Cochlear implantation : experimental and clinical studies

Abstract

Cochlear implantation makes hearing restoration possible in patients with severe to profound hearing loss. However, patients with residual hearing, where a cochlear implant may be combined with acoustic stimulation, and children with malformed cochleae, where the surgery itself as well as language training may be a challenge, are two important groups of patients that require special procedures. These patient groups are the subject of this thesis.

The first study (paper I) examined the effects of cochlear implantation on residual hearing and postoperative histology in a guinea pig model. After mild to moderate levels of surgical trauma, effectuated as a cochleostomy alone or in combination with limited electrode array insertion, hearing recovered after a two-week period of loss (a temporary threshold shift). The intracochlear structures remained unchanged. A second study (paper II) was performed to test the hypothesis that cochlear implantation may induce endolymphatic hydrops, which could lead to hearing loss. The results indicate that hydrops is present during the first week after cochlear implantation.

These experimental studies conclude that the guinea pig cochlea shows high resilience to cochlear implantation and that mechanical damage incurred during surgery does not explain the loss of residual hearing often seen in patients. Secondary mechanisms, such as hydrops, are likely to be involved in the early postoperative period. This information is important as patients with useful residual hearing increasingly receive cochlear implants.

Two clinical studies examined the effects of cochlear implant surgery on children with x-linked inner ear malformation. The first of these (paper III) describes surgical techniques necessary for safe cochlear implantation, and further shows that implantation permits hearing restoration and the development of spoken language in these children. Further analysis of hearing and language outcomes, cognition and mental health (paper IV) revealed poorer outcome in hearing, language and mental health and lower executive functional level, as compared to a control group. Genetic analysis confirmed mutations in the POU3F4 gene on the X-chromosome.

X-linked malformation deafness is usually considered non-syndromic. However, paper IV shows that these children exhibit signs of neuro-developmental problems consistent with attention deficit and hyperactivity, which is likely related to the POU3F4 mutation. Hence, x-linked cochlear malformation should be re-classified as a syndromic form of hearing loss.