Speech Perception of Second Cochlear Implant after 6 Years of Age in Prelingually Deaf Children

Sequential bilateral cochlear implantation in children becomes less effective as the inter-implant interval increases. However, the cause of this and the age at which speech perception becomes impossible are unclear. We examined the cases of 11 prelingually deaf children who underwent unilateral cochlear implantation at our hospitals before the age of 5 years old, followed by a second implantation procedure on the contralateral side when they were aged ≥ 6 years old (6-12 years old). The subjects' hearing thresholds and speech discrimination scores for the second cochlear implant were evaluated at 3 postoperative months and 1-7 years. All of the subjects demonstrated improvements in their hearing thresholds to a mean of 30 dB HL at 1 year. Regarding speech perception, one patient (a 12-year-old), who had developed bilateral hearing loss at 30 months of age after contracting mumps, demonstrated a 90% improvement in his speech discrimination score at 1 year. However, among the other congenitally deaf children, there were two patients whose speech discrimination scores had improved by ≥ 80% at > 4 postoperative years. The congenitally deaf children exhibited poor speech perception despite showing improved hearing thresholds in the ears that received second cochlear implants. Assuming that the auditory pathway beyond the superior olivary complex remained functional, the reduced speech perception abilities associated with the second cochlear implants may have been attributable to the loss of the spiral ganglion and cochlear nucleus cells due to a lack of auditory input since birth.