5Implants When a Cochlear Implant Is Not Possible (ABI, etc.)

FWhen the implant has nothing to stimulate

The previous module made the case for the cochlear implant in single-sided deafness. But that whole logic rests on one assumption: a stimulable cochlear nerve and a cochlea that can hold an electrode array. Sometimes neither exists. The cochlear nerve may be congenitally absent or aplastic, severed by trauma, or destroyed by a tumour such as a vestibular schwannoma or by the surgery to remove it [2025]. The cochlea itself may be completely ossified after meningitis, or malformed beyond use.

In these ears a cochlear implant cannot work — there is no functioning nerve downstream to carry the signal, or no lumen to receive the array. To restore any hearing, stimulation must jump past the cochlea and nerve altogether and be delivered directly to the next station: the cochlear nucleus on the brainstem.

TThe auditory brainstem implant

The auditory brainstem implant (ABI) is a paddle of surface electrodes placed on the cochlear nucleus in the lateral recess of the fourth ventricle, usually via a retrosigmoid or translabyrinthine approach. By stimulating the brainstem directly it bypasses a dead cochlea and an absent nerve entirely [2024].

Its classic indication is neurofibromatosis type 2 (NF2), where bilateral vestibular schwannomas and their removal leave both cochlear nerves non-functional; the ABI is often placed at the time of tumour resection. It is also used in non-tumour adults and in children with cochlear nerve aplasia or labyrinthine malformations who cannot benefit from a CI [2018]. Outcomes are generally more modest than with a cochlear implant: many users gain useful environmental sound awareness and an aid to lip-reading, while open-set speech understanding is achieved by a smaller subset, typically better in non-tumour than in NF2 patients [2024].

CThe honest scope for tinnitus

It is tempting to extend the cochlear-implant-for-tinnitus story to the ABI — if refilling the auditory channel quietens tinnitus at the cochlea, perhaps stimulating the brainstem would do the same when the nerve is gone. The honest answer is that the evidence is thin and the indication is not established. The ABI is implanted to restore some hearing, not as a tinnitus treatment; any tinnitus change is a secondary observation, not a goal [2024].

There has been genuine interest. A formal protocol explored an ABI specifically for intractable unilateral tinnitus in patients without a usable cochlear nerve, on the rationale that direct brainstem input might suppress the phantom signal — but this remains investigational rather than standard care [2019]. For the common SSD-with-tinnitus patient who has an intact nerve, the ABI is the wrong operation: it is more invasive, gives poorer hearing, and has no established tinnitus benefit. The cochlear implant remains the answer whenever a nerve is present.

The counselling message is therefore deliberately restrained. When a CI is impossible, the ABI may restore some hearing and is worth considering on those grounds; but it should not be offered as a reliable cure for tinnitus, and patients must understand that [2025].

CSelecting candidates — and the price of getting it wrong

Candidacy for an ABI turns on a single anatomical fact: a cochlear implant must be genuinely impossible, confirmed on high-resolution MRI that shows an absent or non-stimulable cochlear nerve, or a cochlea that cannot host an array. In an NF2 patient with bilateral schwannomas, the ABI is typically placed at the time of tumour resection, when the brainstem is already exposed and the cochlear nerve is being sacrificed [2018]. In non-tumour adults and children with nerve aplasia or severe malformation, candidacy is decided in a dedicated implant programme after a CI has been formally excluded [2024].

The cost of getting selection wrong is real. The ABI requires posterior fossa surgery, with the attendant risks of cerebrospinal fluid leak, cranial nerve injury and the well-recognised non-auditory side effects of stimulating structures adjacent to the cochlear nucleus. Offering it to a patient who actually has a stimulable nerve — or who is chasing tinnitus relief it cannot reliably deliver — exposes them to those risks for little gain. The discipline is the same as for every procedure in this chapter: match the operation to a confirmed anatomical problem, and counsel honestly about what it can and cannot achieve [2025].