Auditory pathway functioning in prelingual deafness : The clinical consequences for cochlear implantation

Abstract

Cochlear implantation has proven itself as a very successful treatment for both children and adults with severe to profound hearing loss. Although most patients obtain a high level of speech perception, performance is more variable in prelingually deaf patients, i.e., patients with an onset of severe hearing loss during early-language acquisition before the age of two years. One of the major reasons for this variance is thought to be related to differences in the maturation of the auditory pathway. In this thesis we have performed several studies to evaluate clinical functioning of children and adults with prelingual deafness after cochlear implantation. The studies in the adult cochlear implant (CI) users involved neurophysiological measurements assessing the consequences of long-term prelingual deafness.

In the first part of this thesis we focus on the clinical effects of early and bilateral cochlear implantation in prelingually deaf children. Results of a cohort study demonstrate that the introduction of the newborn hearing screening in the Netherlands has resulted in a decline in the age at cochlear implantation in children and a subsequent increase in the number of children who received their implant within their first year of life. Systematic literature reviews indicated that, although randomized controlled trials were lacking, there is increasing evidence suggesting the benefit of bilateral CI in children for especially sound localization and possibly speech- and language development. Besides, multiple studies indicated that the longer the interval between the first and second implantation the poorer the postoperative performance. These results have served as the scientific basis for two reports for the Dutch Health Care Insurance Board, which eventually resulted in the reimbursement of bilateral cochlear implantation for children in the Netherlands.

In the second part we evaluate the postoperative performance of prelingually deaf patients who received their CI in adulthood (because CIs were not yet available in their childhood). A cohort study including only those patients revealed that their average maximum speech perception score was only 25% correct and eventually 21% of this population decided to stop using their implant due to insufficient benefit. Auditory brainstem responses and cortical auditory evoked potentials recorded in these patients revealed that typical potential waveforms could be evoked. However, latencies and amplitudes of the various waveforms differed from those obtained from postlingually deaf CI users. Wave latencies of the brainstem response were prolonged in the prelingually deaf patients, suggesting a slower neural conduction of especially the rostral part of the auditory brainstem. Their cortical N1 through, reflecting activity of mainly the primary auditory cortex, was found to be earlier and larger, possibly reflecting activation of the more immature and less complex components of the auditory cortex. These findings indicate that the elementary structures of the auditory pathway are being developed even in the absence of sufficient auditory stimulation, but refinement of this network is more sensory driven and becomes impaired after long durations of hearing loss. These maturational deficits are likely to contribute to the poor postoperative performance of prelingually deaf CI users with long term deafness.