Factors influencing the speech of children with CI and HA

In the previous paragraphs, the effects of hearing loss on the speech and language development of children with a hearing impairment were briefly discussed. Which factors influence hearing-impaired children’s developmental path? Age at implantation and length of device use are often mentioned. However, there are many more factors that affect

children’s speech and language outcomes. The multitude of factors influencing language and speech outcomes make the group of HI children rather heterogeneous. Before introducing the main topics of intelligibility (§1.4) and identifiability (§1.5), these characteristics will be discussed. The factors can be subdivided into to three categories: auditory related, child related and environment related characteristics (Boons et al., 2013).

Auditory related factors

First, numerous hearing related variables have to be differentiated.

Many of these variables are linked to the unique trajectory of an individual with a hearing loss. The first important factor is the onset of hearing loss.

The onset can be prior to birth (congenital hearing loss), prior to acquiring the first language (prelingual hearing loss) or later in life (postlingual hearing loss). After detecting hearing loss, two additional factors determine further development: the aetiology and the degree of hearing loss. Other than an unknown aetiology (which is quite common, even after extensive testing), the two most frequent aetiologies for congenital hearing loss are a CMV infection of the child’s mother during pregnancy or a genetic (for example connexin 26) mutation (Gillis, 2017). There is still some uncertainty about the effect of the aetiology, but overall, it seems that a hearing loss resulting from a CMV infection has less favourable outcomes with respect to speech and language than for example a connexin 26 associated hearing loss (Ramirez Inscoe & Nikolopoulos, 2004).

Concerning the degree of hearing loss, a lower degree of hearing loss leads to better speech and language outcomes and vice versa, severe hearing

losses have a far more negative impact on children’s speech outcomes (Ching et al., 2018; Svirsky et al., 2000a; Tseng et al., 2011).

In addition to the degree of hearing loss and the aetiology, the type of hearing device and the time at which the HI child receives the device are important contributors to variability (Svirsky et al., 2000b). The type of hearing device that is provided depends on the type and the degree of hearing loss (as was already discussed in §1.1). Consequently, small hearing losses that are treated with an acoustic HA result in relatively good speech and language outcomes, whereas children with a severe hearing loss treated with an acoustic HA would possibly reach better results with a CI (Leigh et al., 2016). Another main influential factor is the age at which HI children receive their hearing device (Castellanos et al., 2014; Peng et al., 2004; Svirsky et al., 2007). Obviously, this factor depends on the onset of the hearing loss, i.e. when the hearing loss occurred. For congenitally HI children, the general agreement is that an earlier activation of a hearing device leads to the best results. For children with CI, the most appropriate age to implant has been under debate for a long time considering that the pros (providing auditory stimulation in the sensitive period and hence, providing access to oral communication) have to be weighed against the cons of the medical risks of a surgery at a very young age (Bruijnzeel et al., 2016; Holman et al., 2013; Moreno-Torres et al., 2016; Szagun & Stumper, 2012). However, in recent years, there is a growing consensus of a so-called sensitive period which is characterised by the high plasticity of the auditory system (Kral & Sharma, 2012). Therefore, an implantation before the child’s second birthday is recommended and has been shown to lead to better speech and language outcomes than later implantation (Habib et al., 2010;

Nicholas & Geers, 2007; Ruben, 2018; Schafer & Utrup, 2016; Svirsky et al., 2007). For acoustic HA users, the situation is less complex and the device is provided at a very young age in order to keep the period of auditory deprivation as short as possible. Also, the fitting of a contralateral device, either simultaneously or sequentially, does not only have a positive impact on directional hearing but also on the speech and language development (Boons et al., 2013; Litovsky et al., 2006; Sadadcharam et al., 2016).

Moreover, some of the technical aspects of the cochlear implantation itself can be a factor as well. The questions of interest here are: was the electrode array fully inserted and could all electrodes be activated? Negative answers to those questions are indicative of poorer speech and language outcome.

Also, the domain of cochlear implantation is still developing. Since the beginning of cochlear implantation, there have been technical advances as well as a shift in the candidacy criteria leading to, for example, more and younger pediatric implant users. With respect to their speech and language outcome, children who were implanted in the early stages of pediatric implantation received a less advanced implant than children receive nowadays, which could lead to differences in their speech. Therefore, the calendar year of implantation has to be considered (Montag et al., 2014;

Ruffin et al., 2013). Moreover, the aided hearing threshold, i.e. the remaining hearing loss while wearing the device, has been shown to affect speech outcomes (Laccourreye et al., 2015). Finally, the length of device use, i.e. the period of time starting at the activation of the hearing device until the moment of testing, is of importance. Because other factors, such as the onset of the hearing loss and the chronological age at implantation

child actually has been wearing a device. Considering that HI children have to get used to the auditory experiences, it is generally assumed that a longer length of device use equals better speech and language outcomes (Khwaileh & Flipsen, 2010; Szagun & Stumper, 2012). This is especially the case for children with CI since they oftentimes have very minimal or no hearing prior to implantation and, thus, the change is the largest.

Child related factors

Secondly, there are child related factors such as gender, chronological age, intelligence and whether the child has additional comorbidities. The latter variable is of particular importance since it applies to 30-40% of the population of HI children and can greatly affect children’s speech and language development (De Raeve, 2006;

Nikolopoulos et al., 2008).

Environmental related factors

Finally, environmental factors have to be considered. These are mostly related to the family and educational situation. With respect to the family situation, the most significant question is whether the child is raised by hearing or deaf parents. Approximately 90% of HI children are born to hearing parents (Kretschmer & Kretschmer, 2010) who tend to have a preference for oral communication. When only considering the linguistic aspects, the use of sign language is under debate. Whereas some studies state that children’s speech and language development profits from bimodal communication (Mouvet et al., 2013), other studies indicate that

“[i]f signs are the more salient aspect of communication, auditory and

speech information will receive secondary attention. Thus, it might be that children who use total communication do not reach their potential in terms of speech development because of problems inherent in their method of communication” (Osberger et al., 1994: 178). The choice between oral, sign and total communication, i.e. a combination of oral and sign language, is also reflected in the educational track of the children.

Whereas mainstream schools are mainly focused on auditory-oral communication, special schools also provide sign support. Studies showed that children enrolled in mainstream schools reach better speech and language outcomes than children in special schools (Geers et al., 2003;

Tobey et al., 2003). It should however be noted that children enrolled in special schools are also more prone to additional disabilities (De Raeve &

Lichtert, 2012). Moreover, the aspect of socio-economic status (SES) should be considered. Studies have shown that a higher income, higher maternal education and/or a generally higher SES are predictors of better speech and language outcomes for NH as well as CI children (Cupples et al., 2018; Vanormelingen, 2016).

Considering the large diversity of factors, it is practically impossible in an experimental context to take into account each one of these factors and, at the same time, to reach a decent number of participants. The present dissertation investigates the long-term speech outcome of children with CI and compares this to peers with NH and HA (chapters 4-6). In order to create two homogeneous comparable HI groups, the CI and HA children met the following criteria. They received their device before their second birthday and their speech was assessed at primary school age after

matched on their aided hearing threshold, i.e. the remaining hearing loss.

Moreover, the children all had hearing parents. Consequently, the communication mode with the parents had a clear focus on oral communication with the use of signs as a support. In order to assure that the results could reasonably be ascribed to the children’s hearing impairment, children with additional comorbidities were excluded from the research reported in this dissertation. Moreover, in chapter 3, two groups of children with CI who were implanted in different (calendar) years were compared. The remaining selection criteria were the same: the children were implanted before the age of two and, at the moment of testing, they were in the first years of primary school.