Ultra-high-resolution CT of the temporal bone before cochlear implantation for pre-operative prediction of chorda tympani nerve management and round window access.

BACKGROUND AND PURPOSE

To evaluate various anatomical parameters and their relationship to chorda tympani nerve (CTN) injury and round window (RW) access during cochlear implantation.

MATERIALS AND METHODS

Ultra-high-resolution CT images of 66 patients were retrospectively reviewed and compared with operative reports. The facial recess and the round window were analyzed, mainly using the chorda-facial angle (CFA), the width of the facial recess, the CTN-tympanic annulus distance, the RW-mastoid portion of the facial nerve angle, and the type of RW.

RESULTS

Surgical management of the CTN was uneventful in 59 patients, whereas surgical difficulty occurred in 7, with significant differences only between the CTN-tympanic annulus distance and the RW-mastoid portion of the facial nerve angle (P ≤ 0.04). The optimal cut-off values for predicting surgical difficulty were 0.95 mm and 19°, respectively, with sensitivity, specificity, positive, and negative predictive values of 0.71, 0.67, 0.2 and 0.95 for the distance, and 0.57, 0.95, 0.57 and 0.94 for the angle, respectively. The RW was accessible in 51 patients and cochleostomy was performed in 15 patients, without significant difference between radiological parameters, especially concerning the CFA.

CONCLUSION

A CTN-tympanic annulus distance greater than 0.95 mm may help to predict a non-negligible risk of CTN surgical damage, and a RW thinner than 1.85 mm may require exploring the possibility of a cochleostomy approach.