Indications and contraindications of auditory brainstem implants: systematic review and illustrative cases

The number of non-neurofibromatosis type 2 (NF2) indications for auditory brainstem implant (ABI) in the literature is increasing. The objective of this study was to analyze and discuss the indications for ABI. Retrospective chart review and systematic review were conducted at Quaternary referral skull base center and referring centers. Analysis of ABI cases with non-NF2 indications and systematic review presenting non-NF2 ABI cases were performed. Fourteen referred cases with ABI were identified. All cases had unsatisfactory results of ABI and all could have been rehabilitated with a cochlear implant (CI). Of these 14 cases, 9 improved with a cochlear implant, and 2 with a hearing aid, two are still planned for CI, one received bilateral CI, no ABI. In literature, we found 31 articles presenting 144 non-NF2 ABI cases with at least 7 different indications other than NF2. ABI should be restricted to those patients who have no other rehabilitation options. Patency of the cochlea and evidence of an intact cochlear nerve should be examined with imaging and electrophysiologic testing. Sometimes a CI trial should be planned prior to proceeding with ABI. We have shown that in many cases a CI is still possible and CI provided better results than ABI. In vestibular schwannoma in the only hearing ear, cochlear otosclerosis, temporal bone fractures, (presumed) bilateral traumatic cochlear nerve disruption, auto-immune inner ear disease and auditory neuropathy primarily CI are indicated. Traumatic bilateral cochlear nerve disruption is exceptionally unlikely. In cochlear nerve aplasia, testing should be performed prior to meeting indications for ABI. In malformations, ABI is indicated only in severe cochlear hypoplasia or cochlear aplasia.     

Auditory brainstem implantation: The first Indian experience

Multichannel auditory brainstem implants (ABI) are currently indicated for patients with neurofibromatosis type II (NF2) involving both vestibulocochlear nerves. The ABI helps bypass the damaged cochlear nerves and restores a level of auditory sensation via the electrical stimulation of the cochlear nucleus. The implant is usually placed in the lateral recess of the fourth ventricle at the time of tumor resection to stimulate the cochlear nucleus. We report a case of ABI done on a 15-year-old girl with bilateral vestibular schwannomas.     

The auditory midbrain implant: a new auditory prosthesis for neural deafness-concept and device description.

The auditory midbrain implant (AMI) is a new central auditory prosthesis designed for penetrating stimulation of the human inferior colliculus. The major group of candidates for the AMI consists of neurofibromatosis type 2 (NF2) patients who develop neural deafness because of growth and/or surgical removal of bilateral acoustic neuromas. Because of the absence of a viable auditory nerve, these patients cannot benefit from cochlear implants. An alternative solution has been the auditory brainstem implant (ABI), which stimulates the cochlear nucleus. However, speech perception performance in NF2 ABI patients has been limited. The fact that the ABI is able to produce high levels of speech perception in nontumor patients (with inaccessible cochleae or posttraumatic damage to the cochlear nerve) suggests that limitations in ABI performance in NF2 patients may be associated with cochlear nucleus damage caused by the tumors or the tumor removal process. Thus, stimulation of the auditory midbrain proximal to the damaged cochlear nucleus may be a better alternative for hearing restoration in NF2 patients. We propose the central nucleus of the inferior colliculus (ICC) as the potential site. A penetrating electrode array aligned along the well-defined tonotopic gradient of the ICC should selectively activate different frequency regions, which is an important elementfor supporting good speech understanding. The goal of this article is to present the ICC as an alternative site for an auditory implant for NF2 patients and to describe the design of the first human prototype AMI. Practical considerations for implementation of the AMI will also be discussed.     

Neurofibromatosis 2: hearing restoration options

Neurofibromatosis 2 (NF2) is an autosomal dominant disease in which hearing loss is predominant. Auditory restoration is possible using cochlear implants (CI) or auditory brainstem implant (ABI).ObjectiveTo assess the auditory results of CI and ABI in NF2 patients and review the literature.MethodsFour NF2 patients were prospectively evaluated. They were submitted to tumor resection followed by ipsilateral CI or ABI depending on cochlear nerve preservation. Long term auditory results were described for CI (12 months) and ABI (48 months).ResultsAll patients achieved auditory perception improvements in their hearing thresholds. The CI patient does not recognize vowels or sentences. The 3 ABI patients discriminate 70% of vowels and 86% in the 4-choice test. One of them does not recognize sentences. The other two recognize 100% of closed sentences and 10% and 20% of open sentences.ConclusionThe choice of implant type to restore hearing to NF2 patients will relay on anatomical and functional cochlear nerve preservation during tumor resection surgery. Although our experience was different, the literature shows that if this condition is achieved, CI will offer better auditory results. If not, ABI is recommended.     

