Auditory brainstem implant (ABI)

When the cochlear nerve itself is absent or non-functional — classically in neurofibromatosis type 2 (NF2), where bilateral vestibular schwannomas and their removal destroy both auditory nerves, and also in cochlear nerve aplasia or severe cochlear ossification — a cochlear implant cannot work because there is no nerve to stimulate. The auditory brainstem implant places a surface electrode paddle on the cochlear nucleus in the lateral recess of the fourth ventricle, bypassing the nerve to stimulate the first central auditory relay directly.

Outcomes are far poorer and more variable than the CI: most NF2 users gain useful sound awareness and lip-reading enhancement but only a minority achieve open-set speech, because the surface array poorly accesses the nucleus's buried, three-dimensionally folded tonotopic map and can trigger non-auditory side effects (from nearby cranial-nerve and cerebellar structures) that force channels to be switched off. Non-tumor and pediatric ABI recipients (for example cochlear nerve aplasia) tend to do somewhat better than NF2 patients, implicating tumor- and surgery-related pathway damage in the poor NF2 results.