The Intratemporal Facial Nerve

From the fundus of the internal auditory canal the nerve enters the labyrinthine segment, the shortest and narrowest part of its course, between the cochlea in front and the vestibule and superior canal ampulla behind. At the geniculate ganglion it turns sharply backwards, the first genu, and gives the greater petrosal nerve forwards. The tympanic segment runs back on the medial wall of the middle ear, below the lateral canal and above the oval window. Behind the pyramidal eminence it turns down at the second genu, just inferior to the dome of the lateral canal, and descends as the mastoid segment to the stylomastoid foramen, giving the nerve to stapedius and the chorda tympani on the way.

Segment lengths and the angles of the genua vary between bones, and the canal may be dehiscent, most often over the oval window. Treat any uncertain soft tissue there as nerve until its identity is established.

Trace a segment you have identified into the next one. Never drill at a point chosen from a diagram.

Begin with the anatomy around the nerve: the lateral canal, the incus, the posterior canal wall and the digastric ridge. They give an estimate of the course, which guides attention; it does not replace identifying the bony canal.

Skeletonising defines the bony covering well enough to understand the course; it does not open the canal. A basic mastoid or facial recess exercise needs awareness and preservation of the nerve, not decompression. Use controlled bone work, enough irrigation to keep the surface readable and a view that shows the contact point; the burr shaft and the suction near the nerve need the same attention as the burr head.

Do not rely on a pale yellow colour: fixation, bone thickness and natural variation all change what shows through thin bone. Stations 4 and 5 are separate exercises, done under specialist guidance and only when the specimen and session allow.

Skeletonisation, decompression and opening the sheath are three different results, not stages that follow automatically from one another. Record which of these you reached.

In a patient, decompression needs an indication, a decision on timing, functional assessment and an appropriate route. Laboratory removal of bone reproduces none of those decisions.

The nerve is one structure, but each segment is reached by a different route and lies against different landmarks. Lengths are approximate and vary between individuals; measure your own specimens and do not rely on a number to locate the nerve.

Dehiscence: expect it, do not be surprised by it. In 1000 temporal bones, 56% had at least one dehiscence of the facial canal, most often at the oval window, and 76% of those were bilateral (Moreano et al., 1994). Earlier anatomical series reported a similar figure of about half (Baxter, 1971; Nager & Proctor, 1991). A dehiscent nerve can bulge over the stapes and be mistaken for granulation tissue or mucosa.

Malformed ears carry most anomalies: an anteriorly displaced mastoid segment, a nerve over or below the oval window, bifurcation or trifurcation, and rarely a persistent stapedial artery running with the nerve (about 0.5% in Moreano’s series). Read the CT before you drill any congenital ear.

Inspect for bony discontinuity, direct instrument contact, traction and tissue disruption. A visually continuous nerve is not proof of preserved function: thermal or pressure injury may not be visible in a cadaver, and in a patient monitoring complements anatomical technique rather than replacing it.

A suspected injury stops the manoeuvre. Preserve the field, obtain instructor review and record the instrument, the site and the likely mechanism, rather than continuing to finish the exposure.

Follow the nerve’s continuity, not its colour.