Experiences from Auditory Brainstem Implantation (ABIs) in four paediatric patients

Introduction: Indications for auditory brainstem implants (ABIs) have been widened from patients with neurofibromatosis type 2 (NF2) to paediatric patients with congenital cochlear malformations, cochlear nerve hypoplasia/aplasia, or cochlear ossification after meningitis. We present four ABI surgeries performed in children at Uppsala University Hospital in Sweden since 2009.

Methods: Three children were implanted with implants from Cochlear Ltd. (Lane Cove, Australia) and one child with an implant from MedEl GMBH (Innsbruck, Austria). A boy with Goldenhar syndrome was implanted with a Cochlear Nucleus ABI24M at age 2 years (patient 1). Another boy with CHARGE syndrome was implanted with a Cochlear Nucleus ABI541 at age 2.5 years (patient 2). Another boy with post-ossification meningitis was implanted with a Cochlear Nucleus ABI24M at age 4 years (patient 3). A girl with cochlear aplasia was implanted with a MedEl Synchrony ABI at age 3 years (patient 4). In patients 1, 2, and 3, the trans-labyrinthine approach was used, and in patient 4 the retro-sigmoid approach was used.

Results: Three of the four children benefited from their ABIs and use it full time. Two of the full time users had categories of auditory performance (CAP) score of 4 at their last follow up visit (6 and 2.5 years postoperative) which means they can discriminate consistently any combination of two of Ling's sounds. One child has not been fully evaluated yet, but is a full time user and had CAP 2 (responds to speech sounds) after 3 months of ABI use. No severe side or unpleasant stimulation effects have been observed so far. There was one case of immediate electrode migration and one case of implant device failure after 6.5 years.

Conclusion: ABI should be considered as an option in the rehabilitation of children with similar diagnoses.     

Zentral-auditorische Implantate

Deaf patients with severe sensory hearing loss can benefit from a cochlear implant (CI), which stimulates the auditory nerve fibers. However, patients who do not have an intact auditory nerve cannot benefit from a CI. The majority of these patients are neurofibromatosis type 2 (NF2) patients who developed neural deafness due to growth or surgical removal of a bilateral acoustic neuroma. The only current solution is the auditory brainstem implant (ABI), which stimulates the surface of the cochlear nucleus in the brainstem. Although the ABI provides improvement in environmental awareness and lip-reading capabilities, only a few NF2 patients have achieved some limited open set speech perception. In the search for alternative procedures our research group in collaboration with Cochlear Ltd. (Australia) developed a human prototype auditory midbrain implant (AMI), which is designed to electrically stimulate the inferior colliculus (IC). The IC has the potential as a new target for an auditory prosthesis as it provides access to neural projections necessary for speech perception as well as a systematic map of spectral information. In this paper the present status of research and development in the field of central auditory prostheses is presented with respect to technology, surgical technique and hearing results as well as the background concepts of ABI and AMI.     

An auditory brainstem implant system

Neurofibromastosis type II (NF2) is a condition that may result in bilateral acoustic neuromas. The tumors and their removal may cause profound bilateral deafness. Because the auditory nerve is compromised, people with NF2 are unable to receive a cochlear implant to restore a sensation of hearing. Electrical stimulation of the auditory pathway can provide hearing in such people. This is possible by means of an auditory brainstem implant (ABI). This article focuses on the MED-EL high-rate multichannel ABI system. The system consists of the implanted and external components. Appropriate placement of the ABI is dependent on electrical auditory brainstem response testing performed intra-operatively. Data on a group of European patients implanted with the MED-EL ABI are presented. Results are promising and include some open-set speech ability.     

Open Set Speech Perception with Auditory Brainstem Implant?

