded in plastic with a synthetic mesh backing, depending on the manufac- Division of Neurological Surgery and †Section of Neurotology, Barrow Neurological Institute, St. turer or model. Joseph’s Hospital and Medical Center, Phoenix, Arizona In 1994 a clinical trial was begun in 40 BARROW QUARTERLY • Vol. 20, No. 4 • 2004 Lekovic et al: Auditory Brainstem Implantation the United States to evaluate the safety the ear above the canthomeatal line. A ing loss is not a deciding factor (i.e., and efficacy of multichannel ABIs. This second component consists of micro- there is no audiological cut-off for ABI trial was concluded in 2000 and culmi- phone, speech processor, and radio- placement). Other factors also distin- nated in the Food and Drug Adminis- transmitter coil that detects sound, con- guish optimal ABI candidates: good tration (FDA) approving the Nucleus verts it to a digital signal, and transmits overall health, acceptable vision (because 24 Contour (Cochlear Corporation, it to the receiver, respectively. The the ABI may aid most with lipreading), Englewood, CO) and Nucleus 22 (Co- transmitter attaches to the receiver-stim- high motivation, excellent family sup- chlear Corporation, Englewood, CO) ulator by a magnet. Once an ABI is im- port, acceptable anatomical status, and for implantation via the translabyrin- planted, monopolar cauterization is an interest in spoken communication. thine approach for the treatment of strictly forbidden from use for fear of Because the ABI creates an ‘unnatural’ deafness related to NF-2. Currently, the damaging the electrode, brain, or both. quality of sound, many users are disap- Nucleus 24 is the most widely implant- Recently, the magnet has been replaced pointed with it at first. High motiva- ed ABI in the world, including North with a nonmagnetic plug at surgery, and tion and excellent family support are America, Australia, and the European the transmitter coil is fixed to the scalp important to ensure that users do not Union. Other available ABIs include the with adhesive. These changes permit drop out of the ABI program. MXM Digisonic (Laboratories MXM, MR imaging from the outset. These issues are exemplified in the Valaurious Cedex, France), Med-EL ABIs are not activated until a patient House experience with placing ABIs in (Med EL Corporation, Research Tri- recovers from surgery. Because of con- teenagers.17 Of 21 teens implanted, four angle Park, NC), and Clarion (Ad- cerns about the potential activation of dropped out of the program. One later vanced Bionics, Sylmar, CA). So far no brainstem structures, many centers pre- returned and has become an above av- evidence suggests that the efficacy of the fer that the electrode be activated in a erage ABI user as a result of strong fam- various multichannel ABIs differs. monitored setting. After successful ABI ily support and encouragement. Qual- implantation, medical and audiological ities most characteristic of program follow-up is performed at least every 3 dropouts included low motivation and Design of ABIs months for the first year and then an- poor family support. However, an im- All ABIs share certain features re- nually thereafter. portant lesson from the House experi- gardless of specifications (Fig. 1). The ence has been that the ideal candidate is basic electrode itself is placed within the self-motivated to hear again rather than lateral recess of the fourth ventricle and Indications for ABIs motivated for the benefit of family connected via a cable implanted into a The FDA has approved the ABI for members. groove in the temporal bone to a re- patients with NF-2 who are 12 years or The only absolute contraindication ceiver-stimulator. The latter is seated in older who have reasonable expectations to the placement of an ABI is an active the bone of the skull behind the helix of and motivation. The degree of hear- infection. Relative contraindications Ground electrode Paddle Microphone Speech processor- digitizer Transmitter coil A Receiver-stimulator B 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. BARROW QUARTERLY • Vol. 20, No. 4 • 2004 41 Lekovic et al: Auditory Brainstem Implantation are rare and include patients who may on the first side. If the device is im- Marangos et al.15 examined 19 ears meet the formal criteria but who lack properly positioned within the lateral with promontory testing in patients adequate support or motivation, as out- recess of the fourth ventricle, does not with profound bilateral deafness. The lined above. As discussed later, if the co- elicit auditory sensations, or creates in- promontory tests were positive in five chlear nerve can be preserved intact after tolerable side effects that prevent its use, of six ears that had not undergone pre- resection of an acoustic neuroma, place- then the contralateral side can be im- vious surgery and in three other surgi- ment of a cochlear implant may be in- planted when surgery is performed on cally treated ears with residual tumor. dicated if promontory testing is positive. the second side. The patient thereby The patient who responded negatively The FDA has approved the use of has a second chance to obtain a func- on promontory testing but who had ABIs only in the setting of NF-2. How- tioning implant. Conversely, if the first undergone no previous surgery had bi- ever, off-label use of the device has been side is implanted and functions well, the lateral acoustic neuromas causing pro- reported (predominantly in Europe) for user can become accustomed to the de- found brainstem compression. A pa- other indications, including deafness re- vice while still able to hear with the tient presenting with deafness one year lated to bilateral skull base trauma,3 spo- other ear. This situation may help the after undergoing a subtemporal ap- radic acoustic neuroma in patients with patient to acclimatize to and interpret proach in which hearing was preserved only one hearing ear,4 cochlear nerve the ABI sound. However, most patients had a positive promontory test. This hypoplasia or aplasia,2,5 cochlear ossifica- implanted during surgery on the first patient was fitted with a cochlear im- tion,11 bilateral auditory nerve deafness,22 side do not become daily users of ABI plant and had an excellent response. or involvement with other neoplastic until their deafness is bilateral.
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