Abnormalities of the Facial Nerve in Temporal Bones With Inner Ear Malformations

Abstract

Background: Bony inner ear malformations (IEMFs) account for ~20% of congenital sensorineural hearing loss, often requiring cochlear or auditory brainstem implants. To ensure safe cochlear implantation in patients with IEMFs, understanding their anatomical features, particularly related to the facial nerve (FN), is crucial.

Methods: We examined 28 TBs obtained from donors with bony IEMFs. We classified cochlear and vestibular malformations and analyzed several anatomical features, including the diameter of the internal auditory canal (IAC), the angle of the first genu, the relationship of FN with the oval window (OW), the facial recess (FR), and the overall development of the FN.

Results: Among the TBs, 5 (17.8%) were cochlear hypoplasia-type II, 20 (71.4%) were cochlear hypoplasia-type III, 2 (7.1%) were incomplete partition-type II, and 1 (3.5%) had an isolated vestibular malformation. The IAC diameter was narrow in 2 of 26 TBs (7.7%). The first genu angle was obtuse or perpendicular in 14 of 20 TBs (70.0%). The FN was abnormally located in 8 of 27 TBs (29.6%). The FR was narrow (< 2.5 mm) in 16 of 27 TBs (59.3%). Additionally, the FN was hypoplastic in 18 of 28 TBs (64.3%).

Conclusion: Our study revealed a high prevalence of FN abnormalities among patients with IEMFs. Some of these abnormalities could pose significant challenges when using the traditional FR approach for cochlear implantation. Our findings underscore the importance of thoroughly evaluating the FN course preoperatively to mitigate the risks of surgical complications.

Evidence level: N/A.

Keywords: cochlear implantation; facial nerve; inner ear malformations; temporal bone; temporal bone pathology.

FIGURE 1

Representative horizontal sections of the human temporal bones with inner ear malformations: (A) Cochlear hypoplasia type II (CH‐II). In CH‐II, while the bony contour is normal, the bony cochlear modiolus is absent, and there is a wide communication with the internal auditory canal. (B) Cochlear hypoplasia type III (CH‐III). In CH‐III, the cochlear contour resembles normal, but the bony cochlear modiolus is short, and it is generally smaller with fewer turns. (C) Incomplete partition type II (IP‐II, Mondini's deformity). In IP‐II, the bony septum is present only in the basal turn of the cochlea, and it is absent in the upper turns. The cochlear modiolus is also present only in the basal turn and is absent in the upper turns. The middle and apical turns of the cochlea appear to be fused into a sac‐like structure.

FIGURE 2

Arrow: Stapes arrow head: Outline of the mastoidRepresentative examples of mastoid development. (A) Well‐developed mastoid with robust pneumatization and clearly defined air cells. (B) Hypo‐developed mastoid predominantly filled with bone and bone marrow, exhibiting significantly limited pneumatization and smaller, less distinct air cells.

FIGURE 3

Star: Apex of Bill's bar or the most prominent point of the bottom of the internal auditory meatus at that level where the branching of the FN and superior vestibular nerve can be identified most clearly evaluation of the diameter and length of the internal auditory canal. The longitudinal axis line from the star (*) to the center of the opening of the middle cranial fossa represents the length of the internal auditory canal. The other one indicates the diameter of the internal auditory canal (Interosseous distance) and is perpendicular to the longitudinal axis line and goes through the midpoint of it. (A) This specimen is from one donor with cochlear hypoplasia type II. In this case, the IAC has a normal diameter (≥ 2.0 mm) and length (≥ 5.0 mm). (B) This specimen is from one donor with cochlear hypoplasia type III. In this case, the IAC has a narrow diameter (1.9 mm) and length (≥ 5.0 mm). (C) This specimen is from one donor with cochlear hypoplasia type III. In this case, the IAC has a normal diameter (≥ 2.0 mm) and a short length (3.8 mm).

FIGURE 4

Star: FN arrow head: First genu representative cases of the angle formed by the facial nerve at the first genu being normal (acute or < 75°) and abnormal (obtuse). (A) This specimen is from one donor with cochlear hypoplasia type‐III. In this case, the angle formed by the facial nerve at the first genu is normal (i.e., an acute angle or < 75°). In this case, no apparent hypoplasia of the facial nerve is observed. (B) This specimen is from one donor with an isolated vestibular malformation. In this case, the angle formed by the facial nerve at the first genu is obtuse. In this case, the diameter of the facial nerve is significantly smaller as compared to that of a normal specimen, indicating facial nerve hypoplasia.

FIGURE 5

Star: FN arrow head: OW. The positional relationship between the facial nerve and the oval window in the tympanic segment. (A) This specimen is from one donor with cochlear hypoplasia type III. In this case, the position of the facial nerve relative to the oval window is normal, but there is partial dehiscence of the facial nerve. (B) This specimen is from one donor with cochlear hypoplasia type III. In this case, the position of the facial nerve relative to the oval window is anterior and very close to normal. Furthermore, there is dysplasia of the oval window. Interestingly, this case exhibited cochlear hypoplasia type III on the right side and cochlear hypoplasia type II on the left side. (C) This specimen is from one donor with cochlear hypoplasia type III. In this case, the position of the facial nerve relative to the oval window is more posterior than normal.

FIGURE 6

Star: FN arrowhead: Chorda tympani representative cases of the facial recess (FR) width at the round window membrane level being normal (i.e., 2.5 mm or more) and narrowed (i.e., < 2.5 mm). (A) This specimen is from one donor with cochlear hypoplasia type III. In this case, the facial recess has a normal width (≥ 2.5 mm). (B) This specimen is from one donor with cochlear hypoplasia type III. In this case, the facial recess has a narrow width (1.5 mm).

FIGURE 7

Presentation of a case with prominent facial nerve hypoplasia and partial aplasia. This is a specimen from one donor with cochlear hypoplasia III and an example of a case that particularly demonstrates prominent facial nerve hypoplasia and partial aplasia in our current study. Interestingly, the facial nerve became aplastic before the first genu in the labyrinthine portion (arrow head).

FIGURE 8

Facial nerve which is significantly anteriorly displaced and dehiscent presentation of a case with prominent facial nerve dehiscence in the tympanic segment. (A) This is a specimen from one donor with cochlear hypoplasia type III and an example from our current study that particularly highlights dehiscence of the facial nerve in the tympanic segment. The course of the facial nerve is also significantly displaced anteriorly. (B) This is a section at the level of the round window in the same case of Figure 8A. In such cases, even if the surgeon chooses a trans‐canal approach, special attention should be paid to the fenestration of the cochlear window.