14When the Cochlea or Nerve Cannot Be Used: ABI Candidates

TWhen the cochlear implant cannot work

A cochlear implant depends on a cochlea that can hold an electrode and a cochlear nerve that can carry the signal; when both are absent or unusable, the implant has no target. The auditory brainstem implant bypasses the cochlea and nerve to stimulate the cochlear nucleus complex directly, conveying salient acoustic cues to the central auditory pathway. The defining adult candidate is neurofibromatosis type 2, in whom bilateral vestibular schwannomas and their removal destroy both cochlear nerves. Non-tumour indications include cochlear nerve aplasia, complete cochlear aplasia, and cochlear ossification or malformation so severe that no electrode can be placed. It is conceived as the next step when a cochlear implant cannot work, and has also been used as a salvage option after a cochlear implant fails to give any useful hearing.[2006][2014][2009]

TThe electrode on the cochlear nucleus

The array of roughly 20 surface electrodes on a flexible paddle is placed in the lateral recess of the fourth ventricle, reached through the foramen of Luschka, and laid over the cochlear nucleus complex. The target spans the dorsal and ventral cochlear nuclei in the floor of the lateral recess, a deeper and more demanding site than the cochlea. Maintaining stable contact with the nucleus and aligning electrodes with a useful population of neurons is harder than positioning an array in the cochlea. Intraoperative electrically evoked auditory brainstem response monitoring guides placement and confirms auditory rather than non-auditory activation. Sound processors and coding strategies are adapted from cochlear implants, though the brainstem is a coarser and less tonotopically accessible target.[2014][2009]

COutcomes: tumour versus non-tumour

Outcomes are generally poorer than with a cochlear implant; for many NF2 recipients the benefit is sound awareness and enhancement of lip-reading rather than open-set understanding. Colletti's series reported open-set speech perception averaging about 59% in non-tumour adults at one year, compared with roughly 11% in NF2 patients, a statistically significant gap. The best non-tumour performers were patients who lost the eighth nerve to head trauma or to severe cochlear ossification, some approaching cochlear-implant-like comprehension. A 2024 systematic review of tumour versus non-tumour recipients confirmed the pattern, with non-tumour patients reaching higher open-set sentence recognition and closed-set word scores. The tumour itself, nerve damage from its removal, and disrupted central pathways are thought to explain why NF2 outcomes lag behind non-tumour cases. Even modest benefit, restoring environmental sound and supporting communication, is meaningful for a patient who would otherwise be totally and permanently deaf.[2006][2024][2014]

CChildren, non-auditory effects, and counselling

In children with cochlear nerve aplasia or severe inner-ear malformation, the brainstem implant offers hearing where no cochlear implant can, and some paediatric recipients achieve open-set perception. Outcomes in children depend heavily on early implantation within the sensitive period and on the integrity of the central auditory pathway, not just the periphery. Because the array sits among brainstem nuclei, non-auditory side effects such as tingling, dizziness, throat sensations, or visual disturbance can occur and are managed by deactivating offending electrodes. Programming may sacrifice some active electrodes to eliminate non-auditory sensations, leaving fewer usable channels than the array provides. Counselling must set expectations honestly: the brainstem implant restores access to sound and aids communication, but should not be promised as equivalent to a cochlear implant.[2014][2009]