9The Decider: The Cochlear Nerve and Its Bony Canal
An implant is a transducer; it needs a wire on the far side. That wire is the cochlear nerve, and whether it exists decides cochlear implant versus auditory brainstem implant far more than the shape of the cochlea does. A beautifully formed cochlea with no nerve cannot hear; a grossly malformed cochlea with a healthy nerve often hears well. This module is about how to find the nerve before you commit — on CT through the bony canal it travels in, and on MRI by looking at the nerve itself.
FWhy the nerve, not the bone, decides
A cochlear implant stimulates the auditory nerve; if the cochlear nerve is absent, there is no afferent pathway for the device to drive, and an auditory brainstem implant (ABI) becomes the option to consider. The corollary is liberating for malformation surgery: a malformed but nerve-bearing cochlea — IP-II, many hypoplasias — can implant well, while a normal-looking cochlea with an absent nerve cannot. Cochlear nerve deficiency (CND) is therefore assessed deliberately and separately from the partition or shape anomaly, and it co-occurs frequently with IP, cochlear hypoplasia, and a narrow internal auditory canal (IAC). CND is graded as aplasia (the nerve is absent) versus hypoplasia (the nerve is present but small), and this distinction, not the cochlear morphology, anchors the candidacy decision.[2017][2011][2016]
TReading the bony cochlear nerve canal on CT
The bony cochlear nerve canal (BCNC), or cochlear aperture, is the channel through which the nerve enters the modiolus from the IAC fundus; its bony width is a proxy for the nerve it carries. A BCNC width below about 1.4-1.5 mm suggests a deficient cochlear nerve and should prompt direct MRI assessment of the nerve itself. CND frequently coexists with a narrow IAC; a small canal and a stenotic aperture together raise suspicion well before the nerve is visualised. CT alone cannot count the nerve — a normal BCNC does not guarantee a normal nerve and a narrow one does not prove aplasia — so the canal width is a flag, not a verdict.[2007][2013][2017]
TSeeing the nerve itself on MRI
High-resolution heavily T2-weighted MRI shows the nerves as dark filling defects in bright CSF; a parasagittal oblique reformat perpendicular to the IAC displays the four nerves in cross-section. In that view the facial nerve lies antero-superior, the cochlear nerve antero-inferior, and the superior and inferior vestibular nerves posteriorly — so an absent or thin antero-inferior nerve is read as cochlear aplasia or hypoplasia. MRI is the reference standard for CND because it images the nerve directly rather than the bone around it, which is why a narrow BCNC mandates an MRI rather than substituting for one. Comparing the suspect cochlear nerve with the ipsilateral facial nerve and the contralateral cochlear nerve gives a practical internal yardstick for hypoplasia.[2002][2017]
CAplasia, hypoplasia, and the CI-versus-ABI call
Cochlear nerve aplasia generally predicts poor or no cochlear-implant benefit, and an ABI should be discussed; hypoplasia gives variable but often useful CI benefit, so a trial of CI is frequently reasonable before considering ABI. Outcomes in CND are more variable and on average lower than in nerve-normal ears, so families are counselled toward realistic expectations and a structured plan if early progress stalls. Where nerve status is uncertain, many programmes implant the cochlea first — the less invasive option — and reserve ABI for documented non-response, since some hypoplastic nerves perform better than imaging predicts. Promote-or-pivot is the practical frame: confirm the nerve, set expectations to its status, implant when a nerve is present, and keep ABI in reserve for true aplasia or a failed CI trial.[2016][2011][2017]
What does the imaging indicate, and how should the device decision be framed?
On CT, what bony cochlear nerve canal width should prompt MRI to look for cochlear nerve deficiency?
In the parasagittal oblique view of the internal auditory canal, where does the cochlear nerve normally sit?
How does the device decision differ between cochlear nerve aplasia and hypoplasia?