Common cavity anomaly lacks separation between the cochlear and vestibular part of the inner ear. In contrast, the cochlear and vestibular regions are individually identified in the inner ear of IP-I and IP-II, but both lack bony partitions within the cochlea, partly or completely. While the osseous structure of the basal turn including the modiolus is formed in IP-II, the bony modiolus is missing in IP-I. Thus, the primary auditory neurons exist at the center of the cochlea in IP-II, while the distribution of auditory neurons of IP-I varies and is not always located at the central region in the cochlea. Since patients with common cavity and IP-I anomalies have profound deafness at birth, cochlear implantation is the only strategy for them to obtain auditory perception. In contrast, patients with IP-II anomaly often have residual hearing, primarily in low frequencies, at birth, and there are children who acquire spoken language with hearing aids. Their hearings, however, usually deteriorate with age, and cochlear implants take over the role of hearing aids. Anatomical differences among these anomalies influence postoperative hearing and spoken language development.

We performed cochlear implantation in 69 ears of 57 pediatric patients with malformations in the inner ear and/or in the internal auditory canal. Among them, 27 patients reached the age range at which speech perception test was possible and had been followed up more than 1 year after surgery. The test results of 27 ears in these patients, 7 ears with CC, 9 with IP-I, and 11 with IP-II anomaly, were studied (Table 12.2). The results of 22 pediatric CI patients whose hearing loss had been confirmed to be due to GJB2 gene mutation and without inner ear malformation were used as controls. Children with mental retardation and pervasive developmental disorders were excluded from the current study.

The mean age at implantation in IP-II was 71.1 months, which was much higher than the other groups (Table 12.2). The delay of CI surgery in IP-II children was due to their usable residual hearings that enabled them to, at least partly, acquire speech. But they lost hearing afterward and underwent cochlear implantation.

The CIs used in the current pediatric patients were all cochlear devices. In principle, electrode arrays with straight configuration (CI24M, CI24RE-ST, CI422) were selected in CC and IP-I patients, with an exception in which pre-curved electrode (CI24R-CS) was used in one IP-I patient. In contrast, pre-curved arrays (CI24R-CS, CI24RE-CA) were used more in IP-II and in control group with three exceptions in which straight-type electrode arrays (two CI24M and one CI422) were selected. The initial values of mapping parameters, pulse width, stimulation rate, and maxima (the number of electrodes for stimulation to extract sound features), are set at 25 μs, 900 Hz, and 8, respectively. The map for each patient is created by gradually raising the sound intensity from the T level (threshold level) until the charge reaches the C level (maximum comfort level) by observing the responses to the sound. If the charge amount corresponding to T level and C level is not attainable within default current range, a pulse width is widened to create a map at lower current levels. Such adjustments are often necessary in anomalous inner ears, and there are even cases in which certain electrodes are determined to be unusable due to lack of auditory responses in spite of thorough adjustments.