Auditory Brainstem Implants/Neurofibromatosis Type II

By David A. Klodd, Ph.D., Janna M. Hoffman, B.A.,Thomas J. Haberkamp, M.D.

• What is an Auditory Brainstem Implant?
• Who is eligible to receive an ABI?
• What is Neurofibromatosis?
• How is NF2 diagnosed?
• How is NF2 treated?
• Can a patient hear right after surgery?
• What is the difference between an ABI and a cochlear implant?
• ABI Research

What is an Auditory Brainstem Implant?

The Auditory Brainstem Implant (ABI) was initially developed at the House Ear Institute in Los Angeles, California in 1979. It received FDA approval in 2000, and has been implanted in more than 500 patients worldwide. The ABI consists of a receiver/stimulator, a 21 electrode array, a stimulating electrode internally and a speech processor externally.

The ABI is the first prosthetic hearing device designed to bypass the cochlea and auditory nerve (which in ABI patients are damaged) and transmit sound directly to the brainstem. In normal hearing, sound is picked up in the cochlea, translated into electrical signals and passed down the auditory nerve to the brainstem. There, it is processed into signals that the brain perceives as sound. The ABI’s electrodes are placed directly at the base of the brain at the cochlear nucleus.

When sound reaches the patient, it is amplified by a small microphone and transmitted to a speech processor. The speech processor analyzes the sound and converts it into a digital signal that can be sent via transmitting coil and subsequently radio signal to the implant. The various electrodes on the implant analyze the sound’s amplitude (loudness), frequency (pitch), and timing, and, once they receive the radio signal, they stimulate the cochlear nucleus in the brainstem. From there, the auditory pathway proceeds as if it had naturally received sound (Kuchta, 2004).

This is what an auditory brainstem implant looks like. The wire with the tip with the black dots is what is implanted into the brainstem.

This image from the House Ear Institute in Los Angeles, California, shows how the ABI is placed.

The ABI was initially intended for patients with Neurofibromatosis Type II (NF2), a condition where a tumor growing on the auditory nerve can cause hearing loss. During surgery to remove auditory tumors in these patients, an ABI can be placed simultaneously in an attempt to restore hearing. Because tumor removal often leads to complete hearing loss in the treated ear, patients with NF2 had virtually no chance to regain their hearing before the advent of ABI.

Who is eligible to receive an ABI?

In October, 2000, the FDA approved this procedure for patients who have NF2-related hearing loss only. Other criteria that ABI candidates need to meet are as follows: that the patient undergoes/will undergo unilateral or bilateral eighth nerve tumor removal; the patient is at least twelve years of age; is deemed psychologically stable by his or her physician; and is willing and able to comply with extensive follow-up training. In addition, the patient must have realistic expectations regarding the implant, and must demonstrate awareness of the continuum of potential post-surgical outcomes (Kanowitz, 2004).

In 2005, Medicare approved the ABI for surgical implantation for patients that have bilateral vestibular schwannomas. In addition to these patients, trauma patients who have suffered damage to the eighth cranial nerve or cochlear nucleus may also benefit from this surgery (Colletti, 2004).

What is Neurofibromatosis?

Neurofibromatosis is a genetic nervous system disorder affecting approximately 100,000 Americans where tumors grow on or around neurons. Most often, the tumors are benign and grow on peripheral nerves. However, they can get quite large if left untreated or if they are not surgically removed. Depending on the genetic mutation, tumor location, and specific symptoms in a given patient, the disorder can be classified as Neurofibromatosis Type I (NF1) or Type II (NF2).

NF1 is the most prevalent form of neurofibromatosis, affecting 1 in 3,000 males and females in all ethnic groups. In this autosomal dominant disorder, neurofibromas are often found growing on the iris of the eye or on the optic nerve. NF2 is a much less prevalent disorder. It is also autosomal dominant and affects one in 33,000 to 40,000 males and females of all races and ethnicities. Patients with NF2 develop tumors on the eighth cranial nerve, which controls hearing and balance.

How is NF2 diagnosed?