OBJECTIVE: Only a small percentage of auditory brainstem implant (ABI) recipients treated for neurofibromatosis type 2 (NF2) have proved capable of identifying words using only the sound from the ABI. Recently, the ABI was applied to a series of patients with no cochlear nerve or with cochlear disorders that could not benefit from a cochlear implant (i.e., cochlear nerve aplasia or posttraumatic avulsion) or whose benefit was or would be severely compromised. A significant number of these patients have proven capable of understanding speech, including effortless telephone use. In the present study, a series of psychophysical tests were administered to determine the cause of the difference in performance between tumor (T) and nontumor (NT) ABI patients. STUDY DESIGN: Retrospective case review. SETTING: Tertiary referral center. PATIENTS: Twenty patients with ABIs participated in the investigation. Ten were NF2 patients and 10 NT subjects. Patient ages ranged from 24 to 61 years. Eleven were males and nine females. INTERVENTION: Auditory rehabilitation in auditory disconnection caused by cochlea or cochlear nerve disorders. RESULTS: There was a significant correlation between modulation detection thresholds and speech understanding and a significant difference in modulation detection between T and NT patients. CONCLUSIONS: The difference in modulation detection between the two groups suggests a difference in the survival of specific cells in the cochlear nucleus that support modulation. The pattern of results indicates a separate pathway of auditory processing that is specialized for modulated sounds, and that pathway is critical for speech understanding. In NF2 patients, the tumor and surgery may selectively damage this pathway, resulting in poor speech recognition with prosthetic stimulation.     

Brainstem electronic implants for bilateral anacusis following surgical removal of cerebello pontine angle lesions.

The multichannel auditory brainstem implant (ABI) has been used successfully to treat deafness in individuals with neurofibromatosis type II. The device has been implanted in nearly 150 recipients worldwide, and clinical trials with the device are approaching completion. The implantation and fitting of the multichannel ABI differ significantly from cochlear implantation, and the processes are illustrated in a series of case studies. Performance data also are included from recipients with up to 7 years experience.     

多道听觉脑干植入的临床应用

目的　探讨多道听觉脑干植入的手术方法及其对恢复双侧听神经瘤所致全聋患者听力的效果。方法　 7例双侧听神经瘤患者 ,在肿瘤摘除后将 2 1道听觉脑干植入 (auditorybrainstemimplant,ABI)电极植入于第四脑室外侧隐窝内 ,直接刺激脑干耳蜗核 ,术中以面神经电图、舌咽神经肌电图及电诱发听觉脑干反应 (electricallyevokedauditorybrainstemresponses,EABR)确定电极的正确位置。术后 6周开通装置并调试 ,定期行言语识别能力测试。结果　 6例患者术中均能准确定位外侧隐窝 ,并能记录到典型的EABR ;术后均能获得不同程度的言语识别能力 ,其中 2例获得开放句识别能力。另 1例患者术中解剖定位困难 ,术中未能记录到EABR。术后电极刺激 1例无非听觉反应、5例部分电极产生非听觉反应、1例全部电极均产生非听觉反应 ,有非听觉反应的相应电极被关闭。结论　多道听觉脑干植入能使因双侧听神经瘤而全聋的患者产生有意义的听觉 ,术中正确定位脑干耳蜗核为手术成功的关键。     

Distribution of Fos-like immunoreactivity in the auditory pathway evoked by bipolar electrical brainstem stimulation

OBJECTIVE: The auditory brainstem implant (ABI) represents a new modality for the treatment of patients deafened as a result of complete excision of a bilateral VIIIth nerve tumor. However, little work has been done on the effect of the ABI on the mammalian auditory pathway. The aim of this study was to demonstrate the effect of the ABI using Fos-like immunoreactivity. MATERIAL AND METHODS: A bipolar electrode was implanted in the dorsal cochlear nucleus of bilaterally deafened Sprague-Dawley rats, and electrical stimulation was presented at an intensity four times that of threshold. RESULTS: Fos-like immunoreactivity was induced in the neurons of various auditory brainstem nuclei and observed in the low-to-middle frequency area. In the ipsilateral dorsal cochlear nucleus, ipsilateral posterior ventral cochlear nucleus and bilateral inferior colliculus, Fos-like immunoreactive neurons were observed as a distinct banding pattern. CONCLUSIONS: This study showed that Fos-like immunoreactivity was observed in the restricted area of the primary brainstem auditory pathway with the appropriate tonotopicity. These results indicate that the ABI can provide auditory information suitable for speech recognition.     

