Round Window Orientation and Relevance for Cochlear Implantation in Infants

Introduction: Cochlear implant (CI) eligibility criteria have recently expanded to include infants as young as 9 months. However, many undergo implantation at younger ages, highlighting the growing need to examine potential anatomical limitations in this population. This study aimed to evaluate differences in cochlear and temporal bone structures relevant to round window (RW) access and cochlear implant electrode insertion between early and late infancy.

Methods: This retrospective chart review was conducted at a tertiary university pediatric hospital. Demographic and otologic data were collected on patients with and without sensorineural hearing loss (SNHL) who had received a high-resolution computed tomography before 13 months of age since 2020. Patients without fully pneumatized mastoid air cells or with cochlear vestibular morphological abnormalities were excluded. Angles and distances were measured between structures relevant to cochlear electrode insertion, including the round window (RW), facial nerve (FN), facial recess, basal turn of the cochlea (BTC), and cochlear axis. Wilcoxon rank-sum tests and linear regressions were performed.

Results: A total of 121 ears with normal hearing and 81 ears with SNHL were analyzed, with median ages of 8 months (IQR: 6) and 6 months (IQR: 5), respectively. Congenital CMV was the most common known cause of SNHL (14%), although most cases had undetermined etiology (66%). The presence of SNHL was significantly associated with narrower angles from RW to BTC (p < 0.01), FN to RW (p < 0.05), and cochlear axis to FN (p < 0.05). Within the SNHL cohort, there were significant linear relationships between age at scan and the angles from the facial recess to BTC (slope: -0.58°/month, p< 0.05), and from the FN to RW (slope: 0.46°/month, p< 0.05).

Conclusions: This study highlighted age-related changes in cochlear and temporal bone anatomy which are essential for effective surgical planning in cochlear implantation to minimize complications and optimize the procedure.