Anatomical Characteristics of the Facial Nerve in Patients With Cochlear Hypoplasia

To quantitatively characterize the spatial relationships between intratemporal facial nerve segments and key cochlear implantation (CI) landmarks in cochlear hypoplasia (CH), and to compare these features with age-matched CI recipients with normal inner ear anatomy, thereby informing preoperative planning and facial nerve preservation strategies.

CI recipients were reviewed retrospectively, and high-resolution temporal bone CT datasets were reconstructed. The measured parameters comprised cochlear-facial partition width (CFPW) of the labyrinthine segment, the distance between the tympanic facial nerve and oval window (FN‑OW), the distance between the mastoid facial nerve and round window (FN‑RW), the angle between the FN‑RW line and the anteroposterior axis (α), and the distance from the mastoid facial nerve to the posterior external auditory canal wall (FN‑EAC).

The CH cohort included 48 patients (91 ears: 40 CH‑Ⅱ, 43 CH‑Ⅲ, 8 CH‑Ⅳ); controls comprised 45 patients (90 ears). The median CFPW was larger in CH patients than in controls (0.7 vs. 0.57 mm; p < 0.01). Cochlear-facial dehiscence occurred at similar rates (CH 4.40% vs. controls 4.44%). FN-OW did not differ significantly between the two groups (2.33 vs. 2.37 mm; p = 0.198). A low‑lying tympanic segment was observed in 23.07% of CH ears but in none of the control ears. For the mastoid (vertical) segment, the CH group exhibited a shorter FN-RW (5.74 vs. 6.08 mm; p < 0.05) and a larger angle α (60.4° vs. 57.8°; p < 0.05). FN-EAC was slightly smaller in the CH group without reaching statistical significance (4.08 vs. 4.35 mm; p = 0.087).

In CH, intratemporal facial nerve segments exhibit substantial positional variability. Anterior displacement of the vertical segment is common and may impede round window exposure via the facial recess. Preoperative HRCT should systematically assess nerve courses to reduce intra- and postoperative facial nerve complications.