Notes

collected by

Prof: Akram Abdel Rahman

Asmaa Salah

THE AUDITORY BRAIN STEM IMPLANT (ABI) Definition: It is an electronic device, which is surgically implanted in lateral recess of the 4th ventricle to bypass the cochlear nerve and directly stimulate the auditory neurons of cochlear nucleus. Components: 1. Microphone. 2. Speech processor. 3. Transmitting coil. 4. Implanted receiver-stimulator 5. and electrode array(consists of 22-nucleus,16(clarion) located on 3 rows on Dacron mish) N.B the receiver stimulator package has removable magnet( as most patients require postoperative MRI(NF2) Candidacy criteria: 1. Diagnosis of NF-2. 2. Imminent need for 8th nerve tumour removal. 3. Age > 12 years old. 4. Competence in language( POSTLINGUAL). 5. Reasonable expectations of potential performance. 6. Ability to comply with follow up protocol. Investigational candidates include:

 Bilateral cochlear malformation with cochlear nerve aplasia.  CN damage after head injury.  Ossified cochleas.  Auditory neuropathy (i.e.: diseases with disconnection of the central auditory system from the external sound environment).

mode of action  The ABI stimulates the auditory system by using electrical impulses. 

A microphone connected to an external speech processor collects and decodes the acoustic signal (i.e. sound) into the various component parts.

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This information is then delivered as a digital signal by radiofrequency to an internal receiver/stimulator implanted under the skin behind the ear.

 A small wire connects the reciever to the implanted electrode attached to the brainstem,The stimulating electrodes exit the internal device and are placed onto the brainstem cochlear stimulating the cochlear nucleus.

Surgery for ABI:  Translabyrinthine or retrosigmoid approach is used for tumor removal and also for implantation. Translabyrinthine provides more lateral view of the brain stem &better view of foramen of Luschka  Placement of ABI is within the lateral recess adjacent to dorsal cochlear nucleus and inferior portion of ventral cochlear nucleus, which provides for low stimulus thresholds, good performance and minimal nonauditory sensations (i.e.: activation of VII, IX or overlying flocculus of cerebellum).  During placement EABR should be assessed to ensure the device is placed over the surface of dorsal and ventral cochlear nucleus.  Complications of surgery include: CSF leak, cerebellar oedema, facial palsy, haemorrhage or meningitis. Processor Fitting and Programming Initial stimulation of the cochlear nucleus: It occurs 6 weeks postoperatively to allow for adequate healing and to ensure oedema has subsided. It is done in the intensive care unit with ECG monitoring and assistance of an anesthetist because of the possible risks in stimulating brainstem structures. The threshold level (TL) and maximum comfortable (MCL) levels: These are established after initial investigation to detect the electrodes that elicit auditory sensations and exclude electrodes that induce unpleasant sounds or nonauditory effects. Pitch scaling and pitch ranking: During pitch scaling, electrodes are stimulated at C-level, and the patient is asked to define the sound by assigning to it a sharpness rating between 1 (lowest) and 100 (highest). The software presents each channel 10 times and once the test is completed, the median and SD for each channel are displayed. During pitch ranking, 2 electrodes are stimulated in succession, and the patient is asked to indicate which sound is the higher in pitch and this information enables to achieve the most appropriate tonotopic arrangement for the subject. Speech processing strategies employed: The SPEAK encoder strategy. ACE encoder strategy. The fundamental difference between the 2 strategies is that ACE has higher stimulation rates, which allows better spectral and temporal resolution of speech signals, obtaining improvement in open-set speech recognition scores within a short period of time.

Follow up: Patients are followed regularly for assessment of the efficacy and safety of their implants at 1 month, 6 months and 1 year after activation and annually thereafter. Follow up include: 1. Medical follow-up. 2. Reprogramming of speech processors. 3. Speech perception testing. OUTCOME OF BRAIN STEM IMPLANT  ABIs had significant and measurable benefit with regard to auditory sensation and enhancement of lip reading. 1. Perceptual performance in patients implanted varies considerably, from sound-only sentence recognition scores to the ability to converse on the telephone; these results are influenced by numerous factors:  Mechanism of the auditory damage associated with the disease (. Poorer performance with an ABI may be caused by damage to the CN, either by the tumor or its removal, or by the trauma that caused the loss of the VIII nerve.)  Duration of the disease.  Treatment of the disease.  Number of active electrodes and their spatial configuration. 2. THERE IS increase closed set discrimination of consonants &vowels 3. Increase open set discrimination of sentences up to 70% 4. Speech quality , dominantely of low frequencies Disadvantages 1-Non auditory stimulation -motor respons e.g tremors of hand -jettering of visual field due excitation of cerebellum 2- pain in the head 3-Convulsive attacks 4- surgical complications e,g CSF fistula bleeding, cranial nerve deficit,cerebellar dysfunction, risk of infection The Auditory Brainstem Implant (internal receiver/stimulator and electrodes)

right: The external microphone and speech processor

PENETRATING MICROELECTRODE ARRAY  Animal studies have shown the feasibility and effectiveness of implanting penetrating electrodes in the cochlear nucleus complex.  With stimulation within the substance of the nuclear complex, lower thresholds and wider dynamic ranges were found.  penetrating electrodes can be safely inserted without significant injury to tissue or blood supply in long-term preparations 

This array is designed to access the cochlear nucleus complex when inserted using a specifically designed spring-powered tool to penetrate the ependyma and assure on-axis insertion of the electrodes

 Theoretically, the penetrating array will be better able to capitalize on the tonotopic gradient, which extends into the matrix of the nucleus.  Following the successful completion of animal experiments,the penetrating array received clearance from theFDA for use in humans.

The auditory midbrain implant (AMI) Definition: a new central auditory prosthesis designed for penetrating stimulation of the human inferior colliculus. Indications: o 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. o 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. Components ,  As with the cochlear nucleus, the tonotopy of the inferior colliculus is arranged perpendicular to its surface( A penetrating electrode array aligned along the welldefined tonotopic gradient of the ICC toactivate different frequency regions, important element for supporting good speech understanding)  A penetrating electrode array has been developed in association with Cochlear Corporation.  This array consists of 20 electrodes organized circumferentially -carrier of electrode array formed of silicone rubber is concentrically hollow - Stiffening element (stylet) made of stainless steel is positioned through the axial center of the carrier to enable insertion of electrode after array is positioned stylet is removed & carrier remain in the tissue - Dacron mesh is used to anchor electrode array in the IC ( array inserted via the supracerebellar, infratentorial route using an introducing stylet)  the central nucleus of the inferior colliculus (ICC) is the potential site

Disadvantages:  more complicated surgical approach  the inabilityto resect an acoustic neuroma at the same sitting. 

The influence of stimulating the auditory system at a more highly processed level is unknown.