Over the last two decades, substantial resources have been dedicated toward the goals of early identification and intervention for children with hearing loss. Notable successes have occurred, including lowering the age of identification and the age of amplification from two years down to a few months of age (Holte. Am J Audiol 2012;21:163-174). However, the evidence to suggest that the early provision of amplification has a positive impact on developmental outcomes in children with hearing loss has been more challenging to document.
The U.S. Preventive Services Task Force noted a lack of prospective, longitudinal research to address the impact of early identification on the development of children with hearing loss (Nelson. Pediatrics 2008;122:e266-e276). Previous studies have largely focused on variables that cannot be altered through intervention, such as the degree of hearing loss or age of identification. However, a recent article by Bruce Tomblin, et al., summarized a longitudinal analysis of language abilities for young children with hearing loss. The goal of the study was to reveal the factors that influence language growth in children who are fit with hearing aids over time and support clinical decisions about amplification. The study emphasized amplification factors, such as whether speech was audible through the child's hearing aid and how much each child wore their hearing aid, which could be targeted for intervention.
The sample for the analysis of language abilities included 302 children with hearing losses that ranged from mild to severe. A cohort of 112 children with normal hearing was also included as a comparison group. All children were between 2 and 6 years old and received norm-referenced, standardized assessments of language annually at a study visit that occurred near their birthday. Different language tests were combined into a single index of language ability over time. The study used growth curve modelling to determine the factors that affected growth in language abilities over time.
The influence of key factors on language development were evaluated, including degree of hearing loss (based on the better-ear pure tone average from the audiogram), audibility of speech through their hearing aids (based on a residualized measure of the Speech Intelligibility Index [SSI]), average amount of daily hearing aid use (based on parent report from a questionnaire about hearing aid use), and the duration of hearing aid use (based on the child's duration of hearing aid use in years). Each predictor represents a different way to quantify the amount of auditory experience that children who wear hearing aids might have. The prediction was that children with lesser degrees of hearing loss, better aided hearing, more consistent hearing aid use, and longer duration of use would have better language abilities than peers with less favorable auditory access across these domains.
Language Outcomes in Young Children with Mild to Severe Hearing Loss
Tomblin, B, Harrison, M, et al
Overall, children with hearing loss had poorer language abilities than peers with normal hearing, as expected. Also not surprising was the impact of the degree of hearing loss on language ability over time because the children with the greatest degrees of hearing loss had the poorest language abilities at each age group. The pattern of results for the amount of speech that was audible through the hearing aids and hearing aid use, however, were more informative for clinical practice. Because each child's degree of hearing loss will influence the amount of speech audible through their hearing aid, a measure of the SII that incorporates both aided and unaided audibility was constructed to independently measure the contribution of amplification to language abilities. This measure was called residualized SII or rSII. The children with hearing loss were separated into four equal groups based on the quartiles of the rSII values. At age 2, the differences between children with the highest and lowest rSII were minimal. By age 6, the difference between the children with the highest and lowest rSII was nearly two-thirds of a standard deviation, reflecting significantly greater growth in language abilities for the children with the best audibility for speech through their hearing aids.
Similarly, children who averaged more than 10 hours per day of hearing aid use by parent report had more positive language growth trajectories than peers with less than 10 hours of use per day. The pattern of language abilities for children with different ages of hearing aid fitting was also informative. Consistent with previous research, children with earlier ages of hearing aid fitting (less than 6 months of age) had better language abilities than peers with later ages of hearing aid fitting (over 12 months of age), particularly at 2 and 3 years of age. The good news was that by 6 years of age, the later-identified children had made progress in closing the language gap with the earlier-identified group.
Collectively, these results have important implications for professionals who serve children who wear hearing aids and their families. The amount of audibility provided by the hearing aid is a crucial predictor of language ability over time, even after the influence of degree of hearing loss has been accounted for. This finding highlights the importance of hearing aid verification and using audibility-based prescriptive formulae to fit amplification for infants and young children. Hearing aid use was also an important factor that professionals can monitor over time. If families report challenges establishing consistent hearing aid use, support can be provided to give families strategies to increase the hearing aid use over time.
These results also suggest that late identification of hearing loss, while not optimal, is also not necessarily a determinant of poor outcomes. Although professionals should continue to strive to identify hearing loss and fit children with hearing aids as early as possible, there is no way to go back in time to change the past when a child is late-identified. These results can encourage professionals and families that progress is possible. Further research in this area should be conducted to see the specific areas of language that might be most susceptible to mild to severe hearing loss, as well as how each of these factors might affect development of those domains.