Many studies that have looked at language outcomes of children with hearing loss have collapsed all children with any degree of hearing loss (mild, moderate, severe, profound) into one group and compared them to children with normal hearing. There haven’t been many studies that have looked at how the degree of hearing loss affects language outcomes, how age at which hearing aids are first fit affect language outcomes, or how these factors might interact with each other.
Tomblin et al. recently published an article sharing results from a longitudinal study that looked at language outcomes from children (aged 2 to 6 years) with varying degrees of hearing loss. (“Language Outcomes In Young Children With Mild to Severe Hearing Loss.” 2015. Ear and Hearing, Vol. 36, p. 76-91).
A really cool thing about this study is that it was a longitudinal study that looked at hundreds of children. A longitudinal study follows the same group of people and collects data from them at different time points, and it can therefore detect changes across time for different groups (for example, how language development changes over time for children with mild hearing loss compared to children with severe hearing loss). Longitudinal studies tend to be quite expensive and time-consuming to run, so this was quite a feat! Here’s more information about longitudinal studies.
Tomblin et al. compared language outcomes for 290 children with hearing loss (grouped into different degrees) with language outcomes for 112 children with normal hearing. The children participated in a battery of age-appropriate language tests that were administered yearly, and the results of the language tests were compared based on factors such as the degree of hearing loss, the age at which children first began wearing hearing aids, and the duration of time per day children typically wore hearing aids.
The study was quite extensive, so I’ll just highlight a few results that I found particularly interesting!
Language Growth Over Time Is Consistent For All Degrees Of Hearing Loss
The researchers found that children with worse hearing (as measured by the Better Ear Pure Tone Average [BEPTA] score) had significantly worse language scores at all age measurements. They also found that language scores significantly improved for all groups of children over time – that is, language scores improved from year to year, for both normal hearing children and children with hearing loss, regardless of the degree of hearing loss.
Interestingly, the researchers also found that the there was no significant difference in the language improvement as a function of degree of hearing loss. So, there was no statistically significant difference in language improvement for children with a severe hearing loss compared to children with a mild hearing loss compared to children with normal hearing – the pattern of growth in language scores was parallel for all groups! (See FIG. 2 of Tomblin, et al., reproduced below).
The Effect of Age of Hearing Aid Fitting On Language Outcomes and Language Growth
The first 2 years of a child’s life are so critical for language development, and so current thinking is that it’s best for a child to start wearing hearing aids as soon as possible to maximize high quality auditory and language input during this critical time, and that fitting hearing aids before 6 months of age is ideal. However, for various reasons, it might not be possible to detect a hearing loss until a child is older, and the results of this study show some interesting results regarding language development for different populations of children who began wearing hearing aids at different ages.
For children who began wearing hearing aids at less than 6 months of age, the researchers found that language outcome growth was fairly stable and flat (relative to the other groups). Recall that in this study, language outcomes were only measured beginning at 2 years of age, so the researchers hypothesized that early hearing aid fitting at < 6 months protected these children from falling behind, or allowed them time to catch up before language outcomes were measured at 2 years. The children who received hearing aids after 1 year had worse language outcomes when measured at 2 years, but showed a steeper increase in language growth over time compared to the earlier fit children – this indicates that even though these children started off with poorer language outcomes compared to the earlier fit children, wearing hearing aids during the critical preschool years allowed them to catch up to their earlier fit peers. (See FIG. 5 of Tomblin, et al., reproduced below).
Overall, the data presented in the article indicates that even though fitting as early as possible is definitely supported by the data, later fitting (done for whatever reason) is also very beneficial and does not necessarily lead to irreversible language deficits.
Morphology and Degree of Hearing Loss
There are many different aspects of language, such as vocabulary, grammar, etc., and childhood acquisition of different aspects of language might be more or less affected by hearing loss. Tomblin et al. hypothesized that there would be a relationship between a child’s degree of hearing loss and their scores on a task that looks at language morphology, and that this relationship would be different than the relationship between degree of hearing loss and a task that looks at vocabulary.
A little bit about morphology: A morpheme is the smallest unit in a language. A simple example is the “s” that turns “dog” into the plural “dogs.” This particular morpheme is particularly subtle – the “s” sound is relatively soft and high in frequency. So, learning the grammatical rule of “one dog” and “two dogs” might be particularly difficult for children with hearing loss (and more difficult than learning the vocabulary word “dog”), because the “s” sound is not as readily accessible to the child in the conversations they hear every day.
Tomblin et al. found that morphology does seem to have a different relationship with degree of hearing loss compared to vocabulary – although both morphology and vocabulary scores were worse with more severe hearing loss, morphology seemed to be more dramatically affected than vocabulary (see FIG. 9 of Tomblin et al., reproduced below).
These results would indicate the importance of children with hearing loss wearing properly fitted hearing aids, so that they have access to the subtle aspects of language in the conversations they hear every day.
Overall, I found the results of this study very encouraging – when hearing loss is identified and properly treated with properly fitted hearing aids, language acquisition and language development, even the more subtle aspects of language, are really very good!