Diagnosis of NF2 usually occurs between the ages 10 and 40. While symptoms are most often noticed between the ages of 18 and 22, almost 20 percent of patients are diagnosed before age 15. Patients with NF2 typically present with bilateral vestibular schwannomas - benign tumors arising from the superior and/or inferior branches of the vestibular division of the eighth cranial nerve.

Patients with NF2 typically first present with symptoms of sensorineural hearing loss (most common type of hearing loss, associated with the eighth cranial nerve and/or the cochlea), tinnitus (ringing in the ears), and changes in vision and/or café-au-lait spots (light browns spots on the skin measuring more than 5 millimeters across in children, and 15 millimeters in adolescents and adults) elsewhere on the body. With tumor growth, symptoms may progress to include headache, visual changes, facial numbness, or facial weakness.

Vestibular schwannomas often involve the internal auditory canal or the cerebellopontine angle, resulting in the symptoms described above. Hearing loss may or may not be present at the time that NF2 is diagnosed. The success for hearing preservation with surgical treatment is approximately 60 percent for intracannalicular tumors (tumors within the auditory canal), and the success for facial nerve preservation is about 90 percent. For tumors more than 3 centimeters, the success rate of surgery to preserve hearing drops to near zero percent, with a 70 percent success rate for preserving the facial nerve.

How is NF2 treated?

NF2 itself cannot be cured. Treatment timing is controversial with observation, imaging, and tumor removal all indicated under certain circumstances. Schwannomas may be monitored closely with imaging; even small tumors can be located at early stages via routine Magnetic Resonance Imaging (MRI). In many cases, particularly in the case of a small tumor, only hearing ear observation may be indicated, although immediate removal with an attempt at hearing preservation is always possible. Removal of the larger tumors may be necessary, as they have the potential to compress the brainstem areas necessary for consciousness and life. Moreover, increasing tumor size may ultimately decrease the probability of hearing preservation once the tumors are removed (Kanowitz, 2004).

Tumor removal can be accomplished in a variety of ways, depending on the tumor size and extent of hearing loss. Two commonly used techniques include surgical removal and gamma knife radiosurgery (non-invasive, via ionizing radiation). Although others prefer the gamma knife technique in many instances, one must be aware that it may lower the probability of success using the ABI. Recent research has indicated that patients who undergo gamma knife radiosurgery to remove schwannomas have a higher incidence of scarring and fibrosis at the site than those who have not. Scar tissue may adhere to the brainstem and the facial nerve (Friedman, 2005).

Can a patient hear right after surgery?

After surgery, the patient needs to wait at least 6 to 8 weeks for the incision to heal and for the central nervous system to stabilize. At that point, he or she will visit the implant center to have the ABI programmed. Afterwards, the patient undergoes extensive training to “re-learn” how to hear again using the implant. It is important to note that with post-implantation programming, patient outcomes can vary significantly.

Recent research performed by Vittorio Colletti, M.D. (University of Verona, Italy) and Robert Shannon, Ph.D. (House Ear Institute, Los Angeles, CA) indicates that success of auditory percept development is related to the degree of auditory pathology and the reason for cochlear nerve damage (and therefore hearing loss). Specifically, post-implantation patients without NF2 tumors (e.g. trauma patients) showed better amplitude modulation and speech perception than did those with tumors. Both groups were comparably able to determine pitch and could detect sounds at a large variety of levels. In addition, non-tumor patients had a higher success rate in open-set sentence recognition. Their work further emphasizes the importance of early detection and removal of NF2 related tumors, as their results suggest that growth and removal of NF2 related tumors may damage parts of the cochlear nucleus (Colletti, 2005).

What is the difference between an ABI and a cochlear implant?

While the devices have similar signal processing technologies and use a comparable number of stimulating electrodes, the key difference between them lies in the location of the implant: the cochlear implant’s electrodes are placed in the cochlea, while ABI electrodes are placed in the cochlear nucleus of the brainstem. The cochlear implant works by bypassing damaged parts of the inner ear (cochlea) to stimulate the auditory nerve. This signal is ultimately recognized by the cochlear nucleus and is interpreted as sound throughout the auditory pathway. Thus, the site of implantation is of crucial significance to NF2 patients. Without a functional auditory nerve (as occurs during tumor growth and/or removal), the cochlear implant cannot work. For this reason, the ABI is their primary opportunity for auditory rehabilitation (Moller, 2006).

