Down syndrome (DS) is a chromosomal genetic disorder that occurs in an estimated 1 in 700-750 live births in the United States (Parker et al., 2010). Trisomy 21 is the most common form of Down syndrome, accounting for 95% of cases. It occurs when there is a complete reduplication of Chromosome 21, resulting in three copies of the chromosome instead of two. DS is a commonly identified cause of intellectual disability (Sherman, Allen, Bean, & Freeman, 2007), and individuals with Down syndrome can demonstrate difficulty with phonological memory, attention, and weaknesses in communication.
Speech and Language Characteristics of Children with DS
A specific behavioral phenotype characterizes the communication ability of individuals with DS, in particular, deficits in expressive language ability (Abbeduto, Brady, & Kover, 2007; Chapman, 2006; Roberts et al., 2007; Roberts, Price, & Malkin, 2007). Studies of individuals with Down syndrome have found greater deficits in receptive syntax than in receptive vocabulary (Abbeduto et al., 2003; Loveall, Channell, Phillips, Abbeduto, & Conners, 2016). Cuskelly, Povey, and Jobling (2016) examined the receptive vocabulary ability of individuals with Down syndrome, ranging in age from age 2;7 to 29;7, on the Peabody Picture Vocabulary Test- Revised (PPVT-R; Dunn & Dunn, 1981) or the Peabody Picture Vocabulary Test- Third Edition (PPVT-III; Dunn & Dunn, 1997). Nonverbal cognitive ability, measured by the Pattern Analysis subtest of the Stanford–Binet Intelligence Scale: Fourth Edition (Thorndike, Hagan, & Sattler, 1986), was related to receptive vocabulary; however, surprisingly, maternal education was not associated with performance. Receptive vocabulary continues to increase for individuals with Down syndrome until approximately 20 years of age, although the rate of increase is slower for individuals with Down syndrome than their typically developing peers (Cuskelly et al., 2016).
Findings regarding expressive language ability have been consistent. Berglund, Eriksson, and Johansson (2001) found that more than half of the children with DS did not have a spoken vocabulary of two words until two-years of age, and a majority did not have a spoken vocabulary of 50 words until four-years of age. Studies report difficulties in expressive morphology and syntax relative to expectations for chronological age and cognitive ability (Chapman, Hesketh, & Kistler, 2002; Chapman, 2006; Fowler, Gelman, & Gleitman, 1994; Miller, 1999). Individuals with Down syndrome are delayed in expressive syntax when compared to other populations with intellectual disabilities such as Fragile X syndrome (Abbeduto et al., 2001; Bellugi, Lichtenberger, Jones, Lai, & St. George, 2000; Ring & Clahsen, 2005; Vicari, Caselli, Gagliardi, Tonucci, & Volterra, 2002), and unknown etiologies (Chapman, 2006). In conversational samples, participants with Down syndrome achieved lower scores on the total score and each of the subscales of the Index of Productive Syntax (IPSYN; Scarborough, 1990) than typically developing (TD) children and males with Fragile X syndrome (FXS) matched for nonverbal cognitive ability.
Similar to receptive vocabulary, studies indicate that children with DS continue to develop expressive language skills throughout adolescence, albeit at a slow pace. In a longitudinal study, 84% of participants with DS showed a positive growth trajectory over six years as measured by mean length of utterance in morphemes (MLUm) in narrative samples (Chapman et al., 2002). Adolescents with DS and TD children matched for MLUm produced narrative samples that were similar in the number of different types of complex sentences as well as the percentage of complex utterances (Thordardottir, Chapman, & Wagner, 2002).
Individuals with DS demonstrate significant delays in speech sound development and production (Dodd & Thompson, 2001; Kent & Vorperian, 2013; Kumin, 2001; Wild, Vorperian, Kent, Bolt, & Austin, 2018). Individuals with DS exhibit delays in the acquisition of speech sounds, particularly consonants, and demonstrate disordered speech production (Kent & Vorperian, 2013). Poor speech intelligibility is a consistent finding in the literature (Kent & Vorperian, 2013; Kumin, 2001; Wild et al., 2018); however, there is some evidence that speech intelligibly may continue to improve over the individual’s life span (Wild et al., 2018). The age at which infants use canonical babbling may be delayed but the delay is relatively smaller when compared to the degree of delay in the acquisition of other developmental milestones for gross motor skills (e.g., walking, standing) (Cobo-Lewis, Oller, Lynch, & Levine, 1996; Kent & Vorperian, 2013; Palisano et al., 2001).
Biological/Physical Characteristics of DS
In addition to an increased risk for communication disorders, individuals with DS are at risk for cardiovascular defects. Approximately 50% of infants with DS are diagnosed with a cardiovascular defect (Bergstrom et al., 2016; Kucik et al., 2013; Vis et al., 2000). Individuals with DS who also have a heart defect have an increased risk of death in the postneonatal period nearly five times that of those children with DS who do not have a heart defect (Kucik et al., 2013). The presence of a heart defect continues to be a significant predictor of mortality through 20 years of age. While the mortality rates have improved across racial and ethnic groups, non-Hispanic White children with Down syndrome continue to have a higher survival rate beyond infancy than non-Hispanic Black children with DS (Kucik et al., 2013).
Studies have also reported that children with DS are at higher risk of hearing loss (Nightengale, Yoon, Wolter-Warmerdam, Daniels, & Hickey, 2017; Phelan, Pal, Henderson, Green, & Bruce, 2016). A retrospective database study of children with DS indicated that 9% of the children had a unilateral hearing loss while 91% had a bilateral hearing loss (Kreicher, Weir, Nguyen, & Meyer, 2018). The most common types of hearing loss were pure conductive and “undefined.” Approximately 19% of the children with DS had a moderate or more severe hearing loss, and 75% of the children had chronic otitis media or multiple episodes of acute otitis media (Kreicher et al., 2018).