|An Auditory Brainstem Implant|
|An example of the path from microphone to electrode in an ABI. Bottom right - an example of
the electrode panel stimulating the cochlear nuclei
Potthoff, M. (2013). ABI how it works image [image format].
retrieved from http://newsroom.hei.org/news/fda-approves-clinical-trial-of-242830
Recently, researchers have been investigating the possibility of implanting electrodes into patients with complete sensorineural hearing loss in order to restore their hearing. The most common approach used is the cochlear implant (CI), although there are some cases where cochlear implantation is not possible. In cases where cochlear implantation would provide no hearing improvement, an auditory brainstem implant (ABI) is the alternative method of hearing restoration and over 500 have been implanted with success since 1979 . An ABI is most commonly inserted in patients whom have neurofibromatosis type 2 (NF 2), an autosomal dominant disease causing tumors of the glial cells in the spinal cord and brain. NF 2 normally causes hearing loss due to bilateral vestibular schwannomas; tumors of the schwann cells supplying the vestibulocochlear nerve. Once inserted, an auditory brainstem implant utilizes an external microphone to pick up sounds from the environment which are then processed into a series of electrical pulses that are directly delivered to the cochlear nuclei by electrodes. Pulses stimulate many different cell types of the cochlear nuclei, and although some cell types are thought to provide upper brain regions with information on pitch or vowel identification, the mechanism behind why ABI provides a degree of auditory restoration in patients is largely unknown. With continuing research the ABI is a growing possibility for hearing restoration in those unable to receive a cochlear implant.
Table of Contents
Indications for ABI implementation
Backous, D. (2011). What is a brainstem implant [video]. Retrieved from http://www.swedish.org/Services/Neuroscience-Institute/Neuroscience-Services/Hearing-and-Skull-Base-Surgery/Videos#axzz2OwppBsSl
Neurofibromatosis type 2 cases
Deafness can occur in patients with neurofibromatosis type 2 (NF 2) as tumors of the schwann cells in the vestibular region, known as vestibular schwannomas, can cause post-cochlear damage to auditory structures due to compression forces from tumor growth or from surgical damage during tumor removal. As there is a high risk of permanent damage to the vestibulocochlear nerve (cranial nerve VIII), NF 2 patients will likely be unable to benefit from a cochlear implant (CI), as the information conveyed by the cochlea will be unable to reach its next target; the cochlear nuclei. As these patients will not benefit from a CI, some audition can be recovered if an auditory brain stem implant (ABI) is implemented. ABIs can be inserted onto the cochlear nuclei, during shwannoma removing surgery, allowing the surgeon to both remove the tumor and circumvent the damaged auditory nerve to restore some of the patients hearing abilities. Although it is possible to insert ABI in tumor patients, it has been shown that the audiological performance of NF 2 patients after ABI insertion was lower than than of non-tumor cases This lowered performance could be due to compression of the cochlear nuclei by the schwannomas or damage from schwannoma removal.
ABI implementation can provide improvement in auditory sensation for more people than just those suffering from NF 2, as there are multiple cases where both hearing is completely lost and cochlear implantation is not an option.
For example, ABI use in indicated in individuals with congenital disorders resulting in the lack of formation of inner ear structures such as the cochlea or vestibulocochlear nerve as these individials would receive no benefit from a CI. In addition to the increased auditory benefit these patients can receive from an ABI, the area of ABI implantation is more easily accessed in non-tumor patients as no compression or distortion of cochlear nuclei areas is evident Congenital non-tumor ABI use allows implantation to be done in young children ) to maximize use of the brains natural plastic abilities; potentially allowing ABI electrode stimulation to restructure the tonotopic mapping in the cochlear nuclei, greatly improving ABI outcome for children when compared to adults.
There are also cases where hearing loss and contraindication of CI develops over time rather than occurring congenitally. A main example of such a case is complete ossification of the cochlea, in which bone develops inside of the cochlea following an infection such as meningitis. Although some of the literature supports ABI use in these ossification cases, other surgeons highly recommend first attempting cochlear implantation by drilling through the newly formed bone, as the speech perception outcomes are usually better with CI and CI use is possible since the pathway from the cochlea to upper brain structures is intact.
|Where does the ABI go?|
|A) Diagramtic representation of an axial slice through the area of ABI implantation.
The proximity of the facial and vestibulocochlear nerve can be seen as well as the major landmark - the foramen of Luschka.
B) A close up of the implantation site, showing how the ventral cochlear nucleus is more easily reached, although both nuclei can be stimulated.
Note how the paddle lies in the lateral recess of the fourth ventricle.
Lekovic, G.P. et al. (2004) [Image of the ABI implantation site]
Retrieved from https://secure.chwhealth.org/stellent/groups/public/@xinternet_con_bni/documents/webcontent/st029191.pdf
Before surgery is performed, the necessity for implantation in the patient must be determined using a variety of imaging techniques. Both computerized tomography (CT) scans and magnetic resonance imaging (MRI) are common preoperative techniques to asses tumors or structural abnormalities in the inner such as cochlear ossification or aplasia of the vestibulocochlear nerve . In additon, patients are screened for speech perception abilities using methods such as the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) and the Meaningful Auditory Integration Scale (MAIS).