Vestibular schwannoma in the only hearing ear: cochlear implant or auditory brainstem implant?

OBJECTIVE: To explore the dilemma faced by neurotologists confronted with the patient who develops a vestibular schwannoma in the only hearing ear, the other having been deaf from birth, and to consider the choice between auditory rehabilitation using a cochlear implant (CI) on the congenitally deaf side and an auditory brainstem implant (ABI) on the tumor side. STUDY DESIGN: A record review of two patients born deaf in one ear and who developed a vestibular schwannoma in the contra lateral ear, who then received a CI in the congenitally deaf ear. SETTING: Tertiary referral center with special experience in vestibular schwannoma surgery, neurofibromatosis type 2 management, and cochlear implantation. RESULTS: Neither patient was a good CI user. At 1 year postimplant, they both scored 0% on abbreviated words and 0% and 7%, respectively, on Bench Kowal Bamford sentences. They scored 54% and 57%, respectively, on City University of New York sentences with lip-reading, and both had fair access to environmental sound scoring at 45% each. CONCLUSIONS: The results from cochlear implantation in the congenitally deaf ear in these patients were poor and suggest that stimulus deprivation in the early stages of the maturation of the auditory pathways is important even for a unilateral hearing loss. Unfortunately, the factors that predict a good ABI result are not known. In these circumstances, the authors advocate the insertion of an ABI at the time of tumor removal, retaining the option of CI in the congenitally deaf ear in the event of a poor outcome with the ABI.     

The transtympanic promontory stimulation test in patients with auditory deprivation: correlations with electrical dynamics of cochlear implant and speech perception

Transtympanic promontory stimulation test (TPST) has been suggested to be a useful tool in predicting postoperative outcomes in patients at risk of poor auditory neuron functioning, especially after a long auditory deprivation. However, only sparse data are available on this topic. This study aimed at showing correlations between the auditory nerve dynamic range, evaluated by TPST, the electrical dynamic range of the cochlear implant and speech perception outcome. We evaluated 65 patients with postlingual hearing loss and no residual hearing, implanted with a Nucleus CI24 cochlear implant device for at least 2 years and with a minimum of 17 active electrodes. Using the TPST, we measured the threshold for auditory perception (T-level) and the maximum acceptable level of stimulation (M-level) at stimulation frequencies of 50, 100 and 200 Hz. General linear regression was performed to correlate 1/speech perception, evaluated using the PBK test 1 year after surgery, and 2/cochlear implant electrical dynamic range, with the age at time of implantation, the duration of auditory deprivation, the etiology of the deafness, the duration of cochlear implant use and auditory nerve dynamic range. Postoperative speech perception outcome correlated with etiology, duration of auditory deprivation and implant use, and TPST at 100 and 200 Hz. The dynamic range of the cochlear implant map correlated with duration of auditory deprivation, speech perception outcome at 6 months and TPST at 100 and 200 Hz. TPST test can be used to predict functional outcome after cochlear implant surgery in difficult cases.     

Brainstem auditory implants

Neurofibromatosis type 2 (NF 2) typically results in deafness due to disruption of the cochlear nerves, making peripheral devices such as cochlear implants ineffective. Auditory brainstem implants (ABIs), for direct electrical stimulation of the cochlear nucleus, have been used to provide auditory stimulation in this group of patients. Currently, an advanced multichannel device is utilized, with direct stimulation of the brainstem surface. Patients undergoing ABI have significant improvement in scores on several audiologic tests over baseline. When used to augment lip reading, improvement is also seen. The degree of improvement varies considerably among patients. ABI is a useful device for use by deaf patients with NF 2. As measured by audiologic testing, many patients receive substantial benefit with regard to sound and speech comprehension.     