ABI Research

Currently, at Rush University Medical Center in Chicago, Janna Hoffman, B.A., David A. Klodd, Ph.D., Valeriy Shafiro, Ph.D, and Thomas J. Haberkamp, M.D. are studying the results of the Familiar Environmental Sounds Test with ABI patients. The purpose of the test is to introduce a subjective measure to evaluate the success of cochlear implants, as accurate identification of the sources of environmental sounds and subsequent behavioral cues are, along with speech perception, major concerns of patients with auditory prosthesis such as cochlear implant and ABI users (Shafiro et al., 2005; Gygi 2006). While the ABI has been shown to improve various elements of sound perception in some patients, little is presently known about ABI patients’ ability to recognize familiar environmental sounds. Previous research also indicates that perception of speech is correlated significantly with the perception of nonspeech environmental sounds. Thus, a test of environmental sound perception may be also potentially predicative of patients’ speech performance following implantation. This possibility will be especially useful in evaluating performance of ABI patients with limited English proficiency, which may constitute a large portion of patient population in large urban areas.

Dr. Colletti and colleagues have also recently indicated that patients who have undergone unsuccessful treatment of sensorineural hearing loss by cochlear implantation may improve auditory perception by subsequently receiving an ABI (Colletti et al., 2004).

Other researchers have advocated the midbrain (inferior colliculus) as a possible alternate site of implantation. The midbrain implant was designed with similar indications as the ABI, however taking into consideration that some NF2 patients receive limited speech understanding with ABIs, potentially due to scarring and/or damage related to tumor removal. Thus, an implant at an alternative site might serve as a route for better word recognition in these patients (Lenarz et al., 2006; Colletti et al., 2007).

 

References
Colletti VC et al. Auditory brainstem implant as a salvage treatment after unsuccessful cochlear implants. Otology and Neurotology, 2004: 25(4): 485-496.

Colletti VC et al. Auditory brainstem implantation: the University of Verona experience. Archives of Otolaryngology – Head and Neck Surgery, 2002: 127(1): 84-96.

Colletti VC et al. Auditory brainstem implant in posttraumatic cochlear nerve avulsion. Audiology and Neurotology, 2004: 9(4): 247-255.

Colletti VC et al. The first successful case of hearing produced by electrical stimulation of the human midbrain. Otology and Neurotology 2007: 28(1): 39-43.

Colletti VC and Shannon RV. Open set speech perception with auditory brainstem implant? Laryngoscope 2005: 115: 1974-1978.

[FDA] “New Device Approval: Nucleus 24 Auditory Brainstem Implant System: P00015”. October 20, 2000. Center for Devices and Radiological Health, Department of Health Services. U.S. Food and Drug Administration. http://www.fda.gov/cdrh/mda/docs/p000015.html. Accessed July 5, 2007.

Friedman RA et al. Surgical salvage after failed irradiation for vestibular schwannoma. Laryngoscope 2005: 115: 1827-1832.

Gygi B and Shafiro V. General functions and specific applications of environmental sound research. Frontiers in Bioscience 2007: 12: 3152-3166.

Kanowitz SJ, Shapiro WH, Golfinos JG, Cohen NL, Roland JT. Auditory brainstem implantation in patients with Neurofibrmatosis Type 2. Laryngoscope 2004: 114: 2135-2146.

Kuchta JK et al. The multichannel auditory brainstem implant: how many electrodes make sense? Journal of Neurosurgery 2004: 100: 16-23.

Lenarz T et al. The auditory midbrain implant: a new auditory prosthesis for neural deafness – concept and device description. Otology and Neurotology 2006: 27(6): 838-843.

Moller AR. History of cochlear implants and auditory brainstem implants. Advances in Otorhinolaryngology 2006: 64: 1-10.

Shafiro V et al. Effects of training on the perception of spectrally smeared environmental sounds. Presentation at the 2005 conference on Implantable Auditory Prosthesis, Pacific Grove, California.