Auditory Performance of Cochlear Implantees

Here's interesting information from National Institutes of Health.

What Factors Affect the Auditory Performance of Cochlear Implant Recipients?

Subject Factors

Auditory performance, defined as the ability to detect, discriminate, recognize, or identify acoustic signals, including speech, is highly variable among individuals using cochlear implants. Since the 1988 CDC on Cochlear Implants, however, some factors associated with outcome variability are now better understood.

Etiology

Because of a larger subject sample, the effects of etiology can now be distinguished from other factors such as the duration of deafness and the age of onset. For example, deafness due to meningitis does not necessarily limit the benefit of cochlear implantation in the absence of central nervous system complications, cochlear ossification, or cochlear occlusion. Children with congenital deafness and children with prelingually acquired meningitic deafness, for example, achieve similar auditory performance if the cochlear implant is received before age 6. In general, etiology does not appear to affect auditory performance in either children or adults.

Age of Onset of Deafness

The age of onset continues to have important implications for success with cochlear implantation, depending on whether the hearing impairment occurred before (prelingual), during (perilingual), or after (postlingual) learning speech and language. At the last CDC, data on cochlear implantation suggested that children or adults with postlingual onset of deafness had better auditory performance than children or adults with prelingual or perilingual onset. Current data about auditory performance in children over longer times support this finding. However, the difference between children with postlingual and those with prelingual-perilingual onset of deafness appears to lessen with time. Large individual differences remain within each group, however.

Age at Implantation

Previous data suggested that prelingually or perilingually deafened persons who were implanted in adolescence or adulthood did not achieve the same level of auditory performance as those implanted during childhood, although individual differences were recognized. Current data continue to support the importance of early detection of hearing loss and implantation for maximal auditory performance. However, it is still unclear whether implantation at age 2, for example, ultimately results in better auditory performance than implantation at age 3.

Duration of Deafness

As deafness endures, even in postlingually deafened individuals, some auditory and linguistic skills may decline and some behavioral traits that work against successful adaptation to a sensory device may develop. Individuals with shorter durations of auditory deprivation tend to achieve better auditory performance from any type of sensory aid, including a cochlear implant, than do individuals with longer durations of auditory deprivation.

Residual Hearing

Initially, cochlear implant use was restricted to persons with profound hearing loss (pure-tone threshold average (PTA) of greater than 100 dB HL and no open-set speech recognition ability with best-fit hearing aids). The average auditory performance of these cochlear-implant users has been better than the average auditory performance of hearing-aid users with some residual hearing, that is, severe hearing loss (PTA > 90 dB HL) and some (<30 percent) open-set speech recognition ability with best-fit hearing aids. Recent data show that auditory performance in people with residual hearing improves after cochlear implantation relative to preoperative auditory performance, although the degree of improvement could not be predicted from preoperative hearing sensitivity. Research is now addressing the critical distinction between the importance of residual hearing sensitivity compared with overall residual auditory capacities and functional communicative status.

Electrophysiological Factors

Some surviving spiral ganglion cells are necessary for auditory performance with a cochlear implant. Degenerative changes occur in both ganglion cells and central auditory neurons following sensorineural deafening. Although a relationship between the number of surviving ganglion cells and psychophysical performance has been demonstrated in animals, a direct relationship between ganglion cell survival and level of auditory performance in humans has not been shown. Animal studies also suggest that electrical stimulation increases ganglion cell survival and also modifies the functional organization of the central auditory system. The implications of these new findings for humans remain to be determined.

Device Factors

The task of representing speech stimuli as electrical stimuli is central to the design of cochlear implants. Designs vary according to (1) the placement, number, and relationship among the electrodes; (2) the way in which stimulus information is conveyed from an external processor to the electrodes; and (3) how the electrical stimuli are derived from the speech input (and other signals). Changes in cochlear implant design/processing strategies and their effects on auditory performance are discussed in the section on technical and safety considerations.

Source: What Factors Affect the Auditory Performance of Cochlear Implant Recipients?