The treatment of pediatric hearing loss depends on cause and degree. Infants and children with mild-to-moderate hearing loss are best managed with hearing aids. Cochlear implants (CIs), which bypass the nonfunctioning hair cells of the inner ear to directly stimulate the auditory nerve, are reserved for children with severe to profound hearing loss in both ears.
Roughly 80,000 pediatric patients have received cochlear implants worldwide, and the vast majority enjoy sound and speech perception (Acta Otorhinolaryngol Ital 2012;32:347-370 ). However, there is a small subset of congenitally deafened children who will not benefit from a CI due to: 1) small or absent cochlea, 2) small or absent auditory nerve, or 3) injury or scarring of the inner ear or auditory nerve due to meningitis, trauma, or tumor (see figure 1 ).
In such cases, an auditory brainstem implant (ABI) is an option to bypass the damaged or absent cochlea and auditory nerve, transmitting electrical impulses to the cochlear nucleus (CN) in the brainstem (see figure 2 ). The CN is the first relay station in the brain where sound information is processed.
In 1979, William Hitselberger, MD, and William House, DDS, MD, performed the first ABI after resection of a tumor of the vestibular nerve (Otolaryngol Head Neck Surg 1984;92:52-54 ). Over subsequent years, improvements in the design of the ABI paralleled advances in the CI (Otol Neurotol 2009;30:708-715 ).
Then in 2000, the United States Food and Drug Administration (FDA) approved the auditory brainstem implant for patients 12 years of age or older with neurofibromatosis type 2 (NF2). NF2 is a genetic disease that occurs in one in 40,000 births (Curr Opin Neurol 2003;16:27-33 ).
One of the defining characteristics of NF2 is the growth of benign tumors along the vestibular nerves, as well as the presence of multiple tumors in the brain and the spine.
Surgical removal of these tumors typically damages the auditory nerve, resulting in deafness. Unlike a cochlear implant, an ABI does not require an intact auditory nerve and can be placed after tumor removal to provide sound sensations to the deafened patient (Prog Brain Res2009;175:333-345).
Research performed over the past several decades has demonstrated that ABIs provide meaningful sound awareness and improve lip-reading scores (Laryngoscope 2005;115:1974-1978 ).
For example, among 26 patients in a study by Barry Nevison, DPhil, and colleagues, all but one patient had auditory sensation at initial ABI activation, and the ABI provided most subjects with some ability to appreciate sound (Ear Hear 2002;23:170-183). However, benefits for individual ABI patients have varied substantially, as noted by Marc S. Schwartz, MD, et al (Stereotact Funct Neurosurg 2003;81[1-4]:110-114).
While US otologists and neurosurgeons have predominately used auditory brainstem implants in the adult NF2 population, European colleagues, including Vittorio Colletti, MD, in Verona, Italy, and Levent Sennaroglu, MD, in Ankara, Turkey, began performing ABI surgery in children and in non-NF2 patients.
Dr. Colletti was the first to implant an ABI in a non-NF2 child, and he has performed auditory brainstem implant surgery in children as young as 9 months (data not published). The indications for the ABI were congenital or acquired malformations of the inner ear (Laryngoscope 2005;115:1974-1978 ).
He has compared non-tumor patients with NF2 patients, hypothesizing that the former population would have better outcomes than the latter (Otolaryngol Head Neck Surg 2005;133:126-138 ). The non-tumor group obtained an average speech recognition of 63 percent, versus 12.2 percent in the tumor group. The investigators concluded that open-set speech perception was possible with ABIs in non-tumor patients.
Other investigators, including Dr. Sennaroglu; Laurie S. Eisenberg, PhD; and their colleagues, have also demonstrated speech perception in some non-tumor pediatric patients who underwent ABI surgery for severe inner-ear abnormalities (Otol Neurotol 2008;29:251-257); (Otol Neurotol 2009;30:708-715).
NEW CLINICAL TRIALS
Based on these encouraging findings in Europe, there has been a resurgence of auditory brainstem implant research in the United States. This work has focused on better understanding the factors that contribute to improved outcomes, beginning new clinical trials to enroll non-NF2 candidates, and refining electrode designs.
Our institution, the Massachusetts Eye and Ear Infirmary and Massachusetts General Hospital, is one of three centers in the United States approved by the FDA to conduct ABI surgery in infants and children who are deaf and cannot receive a cochlear implant.
