4Who Needs an Auditory Brainstem Implant?

FWhen a cochlear implant has nothing to talk to

A cochlear implant works by sending electrical pulses to the spiral ganglion neurons, the first-order cells whose fibres bundle into the cochlear nerve and carry the signal to the brain. That arrangement only works if a cochlear nerve actually exists and can conduct. When the nerve is congenitally absent, surgically sacrificed, or destroyed by disease, electrodes inside the cochlea fire into silence, because there is no cable left to relay the message centrally.

The auditory brainstem implant solves this by skipping the missing link entirely. Its electrode sits not in the cochlea but on the surface of the cochlear nucleus, the first relay station inside the brainstem where the cochlear nerve normally terminates. By stimulating these second-order neurons directly, the ABI reaches the auditory pathway downstream of the break. The trade-off is that this is intracranial surgery and the percept is generally coarser than a well-placed cochlear implant delivers.[2012][2014]

TProving the nerve cannot be used

Candidacy hinges on objective evidence that the cochlear nerve is unusable, because no audiogram alone can establish this. High-resolution imaging is central. Heavily T2-weighted MRI through oblique-sagittal reconstructions of the internal auditory canal lets the surgeon count the nerves crossing it; absence or marked hypoplasia of the cochlear nerve in a stenotic or atretic canal points away from a cochlear implant. CT complements this by showing a totally ossified cochlea after meningitis, or a malformation such as common cavity or cochlear aplasia where no functional neural substrate exists to implant.

Electrophysiology adds functional confirmation. A promontory or round-window electrical stimulation test that fails to evoke any auditory response, or an absent electrically evoked auditory brainstem response, supports a non-functioning nerve. In NF2, the situation is dynamic rather than static: the cochlear nerve may be intact preoperatively but is routinely sacrificed during removal of a vestibular schwannoma, so candidacy is judged around the planned surgery rather than from a fixed baseline.[2014][2025]

CThe classic indications, tumour and non-tumour

Historically the ABI existed for one group: patients with neurofibromatosis type 2, in whom bilateral vestibular schwannomas grow on, and ultimately destroy or require sacrifice of, both cochlear nerves. For these patients an ABI placed at the time of tumour removal is often the only route back to any sound. Since the late 1990s the indications have widened to non-tumour adults and children. These include congenital cochlear nerve aplasia or severe hypoplasia, totally ossified cochleae after meningitis where an array cannot be threaded, severe inner-ear malformations such as common cavity or cochlear aplasia, cochlear nerve avulsion from temporal bone fracture, and selected ears where a cochlear implant produced no useful hearing.

Across these groups a consistent observation drives counselling: non-tumour ABI users tend to outperform NF2 users, because in non-tumour ears the cochlear nucleus has not been disturbed by tumour growth or its removal. A European consensus position has framed candidacy around exactly this distinction, separating prelingual malformation and nerve-deficiency cases from postlingual acquired causes, and favouring the retrosigmoid route in non-tumour patients.[2014][2006][2009]

CThe CI-first principle

The governing rule of candidacy is that the ABI is a fallback, not a first choice. Whenever a cochlear nerve plausibly exists, the team should attempt or expect a cochlear implant, because even a modest cochlear implant result usually exceeds a typical ABI result and avoids a craniotomy. This is most consequential in children with cochlear nerve deficiency, where imaging can be equivocal and some apparently deficient nerves still support useful cochlear implant hearing. Many centres will trial a cochlear implant first in borderline cases and reserve the ABI for genuine non-responders.

Because the ABI commitment is large, candidacy is a multidisciplinary decision rather than an audiological threshold. There are no fixed audiometric cut-offs as there are for a cochlear implant; instead the team weighs imaging, electrophysiology, the patient’s age and plasticity, realistic expectations, and the family’s readiness for prolonged rehabilitation. The decision is reached when the nerve has been shown unusable and the patient is motivated and appropriately counselled about a result that may be sound awareness and lip-reading support rather than open conversation.[2025][2012][2014]