Speech, Language, and Reading Skills After Early Cochlear Implantation

Medical approaches to deafness were set in motion in 1957, when a substantial level of hearing was restored by a simple wire placed in the deaf ear of a 50-year-old man during mastoid surgery for complications related to chronic otitis media.¹ Since then, clinicians have applied understanding of the ear's neurobiology and increasingly sophisticated digital technologies to enable the perception of sound in deafness.

Prosthetic activation of the auditory periphery is based on robust reserves of auditory neurons that endure in deafness. On average, close to half of the neuronal population survives even when deafness is profound and of early onset.² Further, surviving neurons retain their responsivity to electrical currents. Electrical contacts implanted into the cochlea can generate such currents safely and efficiently. When configured across channels (to convey pitch information), variations in the power and tempo of electrical currents can encode sound via spike trains carried by auditory neurons. Advances in information processing software have enhanced the fidelity with which complex sounds are processed into physiologically meaningful codes.

The functional neurology of the deafened auditory pathway and newly designed implantable devices combine to offer clinical opportunities. Through learning in the early, formative years—or by virtue of an established auditory memory—central auditory stations are capable of processing information generated by an implant to enable speech comprehension and sensitive appreciation of environmental sounds.

Evidence of basic perceptual gains of cochlear implantation is found in consistent improvements in hearing thresholds.³ However, improved thresholds for sound awareness represent only a preliminary measure of the intervening effect of a cochlear implant. A vast range of levels of communication ability are observed in children who receive cochlear implants, and true impact is measured by more consequential outcomes than awareness of sound. Perhaps in no scenario is the complexity of the outcome assessment greater, nor the conclusion more important, than in considering whether a deaf infant should receive a cochlear implant.

Because early-onset deafness is typically recessive in its genetic transmission, or is acquired as a result of infection, most deaf children grow up in hearing households. Manual strategies of signing can overcome many barriers to communication. However, the prevalence of signing in society is limited and the depth of engagement with sign language in hearing families does not systematically expose deaf children to abstract semantic content.⁴ The potential communication mismatch between a deaf infant and a hearing family is profound. Less obvious are the implications of this mismatch; for example, the prediction of a potentially significant barrier to the growth of the parent-child relationship with consequences for later psychosocial development.⁵

The notion of a "successful implant" often relates to the parents' perception of how their child will best relate to the outside world. Hearing parents who seek alleviation of their child's deafness will commonly express a goal of providing their child with options for real engagement with the mainstream, specifically in play and school at an early age, and in vocational options and life chances in adulthood. The pervasive nature of communication, within the family and in society, suggests a criterion standard metric of outcome. Because the goal of restored hearing in a deaf child is to enable useful hearing, a key measure of outcome should reflect how a deaf child's experience with a cochlear implant develops into the effective use of spoken language.

Language can be defined as "an internalized, abstract knowledge that is the basis for communication" and less formally as "a window on thought."⁶ Language is a tool used to reveal ourselves to others in establishing and maintaining relationships. Using spoken language with clarity and nuance facilitates participation in the social mainstream.

Models of development propose that language arises through successive, organizational adaptations. Early stages of language learning require a child to extract acoustic representations from speech streams. Through such experiences, a child discovers regularities that enable insight into grammatical rules of spoken language. As acoustic elements are paired with meaning, a child develops semantic awareness, and this understanding provides a scaffold on which effective use of the spoken word can develop to generate linguistic competence. Social cognition and emotional needs likely drive continued language development.

In the May 2004 issue of the Archives of Otolaryngology–Head & Neck Surgery, Geers⁷ provides a long-term follow-up study of language learning in 181 children with cochlear implants from the United States and Canada. The participants had received their cochlear implant at an average age of 3 years and were tested at the age of 8 or 9 years. Although the technology used in this cohort has now been upgraded, and recent clinical trends indicate a younger average age at implantation, the report highlights important time-sensitive aspects of intervening with a cochlear implant in a child. Children who received a cochlear implant at 2 years of age demonstrated a nearly 3-fold greater rate of developing age-normal speech and language skills than children who received an implant at 4 years of age. Earlier use of an updated speech-processor (one that more faithfully represents the sounds of speech) was significantly associated with greater speech intelligibility. Normal speech and language skills were documented in 80% of children who had lost hearing after birth and who received a cochlear implant within a year of the onset of deafness.

The observations by Geers⁷ provide practical guidelines for identifying candidates for cochlear implants and underscore the importance of basic biological constructs in applying implants to young children. Patterns of neuronal change, synaptic abnormalities, and altered connectivity extend across multiple synaptic stations of the auditory pathway in deafness.⁸ The extent is most pronounced in early-onset deafness in keeping with critical period theory. Conversely, relatively modest levels of sound-evoked activity in auditory units enable the stabilization of neural tracts that comprise the central auditory pathway.⁹ Effects on auditory centers of the brain generated by inputs from a cochlear implant likely form a foundation for supporting higher communication outcomes. Current understanding of the interplay of biology and technology in guiding clinical decisions (for example, when and how to use a cochlear implant in a young child) requires the kind of long-term clinical follow-up offered by Geers.

The May 2004 issue of the Archives of Otolaryngology–Head & Neck Surgery is dedicated to recent developments in childhood cochlear implantation. Based on contributions from the Ninth Symposium on Cochlear Implantation in Children held in Washington, DC, in April 2003, the issue provides diverse perspectives on the progress and challenges of applying modern digital technologies to early-onset deafness. From the relevant synaptic neurobiology to device development to effects on cultural identity, a range of results using novel outcome metrics is described.

Deafness in a young child means that a condition with pervasive cognitive and psychosocial effects occurs at a time when developmental milestones are normally reached in rapid-fire sequence. Consequently, the decision to implement a prosthetic approach to early-onset deafness and the ability to monitor its effects are inherently complex. As basic and clinical research continues to examine how the early experience with a cochlear implant does, or does not, turn into effective use of spoken language, the true impact of this novel intervention may be critically assessed.