A Novel Classification: Anomalous Routes of the Facial Nerve in Relation to Inner Ear Malformations

OBJECTIVES/HYPOTHESIS: The objective of this study was to classify anomalous facial nerve (FN) routes and to determine their association with inner ear malformations (IEMs). STUDY DESIGN: Retrospective cross sectional study. METHODS: The computed tomography images of 519 patients (796 ears) with IEMs were retrospectively evaluated, and the abnormal routes of the FN were classified as: Meatal segment: type 1, normal internal auditory canal (IAC); type 2, narrow IAC; type 3, facial canal (FC) only; type 4: separate FC/duplicated IAC. Labyrinthine segment (LS): type 1, normal; type 2a/b/c, mild/moderate/severe anterior displacement; type 3, superior displacement; type 4: straight LS. Tympanic segment (TS): type 1, normal; type 2, superiorly displaced TS; type 3, TS at the oval window; type 4: TS inferior to the oval window; type 5: unclassified. Mastoid segment: type 1, normal facial recess (FR)/normal mastoid segment; type 2: narrow FR; type 3, unclassified. RESULTS: In meatal segment classification, a narrow IAC was common in ears with cochlear hypoplasia (CH) (76.1%), and only FC was common in ears with severe IEMs (62.7%) such as Michel deformity, common cavity, and cochlear aplasia. Incomplete partition-III has its unique superiorly displaced LS (100%). CH-IV also has its unique mild anterosuperior displacement. Ears with a superiorly displaced TS usually (93.1%) had aplastic or hypoplastic semicircular canals. The FR is likely to be narrow in CH and severe IEMs. CONCLUSIONS: The FN route is affected in IEMs, which must be kept in mind when operating on ears with IEMs. Especially in CH cases, all segments of the FN can be abnormal. LEVEL OF EVIDENCE: 4 Laryngoscope, 2020.