Prediction of Difficult Round Window Visibility during Cochlear Implantation via a Reformatted CT Facial Recess View: A Retrospective Study with Surgical Correlation.

Background and purpose: Cochlear implant surgery is performed commonly through the facial recess via the round window (RW) approach. This study aims to evaluate the utility of reformatting the preoperative CT temporal bone scan into a CT facial recess view in alerting surgeons to a potentially difficult surgery with poorly visualized RW.

Materials and methods: This is a retrospective study of 41 patients (43 ears), who had undergone cochlear implant surgery. Intraoperative findings of RW position relative to second genu-mastoid portion of facial nerve, and RW membrane orientation were recorded by the surgeons. Preoperative CTs were analysed by 2 radiologists in both axial and reformatted planes, with the later simulating the surgeon's view via the facial recess. Radiologic assessment markers include the facial nerve-chorda tympani nerve width measured 1.2 mm inferior to the exit point of the chorda tympani nerve into the tympanic cavity, RW position relative to second genu-mastoid segment of the facial nerve, and RW membrane's angle from the vertical axis.

Results: The best predictor for difficult RW intraoperative visibility is the RW position relative to the second genu-mastoid segment of the facial nerve lying lateral to it on CT facial recess reformatted images. A RW that lies partially to completely posterior to the posterior border of the second genu-mastoid segment of the facial nerve had up to 55.6% risk of encountering difficult access, while those positioned anterior to or partially anterior to the anterior edge of the second genu-mastoid segment of the facial nerve had 0% risk of difficult access (P < .05). There are substantial agreements in the intrarater (κ = 0.751, P < .001) and interrater reliability (κ = 0.698, P < .001). There is no significant association between surgical difficulty and facial nerve to chorda tympani distance or RW angle (P > .05).

Conclusions: Identification of RW positions in the reformatted CT facial recess view is a useful tool in predicting potentially difficult RW access in cochlear implant surgery.