CI Atlas · Preoperative Imaging · Module 10

10The facial nerve — course, anomalies & stimulation risk

The facial nerve is the surgeon's constant companion and constant hazard in cochlear implantation. The transmastoid approach to the round window threads the facial recess, working in the immediate shadow of the nerve, so the operation begins with the CT tracing the fallopian canal segment by segment. In a malformed ear the nerve is far more likely to wander — running anteriorly into the cochlear position, or crossing the promontory toward the round window — and even in a normal ear its tympanic segment may be dehiscent. Imaging also looks ahead to a later problem: where the apical cochlear turn and demineralised otospongiotic bone sit close to the nerve, electrodes can stimulate it, causing twitching that may need deprogramming. This module reads the facial nerve before the drill ever touches bone.

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.

TMapping the fallopian canal

HRCT maps the intratemporal facial (fallopian) canal precisely — essential because the transmastoid facial-recess approach works right around the nerve (Surgery chapter), with intra-operative facial-nerve monitoring as the backstop. The canal is traced through its labyrinthine, geniculate, tympanic and mastoid segments.[2022]

HRCT maps the intratemporal facial nerve precisely — essential because the transmastoid facial-recess approach works right around it (Surgery chapter), with intra-operative monitoring as backstop. An aberrant course is far more likely in malformed ears (~15–20%): the labyrinthine segment may run anteriorly into the cochlear position, or the nerve may cross the promontory toward the round window. Even in normal ears, tympanic-segment dehiscence may be met. Recognising these preoperatively can force a modified approach. Schematic.

CAberrant courses in malformed ears

An aberrant course is far more likely in malformed ears (~15–20%): in cochlear aplasia and CLA-without-cochlea the labyrinthine segment runs anteriorly into the normal cochlear position, and in CH4 and CADV it follows an anomalous path. The classic anomaly runs below the processus cochleariformis and across the promontory toward the round window, often with a stapes anomaly.

CDehiscence in normal ears

Even in non-malformed ears, dehiscence of the tympanic segmentmay be met, along with partial agenesis, a narrow canal with a hypoplastic nerve, an inferiorly displaced tympanic segment over the oval window, or lateral mastoid-segment rotation in atresia. Recognising any of these preoperatively may force a modified or labyrinthotomy approach.

CPredicting facial-nerve stimulation

Imaging also anticipates unwanted facial-nerve stimulation: the apical cochlear turn lies close to the pregeniculate facial canal, and otospongiotic bone adjacent to the labyrinthine segment lowers resistance — so CT analysis flags which electrodes may later need deprogramming (Programming / Objective Measures).

Case 12.10 · A nerve where the cochlea should be

In a child with cochlear aplasia, the CT shows the labyrinthine facial segment running anteriorly. The surgeon reviews the approach.

Why does this matter?

Self-assessment — Module 92 questions

Question 1 · Trainee

Why is CT mapping of the facial canal essential before CI surgery?

Question 2 · Clinician

What does proximity of the apical turn and otospongiotic bone to the facial canal predict?

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