Types of Congenital Malformations of the Facial Nerve

In the middle ear, two most common facial nerve anomalies are nerve displacement (Fig. 13-1) and fallopian canal dehiscence. The nerve may be found at the level of the promontory and covered only by respiratory mucosa. It may be found inferior to or at the level of the stapes or oval window. Sometimes the stapes superstructure ends blindly in the soft tissue substance of the facial nerve. The facial nerve may also bifurcate around an intact stapes, and the split nerve may or may not remain separate (Fig. 13-2). Discovering a malformed stapes during the operation should immediately alert the surgeon that the facial nerve could be out of position. If the oval window is absent, in almost 80% of the cases there is facial nerve displacement.5 Lastly, an uncovered and displaced facial nerve may be found in association with a congenital cholesteatoma.

An inferiorly displaced facial nerve may conceal the round window. It is recommended not to transpose the facial nerve simply to locate the round window. If an oval window is present, hearing preservation procedures should be attempted. It is uncommon to encounter an absent round window as an isolated anomaly. If an oval window is absent, a round window may or may not be present. If an oval window is present, a round window is almost always present.3

FIGURE 13 —1 (A-F) Patterns of facial nerve displacement in congenital middle ear malformations. ET, eustachian tube; OW, oval window; RW, round window.

Finding a large chorda tympani nerve warrants a pause and reappraisal of the operative condition. It is best not to sacrifice any large chorda tympani nerve because it may be the real facial nerve with a sharp anterior and lateral curving feature. At least it may herald an abnormal facial nerve. Electrical stimulation is of no diagnostic value as the chorda tympani nerve propagates the electrical stimulation to the facial nerve. The mastoid portion of the facial nerve almost always migrates anteriorly with external and middle ear anomalies.3

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