Facial Nerve Abnormalities

Abstract

The course of FN through the temporal bone is thought to be dependent on the normal development of the related bony structures. Thus, abnormal formation of otic capsule may lead FN to follow a different route. In this chapter, the authors present a comprehensive classification of FN abnormalities in its all segments. Temporal bone computed tomography (CT) has a key role in detecting abnormal routes of FN as in inner ear malformations. The authors of this chapter classified FN abnormalities in 4 segments of the nerve as: Meatal segment: Type 1-Normal meatal segment (normal internal auditory canal, IAC): The midpoint diameter of IAC ≥ 2 mm. Type 2-Narrow meatal segment (narrow IAC): The midpoint diameter of the IAC < 2 mm. Type 3-Only facial canal: The IAC is absent and FN has its own canal. Type 4-Separate facial canal/duplicated IAC: IAC is present but FN has its own bony canal which is separated by the IAC. A narrow IAC (type 2) is generally seen in patients with cochlear hypoplasia. In more severe malformations such as Michel deformity, common cavity, and cochlear aplasia, only FC (type 3) is predominant type. Emphasis Type="Bold">Labyrinthine segment (LS): Type 1-Normal: LS makes a gentle curve between the geniculate ganglion and the IAC around the basal turn. Type 2-Displacement of the LS to the anterior part of the cochlea: This anterior displacement is evaluated into 3 groups according to the degree of anterior course. Type 2a-Mild anterior displacement: Cochlea is present and LS is located at the anterosuperior part of the cochlea instead of its normal posterolateral course. Cochlear hypoplasia type IV(CH-IV) has this type of LS. Type 2b-Moderate anterior displacement: LS occupies the usual location of the cochlea, and cochlea is absent or hypoplastic. Type 2c-Severe anterior displacement: LS is further anteriorly displaced beyond the usual location of the cochlea. Type 3-Superior displacement: In axial CT sections, LS is situated above the level of cochlea. IP-III causes superior displacement of the LS. Type 4-Straight LS: LS courses from IAC to geniculate ganglion in a non-curved, straight way so that FC looks like a continuation of IAC. A normally sized and developed cochlea is associated with a normal LS. Incomplete partition type I (IP-I) and IP-II are usually associated with a normal LS. As cochlea becomes hypoplastic or aplastic, LS displaces anteriorly in various degrees. Tympanic segment (TS): Type 1-Normal: TS is located superior and lateral to the oval window (OW) and stapes and inferior to lateral SCC. Type 2-Superiorly displaced: TS is located above the OW generally accompanying hypoplastic or aplastic lateral SCC. Type 3-At the OW: TS is located at the OW level at coronal CT sections. Type 4-Inferior to OW: TS is located below the OW, generally accompanying cochlear hypoplasia and abnormal SSCs. Type 5-Unclassified: Because of the aplasia of the OW, FN cannot be classified in relation to OW. In cases with cochlear hypoplasia, lateral semicircular canal is usually found to be aplastic or hypoplastic; this causes various degrees of dislocation of TS. Mastoid segment: Type 1-Normal facial recess (FR)/normal mastoid segment: if the space between the external auditory canal (EAC) and the FN is ≥2 mm. Type 2-Narrow FR: The distance between the EAC and the mastoid portion of the FN is <2 mm. Type 3-Unclassified: The mastoid portion of the FN cannot be visualized. The mean FR tends to be narrower in CH and in severe IEMs where cochlea is absent. © Springer Nature Switzerland AG 2022. All rights reserved.