Understanding Dyslexia in the Context of Developmental Language Disorders

Abstract

Purpose: The purpose of this tutorial is to discuss the language basis of dyslexia in the context of developmental language disorders (DLDs). Whereas most studies have focused on the phonological skills of children with dyslexia, we bring attention to broader language skills.

Method: We conducted a focused literature review on the language basis of dyslexia from historical and theoretical perspectives with a special emphasis on the relation between dyslexia and DLD and on the development of broader language skills (e.g., vocabulary, syntax, and discourse) before and after the identification of dyslexia.

Results: We present clinically relevant information on the history of dyslexia as a language-based disorder, the operational definitions used to diagnose dyslexia in research and practice, the relation between dyslexia and DLD, and the language abilities of children with dyslexia.

Conclusions: We discuss 3 clinical implications for working with children with dyslexia in school settings: (a) Children with dyslexia-with and without comorbid DLDs-often have language deficits outside the phonological domain; (b) intervention should target a child's strengths and weaknesses relative to reading outcomes, regardless of diagnostic labels; and (c) those who have dyslexia, regardless of language abilities at the time of diagnosis, may be at risk for slower language acquisition across their lifetime. Longitudinal studies are needed to assess multiple language skills early, at the time of the diagnosis of dyslexia, and years later to better understand the complex development of language and reading in children with dyslexia.

Figure 1.

Three hypotheses about the relation between dyslexia and developmental language disorder (DLD) tested by Catts et al. (2005). Panel a depicts the phonological severity deficit hypothesis (Kamhi & Catts, 1986; Tallal, Allard, Miller, & Curtiss,1997), in which both dyslexia and DLD are caused by phonological deficits, with more severe phonological deficits leading to deficits in nonphonological domains. This hypothesis was rejected because of the existence of numerous children who showed deficits in vocabulary, grammar, and discourse, despite good skills in phonology. Panel b depicts the partial distinction hypothesis (Bishop & Snowling, 2004), in which all children with DLD show poor phonology (and therefore poor word reading), but in addition, they also have deficits in the other aspects of language, including vocabulary, grammar, and discourse. This hypothesis was rejected because of the existence of numerous children who met the standard diagnostic criteria for DLD but did not have poor phonology or poor word reading. Panel c depicts the fully distinct hypothesis, in which dyslexia and DLD are fully distinct disorders, with different underlying deficits. This model was supported by data from a large sample of children drawn from an epidemiologic study investigating the prevalence of DLD (Catts et al., 2005) and has been supported in numerous follow-up studies (e.g., Adlof et al., 2017; Bishop et al., 2009; Fraser et al., 2010; Ramus et al., 2013). Children who are referred to in studies as “poor comprehenders” display poor reading comprehension despite adequate word reading abilities. Studies indicate that approximately one third of poor comprehenders met the standard diagnostic criteria for DLD (Adlof & Catts, 2015; Catts et al., 2006; Nation et al., 2004). The remaining two thirds exhibited moderate deficits in vocabulary, syntax, and discourse, although they did not qualify as having DLD.