Psychophysical measures from electrical stimulation of the human cochlear nucleus

Auditory performance on basic psychophysical tasks was measured in ten deaf patients with electrodes positioned near their cochlear nucleus. The device is called the auditory brainstem implant (ABI). Electrodes were placed during surgery to remove an acoustic neuroma, which results in the removal of the VIII nerve and, thus deafness. In patients who received auditory sensation from electrical stimulation we measured auditory performance on standard psychophysical tasks: thresholds, loudness growth, intensity discrimination, temporal integration, temporal modulation detection, gap detection, and forward masking. Plots of threshold as a function of frequency or biphasic pulse duration were markedly different from those of patients with cochlear implants. The difference in threshold functions is probably partly due to the biophysical difference in the neural elements stimulated. Another possibility is that part of the difference is due to the highly abnormal spatial pattern of activation in the cochlear nucleus from electrical stimulation, which prevents normal spatial integration of activity. The usable range of electrical amplitudes above threshold is comparable with that of cochlear implants, typically 10-15 dB. Little temporal integration occurs over a range of stimulus durations from 2-1000 ms. When compared at equivalent loudness levels, gap detection thresholds are similar to, or a bit longer than, gap thresholds in normal-hearing listeners and cochlear implant patients. Forward masking recovery functions are similar to those of normal listeners and cochlear implant patients. Patients' ability to detect amplitude modulation as a function of modulation frequency is similar to that of cochlear implant patients and normal listeners. Thus, direct electrical stimulation of the brainstem produces temporal resolution that does not significantly differ from that of normal listeners when compared in equivalent amplitude units. This implies that the limiting factors for these tasks are more centrally located, and not directly related to threshold mechanisms. Thus, a properly designed speech processor could preserve the important temporal features of speech for these patients.     

Hearing habilitation with auditory brainstem implantation in two children with cochlear nerve aplasia

Patients with aplasia and hypoplasia of the cochlear nerve have no chance of having their hearing restored by stimulating the periphery of the auditory system using the traditional cochlear implant. A possible approach to auditory rehabilitation may be direct electrical stimulation of the cochlear nuclei with an auditory brainstem implant (ABI). Recently, two children, aged 4 and 3 years, respectively, with bilateral severe cochlear malformations and cochlear nerve aplasia received an ABI. The present paper reports the technique and the preliminary results of this experience. The classic retrosigmoid approach was used. The correct position of the electrodes was estimated with the aid of EABRs and neural response telemetry (NRT). No postoperative complications were observed. High-resolution CT scans with a bone algorithm reconstruction technique were taken postoperatively to evaluate electrode placement before discharge. The ABI was activated 30 days after implantation in both patients. To date 16 and 13 electrodes, respectively, have been activated in the two children. Three months after activation the first patient had achieved good environmental sound awareness, good speech detection and some speech discrimination. The second child, 1 month after activation, had achieved good environmental sound awareness and moderate speech detection. To the best of our knowledge this is the first report of patients with hypoplasia of the cochlea and aplasia of the cochlear nerve, aged below 5 years and treated with an ABI.     

Clinical application of the multichannel auditory brainstem implant]

OBJECTIVE: To investigate the surgical techniques and speech performance of multichannel auditory brainstem implant (ABI) in patients with bilateral acoustic neuromas (neurofibromatosis type 2). METHODS: The nucleus 21 channel auditory brainstem implant was implanted into the lateral recess of the fourth ventricle through the translabyrinthine approach in 7 patients after removal of the tumor. The accurate placement of electrode array was ensured by the electromyogram monitoring of the 7th and 9th nerves and the electrically evoked auditory brainstem responses (EABR). Initial switch-on occurred six weeks postoperatively. Speech evaluation was performed every 3 months for the first year and annually thereafter. RESULTS: During the surgery, the lateral recess could be found and the typical EABR could be recorded in 6 cases. They later reported a significant benefit from the device. Two of the cases have achieved functional open-set speech understanding. In contrast, one patient with no EABR because of difficulty of the anatomic location during the surgery had no sensations postoperatively. CONCLUSION: The multichannel ABI could effectively restore auditory sensations in patients deafened by bilateral acoustic neuromas. The accurate location of the cochlear nucleus complex during surgery was the key factor for the success of the operation.     

