*2.6.2 Design, mode of action & candidacy*

Although in the earlier years the most frequent indication of an ABI was NF2, a genetic disorder with an autosomal dominant inheritance pattern in which the patient develops benign vestibular schwannomas on both eighth cranial nerves; in recent years, the indications for the ABI have expanded to include bilateral cochlear ossification, unilateral vestibular schwannoma with deafness in the contralateral ear that is not amenable to a cochlear implant, congenital cochlear nerve aplasia or hypoplasia, complete cochlear ossification, malformation of the inner ear, and bilateral traumatic avulsions or absence of the cochlear nerve [30]. The current ABI system utilizes 12 platinum disk electrodes aligned on a flexible silicone and polyester mesh backing for implantation into the lateral recess of the fourth ventricle on the cochlear nucleus of the brain. This bypasses a non-functioning auditory nerve and allows signals to be sent to the brain.

## **2.7 The auditory midbrain implant (AMI)**

### *2.7.1 History*

The first reported attempt at stimulation of the inferior colliculus (IC) of the midbrain for hearing restoration was in 1962 by Simmons and colleagues at Stanford University during a tumor removal operation [31]. The second attempt at stimulating the surface of the IC happened much later in 2005 and involved using

*Audio-Vestibular Neurosensory Prosthetics: Origins, Expanding Indications and Future Directions DOI: http://dx.doi.org/10.5772/intechopen.95592*

an ABI array in a NF2 patient to assess if the limited performance with cochlear nucleus stimulation secondary to tumor related damage could be overcome by stimulation in a higher auditory center and they were successful in eliciting a response at much higher thresholds than what is used in cochlear nucleus stimulation [32]. In 2006, Thomas and Minoo Lenarz at the Hannover Medical University developed the first human prototype Auditory Midbrain Implant (AMI) array for penetrating stimulation across the tonotopic gradient of the central nucleus of the IC [33].

### *2.7.2 Design, mode of action & candidacy*

The AMI is a type of central auditory prosthesis that targets midbrain regions beyond the cochlear nucleus, particularly the central nucleus of the Inferior Colliculus (ICC). There are several properties of the ICC that make it a logical choice for a prosthetic target including being a converging center for almost all ascending auditory projections, possessing tonotopic anatomical organization in addition to spatial organization for speech perception. The Cochlear Ltd. CI array was reduced in dimensions to create an AMI array that was small enough to insert into the ICC with the goal of stimulating its different layers. The AMI electrode array measures 6.4 mm long and a diameter of 0.4 mm and possesses 20 linearly spaced platinum ring electrodes. A distal Dacron mesh prevents over insertion of the electrode into the ICC during implantation and anchors the electrode array onto the surface of the inferior colliculus to minimize movement after positioning. The other components of the AMI system are similar to the Cochlear NUCLEUS CI system consisting of behind-the-ear microphone and processor that transmits the electromagnetic signals to the receiver stimulator implanted under the skin.

#### **2.8 The vestibular implant**

#### *2.8.1 History*

Vestibular disorders are widely prevalent and can cause significant morbidity in the form of incapacitating symptoms, missed work days and even the inability to leave one's home. Advancements in the treatment of vestibular disorders are lacking and the surgical treatment of vestibular disorders has remained unchanged in the past two decades. Over the same duration, neural prosthesis have been developed that substitute one modality for another in order to restore a missing sense. Current research in vestibular neurostimulation has finally made possible the clinical reality of treating a wide range of vestibular disorders with electric stimulation.

#### *2.8.2 Design, mode of action & candidacy*

Vestibular implantation is useful in recurrent acute vertigo attacks (in which central compensation does not occur), for chronic bilateral hypofunction and for Meniere's disease, in which vertiginous attacks may be precipitated by an acute loss of vestibular tone. These are ideal candidates for a "pacemaker"-style vestibular implant that replaces the missing neural impulses during attacks. This implant bypasses the vestibular end organs to directly stimulate the vestibular nerve. The goal of vestibular implantation is to provide vestibular functionality and or reduce symptomatology by programmed stimulation of the vestibular nerve. This device consists of a modified Nucleus Freedom system. The receiver-stimulator has a modified trifurcating array of 9 electrodes. Each electrode is implanted 2.5 mm into the perilymphatic space of the semicircular canal adjacent to the ampullary nerve which is the site of stimulation.
