Auditory brainstem implantation (ABI) requires patients to have a sufficient number of residual auditory nerve fibers for the stimulation electrode to be inserted into the cochlea. In cases where the auditory nerve is absent, such as in patients who have undergone removal of an acoustic neuroma, cochlear implantation is not a viable option.
ABI is designed to place stimulation electrodes on the brainstem (cochlear nucleus) instead. External sounds bypass the cochlea and auditory nerve pathways, reaching the brainstem directly. The stimulation activates different auditory neurons in the cochlear nucleus to generate auditory nerve impulses. These impulses undergo signal processing and encoding, eventually reaching the cerebral cortex and producing meaningful auditory perception.
The ABI device consists of both external and internal components. The external components include a sound processor, a portable speech processor, and connecting wires. The internal components consist of a receiver (stimulator), electrode leads, and an electrode array.
The working principle of ABI is similar to that of the cochlear implant, as both rely on a sequence of electrodes. However, the key difference lies in the stimulation site: cochlear implants achieve auditory perception by stimulating auditory nerve fibers within the cochlea, whereas ABI bypasses the cochlea and the auditory nerve, placing electrodes in the lateral recess of the fourth ventricle to directly stimulate auditory neurons in the cochlear nucleus complex of the brainstem.
Indications for ABI include bilateral vestibular schwannomas (NF2), cochlear ossification, narrow internal auditory canals, and underdeveloped auditory nerves.