1When the Cochlea Won't Do: Why a Central Auditory Implant Exists

FThe implant that begins where the cochlear implant ends

A cochlear implant works by placing an electrode array inside the cochlea so that electrical pulses can excite the first auditory neurons, the spiral ganglion cells, whose fibres form the cochlear nerve. The whole strategy depends on two things being intact: a space to put the electrode into, and a nerve to carry the signal onward to the brain. Most profoundly deaf people have both, which is why the cochlear implant has become one of the most successful neural prostheses ever made.

But a small group of patients have neither a usable cochlea nor a usable cochlear nerve. For them, no amount of stimulation inside the ear can produce hearing, because there is no wire left to carry the message. The auditory brainstem implant, or ABI, was created for exactly this situation. Instead of stimulating the nerve, it skips it entirely and places electrodes on the surface of the cochlear nucleus, the first relay station inside the brainstem itself.

The central question of this chapter is therefore a simple one with profound consequences: who is beyond the reach of a cochlear implant, and what can be offered to them instead?[2019][2019]

FFour ways the cochlear pathway can fail

There are four broad reasons a deaf ear cannot accept a cochlear implant. The first is an absent or aplastic cochlear nerve: on imaging the bony canal that should carry the nerve is empty or impossibly narrow, so there is simply no nerve to stimulate. The second is a cochlea that no implant can enter or that contains no surviving neurons, such as a completely ossified cochlea after meningitis or severe inner ear malformations like a common cavity or labyrinthine aplasia.

The third is a nerve that once worked but has been destroyed. The classic example is neurofibromatosis type 2 (NF2), an inherited condition that grows tumours on both vestibular nerves; removing those tumours, or letting them grow, typically severs hearing on both sides. The fourth is traumatic, where a temporal bone fracture avulses or transects the cochlear nerve.

In each case the failure lies at or before the cochlear nerve. Whatever the cause, the consequence is the same: the only remaining target for an implant is upstream, inside the brainstem.[2024][2016]

CHow the ABI is like, and unlike, a cochlear implant

From the patient’s chair the two devices can look almost identical: a microphone and processor behind the ear, a coil held on by a magnet, and an internal receiver-stimulator. Much of the ABI’s hardware and sound-coding software is borrowed directly from cochlear implant systems. The difference is entirely at the business end. A cochlear implant’s electrode is a thin array threaded into a fluid-filled spiral; an ABI’s electrode is a small flat paddle of contacts on a mesh backing, laid against the surface of the brainstem.

That single change cascades into everything else. Placement is a neurosurgical procedure rather than an otologic one, performed during or after tumour removal through a translabyrinthine or retrosigmoid approach. The cochlear nucleus is never directly seen, so the surgeon relies on anatomic landmarks and electrically evoked auditory brainstem responses to position the paddle. And because the device sits among the lower cranial nerves and vital brainstem centres, some electrodes can produce non-auditory sensations rather than sound.

Outcomes also differ. Many cochlear implant users achieve open-set speech understanding; most NF2 ABI users gain environmental sound awareness and a powerful aid to lip-reading, with open-set speech the exception rather than the rule. As later modules show, non-tumour ABI users often do considerably better.[2008][2019]

CThe chapter roadmap

This chapter follows the ABI from concept to clinic. After this overview we descend to the surgical and neural target itself, the cochlear nucleus complex on the floor of the lateral recess of the fourth ventricle, and ask why a flat paddle can only crudely engage a three-dimensional, tonotopically arranged structure. We then trace the history of the device, from the first single-channel implant in an NF2 patient in 1979 to today’s multichannel arrays used in children with malformed inner ears.

Throughout, keep the central distinction in mind. A cochlear implant is a treatment for a deaf ear; an ABI is a treatment for a deaf pathway. The benefit it offers is real but often modest, and the decision to place one is shaped as much by what else is happening to the patient, an only-hearing ear, growing tumours, a deaf child’s developing brain, as by audiometry alone.

For the right candidate the ABI is sometimes the only door to the world of sound. Understanding when that door applies, and being honest about what lies behind it, is the work of the chapters that follow.[2019][2020]