After confirmation that ABI is needed using the above preoperative tests, the surgery is performed using one of two main approaches; the retrosigmoid or translabyrinthine surgical technique. Both approaches use many of the same landmarks to reach the ABI implantation site on the cochlear nuclei, found in the lateral recess of the fourth ventricle . There are advantages and disadvantages to using each approach, and the final decision is largely based on tumor size (if any) and the preference/experience of the surgeon. In both procedures, similar land-marks are used to locate the lateral recess such as the ninth and tenth cranial nerves, and the ventral exit port for cerebral spinal fluid in the fourth ventricle - the foramen of Luschka(see above image). The foramen of Luschka is particularily useful in as it can be identified easily since the ninth cranial nerve leads in its direction and the lateral recess leaks cerebrospinal fluid (CSF) from this area into the subarachnoid space.
This approach can be done with the patient in two positions; supine (laying face up) or semi-sitting. The retrosigmoid approach allows easy monitoring of the auditory nerve during operation for NF 2 tumor removal - increasing the possibility for hearing retention, potentially avoiding the need to even implant an ABI. In addition, retrosigmoid surgeries avoid the need to cut through the mastoid cells of the middle ear to reach the cochlear nuclei, thus the watertight seal provided by the dura membrane between middle and inner ears is preserved, avoiding post operative infections. Finally, the retrosigmoid approach can be done in a smaller amount of time and allows the cord of the ABI to be attached to the temporal bone, providing long-term stability to the device.
Disadvantages to using the retrosigmoid approach include the need to retract the cerebellum to access the lateral recess, although there is controversy regarding this. Some professionals consider the need to retract the cerebellum a downfall of the retrosigmoid approach but studies quantifying the balance and gait of patients after retrosigmoid or translabyrinthine surgery reported no significant difference, indicating cerebellar retraction is less dangerous than it was thought to be. Another potential downfall of the retrosigmoid approach is the fact that the view to the ABI implantation site, the lateral recess, is not ideal. In a recent publication by Sennaroglu and Ziyal, it was stated that the retrosigmoid approach is best in non-tumor patients for this reason.
The common alternative approach to implanting an ABI involves the translabyrinthine surgery. In surgeries using this method, the patient is placed in a supine position and the initial incision is made 1 to 1.5 centimeters from the back of the pinna (outer ear). The incision above the pinna provides a perfect location for the placement of the ABI receiver, under the scalp and
this method allows more direct access to the lateral recess in comparison to the retrosigmoid approach. The translabyrinthine surgery also avoids the need to retract the cerebellum, preventing possible post-operative effects from cerebellar damage. The most important benefit is the fact that any tumors are easier to extract with this method, thus translabyrinthine surgery is preferred for patients needing ABI because of tumors.
Some disadvantages must also be considered for the translabyrinthine approach. To reach the lateral recess using this method, bone must be removed to reach the lateral recess, causing this procedure to be time consuming. This approach also renders the operated side completely deaf, although most patients with tumours in the vestibular area would have already lost a large portion, or all, of their hearing abilities by the time of operation. Finally, this technique does not maintain a constant watertight seal between the middle and inner ear, risking infection of the inner ear and potentially causing post operative meningitis despite sealing of the opened region with abdominal fat.
The benefit of ABI providing improvements in auditory perception come with risks associated with the invasive procedure necessary for implantation, or from stimulation of neurons in regions other than the cochlear nuclei during activation of the ABI electrodes. However, mortality is rare among ABI implantees.
Occasionally, the ABI may not be placed in the correct location within the lateral recess, or shifts after implantation. Proper placement of the electrodes can be determined by measuring the electrically evoked auditory brainstem responce (EABR) after individual electrodes on the ABI are activated. A positive peak in response corresponding to a single ABI electrode activation indicates that that particular electrode is correctly stimulating the cochlear nuclei. A shift of electrode location may result in the electrodes stimulating structures of the brain other than the cochlear nuclei. Incorrect stimulation may result in patients experiencing effects such as ipsilateral facial tingling due to activation of the facial nerve, throat tingling due to glossopharyngeal activation or headaches. In all cases, the non-auditory response can be remedied by the switching off problematic electrodes individually. These non-auditory responses seem to diminish over time and in some cases, the later reactivation of electrodes which originally caused unwanted responses is possible without unwanted effects.
Numerous complications can arise either during or after the surgery and encompass both tumor and non-tumor cases in patients of a large age range. Major complications, classified as those that occurred immediately after the surgery, include facial palsy due to lesions of the facial nerve (Cranial nerve V), meningitis and hydrocephalus.
Minor complications to note (those that were encountered and dealt with during the surgery) include CSF leakage due to tears in the dura membrane of the brain, balance problems and tinnitus (persisting ringing sound within the ear). These complications were more likely to be seen in patients suffering from NF 2 rather than those who required ABI on a non-tumor basis  as tumors cause distortions of the cochlear nuclei region, making implantation more difficult.
