Cochlear implants have considerably improved speech and language outcomes in children with bilateral severe to profound hearing loss [1, 2]. Cochlear implantation is typically offered to individuals who receive limited benefit from conventional stimulation with well-fitted hearing aids. The definition of "limited benefit" for children has changed appreciably in the past 15 years. Early criteria for paediatric cochlear implantation restricted the procedure to children with profound hearing loss who derived essentially no benefit from conventional hearing aids. However, as cochlear implant technology progressed and documented outcomes exceeded early expectations, the audiologic boundaries of candidacy broadened to include children with more residual hearing. Current paediatric audiologic criteria include a sensorineural hearing loss of 90 dB HL or greater and up to 30% or less open-set word speech recognition. However, there appears to be considerable variation in clinical practice regarding the implantation of children with hearing thresholds and/or functional auditory abilities outside these selection criteria. Paediatric cochlear implantation seems now to have arrived at a point of clinical equipoise where there is considerable uncertainty about the boundaries for audiologic criteria in the application of this technology.
The effectiveness of cochlear implantation for children with bilateral severe to profound hearing loss is well documented [1, 3]. However, clinical decisions regarding selection criteria for children with pre-implant residual hearing are complicated by the fact that a wide range of performance has been documented after implantation [4]. A recent systematic review by our research group which analyzed published results of prospective studies of children implanted during the preschool years, found that average open-set speech recognition results ranged between 40% and 70% after 4 -5 years of implant experience (Fitzpatrick et al., unpublished data). Furthermore, the boundary beyond which a cochlear implant offers greater benefit than conventional amplification may also be influenced by other child and family factors such as age of implantation, family involvement, post-implant rehabilitation and educational mode [1, 4–6].
Since cochlear implants first received U.S. Food and Drug Administration (FDA) approval, audiologic selection criteria have been expanded for both adults and paediatric patients. For adults, selection criteria have changed from a profound hearing loss and limited open-set speech recognition in the early 1990s to a 70 dB hearing loss and up to 50% open-set sentence speech perception [7]. Although the broadening of the selection criteria to a 70 dB pure-tone-average hearing loss occurred in 1995 for the adult population, the criteria for children remain a pure-tone- average of less than or equal to 90 dB HL.
Several studies have investigated whether cochlear implantation is beneficial for marginal hearing aid users. In several adult outcome studies, residual hearing and better speech recognition scores before implantation have appeared to be determinants of cochlear implant benefit [8]. Overall, there is empirical support for the implantation of adults with acquired deafness and significant open-set speech perception [7].
Our recent systematic review did not identify any controlled intervention trials comparing the outcomes for groups of "borderline" children who use hearing aids with those who use cochlear implants. Several investigators have compared the auditory capacity of children with cochlear implants to children with hearing aids by using the functional equivalent average hearing level concept. In 1994, Boothroyd and Eran [9] reported that the best cochlear implant users were functioning similar to children with a hearing loss of 70 to 89 dB HL. Several other authors have subsequently explored the equivalent hearing loss concept through a variety of procedures [10–14]. These studies report average functional hearing levels for implanted children ranging from 77 to100 dB HL. The best performers after implantation demonstrate functional abilities comparable to children with hearing loss of 70 to 80 dB HL. This wide range of performance explains the difficulty in drawing conclusions about the benefits of cochlear implantation for any individual child.
Early studies suggested that children with a hearing loss greater than 100 dB HL performed better with a cochlear implant while children with a hearing loss of 90–100 dB HL and early auditory instruction showed results similar to that of hearing aid users. In 1997, Geers [15] cautioned against implanting children with hearing levels less than 90 dB. Recent publications suggest that in some centres, paediatric implantation criteria have broadened to include children whose hearing thresholds and/or pre-implant auditory capacity exceed typical selection criteria [16–18]. These investigators advocate implantation of "select" patients with less severe hearing loss who have previously developed auditory skills with hearing aids. However, identifying these particular patients remains a challenging process supported by little empirical evidence.
The degree of hearing loss or auditory functioning that determines cochlear implant candidacy varies across clinical programs. In some cases, both parents and educators have become strong advocates for implanting children whose auditory skills are outside the normally accepted criteria. This creates a dilemma for the cochlear implant team – how much hearing is too much for a cochlear implant? To address this issue, it is useful to document outcomes after implantation on children who already derive significant benefit from conventional acoustic stimulation. This is particularly important in view of the relatively small number of "borderline" children who have received cochlear implants to date and the apparent increasing demand for this intervention [12]. The purpose of this study was to build on previous reports by examining the improvement in auditory functioning after cochlear implantation for children who were considered "borderline" candidates at our centre.