Auditory brainstem implants: how do they work?

This review covers the design, structure, and function of auditory brainstem implants. Auditory brainstem implants (ABIs) are auditory prostheses initially designed to treat deafness in patients with neurofibromatosis type 2 (NF2). NF2 typically results in deafness due to disruption of the cochlear nerves. When the tumors are removed the auditory nerve is usually cut or nonfunctional anymore. In these cases, patients cannot benefit from peripheral devices such as cochlear implants (CI). Another cause of VIII nerve loss is bilateral temporal bone fracture. Worldwide, more than 500 persons have received an ABI after removal of the tumors that occur with NF2. More recently, some extensions of indications have been proposed to include subjects who would not benefit enough from a cochlear implant (i.e. cochlear ossification). The ABI is similar in design and function to a CI, except that the electrode is placed on the first auditory relay station in the brainstem, the cochlear nucleus (CN). The ABI electrode array is a small paddle that contains plate electrode contacts. The CN has not a single linear tonotopic organization from base to apex like the cochlea but different tonotopic subunits. The CN comprises multiple neuron types that are characterized by specific properties (morphology, regional distribution and cell-membrane characteristics), synaptic input and responses to acoustic stimuli. As the ABI electrode array is placed along the surface of the CN, each electrode likely activates a variety of neuron types, possibly with different characteristic frequencies. Patients undergoing ABI have variable benefit with regard to sound and speech comprehension. For the majority of patients, this improvement is essentially obtained by an augmentation of lip reading performances. Speech comprehension without lip-reading is not as good as with cochlear implants.