Auditory brainstem implantation in patients with neurofibromatosis type 2

OBJECTIVES: Multichannel auditory brainstem implants (ABI) are currently indicated for patients with neurofibromatosis type II (NF2) and schwannomas involving the internal auditory canal (IAC) or cerebellopontine angle (CPA), regardless of hearing loss (HL). The implant is usually placed in the lateral recess of the fourth ventricle at the time of tumor resection to stimulate the cochlear nucleus. This study aims to review the surgical and audiologic outcomes in 18 patients implanted by our Skull Base Surgery Team from 1994 through 2003. STUDY DESIGN: A retrospective chart review of 18 patients with ABIs. METHODS: We evaluated demographic data including age at implantation, number of tumor resections before implantation, tumor size, surgical approach, and postoperative surgical complications. The ABI auditory results at 1 year were then evaluated for number of functioning electrodes and channels, hours per day of use, nonauditory side effect profile and hearing results. Audiologic data including Monosyllable, Spondee, Trochee test (MTS) Word and Stress scores, Northwestern University Children's Perception of Speech (NU-CHIPS), and auditory sensitivity are reported. RESULTS: No surgical complications caused by ABI implantation were revealed. A probe for lateral recess and cochlear nucleus localization was helpful in several patients. A range of auditory performance is reported, and two patients had no auditory perceptions. Electrode paddle migration occurred in two patients. Patient education and encouragement is very important to obtain maximum benefit. CONCLUSIONS: ABIs are safe, do not increase surgical morbidity, and allow most patients to experience improved communication as well as access to environmental sounds. Nonauditory side effects can be minimized by selecting proper stimulation patterns. The ABI continues to be an emerging field for hearing rehabilitation in patients who are deafened by NF2.     

Speech perception ability in a patient with a 8-channel auditory brainstem implant]

Auditory brainstem implant (ABI) is a central prosthesis that directly stimulates the cochlear nucleus in the brainstem for those who have interrupted auditory nerves and cannot benefit from the cochlear implantation. Speech perception in a recipient of the Nuclues 8 channel ABI, the first in Japan, is reported. A 25-year-old man with bilateral acoustic nerve tumors postlingually deafened due to tumor resection received auditory sensations with 5 channels. The correct answer using a coding strategy, SPEAK, was 35% for 5 vowels and 36% for 5 monosyllables. The use of ABI also improved his lip-reading ability on monosyllables and open-set words. This indicated that he benefited from ABI, although it was limited. Even after 1 year and 3 months of follow-up, he had no serious side effects such an infection or implant rejection.     

Results from a European clinical investigation of the Nucleus multichannel auditory brainstem implant

OBJECTIVE: This study was designed to investigate the perceptual benefits and potential risks of implanting the Nucleus(R) multichannel auditory brainstem implant. DESIGN: Between September 1992 and October 1997 a total of 27 subjects received a Nucleus 20- or 21-channel Auditory Brainstem Implant (ABI). All subjects involved in the trial had bilateral acoustic tumour as a result of neurofibromatosis type 2 (NF2) resulting in complete dysfunction of the VIIIth nerve. The study used each subject as their own control without a preoperative baseline because residual hearing, if existing, was destroyed at surgery by tumour removal. A battery of speech tests was conducted to evaluate each patient's performance and communication abilities. Tests were conducted, where possible, in the auditory-only, visual-only, and auditory-visual conditions at 3 days postoperatively (baseline), at 3-mo intervals for the first year and every 12 mo thereafter. A subjective performance questionnaire was administered together with an extensive neurological examination at each test interval. RESULTS: 27 subjects involved in this trial were successfully implanted with a Nucleus ABI. One subject died 2 days postoperatively due to a lung embolism unrelated to the device. Twenty-six subjects underwent device activation and all but one patient received auditory sensation at initial stimulation (96.2%). On average 8.6 (+/-4.2) of the available 21 electrodes were used in the patients' MAPs. Performance evaluation measures showed that the majority of users had access to auditory information such as environmental sound awareness together with stress and rhythm cues in speech that assist with lipreading. Although most subjects did not achieve any functional auditory-alone, open-set speech understanding, two subjects from this series (7.4%) did receive sufficient benefit to be able to use the ABI in conversation without lipreading. CONCLUSIONS: Although the medical risks and surgical complexity associated with ABI device implantation are far greater than those for a cochlear implant, the clinical results from this trial show that the Nucleus multichannel ABI is capable of providing a significant patient benefit over risk ratio for subjects suffering loss of hearing due to bilateral retrocochlear lesions.