CI Atlas · Preoperative Imaging · Module 08

8Cochlear nerve, IAC & the CI-vs-ABI boundary

Everything else in the work-up assumes there is a nerve for the implant to stimulate — and this module is where that assumption is tested. The single most important teaching point is a trap: the bony internal auditory canal can look perfectly normal on CT while the cochlear nerve inside it is hypoplastic or absent, so a child with profound loss needs MRI, not CT alone. On the right oblique-sagittal slice the cochlear nerve is measured against the facial; canal calibres on CT give supporting clues. Where the nerve is merely small, the prognosis is guarded and the implant may still help; where it is truly absent, the cochlear implant has no target and the decision turns to an auditory brainstem implant. And the nerve's calibre, beyond present-or-absent, hints at how many neurons survive. This module is the nerve, and the boundary it draws.

Imaging note

Representative CT and MRI images for this chapter are being added soon. The interactive figures here are original schematic teaching diagrams; to respect copyright we do not reproduce third-party radiographs.

TA normal canal is not a present nerve

HRCT is inadequate for the cochlear nerve: a normal IAC does notguarantee a present nerve. In Adunka's CISS series, most ears with small or absent cochlear nerves had normal-sized IACs — so every child with profound loss should have high-resolution MRI, not CT alone.[2006]

CThe calibre check & Casselman

On the oblique-sagittal CISS/FIESTA view the cochlear nerve should be at least as large as the ipsilateral facial nerve and symmetric with the other side (a common cochleovestibular nerve is normal at 1.5–2× the facial). Casselman classified the anomalies: type I no cochlear nerve (facial only); IIa absent nerve with a dysplastic labyrinth; IIb absent nerve with a normal labyrinth; III isolated vestibular-nerve aplasia.

CBCNC & IAC thresholds

CT supplies bony surrogates. The bony cochlear nerve canal(cochlear aperture) at mid-modiolar level is hypoplastic <1.4 mm (CHARGE association), aplastic when no canal forms, and wide >3 mm in X-linked IP3 (predicting gusher). A narrow IAC (<2 mm) often accompanies nerve deficiency.

The bony cochlear nerve canal (cochlear aperture) at the mid-modiolar level is a CT surrogate for the nerve: <1.4 mm suggests cochlear-nerve deficiency (CHARGE association), aplastic (no canal) points to ABI, and >3 mm a wide canal predicts gusher (X-linked IP3). A narrow IAC (<2 mm) often accompanies nerve deficiency. The essential caveat: a normal-sized canal does NOT prove a present nerve — in Adunka's series most deficient nerves had normal IACs, so high-resolution MRI is mandatory. Schematic.

CHypoplasia, aplasia & the ABI boundary

Hypoplasia (a nerve smaller than the facial) is a relative contraindication with a guarded open-set prognosis; aplasia (no nerve) is absolute and directs to an auditory brainstem implant. Beyond present-or-absent, imaged nerve calibre is a neural- substrate predictor— Nadol showed nerve diameter correlates with total spiral-ganglion-cell count, so MRI helps predict surviving neurons and outcome (Objective Measures / Devices). A pitfall: a “small cochlear nerve” may actually be the nervus intermedius.[1984]

Case 12.8 · Normal canal, absent nerve

A child with profound deafness has a normal-sized internal auditory canal on CT. The family is told the nerve must be fine.

Why is that reassurance premature?

Self-assessment — Module 72 questions

Question 1 · Trainee

Does a normal-sized IAC on CT prove a present cochlear nerve?

Question 2 · Clinician

How do cochlear-nerve hypoplasia and aplasia differ for candidacy?

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