A Clinical Study of Cortical Auditory Evoked Potentials in Cochlear Implantees

Abstract

Normal maturation of central auditory pathways is a precondition for the optimal development of speech and language skills in children. The temporal cortex gets acoustically tagged due to auditory stimulation and important changes occur in the higher auditory centers due to hearing loss of any type and degree. Cochlear implantation increases auditory sensitivity by direct electrical activation of auditory nerve fibers, enabling phonemic awareness, discrimination and identification ultimately yielding speech understanding. Early implantation stimulates a brain that has not been re-organized and will therefore be more receptive to auditory input and greater auditory capacity. Cortical potentials have enabled us to objectively study this phenomenon. To assess the outcomes of Cochlear implants on the auditory cortex by analyzing cortical auditory evoked potentials (CAEPs) in the habilitation period. This prospective clinical study was performed in 30 pre-lingual candidates with varied etiology of deafness who underwent cochlear implantation at our institute over the last 1 year. The study group had two cohorts (group-1: 0-8 years and group-2: 8-15 years) which included candidates with normal inner ear and no syndromes or handicaps. All implantees in the study group underwent CAEP testing at 6 months and 1 year post-implantation and comparison of the CAEP wave parameters (P1 amplitude, P1 latency and P1 morphology) were done between the two cohorts. In children Implanted early (group-1) there was an early onset rapid increase in P1 amplitude along with a decrease in P1 latency during the follow-up period. Significant change in the CAEP wave morphology was also notable in group-1 unlike in group-2. Candidates who experienced less than 3 years of auditory deprivation before implantation showed P1 latencies, which fell into the range of normal children within 6 months of habilitation. Children with more than 6 years of auditory deprivation, however, generally did not develop normal P1 latencies or morphology even after 1 year of habilitation. The overall outcome with CAEP was much better in group-1 as compared to group-2 and the observations were is in comparison with the existing world literature. The advent of CAEP has objectively proved beyond doubt that there is a critical age for stimulating the auditory brain via cochlear implantation. There is considerable evidence for a developmental sensitive period, during which the auditory cortex is highly plastic. If sensory input is deprived to the auditory system during this sensitive period, then the central auditory system is susceptible to large scale reorganization. Restoring input to the auditory system by Cochlear Implant at an early age can provide the stimulation necessary to preserve the auditory pathways. However, if auditory input is not restored until after this developmental period, then the cross-modal reorganized pathways may exhibits abnormal functional characteristics as observed in recorded P1 amplitude, latencies and morphologies of CAEPs.

Developmental and cross-modal plasticity in deafness: Evidence from the P1 and N1 event related potentials in cochlear implanted children

“…From determination of candidacy for cochlear implantation, to verification of the efficacy of the implant in promoting cortical maturation, the developmental status of the auditory cortex can be monitored and used in clinical decision-making (Carter et al, 2010; He et al, 2012; Hossain et al, 2013; Thabet and Said, 2012). For instance, the P1 CAEP has been employed as a tool in determining whether a patient is a candidate for a cochlear implant.…”

Changes in P1 latencies of children with normal hearing and those with cochlear implants

“…Cortical system plasticity decreases with hearing loss that persists after the age of 7 years [9,27]. In the present study, the CI age of all children was below 7 years.…”

Neural Mechanisms of Hearing Recovery for Cochlear-Implanted Patients: An Electroencephalogram Follow-Up Study

“…The third limitation we should consider is the generality of our EEG indicators. The principle of cochlear implants is that the CI increases auditory sensitivity by direct electrical activation of auditory nerve fibers, enabling phonemic awareness, discrimination, and identification, ultimately yielding speech understanding ( Hossain et al, 2012 ). However, the various styles of cochlear implants with different manufacturers and algorithms (spectral peak, continuous interleaved sampler, advanced confined encoding, etc.)…”