In addition, our research group at the Eaton–Peabody Laboratories is working closely with colleagues at École Polytechnique Fédérale de Lausanne (Swiss Federal Institute of Technology, Lausanne) in Switzerland to: 1) develop a flexible ABI electrode array that better conforms to the shape of the brainstem and 2) develop ABI electrodes that deliver blue light to excite photosensitized cochlear nucleus neurons through an exciting technology called optogenetics.
In optogenetics, which is already used in a number of clinical trials in the United States, a virus delivers a gene to neurons that produces a light-sensitive protein. These proteins, called “opsins,” make it possible to activate or silence the neuron using visible light of a particular wavelength. The advantage of light is that it can be focused to produce many more independent channels than possible with existing technology based on electrical current.
Two recent articles have focused on the expanding indications for auditory brainstem implants and investigated the anatomy of the brainstem traditionally implanted with an ABI.
Indications and Contraindications of Auditory Brainstem Implants: Systematic Review and Illustrative Cases
Merkus P, Di Lella F, et al Eur Arch Otorhinolaryngol 2013 Feb 13 (epub ahead of print)
The first article, by Paul Merkus, MD, PhD, et al, analyzed current indications for ABI in non-NF2 patients. The authors first reviewed their internal medical records of patients who were implanted at outside ABI centers between 1986 and 2011 and referred to their ABI center in the Netherlands for follow-up.
They identified 14 patients with ABI and non-NF2 indications—13 adults and one teenager. All cases, Dr. Merkus and colleagues argued, had modest outcomes and may have benefited from a cochlear implant alone.
Of the 14 patients, nine improved with a subsequent CI, two improved with a hearing aid, one had beem referred for ABI but received a bilateral cochlear implant, and, for the remaining two, cochlear implantation is still planned.
In the same paper, the authors further reviewed the medical literature to find 31 articles citing outcomes in a total of 144 non-NF2 patients from 12 different centers. The articles, which were published between 2000 and 2011, each identified at least seven indications for ABI other than NF2—mainly congenital and traumatic malformations of the inner ear.
Auditory brainstem implants should be restricted to patients who are not CI candidates and have no other options for hearing rehabilitation, Dr. Merkus and colleagues concluded.
Structures of the inner ear should be thoroughly investigated prior to ABI to make sure no alternative option is possible, they argued. Assessment of ABI candidates should occur in a stepwise approach, and a cochlear implant trial prior to ABI may be warranted in certain cases, the authors added.
This paper, which tells us that patients need to be carefully selected by a multidisciplinary team using rigorous criteria, is immediately applicable to the pediatric population. Given that CI data indicate improved outcomes with earlier implantation, the expeditious evaluation and referral of pediatric patients who are candidates for ABI also appears reasonable.
No Easy Target: Anatomic Constraints of Electrodes Interfacing the Human Cochlear Nucleus
Rosahl SK, Rosahl S Neurosurgery 2013;72(ONS suppl 1):ons58-ons65
The second recently published article, by Steffen K. Rosahl, MD, PhD, and Sybille Rosahl, MD, sought to determine the possible anatomical constraints of the ABI interface and reasons for variable outcomes.
The size, shape, location, and orientation of the auditory brainstem were assessed in 33 adult human specimens obtained from 20 patients at autopsy. Photomicrographs and three-dimensional renderings were obtained.
From this host of measurements, the principal finding was that the maximum dimensions of the cochlear nucleus were extremely small—less than one centimeter all around. The most significant constraint of the ABI was the suboptimal interface with the brainstem, the authors suggested.
Further, the rotation of the cochlear nucleus potentially reduces electrode contact in close proximity to auditory neurons. In the absence of surface landmarks and image guidance, ABI placement remains a major challenge, Drs. Rosahl concluded.
This paper may provide important insights for ABI design and surgical approach. A similar investigation examining the pediatric brainstem would be as important to conduct and would help identify anatomical differences in children compared with adults.
In addition, the technical challenges demonstrated in this paper highlight the concept that ABI surgery may best be accomplished by a limited number of designated quaternary care centers in the United States and around the world that have surgeons and audiologists with robust technical expertise.
Forty years since the first ABI surgery, auditory brainstem implantation continues to be employed and investigated in an increasing, but highly specific, subset of patients. Over the past decade, new indications in the pediatric population have seen growing support and may demonstrate improved outcomes.