Programming the ABI - How it works
|How the ABI works|
|The main steps an ABI makes to
aid with auditory function
Adapted from 
The ABI allows sound detection in patients by picking up sound via an external microphone and sending this information to a signal processor implanted just under the patient's scalp. This signal processor reorganizes the signal into specific patters which are relayed to the electrode, ultimately creating auditory sensations by the direct stimulation of the cochlear nuclei. Sound sensation in terms of pitch is thought to be due to the evidence of tonotopic mapping within the cochlear nuclei, with dorsal regions conveying high frequencies and ventral regions sides contributing to the sensation of low frequencies. Although ABI placement along these regions should allow the patient to discriminate among frequencies, it is not known if this mapping is altered in individuals lacking signal input to the cochlear nuclei, and thus different for all ABI candidates. To achieve maximal auditory recovery, the ABI electrodes must cover a wide area of the cochlear nuclei in addition to selectively stimulating neurons which would send their projections to the next region of auditory sensation - the inferior colliculus. The neuronal make-up of the cochlear nuclei is complex, consisting of five main groups of neurons: fusiform, octopus, two forms of bushy and stellate neurons, all of which can convey both excitatory and inhibitory signals. For the discrimination of speech sounds, a function which ABI is meant to improve, it has been found that the bushy cells may convey information on vowel sounds, although more research is needed before these findings are conclusive. Despite the fact the general structure of the cochlear nuclei is known and ABI are shown to allow restoration of some auditory function, it is not known exactly what structures the ABI activates in order for it to work.
The programming of the ABI is done after the patient recovers from the implantation surgery - which is usually about six weeks. Upon activation, electrodes of the ABI are tested for non-auditory responses and are switched off if any arise. Once proper stimulation has been confirmed, the maximum and minimum frequency and amplitude values for each electrode are calculated to ensure responses are detectable but not beyond the comfort levels of the patient. Although the tonotopic mapping of the cochlear nuclei are not usually in direct relation with ABI position, a general tonotopic organization of electrodes can be discerned by having the patient rank which electrodes produce higher pitches than others. These ABI tonotopic maps seems to be patient specific as the rankings are done with subjective patient judgement, therefore electrodes eliciting high frequencies in some patients may provide low frequency stimulation in others.
Success of the ABI - speech perception
Colletti, V. (2013). Pediatric ABI - Hearing and Speaking [video]. Retrieved from: https://www.youtube.com/watch?v=MSMb8fbnHe8&list=UUQxKCfkEFKX1AApESP-mY0A&index=1.
For more information on ongoing research, visit - http://newsroom.hei.org/news/fda-approves-clinical-trial-of-242830
Determining whether an ABI has been beneficial to the patient can be assessed by a large battery of audiological tests such as those assessing the patients' ability to distinguish between consonant or vowel sounds or correctly identify which sentence was spoken. These tests can be done using humans for sound generation, allowing the comparison of understanding from visual cues alone (lip reading) to the paired lip reading and ABI auditory input simultaneously. In addition, if the patient has some degree of intact hearing on one side, the test sounds can be directly delivered to the auditory receiver supplying the unilateral ABI, avoiding confounds due to sound detection by the functioning ear. In any case, ABI improvement is lower than that of cochlear implantation, and it is highly recommended to implement CI whenever possible instead of treating a patient with an ABI.
If an ABI was implemented, the performance in auditory tests improves over time, as long as the ABI was being used. Initially, patients were mostly disappointed with their ABI performance, complaining of muffled or garbled sound sensations. However, over time, the ABI users grew used to the implant and experienced some benefits such as environmental sound discrimination (dog barking versus baby crying) and some degree of speech recognition, such as the ability to correctly select the word heard in a auditory-only test. Vowel recognition was difficult among patients, and sentence discrimination was low. The most successful use of the ABI was to augment lip reading, as with both lip reading and ABI use, speech perception scores were above 90% (initially at 28% before ABI activation)
Performance also depends on the patient's status before receiving an ABI. As the NF 2 patients have compression of vestibular structures and may have slight damage to the cochlear nuclei, the performance outcomes in these patients is normally lower than that of a non tumor ABI implantee - 12.2% success in sentence recognition compared to 63%, respectively.
In terms of performance based on the age of the patient, the best window for ABI implementation to improve speech perception seems to be from 18 to 24 months although if the surgeon is comfortable with the difficulty of implantation in the small lateral recess, ABI can be inserted in a child who is as young as 12 months. In general, these pediatric cases seem to have increased auditory performance, achieving 100% on word identification tests and showing improvements in sentence recognition, a difficult hurdle for ABI to jump. However, only two children were involved in this study, which is too small to provide significant data and the true outcomes of these patients will not be known until these individuals are older. Although there is limited information on ABI, preliminary results are promising, especially for pediatric cases for which the auditory improvement seems to be most pronounced. This increase in performance may be because the activity relayed from cochlear nuclei to auditory cortex is restored before a "sensitive period" has passed. If this sensitive period passes, which could be before the age of 3 and a half, the ability for the auditory system to reorganize could be limited. With ABI continuing to be implemented in both non-tumor and tumor cases for both adults and children, the future is bright for those with sensorineural hearing loss who cannot receive a cochlear implant.